Applied Catalysis B-Gao Wenwen

Chemical deactivation of V 2O 5/WO 3–TiO 2SCR catalysts by additivesand impurities from fuels,lubrication oils,and urea solutionI.Catalytic studiesOliver Kro¨cher *,Martin Elsener Paul Scherrer Institute,5232Villigen PSI,SwitzerlandReceived 27March 2007;received in revised form 24April 2007;accepted 25April 2007Available online 27April 2007AbstractThe influence of the combustion products of different lubrication oil additives (Ca,Mg,Zn,P,B,Mo)and impurities in Diesel fuel (K from raps methyl ester)or urea solution (Ca,K)on the activity and selectivity of vanadia-based SCR catalysts were investigated.Standard V 2O 5/WO 3–TiO 2catalysts coated on metal substrates (400cpsi)were impregnated with water soluble compounds of these elements and calcined at 400and 5508C,in order to investigate the chemical deactivation potential of different elements and combinations of them.It was found that potassium strongly reduced the adsorption equilibrium constant K NH 3of ammonia.At small ammonia concentrations in the feed,only part of the active sites were covered with ammonia resulting in a reduced SCR reaction rate.At high ammonia concentrations,the surface coverage and SCR reaction rate increased,but high SCR activity at concurrent low ammonia emissions was impossible.Calcium caused less deactivation than potassium and did not affect the ammonia adsorption to the same extent,but it lowered the intrinsic SCR reaction rate.Moreover,deactivation by calcium was much reduced if counter-ions of inorganic acids were present (order of improvement:SO 42À>PO 43À>BO 33À).Zinc was again less deactivating than calcium,but the positive effect of the counter-ions was weaker than in case of calcium.The degree of N 2O production at T >5008C,which is typical for V 2O 5/WO 3–TiO 2catalysts,was not influenced by the different compounds,except for molybdenum,which induced a small increase in N 2O formation.#2007Elsevier B.V .All rights reserved.Keywords:Selective catalytic reduction;DeNO x ;SCR catalyst;Vanadia;Deactivation;Ammonia adsorption1.IntroductionUrea-SCR (SCR:selective catalytic reduction)is an efficient technology to reduce the nitrogen oxide emissions of Diesel vehicles [1–3].This process has been developed from ammonia SCR,which is used for several decades as a technique to remove NO x from stationary power plants [4–6].The basis of SCR is the comproportionation reaction of ammonia with NO forming harmless nitrogen:4NH 3þ4NO þO 2!4N 2þ6H 2OThe main development steps on the way from the stationary application to SCR for Diesel vehicles were to identify urea as non-poisonous storage compound for ammonia,to develop aurea dosing strategy,which is able to cope with the varying NO x concentrations in dependency of engine load,speed and catalyst temperature,and to increase the volumetric activity of the existing vanadia-based catalysts [7].Catalysts of this type,optimized for Diesel vehicle applications,contain 1.5–2.0wt.%of V 2O 5on anatase TiO 2,promoted with $8wt.%WO 3.The vanadia concentration is higher than in catalysts for stationary applications ($1.0wt.%)to achieve high DeNO x activity.However,vanadia must be present as two-dimensional surface layer on the titania support to be selective.Too high surface concentrations result in the formation of bulk vanadia,which favour unselective oxidation reactions at higher temperatures,i.e.the formation of N 2O or the oxidation of ammonia to nitrogen.This phenomenon is more pronounced when the amount of vanadia exceeds the value that is necessary to form a monolayer on the surface.Tungsta is an important catalyst promoter,which stabilizes the anatase modification of titania,supports the spreading of the vanadia on the surface,increases/locate/apcatbApplied Catalysis B:Environmental 75(2008)215–227*Corresponding author.Tel.:+41563102066;fax:+41563102323.E-mail address:oliver.kroecher@psi.ch (O.Kro¨cher).0926-3373/$–see front matter #2007Elsevier B.V .All rights reserved.doi:10.1016/j.apcatb.2007.04.021the surface acidity and improves the poison resistance to alkali metals [8].This interplay of the different constituents explains the excellent properties of V 2O 5/WO 3–TiO 2systems as SCR catalysts.When considering a catalyst for a mobile SCR system,its durability concerning thermal degradation,mechanical attrition and chemical deactivation has to proven over the lifetime of the vehicle,which can be up to 106km for a heavy-duty truck.These requirements are of course much more severe than for stationary applications,since much higher temperatures are reached,the catalysts are exposed to thermal stress due to the varying exhaust gas temperatures and they have to withstand vibrations during operation of the vehicle.Moreover,SCR catalysts onboard of Diesel vehicles may deactivate by the action of water,which condenses in the catalytic converter after engine start,and by different substances in the exhaust gas,which deposit on the catalyst surface.The origin of these substances are mainly fuel and lubrication oil additives,e.g.detergents,dispersants,viscosity improvers etc.,which are responsible for a series of important product properties(Table 1).These additives are converted to inorganic compounds in the combustion chamber or emitted unchanged into the exhaust gas from the turbocharger lubrication system.Besides these intended additives,impurities,such as the sulphur in fuel and lubrication oils,also play a role in the emissions of Diesel engines.Table 2lists the concentrations of emission-relevant elements in lubrication oils and their most likely combustion products [9].The most emission-relevant elements in lubrication oil additives are sulphur,calcium,zinc and phosphorus,which are emitted as CaCO 3(or CaO),SO 2,ZnO and H 3PO 4from the combustion chamber.Some lubrication oils contain magnesium in addition to or instead of calcium,others contain boron instead of phosphorus compounds and also molybdenum-based additives are some-times used.Magnesium,emitted as MgCO 3or MgO,being less basic than CaCO 3or CaO,respectively,is expected to be less deactivating.It is interesting to note that a zinc-free lubrication oil could be developed for heavy-duty Diesel engines,although zinc dithio dialkyl phosphate is claimed to be a very important multifunctional additive,reducing engine wear,preventing oxidation and bearing corrosion.For a discussion of the functionality of the different additives refer to Keller et al.[10].The increasing part of raps methyl ester (RME)in Diesel fuel,produced by esterification of raps oil by mainly potassium hydroxide,is a potential source of potassium,which is known to be a very strong poison for vanadia-based SCR catalysts.The urea solution,used as reducing agent in the SCR process,poses another problem,since it may contain small amounts of alkali elements.Finally,engine abrasion produces traces of iron,copper,aluminium,chromium etc.,which are also found in the exhaust gas.In general,only a small part of the compounds in the exhaust gas is expected to deposit on the catalyst surface under usual operating conditions,but the major part will slip through the smooth converter channels in form of small aerosols.However,this is not valid for the urea solution in case of a short distance between urea injection and SCR catalyst,which favours the capture of urea droplets at the catalyst entrance.The deactivation of SCR catalysts onboard a Diesel heavy-duty truck has already been investigated in a road test and aboutTable 1Lubrication oil and fuel additives in Germany in 1997from [9]Additive (tons)Engine oilsFuels GasolineDiesel Detergents/dispersants 28,60012,6001,750Viscosity improvers19,700Multifunctional additives(zinc dithio dialkyl phosphate)4,900Antioxidants4,300a1,200Metal deactivators Foam inhibitor Corrosion inhibitor 400150Cetan number improvers 3,700Antiwax additives 7,100Lubrication improver 1,000Subtotal 57,50014,20013,700Total85,400aSum of the four additives.Table 2Concentration of emission-relevant elements in lubrication oils and their most likely combustion products from [9]Element aConcentration in lubrication oil Combustion productsLight-duty (wt%)Heavy-duty (wt%)Carbon 11.6611.09C !gases (CO 2,CO,HC b ),PM c Sulfur 0.220.30S !PM (SO 42À),gases (SO 3,SO 2)Calcium 0.190.30Ca !PM (sulfate,carbonate,oxide)Zinc0.100.12Zn !PM (oxide,pyrophosphate)Phosphorus 0.100.11P !PM (oxide,Zn 3(PO 4)2)Magnesium 0.080.11Mg !PM (sulfate,oxide)Nitrogen 0.070.06N !gases (NO,NO 2)Silicon 0.0020.002Si !PM (oxide)Total12.4212.09a Oxygen and hydrogen not included.b HC:hydrocarbons.cPM:particulate matter.O.Kro¨cher,M.Elsener /Applied Catalysis B:Environmental 75(2008)215–22721610%reduction of the DeNO x activity was found after 535,000km,which was tentatively assigned to phosphorus deposits on the catalyst[11].However,the superposition of the effects of thermal degradation,mechanical attrition,pore blocking,different poisons in various amounts and concentra-tion profiles along the length of the converter and in the catalyst layer prevents any clear conclusion on the pure chemical deactivation effect of the different exhaust gas components. Although some literature can be found on the chemical deactivation of SCR catalysts,they mainly deal with tungsta-free model SCR catalysts and only emissions specific for powerplant application were included,such as halogens, different alkali metals,alkaline earth metals,arsenic,lead, sulphur and phosphorus[12–15].Arsenic was found to be the most severe poison[13],which is fortunately irrelevant for Diesel vehicles.Many studies deal with the strong poisoning effect of alkali metals on vanadia catalysts by decreasing the acidity and preventing the effective activation of ammonia on the surface[16–20].In the present study,we were interested in the pure chemical effect of the emissions specific for Diesel exhaust gas on practice-oriented V2O5/WO3–TiO2SCR catalysts,thereby excluding any other deactivation mechanisms,such as pore blocking.In thefirst part of the work,the influence of the different exhaust gas constituents on the catalytic performance were investigated.The results of the characterization of selected deactivated catalysts as well as a mechanistic explanation of the observed effects are reported in the second part of this publication[21]and a summary of the project can be found in[22].For the deactivation experiments,samples of V2O5/WO3–TiO2SCR catalysts coated on metallic substrates were dip-coated with different concentrations and combinations of the elements typical for Diesel exhaust gas emissions in form of salt solutions,calcined and tested for their catalytic perfor-mance.Since some additives reach the SCR catalyst undecomposed via the turbocharger lubrication system without being combusted,supplementing experiments were carried out with the original organometallic additive compounds in order to investigate if they interact in a similar way with the catalyst as the metal salts.2.Experimental2.1.CatalystsStandard metal substrates with a cell density of400cpsi,a length of21mm and a diameter of21mm(V=7.2cm3)were coated by Wacker Chemie with1.3g of a V2O5/WO3–TiO2 catalyst developed for SCR onboard of heavy-duty Diesel vehicles.The formulation is based on a V2O5/WO3–TiO2 catalyst,which was originally developed in combination with a coating process at the Paul Scherrer Institute.For verification of the test results or continuative experiments,the detailed recipe can be found in[7].Due to the manufacturing process,two large channels remain in the centre of the modules,which were sealed with inert ceramicfibres in order to prevent any uncontrolledflux in the reactor.The volume of these channels was subtracted from the gross volume of the modules resulting in a net reactor volume of7.0cm3.Prior to the tests the catalysts were degreened at5508C for50h.2.2.AnalyticsThe catalyst modules were tested in a laboratory quartz reactor at a GHSV of52,000hÀ1.The model exhaust gas (364l N/h)composed of10%O2,5%H2O,1000ppm NO,0to $2000ppm NH3,and balance N2.All components were dosed with gas or liquid massflow controllers,whereby water was evaporated in a heated box.High resolution FTIR spectroscopy was used to analyze the reaction products NO and NO2with a detection limit of3–5ppm and N2O and NH3with a detection limit of1ppm.The nitrogen balances could always be closed within1–2% accuracy,taking into account the formation of nitrogen with a NH3:NO stoichiometry of1:1according to the standard-SCR reaction:4NH3+4NO+O2!4N2+6H2O.The catalysts were characterized by ammonia slip–DeNO x curves,which were recorded going from450to2008C in508C steps.For every temperature,ammonia dosage is increased stepwise and after reaching steady-state conditions,the DeNO x value as well as the ammonia emission are measured.For evaluation and interpretation of the data,mainly the DeNO x at 10ppm ammonia slip and DeNO x at a=1.2ða¼NH3in=NO inÞwere used.2.3.Doping of catalystsMost of the experiments were carried out with0.4mol%of the doping element based on the sum of titanium,tungsten and vanadium,which is equal to20mol%based on vanadium.That allows us to describe the influence of the single doping elements independent of the atomic mass.The chemicals used for the aqueous impregnations are listed in Table3.The tared catalyst module is dipped for2s in the aqueous solution.After that,the module is shortly blown out to remove excess impregnation solution without alteration of the solution uptake.Moreover,the short impregnation time prevents chromatographic and adsorption effects as well as washing out of the catalyst layer.The catalyst module,after weighing,is dried at908C andfinally calcined at4008C for5h.After the measurement of the catalyst performance,all catalysts were calcined for a second time at5508C for5h and tested again.For the supplementing experiments on the direct influence of the undecomposed lubrication oil additives,Fuchs Europe Schmierstoffe provided a zinc-containing and a zinc-free mixture of commercial organometallic lubrication oil additives dissolved in hexane.The relative molar composition of mixture FES105-1129based on calcium(Ca=1)was Ca=1,Zn=0, P=0.45,S=1.2,resulting in a molar concentration on the catalyst of0.32mol%Ca and0mol%Zn.The corresponding composition of mixture FES105-1130was Ca=1,Zn=0.24, P=0.45,S=1.2,resulting in0.31mol%Ca and0.07mol%ZnO.Kro¨cher,M.Elsener/Applied Catalysis B:Environmental75(2008)215–227217on the catalyst.Prior to the first measurement,both impregnated catalyst modules were sulphated with 100ppm SO 2at 4008C for 5h.3.Resultsparison of undoped and doped SCR catalysts Ammonia slip–DeNO x curves proved to be a suitable graphical representation for the assessment of the quality of SCR catalysts [7,23].Fig.1exemplarily shows the shape of these curves for one of the undoped catalyst samples after degreening at 5508C for 50h.For such a highly active SCR catalyst,the curves run through the lower right corner for temperatures between 350and 4508C,i.e.high DeNO x values are reached at very low ammonia emissions.For low temperatures,the catalyst activity decreases,resulting in ammonia emissions at lower DeNO x values.At temperatures of >=3508C,small variances in the catalytic activity of the different samples hardly affect the slip–DeNO x curves and differences are within the measuring accuracy of +/À1%DeNO x .At 2508C or below,the curve shape is much more sensitive towards the sample activity but the reproducibility isreduced.The large ammonia storage capacity of the catalysts at low temperatures results in a waiting time of up to 10min until steady-state conditions are reached after concentration changes.Small,but unavoidable fluctuations in the reactor temperature and water dosage during this period causes small distortions of the slip–DeNO x curves.Thus,the curve shapes at the intermediate temperature of 3008C are best suited for the purpose of comparison,since it is sensitive regarding the catalyst activity but quickly equilibrated and hardly affected by fluctuations of the water feed concentration.A further reduction of the amount of raw data is possible by extracting only the DeNO x at 10ppm ammonia slip from the curves,representing realistic operating conditions in an SCR system onboard of a Diesel vehicle,and the DeNO x values for constant ammonia overdosage with a =1.2,providing infor-mation about the maximum SCR activity of the catalysts.Although the 32laboratory catalyst samples from Wacker were coated with very similar amounts of active mass,the samples showed a significant variance in activity of up to 20%.The mean DeNO x values at 10ppm NH 3slip were 49.9%with a standard deviation of 7.8%at 2508C,87.3%with a standard deviation of 3.4%at 3008C,and 97.0%with a standard deviation of 0.7%at 4508C.Due to the large deviations in activity of six catalyst samples from the mean value,these catalysts were not included in the study.Thereby,the standard deviation of the remaining 26samples was significantly reduced,which were used for the deactivation experiments.N 2O formation was as expected small with values between 10and 12ppm at 4508C (0ppm at 2508C and 3008C).The significant differences in the catalytic activity of the undoped samples prevented a direct comparison of the deactivated catalysts.Therefore,a relative DeNO x was defined as:rel :DeNO x 10ppm ¼DeNO x cat x deact :DeNO x cat x new :(1)with rel :DeNO x 10ppm ¼relative DeNO x at 10ppm ammonia slip (–),DeNO x cat x deact :¼DeNO x of deactivated catalyst X at 10ppm ammonia slip (%),and DeNO x cat x new :¼DeNO x of new catalyst X at 10ppm ammonia slip (%).The relative DeNO x at a =1.2was calculated accordingly.By multiplying these relativeDeNO x values with the meanFig.1.Ammonia emissions as function of DeNO x for a typical V 2O 5/WO 3–TiO 2catalyst coated on metal support designed for SCR applications onboard of Diesel vehicles.The catalyst sample was degreened at 5508C for 5h before testing.(*)2008C,(*)2508C,(4)3008C,(~)3508C,(Â)4008C,(&)4508C.Table 3List of doping elements used in this study Element PrecursorFormula Quality Supplier Potassium (K)Potassium carbonate K 2CO 3Puriss.Fluka Potassium sulfateK 2SO 4Pro analysi Merck Potassium hydrogensulfate KHSO 4Pro analysi Merck Molybdenum (Mo)Ammonium molybdate(NH 4)6Mo 7O 24Á4H 2O Ultra (>99%)Fluka Phosphorus (P)Orthophosphoric acid (85%)H 3PO 4Pro analysiMerck Sulphur (S)Sulphur dioxide SO 21%in N 2(>99.9%)Carbagas Zinc (Zn)Zinc acetate Zn(CH 3COO)2Á2H 2O Puriss.Fluka Boron (B)Orthoboric acid H 3BO 3Puriss.Fluka Calcium (Ca)Calcium acetate Ca(CH 3COO)2Áx H 2O 96%Merck Magnesium (Mg)Magnesium acetateMg(CH 3COO)2Á4H 2OPurumFlukaO.Kro¨cher,M.Elsener /Applied Catalysis B:Environmental 75(2008)215–227218DeNO x value of all undoped samples,normalized DeNO x values were obtained,which are independent of the scattering activities of the undoped catalyst samples.The deactivation of the catalyst is best described by the decrease of the mass based reaction rate constant,which can be calculated for afirst-order reaction in a plug-flow reactor according to:k mass¼ÀVÃWlnð1ÀXÞ(2)with k mass=mass based reaction rate constant[cm3/(g s)],V*= volumeflow rate at T reactor,p reactor(cm3/s),W=catalyst mass (g),and X=NO conversion(–).All reaction rate constants k mass were calculated from DeNO x at a=1.2,which was found to be equal to the maximum attainable DeNO x for almost all catalysts,i.e.ammonia is present in excess on the catalyst surface and pseudofirst-order conditions are achieved.Relative k mass values were obtainedanalogously to rel.DeNO x10ppm .k mass is only with the intrinsic activity of the catalyst at low temperatures up to2508C.For highly active catalysts,such as used in this study,the SCR reaction increasingly takes place in the upper catalyst layer already at 3008C or higher.Thus,the deactivation of the catalysts might be partly compensated by deeper diffusion of the reactants and, thus,a better exploitation of the catalyst[24].However,the evaluation of the results showed only a moderate increase of the relative k mass for higher temperatures.The relative k mass values for250and3008C should,therefore,be representative of the intrinsic activity loss of the catalyst.3.2.SCR activity after impregnation with waterIn a preliminary investigation,it was tested,if the catalyst activity is affected by water and if water-free solvents have to be used for following investigations.Therefore,the catalysts werefirst tested as delivered and after a pre-treatment at5508C for50h.Then,the catalysts were dip-coated with water and tested again.For the dip coating,the catalysts were immersed into deionized water for2s,shortly blown out and dried in airflow at908C.The loss in catalyst mass was$19mg,which is$2%of the original mass.The blown-out solution had a deep yellow colour.Finally,the catalysts were calcined at4008C for 5h.Subsequently,a second calcination was done at5508C for 5h in order to intensify the solid-state reactions.The catalyst activity increased by the thermal pre-treatment at5508C for50h as shown in Table4.The subsequent dipping in water and the calcination at4008C had almost no influence on the catalyst with a tendency to slightly smaller activities. However,the second calcination at5508C for5h increased the activity again.In general,the differences were very low and near the measuring accuracy and reproducibility.N2O formation was generally low and only detectable at higher temperatures(Table4).Dipping in water caused a slight increase of the N2O formation,but still on a reasonable level.The second calcination at5508C increased the selectivity a little bit again,resulting in a slight decrease in N2O formation.3.3.Doping with single elementsAtfirst,catalyst modules were doped with0.4mol%of the single elements boron,phosphorus,zinc,calcium,magnesium and potassium.Boron was added as aqueous ortho boric acid (H3BO3),phosphorus as ortho phosphoric acid(H3PO4), potassium as carbonate(K2CO3)and zinc,calcium as well as magnesium as acetate(Zn(CH3COO)2,Ca(CH3COO)2, Mg(CH3COO)2).During thefirst calcination at4008C for5h, the compounds decomposed in different ways.Calcium and magnesium acetate decomposed to the corresponding carbo-nates(CaCO3,MgCO3),whereas zinc acetate was likely converted to zinc oxide(ZnO),as zinc carbonate itself decomposes already at1208C.Potassium carbonate decom-posed to potassium oxide(K2O)during calcination.Fig.2shows the DeNO x values at10ppm ammonia slip for 4508C and3008C of the samples in the undoped state,after doping followed by thefirst calcination at4008C and after the second calcination at5508C.The addition of ortho boric acid caused no deactivation for this concentration.Doping with phosphorus resulted in a moderate decrease of activity,i.e.k mass was lowered to80–85%of the original value.At4508C,this had only a small influence on DeNO x(3–4%decrease),while DeNO x was10%lower at3008C and20–25%lower below 3008C.For molybdenum,no deactivation was observed at high temperatures,but DeNO x slightly decreased at200and2508C. The addition of molybdenum increased the N2O formation by a factor of2–3to40ppm at4508C.The addition of zinc caused about50%reduction of the catalyst activity k mass.At high temperatures DeNO x at10ppm ammonia slip was reduced to 80–90%of the original value.At lower temperatures,thisTable4Influence of testing procedure on DeNO x and N2O formationT(8C)DeNO x at10ppm NH3slip(%)N2O at10ppm NH3slip(ppm)New50h at5508C Dipping in water+5h at4008C+5h at5508CNew50hat5508CDipping in water+5h at4008C+5h at5508C4509194929346119 400899696961122 350809393940010 300597677782503043424220011151516O.Kro¨cher,M.Elsener/Applied Catalysis B:Environmental75(2008)215–227219decrease was even more pronounced.By the addition of calcium,the activity of the catalyst dropped to 40%of its original value.Doping with magnesium resulted in a clear deactivation over the entire temperature range.Its deactivation potential was;however,lower than calcium.The addition of potassium caused a very strong deactivation of the catalyst to below 10%of its original value.After the first measurement,the catalysts were calcined a second time at higher temperature (5508C for 5h)in order to intensify the solid-state reactions of the dopants with the catalysts and tested again.The catalyst with boron showed again the same activity.The activities of the other catalysts increased by 10–20%,i.e.the deactivation was partly restored.However,this effect was not very pronounced and the DeNO x values only slightly changed in the range of a few percent.No increase in N 2O formation could be observed for any of the doped catalysts,except for the molybdenum-containing sample.In general,the N 2O formation was affected in parallel to the deactivation.Doping with 0.4mol%potassium resulted in an already strong deactivation,which prevented the reliable assessment of its deactivation potential (Fig.2).By lowering the potassium doping from 0.4to 0.2%clearly higher DeNO x values could be obtained for low temperatures,but at 4508C still only 15%of the original values were measured (Fig.3).For potassium content of only 0.1%,a moderate catalyst deactivation was achieved.The DeNO x values measured with this catalystranged between those for the catalyst with 0.4%Zn and 0.4%Ca.3.4.Effect of counter-ionsThe influence of counter-ions from different inorganic acids on the primary effect of the cations was tested by doping catalysts with the same potassium concentration in form of three different potassium salts (K 2CO 3=basic,K 2SO 4=neu-tral,KHSO 4=acidic).In case of calcium,direct impregnation with calcium sulphate was avoided due to its low solubility.Therefore,the catalyst was impregnated with calcium acetate and treated with 100ppm SO 2in the feed gas at 4008C for 5h,which resulted in the formation of CaSO 4on the catalyst.Moreover,calcium was tested as phosphate and borate in order to compare these compounds with calcium sulphate,which was expected to be relatively inert on the SCR catalysts.Magnesium sulphate was likewise prepared by treatment with SO 2and magnesium phosphate was also tested.Zinc was combined with phosphate and borate.The concentration of each element was 0.4mol%,except for the in-situ prepared sulphates.A simple 1:1ratio was chosen for the cations and anions since the real concentration ratios of the different components in the exhaust gas is unknown and it is not clear,how they meet on the surface and what are the exact reaction products (e.g.zincphosphateparison of catalysts deactivated by different concentrations of potassium.DeNO x at 10ppm NH 3slip (a)after calcination at 4008C for 5h and (b)after calcination at 5508C for 5h.Fig.2.Catalysts doped with single elements.DeNO x at 10ppm NH 3slip (a)after calcination at 4008C for 5h and (b)after calcination at 5508C for 5h.O.Kro¨cher,M.Elsener /Applied Catalysis B:Environmental 75(2008)215–227220Zn 3(PO 4)2or zinc pyrophosphate Zn 2P 2O 7).Moreover,thisratio allowed us to impregnate calcium as well as zinc with phosphate in one step,since the corresponding hydrogen phosphates are sufficiently soluble for the preparation of the impregnation solution.The deactivation potential of potassium is so strong,that the addition of a second component had hardly any effect in our experiments (Fig.4).DeNO x decreased to below 10%of its original value for the catalyst with potassium sulphate.The catalyst with potassium hydrogensulfate was only slightly better,but DeNO x at 10ppm ammonia slip did not exceed 20%of the value before doping.No appreciable influence was observed for phosphorus or boron.The deactivation of the calcium containing catalysts strongly depends on the acidity of the corresponding acid of the second doping element.CaSO 4showed no deactivation effect at high temperatures,but at low temperatures a significant activity loss was observed,comparable to that of calcium phosphate.The addition of orthoboric acid only slightly improved the calcium-doped catalyst.The sulphated as well as the phosphated magnesium doped samples showed a clearly higher activity than the sample without counter-ions,however,the improvement was much less pronounced than in the case of calcium.The activity of the catalyst with sulphated magnesiumis again reduced by the second calcination at 5508C,whereas for the phosphate-containing catalyst it recovers somewhat.The combination of zinc with phosphorus resulted in a distinctly lower deactivation at low and intermediate tempera-tures than doping with zinc alone.At 400and 4508C both catalysts are comparable.After the additional calcination at 5508C the Zn/P catalyst showed a slight decrease in DeNO x at 400and 4508C.The catalyst with zinc and boron was quite similar to that with zinc alone.After calcination at 4008C,somewhat smaller DeNO x values are obtained than with zinc,but after calcination at 5508C they are virtually the same.The activity of the catalysts Zn/B and Zn are clearly improved by the calcination at 5508C,whereas the catalyst Zn/P remain more or less on the same level,so that all catalysts are getting quite similar.3.5.Doping with combinations of cations and anions In addition to the doping with single cations and the doping with combinations of single cations with different counter-ions,we examined the influence of more than one cation or two anions on the catalyst performance (Fig.5).Fig.4.Effect of counter-ions on the deactivation potential.DeNO x at 10ppm NH 3slip (a)after calcination at 4008C for 5h and (b)after calcination at 5508C for 5h.Fig.5.Effect of combinations of elements.DeNO x at 10ppm NH 3slip (a)after calcination at 4008C for 5h and (b)after calcination at 5508C for 5h.The samples Ca/P/SO 4and Ca/Zn/P/SO 4were prepared by impregnation of V 2O 5/WO 3–TiO 2catalysts with mixtures of commercial lubrication oil additives in hexane.O.Kro ¨cher,M.Elsener /Applied Catalysis B:Environmental 75(2008)215–227221。

仿生界面二氧化钛微球光催化剂的制备及其性能叶长英;全学军;蒋丽;张莉;谭林燕【摘要】采用模板-超声-水热法制备仿生界面TiO2微球,并采用场发射扫描电镜(FESEM)、N2吸附仪和X射线衍射(XRD)对制备的TiO2微球样品进行表征.结果表明,在乙醇溶液中水的体积分数为10％,H2O-与Ti物质的量之比为5,Ti与表面活性剂三嵌段共聚物P123 (EO20PO7oEO20)物质的量之比为46,超声作用90 min,水热定型处理后煅烧温度为400℃的条件下,制得的TiO2光催化剂具有较高的催化活性.该光催化材料为具有多级结构的仿生界面TiO2微球,其比表面积达185 m2/g,晶相中锐钛型与金红石型比例为7∶3,样品的实际光催化效率是商业化Degussa-P25催化剂的3倍以上.%TiO2 microspheres with hierarchically biomimetic surface were prepared by template-ultrasonic hydrothermal method and were characterized by means of field emission scanning electronic microscope(FESEM), N2 adsorption and X-ray diffraction(XRD). The results showed that TiO2 microsphere prepared under the conditionsof volume fraction of water 10%, molar ratio of H2O to Ti 5, molar ratio ofTi to PI23 46, ultrasonication time 90 min and calcination temperature 400 ℃ exhibited high activity. The TiO2 microspheres had biomimetic structure,the BET specific surface area reached 185 m2/g, and the ratio of anatase to rutile phase was about 7:3. The practical photocatalytic efficiency of the catalyst was more than 3 times that of the commercial photocatalyst P25.【期刊名称】《化学反应工程与工艺》【年(卷),期】2011(027)006【总页数】6页(P515-520)【关键词】光催化剂;二氧化钛微球;仿生界面;实际光催化效率【作者】叶长英;全学军;蒋丽;张莉;谭林燕【作者单位】重庆理工大学化学化工学院,重庆400050;重庆理工大学化学化工学院,重庆400050;重庆理工大学化学化工学院,重庆400050;重庆理工大学化学化工学院,重庆400050;重庆理工大学化学化工学院,重庆400050【正文语种】中文【中图分类】O643;TQ426.6TiO2作为一种光催化材料，具有良好的理化性能和光催化活性，其制备技术是材料科学与工程领域研究热点之一[1]。

Ce-promoted Ru/SBA-15catalysts prepared by a‘‘two solvents’’impregnation method for selective hydrogenation of benzene to cyclohexeneJian-Liang Liu a,Ling-Jun Zhu a,Yan Pei a,Ji-Hua Zhuang a,*,Hui Li b,He-Xing Li b,Ming-Hua Qiao a,*, Kang-Nian Fan aa Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,Fudan University,No.220Handan Road,Shanghai200433,PR Chinab Department of Chemistry,Shanghai Normal University,Shanghai200234,PR China1.IntroductionThe selective hydrogenation of benzene to cyclohexene is ofgreat industrial interest,because cyclohexene can be facilelyconverted to value-added cyclohexanol,caprolactam,and adipicacid by typical olefin pared to traditional methodsfor the manufacture of cyclohexene by dehydrogenation ofcyclohexane,dehydration of cyclohexanol,and the Birch reduction,selective hydrogenation of benzene is superior in terms ofinexpensive starting feedstock,energy-saving,lower amounts ofundesirable products,and simplified operation[1,2].However,thermodynamically it is difficult to obtain cyclohexene frombenzene,since the standard free energy change for cyclohexeneformation from benzene hydrogenation isÀ23kJ molÀ1,while thatfor cyclohexane formation isÀ98kJ molÀ1.So,for a long period,only cyclohexane was obtained during the hydrogenation ofbenzene[3].Thefirst encouraging result was reported by Drinkard[4],whofound that in the presence of water and employing Ru as thecatalyst in a tetra-phase reactor at403–453K and7MPa of H2pressure,a yield of cyclohexene of30%was obtained,showing theprospect of the industrial application of the catalytic hydrogena-tion of benzene to cyclohexene.Up to now,methods for thecatalytic hydrogenation of benzene to cyclohexene have includedgas-phase[5,6]and liquid-phase[7–11]hydrogenations.The mainadvantage of the liquid-phase hydrogenation method is that it ispossible to achieve much higher selectivity to cyclohexene at highconversion of benzene[4,7–10].Among various metals investi-gated,Ru gives the highest selectivity to cyclohexene[4–7].AsahiChemical has industrialized the process for the selective hydro-genation of benzene to cyclohexene using a proprietary Ru catalyst[12].The performance is80%selectivity to cyclohexene at50%conversion of benzene or70%selectivity to cyclohexene at62%conversion of benzene.However,they have not presented thedetails of this process,and hence scientific studies especially oncatalyst preparation and modification are needed.It was found that the catalyst preparation method and thepromoter have great impacts on the yield of cyclohexene.Mizukami et al.[13]and Niwa et al.[14]prepared a supportedRu catalyst doped with a small amount of Cu by the sol–gelmethod,and obtained a cyclohexene yield of31%at a benzeneconversion of83%.Deng and co-workers[15–17]reported that RuBcatalysts prepared by the chemical reduction method were moreselective to cyclohexene than Ru catalysts reduced by H2,giving acyclohexene yield of33%.Liu et al.[18]performed the reactionwith a RuLaB/ZrO2catalyst,and found that the selectivity tocyclohexene could reach66%at the conversion of81%by furtheradding ZrO2as disperser and byfinely tuning the acidity of theApplied Catalysis A:General353(2009)282–287A R T I C L E I N F OArticle history:Received21July2008Received in revised form31October2008Accepted31October2008Available online12November2008Keywords:BenzeneA B S T R A C TCe-promoted Ru/SBA-15catalysts were prepared by a‘‘two solvents’’impregnation method.In liquid-phase hydrogenation of benzene,the Ce-promoted Ru/SBA-15catalysts exhibited superior catalyticperformance to the un-promoted catalyst.The optimum bulk Ce/Ru molar ratio was determined to be0.25,at which a cyclohexene yield of53.8%was obtained.Based on characterizations,the beneficialeffect of the Ce promoter on the selectivity to cyclohexene was ascribed to the enhancement of thehydrophilicity of the catalyst and to the electron transfer between the Ce(III)species and metallic Ru.ß2008Elsevier B.V.All rights reserved.*Corresponding author.Tel.:+862155664679;fax:+862165641740.E-mail address:mhqiao@(M.-H.Qiao).Contents lists available at ScienceDirectApplied Catalysis A:Generalj o u r n a l h o m e p a g e:w w w.e l s e v i e r.c o m/l o c at e/a p c a t a0926-860X/$–see front matterß2008Elsevier B.V.All rights reserved.doi:10.1016/j.apcata.2008.10.056aqueous solution of ZnSO4.Hu and Chen[19]doped the Ru/SiO2 catalyst with Zn by the co-precipitation method,and found that the yield of cyclohexene increased from17%to32%.On the other hand,Ce has been frequently used as promoter for hydrogenation catalysts.For example,Li et al.[20]prepared Ce-doped PdB catalysts for the selective hydrogenation of phenol to cyclohexanone,on which the maximum yield of cyclohexanone increased from33.1%to83.0%after the promotion with Ce.The promoting effect of the Ce dopant was ascribed to the improve-ment of the dispersion of Pd,the increment of the electron density of Pd,and the enhancement of the surface basicity.Bachiller-Baeza et al.[21]tested the un-promoted and Ce-promoted Ru/AC catalysts in gas-phase crotonaldehyde and liquid-phase citral hydrogenations,and found that CeO2alone and CeO2supported on AC showed low activity but very high selectivity to unsaturated alcohols(93%and100%,respectively).It is suggested that the defect sites on the Ce promoter were highly selective sites for the formation of unsaturated alcohols due to the positive effects on C O bond activation.Regular mesoporous molecular sieves have shown great potential in catalysis,which is closely related to their high surface area,uniform pore structure and narrow pore size distribution [22–24].These unique features of mesoporous molecular sieves offer new possibilities for obtaining highly dispersed metal catalysts with defined particle sizes.However,the traditional incipient wetness impregnation method is not good at dispersing homogeneously the active components in the mesopores;thus we are unable to fully utilize the advantages of the mesoporous structure.The‘‘two solvents’’impregnation method,a derivative of the traditional incipient wetness impregnation technique using a combination of a hydrophobic solvent and water to impregnate the porous materials,has been successfully employed by Huu et al.[25]to disperse uniform CoFe2O4nanoparticles into the pores of carbon nanotubes and by Clerc et al.[26]to synthesize MnO2 nanowires patterned by SBA-15under mild condition.In our preliminary work,we have prepared a series of Ru catalysts supported on different siliceous materials including commercial amorphous SiO2,HMS,MCM-41,SBA-15,and MCF by the‘‘two solvents’’impregnation method,and we have found that the SBA-15-supported catalyst exhibited the highest selectivity and yield in liquid-phase hydrogenation of benzene to cyclohex-ene.Motivated by these results,in this work we prepared the Ce-promoted Ru/SBA-15catalysts by the‘‘two solvents’’impregnation method for liquid-phase hydrogenation of benzene to cyclohexene. It is expected that using this method Ce and Ru could be homogeneously distributed in the channels of the mesoporous SBA-15.The physicochemical properties of the catalysts were characterized and the promoting effect of Ce was discussed.2.Experimental2.1.Catalyst preparationThe mesoporous siliceous SBA-15with hexagonal structure was prepared according to the procedure reported by Zhao et al.[22].The catalysts were prepared by the‘‘two solvents’’impregnation method ly,30ml of cyclohexane was added to1.0g of SBA-15while stirring until all SBA-15was well dispersed.Then3.0ml of0.04g Ru/ml of RuCl3aqueous solution with different amounts of Ce(NO3)3was added dropwise under stirring.After the supernatant was decanted,the solid was dried at373K overnight,then reduced by5vol.%H2in Ar at573K for4h at a ramping rate of2K minÀ1.The loading of Ru was kept at 12wt%relative to the weight of SBA-15.Measurements revealed that the concentration of Ru ions in the supernatant was below the detection limit,showing that the‘‘two solvents’’impregnation method is very efficient in transferring Ru ions from the liquid phase to the channels of SBA-15.The catalyst without Ce was denoted as Ru/SBA-15,while RuCe/SBA-15-x is used for the Ce-promoted catalyst,where x denotes the nominal molar ratio between Ce and Ru.2.2.Catalyst characterizationThe bulk compositions of the catalysts were analyzed by inductively coupled plasma-atomic emission spectroscopy(ICP-AES;Thermo Elemental IRIS Intrepid).N2physisorption was performed on a Micromeritics TriStar3000apparatus at77K.The sample was transferred to the adsorption glass tube and heated at 383K under N2for2h before measurement.The pore size distribution was calculated from the desorption branch of the isotherm by the Barrett–Joyner–Halenda(BJH)method.Temperature programmed reduction(TPR)was carried out on a home-made apparatus.After being heated at473K for1h under Ar,the sample was cooled down,and then raised to$700K at a heating rate of10K minÀ1in5%H2in Ar.The active surface area (S Ru)was determined by temperature-programmed desorption of H2(H2-TPD).After being heated at473K for1h under Ar,the catalyst was cooled to273K.Then H2was introduced by the pulse method at273K until saturation,followed by purging with Ar to remove the gaseous and/or physisorbed H2until that the TCD signal returned to the baseline.The H2desorption curve was acquired at a heating rate of20K minÀ1.The active surface area was calculated based on the desorption area with the assumption of a H:Ru stoichiometry of1:1and a Ru surface atomic density of 1.63Â1019atoms mÀ2[27,28].Powder X-ray diffraction(XRD)patterns were acquired on a Bruker AXS D8Advance X-ray diffractometer using Ni-filtered Cu K a radiation(l=0.15418nm).The tube voltage was40kV,and the current was40mA.The surface morphology and particle size were observed by transmission electron microscopy(TEM;JEOL JEM2011)operating at200kV.X-ray photoelectron spectroscopy (XPS)was performed on a Perkin Elmer PHI5000C instrument with Mg K a radiation as the excitation source(h n=1253.6eV).Spectra were recorded after Ar+sputtering for15min.Because the Ru3d3/2 peak partly overlaps with the C1s line of contaminant carbon,all binding energy(BE)values were referenced to the Si2p peak of SBA-15at103.3eV.2.3.Activity testThe selective hydrogenation of benzene was carried out in a 500ml stainless steel autoclave with a mechanical stirrer.After the desired amount of ZnSO4was dissolved in100ml of distilled water,1.0g of the reduced catalyst and50ml of benzene were introduced.Then the autoclave was sealed and purged with H2 at leastfive times to expel air.The reaction conditions were: reaction temperature of413K,H2pressure of 4.0MPa,and stirring rate of1000rpm to exclude the diffusion effect.The reaction process was monitored by taking a small amount of the reaction mixture at intervals,followed by gas chromatographic analysis with a PEG-20M packed column and a thermal conductivity detector(TCD).3.Results and discussion3.1.Structural and electronic propertiesN2physisorption isotherms(Fig.1)were recorded to assess the textural properties of the as-prepared catalysts.The isotherms of the Ru/SBA-15and RuCe/SBA-15-0.4catalysts are of type IV and are almost identical to that of SBA-15[22],but the specific surfaceJ.-L.Liu et al./Applied Catalysis A:General353(2009)282–287283area and pore volume were decreased due to the incorporation of Ru and Ce to the channels,while the average pore diameter was slightly increased from 6.4to 7.4nm.As seen in the inset of Fig.1,the BJH pore size distributions of the SBA-15,Ru/SBA-15,and RuCe/SBA-15-0.4samples were similar,signifying the preservation of the regular pore structure.Fig.2shows the XRD patterns of the SBA-15,Ru/SBA-15,and RuCe/SBA-15samples.The small-angle XRD patterns (Fig.2a)exhibited three well-resolved reflections in the 2u range of 0.5–3.08,which can be indexed to the ordered hexagonal lattice (p6mm)of SBA-15[22].The hexagonal structure was present after the incorporation of Ru and Ce,although the intensity was attenuated.In the wide-angle XRD patterns (Fig.2b),besides the broad feature at 2u of ca.228ascribable to amorphous silica,there was only a broad diffraction peak at $43.08attributable to the (101)diffraction of hcp Ru [29].From the X-ray line-broadening analysis,the Ru crystallite size of ca.2nm was estimated.Fig.3shows the TEM images of the Ru/SBA-15catalyst and the RuCe/SBA-15-0.4catalyst as a representative of the Ce-promoted catalysts.Fig.3a and c revealed that the Ru nanoparticles imaged as dark dots were uniformly dispersed in the mesopores of SBA-15,with the Ru particle sizes mainly ranging from 4to 5nm.Fig.3b shows the HRTEM image of Ru nanoparticles with a diameter of around 5nm taken from the Ru/ttice fringes with an average spacing of 0.21nm were identified,corresponding well to the (101)plane of hcp Ru [29].But for the RuCe/SBA-15-0.4catalyst,the lattice fringes of Ru were blurred (Fig.3d),possibly due to the coverage of the Ru nanoparticles by the Ce bined with the Ru crystallite size derived from XRD,it can be deduced that the Ru nanoparticles observed by TEM were mainly polycrystalline.The catalysts before reduction were characterized by H 2-TPR to investigate the interactions between Ru and Ce (Fig.4).The appearance of two reduction peaks for the Ru/SBA-15catalyst is in good agreement with the literature descriptions [30–32].The first one,in the range of 320–410K,is attributed to the reduction of RuCl 3.The second peak,in the range of 410–515K,is related to the reduction of ruthenium oxide arising from the hydrolysis of RuCl 3during the impregnation or the drying process.After being doped with Ce,both reduction peaks shifted continuously to higher temperatures below the nominal Ce/Ru ratio of 0.6,indicating the stronger interaction between Ce and Ru.However,when further increasing the nominal Ce/Ru ratio,both reduction peaks shifted backwards,implying that the interaction between Ce and Ru was weakened at higher Ce/Ru ratio.Fig.5displays the Ru 3d and Ce 3d spectra of the Ru/SBA-15and RuCe/SBA-15catalysts.It is found that Ru was in the metallic state with the Ru 3d 5/2BE of 280.0eV [32].The complex spectrum of Ce 3d can be resolved into four components,with the assignments being labeled in the figure.The band labeled v 0at 880.5eV is ascribed to the Ce 3d 5/2level,and the band labeled u 0at 899.2eV represents the Ce 3d 3/2level.The band labeled v 0at 885.1eV is the satellite arising from the Ce 3d 5/2level,while the band u 0at 903.8eV is the satellite arising from the Ce 3d 3/2level [33,34].If one compares the XPS spectrum of pure cerium(III)nitrate hexahydrate and the literature values [35–37],one can conclude that the surface Ce species in RuCe/SBA-15catalysts was present in the three-valence state.Moreover,the Ce 3d BEs of the Ce(III)species were 0.3eV higher than the standard values (880.2,884.8,898.9,and 903.5eV for v 0,v 0,u 0,and u 0,respectively)[36],inferring the electronic interaction between Ru and the Ce(III)species,in which the latter donated electrons to the former.It is noted that only Ru ions were reduced under the present reduction condition,as no metallic Ce was identified in (Fig.5).The surface compositions were determined by analysis of relative peak areas in the XPS spectra.As listed in Table 1,together with the bulk compositions,the Ce-promoted catalysts showed a significant surface segregation of Ce.However,when the nominal Ce/Ru ratio was increased to 0.6,the surface Ce/Ru ratio became lower than the bulk value,which can be interpreted as the aggregation of the Ce(III)species to larger particles at higher doping amounts,thus reducing the intensity of the Ce 3d spectrum.The aggregation of the Ce(III)species is expected to weakenitsFig.1.Nitrogen adsorption–desorption isotherms and pore size distributions (inset)of the SBA-15,Ru/SBA-15,and RuCe/SBA-15-0.4samples.Fig.2.(a)Small-and (b)wide-angle XRD patterns of the SBA-15,Ru/SBA-15,RuCe/SBA-15-0.2,RuCe/SBA-15-0.4,and RuCe/SBA-15-0.6samples.J.-L.Liu et al./Applied Catalysis A:General 353(2009)282–287284interaction with Ru,which rationalizes the lower reduction temperatures of the RuCe/SBA-15-0.6catalyst shown in (Fig.4).3.2.Catalytic performanceFig.6shows the courses of the hydrogenation of benzene over the as-prepared catalysts at 413K and H 2pressure of 4.0MPa in the presence of 0.14M ZnSO 4.The products obtained under the present reaction condition were exclusively cyclohexane and cyclohexene.In Fig.6,both the conversion of benzene and the concentration of cyclohexane increased monotonically with thereaction time.On the Ru/SBA-15catalyst,the reaction rate was so fast that within only 15min the benzene was totally consumed.However,the maximum yield of cyclohexene was only 25.0%at 65.5%conversion of benzene.On the RuCe/SBA-15-0.4catalyst,in contrast,the concentration of cyclohexene increased much faster than that of cyclohexane at the beginning of the reaction,and reached a maximum yield of cyclohexene of 44.8%at 80.0%conversion of benzene with a reaction time of about 20min.The yield of cyclohexene then declined gradually following the known behavior of such consecutive reactions.As can be seen from Table 1,the higher the Ce/Ru ratio,the lower is the active surface area.From the XPS analysis,the Ce(III)species was enriched on the surface of the RuCe/SBA-15catalysts.Those facts indicate the physical blocking of the Ru active sites by the Ce promoter,which decreased the number of the exposed Ru atoms and consequently lowered the hydrogenation activity of the catalysts.Table 2also shows that the maximum yield of cyclohexene increased with the increment of the Ce/Ru ratio,and then declined at higher doping amounts of Ce,with the optimal bulk Ce/Ru molar ratio being 0.25.The mechanism of selective hydrogenation of benzene to cyclohexene over Ru-based catalysts has been studied;results demonstrated that the existence of a layer of water over the catalyst surface was one of the most important factors responsible for the high selectivity towards cyclohexene [38,39].The roles of water played in selective hydrogenation of benzene can be understood based on the following considerations:(1)Water readily occupies active sites where cyclohexane ispreferentially formed [38];(2)water promotes the desorption of cyclohexene from thecatalyst surface by competitive adsorption,and inhibitstheFig.3.TEM and HRTEM images of the Ru/SBA-15(a)and (b),and RuCe/SBA-15-0.4(c)and (d)catalysts.Fig.4.TPR profiles of the Ru/SBA-15,RuCe/SBA-15-0.2,RuCe/SBA-15-0.4,and RuCe/SBA-15-0.6catalysts.J.-L.Liu et al./Applied Catalysis A:General 353(2009)282–287285readsorption of cyclohexene for further hydrogenation to cyclohexane [39].It is expected that the existence of the Ce(III)species on the Ru crystallites in the RuCe/SBA-15catalysts,as inferred by the HRTEM image,could enhance the hydrophilicity of the catalysts,which is beneficial to the selectivity to cyclohexene.Hronec et al.[40]prepared charcoal and anionic cross-linked polymer-supported Ru catalysts for liquid-phase hydrogenation of benzene.The selectiv-ity to cyclohexene was much higher for Ru catalysts supported on a strongly hydrophilic microporous resin than that for those supported on charcoal.Ronchin and Toniolo [41]conducted the reaction on Ru catalysts supported on different oxides (TiO 2,ZrO 2,Yb 2O 3,and Fe 2O 3),and found that the hydrophilicity of the support was related to the selectivity to cyclohexene.In fact,if the organic phase (benzene)surrounds the catalyst,the consecutive hydro-genation of cyclohexene is so fast that the selectivity to cyclohexene is nearly zero [39].Our experiment showed that,for the RuCe/SBA-15-0.4catalyst,only a small amount of cyclohexene (selectivity to cyclohexene less than 7%)was detected during the hydrogenation of benzene under the present condition when absolute ethanol instead of water was used as the solvent,regardless of the addition of ZnSO 4.In addition,according to the XPS spectra in Fig.5,electron transfer occurred between the Ce(III)species and Ru for RuCe/SBA-15catalysts,making the Ce(III)species electron-deficient.Since the electron-deficient Ce(III)species can more easily accept the lone electron pair on the oxygen atom of water,itFig.5.XPS spectra of the Ru 3d and Ce 3d levels of the Ru/SBA-15,RuCe/SBA-15-0.2,RuCe/SBA-15-0.4,and RuCe/SBA-15-0.6catalysts.Table 2Results of the hydrogenation of benzene over the Ru/SBA-15and RuCe/SBA-15catalysts a .CatalystCon.b (%)Sel.b (%)Yield b (%)Time b (min)Ru/SBA-1565.538.125.05RuCe/SBA-15-0.278.150.039.115RuCe/SBA-15-0.480.056.444.820RuCe/SBA-15-0.675.257.742.930aReaction conditions: 1.0g of catalyst,50ml of benzene,100ml of H 2O,temperature of 413K,H 2pressure of 4.0MPa,stirring rate of 1000rpm,and C znso 4of 0.14M.bValues recorded at the maximum yield ofcyclohexene.Fig.6.The courses of benzene hydrogenation over the Ru/SBA-15and RuCe/SBA-15-0.4catalysts.Reaction conditions:1.0g of catalyst,50ml of benzene,100ml of H 2O,temperature of 413K,H 2pressure of 4.0MPa,stirring rate of 1000rpm,and 0.14M of ZnSO 4.Table 1Physiochemical properties of SBA-15,Ru/SBA-15,and RuCe/SBA-15samples.SampleCe/Ru ratio(bulk a /surface b )S Ru(m 2g À1Ru)S BET(m 2g À1)V p(cm 3g À1)d p (nm)SBA-15––5330.96 6.4Ru/SBA-15–794080.757.1RuCe/SBA-15-0.20.13/0.45263710.737.2RuCe/SBA-15-0.40.25/0.38163600.707.4RuCe/SBA-15-0.60.35/0.29113200.687.6a Determined by ICP-AES.bDetermined by XPS.J.-L.Liu et al./Applied Catalysis A:General 353(2009)282–287286would promote the adsorption of water on the catalyst surface.Moreover,the electron-enriched Ru is not favorable for the overlap of the p -electrons of cyclohexene with the empty d-orbitals of Ru,thus promoting the desorption of cyclohexene [38].For the RuCe/SBA-15-0.4catalyst exhibiting the best perfor-mance in selective hydrogenation of benzene,we further explored the effect of the concentration of the ZnSO 4additive on the selectivity to cyclohexene.As seen from Fig.7,with the increase of the concentration of ZnSO 4,the yield of cyclohexene increased greatly,with a maximum yield of 53.8%being obtained at the ZnSO 4concentration of 0.42M.As far as we are aware,this value represents the highest yield of cyclohexene in selective hydrogenation of benzene over heterogeneous Ru catalysts reported in the open literature [18].The positive effect of the zinc salts,according to Fukuoka et al.[42],can be interpreted partly by the stabilization of hydrogenated inter-mediates by forming adducts between cyclohexene and Zn cations,and partly by the hindering of the readsorption of cyclohexene,which both suppress the rapid consecutive hydrogenation of cyclohexene.4.ConclusionsThe RuCe/SBA-15catalysts prepared by the ‘‘two solvents’’impregnation method exhibited better selectivity towards cyclo-hexene than the Ru/SBA-15catalyst in liquid-phase hydrogenation of benzene.The existence of the Ce(III)species decreased the number of exposed Ru atoms,increased the number of electrons on metallic Ru,and enhanced the hydrophilicity of the catalyst.As a result,these improvements generated a high selectivity towards cyclohexene.After the concentration of ZnSO 4was optimized,the maximum yield of cyclohexene of 53.8%was obtained on the RuCe/SBA-15-0.4catalyst.AcknowledgementsThis work was supported by the National Basic Research Program of China (2006CB202502),Shanghai Science and Tech-nology Committee (06JC14009),the Fok Ying Tong Education Foundation (104022),the NSF of China (20673025),and State Key Laboratory of Catalytic Material and Reaction Engineering (RIPP,SINOPEC).References[1]E.Dietzsch,U.Rymsa,D.Honicke,Chem.Eng.Technol.22(1999)130–133.[2]S.C.Hu,Y.W.Chen,J.Chin.Inst.Chem.Eng.29(1996)387–396.[3]P.T.Suryawanshi,V.V.Mahajani,J.Chem.Technol.Biotechnol.69(1997)154–160.[4]W.L.Drinkard,NL Patent 7,205,832(1972)to Dupont de Nemours.[5]E.Dietzsch,P.Claus,D.Honicke,Top.Catal.10(2000)99–106.[6]J.Patzlaff,J.Gaube,Chem.Eng.Technol.21(1998)651–654.[7]L.Fang,L.Jing,X.Jie,Prog.Chem.15(2003)338–343.[8]H.Imamura,T.Nuruyu,T.Kawasaki,T.Teranishi,Y.Sakata,Catal.Lett.96(2004)185–187.[9]O.D.Quintero,S.Martinez,Y.Henriquez,L.Ornelas,H.Krentzien,J.Osuna,J.Mol.Catal.A:Chem.197(2003)185–191.[10]V.Mazzieri,N.Figoli,F.C.Pascual,P.Argentiere,Catal.Lett.102(2005)79–82.[11]E.T.Silveira,A.P.Umpierre,L.M.Rossi,G.Machado,J.Morais,G.V.Soares,I.J.R.Baumvol,S.R.Teixeira,P.F.P.Fichtner,J.Dupont,Chem.Eur.J.10(2004)3734–3740.[12]H.Nagahara,M.Ono,M.Konishi,Y.Fukuoka,Appl.Catal.121/122(1997)448–451.[13]F.Mizukami,S.Niwa,S.Ohkawa,A.Katayama,Stud.Surf.Sci.Catal.78(1993)337–344.[14]S.Niwa,F.Mizukami,S.Isoyama,T.Tsuchiya,K.Shimizu,S.Imai,J.Imamura,J.Chem.Technol.Biotechnol.36(1986)236–246.[15]S.H.Xie,M.H.Qiao,H.X.Li,W.J.Wang,J.F.Deng,Appl.Catal.A:Gen.176(1999)129–134.[16]Z.Liu,W.L.Dai,B.Liu,J.F.Deng,J.Catal.187(1999)253–256.[17]Z.Liu,S.H.Xie,B.Liu,J.F.Deng,New J.Chem.23(1999)1057–1058.[18]S.C.Liu,Z.Liu,Z.Wang,S.H.Zhao,Y.M.Wu,Appl.Catal.A:Gen.313(2006)49–57.[19]S.C.Hu,Y.W.Chen,Ind.Eng.Chem.Res.40(2001)6099–6104.[20]H.Li,J.L.Liu,H.X.Li,Mater.Lett.62(2008)297–300.[21]B.Bachiller-Baeza,I.Rodriguez-Ramos,A.Guerrero-Ruiz,Appl.Catal.A:Gen.205(2001)227–237.[22]D.Y.Zhao,J.L.Feng,Q.S.Huo,N.Melosh,G.H.Fredrickson,B.F.Chmelka,G.D.Stucky,Science 279(1998)548–552.[23]Y.M.Wang,Z.Y.Wu,L.Y.Shi,J.H.Zhu,Adv.Mater.17(2005)323–327.[24]Z.Konya,V.F.Puntes,I.Kiricsi,J.Zhu,A.P.Alivisatos,G.A.Somorjai,Nano Lett.2(2002)907–910.[25]C.P.Huu,N.Keller,C.Estourne`s,G.Ehret,M.J.Ledoux,mun.(2002)1882–1883.[26]M.I.Clerc,D.Bazin,M.D.Appay,P.Beaunier,A.Davidson,Chem.Mater.16(2004)1813–1821.[27]X.F.Shen,L.J.Garces,Y.S.Ding,ubernds,R.P.Zerger,M.Aindow,h,S.L.Suib,Appl.Catal.A:Gen.335(2008)187–195.[28]I.Rossetti,N.Pernicone,L.Forni,Appl.Catal.A:Gen.248(2003)97–103.[29]F.B.Su,F.Y.Lee,L.Lv,J.Liu,X.N.Tian,X.S.Zhao,Adv.Funct.Mater.17(2007)1926–1931.[30]P.G.J.Koopman,A.P.G.Kieboom,H.van Bekkum,J.Catal.69(1981)172–179.[31]D.Gonzalez,E.Lima,N.Martin,Ind.Eng.Chem.Res.46(2007)4335–4340.[32]V.Mazzieria,F.C.Pascualb,A.Arcoya,P.C.L.Argentiere,N.S.Figoli,Appl.Surf.Sci.210(2003)222–230.[33]B.M.Reddy,A.Khan,Y.Yamada,T.Kobayashi,S.Loridant,J.C.Volta,J.Phys.Chem.B 106(2002)10964–10972.[34]A.E.Nelson,K.H.Schulz,Appl.Surf.Sci.210(2003)206–221.[35]L.S.Kasten,J.T.Grant,N.Grebasch,N.Voevodin,F.E.Arnold,M.S.Donley,Surf.Coat.Technol.140(2001)11–15.[36]K.D.Schierbaum,Surf.Sci.399(1998)29–38.[37]J.Z.Shyu,K.Otto,W.L.H.Watkins,G.W.Graham,R.K.Belitz,H.S.Gandhi,J.Catal.114(1988)23–33.[38]J.Struijk,J.J.F.Scholten,Appl.Catal.A:Gen.82(1992)277–287.[39]J.Struijk,M.Angremond,W.J.M.Regt,J.J.F.Scholten,Appl.Catal.A:Gen.83(1992)263–295.[40]M.Hronec,Z.Cvengrosova,M.Kralik,G.Palma,B.Corain,J.Mol.Catal.A:Chem.105(1996)25–30.[41]L.Ronchin,L.Toniolo,React.Kinet.Catal.Lett.78(2003)281–289.[42]Y.Fukuoka,M.Kono,H.Nagahara,M.Ono,J.Chem.Soc.Jpn.11(1990)1223–1226.Fig.7.The conversion of benzene and the yield of cyclohexene over the RuCe/SBA-15-0.4catalyst against the concentration of ZnSO 4.Reaction conditions:1.0g of catalyst,50ml of benzene,100ml of H 2O,temperature of 413K,H 2pressure of 4.0MPa,and stirring rate of 1000rpm.J.-L.Liu et al./Applied Catalysis A:General 353(2009)282–287287。

Applied Catalysis B:Environmental 198(2016)162–170Contents lists available at ScienceDirectApplied Catalysis B:Environmentalj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /a p c a tbCeria supported rhodium nanoparticles:Superb catalytic activity in hydrogen generation from the hydrolysis of ammonia boraneSerdar Akbayrak a ,Yalc ¸ın Tonbul a ,b ,Saim Özkar a ,∗a Department of Chemistry,Middle East Technical University,06800Ankara,Turkey bZiya Gökalp Faculty of Education,Dicle University,21280Diyarbakır,Turkeya r t i c l ei n f oArticle history:Received 12April 2016Received in revised form 21May 2016Accepted 24May 2016Available online 24May 2016Keywords:CeriaRhodium nanoparticles Ammonia borane Hydrogen generation Catalytic hydrolysisa b s t r a c tWe investigated the effect of various oxide supports on the catalytic activity of rhodium nanoparticles in hydrogen generation from the hydrolysis of ammonia borane.Among the oxide supports (CeO 2,SiO 2,Al 2O 3,TiO 2,ZrO 2,HfO 2)ceria provides the highest catalytic activity for the rhodium(0)nanoparticles in the hydrolysis of ammonia borane.Rhodium(0)nanoparticles supported on nanoceria (Rh 0/CeO 2)were prepared by the impregnation of rhodium(III)ions on the surface of ceria followed by their reduction with sodium borohydride in aqueous solution at room temperature.They were isolated from the reaction solution by centrifugation and characterized by a combination of advanced analytical techniques.The catalytic activity of Rh 0/CeO 2samples with various rhodium loading in the range of 0.1–4.0%wt.Rh was also tested in hydrogen generation from the hydrolysis of ammonia borane at room temperature.The highest catalytic activity was achieved by using 0.1%wt.rhodium loaded nanoceria.The resulting Rh 0/CeO 2with a metal loading of 0.1%wt.Rh show superb catalytic activity in hydrogen generation from the hydrolysis of ammonia borane with a record turnover frequency value (TOF)of 2010min −1at 25.0±0.1◦C.The superb catalytic activity of Rh 0/CeO 2is ascribed to the reducible nature of ceria.The reduction of cerium(IV)to cerium(III)leads to a build-up of negative charge on the oxide surface which favors the bonding of rhodium(0)nanoparticles on the surface and,thus,their catalytic activity.Rh 0/CeO 2are also reusable catalysts preserving 67%of their initial catalytic activity even after the fifth use in hydrogen generation from the hydrolysis of ammonia borane at room temperature (TOF =1350min −1.The work reported here also includes the kinetic studies depending on the temperature to determine the activation energy (E a =43±2kJ/mol)and the effect of catalyst concentration on the rate of hydrolysis of ammonia borane.©2016Elsevier B.V.All rights reserved.1.IntroductionAmmonia borane (NH 3BH 3,AB)is one of the most promising solid hydrogen storage materials for on-board hydrogen appli-cations due to its high hydrogen storage capacity (19.6%wt.),non-toxicity,and high stability under ambient conditions [1–6].Ammonia borane can release 3equivalent H 2upon hydrolysis in the presence of suitable catalysts even at ambient temperature according to Eq.(1).H 3NBH 3(aq )+2H 2O(l )catalyst→NH +4(aq )+BO −2(aq )+3H 2(g )Although a large variety of catalysts including noble [7–10]and non-noble [11–14]metal nanoparticles have been tested in∗Corresponding author.E-mail address:sozkar@.tr (S.Özkar).hydrogen generation from the hydrolysis of ammonia borane,the development of efficient and stable catalysts is still an impor-tant challenge in using ammonia borane as solid hydrogen storage materials for the fuel cell applications under moderate conditions [15].So far rhodium(0)nanoparticles supported on carbon nano-tubes have been reported to be the highest activity catalyst with a turnover frequency of 706min −1in hydrogen generation from the hydrolysis of ammonia borane at room temperature [16].Therefore it is quite plausible to give effort for further improving the catalytic activity of rhodium nanoparticles in this industrially important reaction.The catalytic performance of metal nanoparticles depends on the particle size and dispersion of the active sites while the reusability and catalytic lifetime of nanoparticles are affected by their stability against agglomeration [17].Stable metal nanoparti-cles catalysts can be obtained by using suitable stabilizer ligands or supporting materials with large surface area [18–20].Recent stud-ies have shown that metal nanoparticles supported on reducible/10.1016/j.apcatb.2016.05.0610926-3373/©2016Elsevier B.V.All rights reserved.S.Akbayrak et al./Applied Catalysis B:Environmental198(2016)162–170163parison of TOF(turnover frequency in mol H2/(mol Rh×min))values of rhodium nanoparticles supported on different oxides at(a)high and(b)low rhodium loadings of catalysts used in hydrogen generation from the hydrolysis of ammonia borane(10mL,100mM)at25.0±0.1◦C.For all the tests Rh/AB molar ratio of0.0008wasused and no correction has been made for the initial TOF values by the fraction of catalytically active surfacesites.Fig.2.Powder XRD patterns of(a)CeO2,(b)Rh0/CeO2with a0.1%wt.Rh loading, (c)Rh00/CeO2with a3.42%wt.Rh loading.oxides such as ceria(CeO2)provide high catalytic activity in many reactions[21].Cerium oxides have cerium(III)defects which can readily be formed because of the favorable large positive standard reduction potential of Ce4+→Ce3+(1.76V in acidic solution[22]). It is conceivable that the interconversion of two oxidation states cerium(IV)and cerium(III)can occur under the catalytic reaction conditions,that is,ceria can undergo redox cycling in aqueous solu-tion[23].The formation of cerium(III)causes an excess negative charge to build up on the oxide surface which enhances the coordi-nation of metal(0)nanoparticles to the oxide surface and,thus,the catalytic activity through a more favorable substrate-metal inter-action[24].Therefore,ceria has been used to improve the catalytic performance of transition metals through strong metal–support interaction,in particular,of the electron rich late-transition metal nanoparticles[25–27].Although the mechanism of the promoting effect of cerium oxides has not been well understood yet,ceria has found broad applications in thefield of catalysis such as in water splitting reactions[28,29],water–gas shift reactions[30,31], hydrogen generation from ammonia borane[32],decomposition of hydrazine[33],methanol synthesis from carbon dioxide[34], removal of nitrogen oxides from exhaust gases[35,36],and formic acid oxidation[37].Herein we report the preparation,characterization,and catalytic use of rhodium(0)nanoparticles supported on nanoceria,Rh0/CeO2. For comparison,nanopowders of silica(SiO2),alumina(Al2O3),tita-nia(TiO2),zirconia(ZrO2),and hafnia(HfO2)were also employed as support for the rhodium(0)nanoparticles catalyst in hydrolysis of ammonia borane under the same conditions.The comparative study shows that Rh0/CeO2has superior catalytic activity with a turnover frequency of TOF=2010min−1in hydrogen generation from the hydrolysis of ammonia borane at25.0±0.1◦C.Our report also shows that the nanoceria supported rhodium(0)nanoparticles are reusable catalyst providing a TOF value of1350min−1after the fifth run of hydrogen generation from the complete hydrolysis of ammonia borane at25.0±0.1◦C.2.Experimental2.1.MaterialsRhodium(III)chloride hydrate(RhCl3·3H2O),ammonia borane (AB,97%),nanoceria(CeO2,particle size≈25nm),nanotita-nia(TiO2,particle size≈25nm),nanozirconia(ZrO2,particle size≈100nm),hafnia(HfO2,particle size≈100nm),nanoalu-mina(Al2O3,particle size≈13nm),and nanosilica(SiO2,particle size≈12nm)were purchased from Aldrich.Deionized water was distilled by water purification system(Milli-QSystem).All glass-ware and Teflon-coated magnetic stir bars were cleaned with acetone,followed by copious rinsing with distilled water before drying in an oven at150◦C.2.2.CharacterizationThe rhodium content of Rh0/CeO2samples was determined by the Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES,Leeman-Direct Reading Echelle)after each sample was completely dissolved in the mixture of HNO3/HCl(1/3ratio). Transmission electron microscopy(TEM)was performed on a JEM-2100F(JEOL)microscope operating at200kV.Samples were examined at magnification between400K and700K.Scanning electron microscope(SEM)images were taken using a JEOL JSM-5310LVat15kV and33Pa in a low-vacuum mode without metal coating on aluminum support.The X-ray photoelectron spec-troscopy(XPS)analysis was performed on a Physical Electronics 5800spectrometer equipped with a hemispherical analyzer and using monochromatic Al K␣radiation of1486.6eV,the X-ray tube working at15kV,350W and pass energy of23.5keV.11B NMR spec-tra were recorded on a Bruker Avance DPX400with an operating frequency of128.15MHz for11B.164S.Akbayrak et al./Applied Catalysis B:Environmental 198(2016)162–170Fig.3.(a)SEM image and (b)SEM-EDS spectrum of Rh 0/CeO 2with a 0.1%wt.Rhloading.Fig.4.TEM images of Rh 0/CeO 2with a 0.1%wt.Rh loading at different magnifications (a–d)and the corresponding histogram showing the particle size distribution (e).2.3.Preparation of rhodium(0)nanoparticles supported on ceriaCeria (500mg)was added to a solution of RhCl 3·3H 2O (in the amount required for the desired rhodium loading)in 100mL H 2O in a 250mL round bottom flask.This slurry was stirred at room temperature for 4h and,then,10mL of 5mM NaBH 4solution was added dropwise.After 30min stirring Rh 0/CeO 2were formed which were isolated by centrifugation and washed with 100mL ofwater.The remnant was dried under vacuum (10−3torr)at 60◦C for 12h.For comparison,Rh 0/SiO 2,Rh 0/Al 2O 3,Rh 0/TiO 2,Rh 0/ZrO 2,and Rh 0/HfO 2were prepared by following the same procedure as described above,by using silica,alumina,titania,zirconia,or hafnia,respectively,instead of ceria.S.Akbayrak et al./Applied Catalysis B:Environmental 198(2016)162–170165Fig.5.X-ray photoelectron spectrum of Rh 0/CeO 2with a 0.1%wt.Rh loading.The inset shows the high resolution scan and deconvolution of Rh 3dbands.Fig.6.The turnover frequency (TOF)values in hydrolysis of AB (100mM)starting with Rh 0/CeO 2(0.08mM Rh)at different rhodium loading at 25.0±0.1◦C.2.4.Catalytic hydrolysis of AB using rhodium(0)nanoparticles supported on ceriaBefore starting the hydrolysis of AB,a jacketed reaction flask (20mL)containing a Teflon-coated stir bar was placed on a mag-netic stirrer (Heidolph MR-301)and thermostated to 25.0±0.1◦C by circulating water through its jacket from a constant tempera-ture bath.Then,a graduated glass tube (60cm in height and 3.0cm in diameter)filled with water was connected to the reaction flask to measure the volume of the hydrogen gas to be evolved from the reaction.Next,20mg powder of Rh 0/CeO 2was dispersed in 10mL distilled water in the reaction flask thermostated at 25.0±0.1◦C.Then,31.8mg AB (1.0mmol H 3N ·BH 3)was added into the flask and the reaction medium was stirred at 1200rpm.The volume of hydrogen gas evolved was measured by recording the displace-ment of water level every 1min at constant atmospheric pressure of 693Torr.The catalytic activity of Rh 0/SiO 2,Rh 0/Al 2O 3,Rh 0/ZrO 2,Rh 0/TiO 2,and Rh 0/HfO 2was also tested by following the same pro-cedure as described above under the same conditions at the same rhodium concentration.2.5.Determination of the most active rhodium loading for Rh 0/CeO 2in the hydrolysis of ABThe catalytic activity of Rh 0/CeO 2samples with various rhodium loadings in the range of 0.1–4.0%wt.was tested in hydrogen genera-tion from the hydrolysis of ammonia borane starting with 0.08mM Rh and 100mM AB in 10mL solution at 25.0±0.1◦C.The highest catalytic activity was achieved by using 0.1%wt.rhodium loaded ceria.For all the tests reported hereafter,rhodium loading of 0.1%wt.was used unless otherwise stated.2.6.Determination of activation energy for the hydrolysis of AB catalyzed by Rh 0/CeO 2In a typical experiment,the hydrolysis reaction was performed starting with 10mL of 100mM (31.8mg)AB solution and 20mg Rh 0/CeO 2(0.1%wt.Rh),[Rh]=0.02mM)at various temperatures (20,25,30,35◦C)in order to obtain the activation energy.The rate constant for the hydrogen generation reaction was calculated from the slope of the linear part of each hydrogen evolution versus time plot at various temperatures.Activation energy for the hydrolysis of ammonia borane catalyzed by Rh 0/CeO 2was obtained from the slope of Arrhenius plot.2.7.Reusability of Rh 00/CeO 2in the hydrolysis of ABAfter the complete hydrolysis of AB started with 10mL of 100mM AB (31.8mg H 3NBH 3),and 80mg Rh 0/CeO 2(0.1%wt.Rh,[Rh]=0.08mM)at 25.0±0.1◦C,the catalyst was isolated as dark grey powder by centrifugation and dried under vacuum (10−3Torr)at 60◦C after washing with 50mL of water.The isolated samples of Rh 0/CeO 2were weighed (Table 3)and redispersed in 10mL solution of 100mM AB for a subsequent run of hydrolysis at 25.0±0.1◦C.2.8.Durability of Rh 00/CeO 2catalyst in the hydrolysis of ABRecyclability test was started with 10mL of 100mM AB (31.8mg H 3NBH 3),and 80mg Rh 0/CeO 2(0.1%wt.Rh,[Rh]=0.08mM)at 25.0±0.1◦C.When the AB present in the solution was completely hydrolyzed,1mmol AB was added for another run of hydrolysis.3.Results and discussionA series of survey experiments were performed to find out the best support for the rhodium(0)nanoparticles which would pro-vide the highest catalytic activity in hydrogen generation from the hydrolysis of ammonia borane at room temperature.First,rhodium(III)ions were impregnated from the aqueous solution of rhodium(III)chloride on one of the following supports with the average particle size given in parentheses:CeO 2(25nm),SiO 2(12nm),Al 2O 3(13nm),TiO 2(25nm),ZrO 2(100nm),and HfO 2(100nm).Then,the impregnated rhodium(III)ions were reduced by sodium borohydride at room temperature yield-ing Rh 00/CeO 2,Rh 00/SiO 2,Rh 00/Al 2O 3,Rh 00/TiO 2,Rh 00/ZrO 2,and Rh 00/HfO 2,respectively.The metal content of the isolated powders was determined by ICP and given in Table 1.The supported rhodium(0)nanoparticles were tested for their catalytic activity in hydrogen generation from the hydrolysis of ammonia borane.The catalytic hydrolysis of ammonia borane was followed by monitoring the change in H 2volume,which was then converted into the equivalent H 2per mole of AB using the known 3:1H 2/AB stoichiometry (Eq.(1)).Fig.S1a and b shows the vol-ume of hydrogen versus time plots for the hydrolysis of 0.10M ammonia borane solution performed starting with rhodium(0)nanoparticles supported on different oxide nanopowders at high166S.Akbayrak et al./Applied Catalysis B:Environmental 198(2016)162–170Table 1The rhodium contents and turnover frequency values of rhodium(0)nanoparticles supported on different oxide nanopowders at (a)high and (b)low rhodium loading of the catalysts used in hydrogen generation from the hydrolysis of ammonia borane at 25.0±0.1◦C for a fair comparison.CatalystRh loading (%wt.)a[Rh]conc.(mM)aTOF (min −1)aRh loading (wt.%)b[Rh]conc.(mM)bTOF(min −1)b Rh 0/CeO 2 1.610.081610.100.082010Rh 0/SiO 2 2.030.08430.400.08112Rh 0/Al 2O 3 2.070.081010.500.08195Rh 0/TiO 2 1.950.08730.470.08105Rh 0/ZrO 2 2.120.08150.590.08102Rh 0/HfO 2 1.920.08290.480.0824Fig.7.(a)mol H 2/mol H 3N ·BH 3versus time graph depending on the rhodium concentration in Rh 0/CeO 2for the hydrolysis of AB (100mM)at 25.0±0.1◦C.(b)The logarithmic plot of hydrogen generation rate versus the concentration of Rh;ln (rate)=1.01ln [Rh]+6.33.Fig.8.(a)The evolution of equivalent hydrogen per mole of AB versus time plot for the hydrolysis of AB starting with Rh 0/CeO 2(0.02mM Rh)and 100mM AB at various temperatures.(b)The Arrhenius plot for the Rh 0/CeO 2catalyzed hydrolysis of AB.lnk =−5122.6(1/T)+19.68.and low rhodium loadings,respectively,at 25.0±0.1◦C.The ini-tial turnover frequency values were calculated from the initial rate of hydrogen generation and listed in Table 1.As it can eas-ily be seen from the graphical illustration in Fig.1,among the catalysts tested,rhodium(0)nanoparticles supported on nanoce-ria show the highest catalytic activity in hydrogen generation from the hydrolysis of ammonia borane at room temperature.Therefore,we decided to use nanoceria as support for the further investiga-tion on rhodium(0)nanoparticles catalysts in hydrogen generation from the hydrolysis of ammonia borane.Rhodium(0)nanoparticles supported on nanoceria (Rh 0/CeO 2)were isolated from the reaction solution by centrifugation,copi-ous washing with water,and drying under vacuum (10−3Torr)at 60◦C and characterized by ICP-OES,XRD,BET,SEM,SEM-EDS,TEM and XPS.Fig.2shows the XRD pattern of ceria powders and rhodium(0)nanoparticles supported on ceria (Rh 00/CeO 2)with two different rhodium loadings.Powder XRD pattern of Rh 0/CeO 2in Fig.2b exhibits peaks at 28.5◦,33.07◦,47.08◦,56.33◦and 59.08◦assigned to the (111),(200),(220),(311)and (222)reflections of CeO 2,respectively (JPDS =43-1002).Comparison of the XRD pat-terns clearly shows that there is no change in intensity and position of the characteristic diffraction peaks of ceria.This observation indi-cates that ceria remains intact after reduction of rhodium(III)ions and there is no noticeable alteration in the framework lattice orS.Akbayrak et al./Applied Catalysis B:Environmental198(2016)162–170167 Table2Turnover frequency value(TOF)and activation energy(E a)of the most active rhodium,ruthenium,palladium and platinum catalysts reported for the hydrolysis of AB at 25.0±0.1◦C.Catalyst TOF(min−1)E a(kJ/mol)Metal/AB molar ratio Ref.Rh0/CeO2(0.1%wt.Rh)201042.60.0008This study Rh(0)/CNT706320.0025[15]Pt/CNTs-O-HT468–0.0047[42]Ru/CB429.534.810.00425[43]Pt@MIL-10141440.70.0029[44]Pd@Co/graphene408.9–0.02[45]Ru@MWCNT329330.00189[46]Rh/graphene32519.70.004[38]Pt/␥-Al2O3308210.018[47]Rh(0)@TiO226065.50.00116[48]Pd(0)/SiO2-CoFe2O4254520.0031[49] Laurate stabilized Rh(0)nanocluster20043.60.0025[50]Rh0/nanoAl2O3(0.5%wt.Rh)195–0.0008This study Pt/CeO2182–0.0018[32]Ru(0)/SiO2-CoFe2O4172.545.60.00097[51]Ru/HAp137580.00392[52]Ru/X-NW135770.00271[53]Rh/␥-Al2O3128.2210.018[47]Rh0/nanoSiO2(0.4wt.%Rh)112–0.0008This study Rh0/nanoTiO2(0.47%wt.Rh)105–0.0008This study Rh0/nanoZrO2(0.59%wt.Rh)102–0.0008This study Zeolite stabilized Rh(0)nanocluster9266.90.002[54]Rh0/HfO2(0.48%wt.Rh)24–0.0008This studyloss in the crystallinity of host material.There is no observable peak attributable to rhodium nanoparticles in Fig.2,most likely as a result of low rhodium loading on ceria.The BET nitrogen adsorption analysis(Fig.S2in the Supporting information)gave the surface area of ceria and Rh0/CeO2as48.1and 47.32m2g−1,respectively.This slight decrease in the surface area of ceria upon rhodium loading implies the existence of rhodium(0) nanoparticles on the surface of support.Fig.3exhibits the SEM image and SEM-EDS spectrum of Rh0/CeO2indicating that rhodium is the only element detected in the sample in addition to the framework elements of ceria(Ce,O).The morphology and size of rhodium nanoparticles on the sur-face of ceria were investigated by high resolution TEM(Fig.4a–d) which shows rhodium nanoparticles with particle size in the range 1.8–5.3nm(average diameter:3.2±0.8nm,histogram in Fig.4e) are successfully anchored on ceria nanopowder.The composition of Rh0/CeO2and the oxidation state of rhodium were also studied by XPS technique.The survey-scan XPS spectrum of Rh0/CeO2with a rhodium loading of0.1%wt.given in Fig.5shows the presence of all the framework elements of Rh0/CeO2in agree-ment with the SEM-EDS result.High resolution X-ray photoelectron spectrum of Rh0/CeO2sample given in the inset of Fig.5shows two prominent Rh3d bands at307.07eV and312.10eV which can readily be assigned to Rh(0)3d5/2and3d3/2bands,respectively, by comparing to the values of metallic rhodium[38].The bands at 309.8and305.2eV(inset of Fig.5)are attributable to rhodium oxide [39],which might be formed during the XPS sampling.For compar-ison,high resolution XPS spectra of CeO2and Rh00/CeO2with a rhodium loading of0.1%wt.in Ce3d and Ce4d regions are given in Fig.S3of the Supporting Information.There is no observable change in the spectra after rhodium loading.Before starting with the investigation on the catalytic activity of Rh0/CeO2in the hydrolysis of ammonia borane,a control exper-iment was performed to check whether ceria shows any catalytic activity in the hydrolysis of ammonia borane at the same tempera-ture.In the control experiments performed starting with1.0mmol AB and80mg CeO2(the same amount as the one used in catalytic activity tests)in10mL of water at20.0or35.0±0.1◦C,no hydrogen generation was observed in1h at both temperatures.This obser-vation indicates that the hydrolysis of ammonia borane does not occur in the presence of CeO2in the temperature range used in Table3The TOF value of Rh0/CeO2(0.1%wt.Rh)in the subsequent runs of the hydroly-sis of ammonia borane(The material loss has been taken into account for the TOF calculations in each run).Run Rh0/CeO2(mg)[Rh](mM)TOF(min−1) 1800.08020102720.07215403640.06414704610.06114505560.0561350this study.However,Rh0/CeO2is found to be highly active catalyst in the hydrolysis of ammonia borane generating3.0equivalent H2 gas per mole of AB in the same temperature range.AB molecule interacts with the active site of the metal particle and an activated complex is formed.Then,B N bond dissociation takes place by the attack of water molecule on the activated complex,which results in the hydrolysis of the BH3intermediate to form the borate ion together with the release of H2[40,41].The catalytic activity of Rh0/CeO2in hydrogen generation from the hydrolysis of ammonia borane at25.0±0.1◦C was investigated depending on the rhodium loading of catalyst.The plots of equiva-lent H2gas generated per mole of H3NBH3versus time during the catalytic hydrolysis of100mM AB solution using Rh0/CeO2with different loading at25.0±0.1◦C are available in Fig.S4of the Sup-porting Information.The turnover frequency(TOF)values were calculated from the initial rate measured in each of the hydro-gen generation versus time plots given in Fig.S4of the Supporting Information.The variation of TOF with the rhodium loading of the catalyst is illustrated in Fig.6.It is seen that the Rh0/CeO2sam-ple with a rhodium loading of0.1%wt.Rh provides the highest catalytic activity in hydrogen generation from the hydrolysis of ammonia borane at25.0±0.1◦C.Therefore,Rh0/CeO2catalyst with rhodium loading of0.1%wt.Rh was used in all of the further exper-iments performed in this study.The decrease in catalytic activity of Rh0/CeO2with the increasing rhodium loading may be attributed to the aggregation of rhodium nanoparticles as seen from the TEM images in Fig.S5.Fig.7a shows the plots of equivalent H2gas generated per mole of H3NBH3versus time during the catalytic hydrolysis of 100mM AB solution using Rh0/CeO2catalyst in different rhodium168S.Akbayrak et al./Applied Catalysis B:Environmental 198(2016)162–170Fig.10.TEM image of Rh 0/CeO 2sample (0.1%wt.Rh)harvested after the fifth use (a)and fifth cycle (b)in hydrolysis of AB at 25.0±0.1◦C.Fig.9.Graph of the evolution of equivalent hydrogen per mole of AB versus time plot for the first and fifth use of Rh 0/CeO 2(0.1%wt.Rh)in catalytic hydrolysis of AB for at 25.0±0.1◦C.concentration at 25.0±0.1◦C.The hydrogen generation rate was determined from the linear portion of each plot in Fig.7a and plot-ted versus the initial concentration of rhodium,both in logarithmic scale,in Fig.7b,which gives straight line with a slope of 1.01indi-cating that the catalytic hydrolysis of ammonia borane is first order with respect to the rhodium concentration.The TOF values for hydrogen generation from the hydrolysis of ammonia borane (100mM)at 25.0±0.1◦C were determined from the hydrogen generation rate in the linear portion of the plots given in Fig.7a for experiment performed starting with 100mM AB plus Rh 0/CeO 2(0.1%wt.Rh)in various rhodium concentrations.The TOF value of Rh 0/CeO 2catalyst in rhodium concentration of 0.08mM Rh is as high as 2010min −1in hydrogen generation from the hydroly-sis of ammonia borane at 25.0±0.1◦C.TOF values of the most active rhodium,platinum,ruthenium catalysts used in hydrogen genera-tion from the hydrolysis of ammonia borane are listed in Table 2for comparison.As clearly seen from the TOF values listed in Table 2,rhodium(0)nanoparticles supported on nanoceria (Rh 0/CeO 2)pro-vide the highest catalytic activity (TOF =2010min −1)ever reported for the room temperature hydrolysis of ammonia borane in litera-ture.This high catalytic activity of Rh 0/CeO 2can be attributed to the fact that ceria is a reducible oxide support [20].Because of large standard reduction potential of Ce 4+→Ce 3+(1.76V in acidic solution)[21],Ce 3+defects can easily be formed in ceria under the reaction conditions leading to the build up of excess negative charge on the oxide surface.This negative charge on the oxide sur-face can bind the rhodium(0)nanoparticles strongly making thesurface rhodium sites catalytically more active in hydrogen gener-ation from the hydrolysis of ammonia borane.Catalytic hydrolysis of ammonia borane was also performed starting with 100mM AB in the presence of Rh 0/CeO 2(0.02mM Rh)at various temperatures.Fig.8a shows the evolution of equivalent H 2per mole of ammonia borane versus time plots for the hydrolysis of AB at various temperatures in the range 20–35◦C.The rate con-stant for the hydrogen generation reactions was calculated from the slope of the linear part of each hydrogen evolution versus time plot at various temperatures (Fig.8a).From the slope of Arrhenius plot in Fig.8b,activation energy for the hydrolysis of ammonia borane catalyzed by Rh 0/CeO 2was found to be E a =43±2kJ/mol,which is comparable to the literature values reported for the other catalysts used in the same reaction (Table 2).Reusability of Rh 0/CeO 2catalyst was tested in successive exper-iments performed using the catalyst isolated from the reaction solution after a previous run of hydrolysis of ammonia borane.After the completion of hydrogen generation from the hydrolysis of ammonia borane starting with 0.08mM Rh 0/CeO 2plus 100mM AB in 10mL aqueous solution at 25.0±0.1◦C,the catalyst was isolated by centrifugation and dried under vacuum (10−3torr)at 60◦C after washing with 50mL of water.The isolated solid sample of Rh 0/CeO 2was weighed and redispersed in 10mL solution of 100mM AB for a subsequent run of hydrolysis which was then started immedi-ately and continued until the completion of hydrogen evolution at 25.0±0.1◦C.This hydrogen generation process was repeated 5times in the same way at 25.0±0.1◦C and the results are listed in Table 3.The results of reusability tests reveal that Rh 0/CeO 2are still active in the subsequent runs of hydrolysis of ammonia borane providing 100%conversion (yielding 3equivalent H 2per mole of AB).Fig.9shows the evolution of equivalent H 2per mole of H 3NBH 3versus time plot for the first and fifth use of Rh 0/CeO 2(0.08mM Rh)in catalytic hydrolysis of ammonia borane at 25.0±0.1◦C.After the fifth run of hydrolysis of ammonia borane,Rh 0/CeO 2still exhibits a record TOF value (1350min −1),preserving 67%of the initial cat-alytic activity.The decrease in catalytic activity in successive runs (Table 3)can be attributed to the agglomeration of nanoparticles on the surface of ceria during the isolation,drying and redispersion processes.Indeed,the TEM image in Fig.10a for the sample har-vested after the fifth run of hydrolysis indicates the agglomeration of rhodium(0)nanoparticles on the surface of ceria.Durability of the Rh 0/CeO 2(0.1%wt.Rh)catalyst was also tested in hydrogen generation from the hydrolysis of ammonia borane.For the recyclability test,a standard hydrolysis of ammonia borane cat-alyzed by Rh 0/CeO 2was performed starting with 10mL solution of 100mM AB and 0.08mM Rh at 25.0±0.1◦C.When the ammonia borane present in the solution was completely hydrolyzed,1mmol。