the reactions of ethanol over au/ceo2

Applied Catalysis A: General 261 (2004) 171–181

The Reactions of Ethanol over Au/CeO2

P.-Y. Sheng, G.A. Bowmaker, H. Idriss∗

Department of Chemistry, The University of Auckland, P.O. Box 92019, Auckland, New Zealand

Received in revised form 21 October 2003; accepted 31 October 2003

Abstract

The reaction of ethanol has been investigated on the surface of Au/CeO2 by temperature programmed desorption (TPD), infra red (IR)absorption and in steady state catalytic conditions. The objective of this study is to compare Au/CeO2 to the other previously studied CeO2

[J. Catal. 186 (1999) 279], Pd/CeO2 [J. Catal. 186 (1999) 279], Pt/CeO2 [J. Catal. 191 (2000) 30] and Rh/CeO2 [Catal. Today 63 (2000) 327]for the same reaction. At 300 K, the surface is covered with both ethoxide species and weakly bonded ethanol. Most of these species desorbgiving back ethanol (about 50%; TPD) by 400–450 K with some formation of acetaldehyde (7.5%; TPD). A small part of the remainingethoxides gives bridging CO (ν: 1916 cm−1). Most of CO is oxidized to CO2 (CO2/CO ≈ 25, TPD) translating the powerful nature of Aufor the oxidation process. In the absence of Au, the same ratio drops to 0.7 [J. Catal. 186 (1999) 279]. By 600 K, the surface is coveredwith carbonates species (ν: 1524 cm−1). These carbonates are mainly decomposed to CO2; IR and TPD. Steady state reactions in presenceof oxygen showed the formation of mainly three hydrocarbon-products with a distribution depending on the reaction temperature. At 573 K,acetaldehyde was the main reaction products. Methane desorbed in two distinct reaction-temperatures (673 and 973 K) while acetone was thelargest reaction product at ca. 773 K. Among the non-hydrocarbon products (CO2, CO and H2), CO2 was the largest at low temperatures (upto 700 K with a peak at ca. 573 K (52% molar yield)). Increasing the reaction temperature up to 1073 K resulted in increasing amounts ofH2 and CO ca. 10 and 30% molar yield, respectively. Insight into the reaction mechanism is given with the potential role of direct oxidation,water gas shift and reforming of methane discussed.© 2003 Elsevier B.V. All rights reserved.

Keywords: Ethanol-TPD; Au/CeO2; Hydrogen production; Ethoxide species; Water gas shift reaction

1. Introduction

The reactions of ethanol over oxides and metal/oxides sur-faces are receiving increasing attention because of the po-tentially efficient production of hydrogen by oxidation andsteam reforming[1–10]. Ethanol (now viewed as a bio-fuelwith potential for making hydrogen) has received consid-erable attention in the past because it is a simple probemolecule in studying surface reactions on metals[11–13]and oxides[14–16]. The following equations [Eqs. (1)–(7)]summarize the different reactions observed on solid sur-faces. On almost all surfaces, ethanol molecules are firstdissociated to ethoxides (Eq. (1)). Ethoxides may either befurther oxidized to acetaldehyde (Eq. (2a)), dehydrated toethylene (Eq. (2b)) or (as in the case of Rh) may give anoxametallacyle intermediate (Eq. (2c)) [17,18]. On oxidesurfaces, acetaldehyde may be further oxidized to acetates

∗ Corresponding author. Fax:+64-9-373-7422.E-mail address: [email protected] (H. Idriss).

(Eq. (3)). Acetates can also undergo a coupling reaction (ke-tonisation) to give acetone and CO2 [19] (Eq. (3a)). On met-als, acetaldehyde either desorbs or decomposes to CO andmethane[20] (Eq. (4)). Other secondary reactions may alsooccur depending on the nature and degree of oxidation ofthe metal cations. For example, on stoichiometric surfacesof some oxides, acetaldehyde yields predominately croton-aldehyde, via aldol condensation[21] (Eq. (5)), while onsub-stoichiometric surfaces acetaldehyde may give buteneand butadiene, in a reductive coupling process[22] (Eq. (6)).

CH3CH2OH(g) → CH3CH2O(a) + H(a) (1)

on most surfaces

CH3CH2O(a) → CH3CHO(g) + H(a),

�Hr([1] + [2a]) = 111.5 kJ/mol (2a)

on most metals and large numbers of oxides

CH3CH2O(a) → CH2CH2(g) + OH(a),

�Hr([1] + [2b]) = 88.4 kJ/mol (2b)

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2003.10.046

172 P.-Y. Sheng et al. / Applied Catalysis A: General 261 (2004) 171–181

on some oxides, usually acidic[23] or sub-stoichiometricoxides[24,25]

CH3CH2O(a) → CH2CH2O(a) + H(a) (2c)

on Rh[18,26]

CH3CHO(g) + O(s) → CH3COO(a) + H(a) (3)

mainly on basic oxides, such as CeO2 [27] and ZnO[28]

2CH3COO(a) → CH3C(O)CH3 + CO2 + O(a),

�Hr (acetic acid(g) to acetone, CO2 and water)=13 kJ/mol

(3a)

on stoichiometric oxides such as TiO2 [19], CeO2 [27] andFe2O3 [29]

CH3CHO(g) → CH4(g) + CO(g), �Hr = −18.7 kj/mol

(4)

mainly on metals[20]

2CH3CHO(g) → CH3CH=CHCHO(g) + H2O(g),

�Hr = −10.3 kJ/mol (5)

on stoichiometric oxides such as TiO2 [22], CeO2 [27], andMgO [30]

2CH3CHO(g) + 2VO → CH3CH=CHCH3(g) + 2O(a),

�Hr (acetaldehyde to trans− butene and O2)=321 kJ/mol)

(6)

(VO: oxygen vacancy) on some reduced oxides such asTiO2−x [22] and UO2 [31]

3CH3CHO(g) → C6H6(g) + 3H2O(g),

�Hr = −144.2 kJ/mol (7)

Pt/CeO2 [32] and Ar-sputtered UO2 (1 1 1) single crystal[33]

In Eqs. (1)–(7), (a) for adsorbed, (g) for gas phase; the ad-sorption and desorption reactions are omitted for simplicity.

In the present work, we examine the metal supported ox-ide system, which was chosen for the following reasons.

CeO2 is a very active oxide for oxidation of CO to CO2,for water gas-shift reactions[34,35]and for making H2 fromethanol by reforming[9] or oxidation [6], alone or whenused as a support for metals. We have previously investi-gated the reactions of ethanol on CeO2 [32], Pd/CeO2 [32],Pt/CeO2 [36], Rh/CeO2 [37], and Rh-Pt/CeO2 [6]. Althoughthe results in details can be found elsewhere, it is worth indi-cating that Rh was important in breaking the carbon–carbonbond of ethanol[6]. Yet the most efficient catalyst for thetotal decomposition contained Rh and Pt (or Pd) becauseboth Pt and Pd show a higher activity for oxidation (as wellas hydrogenation to methane) of carbon type species on thesurface and thus prevents coking. However, in all conditions

investigated, non-negligible amounts of CO were detected.The fact that Au supported on several oxide surfaces includ-ing CeO2 [34], Co3O4 [38], TiO2 [39,40]and Fe2O3 [41] isvery active for CO oxidation reactions at low temperatureshas motivated the investigation of the Au/CeO2 system visa vis ethanol reactions. Equally important is the fact that theeffect of Au (on CeO2) on the decomposition pathway of thecarbon–carbon bond (the bottle neck of any efficient com-bustion process) has not yet been addressed. In this work,the reactions of ethanol on Au/CeO2 were followed by tem-perature programmed desorption, in situ by infra-red spec-troscopy and under steady state catalytic conditions.

2. Experimental

2.1. Catalyst preparation

CeO2 was prepared by precipitation from a solution ofCe(NO3)3·6H2O, in de-ionized water, by addition of ammo-nia (0.91 mol l−1) with stirring at 373 K (pH 8–8.5). The re-sulting white slurry was filtered, washed, and dried at 373 K(12 h), then calcined at 773 K (4 h) under air-flow. Au/CeO2was prepared by impregnation of CeO2 with a solution of themetal precursor (HAuCl4, 1 g(Au)l−1), in HCl (1 mol l−1), at300 K with stirring. The temperature of the mixture was thenraised to 373 K. The resulting paste like material was driedat 373 K (12 h), then calcined at 673 K (4 h) under air-flow.

2.2. Catalyst characterisation

The surface area of the catalyst was determined by BET(Quantasorb JR). The phase and average particle size wereanalyzed by XRD (Philips 1130 generator and a Philips1050 goniometer: Cu K� (broad focus) at a 44 kV and20 mA). The surface and near surface composition was mon-itored by X-ray photoelectron spectroscopy (XPS) (KratosXSAM-800 model, Al K�), pass energy= 20 eV. Resultsof catalyst characterization are given inTable 1.

2.3. Fourier transform infra-red (FT-IR) spectroscopy

Infrared spectra were recorded using a Digilab FTS-60Fourier transform spectrometer at a resolution of 4 cm−1

(256 scans). Adsorption of ethanol was performed in a stain-less steel IR cell (at 10−3–10−4 Pa) equipped with remov-able CaF2 windows (32 mm diameter, 4 mm thick). A typeK thermocouple, welded into the center of the cell in closeproximity to the catalyst disc, was used to monitor the tem-perature. Catalysts were pressed into a self-supporting disc(ca. 15 mm in diameter), and mounted into a gold-platedbrass sample holder at the center of the cell. CH3CH2OH,and O2 were contained in separate glass bulbs attached tothe vacuum line. Cleaning was performed by annealing un-der 20 Torr of O2 at 673 K followed by evacuation for ca.1 h (while cooling) prior to adsorption. Prior to dosing,

P.-Y. Sheng et al. / Applied Catalysis A: General 261 (2004) 171–181 173

Table 1Characterization of CeO2 and Au/CeO2 catalysts

Catalyst BET, m2 g−1 Catal Mean particle size (CeO2), nm Binding energy (XPS), eV Au/Ce (XPS) Atomic ratio

CeO2 33 15.4 *Ce 3d5/2, *Ce 3d3/2 –1 wt.% Au/CeO2 29 18 Au 4f7/2,5/2 85.6 and 88.8 0.0063 (0.0087)a

*Ce 3d5/2 V V ′ V′′ V′′′

883.3 886.9 890.3 899.7

*Ce 3d3/2 U U′ U′′ U′′′

902.2 904.6 908.7 917.8

a Expected based on 1 wt.%.

ethanol was subjected to several freeze–pump–thaw cycles.Ethanol vapor (103 Pa) was exposed to the sample catalystsat room temperature for 3 min. The cell was pumped downto 10−2–10−3 Pa in order to remove excess and loosely ad-sorbed ethanol on the surface. The temperature was thenraised, in increment steps, but spectral collection was con-ducted ca. 300 K. Spectra presented are obtained by subtract-ing the spectrum of the catalyst sample prior to adsorptionfrom that of the adsorbed sample.

2.4. Temperature programmed desorption (TPD)

Samples (∼50 mg) were loaded onto the TPD reactor.Prior to reaction, sample catalysts were oxidized at 573 Kunder an O2 stream for 3 h. The reactor was allowed to coolto 300 K under O2, then pumped down to base pressure(∼1 Pa), in order to remove any loosely adsorbed oxidizinggas on the surface of the catalyst and to clean the reactionline. Ethanol vapor was dosed onto the catalysts for a pe-riod of 3 min. Before TPD runs, the signal atm/z = 31 (thedominant fragment of ethanol) was monitored. The experi-ment started when no further decrease was observed (com-plete removal ethanol in the dosing line); this typically tookan hour. Temperature ramping was conducted up to 773 Kwith a ramping rate of 20 K min−1. the desorption profilewas monitored with a Spectra Vision quadrupole mass spec-trometer. Initially monitoring of all masses up to 200 amuwas conducted at a cycling rate of ca. 60 s. Once all desorp-tion masses were identified analyses with a smaller numberof masses (typically 12) were conducted in order to achieve abetter signal to noise ratio at a cycling rate of≈5 s per scan).

2.4.1. Quantitative analysisRelative yields were calculated for individual desorption

products by quantitatively analyzing the desorption spectrafor the respective mass fragments. Due to the possibility ofmore than one product with the same mass fragment sig-nal desorbing at same temperature, every peak area fromthe TPD profile was subtracted according to the method de-scribed elsewhere[27]. The reaction yield,Yi, was calcu-lated according to.

Yi = PAi × CFi∑

PAi × CFi

(8)

where PAi is the area under a peaki, and CFi is its correctionfactor (relative to CO).

Normalization of the desorption profiles were obtainedby multiplying the product desorption spectrum by the cor-rection factor. The raw area of the TPD peak of a producti was computed by the Trapezoidal method (an approxima-tion for determining total area under numerous data points)after background subtraction.

2.5. Steady state reactions

Reactions were conducted in a quartz fixed-bed micro-reactor (8 ml) at atmospheric pressure. Prior to each analy-sis, catalysts (50 mg) were regenerated at 573 K under dryair (3 h). Ethanol was filled into a saturator and kept at aconstant temperature depending on the desired vapor pres-sure. A total flow of 200 ml min−1 composed of dry air (107or 143 ml min−1) and nitrogen (93 or 57 ml min−1), passedthrough the saturator with an effluent ratio of C2H5OH: O2at 1:1.5 or 1:2. The mixture effluent was allowed to flowthrough the catalyst bed for 10 min before monitoring thereaction products. Steady state data collection was doneat each desired temperature between 373 and 1073 K aftermaintaining the condition for at least 1 h. Data collectionover periods of 8–12 h on-stream were obtained for each setof reaction conditions. Four different gas chromatographs(GC) were used for product analysis. For CO: Gow-Mac Se-ries 150 TCD with a molecular sieves 5A (60/80 mesh) stain-less steel column (2 m×0.3 mm). For CO2: a Varian Model3700 TCD with a Chromosorb 102 (60/80 mesh) stainlesssteel column (2 m× 0.3 mm). For both CO and CO2 analy-ses He was used as carrier gas at a flow rate of 15 ml min−1

at 300 K. For H2, O2, N2, CH4, and CO2 separation a 8700BasicTM GC TCD with a molecular sieves 5A (60/80 mesh)stainless steel column (4 m× 0.3 mm); N2: carrier gas ata flow rate of 15 ml min−1 at 295 K. CH4, CH3OH, C2H4,CH3CHO, C2H5OH, CH3C(O)CH3, and C4 were separatedby a stainless steel column (2 m× 0.3 mm) packed withChromosorb 102 (80/100 mesh) at 373 K of a Philips PyeUnicam series 304 FID. Data processing were carried outby online computer using Peak Simple II software. Prior toruns TCD and FID were previously calibrated by injectingknown amounts of reaction products. The rate of formation


of H2, CO, CO2 and hydrocarbons were estimated as follow:

rx = Cx × F

W(9)

where rx is the rate of the speciesx (mol g−1 s−1), F theflow rate through the catalyst (l s−1), Cx the concentrationof the speciesx (mol l−1), andW the catalyst weight (g).

3. Results

The results section is divided into four parts. The firstdeals with characterization of Au/CeO2. The second is a de-tailed study of the surface adsorbates following ethanol ex-posure at saturation as a function of reaction temperature.The third part is the reaction products in stoichiometric con-ditions (TPD). The fourth part is catalytic test and trying torelate the observed products to those of surface adsorbates(IR) and transient conditions (TPD).

3.1. Characterization

Table 1shows some of the results of characterization ofCeO2 and Au-CeO2 catalyst. A small decrease in the sur-face area of CeO2 following impregnation with Au is seen.The mean particle size of CeO2 was calculated using Sher-rer equation from the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) lines(the most intense CeO2 XRD lines). A very small diffractionline at 2θ = 38.34◦, that can be attributed to the (1 1 1) planeof Au was seen in the case of Au/CeO2 (Fig. 1); this is notunusual and indicates that the small Au atoms loading are ofsmall clusters (if not a sharp peak would be present).Fig. 2shows XPS Ce 3d (and XPS Au 4f, inset) regions of theas-prepared Au/CeO2 catalyst. The presence of Ce3+ linesis clearly evident, unlike the case of CeO2 alone (not shown- containing only Ce4+ ions). This indicates that addition ofAu results in partial reduction of surface (and near surface)Ce4+ ions. This is similar to other XPS results on the effectof Pt [36], Rh [37], and Pd[32] on the oxidation state of

Fig. 1. XRD of CeO2 and Au/CeO2 catalysts.

Fig. 2. XPS of Ce(3d) and Au(4f) (inset) of Au/CeO2.

Ce atoms. The assignment of the different XPS lines wasmade by referring to work conducted by several other work-ers[42–45]. The lines V, V′′ and V′′′ are attributed to Ce4+(CeO2). The hybridization between the Ce 4f and the O 2plevels results in the V and V′′ lines, which are due to a mix-ture of Ce(5d 6s)0 4f2 O2p4 and Ce(5d 6s)0 4f1 O2p5 con-figurations, respectively[36]. The V′′′ line is a pure Ce(5d6s)0 4f0 O2p6 final state. On the other hand, VO and V′ aredue to mixture of Ce(5d 6s)0 4f2 O2p4 and Ce(5d 6s)0 4f1

O2p5 configurations in Ce2O3 (VO is not well resolved inour case). The U lines of Ce 3d3/2 level are explained in asimilar way. The inset inFig. 2shows XPS of the Au 4f re-gion (binding energy of Au 4f7/2 = 85.6 eV). The FWHMof the XPS Au 4f7/2 states was found to be 1.56 eV (muchhigher than that expected from Au metal). Both the bindingenergy and FWHM are close to those of Au2O3.

3.2. Infra-red

The complete IR spectra of ethanol adsorbed on Au/CeO2at 295 K followed by evacuation at the indicated tempera-tures are presented inFig. 3 (these spectra are presented inmore detail inFigs. 4 and 5). At 295 K, coexistence of ph-ysisorbed ethanol and dissociatively chemisorbed ethoxidespecies is observed.

(i) The bands at 1046, 1096, 1148 cm−1 (Fig. 4A) and1269, 1316, 1399 cm−1 (Fig. 4B) (observed at 295 K)are ascribed toν(OC), �op(CH3), �ip(CH3), δ(OH),ω(CH2), andδs(CH3) of adsorbed ethanol, respectively.The presence of OH bending vibration is attributed tothe OH· · · O in plane deformation caused by weak hy-drogen bonding with surface oxygen anions of CeO2.

(ii) Absorption bands corresponding to adsorbed ethoxidespecies at 295 K were identified at 2971, 2933, 2904,2875, 1478, 1449, 1383, 1362, 1333, 1109, 1065, and1038 cm−1.


Fig. 3. FT-IR spectra after ethanol adsorption on Au/CeO2 at (a) 295 Kand subsequent heating to (b) 360 K, (c) 420 K, (d) 460 K, (e) 520 K, (f)565 K, (g) 615 K, and (h) 673 K. All spectra are recorded at 300 K.

(iii) The assignments of the ethoxide bands are presentedin Table 2. Ethoxide species on various metal oxideshave been observed and these include Al2O3 [46], TiO2[47,48], ZrO2 [49] MoO3 [50], and CeO2 [32].

3.2.1. Ethoxide speciesThe bands at 1109 and 1038 cm−1 are analogous to those

observed for ethanol on Pd/CeO2 [32] and Pt/CeO2 [36] sur-faces (Fig. 4A). They are due to two different types of sur-

Table 2IR vibrational frequencies and mode assignments for ethoxide species from the adsorption of ethanol on Au/CeO2 and various catalysts

Vibrational mode CeO2 [32] Al2O3 [46] TiO2 [48] Pd/CeO2 [32] Pt/CeO2 [36] Rh/CeO2 [37] Au/CeO2 (this work)

νas(CH3) 2960 2970 2971 2982 2977 2981 2971νas(CH2) – 2930 2931 2934 2933 2934 2933νs(CH3) 2836 2900 – 2909 2912 2911 2904νs(CH2) – 2870 2870 2880 2878 2878 2875δas(CH2) 1473 – 1473 1478 1480 1478 1478δas(CH3) – 1450 1447 1451 1451 1450 1449δs(CH3) 1383 1390 1379 1397 1399 1399 1399δs(CH2) – – – – – – 1362ω(CH2) – – 1356 – – – 1333ν(OC) mono- 1107 1115 1119 1078 1081 1080 1109ν(OC)/ν(CC) – – – – – – 1065ν(OC) bi- 1057 1070 1042 1037 1037 1038 1038

face ethoxide groups. The higher wavenumber band has beenassigned to an ethoxide coordinated to a single metal cation(monodentate), and the lower wavenumber to an ethoxidebridged bonded (bidentate) to two adjacent cations. The1269 and 1399 cm−1 bands (Fig. 4B) disappear when thesample temperature is raised to 460–520 K, suggesting com-plete removal of molecularly adsorbed ethanol. This impliesthat the remaining absorption bands in the spectrum are fromadsorbed ethoxide surface species. From 520 K onwards, ad-sorbed carbonate species were the dominant species on thesurface.

3.2.2. Carbonate speciesOwing to its basic character, CeO2 strongly binds car-

bonate entities. The nature of adsorption sites is expectedto change with the atmospheric environment and the priortreatment conditions. The frequency of the vibrationalmodes is sensitive to the degree of coordinative unsaturationof Ce ions and oxidation state (+3 or +4) because of therole of the polarization power of the ion. Hence the detailedassignment of all bands can only be tentative. The natureof coordination of carbonate species is usually assigned onthe basis of the splitting of the asymmetric COO stretchingband[51] and depending on the splitting, one can distin-guish between mono- or bi-coordinated carbonates on thesurface. Mono-dentate carbonates (O–CO2) show two bandsin the region 1530–1470 cm−1 (ν4) and 1370–1300 cm−1

(ν1) whilst the corresponding bands for bi-dentate carbon-ates occur in the regions 1630–1590 and 1270–1260 cm−1.Bicarbonate (hydrogeno-carbonate) species exhibit bandsin the 1660–1620 cm−1 (ν4), 1410–1290 cm−1 (ν1) and1215–1240 cm−1 (�OH) regions. Accordingly, the bands inthis work at 1524 and 1360 cm−1, with a 164 cm−1 splitting,are assigned to mono-dentate carbonate. The weak bands at1574 and 1320 cm−1, having a 254 cm−1 splitting can beassigned to bi-dentate carbonate. The band at 1437 cm−1 isattributed to symmetric carbonate.

3.2.3. Adsorbed COTwo broad bands at ca. 1916 cm−1 (moderate) and at ca.

2120 cm−1 (weak) were observed during the initial ethanol


Fig. 4. (A) FT-IR spectra 1000–1200 cm−1 after ethanol adsorption on Au/CeO2 at 295 K and subsequent heating as indicated inFig. 3. (B) FT-IRspectra 1200–1600 cm−1 after ethanol adsorption on Au/CeO2 at 295 K and subsequent heating as indicated inFig. 3.

adsorption at 295 K (spectra (a) and (b) ofFig. 5A). The in-tensity of the 1916 cm−1 band (bridging CO) decreased withincreasing temperature, and vanished at 565 K. The intensityof the band at ca. 2120 cm−1 (linear CO) initially decreasedwith heating (compare spectra (a) and (d)) then another band,at a slightly higher wavenumber, ca. 2130 cm−1, emergedand increased in intensity up to 673 K. It has been suggestedthat on a reduced CeO2 a 2F5/2 → 2F7/2 electronic transi-tion due to Ce3+ can be seen at 2127 cm−1 [52], and thismay as well be the assignment of the 2130 cm−1 band in thiswork.

3.2.4. Evolution of adsorbed species with surfacetemperature

The intensities of the IR bands due to adsorbed ethox-ide, carbon monoxide, and carbonate species with respectto surface temperature is shown inFig. 6. Heating theethanol-adsorbed surface to 360 K, resulted in a signif-icant increase in intensity of mono-dentate (1040 cm−1)(not shown) and bi-dentate (1109 cm−1) ethoxides. Thisincrease was likely due to dehydrogenation of physisorbedethanol molecules on the surface from initial adsorption.Further heating of the surface caused a decrease of the in-tensity of both bands. The sharp decrease of the ethoxide

bands after 500 K is concomitant with a sudden increase ofcarbonates. It is also worth noting that bridging CO alsosharply decreased by this temperature. By 615 K, the surfacewas mainly covered with carbonates (slightly decreased at673 K).

3.3. Temperature programmed desorption (TPD)

TPD following ethanol adsorption at 305 K is shown inFig. 7. The desorption products are quantified as shown inTable 3.

Three types of reactions are observed: decomposition(main), dehydration and coupling (minor).

Unreacted ethanol (CH3CH2OH, m/z = 31, 29, 46) des-orbed in three temperatures centered at 365, 420, and 485 Kand accounted for nearly 55% of the total carbon yields. Aco-desorption of m/z 44 with 29 was clear at 485 K. Aftersubtracting the ethanol fragment from m/z 29, these wereclearly assigned to acetaldehyde. Acetaldehyde molecules,produced by dehydrogenation of ethanol via selective C–Hbond scission of surface ethoxides, accounted for nearly 17%of the carbon selectivity. Evidence of decomposition prod-ucts was seen by the desorption of CO, CO2 and CH4. C–Cbond dissociation of surface ethoxides result in the formation


Fig. 5. (A) FT-IR spectra 1800–2400 cm−1 after ethanol adsorption on Au/CeO2 at (a) 295 K and subsequent heating as indicated inFig. 3. (B) FT-IRspectra 2800–3000 cm−1 after ethanol adsorption on Au/CeO2 at (a) 295 K and subsequent heating as indicated inFig. 3.

of CH4 and CO (CO was seen at temperature as low as 300 Kby IR, Fig. 5A). Methane desorption accounted for nearly11% of carbon selectivity. Most of CO was oxidized to CO2before desorption (CO2/CO ≈ 25). This is in large contrastto a ratio of 0.7 observed in the absence of Au[32].

Hydrogen desorbed in three temperature-domains, at 365(very small), 540, and 685 K. The 540 K desorption is as-sociated with CO, CO2 and H2O desorption and is proba-bly due to a total decomposition process. The 685 K-peakis most likely due to water gas shift reaction (mainly CO2

Fig. 6. IR absorption intensities of main detected species formed uponethanol adsorption/reaction on Au/CeO2 as a function of temperature.

and H2). Water (H2O, m/z = 18) desorption was identifiedin four temperature domains, 385, 450, 515, and 550 K witha high relative yield. The absence of the dehydration prod-uct (ethylene,m/z = 27, 26 or ethane,m/z = 30) indicatesthat most of water desorption might be due to non-selectivedecomposition (H2 oxidation).

Small amounts of coupling products were seen. Furandesorbs at 485 K (same temperature as acetaldehyde) and ismost likely formed from the latter. Furan is the main productformed from acetaldehyde on UO3 surfaces[53]. C4 andC6 hydrocarbons are coupling reactions on oxygen defectedsites and are similar to those previously observed on reducedCeO2 [27] and reduced TiO2 [22]. Small amounts of acetonewere also observed.

3.4. Steady state reactions

Figs. 8A and Bshow the product concentrations as afunction of temperature for a feed stream of 1:1.5 ethanolto molecular oxygen ratio (C2H5OH:O2) over Au/CeO2(changing the ratio to 1:2 caused only minor modifications).The main hydrocarbon products observed were CH3CHO,CH3C(O)CH3, and (CH4) (Fig. 8A). At 473–573 K, aconsiderable conversion of ethanol is seen, mainly to


Fig. 7. TPD profile of ethanol adsorption on Au/CeO2 at 305 K.

acetaldehyde and methane. At 673 K, increasing amountsof methane and acetone were seen at the expense of ethanoland acetaldehyde. By 973 K, methane has become the dom-inant species, while the other species have vanished, andthis trend continued on to 1073 K. H2, CO, and CO2 pro-files as a function of temperature are displayed inFig. 8B.

Table 3Relative yield, carbon yield and selectivity of products desorbing during ethanol–TPD on Au/CeO2

Product Desorption temperature (K) Relative yield % carbon yield % carbon selectivity

Ethanol 365, 420, 485 1.0 54.5 –

DecompositionH2 365, 540, 685 0.008 – –H2O 385, 450, 515, 555 1.162 – –CH4 365, 430 0.179 4.9 10.7CO 405, 525–555 0.042 1.2 2.5CO2 700–750 1.059 28.9 63.5

Total 35 76.7

DehydrogenationAcetaldehyde 485 0.138 7.5 16.6

CouplingButadiene 510 0.018 2.00 4.4Butene 520 0.001 0.1 0.3Acetone 645 0.001 0.1 0.2Furan 485 0.004 0.5 1.0Benzene 365, 495 0.002 0.3 0.7

Total 3.0 6.6

Table 4Mole percent yield of products from ethanol reaction over Au/CeO2 atthe indicated temperatures

473 K 573 K 673 K 773 K 873 K 973 K 1073 K

CH3CHO 56.4 25.5 9.9 6.5 5.0 0.1 0.1CH4 7.5 5.7 18.5 7.3 8.8 24.8 24.7CH3C(O)CH3 – 3.2 6.7 37.0 26.8 0.8 –CO – 8.3 10.2 16.8 22.0 31.1 31.3CO2 31.9 55.5 52.3 27.9 32.5 34.1 35.0H2 4.1 1.9 2.4 4.7 4.8 9.1 8.9Conversion 31.8 70.8 76.6 77.4 81.9 98.5 99.1

Trace amounts of hydrogen were observed at initial heatingat 373 K, with no sign of CO and CO2 (this is most likelyassociated with dehydrogenation of ethanol to acetaldehydeand is also seen during TPD). Further heating resulted inan increase in production of all three products. CO2 pro-duction showed a significant increase in concentration from473 to 673 K, followed by a sharp drop at 773 K. H2 andCO showed stable production from 973 to 1073 K. Ethanolconversion rates and mole percent yield of products aregiven inTable 4.

4. Discussion

The discussion will focus on the following points. (i)First, comparing Au/CeO2 to CeO2. (ii) Second, ComparingAu/CeO2 to the other metals deposited on CeO2. (iii) Third,an attempt to understand the formation of H2 from ethanolfrom the observed experimental data. Points (i) and (ii) willbe discussed together.

Comparison of the results for Au/CeO2 with those forCeO2 indicates that the presence of Au even with such asmall ratio (1 Au to 166 Ce atoms as computed from XPS


Fig. 8. (A) Plot of concentrations of (�) ethanol, (�) acetaldehyde, (�) methane, and (�) acetone as a function of temperature for ethanol/O2 reactionover Au/CeO2. (B) Plot of concentrations of H2, CO, and CO2 as a function of temperature for ethanol/O2 reaction over Au/CeO2.

Ce3d and Au4f corrected peak area ratios) has a considerableeffect on the surface reaction.

Among the clearest effects are

(a) the absence of surface acetaldehyde (contrasted toRh/CeO2 [37] and Pd/CeO2 [32] and

(b) the absence of surface acetates contrasted to CeO2 [32].Ethanol[32] or acetaldehyde[27] gave mainly acetateson CeO2.

The absence of adsorbed acetaldehyde indicates that theenergy barrier for the formation of acetaldehyde from ethox-ide species (stable surface species) is higher than the adsorp-tion energy of acetaldehyde (absence of vibrational bands ofacetaldehyde, IR): simply as soon as acetaldehyde is formedit desorbs (compareFig. 7 to see for acetaldehyde forma-tion to Fig. 3 where C=O of aldehyde (in a�1-C, O mode)should give a band at about 1700 cm−1). The absence ofacetates (by IR) indicates that the effect of Au on the re-duction of the surface of CeO2 (XPS Ce3d,Fig. 1) is sim-ilar to that of the other noble metals: suppression of theoxidation route of acetaldehyde with surface oxygen. Theconsiderable amount of acetone (formed from acetates) un-der steady state conditions is not contradictory to the IRdata. IR experiments are stoichiometric and conducted invacuum (ca. 10−3 Pa) while the steady state reactions arecatalytic and conducted in a flow of O2. Thus, the rela-tively high concentration of acetone formed (in the steadystate oxidation of ethanol) is due to the favored oxidation

route of acetaldehyde to acetates, the latter being coupled toacetone.

Steady state reactions show the desorption of consider-able amounts of CH4 in two temperature domains. The firstdesorption is clearly due to the total decomposition of ac-etaldehyde to CH4 and CO. Yet as seen inFig. 8B most ofthe carbon in this temperature domain is desorbing as CO2.In other words, the oxidation of CO to CO2 can naturally belinked to the high capacity of Au particles to oxidize CO toCO2. [Water gas shift reaction, WGSR, may as well occurand Au/CeO2 is an efficient catalyst for water gas shift reac-tion [54]]. We have not analyzed for water so we cannot di-rectly investigate the effect of WGSR. However, inspectionof the H2, CO and CO2 profiles (as well as the mass balanceand equilibrium constants,Table 5) may help in tracking theextent of the reaction, and by difference account for water(see below for more analyses)].

4.1. H2 production

It appears that there are two regimes for H2 production.One up to 700 K and the other starts to peak by 900 K. Thereare also two regimes for methane. The first peaks at 673 Kwhile the second by 900 K. The first methane peak is at thesame temperature as that of CO2. A simple reaction mech-anism for the first regime can then be depicted inScheme 1as follows:

H2 desorbing in this regime (the rising slope up to ca.773 K (Fig. 8B) is mainly due to the oxidative dehydrogena-


Table 5Products weight in g/ml at the indicated temperatures

473 K 573 K 673 K 773 K 873 K 973 K 1073 K

Ethanol (×105) 7.92 3.39 2.71 2.63 2.11 0.167 0.10Acetaldehyde (×105) 1.05 1.89 1.09 0.78 0.65 0.01 0.008Methane (×105) 0.10 0.30 1.48 0.63 0.83 2.23 2.24Acetone (×105) 0.00 0.21 0.65 3.90 3.05 0.08 0.00

Total (1) (105) 9.07 5.79 5.93 7.93 6.64 2.49 2.35H2 (×106) 0.10 0.19 0.36 0.76 0.850 1.54 1.52CO (×105) 0.00 0.785 1.43 2.56 3.63 4.90 4.98CO2 (×105) 1.18 8.22 11.5 6.68 8.42 8.44 8.74

Total (2) (×105) 1.19 9.02 1.30 9.32 1.21 1.35 1.39(1) + (2) (×104) 1.03 1.48 1.89 1.72 1.88 1.60 1.62Inlet (×104) 1.97 1.97 1.97 1.97 1.97 1.97 1.97Difference (×104) 0.94 0.49 0.08 0.25 0.09 0.37 0.35Kp 210 38.8 11.7 4.9 2.55 1.54 1.04H2Oa (×104) 0.11 0.16 0.11 0.11 0.15 0.15

Kp = pH2pCO2/pCOpH2O, values ofKp at different temperatures are taken from[55].a Calculated frompH2, pCO, pCO2 and Kp.

tion character of Au/CeO2 as well as to WGSR. In the abovescheme, one would expect equal amounts of H2, (CO+CO2)and CH4 in this regime. At 673 K (fromFig. 8B), the ratiois close to 3 (CO+ CO2), 1 CH4 and 0.2 for H2. Thus, withthe assumption that a large amount of H2 has been oxidizedto H2O, large amounts of water have reacted with methaneto give CO and most of the CO was oxidized to CO2.

The second regime involves the formation of acetone fromacetates (Eq. (3a)), with CO2 and water as side products(moles of acetone= moles of CO2). The sudden rise ofCO2 above 700 K can be attributed to this reaction path-way. Above 900 K, the reaction of acetate species to acetonemolecules seem not favored any more and most of the reac-tion is probably due to acetaldehyde decomposition to CH4and CO (note the positive slope of CO at high temperatures).H2 production is now becoming important but is most likelyformed by reforming of CH4.

Scheme 2.The high concentration of CO together with the large dif-

ference in mass balance above 973 K (attributed to water) isa further indication that WGSR is no longer effective (ther-modynamically not favored at high temperatures). This canbe seen from the values ofKp of WGSR (Table 5). Com-paring the expected amount of water from WGSR (last rowin Table 5) to that calculated from the mass balance (differ-ence,Table 5) indicates that there is a reasonable agreementat high temperatures (from 673 K and above) but that at lowtemperatures (below 673 K) most of water formation is dueto direct oxidation of H2.

Scheme 1. Formation of H2, CH4, CO and CO2 from ethanol oxidationover Au/CeO2 up to 773 K (fromFig. 8).

Scheme 2. Formation of H2, acetone, CH4, CO and CO2 from ethanoloxidation over Au/CeO2 in the 773–1073 K temperature domain (fromFig. 8); [O] denotes oxygen from the surface and does not account forstoichiometry.

5. Conclusions

Au/CeO2 is an efficient catalyst for ethanol oxidation.Although its activity is comparable to other M/CeO2 cat-alysts, it has one main advantage: high concentrations ofCO2. However, the CO2/CO decreases with increasing re-action temperature and indicates the limitation of Au/CeO2for an efficient H2 production from ethanol (with low CO).Two routes for making hydrogen from ethanol are observed.The first (minor) is due to dehydrogenation of ethanol toacetaldehyde (detection of hydrogen without CO and CO2at 373 K and without CO at 473 K). The second (major) ismore complex and appears tracking the reforming/oxidationof methane (where methane is formed from acetaldehyde).

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the reactions of ethanol over au/ceo2

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