ORIGINAL PAPER
Low-Temperature Complete Oxidation of Ethyl Acetate Over CeO 2-Supported Precious Metal Catalysts
Tomohiro Mitsui ÆToshiaki Matsui ÆRyuji Kikuchi ÆKoichi Eguchi
Published online:3March 2009
ÓSpringer Science+Business Media,LLC 2009
Abstract Catalytic combustion of ethyl acetate was investigated over various CeO 2-supported precious metal catalysts prepared by impregnation method,and the effect of reduction treatment on the activity was examined.Among the catalysts tested,Ru/CeO 2achieved the highest activity for ethyl acetate combustion,and the activity was almost unchanged by the heat treatment in a hydrogen atmosphere.In the cases of Pt/CeO 2,Pd/CeO 2,and Rh/CeO 2,the catalytic activity was enhanced by the reduction treatment at 400°C,though the activity of the reduced catalysts was still inferior to that of Ru/CeO 2.It was con-firmed by temperature-programmed reduction that the reduction of the ruthenium species was initiated at the lowest temperature among the CeO 2-supported precious metals.The precious metal species reducible at lower temperatures should be responsible for the high activity in the complete oxidat
ion of ethyl acetate.Keywords Volatile organic compounds ÁCatalytic combustion ÁCeO 2ÁEthyl acetate
1Introduction
Volatile organic compounds (VOCs)have relatively high-vapor pressure,and thus readily vaporize under ambient conditions.These compounds are known as a major cause of photochemical smog,ground-level ozone,sick house
syndrome and chemical sensitivity [1–3].Among VOCs,ethyl acetate is widely used as a solvent for varnishes,coatings,plastics,and so on.Consequently,even through the concentration of ethyl acetate at application sites in dilute,the amount of ethyl acetate emitted into the atmo-sphere will eventually be huge in total.Therefore,it is desired to abate the emissions totally.
Catalytic combustion is regarded as one of the effective methods for VOC removal,since complete combustion of dilute fuel proceeds stably at low temperatures,leading to low emission of NO x and unburned fuels [4].Alumina is the most popular support material for precious metals to disperse on it,and many researchers have reported that Al 2O 3-supported precious metal catalysts exhibit high activity for the combustion of VOCs [5–10].However,higher temperature is required for the catalytic oxi
dation of ethyl acetate over this catalyst system as compared with that of the other VOCs such as alcohol and aromatic compounds [11].Thus,it is required to develop combustion catalysts with high activity so as to reduce the operating temperature.As an alternative to alumina,cerium oxide is one of the attractive supports due to its high oxygen transport and storage capacities.Ceria-supported precious metal catalysts exhibit high activity for various reactions such as purifying of automotive exhaust,water–gas shift reaction,CO oxidation,and hydrocarbon combustion [12–16].Accordingly,it can be expected that this catalyst system is a promising candidate for ethyl acetate combustion.
In this study,combustion characteristics of ethyl acetate over CeO 2-supported precious metal catalysts were inves-tigated,and the catalytic activity of the catalysts was compared with that of Al 2O 3-supported precious metal catalysts.In addition,the influence of oxidation–reduction treatment on the catalytic activity was also examined.
T.Mitsui ÁT.Matsui ÁR.Kikuchi ÁK.Eguchi (&)
Department of Energy and Hydrocarbon Chemistry,Graduate School of Engineering,Kyoto University,Nishikyo-ku,Kyoto 615-8510,Japan
e-mail:eguchi@scl.kyoto-u.ac.jp
Top Catal (2009)52:464–469DOI 10.1007/s11244-009-9186-4
2Experimental
2.1Catalyst Preparation
Ceria-supported precious metal catalysts were prepared by the impregnation method.For comparison,c-Al2O3(JRC-ALO-8,The Catalysis Society of Japan)was used as a sup-port for precious metal catalysts,and c-Al2O3-supported samples were prepared in the same way.A solution of Pt(NO2)2(NH3)2(Tanaka Kikinzoku Kogyo),Pd(NO2)2 (NH3)2(Tanaka Kikinzoku Kogyo),Ru(NO3)3(Tanaka Ki-kinzoku Kogyo),or Rh(NO3)3(Tanaka Kikinzoku Kogyo) was used as a precious metal source.Ceria(CeO2,Aldrich) was impregnated with the solution.The mixture was kept on a steam bath at80°C until the solvent was evaporated. Subsequently,the resulting powder was calcined at400°C for30min in air.Metal loading in the samples was1.0or 10wt%.The catalysts with high loading were prepared to clarify the changes in crystalline phase and electronic state of precious metals.Part of the calcined catalysts was heat-treated at400°C for15min in50%H2/N2prior to charac-terizations and catalytic reactions.
2.2Catalytic Combustion of Ethyl Acetate
Afixed-bedflow reactor made of quartz tubing of8mm inner diameter was used,and the prepared catalyst (600mg)was set in the reactor.Each catalyst was tabletted and pulverized into0.85–1.7mm before catalytic reaction tests.A gaseous mixture composed of0.1%ethyl acetate and99.9%air was fed with aflow rate of100cm3min-1 (space velocity:10,000L kg-1h-1).The catalyst bed length of Al2O3-supported metal catalysts was longer than that in CeO2-supported metal catalysts:GHSV(Al2O3 -supported catalysts)=4,000h-1;GHSV(CeO2-supported catalysts)=15,000h-1.The outlet gas compositions were analyzed by an on-line micro-gas chromatograph with a thermal conductivity detector(TCD)(VARIAN,CP-4900) and aflame ionization detector(FID)(Shimadzu,GC-8A). The temperature was raised from room temperature up to 250°C in a heating process.The measurements were car-ried out at afixed temperature.Ethyl acetate conversion to carbon dioxide was defined as follow:
Ethyl acetate conversion to CO2ð%Þ¼
F CO
2
out
4ÂF EAin
Â100
where F EA in is the influent molarflow rate of ethyl acetate,
and F CO
2out
is the effluent molarflow rate of CO2.
2.3Catalyst Characterization
The samples were characterized by X-ray diffraction(XRD), X-ray photoelectron spectroscopy(XPS),temperature-programmed reduction(TPR),and BET surface area.XRD patterns were recorded by Cu K a radiation on a RIGAKU Rint2500diffractometer for phase identification in the samples.XPS measurements were conducted on Shimadzu ESCA-850using a Mg K a source.In the case of the reduced catalysts,the samples were transferred directly into XPS chamber without exposure to air after the reduction treat-ment.The binding energy was referenced to the C1s peak (284.
ac reactor3eV).BET surface area was determined by N2 adsorption at the liquid nitrogen temperature using a Shi-madzu Gemini2375analyzer.TPR measurements were conducted using a Quantachrom CHEMBET3000system, and the amount of consumed hydrogen was measured by a thermal conductivity detector(TCD).A weighed amount (25mg)of the as-calcined catalysts was placed in a quartz tube reactor,and then a gaseous mixture of5%H2-95%Ar was fed to the reactor at30mL min-1.The temperature was raised up to800°C at a heating rate of10°C min-1.
The size and dispersion of precious metal particles on CeO2support were determined from the chemisorption of carbon monoxide.The CO adsorption was carried out by an O2-CO2-H2-CO pulse method(Quantachrom CHEMBET 3000system)because the conventional method is ineffec-tive for the supports with large oxygen storage capacity due to the overestimation of adsorbed carbon monoxide[17]. First,the sample(25mg)was heat-treated at300°C in an oxygen atmosphere,and then cooled down to room tem-perature.The resultant sample was secondly reduced at 400°C in a hydrogen atmosphere and cooled down to room temperature.Subsequently,O2,CO2,and H2gases were fed to the sample in a sequential manner at room temperature for5min,and carbon monoxide(0.224mL) was pulsed repeatedly to the sample until the amount of CO at the outlet reached a constant value at room temperature. The dispersion and the diameter of precious metals were calcul
ated by assuming the adsorption stoichiometry of CO/M s=1(M s:Surface atom of the precious metal).
3Results and Discussion
3.1Ethyl Acetate Combustion Over As-Calcined
Catalysts
The results for the catalytic activity tests over CeO2and as-calcined1.0wt%Pt/CeO2,Pd/CeO2,Ru/CeO2,and Rh/ CeO2are shown in Fig.1.Complete oxidation of ethyl acetate over CeO2was not achieved even at high temper-atures,and many by-products such as hydrogen, hydrocarbon,and so on,were formed at above400°C.On the other hand,ethyl acetate was completely oxidized over CeO2-supported metal catalysts below250°C,and only carbon dioxide was detected as afinal product.Among the
catalysts,Ru/CeO2exhibited the highest activity at low temperatures;ignition temperature was ca.130°C and the conversion of90%was attained at180°C.The results for the catalytic activity tests over as-calcined1.0wt%Ru/ CeO2,Pt/c-Al2O3,Pd/c-Al2O3,and Ru/c-Al2O3are shown in Fig.2.The diameter of platinum particles on c-Al2O3 was2.0nm,and the BET surface area was higher than 140m
2g-1.The activity of Ru/CeO2was higher than that of Al2O3-supported precious metal catalysts,in spite of the low surface area as summarized later in Table2.
The effect of precious metals loaded on CeO2was also confirmed in the reaction products at low temperatures.The unreacted ethyl acetate and products selectivity at210°C over CeO2,Pt/CeO2,Pd/CeO2,Ru/CeO2,and Rh/CeO2are summarized in Table1.The main by-products were etha-nol and acetaldehyde at low temperatures,which started to form below100°C.In the cases of Pt/CeO2,Pd/CeO2,and Rh/CeO2,the selectivity to ethanol was lower than that to acetaldehyde,whereas over CeO2,the ethanol selectivity was higher.A small amount of methanol and acetic acid was also detected.Thus,it can be expected that precious metals loaded on CeO2promoted the ethanol oxidation as well as ethyl acetate oxidation.In ethyl acetate combustion over Al2O3-and TiO2-supported metal catalysts,it has been reported that acetaldehyde was formed by ethanol oxidation[18,19].In this study,therefore,a part of ethyl acetate oxidation over CeO2-supported metal catalysts should proceed via the formation of ethanol.
3.2Characterization of As-Calcined Catalysts
Surface characteristics of the as-calcined samples are summarized in Table2.The diameter and disper
sion of precious metal particles on CeO2were confirmed to depend on the metal species,although BET surface area was
Table1Selectivity of unreacted ethyl acetate and products at210°C over CeO2,Pt/CeO2,Pd/CeO2,Ru/CeO2,and Rh/CeO2
Catalyst Ethyl acetate(%)Acetaldehyde(%)Ethanol(%)Methanol(%)Acetic
acid(%)Carbon dioxide(%)
CeO245.7  1.27.20.40.240.8 Pt/CeO2  6.5  3.5  1.4––85.6 Pd/CeO230.38.2  1.2––60.2 Ru/CeO2000––100 Rh/CeO20  2.80.9––96.6
comparable in each sample.The particle size of ruthenium on CeO2was the largest among all samples despite the high activity for ethyl acetate combustion.
Temperature-programmed reduction profiles of pre-cious metal/CeO2catalysts are shown in Fig.3.All profiles consisted of three main peaks.The sharp peak below200°C corresponds to the reduction of precious metal oxide,and the other two peaks at high temperatures are ascribed to the reduction of the surface capping oxygen of CeO2[20].The reduction of ruthenium oxide on CeO2was initiated at the lowest temperature among the samples tested.Rh/CeO2exhibited a broad peak starting to rise at low temperature,and a maximum of this peak appeared at higher temperature as compared with that of Ru/CeO2.Thus,the precious metal species reducible at lower temperatures should be responsible for the high activity to the complete oxidation of ethyl ace-tate,as can be seen in Fig.1.These results indicate that ethyl acetate combustion proceeds through the reduction of the precious metal oxide.3.3Ethyl Acetate Combustion and Characterization of
the Reduced Catalysts
The reduction treatment in a hydrogen atmosphere has significantly affected the activity.The results for the cat-alytic activity tests over reduced1.0wt%Pt/CeO2,Pd/ CeO2,Ru/CeO2,and Rh/CeO2are shown in Fig.4.As in the case of the as-calcined catalysts,Ru/CeO2exhibited the highest activity.Furthermore,the activity was almost unchanged before and after the reduction treatment.We have previously reported that the reduced ruthenium spe-cies readily reacted with the lattice oxygen of CeO2[21]. Thus,this can be the reason for the ineffectiveness of the reduction treatment.In contrast,the catalytic activity of other reduced catalysts was enhanced as compared with that over the as-calcined catalysts.In the case of Pt/CeO2,a remarkable effect of the reduction treatment was observed at lower conversion.For acetaldehyde combustion over Pt/ CeO2and Pd/CeO2,the activity was also improved by the reduction treatment[22].
The electronic state of precious metal species on CeO2 was investigated by XPS measurement(Table3).All reduced samples were not exposed to air in this experi-ment,while the combustion was conducted in an oxidizing atmosphere.The binding energy of Pt4f7/2,Pd3d5/2,Ru 3d5/2,and Rh3d5/2was recorded.The energy of the as-calcined catalysts agreed well with those reported for precious metals in the oxidized state:PtO(Pt4f7/2= 73.8eV),PdO(Pd3d5/2=336.3eV),RuO2
(Ru3d5/2= 280.7eV),and Rh2O3(Rh3d5/2=308.8eV)[23].On the
Table2Physical properties of CeO2-supported1wt%metal catalysts
Catalyst Surface area
(m2g-1)Particle size of
precious metal(nm)
Dispersion
(%)
CeO265––Pt/CeO270  2.839 Pd/CeO272  2.741 Ru/CeO265  4.530 Rh/CeO269  2.839
other hand,the reduced catalysts exhibited binding energy almost identical to that of Pt0(Pt4f7/2=71.2eV),Pd0(Pd 3d5/2=335.1eV),Ru0(Ru3d5/2=280.1eV),and Rh0 (Rh3d5/2=307.2eV)[23].Accordingly,by the reduction treatment,the precious metal species were reduced to the metallic state,resulting in the enhancement of the activity for ethyl acetate combustion.In the case of Ru/CeO2,
however,different redox characteristics of ruthenium spe-cies were observed:reoxidation of metallic Ru is facilitated by the lattice oxygen of CeO2.As shown in Fig.3,this can be explained by the partial reduction of CeO2at400°C in a hydrogen atmosphere,leading to stabilization of ruthe-nium species in the metallic state.
3.4Reoxidation Effect of the Reduced Samples
The influence of reoxidation treatment for Pt/CeO2reduced at400°C was studied because the catalytic activity was significantly enhanced by the initial reduction treatment. The reduced Pt/CeO2was heat-treated at400°C for 30min in air prior to catalytic reaction(reoxidized-sam-ple).The results for the catalytic activity tests over 1.0wt%Pt/CeO2heat-treated in various conditions are shown in Fig.5.The high activity for the reduced-sample was deteriorated significantly by the subsequent reoxida-tion treatment.However,the activity was regenerated when the reoxidized-sample was subjected to the second-reduc-tion(reduced2)at400°C for15min in a hydrogen atmosphere.
Then,the characterizations for the reoxidized and sub-sequently reduced samples were conducted.Figure6 shows the XRD patterns of10wt%Pt/CeO2heat-treated in various conditions.The diffraction patterns of as-calcined, reduced,and reoxidized samples were consistent with that of CeO2,
and the intensity was increased as the oxidation–reduction treatment was repeated.In the case of the Pt/CeO2sample after the second-reduction(reduced2),a new phase of metallic platinum was observed.The XPS spectra of10wt%Pt/CeO2heat-treated in various condi-tions are shown in Fig.7.The two peaks at high and low binding energy correspond to Pt4f5/2and Pt4f7/2, respectively.As summarized in Table3,both peaks were negatively shifted by thefirst-reduction treatment.The transition between platinum oxide and platinum on CeO2 surface was induced by the repetitive oxidation–reduction treatment:the positively shifted peaks in the reoxidized-sample were moved to negative side again by the second-
Table3Binding energy of the CeO2-supported10wt%metal catalysts
Catalyst Binding energy(eV)
As-calcined Reduced
Pt/CeO2a72.971.1 Pd/CeO2b337.1335.1 Ru/CeO2c280.7279.6 Rh/CeO2d308.5307.4
Binding energy is value of a Pt4f7/2,b Pd3d5/2,c Ru3d5/2,and d Rh 3d5/2

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