Cite this:Chem.Soc.Rev.,2014,43,7040
Catalytic applications of layered double hydroxides:recent advances and perspectives
Guoli Fan,Feng Li,*David G.Evans and Xue Duan
This review surveys recent advances in the applications of layered double hydroxides (LDHs)in heterogeneous catalysis.By virtue of the flexible tunability and uniform distribution of metal cations in the brucite-like layers and the facile exchangeability of intercalated anions,LDHs—both as directly prepared or after thermal treatment and/or reduction—have found many applications as stable and recyclable heterogeneous catalysts or catalyst supports for a variety of reactions with high industrial and academic importance.A major challenge in this rapidly growing field is to simultaneously improve
the activity,selectivity and stability of these LDH-based materials by developing ways of tailoring the electronic structure of the catalysts and supports.Therefore,this Review article is mainly focused on the most recent developments in smart design strategies for LDH materials and the potential catalytic applications of the resulting materials.
1.Introduction
Layered double hydroxides (LDHs)or hydrotalcite-like com-pounds are a large family of two-dimensional (2D)anionic clay materials which can be represented by the general formula [M 1àx 2+M x 3+(OH)2]x +[A x /n ]n àám H 2O.1–3As shown in Fig.1,LDHs are composed of brucite-like layers in which a fraction of the divalent metal cations (e.g.,Mg 2+,Fe 2+,Co 2+,Cu 2+,Ni 2+,or Zn 2+)
coordinated octahedrally by hydroxyl groups have been replaced isomorphously by the trivalent metal cations (e.g.,Al 3+,Cr 3+,Ga 3+,In 3+,Mn 3+or Fe 3+),giving positively charged layers.4The value of x is equal to the molar ratio M 2+/(M 2++M 3+)and is generally in the range 0.2–0.33;water and exchangeable inorganic or organic charge-compensating anions are present in the interlayer galleries.Each hydroxyl group in the LDH layers is oriented toward the interlayer region and may be hydrogen bonded to the interlayer anions and water molecules.LDH materials have a relatively weak interlayer bonding and as a consequence exhibit excellent expanding properties.The wide tunability of the types of metal cations,the M 2+/M 3+molar ratios,
State Key Laboratory of Chemical Resource Engineering,Beijing University of Chemical Technology,P.O.Box 98,Beijing 100029,P.R.China.
E-mail:lifeng@https://www.doczj.com/doc/ca10093777.html,;Fax:+86-10-64425385;Tel:+
86-10-64451226Guoli Fan
Guoli Fan received his BS (2006)and PhD (2011)degrees from Beijing University of Chemical Technology,where he mainly studied synthesis of new cata-lysts based on layered double hydroxides,under the supervision of Prof.Feng Li.After obtaining his PhD in 2011,he continued working at Beijing University of Chemical Technology.His research interests focus on design and fabrication of cata-lytic materials and environmentally friendly
catalysis.
Feng Li
Feng Li studied at East China University of Science and Technology for both his under-graduate and graduate degrees.He was awarded a PhD degree from Beijing University of Chemical Technology (BUCT)in 1999.He was
a visiting scholar at Universite
′Blaise Pascal in 2001.He was appointed Professor of Chemistry at BUCT in 2005.His research interests are heterogeneous catalysis and materials chemistry.In 2004he was selected to participate in the
‘New Century Outstanding Talent’scheme of the Ministry of Education.In 2013he was supported by the National Science Foundation for Distinguished Young Scholars.
Received 9th May 2014DOI:10.1039/c4cs00160e
https://www.doczj.com/doc/ca10093777.html,/csr
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and the nature of interlayer compensating anions results in a huge variety of host–guest assemblies and nanoarchitectures with versatile physical and chemical properties,and the increasing interest in these materials is driven by this compositional flexibility.An additional advantage is that LDHs can be synthesized by well-established synthetic protocols—such as co-precipitation and homogeneous precipitation by urea hydrolysis—which are simple and amenable to scaling up for industrial production.1
Over the past two decades,the properties of LDHs have been tailored to produce materials designed to fulfil specific require-ments for practical applications in a wide variety of fields,including as additives in polymers,adsorption materials,pre-cursors for functional materials,and in pharmaceutics,photo-chemistry and electrochemistry.5In particular,LDHs—both as directly prepared or after thermal treatment and/or reduction—have been widely employed in heterogeneous catalysis as actual catalysts,catalyst precursors and catalyst supports for various reactions including organic transformations,photodegradation of organic wastes,and the total decomposition of volatile organic compounds.6In the past five years,there has been an
extremely rapid growth in publications related to the catalytic applications of LDH-based materials.Recently,An and co-workers summarized advances in the design of nanocatalysts using LDHs as precursors.7However,the rapid growth of the use of LDH materials in a range of fields related to heterogeneous catalysis warrants a broader assessment and critical discussion of recent developments.
In this Review article,we summarize the most recent advances in the smart design and synthesis of LDHs and derivatives for use as catalysts and catalyst supports in the field of heterogeneous catalysis.Especially,a variety of types of LDH-based catalysts,which can be obtained by di?erent synthetic pathways (Fig.2),are emphatically reviewed in the following paragraphs.A major challenge in this rapidly growing field is to improve the function-alities of these LDH-based catalytic materials.Therefore,the advantages of LDHs and derivatives as catalysts and catalyst supports are highlighted and the structure–property correlations are addressed,which provides useful insights into ways to develop robust and promising catalysts and catalyst supports based on LDH materials.In addition,we also look at potential future developments in the use of LDH-derived materials in catalysis.It should be noted that throughout the paper,LDHs with different metal cations are usually abbreviated in the form M 2+M 3+–LDH (M 2+denotes divalent cations,M 3+denotes trivalent cations),if anions are not specified.
2.LDH-based catalysts
2.1.
Simple LDHs as catalysts in their own right
LDHs have two attractive features which impart catalytic pro-perties to the materials.Firstly,the brucite-like layers have an abundance of basic sites allowing the materials to be used as heterogeneous solid base catalysts.Secondly,the two or more metal cations within the brucite-like layers are uniformly distri-buted at the atomic level without a segregation of ‘‘lakes’’of separate cations,and where one of the cations is a
catalytically
Fig.1The idealized structure of carbonate-intercalated LDHs with dif-ferent M 2+/M 3+molar ratios showing the metal hydroxide octahedra stacked along the crystallographic c -axis,as well as water and anions present in the interlayer
region.
David G.Evans
David G.Evans studied as both an undergraduate and a research student at Jesus College,Oxford,and obtained a DPhil under the supervision of Prof. D.M.P.Mingos FRS.After postdoctoral work at Bristol University with Prof.F.G.A.Stone FRS,he was appointed as a lecturer at Exeter University in 1985.He moved to Beijing University of Chemical Technology in 1996.His research interests focus on intercalation in layered solids.
He was awarded an International Scientific and Technological Cooperation Award of the People’s Republic of China in
2005.
Xue Duan
Xue Duan was elected as an Academician of the Chinese Academy of Sciences in 2007and a Fellow of the Royal Society of Chemistry in 2009.He was awarded his BS degree from Jilin University in 1982and his PhD degree from the Chinese Academy of Sciences in 1988.He was subsequently appointed to the sta?of Beijing University of Chemical Technology (BUCT)and established the Applied Chemistry Research Institute in
1990.He was promoted to full professor in 1993.He is currently Executive Vice-Chair of the Academic Committee of the State Key Laboratory of Chemical Resource Engineering.
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active transition metal this can lead to high catalytic activity and selectivity.
2.1.1Solid base catalysts.By virtue of the abundance of hydroxyl groups,simple binary as-prepared materials such as MgAl–LDHs are potential heterogeneous solid base catalysts for a wide variety of organic transformations.Furthermore,after calcination at intermediate temperatures (450–6001C),LDHs can be converted into well-dispersed mixed metal oxides (MMOs)with large surface area and numerous Lewis base sites.8–11Through subsequent rehydration in the absence of CO 2,MMOs can reform the layered structure with hydroxyl anions incorporated in the interlayer region producing acti-vated LDHs with abundant Br?nsted-type basic sites.12The conditions under which the rehydration of the MMO is carried out control the number and the strength of OH àsites.As a result,LDHs,MMOs or activated LDHs acting as heterogeneous solid base catalysts are currently attracting increasing attention from the viewpoint of environmental and economical concerns,because they can replace homogeneous base catalysts as environ-mentally benign and recyclable catalysts for several types of important organic reactions.These reactions involve C–C and C Q C bond formation,such as Knoevenagel condensations,13,14Michael additions,15aldol and Claisen–Schmidt condensations,16–19cyanoethylation of alcohols,20as well as transesterification of
triglycerides with methanol for synthesis of biodiesel.21–26The catalytic performance of different materials depends strongly on structural features,such as the nature,strength and relative amounts of base sites.
The catalytic performance of binary LDHs can be tailored by incorporation of a third cation.For example,Pavel et al.27recently reported that modification of MgAl–LDH with Y 3+ions followed by calcination/rehydration gave improved activity and selectivity for styrene oxide by styrene epoxidation with hydro-gen peroxide in acetonitrile as the solvent.Since its ionic radius is significantly larger than those of Mg 2+or Al 3+,the Y 3+cations are present in the form of hydroxide species either in the interlayer galleries or on the LDH surface rather than being isomorphously substituted in the brucite-like layers,and the lower electronegativity of yttrium compared with that of aluminium resulted in an increase of the basicity,accounting for the observed enhanced catalytic activity.
In addition to elemental composition,the surface structure and morphology of LDH-based solid base catalysts also have a significant e?ect on their catalytic behaviour.For example,Lei et al.28reported a green route to the synthesis of fatty acid monoethanolamides using fatty acid methyl esters as the raw materials and activated MgAl–LDHs as solid base catalysts.They prepared two activated MgAl–LDHs—U-RLDH and
S-RLDH—by
Fig.2Summary of the synthetic pathways for preparation of the LDH-based catalysts.
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calcination/rehydration of LDH precursors prepared by the urea decomposition method and the separate nucleation and aging steps (SNAS)method 5developed in our laboratory,respectively.They found that S-RLDH showed a higher catalytic activity than U-RLDH in the amidation of methyl stearate,with 87.0%conversion at 1201C.It was concluded that the smaller crystallite size and larger surface area of S-RLDH resulted in there being more basic sites on the catalyst surface,as well as the easier desorption of the final product.
Although powdered LDHs can be used as catalysts on a laboratory scale,for large scale practical applications they need to be supported on a suitable substrate to facilitate mass-transport.For example,Lei et al.fabricated structured MgAl–LDH films on a muscovite mica substrate through an epitaxial growth approach,29as illustrated in Fig.3.Atomic force micro-scopy (AFM)revealed that half-hexagonal shaped LDH platelets grew obliquely on the substrate with a high degree of dispersion.After being activated by a calcination–rehydration procedure,the rehydrated LDH (RLDH)platelets remained firmly immobilized on the muscovite substrate and retained the half-hexagonal platelet morphology (Fig.3b).In the aldol condensation of acetone,a conversion of 23.0%was achieved over the RLDH/muscovite acting as a structured solid base catalyst after 460min.However,the powdered RLDH analogue synthesized by the same procedure exhibited an identical conversion after a longer reac-tion time of 560min.
In addition,it has been reported that small activated MgAl–LDH nanocrystallites supported on carbon nanofibers (CNFs)showed a catalytic activity almost 300times higher than that of the bulk catalyst in the transesterification of glycerol with diethyl carbonate to obtain glycerol carbonate.30In base-catalyzed reactions,the active sites are usually at the edges of LDH crystal-lites.Therefore,the significantly improved activity was attributed to the small size of the MgAl–LDH crystallites and the accessible pores of the CNFs,which gave rise to a large number of accessible active OH àsites in catalysts.
In an attempt to overcome the severe mass-transport limita-tions in biodiesel synthesis from viscous oils,Woodford et al.31fabricated an alkali-free macroporous activated MgAl–LDH catalyst via physical templating around a sacrificial polystyrene
bead template (ca.350nm in diameter).The resulting hierar-chical macroporous–microporous solid base displayed exceptional activity in the transesterification of long-chain triglycerides and methanol with 24.7%conversion of trilaurin and 26.1%con-version of triolein after 24h reaction time,due to a great improvement of the diffusion of bulky triglycerides and accessibility of active sites within the hierarchical macropore–micropore architecture.
The MMOs with a rock-salt-like structure formed by calcina-tion of multi-component LDHs,as mentioned above,possess abundant acid and basic sites associated with the presence of O 2à–M n +acid–base pairs.The nature,strength and relative amounts of these basic sites depend on the identity and molar ratio of the metal cations,the preparation method,the calcina-tion temperature,and type of interlayer anions.32–39For example,a LiAl–MMO was recently synthesized from a uniform LiAl–LDH precursor prepared by a separate nucleation and solvothermal aging steps method.40The synthesis of the LDH precursor involved a very rapid mixing and nucleation process in a modified colloid mill 5followed by a separate ethanol-aided solvothermal aging process.The resulting MMO possessed higher surface basicity than the corresponding material pre-pared without the addition of ethanol,as a result of both the smaller crystallite size and the higher surface area,and exhibited higher catalytic activity for the Knoevenagel condensation of benzaldehyde (BA)and ethyl cyanoacetate with 83.8%conversion of BA after 12h.Recently,Meyer et al.41investigated the influence of the nature of the interlayer anions in MgAl–LDH precursors on the catalytic activity of the resulting MMO in the transesterification of glycerol trioctanoate with methanol.By comparing different intercalated anions,they concluded that MgAl–MMO derived from precursors with highly charged anions exhibited higher catalytic activity than those derived from pre-cursors with long-chain anions.They attributed the higher activity to wider pores,enabling better accessibility for bulky triglyceride molecules.In addition,Ruiz and colleagues found that the solid base catalyst obtained by calcining delaminated MgAl–LDH containing interlayer hydroxyl ions provided excellent catalytic activity (100%conversion after 2h)in the Meerwein–Ponndorf–Verley (MPV)reaction of benzaldehyde with 2-propanol,due to the high surface basicity of the Lewis type.42
Recently,Tichit and colleagues also prepared a series of new MMOs via alkaline-earth cations or La 3+containing anionic complexes intercalated MgAl–LDH precursor route.The forma-tion of stable complexes was achieved using EDTA or citrate as chelating agent.They found that Sr-and Ba-containing MMOs exhibited basicity close to the ones of MgO and Mg(La)O mixed oxide possessing strong basicity,owing to the improved metal dispersion in precursors.43
2.1.2Catalytic properties of LDHs with transition metal cations in the layers
2.1.2.1Electrocatalysts.Replacing rare and expensive noble metal catalysts with inexpensive and abundant ones for various renewable energy-related chemical processes,as well as for the production of high value chemicals,is of considerable importance in making chemical processes more sustainable.LDH
materials
Fig.3(a)Proposed structure of LDH/muscovite based on the lattice match between MgAl–LDH and the muscovite phases,and (b)an AFM image of RLDH/muscovite.Reproduced with permission from ref.29.Copyright 2012American Chemical Society.
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containing transition metal cations,particularly Ni-and Co-containing LDHs,have been widely used in electrocatalytic reactions.44–49For example,ZnCo–LDH has been used as an electrocatalyst and catalyst for water and alcohol oxidation,respectively.48Compared with traditional monometallic Co(OH)2and Co 3O 4systems,ZnCo–LDH possessed a lower overpotential (by B 100mV)for the electrochemical oxidation of water,and the turnover frequency (TOF)of the LDH was 410times that of Co(OH)2and Co 3O 4at the same applied potentials.Moreover,this material also exhibited good catalytic activity and selecti-vity for oxidation of alcohols at relatively low temperature.The authors claim that the relatively inactive Zn 2+cations can be regarded as a structural support which provides a synergistic effect in the catalytic system (possibly by assisting the inter-actions between the reactants and the catalyst).Recently,a core–shell structured CoNi–LDH was built from a Co–Ni hydroxide precursor in a topotactic process using iodine as an oxidizing agent.49A low overpotential (B 0.5V,at 1.4V vs.NHE)and high current density (B 1mA cm à2)were achieved using the as-constructed CoNi–LDH as a catalyst for the oxygen evolution reaction (OER)without any sacrificial agent being required,which was attributed to both the presence of more Co active sites and the rapid movement of interlayer ions within the layered structure.
In addition to the composition of LDH materials,the morphology of electroactive species can also play an important role in improving the electrocatalytic performance.For instance,Sun and colleagues successfully fabricated NiFe–LDH with a three-dimensional (3D)architecture.50Compared with the con-ventional 2D planar architecture,an electrode based on the 3D porous architecture could facilitate electron transportation,pro-mote electrolyte penetration and increase the electrochemically active surface area.As a result,the onset potential of the electrode in OER was significantly reduced to B 1.46V vs.RHE,yielding a high current density of B 60mA cm à2at small overpotentials;in addition the 3D architecture showed outstanding stability.2.1.2.2Photocatalysts.In view of the increasingly serious problems of environmental pollution and energy shortage,the search for suitable semiconductor photocatalysts for the decom-position of organic dyes and colourless pollutants,as well as water splitting by using solar energy,is one of the primary missions of materials science.51,52It is well known that many layered compounds that contain Ti 4+,Nb 5+,or Ta 5+cations possess photocatalytic capability for water splitting,since they may provide spatially separate reduction and oxidation reaction sites.53,54However,the biggest problem with these layered photocatalysts is the aggregation of photo-active species and thus the low energy-conversion e?ciency.Fortunately,disper-sing the active species in an inorganic LDH matrix can provide an e?ective solution to this problem.The appropriate incor-poration of photo-active components (e.g.,Zn,Ti,Fe or Cr)into LDHs can a?ord a variety of transition metal-containing LDHs which are semiconductor photocatalysts with a band gap normally in the range from 2.0to 3.4eV.There have been numerous investigations on the application of LDH-based
photocatalysts in the elimination of organic pollutants,and water reduction and water oxidation using UV or solar light as the irradiation source.
(a)Pollutant elimination.The growing demand for the removal of organic pollutants in the e?uent from chemical and pharmaceutical industries has stimulated the development of e?ective photocatalysts.Due to their excellent visible light absorbing abilities,ZnTi–LDH and ZnFe–LDH afford good visible-light-induced photocatalytic performance in the degradation of organic pollutants.55,56Recently,Zhao et al.57prepared a series of MCr–X–LDHs (M =Cu,Ni,Zn;X =NO 3à,CO 32à)by the SNAS 5method.They demonstrated that the highly dispersed Cr 3+ions in the octahedral sites in the LDH matrix played a significant role in the photo-excitation of electrons.Notably,MCr–NO 3à–LDHs (M =Cu,Ni)showed 20-times higher activity than a standard Degussa P25titania catalyst in the decomposition of dye and phenols under visible light irradiation,as well as excellent recycling ability.In addition,with the introduction of Co 2+into CuCr–LDH,Mohapatra et al.found that active superoxide anion radical species could be generated through the visible-light-induced metal-to-metal charge transfer (MMCT)transitions of Cr(III )–O–Co(II )to Cr(IV )–O–Co(I )moieties,58thus giving rise to the higher photocatalytic activity.
For the purpose of obtaining immobilized and recyclable photocatalysts for practical water treatment application,CuCr–LDH films immobilized strongly on copper substrates were fabricated by an electrophoretic deposition method.59Due to the large specific surface area and rich macro-/mesoporous structure,a structured CuCr–LDH film,with an optimized thickness of ca.16.5m m and high orientation along the c -axis,showed higher visible-light-driven photocatalytic activity for the decomposition of three representative organic contaminants (about 97%degradation percentage for 2,4,6-trichlorophenol and above 90%degradation percentage for sulforhodamine B (RB)and Congo red after 200min irradiation)than the corresponding powder sample,as well as excellent recyclability and convenient manipulation.
(b)Water reduction for H 2generation.The search for suitable semiconductor photocatalysts for water splitting using solar energy is one of the most important areas of contemporary physical science.In the case of a carbonate-intercalated ZnCr–LDH,60the ligand-to-metal charge transfer (LMCT)transitions of CrO 6octahe-dra visible in the UV-vis spectra are thought to be the origin of its ability to photodegrade organic pollutants.More importantly,this material was subsequently found to be a visible-light-active photo-catalyst for water splitting to generate hydrogen,61because the carbonate radicals originating from the oxidation of intercalated carbonate anions by photogenerated holes were found to improve the light-induced electron–hole separation.Furthermore,replacing part of the Zn 2+with Ni 2+also resulted in MMCT transitions of Zn(II )/Ni(II )–O–Cr(III )to Zn(I )/Ni(I )–O–Cr(IV ),62accounting for the enhanced photocatalytic activity for hydrogen evolution.In addition,the high dispersion of TiO 6octahedra confined in the 2D matrix of a series of MTi–LDH (M =Ni,Zn,Mg)can suppress the carrier recombination under visible-light excitation,63by taking advantage of the large number of surface Ti 3+–O defects
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serving as trapping sites to improve the electron–hole separa-tion.The modified band gap of about 2.1eV obtained by the formation of the M 2+–O–Ti network greatly facilitated water splitting,accounting for the 1.23eV thermodynamic over-potential and other kinetic overpotentials for both hydrogen and oxygen evolution.As a result,Ti-containing LDHs showed an extremely high photocatalytic H 2-production rate of 31.4mmol h à1,which was 18times higher than that of the control K 2Ti 4O 9sample.
(c)Water oxidation for O 2evolution.In both thermodynamic and kinetic terms,water oxidation is the uphill step in solar-driven water splitting.The formation of oxygen from water is challenging since it has to occur through several steps requiring four positive holes and the formation of O–O bonds.64Recently,Ti-or Zn-containing LDH materials have been tested for their photocatalytic abilities in water oxidation.65,66LDH-based photo-catalysts for O 2evolution under visible irradiation,such as ZnCr–LDH,ZnTi–LDH and ZnCe–LDH,were first reported by Silva et al.65Of these,ZnCr–LDH displayed two visible absorption bands centred at 410and 570nm,which are assigned to the LMCT transitions in the octahedral Cr sites,and possessed the highest apparent quantum yield of 60.9%for O 2generation.This is one of the highest values ever reported for a bifunctional chromophore–catalyst material without the involvement of a foreign photosensitizer.Recently,high-surface-area NiTi–LDH with two visible absorption bands in the blue and red wave-length regions was found to show a higher O 2generation rate than that of CuTi–LDH under visible excitation (49vs.31m mol of O 2)by using 200mg of the photocatalyst and 1mmol of AgNO 3as a sacrificial agent,66indicative of the good water oxidation activity of Ti-embedded LDHs.
In addition,Kim et al.67demonstrated that the Co–O–Fe oxo-bridges connecting the different metal ions in CoFe–LDH helped to prevent the recombination of holes with electrons.In particular,an Fe-rich compound displayed the highest photocatalytic efficiency for water oxidation (about 45m mol of O 2for 3h)under visible light,which was attributed to its good crystallinity,enhanced light absorbance and low charge carrier recombination originating from the increase in the number of oxo-bridges.
It is well known that the size,shape and morphology of semiconductors are key factors in determining the nature of their surface defects,which a?ects both the e?ciency of charge separation and the photoconversion capability.68–70In a very recent research,NiTi–LDH nanosheets with lateral dimensions in the range 30–60nm were synthesized using a reverse micro-emulsion method,71in order to increase the density of surface Ti 3+defects in Ti-based LDH materials.This material showed extraordinarily high photocatalytic activity for oxygen evolution from water (B 2148m mol g à1h à1)using visible light,which is currently the most effective visible-light photocatalyst reported for O 2production.Moreover,the quantum yields reached 65.0%and 20.0%under monochromatic irradiation at 400and 650nm respectively.A proposed scheme that relates the structure and O 2evolution process is shown in Fig.4.Such LDH
materials can be expected to act as a useful stepping stone for the exploration of new efficient photocatalysts for water oxidation.
(d)CO 2reduction.Photocatalytic reduction of CO 2to CO,methanol and other organic species is an attractive way to meet the demands for green energy and environmental sustainability.In this regard,a study of the remarkably highly selective CO 2photoconversion to methanol (88mol%)was reported using H 2and ZnCuGa–LDHs.72–74The high catalytic selectivity was mainly attributed to the binding of CO 2at the Cu–O–Zn and Cu–O–Ga sites to form hydrogen carbonate species.In addition,MgIn–LDHs were found to exhibit better activity (B 16.4m mol of O 2and B 2.4m mol of CO after 10h irradiation)than a simple metal hydroxide for the photocatalytic conversion of CO 2into CO in an aqueous system,75by taking the advantage of high water toler-ance of the surface base sites in MgIn–LDHs.2.2
Intercalation catalysts
LDH materials have relatively weak interlayer bonding and as a consequence exhibit excellent expanding properties.During the past two decades,increasing interest has been devoted to the use of these layered inorganic solids as host materials in order to create
host–guest supramolecular intercalation structures with desirable physical and chemical properties,usually via an ion-exchange route based on the electrostatic interactions between the intercalated anions and the positively charged layers.The versatile intercalation ability of LDHs greatly enlarges the family of available LDH materials.Specifically,the intercalation of catalytically active species (e.g.,simple inorganic anions,complex anions,and biomolecules)in the interlayer galleries of LDHs is an effective immobilization approach,and has been proved to be capable of improving catalytic stability and recyclability when compared to the corresponding homogeneous catalyst.More importantly,the brucite-like layers may impose a restricted geometry on the cata-lytically active guests in the interlayer galleries leading to enhanced control of stereochemistry and the dispersion of the active species,and thus lead to increased reaction rates and modified product distributions.
Fig.4(A)Proposed structural model of energy states for NiTi–LDH and (B)schematic illustration of the O 2evolution process over NiTi–LDH nano-sheets under visible-light irradiation.Reproduced from ref.71.Copyright 2014The Royal Society of Chemistry.
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2.2.1Polyoxometalate-intercalated catalysts.Polyoxometalates (POMs)are a subset of metal oxides with unique physical and chemical properties,which can be reliably modified through various techniques and have been used to develop sophisticated materials and devices.76Since Pinnavaia first reported the syn-thesis of nanocomposites of bulky and stable POMs and LDHs,77increasing attention has been paid to POM–intercalated LDHs (POM–LDHs).78–82Their synthesis has been discussed in a recent review paper.83POM–LDHs can be used widely as heterogeneous catalysts for various organic transformations including oxidation of alcohols,84alkenes,85–87thioethers 88and tetrahydrothiophene,89and acid-catalyzed esterification,90and have shown superior cata-lytic properties to pure POMs,owing to the large gallery height and unique interlamellar chemical environment,as well as the high dispersion of catalytically active species.
Recently,a series of POM–intercalated LDHs were reported to exhibit much higher epoxide selectivity than the corre-sponding homogeneous Na–POM catalysts in the epoxidation of allylic alcohols with aqueous H 2O 2as the oxidant without using an organic solvent.91In particular,the combination of a sandwich-type [WZn 3(ZnW 9O 34)2]12à(ZnWO)guest and a MgAl–LDH host endowed the resulting POM–LDH catalyst with 99%selectivity for the epoxide,95%H 2O 2efficiency and a TOF of 37200h à1without the need for any basic additives or pH control.The authors attributed the excellent catalytic performance to the beneficial effect of the basic LDH host in suppressing the acid-catalyzed hydrolysis of the epoxide.Although pristine ZnWO can be used as a catalyst for the oximation of aldehydes,its inherent acidity can cause subsequent acid-catalyzed Beckmann rearrangement and dehydration of the target oxima-tion products leading to nitrile and amide by-products.In an attempt to solve this problem,Song and colleagues 92,93pre-pared a ZnWO-intercalated ZnAl–LDH with a Zn/Al molar ratio of 3:1in the brucite-like layers,and found that the material showed significantly enhanced selectivity towards aldoximes and ketoximes in the oximation of aromatic aldehydes under organic solvent-free conditions with about 90%conversion of thiophene-2-carboxaldehyde and 85%selectivity to the oxime,showing that the surface basic sites of the ZnAl–LDH host (Fig.5)do indeed suppress the acidity of the POM guest.The ZnWO–LDH catalyst could be reused at least ten times without significant loss of activity,indicative of its high chemical and structural stability.
The intercalation of simple metal oxyanions and POMs in LDH hosts can also lead to photocatalytic capability.For example,when MoO 42àand WO 42àanions are intercalated in ZnY–LDHs,the MMCT transitions from the Zn–O–Y oxo-bridged linkages leads to an absorption band in the visible region.94Moreover,the MoO 42àintercalated LDH exhibited excellent photocatalytic activity with about 98%degradation percentage of the dye after 2h irradiation,higher than that by the WO 42à-intercalated analogue (only 80%degradation percentage),due to the contribution of unoccupied Mo 4d orbitals to the conduction band resulting in a smaller band gap.
2.2.2Biomolecule-intercalated catalysts.In recent years,much attention has been paid to the intercalation of bio-molecules (e.g.,proteins,95,96enzymes,97and amino acids 98–101)into the interlayer galleries of LDH nanomaterials,because the adjustable height of the 2D interlayer galleries in LDHs can provide great opportunities for a significant enhancement of their efficiency.He et al.102presented a detailed investigation of the immobilization of L -proline in the interlayer galleries of LDHs using the calcination–rehydration method of synthesis,along with its application in a typical asymmetric aldol reaction between acetone and benzaldehyde.It was noted that the resulting L -proline-LDHs gave 94%enantioselectivity and 90%yield even when exposed to rigorous conditions (e.g.UV irradia-tion and thermal treatment).Moreover,the immobilization of L -proline in LDHs significantly inhibited its racemization,and thus enhanced its thermal and optical stability.Similarly,L -proline-LDHs also showed improved catalytic enantioselectivity compared with pure L -proline in the asymmetric Michael addi-tion reaction between b -nitroalkene and acetone.103
Recently,Pitchumani et al.104prepared sixteen MgAl–LDHs containing different amino acids (including leucine,isoleucine,phenylalanine,methionine,and tyrosine)intercalated in the interlayer galleries,and found that the intercalated amino acids exhibited higher activity in the chemoselective O -methylation of phenols and S -methylation of thiophenol with dimethyl carbonate (DMC),whereas the free amino acids showed negligible activity.The intercalation of amino acids thus clearly created an environment facilitating the reaction in a facile manner with high conversion and selectivity.Moreover,the as-obtained catalysts could be easily recycled and reused without decreasing the catalytic activity.Taking the leucine-intercalated LDH catalyst (Leu-LDH)as an example,a plausible reaction pathway was proposed.As shown in Fig.6,the amine group of leucine was exposed and the elongated side chain created a hydrophobic pocket in Leu-LDH.The free amine could abstract a proton from phenol generating a phenolate anion,which in turn attacked DMC forming a six membered transition state,which further rearranged to give the anisole product.
2.2.3Metal complex-intercalated catalysts.Many years ago,Pinnavaia and colleagues used LDHs as layered host systems for interlayer gallery immobilization of complex anions such as [Ru(4,7-diphenyl-1,10-phenanthrolinedisulfonate)]4àvia an inter-calation pathway.105The intercalated complex anions are highly dispersed within the structure of LDHs owing to the confinement e?ect of the 2D LDH layers.In recent years,the intercalation
of
Fig.5A sandwich POM-intercalated ZnAl–LDH active as a catalyst for highly selective oximation reactions.Reproduced from ref.92.Copyright 2011The Royal Society of Chemistry.
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various metal complex anions (e.g.,metal porphyrins,106metal salens (salen-N ,N 0-ethylenebis(salicylimine)),107,108and metal–Schiff base complexes 109)into the interlayer space of LDHs has been a versatile and important approach for the immobilization of active metal complex species subsequently employed in a number of organic transformations.For instance,Bhattacharjee et al.110reported that a chiral sulfonato-salen manganese(III )-complex-intercalated ZnAl–LDH was a useful stereoselective epoxidation catalyst,and found that the catalyst gave 100%conversion,91.7%selectivity,and 98.1%enantiomeric excess in the oxidation of (R )-limonene.Also,a Ce(III )-complex-intercalated ZnAl–LDH catalyst could efficiently catalyze the liquid phase oxidation of primary alcohols to their corresponding aldehydes using molecular oxygen at room temperature,111and showed excellent yields and good recyclability.However,in same cases,the steric hindrance to the access of the substrates to the active sites of the intercalated species could occur,112leading to the lowered activity.
Homogenous catalysts su?er from the problems of low stability and selectivity,and di?culty in separating the cata-lytically active component from the product at the end of the reaction.Wei et al.113synthesized a ZnAl–LDH cointercalated with trans -RhCl(CO)(TPPTS)2and TPPTS (TPPTS =trisodium salt of 3,30,300-phosphanetriyl benzenesulfonic acid),and found that the LDH-intercalated catalyst showed higher activity toward aldehyde formation in the hydroformylation of higher olefins with TOF values of 80–150h à1,compared with the corresponding water–oil biphasic catalytic system (TOFs =20–25h à1)under similar reaction conditions,as well as better reusability because of the strong interactions between the LDH layers and the active rhodium complex species.Similarly,ZnAl–LDHs cointercalated with a Pd–TPPTS complex and PdCl 42àwere also found to be efficient green catalysts in the cycloisomerisation reaction of acetylenic carboxylic acids to the corresponding five-membered ring heterocycles with 100%atom efficiency and exclusive forma-tion of the exo -isomer under heterogeneous conditions.114The catalytic results were superior to those obtained using other heterogeneous systems for this reaction,and were comparable with those obtained under homogeneous conditions.Since no
external base or external ligand was required,it was proposed that the basicity of the LDH support promotes a clean and selective reaction of acetylenic carboxylic acids.
Recently,chiral sharpless titanium tartrate catalyst-intercalated MgAl–LDHs have been successfully prepared as catalysts for the sulfoxidation of the prochiral methyl phenyl sulfide.115–117After intercalation in the LDH host,the catalysts showed significant increases in enantiomeric excess of up to B 50%.Moreover,a significant further improvement in catalytic performance could be achieved by incorporating more highly dispersed active guest anions and/or increasing the interlayer spacing by in situ swelling with organic solvents.In these cases,the confinement effect of the LDH layers played a key factor in enhancing the catalytic properties of the intercalated active species.The formation of such swollen intercalated LDH structures opens up new possibilities for synthe-sising catalytic materials with enhanced performance.
Inspired by the crucial role of the nature of hydrophobic regions in highly e?cient enzymatic catalysis,He’s group have recently developed a heterogeneous Mn catalyst by encapsulating Mn(TPP)OAc (TPP =tetraphenylporphyrin dianion)in the bio-mimetic flexible 2D hydrophobic region of amphiphilic dodecyl sulfonate (DDS)-intercalated ZnAl–LDHs (Fig.7),118which has a similar bilayer structure to that of naturally occurring phospho-lipids.In the DDS-intercalated ZnAl–LDHs,the aliphatic tails of DDS were elongated and adopted a bilayer arrangement in order to maximize guest–guest dispersion interactions,which consequently enlarged the interlayer gallery height and weakened the interlayer interactions,thus facilitating the accommodation of Mn(TPP)OAc.The resulting catalyst exhibited superior catalytic activity and comparable selectivity to its homogeneous counterpart in the epoxidation of a variety of alkenes including cyclohexene,heptylene,phenylethylene,3-methyl-3-buten-1-ol,and even func-tional alkenes such as ethyl cinnamate and chalcone.2.3
Nanocomposite catalysts
Although confinement in the interlayer galleries of LDHs has been reported to be a successful way of enhancing the selectivity toward the desired products for some catalysts,this approach is not always e?ective owing to the complicated geometrical and chemical microenvironment in the interlayer galleries,as well as the inaccessibility of the inner surfaces of the host layers to the reagents.In this regard,the fabrication of heterostructured LDH hybrid nanocomposites from two functional components is another approach for the design and synthesis of e?cient heterogeneous catalysts,which may also benefit from synergistic e?ects between the two
components.
Fig.6Plausible reaction mechanism for Leu–LDH catalyzed O -methylation of phenol with DMC.Reproduced with permission from ref.104.Copyright 2013
Elsevier.
Fig.7Phospholipid bilayer in nature (A)and the schematic structure of the proposed biomimetic catalyst (B).Reproduced with permission from ref.118.Copyright 2013American Chemical Society.
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2.3.1Nanocatalysts based on LDH nanosheets.Another advantage of LDH (and other layered materials)is that under appropriate conditions they may be delaminated to give single highly anisotropic LDH nanosheets.Due to their high specific surface area and large number of surface functional groups,these nanosheets can be used as supports for immobilization of some macromolecules and biomolecules,including iron porphyrins and amino acids.119,120Catalytically active anions can also be immobilized on the exfoliated LDH nanosheets to form novel nanocatalysts.However,constraining catalytic cen-ters on the surface of solids usually reduces the reaction rate,due to the emergence of liquid/solid interfaces and di?usion limitations.
To solve the above problem,He and co-workers successfully immobilized a vanadium(V )precursor coordinated to amino acid ligands on host ZnAl–LDH nanosheets formed by delami-nation of the amino acid-intercalated ZnAl–LDHs (Fig.8).121–123Their studies indicated that anchoring amino acid ligands on the delaminated ZnAl–LDH nanosheets led to an excellent catalytic performance with a product yield of 93%and enantio-meric excess values of 96%for trans isomers and 63%for cis isomers after 520min in the asymmetric epoxidation of allylic alcohols.In contrast,the heterogeneous catalysts based on pure amino acid-intercalated only gave a product yield of 83%after a longer reaction time of 1050min.The impressive enhancement in catalytic performance was explained as follows:(i)the delami-nated LDH nanosheets provide a stable and rigid environment around the chiral centre,which significantly improves the enantiomeric selectivity by restricting or directing the access trajectory of reactant molecules;(ii)the easy access of the sub-strate to the catalytic centers located at the interface between the delaminated nanosheets and the solution led to the great increase in the reaction rate.Interestingly,such colloidal pseudo-homogeneous catalysts obtained by delamination in water could be directly recovered by simple liquid–liquid separation,and therefore reused with negligible loss of catalytic activity and enantioselectivity.Furthermore,the chiral induction
of amino acids in the direct aldol reaction was significantly improved by their attachment to the rigid LDH nanosheets,regardless of whether the amino acids acted as chiral ligands for Zn(II )centers or as the asymmetric catalysts in their own right.Theoretical calculations revealed that the nanosheet not only exerted steric effects promoting the chiral induction,but also assisted the formation of transition states via hydrogen bonds.Recently,the direct combination of MgAl–LDH nanosheets and an ionic Mn(II )–salen complex to afford an ordered nano-sized sandwich-structured composite was achieved via a modified flocculation method using oppositely charged compounds.124In this way,the interlayer gallery of the LDH was completely opened up,resulting in no steric hindrance for the immobilization of large guests.In the N -oxidation of 4-picoline using H 2O 2as an oxidant,the material exhibited superior thermal stability and 2–10fold activity over both the corresponding LDH material prepared using a traditional ion exchange method,and the homogeneous analogue.It is believed that the weak basicity of the nanosheets helped to cleave the peroxide bond in H 2O 2through the formation of hydrogen bonds between LDH and H 2O 2,while the nanoscale thickness of the materials significantly decreased the distance needed for diffusion of the substrate to the active site.
By taking advantage of the electrostatic forces or/and hydrogen bonding between oppositely charged particles,layer-by-layer (LBL)assembly of LDH nanosheets with negativity charged 2D sheets can be employed to synthesize nanocomposites with excellent catalytic performances.For example,Gunjakar et al.125reported the assembly of exfoliated positively-charged ZnCr–LDH nano-sheets with negatively charged layered titanate nanosheets to form heterolayered nanohybrids (ZCT)by the LBL approach.As shown in Fig.9,the cross-sectional high resolution transmis-sion electron microscopy (HRTEM)lattice image of the sample exhibited parallel-aligned dark lines with two different spacings of B 0.5nm and B 0.7nm,which can be attributed to the ZnCr–LDH layers and titanate layers,respectively.The effective electrostatic coupling between cationic and anionic inorganic nanosheets resulted in a strong visible light absorption ability and a remarkably depressed photoluminescence signal;the layer-by-layer stacking of the two layered materials also resulted in an enhanced porosity,which could be controlled by changing the ratio of layered titanate to ZnCr–LDH.The resulting ZCT heterolayered nanohybrids were found to be extremely
active
Fig.8The a -amino acids used as ligands attached to the brucitelike layers.(K :Zn,K :Al,K :N,K :C,K :O,K :H;blue:Zn–O octahedron,pink:Al–O octahedron).L -Glutamic acid (1),L -alanine (2),and L -serine (3)in pristine (homogeneous)state are labeled a;their anions (green)in inter-calated (heterogeneous)or delaminated (colloidal)states are labeled as b and c,respectively.Reproduced with permission from ref.121.Copyright 2011
Wiley-VCH.
Fig.9Cross-sectional HRTEM image of the ZCT nanohybrid (a)and its enlarged view and structural model (b).Reproduced with permission from ref.125.Copyright 2011American Chemical Society.
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catalysts for O 2generation with a rate of B 1.18mmol h à1g à1under visible light,which is higher than that for the pristine ZnCr–LDH (B 0.67mmol h à1g à1).The superior photocatalytic activity for O 2production was rationalized by considering the ordering of the heterolayered nanosheets.In addition,the chemical stability of the LDH was greatly improved due to the protection afforded by the layered titanate.This work verified the efficacy of hybridization between two types of nanosheets in constructing performance-enhanced nanocatalysts,and this approach should be capable of extension to hybrids with a range of other layered materials.
The exfoliated positively-charged LDH nanosheets have the ultimate in 2D anisotropy which facilitates their use as building blocks for assembly into functional multilayer nanostructures with active species by employing the LBL technique.126,127Wei’s group assembled cobalt phthalocyanine/LDH ultrathin films (UTFs)using an electrostatic LBL technique.126They found that the surface of the UTFs was continuous and uniform with a long range stacking order in the direction normal to the substrate.A modified electrode based on this material exhi-bited a low detection limit of 3.2?10à7mol L à1,fast response of B 2s and excellent long-term stability for the determination of dopamine.
In addition,the LDH nanosheet can be used as a 2D matrix to immobilize metal nanoparticles (NPs).For example,Zhao et al.127successfully fabricated LDH nanosheets/Au NPs UTFs using the LBL technique.The LDH nanosheets provided a confined and stable microenvironment for the immobilization of Au NPs with a long range stacking order,and the uniform dispersion of positive charge in the LDH nanosheets resulted in the high dispersion of Au NPs.As a result,the multilayers possessed higher electrocatalytic activity and selectively for the oxidation of glucose than that of previously reported Au NP-based glucose sensors.Also,Lee et al.128immobilized Pt NPs on exfoliated MgAl–LDH nanosheets through an electrostatic self-assembly pro-cesses.The resulting catalyst exhibited higher catalytic activity for the reduction of p -nitrophenol into p -aminophenol by NaBH 4with a TOF value of 0.75min à1,in comparison with the commercial Pt/C catalyst (TOF =0.56min à1).
2.3.2Nanocatalysts based on multifunctional nanocompo-sites.Co-assembly of nanocomposites consisting of various LDHs and other functional materials is an e?cient strategy for developing multi-component or bifunctional heterogeneous catalysts.These functional components usually are semi-conductors,metals and carbon-based materials.
(a)Semiconductor/LDH nanocomposites.By hybridization with other semiconductors in composites,the photocatalytic activity of LDHs is expected to be enhanced because of the resulting increase in the lifetime of electrons and holes.Recently,a mesoporous zinc-based LDH (ZnLDH)-supported TiO 2was constructed by reconstruction of ZnLDH in an aqueous solution of TiOSO 4.129Notably,the high surface area of TiO 2/ZnLDH favoured light harvesting,while the complex heterojunctions between the two components facilitated the transfer of photogenerated electron–hole pairs during the course of photocatalysis.As a result,
the degradation percentage of phenol increased significantly from 27%over the pristine ZnLDH to 85%over the hybrid TiO 2/ZnLDH after 240min UV illumination.
In addition to the most widely used semiconductor TiO 2,129–131CeO 2has also been assembled with MgAl–LDH to form efficient CeO 2/MgAl–LDH photocatalysts for the degradation of phenol and chlorinated phenols under UV irradiation.132Photocatalysis by these bifunctional basic/semiconducting photocatalysts with a band gap energy of B 3.2eV was shown to be most likely triggered either by UV-induced excitation of electrons from the valence band (VB)to the conduction band (CB)of CeO 2or by the formation of a charge-transfer complex at the defect sites near to the CeO 2/LDH interfaces.CeO 2has also been incorporated into ZnTi–LDH to form an efficient hybrid CeO 2/Zn 2TiO 4photocatalyst via sponta-neous formation and organization of CeO 2NPs on a ZnTi–LDH matrix using a method based on LDH calcination plus hydration in an aqueous solution of Ce(SO 4)2,followed by a final calcination process.133,134Compared with CeO 2/MgAl–LDH,the resulting CeO 2/Zn 2TiO 4photocatalyst exhibited a much high efficiency for phenol removal (about 90%).It was argued that the insertion of Zn 2+and Ti 4+within the LDH network resulted in a good disper-sion of the metal cations within the layers which acted as charge separation centers enhancing the degradation efficiency.
In a recent report,Shao et al.135prepared novel hierarchical nanoarrays based on a ZnO nanowire core and a CoNi–LDH platelet shell on fluorine-doped tin oxide (FTO)electrodes.The resulting ZnO@CoNi–LDH core–shell nanoarray showed a high photocurrent density of 1.5mA cm à2at +0.5V under illumina-tion,about three times larger than that of pristine ZnO nano-wires,as well as excellent stability in photoelectrochemical water splitting.This was attributed to the successful photo-generated electron–hole separation originating from the intimate contact at the interface between the ZnO core and the electro-catalytically active Co-containing LDH shell.
In addition,Ag/AgBr nanomaterials with plasmonic photo-catalytic performance were combined with a nitrate-intercalated CoNi–LDH to form bifunctional visible-light-induced nano-composite photocatalysts for degradation of anionic dyes.136In this nanocomposite structure,‘‘hot’’electrons from the Ag surface plasmon resonance induced by incident visible light were transferred to the CB of AgBr under visible light,and trapped by the dioxygen molecule to form O 2àor other reactive oxygen species.Moreover,the intimate attachment of AgBr to the surface of the LDH further favoured the formation of OH radicals from OH groups of LDH induced by holes in the VB of AgBr.(b)Carbon-based nanocomposites.Carbon-based materials,such as carbon nanotubes (CNTs),carbon fibres and graphene,have been widely used as catalyst supports in a variety of hetero-geneous reactions,due to their fascinating physicochemical properties including their high aspect ratio,high specific surface areas,versatile and abundant surface functional groups after surface modification,high mechanical strength,excellent thermal stability,and good electronic conductivity.
Among carbon-based materials,CNTs are commonly consid-ered as ideal candidates for assembling functional nanocomposites
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used in photoelectrochemical or electrochemical applications.Incorporating CNTs and LDHs into nanocomposites has attracted the interest of many researchers.In LDH/CNT nano-composites,LDHs can retain their intrinsic structure,while the CNT support enhances the dispersion of LDHs and the heat and mass transfer during the reaction,and favours electron transfer throughout the composites.Therefore,combining CNTs with LDHs can result in enhanced performance to meet the requirements of advanced catalytic applications.For example,multi-walled CNTs have been explored as a functional component to enhance the electrocatalytic activity of NiAl–LDH.137By means of a simple in situ coprecipitation method,the NiAl–LDH nanocrystallites were highly dispersed and anchored on the surface of a modified CNTs matrix by virtue of the strong interfacial electrostatic interactions between them.An electrode modified by the NiAl–LDH/CNTs nanocomposite exhibited eight times higher electrocatalytic activity for glucose electrooxidation than those modified by either pristine NiAl–LDH or CNTs.The enhanced electrocatalytic activity was attributed to the fact that CNTs efficiently promoted the charge transport between the active Ni centers and the electrode,and the CNTs also con-structed a porous network-like structure which enhanced the diffusion of the reactant.
Recently,a NiFe–LDH/CNTs hybrid was assembled by direct nucleation and growth of ultrathin NiFe–LDH nanoplates on the functional groups of mildly oxidized multi-walled CNTs.138Fig.10shows the interconnected electrically conducting networks of the NiFe–LDH/CNTs nanocomposite,where the NiFe–LDH nanosheets were dispersed on the surface of CNTs.The resulting hybrid NiFe–LDH/CNTs exhibited higher electro-catalytic activity for oxygen evolution in alkaline media,and higher stability,than either commercial precious metal Ir cata-lysts or the NiFe–LDH nanosheets themselves.Moreover,the TOF value of the NiFe–LDH/CNTs catalyst was about 3times higher
than that of mixed nickel and iron oxide electrocatalysts pre-viously reported and also comparable to that of the most active perovskite-based catalyst.139The authors attributed the high activity of the nanocomposite catalyst to the strong interactions between NiFe–LDH and CNTs facilitating electron transport.As a zero band gap semiconductor carbon-based material,graphene with a special 2D sheet-like structure possesses remarkable electrical,thermal and mechanical properties.140,141In particular,its robust and flexible structure with high carrier mobility a?ords an excellent electrical conductivity.140Therefore,graphene has been intensively investigated as a substrate to grow and anchor functional nanomaterials.Recently,Hwang et al.reported the immobilization of ZnCr–LDH nanoplates on the oppositely-charged graphene nanosheets.142The strong electronic coupling between the two components in the hybrid ZnCr–LDH/graphene nanocomposite gave rise to significantly improved visible light absorption.Consequently,the nanohybrid exhibited excellent photocatalytic activity for O 2generation under visible-light irradiation with a rate of B 1.20mmol h à1g à1,much higher than that over pristine ZnCr–LDH (B 0.67mmol h à1g à1).More recently,a nanocomposite of NiTi–LDH/reduced graphene oxide (RGO)nanosheets was synthesized by an in situ growth approach.143The hybrid composite possessed excellent photo-catalytic activity for water oxidation under visible light irradiation,showing an O 2generation rate of 1.968mmol g à1h à1and a very high quantum efficiency of 61.2%at 500nm,owing to the efficient electron transfer from NiTi–LDH to the RGO matrix.In addition,Xu et al.144also reported the fabrication of NiAl–LDH/graphene nanocomposites using a conventional coprecipitation method under low-temperature conditions and subsequent reduction of the supporting graphene oxide.Notably,the NiAl–LDH/graphene modified electrode exhibited an oxidation current of 2.06mA for dopamine electrooxidation,which was 2.8times higher than that over the pristine NiAl–LDH modified electrode.The above results confirm the benefits arising from hybridization between photoactive or electroactive LDH materials and electron-accepting graphene nanosheets which lead to highly efficient catalysts and electrocatalysts with improved activity.
(c)Magnetic nanocomposites.Magnetically separable nano-composite catalysts have attracted tremendous interest because of the minimization of the consumption of additional materials,energy and time required for their separation from a reaction mixture,which can give significant economic and environmental benefits.145,146Particular interest has focused on magnetic nano-catalysts with the core@shell structure,which combine the advantages of both a magnetic core and an active shell and the possible synergistic e?ects between them.147
In recent years,the synthesis of core–shell type magnetic LDH composite has been developed by several research groups.For example,Li et al.148assembled core–shell Fe 3O 4@MgAl–LDH using a LBL method with delaminated LDH nanosheets and a magnetic matrix.After loading W 7O 246àanions,the magnetic nanocomposite showed 93.6%degradation percentage of hexa-chlorocyclohexane after 12h under visible light
irradiation,
Fig.10Ultrathin NiFe–LDH nanoplates grown on CNTs.(a)Scanning electron microscopy (SEM)image of NiFe–LDH nanoplates grown on a network of multi-walled CNTs.(b)and (c)Transmission electron micro-scopy (TEM)images of a NiFe–LDH/CNTs hybrid.(d)X-ray diffraction (XRD)patterns of NiFe–LDH/CNTs and a control b -Ni(OH)2/CNTs sample.(e)Schematic showing the hybrid architecture and LDH crystal structure.Reproduced with permission from ref.138.Copyright 2013American Chemical Society.
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and the activity remained unchanged after six cycles or reuse.Another example involves a hierarchical core–shell Fe 3O 4@MgAl–LDH with a honeycomb-like structure,which was prepared by the selective deposition and formation of MgAl–LDH onto the surface of Fe 3O 4submicrospheres (220or 450nm in diameter).149Interestingly,the loading of Au NPs on the surface of resulting nanocomposite afforded a material giving 99%yield of aceto-phenone in 1-phenylethanol oxidation.The same research group also synthesized two hierarchical core–shell Fe 3O 4@CuMgAl–LDH magnetic nanocatalysts,150including LDH particles horizontally coated onto the core surface (Fe 3O 4@LDH-1)and hexagonal plate-like LDH particles vertically grown on the core surface (Fe 3O 4@LDH-2)(Fig.11).The two catalysts both showed good magnetic susceptibility and significantly higher catalytic activity in phenol hydroxylation than the pure CuMgAl–LDH.The improved performance was explained by a Fenton-like oxidation process,which was enhanced by the HO–Cu 2+–O–Fe 2+–O–linkages formed at the shell–core interface.It is worth noting that the horizontal growth of CuMgAl–LDH particles afforded a more effective nano-catalyst,which can be attributed to the larger synergistic effect between the thin LDH shell and the Fe 3O 4core.In addition,the as-prepared nanocatalysts showed excellent ease of separation,and high stability and efficiency of reuse.2.4
Complex and mixed metal oxide catalysts
LDHs can be used as precursors to generate a variety of MMO materials by controlled thermal decomposition.The trans-formation of the LDH precursors into MMOs is topotactic owing to the ordering and uniform distribution of metal cations in the layers.MMOs have large specific surface areas (100–300m 2g à1),basic properties,a homogeneous and thermally stable disper-sion of the metal ion components,synergistic effects between
the elements,and the possibility of structure reconstruction under mild conditions,all of which are very important attri-butes for catalysts.Furthermore,the structures and properties of MMOs can be tuned by changing the composition of the LDH precursors,resulting in a huge family of new materials.151,152The resulting multi-metal MMOs,especially those containing transition metals,can be used as heterogeneous catalysts for photodegradation of organic pollutants,reduction of NO x and SO x emissions,oxidation/total combustion of (volatile)organic compounds,and chemical transformations for the synthesis of fine chemicals,and usually exhibit higher activities and longer lifetimes than catalysts prepared by traditional methods.
2.4.1Photodegradation of organic pollutants.In recent years,environmentally friendly complex metal oxide photo-catalysts with long life and excellent catalytic activity derived from LDHs have attracted increasing attention.For example,Li and colleagues reported a novel synthesis of high-surface-area ZnAl 2O 4complex metal oxides with well-developed inter-connected mesopore networks.153The synthesis procedure involved the formation of a two-phase composite (spinel and zinc oxide)by thermal conversion of single-source ZnAl–LDH precursors at 5001C or above,followed by selective leaching of the ZnO phase from the resulting calcined https://www.doczj.com/doc/ca10093777.html,pared with materials prepared by the conventional solid-state method,the as-prepared spinels showed better photocatalytic activity for degradation of phenol in aqueous solution under UV irradiation due to their high surface areas of 253m 2g à1and small pore size.Furthermore,to facilitate practical applications,a ZnFe 2O 4film supported on a sulfonated silicon substrate was also fabricated using the above method,154and exhibited greatly enhanced photocatalytic activity in the degradation of rhodamine B com-pared with pure ZnFe 2O 4powder prepared by the conventional solid-state method.Such structured ZnFe 2O 4-based film photo-catalysts may have potential applications because of their high thermal stability,strong resistance to acid and alkali,and ease of separation.
Recently,Zhao et al.155demonstrated that ZnAl 2O 4particles were uniformly dispersed in a ZnO matrix after controlled thermal conversion of ZnAl–LDH.The heterojunctions formed,and the band gap coupling between them,improved the spatial separation of photogenerated carriers under UV irradiation leading to much enhanced photocatalytic performance in the degradation of methyl orange in comparison with ZnO or ZnO/ZnAl 2O 4catalysts prepared by coprecipitation or physical mixing.In addition,a new strategy for constructing efficient and recyclable MMO-type photocatalysts with hierarchical biomimetic structures was reported by Wei and colleagues via a biotemplate-assisted LDH precursor route.156They used a legume as a sacrificial biotemplate to prepare a biomorphic hierarchical ZnAl–MMO with a meso/macroporous framework (Fig.12).The as-fabricated hierar-chical structured MMO material was applied for UV-light-degradation of two dyes,RB and a azobenzene-containing polymer poly{1-4[4-(3-carboxy-4-hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl sodium salt}(PAZO).The catalytic degradation by the MMO powder was 8.1%for PAZO and 74%for RB in water after 2h.In contrast,the fraction of the decomposed
dye
Fig.11SEM (a)and (c)and TEM (b)and (d)images of Fe 3O 4@LDH-1(a)and (b)and Fe 3O 4@LDH-2(c)and (d).Reproduced from ref.150.Copyright 2013The Royal Society of Chemistry.
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was 29%for PAZO and 81%for RB by the biomorphic MMO framework.The enhanced photocatalytic activity was attributed to the rather high specific surface area as well as the presence of hierarchical porosity.
As in the case of semiconductor photocatalysts,the high exposure of active facets and a high degree of dispersion also play important roles in improving the catalytic performance.For example,hexagonal ZnO nanoplatelets with a high percen-tage of exposed (0001)facets and high dispersion were grafted on a hierarchical flower-like matrix,157based on an in situ topotactic transformation of a ZnAl–LDH precursor grown on amorphous alumina (Fig.13).The resulting ZnO-based nano-structure possessed a lower band gap of 2.90eV than the control samples including ZnAl–MMO powder (3.0eV),ZnO nanoplates (3.04eV)and ZnO nanorods,and exhibited much
higher photocatalytic performance for visible-light-degradation of RhB compared with the above control samples,mainly because of the presence of more defects at the highly active exposed (0001)facets.
On the other hand,semiconductor photocatalysts formed by coupling two kinds of metal oxides together were found to be able to improve the separation e?ciency of electron–hole pairs through the proper modulation of electronic structure.Recently,Fan et al.158reported the synthesis of visible-light-induced heterostructured ZnAlIn–MMO nanocomposite photo-catalysts from ZnAlIn–LDH precursors.In the composites,well dispersed amorphous In 2O 3domains were intimately attached to ZnO nanocrystallites,thus forming the heterostructure of ZnAlIn–MMO.Under visible light irradiation,the fraction of the decomposed methylene blue could reach 73%by the ZnAlIn–MMO nanocomposite with an estimated band gap of B 2.50eV.In constrast,the catalytic degradation by pure In 2O 3was only 48%.This was attributed to the efficient separation and trans-portation of photogenerated charge carriers originating from the heterostructure,as well as the large specific surface area.Besides MMOs containing Zn,In and Fe,other photo-responsive MMOs can be obtained when active components (e.g.,Ni,Ti)are incorporated into LDH precursors.Ti-containing inorganic solids have attracted much attention owing to their multifunctionality,especially in energy conversion and catalytic reactions.One example is a Ti-containing NiO–NiTiO 3nano-composite obtained by the thermal conversion of a NiTi–LDH precursor at 9001C and above.159,160The Ti-based nanocomposite exhibited good photocatalytic activities for the degradation of methylene blue under both UV-and visible-light irradiation,which was attributed to the synergistic effect between the mixed wide band gap of semiconductor NPs in the UV region and the photo-absorption of NiTiO 3in the visible-light region.
In general,the studies addressed above provide very useful design and synthesis strategies for e?cient and versatile MMO materials by the rational combination of the elements in the LDH layers and the structural tunability of LDH precursors,leading to new and e?ective photocatalysts for the elimination of hazardous pollutants.
2.4.2Elimination of NO x and SO x emissions.The increas-ing use of fossil fuels such as coal,natural gas and petroleum for power generation and transportation has greatly increased NO x (N 2O,NO and NO 2)and SO x (SO 2and SO 3)emissions,leading to serious environmental problems.Thus,the elimina-tion of NO x and SO x is becoming a more and more urgent mission for chemists and related researchers.
In recent years,many researchers have explored di?erent types of transition metal-containing MMOs derived from LDH precursors for the reduction of NO x emission.In particular,LDH-derived catalysts containing di?erent transition metals (e.g.,Co,Mn,Ni,Cu)that can either directly decompose NO x or e?ectively reduce NO x in the presence of excess O 2are widely sought.161–166The NO x storage/reduction MMOs derived from Co–and Ca–LDHs,167,168which are used in engines that alternately operate under lean-burn and rich-burn conditions,may be good candidates.The increased surface redox
species,
Fig.12Schematic representation of the fabrication of a biotemplated LDH film and MMO framework from a legume.A uniform Al 2O 3coating was deposited on the surface of the biotemplate with an atomic layer deposition (ALD)process;the film of ZnAl–LDH was prepared by an in situ growth technique;finally,the ZnAl–MMO framework obtained by calcina-tion of the LDH precursor.Reproduced with permission from ref.156.Copyright 2009American Chemical
Society.
Fig.13SEM images of ZnAl–MMO:(a)an overall view of the ZnAl–MMO microspheres;(b)a single ZnAl-microsphere;(c)the nanoflakes of a ZnAl–MMO microsphere;(d)a portion of a ZnAl–MMO nanoflake with hexago-nal ZnO nanoplatelets embedded on its surface (inset:a cross-section of the ZnAl–MMO nanoflake).Reproduced from ref.157.Copyright 2011The Royal Society of Chemistry.
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alkalinity and reducibility of the catalysts are believed to be associated with their improved catalytic activity.
Recently,alkali metal (Li,Na,K,Rb,Cs)promoted CoMnAl–MMOs (with a molar ratio of alkali metal/Co =0.037),derived from CoMnAl–LDHs with the molar ratio Co :Mn :Al =4:1:1,were found to show an increased catalytic activity for N 2O decomposition in inert gas and simulated waste gas from HNO 3production,which decreased in the order Cs 4Rb 4K 4Na =Co 4MnAlO x 4Li,i.e.,the catalytic activity increased with the increasing ionic radius of the alkali metal cation for the same surface coverage.169,170The promotional effect of alkali metals was related to their ionization potential,the charge donation from alkali metal cations to the oxide form of surface oxygen and further to cobalt and manganese,and a decrease in the binding energies of the catalyst components.This work strongly indicated that transition metal-containing MMOs for effective NO x transformation can be rationally syn-thesized by the combination of transition metals in the LDHs precursor and other promoters,and are potential important catalysts for environmental applications.
Calcined transition metal-containing LDHs have been investigated as potential materials for the reduction of SO x emission from the fluid catalytic cracking (FCC)process in oil refineries.171Corma et al.found that CuMgAl–MMO could catalyze both the oxidation of SO 2to SO 42àin the FCC regenerator,and the reduction of sulfates to H 2S in the reducing atmo-sphere of the cracking zone.Notably,CuMgAl–MMO 172and CoMgAl–MMO,173subsequently activated by heating under H 2,could simultaneously remove SO x and NO x .Moreover,CuAl–MMOs and MgAl–MMOs were found to be able to capture a large amount of SO x .174,175Introduction of different metal species (e.g.Ce,Fe,Cu,Mn)to LDHs,either by impregnation or coprecipitation,has been proven to be the best way to generate the mixed oxides/spinel solid solutions with both basic and redox properties for the SO x emissions.174–178Polato et al.179demonstrated that the SO x storage capacity of transition metal doped MgAl–MMOs with a transition metal/total metal molar ratio of around 0.11decreased gradually in the order Cu(II )4Co(II )4Fe(III )4Cr(III ).Polato et al.180also prepared Mn,Mg,Al-spinels from LDHs,and found that the SO 2uptake capacity of Mn,Mg,Al-spinels could reach 2890m mol of SO 2per g only after 10min time on stream (TOS),much higher than that of Mg,Al-spinels (about 400m mol of SO 2per g after 4h TOS).In addition,the incorporation of CeO 2in Mn,Mg,Al-spines could not only increase the amount of SO 2uptake but also favour the catalyst regeneration efficiency.181
2.4.3Oxidation of volatile organic compounds.Emission of volatile organic compounds (VOCs)from many industrial processes and transport vehicles represents a serious environ-mental https://www.doczj.com/doc/ca10093777.html,pared with the elimination of VOCs by thermal combustion,the catalytic total combustion of the pollutants to carbon dioxide and water is more e?ective and economical in terms of energy and cost.In general,noble metals exhibit excellent catalytic oxidation properties.However,the high cost and low stability of noble metal catalysts greatly limits their large-scale practical applications.Therefore,in terms of
economic and environmental sustainability,numerous e?orts have been made to design e?cient and recoverable MMOs for the total combustion of some commonly encountered VOCs,such as methane and ethanol,by employing transition metal (Cu,Mn,Co,and Ni)-containing LDHs as catalyst pre-cursors.182–191Recently,Li et al.192found that Pd-modified CoAl–MMO derived from a CoAl–LDH with the molar Co/Al ratio of 3.0was much more active in toluene elimination than the catalyst prepared by a traditional thermal combustion method,owing to the combined effects of the high surface area,small mean crystallite size of the nano-sized support and highly dispersed PdO particles,as well as the presence of a large number of oxygen vacancies on the support surface serving as the site of oxygen activation.Moreover,the strong synergistic effects between Co 3O 4and PdO also contributed to the high catalytic activity.In another example of use of LDH-derived
MMOs for elimination of VOCs,Urda
?et al.193synthesized a range of rare-earth metal-promoted MgAl–MMOs derived from LDH precursors,and found that the most active and highly stable catalyst was a Ce-containing material containing about 4.5at%surface Ce with respect to total cations,which gave total conversion in methane oxidation at 7001C.The correlation observed between the intrinsic catalyst activity and H 2con-sumption in temperature-programmed reduction experiments suggested the involvement of an oxidation–reduction process based on the involvement of the Ce(IV )/Ce(III )redox couple in the catalytic reaction.2.5
Metal catalysts
In heterogeneous catalysis,synthesis of catalyst NPs with con-trolled size and uniform distribution is a major challenge.In both academia and industry,the most widely used preparation approaches for supported metal catalysts are deposition–pre-cipitation and incipient wetness impregnation.In most cases,however,it is difficult to obtain well-dispersed metal NPs with uniform size and good thermal stability,especially at high loadings.This results from the inhomogeneous distribution of active precursors on the supports as well as the weak interaction between them.In addition,the agglomeration of metal NPs during the catalytic reaction—which can further decrease the dispersion of metal species—is always a problem.Therefore,the design and synthesis of heterogeneous metal catalysts with high dispersion and stability is very necessary.
The layered matrix of LDHs o?ers multiple advantages as a host for NPs,such as ease of manipulation of the brucite-like layer composition by incorporating di?erent active metal anions,and the good dispersion of the metal cations within the layers.Upon calcination and subsequent reduction,such LDHs usually give rise to metal particles well-dispersed in a mixed oxide matrix.This structural transformation endows LDH materials with extraordinary capability as a precursor to metal catalysts for use in a variety of https://www.doczj.com/doc/ca10093777.html,pared with the traditional methods for the synthesis of supported metal catalysts,the use of LDH precursors has three advantages:(i)as noted above,the uniform distribution of active metal cations at the atomic level without a segregation of ‘‘lakes’’of separate cations within the
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brucite-like layers,as verified by Grey et al.2,194and Cadars et al.195by means of multinuclear nuclear magnetic resonance spectroscopy for MgAl–LDH,ensures high homogeneity in a LDH precursor,which enhances the catalytic activity of transi-tion metal containing LDHs as catalysts in their own right;however,it also facilitates the formation of highly dispersed and stable metallic species upon calcination and subsequent reduction;(ii)the strong metal–metal oxide support interactions between the metal oxide and metal NPs—formed concomitantly during calcination of the LDH precursor—can prevent sintering/aggregation of the metal NPs during the reaction;(iii)the particle size and specific morphology of metal NPs can be controlled in situ by the topotactic nature of the transformation of LDHs into MMOs,and/or the effect of confinement of a catalyst precursor in the LDH interlayer galleries.196The above characteristics of LDHs open opportunities for developing powerful new metal-based catalyst systems.
2.5.1Integrated monometallic nanocatalysts
(a)H 2generation and hydrogenation.Hydrogen represents an important alternative energy feedstock for both environmental and economic reasons.Over the past decade,due to their high activity and low cost,numerous transition metal catalysts such as copper,nickel,cobalt on a metal oxide matrix have been prepared by calcination and reduction of LDH precursors,and employed in a variety of heterogeneous reactions for the purpose of H 2production;these include the partial oxidation of methane,197steam reforming of methane,198,199steam reforming of methanol and ethanol,200–205and the water–gas shift reaction.206,207Moreover,the use of these LDH precursors allows the concentration of each metal component in the mixed oxides to be controlled,while a more homogeneous active phase distribution can lead to better catalytic activity.
Recently,Montane
′et al.208investigated the steam reforming of ethanol (SRE)over
Ni-based catalysts derived from NiFe–LDHs.It was found that the best catalyst,with a Ni/Fe ratio of 1and calcined at 5001C,rendered high and stable hydrogen selectivity of up to ca.60%,low methane content,and consisted of a mixture of Ni(Fe)O x solid solution and NiFe 2O 4with very small crystallites.This was because lower Ni/Fe ratios strongly promoted ethanol dehydrogenation and acetaldehyde decarbonyl-ation followed by steam reforming likely due to the promotion effect of Fe/Fe 2+on the reducibility of nickel species and the formation of small nickel particles.However,crystallization and growth of NiFe 2O 4present in catalysts at high calcination temperature (8001C)led to lower activity and fast deactivation,due to active Ni 0sintering and higher carbon deposition.Moreover,the promoter effect of various metals (La,Ce,Ca,etc.)has also been assessed in some cases.209,210For example,Assaf et al.209demonstrated that the intercalation of Ce-and La-containing complexes into NiMgAl–LDH precursors by anion-exchange resulted in better H 2production per mol of ethanol in the feed at 5501C on the resultant Ni-based catalysts in ESR.Especially,on the Ce-modified Ni catalyst,the highest ethanol and acetaldehyde conversion rates were achieved,possibly due to a favoring of water adsorption on the weakly interacting clusters of CeO 2on the surface.More importantly,
the catalysts did not favor the formation of significant carbon deposits throughout the time on stream.
The development of highly selective,e?cient and economical catalysts for H 2generation from hydrogen storage materials (e.g.,NH 3BH 3,N 2H 4áH 2O)has also been one of the most active research areas.Transition metals such as metallic Co,Cu and Ni are very e?cient catalysts for NH 3BH 3dehydrogenation.Recently,Wei’s group reported a series of Cu-based catalysts derived from CuCoAl–LDH precursors for the dehydrogenation of NH 3BH 3,and found that the catalyst with a Cu/Co molar ratio of 1/1yielded a hydrolysis completion time of less than 4.0min with a rate of 1000mL min à1g cat à1under ambient conditions,211which is comparable to that reported for the best noble metal catalysts (e.g.,Ru,Pt).It was shown that the synergistic effects between highly dispersed metallic Cu and Co 3O 4spinel species played a key role in the significantly enhanced activity of these Cu-based catalysts.In addition,the synergistic effect between the small highly dispersed Ni NPs and the strong basic sites on the surface could be formed in noble-metal-free Ni-based catalysts derived from NiAl–LDH precursors,212giving 93%selectivity to H 2for the decomposi-tion of N 2H 4áH 2O at ambient temperature.
Recently,the hydrogen-free synthesis of 1,2-propanediol (1,2-PDO)from glycerol was performed e?ciently over Cu-based catalysts derived from CuMgAl–LDHs.213It was found that the dehydration of glycerol occurred mainly on the basic sites of the catalyst,and the acetol intermediate formed was further hydro-genated to 1,2-PDO over metallic copper NPs using active hydrogen atoms from the dehydrogenation of ethanol as the hydrogen source.The activity of catalysts depended mainly on its basicity,while the presence of metallic copper NPs was indispensable for dehydro-genation of the ethanol.The maximum conversion of glycerol reached 95.1%,with a selectivity to 1,2-PDO of higher than 90%.Recently,Li and co-workers employed a facile single-source precursor route to achieve green,well dispersed supported Cr-free Cu-based catalysts from CuMgAl–LDH precursors and then used these for the vapour-phase hydrogenation of dimethyl-1,4-cyclohexane dicarboxylate (DMCD)to 1,4-cyclohexane dimethanol (CHDM),a commercially important diol.214Their studies indicated that the catalyst could achieve a long-lasting (up to 200hours)100%conversion of DMCD with 99.8%selectivity to CHDM.They ascribed the unprecedented catalytic hydrogenation per-formance to the synergistic effect between the active Cu 0sites highly dispersed on the surface and the surface Lewis base sites from neighbouring MgO (Fig.14).It was demonstrated that Cu 0species were effective catalytic centers for the hydrogenation
Fig.14Schematic representation of the hydrogenation mechanism over Cu-based catalysts derived from CuMgAl–LDH precursors.Reproduced from ref.214.Copyright 2013The Royal Society of Chemistry.
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of DMCD.The quantity of dissociative hydrogen on the active sites directly determined the rate of the reaction.Additionally,surface Lewis basic (LB)sites coming from MgO,especially LB sites in intimate contact with metallic Cu 0sites,could easily interact with the p *-acceptor orbital of the C Q O group by forming a zwitterionic,tetrahedral intermediate with nucleo-philic reactivity at the oxygen atom,thus improving the reac-tivity of the ester group in the DMCD molecule.
(b)Growth of carbon materials.The use of the regularly arranged transition metal cations in the brucite-like layers of LDHs as precursors for the in situ formation of highly dispersed metal NPs (Fe,Co,and Ni)through calcination and reduction,has been proposed as a valid approach for the growth of carbon nanomaterials from catalytic chemical vapour deposition (CCVD).The chemical composition,size,shape,and crystallo-graphic orientation of the NPs can be easily tuned through controlled calcination and reduction of LDHs and selection of the metal cation combination,and has a significant e?ect on the microstructures and morphologies of the carbon materials formed.Meanwhile,the interaction between the active metal and the support is responsible for the quantity and quality of carbon materials.
Li and colleagues first synthesized multi-walled CNTs with a uniform diameter ranging from 20to 30nm by CCVD of acetylene at 7001C over highly dispersed Co NPs derived from a CoAl–LDH precursor.215Such an LDH precursor route for a Co-based catalyst enables scalable preparation of CNTs at low cost.Furthermore,three types of carbon materials (nanotubes,caterpillar-like fibers and interwoven spheres)could be grown on a silicon-supported Co-based catalyst derived from a CoAl–LDH precursor.216The morphologies of the carbon materials could be easily tuned by simply adjusting the growth duration.A growth mechanism was proposed,based on an overgrowth of the initially formed tubular nanostructures.
Zhang et al.217investigated the e?ect of the composition of NiMgAl–LDHs on the formation of CNTs from CCVD of acetylene.The yield of carbon increased with increasing Mg content,because the spinel phases (Ni 1àx Mg x Al 2O 4)formed by calcination of LDHs inhibited the growth of active Ni particles,thus leading to good metal dispersion and small catalyst particle sizes.Moreover,larger amounts of Mg also induced the formation of coiled CNTs with a helical structure and an outer diameter of ca.20nm.The same workers also synthesized uniform twist-shaped single-helical CNFs with a coil pitch of B 80nm,and herringbone-type double-helical carbon fibers with a coil diameter of B 1–2m m over NiCuAl–LDH and NiCuMgAl–LDH,respectively.218The growth of fibrous carbons with different morphology was correlated with the diffusion of carbon species on different crystal planes of the two different active metals (Ni and Cu),which controlled the deposition of graphitic layers.After loading with Pt,an electrode supported on the herringbone-type double microcoiled carbon fibers showed much larger Pt active surface area than that supported on commercial carbon black.Furthermore,compared with the commercial material,the new electrocatalyst exhibited a four-fold enhancement of activity
and excellent anti-poisoning ability in the electrocatalytic oxida-tion of methanol,which was attributed to the combined beneficial effects of the novel microstructure and the special composition of helical carbon fibers.In addition,new submicrometer-scale flat carbon fibers (SFCFs)were successfully synthesized over a NiAl–LDH compound by CCVD of acetylene in the absence of H 2.219During CCVD,the active crystal facets of the NiAl 2O 4spinel phase formed favoured deposition of carbon atoms which grew into SFCFs.After supporting Pt on the SFCFs,the resulting Pt/SFCFs electrocatalyst exhibited much higher activity for methanol oxida-tion than a commercial Pt/carbon black catalyst,owing to the combined beneficial effects of the microstructure of the carbon support,the improved electrical conductivity,and the enhanced dispersion of Pt particles.
When a small amount of active metal component is intro-duced into the LDH layers,metal NPs formed after calcination and reduction can also catalyze the growth of CNTs with fewer walls and smaller diameter,such as single-walled CNTs and double-walled CNTs.Zhao et al.220demonstrated that the Fe NPs derived from MgAlFe–LDH with a Mg/Al/Fe molar ratio of 2:0.9:0.1could catalyze the growth of single-walled CNTs,because the incorporation of Mg into the LDH layers stabilized the active metal NPs.Furthermore,Wei and colleagues 221,222reported the synthesis of single-walled CNT-array double helices by well dispersed and stable active Fe NPs with a small diameter of 1–14nm,an extremely high density (1014to 1016particles per m 2)and good thermal stability at 9001C,which were obtained from calcined MoO 42à-intercalated MgAlFe–LDHs based on a ‘‘pinning effect’’of Mo around the Fe NPs (Fig.15).In comparison,a carbonate-type CoMgAlFe–LDH was demonstrated to be effective for the growth of multi-walled CNT double helices.223Furthermore,the same research group fabri-cated nanocomposites of single/double-walled CNTs and MMO flakes using MgAlFe–LDH precursors with different Fe contents,224where single/double-walled CNTs were interlinked between 2D MMO flakes.When incorporated in a polyimide matrix,the resulting nanocomposite possessed much higher tensile strength and Young’s modulus than the corresponding composite contain-ing pure CNTs.In addition to Fe-containing LDH precursors,Co-and Ni-containing LDHs have been employed for the growth of single-walled CNTs in a fluidized bed reactor.225,226In particular,high selectivity for metallic single-walled CNTs was achieved over the CoMgAl–LDH catalyst.
It is not only transition metal cations located in the layers of LDHs that can give rise to metal particles after calcination and reduction—noble metal complex-intercalated LDHs can
also
Fig.15Schematic illustration to show the formation of high density Fe NPs by the incorporation of MoO 42àin MgAlFe–LDH.Reproduced with permission from ref.221.Copyright 2010American Chemical Society.
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a?ord metal particles with a high degree of dispersion on mixed oxides when they are subjected to similar post-synthesis treat-ment for the growth of carbon materials.227,228For example,recently,Li and colleagues demonstrated that noble metal Pd nanocatalysts on MgAl–MMOs derived from PdCl 42à-intercalated MgAl–LDH precursors efficiently catalyzed the growth of uniform bamboo-like CNTs with few defects by CCVD of methane.228They found that the number of structural defects and the diameter of the CNTs were reduced with increasing metal dispersion.These results suggest that the synthesis of other metal oxide supported noble metal catalysts from LDH precursors will make it possible to prepare nanocomposites of CNTs and noble metals,which should be promising catalysts with many applications in view of the availability of complex geometric shapes of noble metal particles with specific facets exposed and their high degree of dispersion,as well as the high mechanical strength of CNTs as supports.2.5.2Supported metal nanocatalysts
(a)Alumina as a support.Fabricating supported metal NPs with simultaneously enhanced activity and stability is of vital importance in heterogeneous catalysis and remains a challenging goal.Because of its good stability,high specific surface area,and excellent physical strength,alumina has been extensively used as a catalyst support in a variety of reactions.
Recently,Li et al.reported the in situ growth of NiMgAl–LDH microcrystallites on a microspherical g -alumina support through surface activation of the support followed by a homogeneous coprecipitation process.229They found that as compared with the analogous material prepared by conventional impregnation,Ni/MgO–Al 2O 3obtained from NiMgAl–LDH/g -Al 2O 3showed higher metal dispersion and stability,due to the formation of well-developed 2D platelet-like LDHs grown perpendicularly on the support surface,as well as the presence of lattice orientation and confinement effect of LDH platelets.With the introduction of Mg into the LDH layers,the ordered arrangement of larger NiMgAl–LDH platelets stabilized active nickel clusters,and the amorphous phases,including MgO and Al 2O 3,formed around the Ni particles impeded their agglomeration.Consequently,the highly dispersed small Ni NPs efficiently catalyzed the growth of uniform CNTs.
In a similar way,Feng and colleagues synthesized Ni and Pd monometallic catalysts supported on spherical g -alumina through in situ growth of active NiAl–LDH and PdCl 42à/MgAl–LDH precursors.230,231Because the introduction of the LDHs lowered the acidity of the support and strengthened the metal–support interaction,compared with materials prepared by conventional impregnation methods the resulting Ni-based and Pd-based catalysts exhibited not only higher activity but also better selectivity and stability in the vapour phase hydro-dechlorination of chlorobenzene and the selective hydrogena-tion of acetylene,respectively.
In addition,amorphous spherical alumina was utilized to synthesize a surface defect-promoted Ni nanocatalyst with a high dispersion and high particle density embedded on a hierarchical Al 2O 3matrix.232The high dispersion of active Ni species in the LDH precursor induced a uniform size distribution of Ni particles
(5.0?0.9nm)and extremely high particle density (B 2.4?1016particles per m 2)(Fig.16),far exceeding the densities of metal NPs reported previously.221The as-fabricated supported Ni nanocatalyst exhibited excellent low temperature activity for CO 2methanation,owing to an abundance of clusters on surface vacancies which served as active sites.Moreover,the intrinsic topotactic nature of the phase transformation from the LDH matrix to supported Ni NPs ensured a strong anchoring e?ect,which gave rise to extremely high reaction stability,with no sintering and/or aggregation of active species being observed during long-term use up to 260hours.
(b)Carbon materials as supports.In the field of hetero-geneous catalysis,carbon materials have been extensively investigated as catalyst supports to disperse and stabilize metal NPs.233,234For example,Li and colleagues 235successfully pre-pared highly-dispersed,carbon-supported nickel catalysts with Ni loadings from 6.8to 20.2wt%via in situ self-reduction of NiAl–LDH/carbon nanocomposite precursors with interwoven hybrid structure,which involved the crystallization of Ni/Al–LDH with concomitant carbonization of glucose under mild hydrothermal conditions.Notably,the carbon component in precursors serving as the reducing agent could reduce Ni 2+species to Ni 0in situ upon heating under an inert atmosphere.A Ni nanocatalyst with a Ni loading amount of 18.7wt%displayed excellent catalytic performance in the liquid phase hydrodechlorination of chlorobenzene,giving a very
high
Fig.16HRTEM images of Ni/Al 2O 3:(a)an overall image,(b)one nanoflake with Ni NPs embedded onto its surface (inset,a single-crystalline Ni NP),(c)a cross-sectional image of one nanoflake with Ni NPs embedded on both sides of the substrate (inset,a cross-section of a single-crystalline Ni NP),and (d)scanning transmission electron microscopy (STEM)image of the Ni NPs dispersed on the nanoflakes (inset,a portion of the nanoflake and the size distribution based on more than 200particles counted).Reproduced with permission from ref.232.Copyright 2013American Chemical Society.
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conversion of 99.3%.Li and co-workers further prepared a series of highly-dispersed,ZnO-modified supported nickel nanocatalysts (Ni–ZnO/C)via in situ self-reduction of hybrid NiZnAl–LDH/carbon (NiZnAl–LDH/C)nanocomposite precursors.236They found that ZnO addition significantly modified the catalytic properties in the hydrogenation of citral,giving an improved selectivity for citronellol;the maximum yield of citronellol achieved was B 92%.The promotional effect of ZnO addition was mainly attributed to the presence of a ZnO–metal interaction,which was proposed to be responsible for enhanced adsorption of the C Q O bond in the citral molecule on the catalyst surface and thus activation of the C Q O bond.Since such as-synthesized supported Ni catalysts have the advantages of low cost,excellent chemical stability,and facile separation and recovery using a magnetic field,they may have many potential applications in heterogeneous catalysis.
In addition,Li and co-workers have made great e?orts to improve the dispersion and activity of metal catalysts by com-bining the unique properties of one-dimensional CNTs with LDHs.237,238First,they established a novel approach to synthe-size highly dispersed Ni NPs by reduction of NiAl–LDH sup-ported on nitric acid modified multi-walled CNTs in the presence of L -cysteine as a bridging linker.237Due to the support effect of CNTs and the strong metal–support interaction,a high disper-sion of Ni NPs having a small modal diameter of about 5nm and narrow size distribution in the range 3–7nm was obtained.The resulting Ni nanocatalysts exhibited excellent catalytic performance in liquid phase hydrogenation,with 97.1%con-version of o -chloronitrobenzene and 98.3%selectivity toward o -chloroaniline in 300min,a much better performance than that of CNT-supported Ni catalysts prepared by incipient wet-ness impregnation.Secondly,to obtain dispersion-enhanced Ni-based nanocatalysts,poly(acrylic acid)-functionalized CNTs were also utilized to support NiAl–LDH (Fig.17),238and the resulting catalyst exhibited superior catalytic performance in the liquid phase selective hydrogenation of o -chloronitrobenzene to
o -chloroaniline when compared with a catalyst prepared by the conventional impregnation method,with the highest yield being 98.1%in 150min.The superior performance of the new catalyst can be attributed to the higher degree of dispersion of the metal particles,as well as the presence of both the electron-rich support and electron-deficient metal species.
In a very recent work,2D graphene has also been utilized as a catalyst support to synthesize a supported Ni nanocatalyst (Ni/G)via self-reduction of a hybrid NiAl–LDH/graphene precursor.239It was found that NiAl–LDH nanoplatelets were homogeneously dispersed on both sides of an exfoliated,structurally flexible graphene sheet in the hybrid precursor.Furthermore,the graphene component,serving as a reducing agent,reduced Ni 2+species in situ to Ni 0upon heating under an inert atmosphere,thus facilitating the formation of highly dispersed Ni NPs (Fig.18).This work provides a simple approach to fabricate a large number of graphene-supported,metal-based heterogeneous catalysts with many potential applications.
2.5.3Intermetallic nanocatalysts.In recent years,intermetallic compounds (IMCs)have attracted extensive research interest due to their unique geometric/electronic e?ects,specific com-positions and fascinating catalytic properties.240–242The wide versatility of LDH compositions and architectures makes a large number of element combinations accessible,which renders LDHs as a platform precursor for many IMC systems.Some IMCs,with or without noble metal being present in the LDH layers or in the interlayer space—such as Pt–Cu,243,244Pt–Ga,245,246Pt–In,247Pd–Cu,248Pd–Ga,249and Pd–Zn 250—have been formed from LDH precursors and shown to exhibit superior catalytic activity to the conventional monometallic catalysts.
Chemoselective hydrogenation is of great importance in the chemical industry,and IMCs have attracted extensive interest as e?cient catalysts.Recently,several Ni–In IMCs with a tunable particle size and Ni/In ratio were prepared successfully by a reduction process carried out during the topotactic trans-formation of NiIn–LDH precursors.251The Ni–In IMCs exhibited excellent catalytic activity and selectivity toward the hydrogenation of various a ,b -unsaturated aldehydes (e.g.,furfural,1-phenyltanol,crotonaldehyde,and 2-hexenal).Ni K-edge X-ray-absorption fine-structure spectroscopy (XAFS)characterization and periodic density functional theory (DFT)calculations revealed that electron transfer from the In atom to the Ni atom,as well as
active-site
Fig.17Schematic illustration of the synthesis of highly dispersed Ni NPs over poly(acrylic acid)–functionlized CNTs.Reproduced from ref.238.Copyright 2013The Royal Society of
Chemistry.
Fig.18SEM (a)and TEM (b)images of Ni/G.Inset in (b)shows a HRTEM image of a single crystalline Ni nanoparticle.Reproduced from ref.239.Copyright 2014The Royal Society of Chemistry.
Review Article Chem Soc Rev
P u b l i s h e d o n 08 J u l y 2014. D o w n l o a d e d b y S h a a n x i N o r m a l U n i v e r s i t y o n 17/11/2015 08:48:49.