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非铂电催化剂综述
非铂电催化剂综述

A review on non-precious metal electrocatalysts for PEM fuel cells

Zhongwei Chen,*a Drew Higgins,a Aiping Yu,a Lei Zhang *b and Jiujun Zhang b

Received 13th October 2010,Accepted 26th May 2011DOI:10.1039/c0ee00558d

With the approaching commercialization of PEM fuel cell technology,developing active,inexpensive non-precious metal ORR catalyst materials to replace currently used Pt-based catalysts is a necessary and essential requirement in order to reduce the overall system cost.This review paper highlights the progress made over the past 40years with a detailed discussion of recent works in the area of non-precious metal electrocatalysts for oxygen reduction reaction,a necessary reaction at the PEM fuel cell cathode.Several important kinds of unsupported or carbon supported non-precious metal

electrocatalysts for ORR are reviewed,including non-pyrolyzed and pyrolyzed transition metal nitrogen-containing complexes,conductive polymer-based catalysts,transition metal chalcogenides,metal oxides/carbides/nitrides/oxynitrides/carbonitrides,and enzymatic compounds.Among these candidates,pyrolyzed transition metal nitrogen-containing complexes supported on carbon materials (M–N x /C)are considered the most promising ORR catalysts because they have demonstrated some ORR activity and stability close to that of commercially available Pt/C catalysts.Although great progress has been achieved in this area of research and development,there are still some challenges in both their ORR activity and stability.Regarding the ORR activity,the actual volumetric activity of the most active non-precious metal catalyst is still well below the DOE 2015target.Regarding the ORR stability,stability tests are generally run at low current densities or low power levels,and the lifetime is far shorter than targets set by DOE.Therefore,improving both the ORR activity and stability are the major short and long term focuses of non-precious metal catalyst research and development.Based on the results achieved in this area,several future research directions are also proposed and discussed in this paper.

1.0.

Introduction

Proton exchange membrane or polymer electrolyte membrane (PEM)fuel cells,which can ef?ciently convert chemical energy into electricity through electrochemical reactions,are considered ideal power sources for future mobile and stationary applications due to their high energy ef?ciency,high power density,as well as low/zero emissions.With the approaching commercialization,

a

Department of Chemical Engineering,Waterloo Institute for Nanotechnology,Waterloo Institute for Sustainable Energy,University of Waterloo,Waterloo,Ontario,Canada N2L 3G1.E-mail:zhwchen@uwaterloo.ca;Tel:+15198884567ext.38664b

Institute for Fuel Cell Innovation,National Research Council of Canada,4250Wesbrook Mall,Vancouver,BC,Canada V6T 1W5.E-mail:lei.zhang@nrc-cnrc.gc.ca;Tel:+16042213000ext.

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Cite this:Energy Environ.Sci.,2011,4,https://www.doczj.com/doc/4b2971447.html,/ees

REVIEW

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several challenges have been identi?ed,including high cost and insuf?cient durability.Although intensive effort has been put on

both cost reduction and durability improvement,with some progress being made in recent years,major breakthroughs in PEM fuel cell technology are still necessary in order to make a sustainable commercialization feasible.

With respect to both the high cost and low durability,PEM fuel cell electrocatalysts are identi?ed to be largely responsible.According to a United States Department of Energy’s (DOE’s)study in 2007,56%of the cost in a PEM fuel cell stack comes from the platinum-based catalyst layers based on a projected cost for large scale fuel cell production.1This results from a signi?cant amount of platinum-based catalysts required in (i)the anode catalyst layer to catalyze the hydrogen oxidation reaction (HOR)or liquid fuel oxidation reaction and (ii)the cathode catalyst layer to catalyze the oxygen reduction reaction (ORR).In particular,when compared to the anodic HOR,the cathode suffers from relatively sluggish ORR kinetics and therefore high overpotential,limiting the performance of a PEM fuel cell.At the current technology status,Pt-based electrocatalysts are

the Zhongwei Chen Zhongwei Chen is an Assistant Professor of chemical engi-neering at the University of Waterloo,where his research focuses on the development of nanostructured materials for PEM fuel cells,lithium-ion and metal-air batteries.Prior to joining Waterloo,Dr Chen worked as a researcher at the Los Alamos National Labora-tory,USA,where he was responsible for developing

advanced non-precious metal

catalyst for PEM fuel cell in a DOE sponsored project.He

has authored over 40peer-reviewed papers and over 30conference presentations.Dr Chen received his PhD in Chemical Engineering from the University of California at Riverside in

2008.

Drew Higgins

Drew Higgins is currently doing his PhD at the University of Waterloo in Dr Zhongwei Chen’s research group.He completed his Masters in 2011on the application of nitrogen-doped carbon nanotubes and their composites as fuel cell catalyst materials.His current research interests lie in the development of (i)non-precious metal and (ii)Pt-alloy mate-rials with controlled nano-structures for fuel cell and battery

applications.

Aiping Yu Aiping Yu is an assistant professor at the University of Waterloo.She received her PhD degree from University of Cal-ifornia,Riverside in 2007.Before joining Waterloo,she was a research scientist at GE.Her expertise in carbon nano-tubes and graphene has enabled her to design the proper porosity and polarity of these nano-materials for the high energy storage battery/supercapacitor

and multi-functional nano-composites.She has published

over 30papers in peer-reviewed

journals,such as Science,and written one book chapters relating to supercapacitor.Her work has been featured by major media reports such as Nature Nanotechnology,https://www.doczj.com/doc/4b2971447.html,,and

https://www.doczj.com/doc/4b2971447.html,.

Lei Zhang Lei Zhang is a Research Council Of?cer at National Research Council of Canada Institute for Fuel Cell Innovation.She received her ?rst MSc majoring in Materials Chemistry from Wuhan University,China in 1993and her second MSc in Materials/Physical Chemistry from Simon Fraser University,Canada in 2000.Ms Zhang’s main research interests include PEM fuel cell electrocatalysis,

catalyst layer/electrode struc-ture,metal-air batteries and

supercapacitors.Ms Zhang has

co-authored over 90publications including 40refereed journal papers,20conference and invited presentations,1book chapter,1book edition,and 30industrial technical reports.She holds 3US patent

applications.

Jiujun Zhang Jiujun Zhang is a Senior Research Of?cer and PEM Catalysis Core Competency Leader at the National Research Council of Canada Institute for Fuel Cell Innovation (NRC-IFCI).Dr Zhang received his BS and MSc in Electrochem-istry from Peking University in 1982and 1985,respectively,and his PhD in Electrochemistry from Wuhan University in 1988.Dr Zhang has co-authored 240

publications including 160

refereed journal papers,6edited books,12book chapters,as well

as 50conference and invited oral presentations.He also holds over 10US/EU/WO/JP/CA patents,9US patent publications,and produced in excess of 80industrial technical reports.

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practical materials in terms of both activity and stability,although their performance is still insuf?cient and needs further improvement.The major drawback of these Pt-based catalysts is their limited availability and high cost,contributing to the excessive production costs of fuel cell systems.With respect to the availability of Pt materials for PEM fuel cells,some argu-ments indicate the importance of Pt recycling.If the Pt in the catalyst layer can be recycled at the end of the fuel cell life with an ef?ciency of 100%,Pt availability may not be a severe issue facing fuel cell commercialization.However,even while excluding the process costs of Pt recycling,the material loss for every full life cycle will draw heavily on global Pt supplies due to realistic recycle ef?ciencies being less than 100%.In addition,the price of Pt has been steadily increasing with no break in this trend expected in the near future.This will only serve to escalate this challenge facing fuel cell commercialization.Various other noble metal based catalysts (i.e.Pd,Ru)have been investigated as alternatives to Pt.These metals have minimal applicability however,as it involves replacing one expensive metal catalyst with another less expensive metal with diminished performance.Thus,developing inexpensive catalyst materials with both high performance and durability has emerged at the forefront of modern research and development efforts towards PEM fuel cell technology and its commercialization.

For PEM fuel cell cost reduction,the search for inexpensive,high performance electrocatalysts for PEM fuel cells has taken two different approaches.The ?rst approach is to reduce catalyst usage through increasing Pt utilization in the catalyst layers.This can be achieved by alloying Pt with inexpensive metals (Co,Fe,etc.),and/or by utilizing unique support materials for Pt nano-particle deposition.In the last two decades,the required Pt loading has been reduced signi?cantly to approximately 0.4mg cm à2.2Unfortunately,the increasing Pt price during this time has totally offset the Pt loading reduction,rendering the efforts made towards Pt loading reduction in the last two decades ineffective.Therefore,it seems that this approach may not be the long-term solution for cost reduction of PEM fuel cells.The other approach is to develop non-precious metal-based electrocatalyst materials.This approach has been stimulated extensively by the recent push for PEM fuel cell commercialization.Thus,development of high performance,non-precious metal-based catalyst materials seems to be the solution for a long-term and sustainable commerciali-zation of PEM fuel cells.Unfortunately,until today,the performance of the best non-precious metal catalysts (generally carbon supported Fe-and/or Co-N catalysts)is still inferior when compared to Pt-based catalysts in terms of both activity and stability.However,the incremental improvement in both activity and stability of non-precious metal catalysts towards their practical usage has been seen in recent years,making this approach more active and promising.

Regarding the development of non-precious metal based catalysts for PEM fuel cell applications,research efforts have become more extensive,covering a broad variety of potential materials,several of which have been reviewed in recent years.3–9The most promising catalysts investigated thus far are carbon-supported M–N (M–N x /C)materials (M ?Co,Fe,Ni,Mn,etc.)formed by the pyrolysis of a variety of metal,nitrogen and carbon precursor materials.4Other non-precious metal electro-catalyst materials investigated include non-pyrolyzed transition

metal macrocycles,conductive polymer based complexes (pyro-lyzed and non-pyrolyzed),transition metal chalcogenides,metal oxide/carbide/nitride materials,as well as enzymatic based elec-trode materials,all of which will be discussed in the ensuing sections.For a practical non-precious catalyst for use in PEM fuel cells,the DOE has set up the activity and stability goals for 2015,which are:(i)the ORR activity must reach a volumetric current density of 300A cm à3at 0.8V iR-free,and the stability must reach 5000hours at operating temperatures below 80 C.10For clarity,catalyst activity refers to the electrokinetics of the ORR occurring on these materials,generally measured at high electrode potentials.The activity goal is evaluated on a volu-metric basis,as non-precious metal catalysts are relatively inex-pensive compared with Pt.Thus,increased loading is not a signi?cant issue unless catalyst layers approach the thickness of $100m m,at which point mass transport issues will arise.2The performance of these catalysts takes into consideration the current densities produced over a broad range of potentials encountered at the cathode of fuel cells during transient condi-tions.Performance thus not only provides a metric of the elec-trokinetics of the ORR occurring in a broad potential range,but also takes into account mass transport,electronic transport and structural/chemical factors arising from the distinct catalyst con?gurations.Stability and durability of electrocatalyst mate-rials comprise the activity and performance retention following accelerated degradation testing or long term operation.Despite signi?cant progress in the ?eld of non-precious metal electro-catalysts for PEM fuel cell applications,both the speci?c activity and stability requirements have not been met yet.Therefore,there is still a long way to go in order to reach the practical usage of non-precious metal catalysts in PEM fuel cell applications.In this paper,insight into the progress made on non-precious metal catalysts with a variety of different structures and properties will be provided,along with the proposed catalytically active sites and reaction mechanisms from various authors.Discussion of the factors in?uencing catalyst performance and strategies for future improvements will also be provided.

2.0.

M–N x /C catalysts

One of the most promising non-precious metal electrocatalysts in PEM fuel cell application is carbon-supported transition metal/nitrogen (M–N x /C)materials (M ?Co,Fe,Ni,Mn,etc.,and normally x ?2or 4),which have gained increasing attention due to their promising catalytic activity displayed towards the ORR,along with the utilization of abundant,inexpensive precursor materials.It was initially demonstrated in 1964that transition metal porphyrins,namely cobalt phthalocyanine (CoPc)could act as ORR electrocatalysts in alkaline conditions.11Later,the catalytic activity of various metal–N 4complexes supported on carbon was demonstrated in acidic media.However,stability issues arose as the catalyst structures were found to decompose in the presence of acid,resulting in a loss of catalytic activity.12A signi?cant breakthrough was achieved years later when high temperature heat treatment procedures ($400to 1000 C)were introduced to the catalyst synthesis process.This approach was demonstrated to modify the carbon supported M–N x /C catalyst materials,increasing the concentration of available ORR active sites while at the same time improving the catalyst stability as

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well.13–18These structural transformations are a subject of extensive debate,where at high temperatures the atomic con?g-uration of the macrocycle compounds was found to partially or completely decompose.These transformations have been

con?rmed by various methods including M €o

ssbauer spectros-copy,19–22

X-ray photoelectron spectroscopy (XPS),20–26extended X-ray absorption ?ne structure (EXAFS),22,27time of ?ight secondary ion mass spectroscopy (ToF-SIMS)24,28–31or ther-mogravimetric mass spectroscopy (TG-MS),26,32where often these characterization techniques are coupled to provide further insight regarding the nature of surface con?gurations remaining following pyrolysis.Interestingly,different catalytically active site structures have been proposed from these studies including M–N 4/C,19,20,22,28,29M–N 2/C 29and N–C 23,26species.These con-?icting reports can most likely be attributed to differences in the synthesis procedures (i.e.type of macrocycle precursor,pyrolysis temperature and duration)and the nature and sensitivity of the characterization techniques utilized.

Due to different structures of the synthesized catalysts after high temperature decomposition,it was proposed that expensive tran-sition metal macrocycle compounds were not actually required,and that catalytically active M–N x /C moieties could be formed with a variety of different precursor materials.17,33,34The only require-ment was a proper metal–nitrogen coordination occurring on a carbon support resulting from heat treatment,to achieve ORR active catalyst materials using simple metal,nitrogen and carbon precursor materials.Verifying this,in 1989,some active ORR electrocatalysts were successfully prepared using polyacrylonitrile (PAN),Co or Fe salts,and a Vulcan XC-72carbon black support.17This provided a new direction of research involving inexpensive precursor materials.Since then,several different types of transition metal-ion centers have been investigated,including Fe,Co,Ni and Cu,along with various non-macrocycle nitrogen source materials such as organic complexes,nitrogen containing salts and even gaseous nitrogen precursors.

In an effort to produce highly active and stable non-precious metal catalysts,ample approaches have been pursued with signi?cant emphasis placed on elucidating the exact effect of synthesis conditions and the nature of the catalytically active sites.Progress in this ?eld of research will be discussed in this section.Non-pyrolyzed M–N x /C catalyst materials will initially be discussed,as they provide favorable structural control in the absence of high temperature treatment procedures and are the fundamental building blocks of pyrolyzed M–N x /C catalysts,which will be discussed thereafter.2.1.

Non-pyrolyzed M–N x /C materials

Transition metal macrocycle compounds have been employed in many applications,speci?cally as catalysts for a variety of different processes.11,35As previously mentioned,CoPc was initially demonstrated to possess ORR activity in alkaline conditions,11which opened a new direction for research in the ?eld of fuel cell catalysis,leading to numerous investigations focusing on the use of transition metal macrocycle compounds as ORR electrocatalyst materials.36–52For fuel cell applications,these catalysts are usually supported on high surface area carbon supports to create suf?cient reaction surface area.The activity of these complexes has been found to be directly related to the metal

ion center and encompassing ligand structure.35,37,44,53–55Simple cobalt-based complexes (i.e.CoPc or Co porphyrin)have cata-lytic activity towards the reduction of oxygen by a 2-electron process to produce H 2O 2,whereas a 4-electron reduction process forming H 2O is commonly observed for Fe-based complexes.39,44,53However,the activity and stability of these non-pyrolyzed M–N x /C catalysts are too low to be used as ORR catalyst materials in a fuel cell.As mentioned previously,a heat treatment process can improve their ORR activity to a promising level.The bene?ts of utilizing non-pyrolyzed macrocycle compounds,however,are important in this ?eld of scienti?c research for fundamental understanding.During high tempera-ture pyrolysis procedures in inert environments,the structures of the catalyst precursors are signi?cantly altered,leading to uncertainty regarding the nature of the catalytically active sites and the ORR mechanism.In the case of non-pyrolyzed macro-cycle complexes,the well-de?ned structure is preserved during simple synthesis procedures.This allows a direct correlation between the catalyst structure and the resulting ORR activity and stability.Thus,numerous studies have been aimed at these types of investigations in order to provide a relationship between the M–N x /C catalyst structures and observed electrocatalytic prop-erties.These studies are in an attempt to elucidate the precise structural effects on the ORR activity and reaction mechanism,which will provide insight for future progress in the synthesis of high performance,non-precious metal electrocatalysts.2.1.1.Metal-ion center effect on the ORR activity and stability.The electrochemical properties of non-pyrolyzed tran-sition metal macrocycle compounds are dependent on a variety of factors.The metal ion center has a signi?cant in?uence on the redox and electronic properties of these compounds.Fe and Co metal-ion centers have been found to display the optimal elec-trocatalytic properties,attributed to their distinct redox prop-erties and have been proposed as the active site for the ORR.44,53These two particular metal types have thus been the focus of the majority of investigations.The nature of the ligand–metal interaction also plays an important role in the ORR activity of these compounds,with various macrocycle structures (i.e.Pc or porphyrins)available,which possess signi?cantly different chemical and electronic properties.Particularly,a high ioniza-tion potential of the metal-ion centers has been deemed an important factor in?uencing ORR activity.54,56,57In one partic-ular study based on density functional theory calculations,Co porphyrins were found to have a higher degree of ionization potential compared to their Fe counterparts (Fe porphyrins).54However,FePc was determined to possess a signi?cantly higher ionization potential than CoPc,elucidating the effect of ligand on these materials’properties and ORR activity.In general,for all of these metal-ion centers investigated,higher ORR activity could be correlated to increased oxygen binding capabilities and higher ionization potentials.

2.1.2.Ring substituent group effect of the non-pyrolyzed transition metal macrocycles on their ORR activity and stability.Moreover,and most signi?cantly,the presence of substituent functional groups on the outer rings of the macrocycle compounds can also serve to tailor the electrochemical and chemical properties of these materials.41–43,46,49–51,58Several

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interesting observations have been drawn from studies investi-gating substituent groups on transition metal macrocycle complexes.Recently,functional group substituents,which are attached on the outer rings of transition metal macrocycle complex materials and their effect on ORR activities were studied by Baker et al.51In this study,FePc/C and FePc/C substituted with three different functional groups as illustrated in Fig.1were investigated.Different substituent groups were shown to have an effect on the redox behavior,the ORR activity and the stability of these complexes in acidic media at different temperatures.Furthermore,substituent groups were found to play an important role in?uencing the exact mechanism of the ORR,with the number of electrons transferred varying between 1and 3for different samples.One possible explanation is that the substituent could affect the mode of macrocycle adsorption on the carbon support which could exert an in?uence on the elec-trochemical activity.This is in agreement with earlier studies highlighting the in?uence that the size and shape of substituted macrocycle complexes will have on the resulting catalytic prop-erties,due to varying alignments on the carbon support and steric effects.37,44Moreover,Baker et al.51further claimed that the varying ORR activities and redox properties of these substituted FePc/C complexes would be caused by altered elec-tron densities of the metal-ion centers and ring structures resulting from the substituent groups.Previous reports have in fact indicated that the electron withdrawing or releasing prop-erties of the substituent groups would result in lower or higher electron densities of the metal-ion center,respectively,59,60which has been found by other groups to result in enhancement of the ORR activity.41,44Moreover,a recent report by Li et al.61indi-cated that functionalizing carbon supported FePc with thioether phenyl groups (Fe-SPc/C)with a structure inspired by naturally occurring ORR catalysts would lead to exemplary stability

enhancements under potentiodynamic conditions in acidic elec-trolyte.This is due to an electron supplying mechanism provided by the functional groups in Fe-SPc/C,reducing the lifetime of highly oxidizing intermediates.This provides indication that careful design of these non-pyrolyzed complexes can not only serve to improve the ORR activity but also have a signi?cant impact on the operational stability of non-pyrolyzed M–N x /C materials.

In summary,although non-pyrolyzed M–N x /C catalyst materials may never ?nd application in PEM fuel cells due to their modest ORR activity and stability,they provide a basis for investigations into the nature of ORR active sites and reaction mechanisms on carbon supported metal–nitrogen complexes.Fundamental understanding of these factors can provide a plat-form for future studies directed at the synthesis of inexpensive,stable,highly active ORR electrocatalysts.2.2.

Pyrolyzed M–N x /C

The discovery that ORR catalytically active M–N x /C materials could be synthesized by the simple pyrolysis of transition metal,carbon and nitrogen containing precursor materials,paved the way for ample investigations of this variety.In order to develop M–N x /C catalysts with the highest catalytic activity and stability under fuel cell operating conditions,the main focus has been placed on systematic trial and error investigations based on optimization of synthesis conditions,precursor materials and catalytic structures.Several factors were deemed important to the activity and stability of pyrolyzed M–N x /C electrocatalysts,including transition metal type and loading,carbon support surface properties and nitrogen content,and heat treatment conditions and duration.Although signi?cant progress over recent years has been achieved in elucidating these factors,the overall understanding is still limited.Catalysts of this class were reviewed in detail in 2008.4Herein,results obtained prior to 2008will be highlighted,along with the progress made recently regarding the in?uence of precursor materials and synthesis procedures utilized on the ORR activity and operational stability of pyrolyzed M–N x /C catalysts.This insight will provide researchers the ability to properly design synthesis procedures to adequately tailor catalytically active sites,with the goal of producing highly active and durable electrocatalyst materials for PEM fuel cell applications.

2.2.1.Non-precious metal types and source materials.Several different non-precious transition metals have been investigated as potential active centers for M–N x /C chelates,including Fe,Co,Mn,Ni,Cu and Cr.Preliminary investigations have indi-cated that for pyrolyzed M–N x /C catalysts,Fe and Co were the most active transition metal-ion centers for ORR catalysis.33,37,62Furthermore,if one catalyst contains two or more different types of metal-ion centers,its ORR activity can be enhanced notably.In an early study investigating the effect of combining metal-ion centers,Chu and Jiang 63revealed that heat treatment of various binary metal macrocycles rendered ORR electrocatalysts more active than those that just had one type of metal-ion center,such as Fe,Co,Ni,or Cu.Additionally,among binary metal catalysts investigated (Co/Fe,V/Fe,Ni/Fe or Cu/Fe),heat treated Co/Fe macrocycles had signi?cantly higher ORR activity than

other

Fig.1Molecular structure of (a)FePc and (b,c and d)various substituted FePc molecules investigated for ORR catalytic activity and stability.Reprinted from ref.51,with permission from Elsevier.

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combinations.Considering Fe and Co centers alone possess the highest ORR activity of pyrolyzed M–N x /C catalysts,it is understandable that a combination of the two is superior to other formulations investigated.These ?ndings have inspired modern research on the development of pyrolyzed Fe/Co catalyst mate-rials,64–71in an attempt to couple the high ORR activity of Fe-based electrocatalysts with the signi?cant stability of Co-based electrocatalysts.When combined,these materials have not only been observed to improve the ORR activity compared to each individual constituent,64–66,68–70,72but have also been demon-strated to result in improved stability.65,68In one example,Zelenay et al.67successfully synthesized heat treated CoFe-pol-ypyrrole supported on carbon.As displayed in Fig.2,this complex had very high ORR performance in a fuel cell setup and only 10%performance degradation after 130h at 0.4V,attrib-uted primarily to catalyst layer ?ooding.This work was based on pyrolyzed conductive polymer materials which will be discussed speci?cally in Section 3.4,however adequately demonstrates the exemplary performance of mixed metal species.Despite signi?-cant work in recent years investigating the ORR activity and stability of binary Fe/Co-based M–N x /C catalysts,the exact nature of the activity and stability enhancements remains unknown.Several authors have proposed that it is a result of some type of complementary mechanism occurring between the two metal-ion centers.63,69Future work should be directed at elucidating these factors in order to insightfully tailor synthesis parameters (i.e.Fe :Co ratios,precursor materials,fabrication

procedures)to develop binary Fe/Co-based M–N x /C catalysts with improved ORR performance and stability.

With respect to the nitrogen precursor materials utilized for pyrolyzed M–N x /C materials,several different macrocycle structures have been investigated including phthalocyanines (Pc),tetramethoxyphenylporphyrin (TMPP),tetraphenylporphyrin (TPP)as well as tetraaza annulene (TAA).As previously mentioned,due to the uncertainty regarding the exact nature and speci?c role of the macrocycle metal-ion centers as the ORR active sites following high temperature decomposition,14,31,73,74several non-macrocycle compounds such as inorganic salts and organometallic complexes have also successfully been employed as catalyst precursor materials.Highly active ORR catalysts have been developed using these simple,readily available precursor materials,which is considered a breakthrough in the targeted research area.Table 1provides several examples of simple inorganic and organic compounds that have been used as precursor materials for Fe-and Co-based pyrolyzed M–N x /C ORR electrocatalysts.These compounds either contain nitrogen in their structure (i.e.transition metal macrocycles)or require the addition of a supplementary nitrogen containing compound due to the absence of nitrogen in their inherent structure (i.e.various inorganic salts or organometallic complexes).Upon pyrolysis,ORR active moieties are formed,with the exact nature of these site structures a subject of debate.The most prevalent uncer-tainty in the literature surrounds whether or not the transition metal species are present in the active site structures,or merely facilitate the incorporation of catalytically active N–C species.This will be discussed in Section 2.2.2,although Table 1also highlights whether or not the speci?c researchers indicated the presence of metal species in the active site structures.

2.2.2.Proposed structures of the M–N x /C catalytic active site for ORR.Regarding the exact nature of the ORR catalytically active site,there are several different structures and formation mechanisms that have been proposed in the literature.Through ToF-SIMS,Dodelet et al.initially proposed that the nature of the catalytic sites involved a coordination between the transition metal ion,nitrogen functional groups and the carbon support,in the form of MN x C y +.29,95In the case of Fe based catalysts synthesized from either FeAc or ClFeTMPP,two different catalytically active sites were observed,namely FeN 2/C and FeN 4/C (Fig.3a and b,respectively).The former was observed to have a higher ORR catalytic activity and its formation was favored at pyrolysis temperatures between 700and 900 C.29Recently however,Dodelet’s group has revised their proposed active site con?gurations claiming that the majority of active site structures consist of an Fe–N 4/C (labeled by the authors as FeN 2+2/C)con?guration bridging two adjacent graphene crys-tallites (Fig.3c).99These are primarily hosted in the micropores of the catalyst materials and with deliberate carbon support selection and synthesis methods to obtain ideal nanostructure con?gurations,active site densities can be improved.100This would adequately explain previous studies correlating ORR activity to the presence of Fe ions coordinated by four nitrogens

observed through M €o

ssbauer analysis.19,20,22,73,144Kramm et al.20have recently attributed improved ORR kinetics of these Fe–N 4centers to Fe-ion centers with higher electron densities.This could possibly be tied back to fundamental investigations

on

Fig.2Fuel cell polarization curve (top)and stability life test (bottom)of a heat treated CoFe-polypyrrole carbon catalyst material.Figur-e obtained from DOE Annual Review Presentation.67

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non-pyrolyzed M–N x /C catalysts discussed previously,in that the distinct chemical properties of the carbon support and nitrogen ligands could exert electron releasing/withdrawing effects on the metal-ion center,in?uencing the catalytic activity observed.These ?ndings reveal that while a focus should always be maintained on the development of M–N x /C catalysts with improved active site densities,it is possible to tune the electronic

and structural properties of these active site structures in order to enhance the ORR electrokinetics.

ToF-SIMS indicated a slightly different nature of catalytically active sites for Co-based M–N x /C catalysts synthesized from either CoAc or CoTMPP.30The presence of CoN x C y sites was observed,similar to those observed for Fe-based M–N x /C cata-lysts;however,only the presence of CoN 4/C species could be con?rmed,and there was no clear conclusion on the existence of other similar CoN x C y arrangements such as CoN 2/C.It was also observed that there was no dominant ORR active site and that all Co-containing groups present possessed the same degree of ORR activity.Interestingly,these ?ndings were different from the conclusions drawn for Fe-based catalysts through ToF-SIMS,however are consistent with other studies correlating ORR activity with Co–N 4species detected with EXAFS.27,141

In the literature,some authors readily support the claims attesting the ORR activity of M–N x /C catalysts to the existence of MN x C y moieties,14,19,20,22,88whereas other authors had a differing opinion regarding the possible role of the transition metal towards the ORR active site.Instead of being a part of the active site structure,it has been proposed that the presence of the metal-ion center serves to catalyze the formation of N/C cata-lytically active sites during the pyrolysis proce-dures.76,108,110,113,119,129,145Speci?cally,the ORR activity is attributed to the formation of graphitic nitrogen and/or pyridinic nitrogen functional groups,or the degree of edge plane exposure the latter represents.This claim has been supported in the liter-ature with ORR activity being reported for metal free N/C catalyst materials.146,147This further demonstrates the convo-luted understanding regarding the exact nature of the catalyti-cally active moieties with respect to ORR.

Table 1Transition metal containing precursors used in the synthesis of M–N x /C catalysts for the ORR and whether or not the corresponding authors deemed metal as part of the active site structures a ,b

Metal Precursor type Compound Active site structures

Metal based Metal free Not speci?ed Iron

Macrocycle

FePc 2426,75,76

77–79

FePcTc 24

FeTPP 22,64,80,81FeClTPP 21,80,82,83

FeClTMPP 19,20,29,73,84–92

74

FeTAA 13

FeTPTZ 93

Inorganic salts

FeAc 17,29,63,87,88,91,92,94–106107–114

115,116FeSO433,117,11823,71,119,12093,121–124Fe(CN)6102

121FeCl x

125–128Organometallic complex Ferrocene 129Cobalt

Macrocycle

CoPc 31,34

7577CoTPP 28,64,82,83,130,131CoTMPP 30,69,132,133134,135CoTAA 13625

137CoTPTZ 138CoTETA 139

140Inorganic salts

CoAc 17,30,34,141113

Co(NO 3)214223,71,119,120

128,143CoSO 433

121CoCl x

127

a

Arguments regarding whether or not metal comprises the active site structures are not always exclusively con?rmed.b Several of these references come from the same groups with multiple works supporting their

hypothesis.

Fig.3(a and b)Earlier proposed catalytically active FeN 2/C and FeN 4/C structures,respectively.(c)Revised FeN 2+2/C (overall FeN 4/C)active site structure bridging two adjacent graphitic crystallites.Adapted and reprinted from ref.99with permission from Elsevier.

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These contradicting reports likely arise due to fundamental and sensitivity variations between material characterization techniques.Insight into active site formation mechanisms and structures is still insuf?cient at the current technology state.It can however be concluded that the type of metal-ion center utilized for M–N x /C catalysts has a signi?cant impact on the properties and ORR activity of these pyrolyzed materials.The exact role of the metal-ion center regarding the ORR active site as well as the structures of the active site is a controversial subject however,and future detailed investigations are required in order to provide further insight into this topic.

2.2.

3.Non-precious metal loading and content.It has been recognized that not only does the type of metal and metal-precursor materials used affect the ORR activity of pyrolyzed M–N x /C catalysts,the relative metal content present in the catalyst also plays an important role in its catalytic activity.Investigations into the in?uence of metal content on the ORR catalytic activity are ample,29,74,87,95,117,122,148–150and this phenomenon has been discussed in detail recently.4Generally,the optimal metal content of pyrolyzed M–N x /C catalysts varies from study to study.For example,in one particular study,149the optimal Co loading in pyrolyzed Vulcan XC-72carbon black supported CoPc catalyst was found to be 3.5wt%.Conversely,another study indicated that a Co content of 7.18wt%in pyro-lyzed Co-tripyridyl triazine (CoTPTZ)supported on Black Pearls 2000resulted in the highest ORR activity of the resultant materials.138These diverse results could be an effect of the varying surface areas and properties of the carbon support utilized (Black Pearl 2000has signi?cantly higher surface areas),the size and quality of dispersion of the metal precursor mate-rials,or to the distinct synthesis and heat treatment procedures https://www.doczj.com/doc/4b2971447.html,monly however,a distinct trend is observed relating metal contents to the ORR activity of pyrolyzed M–N x /C cata-lysts.At low relative metal contents (study dependent)the ORR activity of these materials will increase with increasing metal content up to a maximum saturation point in which a plateau of ORR activity is observed.117,122,148,151This commonly observed trait is illustrated in Fig.4,where for pyrolyzed Black Pearl 2000supported Fe-tripyridyl triazine (TPTZ)electrocatalyst mate-rials,an optimal Fe loading of

4.7wt%can be observed.122Beyond this plateau region,the catalytic activity can actually decrease with increasing metal loadings,117,148attributed to the formation of ORR inert metal particles and/or metal oxide/carbide materials blocking reactant access to the carbon support pores and active sites.74,117,122,151Further investigations indicated the effectiveness of acid leaching procedures to remove these metallic species.22,145,152Improved ORR activity was observed following acid leaching,providing evidence supporting the notion of active site blockages by inert metallic species.Thus,there is no doubt that the transition metal content in pyrolyzed M–N x /C catalysts has a signi?cant impact on the ORR activity.The optimal metal loading is material dependent and determined through systematic investigations over a broad range of values.Future investigations should be directed at determining optimal metal contents to produce the highest density of active sites in M–N x /C catalysts,without too much regard to the formation of inert metallic species.These species can readily be removed by an acid washing post-treatment procedure (which will be discussed

further in Section 2.2.5)without deterioration or removal of the catalytically active site structures demonstrated previously.222.2.4.Carbon support properties:surface nitrogen content and microporosity.The nature and surface properties of the carbon support present in pyrolyzed M–N x /C catalysts have a signi?cant impact on the resulting ORR activity and stability.The surface nitrogen content of the catalyst materials was initially deemed the most critical factor in?uencing the performance,with nitrogen presence an utmost requirement for obtaining materials with ORR activity.153With respect to this,several reports have demonstrated that increases in the surface concentration of nitrogen can be directly linked to an increase in the ORR activity.75,88,95–97Various carbon support materials have been investigated including carbon black materials,17,80,85,86,154oxidized 142or activated carbons,13,25,101,155as well as even carbons doped with nitrogen prior to catalyst synthesis.71,88,95,102Nitrogen incorporation into the surface of the carbon support materials can be obtained by pyrolysis of various nitrogen precursors in the presence of carbon.Nitrogen precursor materials commonly used include:(i)macrocycle complexes such as Pc or TMPP,(ii)organic compounds such as ethylenediamine (EDA),(iii)nitrogen containing polymers such as PAN,and (iv)gaseous precursors such as NH 3or CH 3CN.Furthermore,oxidizing the nitrogen-containing carbon supports prior to pyrolysis could also be effective in facilitating the formation of catalyst active sites due to the preferential adsorption of metal ions on oxidized carbon particles.71,96,101,117,134,142To demonstrate the wide range of nitrogen precursor materials that can be used for synthesis

of

Fig.4(a)ORR polarization curves for pyrolyzed FeTPTZ/C catalyst obtained at a rotation speed of 400rpm in 0.5M H 2SO 4solution and (b)ORR potential at a current density of 0.5mA cm à2as a function of Fe content,displaying an optimal content of 4.7wt%.Reprinted from ref.122,with permission from Elsevier.

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pyrolyzed M–N x /C catalysts,a summary is provided in Table 2.Moreover,Table 2is broken down to whether or not the authors attributed metal species to being part of the active site or not,in order to further highlight this extensive debate present in the literature.

It is worthwhile to point out that the use of NH 3as a gaseous precursor for nitrogen opened up an interesting direction in the ?eld of pyrolyzed M–N x /C catalysts for the ORR.The use of NH 3during pyrolysis above can not only result in the formation of nitrogen functional groups on the carbon support,but also it can lead to the partial gasi?cation of the support during the heat treatment.This support gasi?cation emanates from the disor-dered carbon phase,97,106,162and is demonstrated to be an important factor enhancing the ORR activity of the NH 3derived M–N x /C catalysts.It was demonstrated that in the presence of NH 3,the gasi?cation of the disordered carbon phase could occur at a rate ten times faster than on graphitized carbon 105according to the following reaction mechanism:97

C +NH 3/HCN +H 2

(1)C +2H 2/CH 4

(2)

It was observed that gasi?cation of the disordered phase present in carbon supports could result in the formation of micropores.Recently,the presence of these micropores in the carbon support has been considered a critical factor in devel-oping more active NH 3derived M–N x /C catalysts,as long as there is a suf?cient amount of nitrogen present.94,97,105,106,162,163Studies have indicated that the mass loss accompanying NH 3etching was dependent on the heat treatment time 162and a maximum ORR activity could be usually observed for a mass loss of 30–50%.97,99,105,162These catalysts were prepared ?rst by impregnation of a carbon support with FeAc,followed by a high temperature pyrolysis treatment in NH 3.Calculated weight los-ses were due to NH 3etching of the carbon support and pore

formation,thus the observed activity dependency is speci?c to this class of NH 3derived catalysts.It has been demonstrated that in the presence of a transition metal,nitrogen functional groups formed on the walls of these micropores during NH 3etching could serve to bridge metal ions in creating catalytically active sites.91,99,106

The promising results observed with NH 3etching during M–N x /C pyrolysis have led authors to investigate various ways in order to optimize micropore formation and increase active site densities.Due to the preferential gasi?cation of disordered carbon,the importance of a high degree of disordered carbon has been demonstrated.97,99,100One particular study,based on the investigation of ?fteen different carbon black support materials with varying properties,resulted in the conclusion that increases in the amount of disordered phase carbon and/or optimizing the size of the graphitic particles in the carbon support would result in an improved electrocatalytic ORR activity after NH 3etching.99For further improving the catalytic activity of the M–N x /C material,ball-milling of graphitic based carbon supports has also been investigated in order to induce a higher degree of structural disorder.98,103While CO 2can effectively be used to induce carbon support etching as well and improve ORR activity,the speci?c activity enhancements are inferior compared to NH 3etching which simultaneously incorporates nitrogen groups and facilitates active site formation.159In summary,NH 3etching during heat treatment can effectively increase the microporous volume of the carbon support through preferential gasi?cation of disordered carbon,resulting in a higher nitrogen loading and ORR active site formation.

Utilizing this knowledge gained from previous investigations,in 2009Lefevre et al.94synthesized the best non-precious metal catalysts to date with respect to ORR activity.Due to the recent emphasis placed on active site containing micropores,these authors utilized highly microporous Black Pearl 2000carbon supports.As an alternative approach to active site impregnation with NH 3pyrolysis in the presence of metal precursor,the

Table 2Nitrogen-containing compounds employed in the synthesis of pyrolyzed M–N x /C ORR catalyst materials and whether or not the corre-sponding authors deemed metal as part of the active site structures a ,b

Precursor type Compound

Active site structures

Metal based Metal free

Not speci?ed

Organic

Ethylenediamine 142

23,71,119,120,156

1,10-Phenanthroline 94,117,157

148Nitroaniline 126Aminosilane 125Histidine 115

Cyanamide 118123,1242,2-Bipyridine 104116

N -Methylpyrrole 1412,5-Dimethylpyrrole 1412,5-Dimethyl-3-pyrroline 141Maleimide 141Imidazole

141

Organic polymer Polyacrylonitrile 17,33,34,141Thiourea

141

Inorganic salt Ammonium iron sulfate 93,122,158Gas

Ammonia 30,87,88,90,91,95–103,105,106,151,159

147

148

Acetonitrile

108–114

160,161

a

Arguments regarding whether or not metal comprises the active site structures are not always exclusively con?rmed.b Several of these references come from the same groups with multiple works supporting their hypothesis.

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micropores of the carbon support were ?lled with 1,10-phenan-throline (phen)and iron acetate as pore ?llers by planetary ball-milling.In this way,the best performing electrocatalyst was obtained by exposing this ?lled carbon support to pyrolysis at 1050 C in an Ar environment in order to induce catalytically active site formation into the micropores,anchored between two adjacent graphitic crystals present in the carbon support as illustrated in Fig.5.Subsequently,a second pyrolysis in NH 3was carried out at 950 C,which was found to be dramatically

effective in improving the ORR activity of the ?nal M–N x /C catalyst.Additionally,in this study,two different pore ?ller materials were used,either nitrogen containing (phen),or nitrogen free (perylene-tetracarboxylic dianhydride).In the latter case,NH 3was utilized during the initial pyrolysis in order to provide the required nitrogen content.The activity of the synthesized catalysts was found to be dependent on the type of pore ?ller and carbon support used.157As previously mentioned,this M–N x /C catalyst displayed the highest ORR activity to date,with a volumetric current density of 99A cm à3at an iR-free cell voltage of 0.8V,signi?cantly higher than 2.7A cm à3determined for the previously presumed best non-precious metal catalyst as displayed in Fig.6.94This ORR activity is approaching the target of 130A cm à3set by the DOE for 2010.10Despite signi?cant ORR activity,the stability of these materials under fuel cell operation seemed to be insuf?cient and needs further improve-ments if it is to be used as a cathode catalyst in fuel cells.

The limited stability of NH 3pyrolyzed M–N x /C catalysts is an ongoing issue overshadowing the promising ORR performance observed with these materials.90For example,one study indi-cated that using even small amounts of NH 3during the pyrolysis (only 1.3%NH 3in an NH 3/Ar mixture)of furnace grade carbon black supported ClFeTMPP could result in a considerable decrease in catalyst stability when compared with that pyrolyzed in pure Ar.89A signi?cant increase in ORR activity was observed however,consistent with a previous study indicating reduced activation overpotentials and enhanced kinetic current densities for carbon supported Fe phenanthroline complexes pyrolysed in NH 3compared to Ar.148Clearly there is a trade-off between ORR activity and catalyst stability if NH 3is used during the pyrolysis.89The higher stability of Ar pyrolyzed catalysts has been attributed to a higher degree of graphitization in the carbon support,a factor previously deemed important to catalyst stability.90

Various approaches have also been investigated in order to produce M–N x /C catalysts with a higher degree of porosity

and

Fig.5Schematic of the micropore ?lling mechanism and active site formation displaying (a)two adjacent graphitic crystallites encompassing a slit pore,(b)cross-section of the empty slit pore,(c)the same slit pore after impregnation of pore ?ller and metal precursor materials and (d)active site formation and graphitic growth between graphitic crystallites after heat treatment.From ref.94.Reprinted with permission from

AAAS.Fig.6ORR volumetric current densities at iR-free cell voltages.Dia-monds refer to the most active M–N x /C catalyst formed previously based on pyrolysis of metal salt-nitroaniline precursors in NH 3,126and circles refer to the most active M–N x /C catalyst in the discussed work.94Large symbols correspond to corrected polarization curves for DOE fuel cell reference conditions.From ref.94.Reprinted with permission from AAAS.

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active site densities.Choi et al.120recently employed both Ketjen Black 600and Ketjen Black 300as carbon supports for pyrolyzed M–N x /C catalysts,with micropore areas of 591.9m 2g à1and 154.2m 2g à1,respectively.Fe and Co salts were employed as the metal precursor materials and EDA as a nitrogen source.Ketjen Black 600supported catalysts were found to display higher ORR activity after synthesis,which was attributed to the higher pore volume and surface area.The authors indicated that allowing a suf?cient amount of time for polymerization of the M–N x complex in the micropores prior to pyrolyzation was a critical parameter leading to improved ORR activity.The best per-forming electrocatalyst reported in this work displayed a maximum power density of 0.44W cm à2in a fuel cell setup,along with no performance degradation after being held at 0.4V vs.RHE for 100h as displayed in Fig.7.

The use of foaming agents such as FeC 2O 4during transition metal macrocycle pyrolysis has also been demonstrated to result in porous carbon based catalyst particles with higher catalytic activities due to their signi?cant surface areas.70,164Moreover,another approach for developing carbon supported catalyst materials with high surface area is silica hard template synthesis methods.After heat treatment,highly porous catalyst materials formed by this method were found to display promising ORR activity either in the presence of a transition metal (M–N x /C catalysts)23,107,115,135or even metal free N–C catalysts.147

It is obvious that the properties and surface characteristics of the carbon support have a direct in?uence on the resulting ORR electrocatalyst materials.In particular,it has been determined that nitrogen incorporation has a signi?cant impact,where higher surface nitrogen contents on the carbon support produce catalysts with higher ORR performance.The signi?cant impact of carbon support porosity should also be acknowledged,along with the high degree of nitrogen incorporation and active site formation inside the catalyst micropores.

2.2.5.Heat treatment temperature and conditions.It has been recognized that the high temperature pyrolysis of M–N x /C catalysts in an inert environment can result in heat annealment and active site formation,leading to catalytic materials with both higher ORR activity and stability when compared to their non-pyrolyzed counterparts.55This occurrence was reviewed in 2007,5with the focal points of this review and recent advances highlighted in this section.In general,the optimal heat treatment temperature of M–N x /C catalysts is material dependent and

mainly determined by trial and error based experiments.Different pyrolysis temperatures in inert environments have varying effects on both ORR activity and stability of the synthesized catalysts.An example of the typical pyrolysis temperature effects observed is displayed in Fig.8for a CoTPTZ/C based catalyst pyrolyzed at temperature between 600and 900 C in an N 2environment.138In this aspect,ample systematic investigations have been carried out on the effects of pyrolysis temperature (incomplete

reference

Fig.7(a)Fuel cell performance of a pyrolyzed FeCo–EDA complex supported on carbon black and (b)durability test at a cell voltage of 0.4V for 100h of operation.Adapted reprint with permission from ref.120.Copyright 2010American Chemical

Society.

Fig.8(a)ORR polarization curves obtained at an electrode rotation of 400rpm in 0.5M H 2SO 4for a pyrolyzed CoTPTZ/C catalyst and (b)pyrolysis temperature effect on the ORR potential obtained at a current density of 0.5mA cm à2.Reprinted from ref.138,with permission from Elsevier.

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list).13,23–26,28,31,34,71,81,82,85,93,119,130,138,139,143,155Due to the tempera-ture variations observed from sample to sample,there are no generalized temperature values at this moment.

Normally,M–N x /C catalysts displaying the highest activity towards ORR can be synthesized at pyrolysis temperatures between 500and 800 C in inert environments (N 2or Ar).In this temperature range,it has been proposed that a thermal anneal-ment process occurs between the carbon,nitrogen and metal groups,creating some catalytically active sites responsible for the ORR activity observed through electrochemical measurements.This observed ORR activity is commonly attributed to the pres-ence of coordinated metal–nitrogen chelate complexes or their remaining fragments well dispersed and annealed on the surface of the carbon support material.As previously discussed,the exact nature of the catalytically active sites formed during pyrolysis is a subject of extensive debate.At temperatures approaching and exceeding 800 C,the metal–nitrogen chelate bonds are known to decompose and a reduction in ORR activity was commonly observed.13,26,29,34,81,85,93,143,151,153Various reports have attributed this decrease in ORR activity to either:(i)formation and growth of relatively inert metallic or metal oxide/carbide particles at these excessive temperatures 28,31,34,93,138or (ii)a decrease in the surface nitrogen content of the pyrolyzed M–N x /C catalysts.23,26,119,165

In the case of (i),inert metallic contaminants on the surface of M–N x /C catalysts can serve to block the catalytically active ORR sites themselves,or the micropores they reside in.To combat this,recent reports investigated the effect of acid leaching and repyrolysis on synthesized M–N x /C catalysts.79,107,119,159In one particular investigation,an M–N x /C catalyst synthesized by pyrolyzing iron acetate with a nitrogen doped porous carbon support was developed.107The authors found that the initial pyrolysis step involved the formation of the catalytically active sites,whereas acid leaching and re-pyrolyzation could remove inactive residues (metals and/or metal oxides)and resulting contaminants,respectively.The latter observation could provide explanation for the exemplary ORR performance increment observed by Koslowski et al.159by exposing pyrolyzed CoTMPP/Fe oxalate based catalysts to a second pyrolysis in N 2.This enhancement was coupled with a weight loss of 15%and higher BET surface areas,which could be attributed to the burnoff of active site/pore blocking contaminants remaining from HCl washing.This mechanism however was not discussed in detail.Referring back to the work of Liu et al.,107the initial post-treatment acid leaching step actually resulted in an reduced ORR activity of the sample as seen in Fig.9(Sample Fe–CN x –L),however after re-pyrolysis (sample Fe–CN x –LH),the observed ORR activity was increased slightly compared to that of the catalyst material prior to puri?cation (Fe–CN x ).This demon-strates the bene?t of the contaminant removal and re-pyrolysis procedures.A similar acid leaching procedure to remove surface metals had also been demonstrated in improving catalytic stability of a pyrolyzed CoFeN/C electrocatalyst,71which was attributed to a lower amount of H 2O 2generated in the absence of surface metallic particles.Wu et al.79effectively coupled acid leaching procedures in a FePc/phenolic resin pyrolyzed catalyst,followed by NH 3pyrolysis and etching (previously discussed)to synthesize non-precious catalyst materials with exemplary performance to date,displaying a maximum power density of 560mW cm à2in an H 2–O 2fuel cell setup.

There also appears to be a trade-off between catalyst activity and stability as a result of the heat treatment temperature utilized in inert environments.At lower temperatures (500–800 C),catalysts exhibiting the highest electrocatalytic activity can be created,however these catalysts were found to have limited stability.Improved stability has been observed for catalysts synthesized at higher pyrolysis temperatures in N 2or Ar,however these catalysts displayed an initial ORR activity inferior to that resulting from lower temperature heat treatment proce-dures.The rationale behind these observations is debatable,due to the fact that the catalyst degradation mechanisms are inter-related to the nature and structure of the catalytically active sites,a controversial subject on its own.Therefore,most investigations could only provide empirical results pertaining to the stability of these pyrolyzed M–N x /C catalysts,with little insight provided in order to elucidate the root cause behind these observations.A recent report 23provided new insight into this topic by attributing the ORR active sites of M–N x /C catalysts to pyridinic and graphitic nitrogen species,with the former displaying a higher degree of activity.It was observed that at elevated pyrolysis temperatures,these catalysts contain a higher amount of quaternary nitrogen and less pyridinic nitrogen.This serves to explain the decrease in ORR performance at elevated tempera-tures.The enhanced stability in the presence of more quaternary nitrogen functionalities could be attributed to the fact that,unlike pyridinic nitrogen groups,quaternary nitrogen function-alities do not possess a lone pair of electrons,and are thus less prone to degradation by the protonation reaction.

Non-traditional heat treatment methods have also been developed.These include chemical vapor deposition using either vapor precursor feeds or target sputtering,109,110,165–170sol–gel methods,171–173low temperature plasma carbonisation,174,175ultrasonic spray pyrolysis,176,177as well as microwave based methods.134These methods all provide alternative approaches

to

Fig.9ORR polarization curves for a pyrolyzed Fe-based M–N x /C catalyst indicating the effect of heat treatment,acid leaching and sample repyrolysis.Reprinted from ref.107,with permission from Elsevier.

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synthesizing catalytically active M–N x /C which have shown some success.Vapor deposition and target sputtering methods provide one approach to synthesize homogeneous materials;however deliberate control of catalyst morphologies and nano-structures with this method is a challenge.Sol–gel preparation procedures allow distinct nanostructure control in synthesizing homogeneous,highly porous structures in the absence of a carbon support;however these methods must be coupled with thermal treatments and thus are more complicated multi-step approaches.Low temperature plasma carbonization is an effec-tive method to prevent catalyst particle sintering and surface area loss accompanying traditional pyrolysis procedures;however these methods require complex equipment,are complicated and costly.With one of the major challenges for non-precious metal ORR catalysts being their low active site densities,one important approach is to develop synthesis routes for self-supported cata-lysts.As mentioned earlier,vapour deposition and sol–gel pyrolysis methods are effective in producing homogeneous,self-supported catalyst materials.Recently,Liu et al.176have also applied ultrasonic spray pyrolysis techniques toward the devel-opment of homogeneous,self-supported catalyst materials with controlled morphologies and structures.It was demonstrated that the synthesized self-supported catalysts in this work had high active site densities,with well controlled porous catalyst morphologies and structures.Fig.10a shows a self-supported Fe-polypyrrole catalyst successfully prepared by ultrasonic spray pyrolysis,with a well-de?ned honeycomb-like mesoporous sphere structure,and higher active site densities than a conven-tional carbon-supported catalyst.This self-supported catalyst demonstrated 3.4times higher peak power density than the carbon-supported one in an un-optimized H 2–air fuel cell single-cell test (Fig.10b).Note that this performance was fed with air rather than pure O 2.

2.2.6.Stability of pyrolyzed M–N x /C catalysts.Although the ORR activities of these pyrolyzed M–N x /C catalysts are prom-ising,compared to DOE targets,their performance with respect to stability is still insuf?cient and considered an ongoing chal-lenge.The exact source of the instabilities of these materials is still a subject of controversy in the literature.Some works have reported that detrimental H 2O 2byproducts may result in the degradation of active site structures,elucidated by treating pyrolyzed M–N x /C catalyst materials with H 2O 2solutions.Lefevre and Dodelet 87treated pyrolysed M–N x /C catalysts

(carbon supported Fe macrocycles or salts,prepared under various conditions)with a 5%H 2O 2in H 2SO 4.They reported a loss of ORR activity for all materials,with the magnitude of activity loss being sample dependent.This degradation was attributed to the oxidation/attack of the active site structures by H 2O 2itself,or radical species formed by the simultaneous pres-ence of H 2O 2and Fe ions.Other authors have indicated similar ORR activity losses following H 2O 2treatment procedures.68,73Interestingly,Koslowski et al.19obtained contradicting results for pyrolyzed N 4-macrocycle complexes,where negligible performance loss was observed after treating M–N x /C catalyst materials in concentrated H 2O 2for 10minutes at room temperature.Moreover,increased H 2O 2yield during the ORR is not always linked to catalyst stability when utilizing heat treated M–N x /C materials with different carbon supports 68or different transition metal-ion centers 178(ref.82,unpublished).This indi-cates that instability issues are dependent on the type of precursor materials and synthesis method utilized and can arise due to factors other than H 2O 2production,including catalyst surface properties (i.e.hydrophobic or hydrophilic,porosity and pore size distribution),the degree of graphitization,or the speci?c surface functional groups present.Highlighting the latter point,Popov’s group has recently attributed ORR activity loss to protonation of the catalytically active pyridinic functional groups under acidic conditions.23,107,119These con?icting reports highlight the necessity for future investigations regarding the instability of pyrolyzed N–M x /C catalysts.

In summary,pyrolyzed M–N x /C materials are promising non-precious metal catalysts for the ORR in fuel cell applications.It is well accepted that a high degree of graphitization is important for catalyst durability as these species comprise a very stable form of carbon.The instability issues that arise using even small amounts of NH 3have also been demonstrated 89as discussed previously and are reported to derive from a reduced degree of graphitization.Careful design and synthesis of these materials is very important in order to produce highly active,stable catalysts.In this section,the importance of a variety of factors has been illustrated.The type of precursor materials utilized,including the type of metal and its nominal content in the ?nal catalyst structure are very important parameters in?uencing ORR activity.Furthermore,the carbon support and the resulting surface properties after pyrolysis signi?cantly alter the ORR activity and stability of pryolyzed M–N x /C catalysts,with the importance of a high surface nitrogen content and microporosity illustrated.Finally,the heat treatment conditions,speci?cally the pyrolysis temperature employed will result in performance vari-ations,with the optimal conditions being material dependent and determined by systematic investigations.It should be noted that due to the structural changes during catalyst synthesis,the exact nature of the catalytically active ORR site and reaction mecha-nism are a subject of immense debate.Regardless,the best per-forming non-precious ORR catalyst materials reported to date are generally pyrolyzed M–N x /C complexes.A volumetric current density of 99A cm à3at an iR-corrected voltage of 0.8V was reported for an iron based pyrolyzed M–N x /C catalyst,approaching the DOE 2010target of 130A cm à3.94In a fuel cell setup,maximum power densities approaching up to 450mW cm à2have been obtained for various binary FeCo–N/C pyro-lyzed materials 23,67,71,120and up to 560m W cm à2for a

multistep

Fig.10(a)SEM image and (b)fuel cell polarization curve of a self-supported iron-polypyrrole catalyst synthesized by ultrasonic spray pyrolysis.176Reproduced by permission of The Royal Society of Chemistry.

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FePc/phenolic resin based catalyst pyrolyzed in NH 3.79These are the highest power densities obtained to date for non-precious metal ORR electrocatalyst materials.However,when comparing results of this nature,the effect of fuel cell operating conditions (temperature,gas feed rates and backpressure,anodic/cathodic catalyst loading,etc.)is signi?cant and should always be taken into account.Despite promising ORR activity observed for some of these pyrolyzed M–N x /C catalysts,very little is understood regarding the stability of these materials.179This remains a considerable challenge that must be overcome if these materials are to be used as fuel cell cathode catalysts.3.0.Conductive polymers

Conductive polymers such as polypyrrole (PPy),polyaniline (PANI),polythiophen (PTh),poly(3-methyl)thiophen (PMeT),as well as poly(3,4-ethylenedioxythiophene)(PEDOT)charac-teristically display mixed metal and polymer like properties and are considered ideal for many applications due to their low cost,high electronic conductivity and distinct redox properties.180–182Recently,conductive polymers have been also investigated for application towards non-precious ORR electrocatalysis in three different ways:(i)utilizing conductive polymers as ORR elec-trocatalysts on their own,180,182–184(ii)incorporating non-precious metal complexes into the conductive polymer matrix,180,182,184–193and (iii)employing conductive polymers as a nitrogen/carbon precursor material for pyrolyzed M–N x /C catalysts.66,68,128,194–1963.1.

Catalytic ORR activity of conductive polymer materials

In 2005,Khomenko et al.183investigated the ORR activity of PPy,PANI,PTh,PMeT as well as PEDOT.In order to provide further insight into the nature of ORR catalyzed by these conductive polymers,quantum-chemical calculations were also carried out.With the exception of PEDOT,each of these elec-tronically conducting polymers displayed ORR activity at high overpotentials.It was argued that in order for these conducting polymers to interact with oxygen,they must be in their reduced states which normally occur at these low electrode potentials.In the case of PEDOT,in the potential range required for ORR,this polymer cannot be reduced,resulting in inertness towards the ORR.Based on calculations and consistent with experimental observations it was claimed that the presence of electron donor heteroatoms in?uence the catalytic activity,with nitrogen atoms (PPy and PANI)providing more activity enhancements than sulfur atoms (PTh and PMeT).The fundamental conclusions drawn from this particular study are strikingly similar to those for M–N x /C catalysts and can provide a basis for investigations focused on improving the ORR activity of conductive polymers and their composites.

Later studies reported the ORR activity of various conductive polymer materials supported on carbon.These polymers have either contained nitrogen or sulfur in their inherent structure.Martinez-Millan et al.180,184investigated the ORR activity of carbon supported PPy,PANI,as well as PMeT.For these three carbon supported polymers,ORR occurred at very high over-potentials.The obtained ORR open circuit potentials were 0.62,0.48and 0.63V vs.NHE,respectively.For ORR stability,PPy/C

was tested for 48hours using chronoamperometry,with a steady decrease in activity observed,implying a slow degradation of the catalyst structure.Sulub et al.182investigated the activity of carbon supported PTh,where an open circuit potential of 0.72V vs.RHE was observed,along with a very low onset potential for ORR.Interestingly,the results from these studies 180,182,184differed from the results presented by Khomenko et al.,183where nitrogen based conductive polymers did not necessarily provide higher activity than sulfur based ones.This could indicate some sort of interaction with the carbon black support materials or different preparation techniques.

3.2.Incorporation of transition metals into conductive polymers for ORR activity improvement

Investigations on the incorporation of transition metal complexes into the conductive polymer matrices were proposed in order to further improve the activity of carbon supported conductive polymer composites.Typical examples are transition metal compounds incorporated into PANI or PPy polymers (M–N x con?guration),or incorporated into PTh or PMeT polymer composites (M–S x con?guration).180,182,184,185This method was ?rst proposed by Bashyam and Zelenay,185where they utilized a carbon supported PPy (PPy/C)to entrap Co ions.After successful incorporation,the synthesized catalyst showed steady performance increases during the initial 30hours of fuel cell testing.During this time,the coupling between Co and adjacent nitrogen groups in the polymer was evidenced by the transition of metallic Co to ionic Co,after which promising fuel cell performance was achieved with a maximum power density of ca.0.14W cm à2displayed in an H 2–O 2fuel cell (2.8atm absolute pressure of O 2).Therefore,the observed ORR activity was attributed to the strong Co–PPy interactions,possibly forming Co–N catalytically active sites with the proposed structure shown in Fig.11.In addition,signi?cant performance stability after 100hours of operation was also observed using this

Co–PPy

Fig.11Proposed structure of catalytically active Co–PPy composite formed after entrapment and reduction of the Co precursor in PPy.Reprinted by permission from Macmillan Publishers Ltd.185

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complex as the fuel cell cathode catalyst in an H 2–air fuel cell operating at 0.4V.

Martinez Millan et al.180,184also investigated the ORR activity of carbon supported PPy,PANI,as well as PMeT incorporated with Ni and Co.For all of these conductive polymers,the addition of Ni was found to improve the ORR performance (either in terms of open circuit potential or catalytic current density).In the case of PPy/C,improved stability after Ni incorporation was also observed.184When incorporated with Co,both PPy/C and PMeT/C displayed signi?cantly improved catalytic activities towards the ORR,whereas PANI/C displayed a higher current density,but a slightly lower open circuit potential.180For PPy/C,it was demonstrated that the incorpo-ration of Co could result in a signi?cantly improved stability.The application of PTh as an ORR electrocatalyst was also investigated by the same group.182Using a pure PTh material,the ORR activity observed is insigni?cant,but when it was sup-ported on carbon black,an enhanced activity at high over-potentials could be observed.The addition of Ni or Co into this conductive polymer complex was also investigated,however no obvious improvement with respect to ORR activity was observed.Regarding the ORR stability,the incorporation of Co and Ni into PTh/C had less stability when compared to that of metal free PTh/C.Out of all the materials investigated by this group in a systematic fashion,including PPy/C,PANI/C,PTh/C,and PMeT/C (either pure or incorporated with Ni or Co),Co-incorporated PPy/C was found to display the most promising properties in terms of both ORR activity and stability.180This study reinforces the performance and stability of CoPPy/C catalysts demonstrated by Bashyam and Zelenay,185however does not investigate the fuel cell performance,nor the effects of electrochemical activation that were reported to have a signi?-cant impact on ORR activity of this class of catalyst.Attempts to improve the catalytic activity of CoPPy/C complexes have also been carried out through the synthesis of PPy with three different counter ions (Cl,dodecylbenzenesulfonic acid (DBS)or dode-cylsulfonic acid (DS))in order to optimize the nanostructure and properties of the synthesized polymer.189The authors observed that increasing the interchain distance of the PPy complex could allow more Co incorporation and thus a higher concentration of Co centered catalytically active sites could be obtained.For example,?ve times of amount of Co was incorporated into PPy synthesized using DBS,and the resulting catalyst displayed the highest ORR activity with an onset potential of 0.76V vs.RHE.Reddy et al.190employed multiwalled carbon nanotubes (MWNT)as a support for CoPPy compounds.Improved cata-lytic ORR activity was observed compared with previously reported results in PEM,direct methanol as well as direct ethanol fuel cells.Furthermore,a high stability over 50hours of opera-tion in a PEM fuel cell setup was demonstrated for these mate-rials.Clearly there are methods to tailor the activity and stability of Co-based conductive polymer complexes by chemical func-tionalization,utilizing different carbon supports,etc.and these approaches can be investigated in order to further understand and improve the ORR electrokinetics occurring on these complexes.

Simple inorganic transition metal compounds are not the only materials that have been utilized as ORR catalysts entrapped in conductive polymer matrices.A copper manganese oxide

complex was investigated,immobilized into PPy to form transi-tion metal oxide-conductive polymer composites (Cu 1.4Mn 1.6O 4/PPy)for ORR catalysis.187The electrochemical testing showed that this Cu 1.4Mn 1.6O 4/PPy composite displayed ORR activity at low potentials.Furthermore,the effect of doping anions used during the electropolymerization of this complex was also investigated.Five different anions with the structure of KA (A ?Cl à,PF 6à,ClO 4à,NO 3àor SO 42à)were investigated,and the use of Cl àresulted in materials with the highest ORR activity.It was argued that these doping anions had an effect on the charge transfer properties of the catalyst complexes,resulting in the observed differences in the ORR activity.This highlights the importance of the electronic properties of conductive polymer complexes on the ORR activity and further efforts should be focused on their enhancement.Similar experiments were carried out with carbon cloth supported transition metal oxides encap-sulated coated in PPy.191It was found that copper manganese oxide particles could display some ORR activity with signi?cant stability,with PPy considered a protective electron conducting shell rather than the catalytically active material.3.3.Incorporation of transition metal macrocycles into conductive polymers for ORR activity improvement

The immobilization of various catalytically active non-precious metal macrocycle materials in a surrounding conductive polymer matrix has also been investigated.In 1995,Coutanceau et al.188investigated the immobilization of iron and cobalt tetrasulfo-nated phthalocyanines (CoTsPc or FeTsPc)into PANI and PPy.They observed an increased ORR activity for PPy entrapped Pc macrocycles,however in the case of PANI,very little differences were observed.Over ten years later,various studies have inves-tigated the encapsulation of CoTPP macrocycles 186,192or CoTPPS in PPy on various supports,and some ORR activities were observed.Although these materials all showed catalytic activity towards ORR at high overpotentials,the activities were fairly low as commonly observed on non-precious macrocycle catalysts in the absence of heat treatment.With respect to ORR performance and stability,these materials offer little competition to the development of pyrolyzed M–N x /C catalysts unless drastic performance increments are realized.

3.4.Conductive polymers used as nitrogen and carbon precursors in synthesizing pyrolyzed M–N x /C catalysts

Conductive polymers have also been utilized as precursors in synthesizing highly active M–N x /C catalysts to provide both carbon and nitrogen sources during high temperature pyrolysis.Despite inherent ORR activity observed on transition metal complexes immobilized in conductive polymers,their catalytic ORR activities were still inferior to that of heat treated compounds.Lee et al.194investigated the effect of heat treatment on Co-PPy/C catalysts,along with the in?uence of heat treat-ment temperature.It is obvious that heat treatment on their Co-PPY/C sample can effectively increase the ORR activity,as shown in Fig.12.The authors attributed the enhanced ORR activity to the formation of catalytically active pyrolic and quaternary nitrogen functionalities generated during the heat treatment.A shift in the ORR mechanism was also observed

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from a 2-electron dominant pathway to a more ef?cient 4-elec-tron dominant pathway as a result of heat treatment.

Wu et al.66carried out investigations on pyrolyzed PANI/C complexes,doped with Fe and/or Co.Results from this study indicated that the ORR overpotential of these complexes could be reduced signi?cantly by the incorporation of these metals,showing an onset potential of 0.9V vs.RHE for FeCo-PANI/C.The product selectivity was also increased dramatically,with 99.6%H 2O produced at an electrode potential of 0.4V vs.RHE.This is a signi?cant improvement compared to the 84%H 2O produced by pryolyzed PANI/C.Furthermore,a high stability of this catalyst was also observed with less than a 2%activity loss during 450hours of fuel cell lifetime testing.A later study 68indi-cated superior stability of these FeCo-PANI/C ORR catalysts compared with similar PPy based materials.This enhanced stability was attributed to the aromatic nature of PANI,which has stronger interactions with the metal-ion centers.Moreover,ordered graphitic like structures could be readily formed which are important for catalyst stability.Upon further optimization,these FeCo-PANI/C materials have recently been reported to demonstrate exemplary fuel cell performance with a maximum cell power density of 550mW cm à2and 700hours of operational stability at a cell voltage of 0.4V.128Yuasa et al.196also pyrolyzed a CoPPy/C sample and achieved an enhanced catalytic ORR activity.At a pyrolysis temperature of 700 C,utilizing PPy as the coordinating ligand,Co ions could be entrapped to form Co–N x active centers without metallic Co formation.In a recent study,195p -toluenesulfonic acid (TsOH)was incorporated into pyrolyzed CoPPY/C materials and found to result in improved ORR activity of these electrocatalyst materials,displaying a current density of 0.21A cm à2at a fuel cell voltage of 0.4V,almost double that of pure unmodi?ed pyrolyzed CoPPY/C.Moreover,a higher product selectivity for H 2O production was also obtained.The authors attributed this improvement in ORR activity to the higher amount of nitrogen incorporation into the catalyst materials,which was facilitated by the presence of TsOH.

In summary,electron conducting polymers provide interesting metal/polymer like properties that can be bene?cial in the

synthesis of non-precious metal ORR catalysts.These polymers can be used for both pyrolyzed and non-pyrolyzed catalyst materials.When used as precursors for pyrolysis,these materials display very promising ORR activity and stability,with results and electrocatalyst properties similar to the pyrolyzed M–N x /C catalyst discussed in Section 2.2.When utilized as non-pyrolyzed catalyst materials,limited catalytic ORR activity and stability are still not suf?cient for fuel cell cathode applications.

4.0.Non-precious transition metal chalcogenides for ORR catalysis

Transition metal chalcogenides are one group of ORR catalyst materials which have been studied extensively in the last several decades.There are many reports about the high ORR catalytic activity of Mo–Ru–Se chalcogenide complexes,with perfor-mances approaching that of platinum.197–204However,Ru is considered a precious metal and its high cost and scarce avail-ability hinder its potential application as cathode catalysts in PEM fuel cells.Thus,efforts have been directed at investigating non-precious transition metal chalcogenides,with some progress being made in recent years.

Behret et al.205initially demonstrated the potential application of non-precious transition metal chalcogenide materials as ORR electrocatalysts.These authors prepared various M (a)z M (b)3àz X 4thiospinels,where M (a)?Mn,Fe,Co,Ni,Cu or Zn,M (b)?Ti,V,Cr,Fe,Co or Ni and X ?S,Se or Te.A systematic study on ORR activity was carried out using these various thiospinel complexes in 2M H 2SO 4vs.a hydrogen electrode in the same solution.Co 3S 4complexes synthesized by an aqueous precipi-tation reaction at moderate temperatures ($400to 450 C)dis-played the most promising ORR activity and performance with an open circuit voltage of $800mV vs.RHE,and a current density of 20mA cm à2at 600mV vs.RHE.However,this activity was found to degrade rapidly when compared with other less active compounds synthesized at higher temperatures.Results from this investigation indicated that the ORR activity of the thiospinel compounds was directly related to the type of metal utilized,with an order of Co >Ni >Fe.Moreover,performance decrease was also observed when sulfur was partially replaced with O,Se or Te.This study provides insight into the activity of non-precious transition metal chalcogenides,initiating further investigations into these inexpensive materials as ORR catalysts several decades later.

This inspired theoretical investigations by Sidik and Ander-son 206in order to explain the exemplary activity of Co sul?de species,where three different Co 9S 8surface structures were studied.It was determined that the partially OH covered surface of Co 9S 8(202)should be active for ORR,with an onset potential of 0.74V vs.RHE.No investigations were carried out with respect to ORR stability,thus experimental results would be needed in order to further characterize and optimize surface structures and properties.These interesting ?ndings led to another similar theoretical investigation carried out on Co 9Se 8phase structure,in an attempt to explain the inferior ORR activity of this complex compared with sulfur-based chalcogen-ides.207Results from this quantum computational approach indicated that Co 9Se 8should in fact display a higher over-potential compared to Co 9S 8(>0.22V),which was

consistent

Fig.12Effect of heat treatment and heat treatment temperature on the ORR activity of Co–PPy/C catalyst materials.UT indicates untreated catalyst material.Note:In ?gure legend,O 2refers to half-cell testing conditions;all pyrolysis was done in N 2.Reprinted from ref.194,with permission from Elsevier.

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with the experimental observations reported by Behret et al.205decades earlier.To supplement the theoretical investigation,the authors synthesized CoSe electrodes with unspeci?ed stoichi-ometries by re?uxing dicobalt octacarbonyl (Co 2(CO)8)and selenium powder in a xylene solvent.207The synthesized compound displayed negligible ORR activity in 0.5M H 2SO 4.However,after subjecting the CoSe complex to heat treatment at 900 C in an NH 3environment,considerable ORR activity was observed,with an onset potential of 0.5V vs.RHE.While NH 3pyrolysis of transition metals has been demonstrated to result in the formation of active site structures (Section 2.2.4),in this study there is an absence of a carbon support to anchor these functional groups and XRD analysis indicated improved crys-tallization as opposed to ligand coordinated ion formation.Thus,the authors attributed this increase in ORR activity to the enhanced crystallization of nonstoichiometric CoSe as a result of heat treatment.The observed ORR activity however was still signi?cantly lower than that of Pt-based electrodes.

At around the same time,Susac et al.208synthesized three thin ?lms of CoSe with varying Co/Se ratios,using magnetic sput-tering of different targets.For comparison,the authors also developed an XC72R carbon supported CoSe powder (CoSe/C).The ORR activity of all materials was investigated in 0.1M H 2SO 4.Without any optimization to the surface structure or properties of the complexes,CoSe/C displayed an open circuit potential of 0.78V vs.RHE,slightly higher than the most active CoSe thin ?lm (0.74V vs.RHE).Cyclic voltammetry (CV)was carried out on these materials,and the results indicated that after initial CV sweeps,some surface Co was leached out,leaving a chalcogen rich active surface.The authors proposed that the ORR active sites consisted of a Co de?cient form of CoSe sur-rounded by chalcogen atoms.It was also noted that during electrochemical testing,some Se–S exchange occurred between the surface of the thin ?lms and the H 2SO 4electrolyte,however the exact effects of this occurrence are still unknown.Due to the fundamental differences between the CoSe/C and Pt/C materials,speci?cally the unknown CoSe/C surface area,the authors argued that appropriate comparison with platinum based cata-lysts is obscured.The CoSe/C particles were also very large and non-uniform (ca.10–100nm)with numerous agglomerates observed,thus development of carbon supported CoSe nano-particles with uniform size and proper dispersion could poten-tially increase the applicability of this class of catalyst.

A year later,Susac et al.209expanded on their investigations,preparing FeS 2and (Fe/Co)S 2thin ?lms by a similar magnetic sputtering procedure.The synthesized FeS 2thin ?lms were found to display higher ORR activity compared to naturally occurring mineral FeS 2(pyrite),which had previously been shown to possess ORR activity in acidic conditions.210The variation in ORR activity of these two materials was attributed to differing surface sulfur contents,however the exact mechanism of the activity enhancement is still unknown.In addition,the ORR activity of the synthesized thin ?lms was compared with the ORR activity of a thin ?lm of Pt,allowing adequate comparison based on similar reaction surface areas.Unfortu-nately,this FeS 2thin ?lm displayed a much lower open circuit potential of 0.78V vs.RHE than that of the Pt electrode which had an open circuit potential of 1.02V vs.RHE.Moreover,in terms of catalytic current density at 0.6V vs.RHE,the Pt

electrode had a current density three orders of magnitude larger than that of the FeS 2electrode.In addition,the cobalt containing (Fe/Co)S 2thin ?lm displayed a slightly higher open circuit potential (0.8V vs.RHE)and enhanced catalytic current densi-ties than that of FeS 2thin ?lm electrode.Despite performance still signi?cantly lower than a Pt thin ?lm electrode,this work indicated the possibility of combining transition metal species for improved ORR activity and performance.Whether the observed ORR enhancement arises due to a complementary reaction mechanism occurring between Fe and Co or simply due to the addition of Co sul?de species with a higher activity is unknown.Elucidating this relationship along with investigating other binary transition metal catalysts could provide valuable infor-mation and progress towards the development of transition metal chalcogenide catalysts.

Feng et al.211used a Vulcan XC-72carbon as a dispersing agent and support material in preparing Co 3S 4and CoSe 2carbon supported chalcogenides (Co 3S 4/C and CoSe 2/C,respectively).Co 3S 4/C (20wt%)complexes were synthesized by three different re?ux methods using cobalt carbonyl and sulfur dispersed in p -xylene,followed by a heat annealment at 250 C in pure nitrogen.The two samples synthesized by in situ surfactant free methods displayed promising ORR activity with onset potentials of $0.66to 0.68V vs.RHE in 0.5M H 2SO 4.This activity was signi?cantly higher than that of the third Co 3S 4/C material synthesized by a surfactant based method (onset potential of $0.3V vs.RHE).This poor activity was attributed to the presence of surfactants blocking the ORR active sites.These materials were shown to have electrochemical stability below 0.8V vs.RHE,212however a later report by these authors suggested that these compounds were unstable under long term exposure to air,where phase conversion from Co 3S 4to CoSO 4was observed by XRD anal-ysis.6In addition,the CoSe 2/C compounds developed in this study were also synthesized by a similar organic solvent based re?ux procedure.211After slight modi?cation of these materials,an onset potential of $0.70V vs.RHE and an open circuit potential of 0.81V vs.RHE were observed through electro-chemical testing.213These methods led to more uniform sized particles dispersed on the carbon support and minimal agglom-erates observed compared with the work of Susac et al.;208however these particles were still relatively large (>50nm).Development of synthesis procedures to load carbon support materials with smaller transition metal chalcogenide nano-particles with controlled surface morphologies and properties could provide valuable ORR activity enhancements and should be a focus of future investigations.

Lee et al.214took a novel approach,synthesizing W–Co–Se chalcogenide materials for use as ORR catalysts.W–Co–Se was synthesized by a chemical precipitation method,utilizing xylene as the solvent under re?ux conditions.In this synthesis,W(CO)6,Co 4(CO)12and elemental Se were used as the precursor mate-rials.Through ORR testing in 0.5H 2SO 4,W–Co–Se displayed an onset potential of 0.755V vs.RHE.This chalcogenide was also found to be electrochemically stable between 0.05and 0.8V vs.RHE.The electrochemical stability along with signi?cant ORR activity were not shown when using individual W or Co,indi-cating that the presence of Se might serve to modify the electronic properties of the materials,enhancing the stability and rendering this W–Co–Se catalysts with an ORR activity.Unfortunately,

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the ORR activity and stability of this W–Co–Se catalyst were still signi?cantly lower than those of commercially available Pt/C.Despite this observation,investigating binary transition metal combinations offers an interesting direction for non-precious chalcogenide electrocatalysts.With improved activity of Fe/Co binary combinations previously demonstrated,209other possible binary and even ternary combinations can be investigated systematically including differing formulations of transition metal and chalcogen species.

In summary,although non-precious transition metal chalco-genides have displayed some promise as alternative catalyst materials for PEM fuel cell applications,the observed ORR activity and stability are still signi?cantly lower than those of commercially available Pt/C catalyst,and furthermore,lower than those of precious metal chalcogenide compounds.There-fore,developing novel non-precious chalcogenides (including new binary and ternary combinations)along with the optimiza-tion of bulk and surface properties should be on the forefront of research in this area.The use of a carbon support could also serve to improve the dispersion and reactive surface area of transition metal chalcogenide particles and is an attractive approach.Future work is still required directed at reducing particle sizes while enhancing their surface properties,particle morphologies and dispersion on the carbon support.

5.0.Metal oxides,carbides,nitrides,oxynitrides and carbonitrides

Non-precious metal carbides,oxides,oxynitrides and carboni-trides have been considered promising inexpensive electro-catalyst alternatives for fuel cell applications in recent years,as reviewed by Ishihara et al.8In this ?eld of research,signi?cant advancements have been made in developing this class of tran-sition metal based catalysts with steady performance improve-ments over recent years,as illustrated in Fig.13.5.1.

Metal oxides

Non-precious transition metal oxides possess reasonable ORR activity and stability in alkaline media.However,despite suitable stability in acidic and oxidizing conditions,their limited ORR

activity still remains a challenge.215,216The low ORR activity is attributed to the inherently large band gap (low conductivity)of these materials and arduous adsorption of oxygen on the surface suggesting they may not be suitable as cathode catalysts for acidic fuel cells such as PEM fuel cells.Regardless,numerous studies have been reported in recent years investigating the potential application of metal oxide composites as ORR elec-trocatalysts.For example,Liu et al.217reported that zirconium oxide (ZrO x )thin ?lm catalysts were shown to possess ORR activity at an electrode potential below 0.9V vs.RHE.A later study by this group 218explored other transition metal compounds and found that the ORR activity of these compounds had an order of ZrO 2àx >Co 3O 4àx >TiO 2àx ?SnO 2àx >NbO 5àx .In their most recent study,219ZrO 2àx thin ?lms were synthesized by radio frequency sputtering,and they found that the ORR activity of these samples could be improved if a higher radio frequency power was used during synthesis.These ORR activity increments were linked to a higher degree of oxygen defected state on the catalyst surface and improved electronic conductivity of these materials.

Tungsten oxide based materials (WO x and M x –WO 3where M ?Na,K,Ba,Pb,Tl,U or Cd)have also been employed both as anode HOR and cathode ORR electrocatalysts.220–224However,it was concluded that the electrochemical activity of these materials was too low for application as fuel cell catalysts.

Tantalum oxides,even with a high ORR onset potential,seem to be not promising as potential fuel cell catalysts due to their extremely low bulk electronic conductivity.In order to improve their electronic conductivity,Kim et al.225deposited Ta 2O 5nanoparticles onto a conducting carbon support.These composite materials were found to display an onset potential very similar to that of Pt.Unfortunately,the observed mass speci?c current densities were still insuf?cient,most likely due to poor electron transport to reaction sites.Unless this can be overcome and current densities improved signi?cantly applica-tion of these materials as ORR catalysts may not be possible.In summary,due to the low inherent activity and insulator properties of metal oxides,their performance is still not suf?cient enough yet for practical usage as acidic fuel cell cathode cata-lysts.For developing more active and stable ORR catalysts,more research is necessary in order to provide facile O 2adsorp-tion and enhanced electronic conductivity,coupled with improved stability in acidic media.It appears as if simple metal oxides without further modi?cation to overcome these challenges will not be suitable ORR electrocatalysts.Some of the approaches taken to overcome these challenges will be discussed in the following sections.5.2.

Metal carbides

Tungsten carbide (WC)was reported to possess a similar elec-tronic structure to that of platinum,226,227which inspired several investigations exploring this material and other metal carbide compounds as possible catalyst materials for PEM fuel cell applications.A majority of these studies were focused on the application of WC-based materials as HOR anode cata-lysts.228–233In these studies,high HOR activity of WC materials was observed in a PEM fuel cell setup,233displaying a current density of 0.9A cm à2at a cell voltage of $0.14V,temperature

of

Fig.13Summary of the progress over recent years with respect to transition metal oxide,carbide and nitride based materials as ORR electrocatalysts.Reprinted from ref.8,with permission from Elsevier.

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80 C and backpressure of 3atm.This performance was still inferior to that of commercial Pt/C.

Due to the limited stability of carbide materials in acidic and highly oxidizing environments such as those encountered during operation of a PEM fuel cell cathode,the potential application of pure WC and other carbide materials as ORR electrocatalysts seems to be unfeasible if the composition and structure of such materials are not modi?ed.To address these composition and structure modi?cations,two different efforts have investigated the effect of incorporating Ta into WC materials.234,235Enhanced electrocatalytic activity and stability towards the ORR were observed in the presence of Ta,235attributed to the formation of a W–Ta alloy phase.

Ni–Ta–C compounds formed by sputtering techniques were also tested as ORR electrocatalyst materials.236,237These synthesized materials were found to have signi?cant stability which was attributed to the formation of a thin TaO protective ?lm on the catalyst surface.236Using this material,adequate ORR activity was also observed,indicating that this ?lm did not hinder electron transport.The effect of heat treatment on N–Ta–C was also reported to provide an enhancement on the observed ORR activity,however the activity was still extremely low (ORR onset potential of 0.45V vs.RHE).2375.3.

Metal nitrides

Transition metal nitrides have relatively good stability in acidic conditions and under high electrochemical potentials;therefore they could be considered as potential ORR electrocatalyst materials.8For example,carbon supported tungsten nitride (W 2N/C)was investigated using a single PEM fuel cell setup,where signi?cant stability after 100hours of operation was observed.238Although the ORR activity of this material was relatively low (ORR onset potential of 0.6V vs.RHE),its high stability renders it as a promising non-precious electrocatalyst.Molybdenum nitride materials were also investigated as ORR electrocatalysts.239When supported on carbon black,these MoN/C catalyst materials were found to display an open circuit potential of over 0.7V in a single fuel cell setup.Notably,signi?cant stability of this material was observed during 60hours of operation.Xia et al.240further demonstrated the catalytic activity of MoN/C catalysts,synthesized using an ammonia heat treatment procedure of MoTPP/C.The effective reduction of oxygen by a 4-electron pathway to produce water was attributed to the hexagonal MoN structure formed during the synthesis.No mention was made however to the similar preparation proce-dures to pyrolyzed M–N x /C https://www.doczj.com/doc/4b2971447.html,ly,a heat treatment of a transition metal macrocycle was utilized using NH 3treat-ment in the presence of a carbon support.The possibility of supplementary M–N x /C or metal free N–C active site structures could potentially contribute to the observed ORR activity and could be probed by more sensitive characterization techniques.Carbon supported niobium nitride complexes formed by a polymerized complex method and subsequent nitration pyrol-ysis under ?owing NH 3have also been investigated.241,242Once again heat treatment of carbon supported transition metal species in NH 3was utilized where ORR activity was observed,however the authors attributed it to the formation of high degree of crystallinity and surface concentration of Nb 5+ions.

Moreover,it was found that the formation of surface Nb 5+could be facilitated by the incorporation of Ba.This could serve to hinder the formation of competing Nb 4+ions,resulting in an ORR onset potential of 0.77V vs.RHE.241

Carbon supported Co–W (CoW/C)treated with NH 3at high temperatures was also found to possess some catalytic activity towards the ORR.243It was found that the catalytic ORR activity was strongly dependent on the synthesis conditions.At the optimized synthesis conditions,highly active CoW/C catalyst was obtained,which was composed of CoW nitride and CoW oxynitride as determined by XRD analysis.Through half cell testing,an onset potential of 0.749V vs.RHE and a current density of à0.158mA cm à2at an electrode potential of 0.6V vs.RHE were obtained for the most active catalyst treated at Co–W/C catalyst treated in NH 3at 550 C.The ORR activity was attributed to the metal nitrides and pyrollic nitrogen species present in the carbon support.5.4.

Metal oxynitrides

Metal oxynitrides are another kind of materials which show some ORR activity.These kinds of materials are commonly formed by substitutionally doping nitrogen into metal oxides.It was reported that this doping could effectively reduce the high inherent band gap of metal oxides,leading to enhanced electron conducting properties important to electrocatalysis.244Ta oxy-nitrides were investigated as ORR electrocatalysts and found to have a promising onset potential of 0.8V vs.RHE.216In addition,the incorporation of carbon into this kind of material could also further enhance their ORR activity.245

Another example is carbon supported zirconium oxynitride [ZrO x N y /C]catalysts formed by pyrolyzing ZrO 2/C at a high temperature in the presence of ammonia.246This catalyst mate-rial gave an ORR onset potential of 0.7V vs.RHE.In a later study,247ZrO x N y thin ?lms synthesized using a reactive sput-tering method displayed an ORR onset potential of 0.8V vs.RHE.In terms of ORR activity,the most favorable catalyst structure present in these types of materials was found to be crystalline Zr 2ON 2phase atomic structures.In these studies,utilizing ammonia did not result in improved ORR activity compared with the reactive sputtering method in an N 2/O 2environment,indicating that the same active site formation mechanisms and ORR performance enhancements of NH 3derived M–N x /C catalysts are not as readily applicable to this class of materials.5.5.

Metal carbonitrides

Transition metal carbonitrides have also been explored as ORR catalyst materials in the literature.For example,a Co x C 1àx ày N y thin ?lm was synthesized by Yang et al.,170using a method of magnetron sputtering coupled with heat treatment.ORR measurements showed that this thin ?lm had some catalytic ORR activity with mainly a 2-electron pathway producing H 2O 2.This approach was further expanded to other transition metals,where one study indicated that Cr,Co and Ni carbonitrides could display ORR activity,whereas V and Mn carbonitrides’activity was negligible.248In addition,Fe carbonitride materials synthesized by a similar sputtering/heat treatment procedure

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were also explored,and some ORR activity was found.249In the case of Fe and Co carbonitrides synthesized by this method,nitrogen content in the catalysts was found to have an effect on the corresponding ORR activity.170,249ORR activity was found to improve steadily with increasing nitrogen content up to approximately 3at.%(material dependent),after which,incre-ments in the nitrogen content had a minimal effect as illustrated in Fig.14.This approach and ensuing results are similar to those obtained for pyrolyzed M–N x /C catalysts discussed in Section 2.2.The activity saturation with nitrogen content observed indicates some other limiting factors present in this type of catalyst which could potentially be low surface areas due to their thin ?lm morphologies obtained using magnetron sputtering procedures.

It was found that the partial oxidation of various transition metal carbonitride materials,including Ta,Zr and Nb carboni-tride composites,had a positive effect on their catalytic ORR activity.250–255This partial oxidation could give high ORR activity with onset potentials of 0.97,0.9and 0.89V vs.RHE for partially oxidized ZrCN,253TaCN 255and NbCN,251respectively.Highly active ZrCN was obtained by pyrolysing zirconium oxide and carbon powder at 1800 C.This material was then partially oxidized at temperatures between 1000and 1400 C,with maximum ORR activity obtained at 1200 C.Moreover,the stability of this highly active partially oxidized ZrCN was also demonstrated by a 100hour stability test at an electrode poten-tial of 0.6V vs.RHE.252This indicates that these materials can be considered one of the most promising non-precious metal cata-lysts for ORR.Future work is necessary to further optimize and improve both their ORR activity and stability which could render them very competitive candidates as electrocatalyst materials.

6.0.Enzymatic compounds

In recent years,enzyme modi?ed electrodes have been explored in an effort to develop alternative non-precious fuel cell cathode materials.These studies were stimulated by the extremely high onset potential and ORR activity observed on several naturally occurring oxygen reducing oxidase enzymes.256–258Laccase in

particular has been studied as an ORR electrocatalyst due to its high activity and astounding selectivity for the 4-electron ORR to produce water.Early investigations involved adsorbing lac-case directly onto carbon based electrodes,then testing its ORR activity.256,259–261In this way,a direct electron transfer between the electrode surface and enzyme molecules could occur.However,these laccase molecules were only weakly adsorbed on the electrode,259and could only provide a monolayer of surface coverage,leading to both insuf?cient ORR activity and stability.Further improvements involved the immobilization of the enzyme in a redox active polymer hydrogel,262–273such as osmium based complexes,which could be ‘‘wired’’to the surface of the cathode electrode.These redox polymers allowed the effective transport of electrons,reactants and products to multiple layers of the ORR active enzyme,resulting in signi?cantly higher active surface areas and ORR activity over the traditional monolayer approach.266,267,271,272It was found that the ORR activity was strongly dependent on the redox potential of the immobilization polymer hydrogel.267

There are however issues when dealing with enzymatic based electrodes.Speci?cally,the signi?cant ORR dependency on system temperature and pH remains a challenge.260,264–266Nor-mally,enzyme based catalysts could only give optimal activity at mildly acidic conditions.256,263–265,269These materials would thus require the presence of a buffer compound,a feat that may not be feasible for PEM fuel cells.Moreover,considering their issues with stability,utilizing these types of materials as PEM fuel cell cathode catalysts may not be realistic.

Interestingly,high ORR activity and product selectivity of naturally occurring tri-nuclear copper based enzyme complexes have also been found.This inspired the synthesis of copper based materials with a similar atomic con?guration in an attempt to mimic the natural rates of ORR on these complexes.257,258,274Signi?cant progress in this area has yet to be realized,however,in 2008,a copper coordinated complex supported on carbon black was reported to display high ORR activity and reduced pH dependency.257Thus,this ?eld of research offers an interesting non-traditional approach to developing ORR catalysts,with further modi?cations of these copper based complexes necessary in order to obtain adequate performance and stability under fuel cell operating conditions.

7.0.Conclusions

For a sustainable PEM fuel cell commercialization,developing active,inexpensive non-precious metal ORR catalyst materials to replace currently used,expensive Pt-based catalysts is a necessary and essential requirement.This review paper outlines the prog-ress in this ?eld of research over the past 40years.Several important kinds of carbon-supported non-precious metal elec-trocatalysts for ORR are reviewed,including non-pyrolyzed and pyrolyzed transition metal nitrogen-containing complexes,conductive polymer-based catalysts,transition metal chalco-genides,metal oxides/carbides/nitrides/oxynitrides/carbonitrides,as well as enzymatic compounds.Among these candidates,pyrolyzed M–N x /C materials are considered the most promising ORR catalysts because they have demonstrated some ORR activity and stability close to those of commercially available Pt/C catalysts.Although other types of non-precious metal

catalysts

Fig.14Effect of nitrogen content on the ORR onset potential of Fe-and Co-based carbonitride complexes.At low nitrogen contents,an increase in onset potential with increasing nitrogen is observed,followed by a plateau in performance enhancement.Reprinted from ref.8,with permission from Elsevier.Based on data summarized from ref.170and 249.

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有机催化剂的应用及发展

催化化学综述 综述题目:有机催化剂的应用及发展 学院:_ 专业:_ 班级:___ 学号:_ 学生姓名:_ 2013年 6月16日

有机催化剂的应用及发展 前言 在化学反应里能改变其他物质的化学反应速率(既能提高也能降低),而本身的质量和化学性质在化学反应前后都没有发生改变的物质叫催化剂(也叫触媒),在现代有机合成化学及化工中有着举足轻重的地位。现代化学工业产品的85%都是通过催化过程生产的,每种新催化剂的发现及催化工艺的研制成功,都会引起化学工业的重大革新。有机催化剂作为其中非常重要的一种,和我们生活的各个方面都有着联系,其发展历史也是几经波折,最终也取得了不错的成果。有机催化剂主要分为金属有机催化剂和非金属有机催化剂,其在社会生产中具有重要作用。

1.非金属有机催化剂 金属有机催化剂相反,非金属有机催化剂是指具备催化剂基本特征的一类不包含金属离子配位的低分子量有机化合物.此类非金属有机催化剂不同于通常的单纯以质子酸中心起主导作用的有机羧酸类、苯磺酸类有机催化剂,它是通过分子中所含的N,P等富电子中心与反应物通过化学键或范德华力形成活化中间体,同时利用本身的结构因素来控制产物的立体选择性。 1.1、非金属有机催化剂的种类 1、有机胺类:脯氨酸、咪唑啉酮类、金鸡纳碱类、Ⅳ杂环卡宾类、二酮哌嗪类、胍类、脲及硫脲类等; 2 、有机膦类:三烷基膦类、三芳基膦类等; 3 、手性醇类质子催化剂:如TADDOL类催化剂。 非金属有机催化剂和金属有机催化剂以及生物有机催化剂有着非常密切的联系,有的非金属有机催化剂例如叔膦本身又是金属有机催化剂很好的配体,还有些非金属有机催化剂显示出类似于酶的特性和催化机理.大量的研究发现大多数非金属催化剂有较高的催化活性,尤其是应用在不对称合成中,经其催化的反应大都有很好的收率和对映选择性,并且具有毒性低、价格低廉、容易制备、稳定性好、易于高分子固载等一系列优点,所以越来越受到各国化学家的重视。 1.2、非金属有机催化剂的应用 1.2.1.松香酯化催化剂 松香是自然界极其丰富的一种天然树脂 ,分为脂松香、浮油松香和木松香三种 ,松香具有防腐、防潮、绝缘、粘合、乳化、软化等特性 ,广泛应用于食品工业、胶粘剂工业、电子工业、医药和农药等 ,但松香性脆、易氧化、酸值较高、热稳定性差等缺点严重妨碍了它的应用。研究发现可以通过对松香进行化学改性 ,人为地赋予它各种优良性能 ,使其得到更广泛的应用。松香化学反应主要在枞酸型树脂酸分子的两个活性基团——羧基和共扼双键上进行。它的主要反应有:异构、加成、氢化、歧化、聚合、氨解、酯化、还原、成盐反应和氧化反应。松香的氢化和酯化是其中最主要的改性手段。

含钼催化剂研究进展

含钼催化剂研究新进展 摘要含钼催化剂广泛用于多种化工生产过程,在含钼精细化学品的研究与开 发中占有重要地位。简要介绍了我国近年来一些含钼催化剂的研究进展和有关文献1前言 催化是现代十分重要的化工技术,据统计,发达国家近三分之一的国民经济总 产值来自催化技术。含钼催化剂在催化领域占有重要地位,广泛用于石油加工和化 工生产,如合成气制造、基本有机合成和精细化工产品等的的生产。因此,长期以 来国内外对含钼催化剂的创新和改进不断进行。这也引起我国钼业界的广泛关注, 逐渐成为我国钼深加工领域的一个新的发展方向。现仅就我国近年来含钼催化剂的 一些新进展作简要介绍。 2烷烃的化学加工催化剂 2.1烷烃芳构化催化剂 四烷无氧脱氢芳构化,为甲烷活化和转化的一个新的研究热点。王林胜等在1 993年首次报道一种以HZSM-5分子筛为载体的含钼催化剂使甲烷于无氧条件下高选择性地转化为苯。该催化剂是甲烷芳构化反应的典型催化剂。此后,对这种催化剂 的研究活跃。舒玉瑛等用机械混合、机械混合后焙烧、机械混合后微波处理等方法 制备这种催化剂,并考察了其对甲烷芳构化反应的催化性能。结果表明:机械混合 法、固相反应法和微波处理法制备的Mo/HZSM-5催化剂,比一般浸渍法能明显提高 芳烃的选择性和减少积碳生成;在不同制法的Mo/HZSM-5催化剂上,Mo物种落位不同,机械混合法、固相反应法和微波处理法能使Mo物种较多地落位于分子筛外表面 ,这对甲烷芳构化反应有利,并明显减少积碳的生成。 王军威等用浸渍法、机械混合法和水热法制备了Mo/HZSM-5催化剂,并考察了 钼含量和反应时间对丙烷芳构化反应的影响,深入研究了Mo物种对HZSM-5分子筛结构和酸性的作用。 最近,田丙伦等报道了对Mo/MCM-22催化剂用于甲烷无氧芳构化的研究结果。MCM-22为晶粒呈片状、含两种孔道结构的高硅沸石分子筛。同Mo/HZSM-5催化剂相比,Mo/MCM-22催化剂稳定性更好,苯产物的选择性较高 。用浸渍法制备的Mo担载量为6%的Mo/MCM-22催化剂性能最佳。此外,还研究了添加钴对Mo/MCM-22催化反应性能和催化剂积碳性质的影响。 2.2烷烃选择氧化催化剂 甲基丙烯酸(MAA)是重要的有机化工原料,当前主要用烯烃为原料生产。然而,饱和烃较烯烃来源广泛,更经济易得,故近年来由异丁烷氧化制MAA已成研究 与开发的新方向。采用一般热表面催化法由异丁烷选择氧化制取MAA主要存在的问 题是MAA选择性低,浓度反应产物(COx)高达40%。激光促进表面反应法是很有应用前景的光催化合成新技术。最近,陶跃武等分别采用在铋钼复合氧化物、钒钼复 合氧化物表面上激光促进异丁烷选择氧化制MAA,取得选择性达到90%和无COx产生的良好结果。

有机催化剂的应用及发展

https://www.doczj.com/doc/4b2971447.html,/sundae_meng 催化化学综述 综述题目:有机催化剂的应用及发展 学院:_ 专业:_ 班级:___ 学号:_ 学生姓名:_ 2013年 6月16日

有机催化剂的应用及发展 前言 在化学反应里能改变其他物质的化学反应速率(既能提高也能降低),而本身的质量和化学性质在化学反应前后都没有发生改变的物质叫催化剂(也叫触媒),在现代有机合成化学及化工中有着举足轻重的地位。现代化学工业产品的85%都是通过催化过程生产的,每种新催化剂的发现及催化工艺的研制成功,都会引起化学工业的重大革新。有机催化剂作为其中非常重要的一种,和我们生活的各个方面都有着联系,其发展历史也是几经波折,最终也取得了不错的成果。有机催化剂主要分为金属有机催化剂和非金属有机催化剂,其在社会生产中具有重要作用。

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