Adsorption and Reaction of Acetaldehyde over CeOX(111) Thin Films

Feb 7, 2011 - The TPDs occurred in a “line-of-sight” geometry with the sample face ca. .... the methyl and carbonyl peaks are nearly gone, the pea...
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Adsorption and Reaction of Acetaldehyde over CeOX(111) Thin Films T.-L. Chen and D. R. Mullins* Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6201, United States ABSTRACT: This study reports the interaction of acetaldehyde with well-ordered CeOX(111) thin film surfaces. The fully oxidized CeO2(111) surface shows a weak interaction with acetaldehyde with the sole desorption product (TPD) being the parent molecule at 210 K. The chemisorbed molecule binds to the surface as the η1-acetaldehyde species rather than through a bridge-bonded dioxy configuration. Acetaldehyde chemisorbs strongly on reduced CeO2-X(111) with nonrecombinative and recombinative acetaldehyde desorbing at 405 and 550-600 K, respectively. Deoxygenation and dehydration also occur, producing ethylene and acetylene at 580 and 620 K, respectively. Acetaldehyde initially adsorbs in the η1 configuration and then converts to a carbanion species with both CdC and CdO bond character above 300 K.

1. INTRODUCTION The adsorption and reaction of various oxygenated hydrocarbons on cerium oxide surfaces are of interest from the perspective of understanding the catalytic properties of the material. Studies on the interactions of alcohols,1-5 formaldehyde,6,7 acetone,8 and carboxylic acids9,10 on well-defined CeO2 surfaces have been primarily motivated by the rich chemistry produced by the variation in the Ce oxidation state and the associated O vacancies. Alcohols and carboxylic acids typically adsorb on oxide surfaces through deprotonation of the molecule, forming adsorbed alkoxy or carboxylate, respectively.11 The alcohols then undergo either dehydrogenation to form an aldehyde or dehydration to produce an alkene. On a reducible oxide, such as CeO2(111), dehydrogenation may lead to H2O desorption by abstracting O from the substrate and dehydration can lead to H2 desorption via oxidation of the substrate.1,2 In a recent study of C2-C3 alcohols on fully oxidized CeO2(111), it was also observed that primary alcohols, such as ethanol and 1-propanol, produce a mixture of aldehydes and alkenes, whereas the secondary alcohol, 2-propanol, produced almost exclusively propene.2 Carboxylic acids also undergo dehydration and dehydrogenation with formic acid, producing CO and CO2, respectively.9-11 Longer-chain carboxylic acids can also undergo ketonization, with acetic acid producing acetone, for example.10,11 Aldehydes and ketones, as an intermediate class of oxygenates between alcohols and carboxylic acids with respect to their degree of oxidation, play an important role in bridging the catalytic chemistry between the two. A previous study6 of the simplest aldehyde, formaldehyde, provided a good comparison to the chemistry of the other C1-oxygenates: methanol and formic acid. However, there is a need to address the chemistry of how the larger aldehydes interact with various oxidation states of the cerium oxide surface as it has been demonstrated that the r 2011 American Chemical Society

reactions of C2 and C3 alcohols differ significantly from those of methanol.2 The adsorption and reaction of acetaldehyde has been investigated on polycrystalline CeO2 powders by Idriss et al.12 On partially reduced ceria, the acetaldehyde is reduced to ethoxide and oxidized to acetate upon adsorption at room temperature. The ethoxide reacts to produce acetaldehyde, crotonaldehyde, crotyl alcohol, and ethanol near 390 K. Nearly 80% of the chemisorbed acetaldehyde reacts and desorbs at 390 K with the majority of the product leaving as recombined acetaldehyde. The acetate decomposes above 600 K to produce CO, CO2, and CH4 as well as small amounts of coupling products, such as acetone and butadiene. Our previous report on the reaction of acetone provides part of the information in characterizing the reaction of the carbonylfunctionalized oxygenates on highly ordered CeOX(111).8 On fully oxidized CeO2(111), acetone is unreactive and only weakly chemisorbs on the surface, and on a reduced surface, it binds strongly as a carbanion species. We might anticipate some similarity in the reaction mechanisms of acetone and acetaldehyde due to the carbonyl component and some differences because acetaldehyde has only one methyl group instead of two. There have been other studies on acetaldehyde reactions on several single-crystal metal oxide surfaces, including ZnO(0001)-Zn,13,14TiO2(001),15-17 SrTiO3(100),18 and UO2(111).19 TiO2(001) is one of the most studied model systems, where the reduction to ethanol has been observed on both reduced and fully oxidized surfaces.15 Modification of the surface has shown two other reactions, aldol condensation and reductive carbonyl coupling, of which the selectivity is highly dependent on the oxidation state of the surface.15-17 In the study of UO2(111), a small amount of benzene production was observed on the Received: November 1, 2010 Revised: January 11, 2011 Published: February 07, 2011 3385

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The Journal of Physical Chemistry C stoichiometric surface.19 On the H2-reduced surface, dehydrogenation to ketene (CHdCdO) and aldolization to crotonaldehyde (CH3CHdCHCHO) were also observed in addition to benzene formation. Surfaces reduced by electron and ion sputtering increased the selectivity toward both ketene formation and reduction to ethanol. Formation of the carboxylate species was seen in the case of ZnO(0001)-Zn,13,14 where acetaldehyde underwent nucleophilic attack by the lattice oxygen on the surface,20,21 followed by either the hydride or the alkyl elimination to form the acetate and formate species, respectively. In this study, we have observed that acetaldehyde has a reaction mechanism similar to acetone on both oxidized and reduced CeOX(111) thin films. It chemisorbs weakly on oxidized CeO2(111) but binds strongly as a carbanion on the reduced CeO2-X(111). The carbanion species can go through either a deoxygenation or a dehydration process, which lead to the production of alkenes or alkynes, respectively. We also re-examined the decomposition of acetone on reduced CeO2-X(111) and discovered a previously unobserved reaction pathway leading to alkyne formation.

2. EXPERIMENTAL SECTION The experiments were performed in two different UHV chambers. Soft X-ray photoelectron spectroscopy (sXPS) and near-edge X-ray absorption fine structure (NEXAFS) were performed in a chamber at the National Synchrotron Light Source on beamline U12A. C 1s sXPS spectra were recorded using 400 eV excitation, and the instrumental resolution was ca. 0.5 eV. The Ce 4d photoemission was used for binding energy calibration using the 122.3 eV satellite feature.1 NEXAFS was performed at the C K-edge. The energy resolution was less than 0.5 eV, and the photon energy was calibrated using the dip in the photon flux at 284.7 eV.22 The X-ray absorption was recorded using a partial yield electron detector. The high-pass retarding grid was set at -230 V. Higher-order X-ray excitation created apparent absorption features from the ceria substrate due to the O K-edge and the Ce MIV and MV edges. The absorption due to only higher-order radiation was determined by recording spectra with a retarding grid voltage of -307 V, that is, greater than the first-order photon energy. The background resulting from the higher-order excitation was subtracted from the NEXAFS spectra. The temperature-programmed desorption (TPD) experiments were performed in a chamber at ORNL. The temperature was ramped at 2 K/s, and the sample was biased -70 V to prevent electrons generated by the mass spectrometer ionizer from stimulating reactions at the surface. A Hiden HAL/3F 301 mass spectrometer was used in these experiments. The TPDs occurred in a “line-of-sight” geometry with the sample face ca. 2 cm from the mass spectrometer aperture. CeO2(111) films were grown in situ on Ru(0001) as has been described previously.23 Briefly, CeO2(111) was produced by depositing Ce metal in an 16O2 ambient of 2  10-7 Torr while the Ru was at 700 K. After deposition, the sample was annealed to 900 K. The ceria films were estimated to be ca. 5 nm thick based on the attenuation of the Ru AES or Ru 3d XPS intensities. This thickness is believed to minimize the influence of the Ru substrate on the chemical properties of the ceria film.24 In general, the films used in the sXPS and NEXAFS experiments may have been slightly thicker than those used in the TPD experiments. Reduced cerium oxide films were produced by

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Figure 1. Ce 3d XPS spectra from oxidized CeO2(111) and 60% reduced CeO1.70(111). A Shirley background has been subtracted from the spectra, and they have been normalized to the same total integrated intensity. The features labeled on the spectrum from the oxidized surface are associated with Ce4þ, whereas those labeled on the reduced surface are associated with Ce3þ40,41.

exposing a CeO2(111) film to methanol (>100 L) at 700 K and then annealing to 900 K.1 A new ceria film was produced before each series of experiments. An old film was completely removed by Ar ion sputtering, as indicated by Ce XPS or AES. A fully oxidized film was then grown, and acetaldehyde was adsorbed for TPD, XPS, or NEXAFS. The film was then reduced by methanol, and acetaldehyde was adsorbed to perform the same experiment on a reduced surface. The acetaldehyde (Alfa Aesar, 98.5%) was freeze/pumped in liquid nitrogen to remove air. At ORNL, the ceria film was exposed to acetaldehyde through an effusive gas doser with an estimated flux of 1  1014 molecules cm-2 s-1.25,26 Although the mass spectrometer was not differentially pumped, the use of the effusive gas doser largely restricted adsorption to the face of the sample and minimized the background signal during TPD. A 60 s exposure was sufficient to saturate the first monolayer. At beamline U12A, the adsorbate was introduced to the UHV chamber by way of a variable leak valve using a “backfilling” method.

3. RESULTS 3.1. TPD. Acetaldehyde was deposited on oxidized and reduced CeOX(111). The ceria surface was initially fully oxidized and then was reduced by methanol to 60% Ce3þ. The Ce 3d XPS spectra for these surfaces are shown in Figure 1. In principle, the degree of reduction could also be determined by monitoring the O intensity by AES, XPS, or sXPS. The O intensity should decrease by 25% between CeO2 and Ce2O3. To obtain a meaningful result, instrumental and structural influences on the intensity would need to be considered. We have not conducted a systematic study of the O intensity as a function of the degree of reduction. Desorption products following acetaldehyde exposure at 175 K are shown in Figure 2. Exposures conducted at 100 K indicated that the second layer of acetaldehyde desorbed at 120 K and the multilayer desorbed at 110 K (not shown). On fully oxidized CeO2(111) (black lines in Figure 2), only the parent molecule was observed. Mass 29 is the most intense fragment, but intensity was also seen in the 26, 43, and 44 amu fragments. The relative 3386

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Figure 2. TPD spectra from acetaldehyde adsorbed at 175 K on fully oxidized CeO2(111) (solid black line) and 60% reduced CeO1.70(111) (dotted black line).

intensities of the various fragments match those seen in the mass spectrum for acetaldehyde in our system. The chemisorbed acetaldehyde desorbed at 210 K with a tail that extends beyond 300 K. We note that, if the sample was not biased to prevent exposure to electrons from the mass spectrometer, a significant amount of ketene, CH2CO, was observed during the TPD from the oxidized surface. When the ceria is reduced to 60% Ce3þ (dotted lines in Figure 2), the chemisorbed acetaldehyde peak at 210 K virtually disappears. New peaks appeared at 405 K and a doublet between 550 and 600 K in the Mass 29 desorption. These features also occurred at Mass 43 and Mass 44, so they are assigned to acetaldehyde. No products were observed at either Mass 30 or Mass 31, indicating that formaldehyde, methanol, and ethanol were not produced. Other reaction products were observed on the reduced surface. H2 desorbs in a broad peak between 500 and 700 K. Ethylene desorbed at 580 K, as seen in the Mass 27 desorption. Mass 26 was monitored to determine if acetylene desorbed in addition to ethylene. In Figure 2, the ethylene contribution to Mass 26 was subtracted using the desorption at Mass 27. The resulting acetylene desorption occurred at 620 K, which is a higher temperature than the ethylene desorption. Desorption at Mass 28 (data not shown) could be completely attributed to ethylene; therefore, no CO was produced. To provide a semiquantitative estimate of the relative yield of acetaldehyde, ethylene, and acetylene, the spectra in Figure 2 have been multiplied by scale factors using a method suggested by Ko et al.27 These scale factors are based on the fragmentation and ionization probabilities of the molecules and the mass dependent response of the mass spectrometer. Applying these scale factors and integrating under the desorption peaks in Figure 2, ca. 40% of the acetaldehyde desorbs in the peaks at 405 K and between 550 and 600 K, 45% reacts to produce ethylene, and 15% reacts to produce acetylene. Idriss et al have reported coupling reactions resulting in crotonaldehyde and crotyl alcohol on faceted TiO2(001)15,16

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Figure 3. TPD of Mass 40 and Mass 41 produced from acetone adsorbed at 140 K on 70% reduced CeO1.65(111).

and 2-butene on sputtered TiO2(001).15,17 We screened for these products by monitoring Mass 41, which is a major fragment of the coupling products, but only a minor fragment (ca. 5% of Mass 29) of acetaldehyde. All of the desorption observed at Mass 41 can be attributed to acetaldehyde; therefore, we conclude that coupling products were not produced. The production of deoxygenation products from acetaldehyde and, in particular, the dehydration product, acetylene, led us to re-examine the reaction of acetone on reduced CeOX(111). The formation of a trace amount of propene was suggested in our previous investigation,8 but we did not look for the dehydration product propyne (CH3CCH). Figure 3 shows the desorption at Mass 40 and Mass 41 from acetone adsorbed on CeO1.65(111) at 175 K. Mass 41 is indicative of propene desorption, whereas Mass 40 is primarily propyne with a smaller contribution from propene (less than half the intensity of the propene fragment at Mass 41). A small amount of propene was detected near 560 K, whereas considerably more propyne desorbed near 610 K. Therefore, unlike acetaldehyde, alkyne formation is heavily favored over alkene formation when acetone decomposes on reduced CeOX(111). 3.2. sXPS. C 1s sXPS spectra were recorded after CeO2(111) was exposed to acetaldehyde. The spectrum from a multilayer of acetaldehyde produced from a 20 L (1 L = 10-6 Torr s) exposure at 95 K is shown at the top of Figure 4. Two peaks of equal intensity at 286.8 and 289.5 eV are assigned to the methyl and carbonyl carbon atoms, respectively.1,2,8 Chemisorbed species were analyzed by exposing the surface to 2 L of acetaldehyde at 120 K and then annealing the sample to progressively greater temperatures. After heating to 175 K, most of the physisorbed species have desorbed, leaving primarily two features at ca. 286.0 and 288.5 eV. These peak positions are in good agreement with those observed for acetaldehyde on TiO2(001) at 170 K.16 The asymmetry of these features indicates that some of the physisorbed acetaldehyde may still have been present. After annealing to 200 K, the two main peaks became sharper; however, an additional peak is clearly evident near 290 eV. When the sample was annealed to 300 K, the methyl and carbonyl peaks are nearly gone, the peak at 290.2 eV increases in intensity, and a new feature is apparent at 286.7 eV. These features disappeared between 500 and 600 K. 3387

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Figure 4. C 1s sXPS spectra following adsorption of acetaldehyde on CeO2(111) at 95 K and then annealed as indicated. The gray line was recorded in a different place on the surface that had not been previously exposed to X-rays before the sample was annealed to 300 K.

Figure 5. (A) C 1s sXPS spectra of acetaldehyde adsorbed on CeO2(111) at 95 K and annealed to 200 K (black line) and 400 K (gray line). The 400 K spectrum has been scaled by 70% so that the intensities of the two spectra at ca. 290.5 eV are the same. (B) The difference spectrum between the two spectra in (A).

Because the TPD experiment showed no appreciable desorption above 300 K from an oxidized surface (Figure 2), the C 1s features observed at 300 K may have been from X-ray beam damage, as was previously observed for acetone8 and formaldehyde.6 Following the spectrum that was recorded after the sample was annealed to 300 K, the sample was moved so that an area that had not been previously exposed to X-rays was analyzed. The resulting C 1s spectrum is indicated by the gray curve in Figure 4. The peak at 290.2 eV is significantly smaller, whereas peaks at lower binding energy have changed their relative intensities. The intensity near 286.7 eV is smaller, whereas the intensity near 285.8 eV is bigger. This indicates that the peaks at 290.2 and 286.7 eV resulted from beam damage. The spectrum of the undamaged chemisorbed acetaldehyde can be obtained by subtracting the spectrum of the beamdamaged components observed at higher temperatures from the spectrum at 200 K that contains both damaged and undamaged components. Figure 5A shows the spectrum after annealing to 400 K (gray curve) scaled by 70% and superimposed on the one obtained after annealing to 200 K (black curve). In Figure 5B, the difference spectrum between the two is shown. The two features at 288.4 and 285.9 eV are associated with an acetaldehyde-like species chemisorbed on the ceria. As in the acetaldehyde multilayer, the higher binding energy peak is associated with the carbonyl end of the molecule and the lower binding energy peak is associated with the methyl end. Both peaks are shifted toward lower binding energy relative to the multilayer. The carbonyl is shifted by 1.6 eV, whereas the methyl group is shifted by 0.9 eV. The greater shift at the carbonyl end indicates that this part of the molecule is affected more by chemisorption and suggests bonding to the surface through the carbonyl. A CeO2(111) film was exposed to more than 100 L of methanol while the temperature was cycled between 600 and 900 K. The sample was reduced to ca. 70% Ce3þ (CeO1.65) based on the Ce 4d spectrum. The reduced ceria was exposed to a 2 L

dose of acetaldehyde at 100 K. The C 1s spectra were recorded after the sample was heated to sequentially higher temperatures. Upon heating to 175 K, there are two prominent peaks at 286.5 and 289.1 eV (Figure 6). These peaks shift to higher binding energy by ca. 0.4 eV when the sample is annealed to 300 K. These peaks are again assigned to the methyl and carbonyl ends of the chemisorbed acetaldehyde. The peak positions are shifted by ca. 1 eV to greater binding energy compared to the chemisorbed species on the oxidized surface (Figure 5B). This shift to higher binding energy for nominally the same species adsorbed on a reduced surface is consistent with what has been observed for many other molecules chemisorbed on oxidized and reduced CeOX(111).1,2,6,8 After recording the 300 K spectrum, the sample was tested for X-ray beam damage by shifting to a different position. The spectrum obtained from the new position (Figure 6, gray line) was virtually identical to the original spectrum, indicating little beam damage. After the sample was annealed to 400 K, the total C 1s intensity decreased and the peaks shifted to 286.2 and 288.1 eV. The decrease in C on the surface is consistent with the desorption of acetaldehyde that occurred between 300 and 400 K on the reduced surface (Figure 2, dotted line). Annealing to 500 K, which presumably removes more acetaldehyde, results in an additional shift of ca. 0.2 eV to lower binding energy. The shift in binding energy and the decrease in peak separation between 300 and 500 K suggest that a different chemisorbed species was present on the surface at these two temperatures. When the sample was annealed to 600 K, most of the C 1s intensity disappears. This coincides with the desorption of acetaldehyde, acetylene, and ethylene. Note that the spectrum recorded after annealing to 600 K on reduced ceria looks very similar to the non-beam-damaged spectrum recorded at 300 K on oxidized ceria (Figure 4, gray line). This suggests that the species that persisted to higher temperatures on oxidized ceria was adsorbed on a site that was also on reduced ceria, for example, isolated O-vacancy sites. 3388

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Figure 6. C 1s sXPS spectra following adsorption of acetaldehyde on reduced CeO1.65(111) at 100 K and then annealed as indicated. The gray line was recorded in a different place on the surface that had not been previously exposed to X-rays before the sample was annealed to 300 K.

After annealing acetaldehyde on reduced ceria to 700 K, a peak persisted at 285.6 eV. This peak persisted to 800 K and is assigned to a CHX fragment bound to Ce. It presumably reacts with lattice O and desorbs as CO at higher temperatures, but a desorption peak was not evident in the CO TPD due to a rising background that occurred as the sample leads get very hot. 3.3. NEXAFS. X-ray absorption spectroscopy at the C K-edge (NEXAFS) was used to further identify the adsorbed species on the oxidized and reduced CeOX(111) surfaces. Resonances in the region before the absorption edge/ionization potential are indicative of bonding structures, such as alkenes (CdC), alkynes (CtC), carbonyls (CdO), and carboxylates (COO).22 Figure 7 shows acetaldehyde adsorbed on fully oxidized CeO2(111). The most intense spectrum is from an unannealed multilayer (20 L) adsorbed at 95 K. Three distinct features are evident at 286, 288.2, and 291 eV. The ionization potentials for gas-phase acetaldehyde are at 291.4 eV (CH3) and 294 eV (CdO).28 The first two resonances are assigned to π* (CdO) and π* (CH3) and are in the same positions as were observed in the gas phase.28 The nature of the feature near 291 eV is less clear. In the gas phase, it was assigned to a higher-lying π* (CH3) state or to σ* (CO).28 When the acetaldehyde was annealed to 175 K to desorb the multilayer, the NEXAFS spectrum remained largely the same as for the multilayer except that the intensity decreased significantly. Note that the π* (CdO) resonance remained at 286 eV. This indicates that chemisorbed acetaldehyde retained the CdO bond and bonds as η1-acetaldehyde.29 Although the NEXAFS indicates that the η1 species is present, it does not exclude the possibility that the bridge-bonded dioxy/η2 species observed in formaldehyde6 might also be present. The π* (CH3) at 288 eV appears to be relatively more intense than in the multilayer. However, the C 1s sXPS spectrum (Figure 6) indicates that carboxylate may also have been present. The π* (COO) resonance also occurs near 288 eV and may have contributed to the intensity at this energy at 175 K.28 The presence of carboxylate is

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Figure 7. C K-edge NEXAFS spectra of acetaldehyde on CeO2(111) recorded at normal incidence. The top spectrum is from a multilayer adsorbed and recorded at 95 K. The bottom spectra were from a lower exposure (2 L) adsorbed at 95 K and annealed as indicated.

Figure 8. C K-edge NEXAFS spectra recorded at normal incidence from 2 L of acetaldehyde adsorbed on CeO1.7(111) at 95 K and annealed as indicated.

clearly evident at 300 K where the 288 eV peak is very intense relative to the overall C absorption above 290 eV, and the C 1s sXPS indicated that mostly X-ray-induced species remain on the surface. Also note that, at 300 K, the π* (CdO) resonance has virtually disappeared. Figure 8 shows the NEXAFS spectra from acetaldehyde adsorbed on a ceria surface that was reduced to 60% Ce3þ (CeO1.7(111)). After annealing to 175 K, the spectrum retains many of the features observed for acetaldehyde adsorbed on CeO2(111). In particular, the peaks at 286, 288, and 290.4 eV are observed. However, these peaks are broader and have less separation between them compared with what was seen on the oxidized surface. There is also a new feature near 284.6 eV. This is tentatively assigned to an alkene bond (π* (CdC)).22 3389

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The Journal of Physical Chemistry C When the sample is annealed to 300 K, there is little change in the NEXAFS spectrum (note that the spectra have been offset) except the resolution on the various peaks may be slightly better. This reflects what was observed in the C 1s sXPS where there was also little change between 175 and 300 K and small changes might reflect the desorption of weakly bound acetaldehyde. At 450 K, the peaks at 284.7, 286.5, and 288.3 eV became much sharper and more intense relative to the total C absorption above 295 eV. Although there are some slight shifts in the peak positions, the first two peaks are still indicative of CdC and CdO. The peak at 288.3 eV may still have some contribution from COO, but the sXPS (Figure 6) indicates that the carboxylate at 290 eV was weak or absent in this temperature range. Therefore, the 288.3 eV peak is assigned primarily to π* (CHX). Another major change in the spectrum is the disappearance of the peak between 290 and 291 eV. Because the identity of this peak is unclear, we speculate that the absence of this peak is related to a major change in the C-O or C-C bonding. After annealing to 600 K, the overall intensity was greatly reduced but the same three pre-edge peaks are still evident.

4. DISCUSSION Acetaldehyde does not dissociate on a fully oxidized CeO2(111) surface. The adsorption model is shown in Scheme 1. It adsorbs at low temperatures without breaking the carbonyl bond, as indicated by the π* (CdO) NEXAFS absorption resonance in the chemisorbed species (Figure 7). The adsorption presumably occurs atop the coordinatively unsaturated Ce cation in the second layer. We have previously shown the same kind of adsorption model in the case of acetone where an intact carbonyl bond is also observed.8 Furthermore, both acetone and acetaldehyde produce only low-temperature desorption of the parent molecules after adsorption on the CeO2(111) surface as no further reaction products appear at higher temperatures (Figure 2). The adsorption and reaction of acetaldehyde on reduced CeOx is depicted in Scheme 2. The π* (CdO) resonance is again evident at low temperature in the NEXAFS spectrum as on the oxidized surface, which indicates that most of the acetaldehyde absorbs without breaking the carbonyl bond (Figure 8). In addition to the prominent π* (CdO) and π* (CHX) resonances, the presence of unsaturated CdC bonds is indicated by a π* Scheme 1. Adsorption and Reaction of Acetaldehyde on Oxidized CeO2(111)

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(CdC) resonance. The presence of this feature, in addition to broad C 1s sXPS features at 300 K (Figure 6), suggests the existence of an additional intermediate. Above 400 K, the π* (CdC) resonance becomes more intense, the NEXAFS spectrum becomes better resolved, and the C 1s peaks shift to lower binding energy. These observations suggest that the minority species that was present on the surface at 300 K became the dominant species above 400 K. The NEXAFS spectrum of acetone on CeOX(111) has the same π* (CdO), π* (CHX), and π* (CdC) features as acetaldehyde.8 Because acetaldehyde is structurally similar to acetone, differing by only one methyl group, the similarity in their NEXAFS spectra on the reduced surface suggests a similar bonding structure for the adsorbates. A carbanion intermediate was proposed for acetone.8 This was consistent with a species having both CdO and CdC bonding character and having three distinct C species, as indicated by the C 1s spectra. A similar species is proposed for acetaldehyde as is depicted in Scheme 2 at 400 K. This species contains both the CdO and the CdC bonding character. Unlike acetone, the C 1s spectrum at 400 K (Figure 6) contains only two C species because the acetaldehydederived carbanion lacks the noninteracting methyl group that occurs in acetone. However, the C 1s peak positions are shifted to lower binding energy at 400 K compared with 300 K. This is due to a decrease in the electron transfer from C to O at the carbonyl end and an increase in the electron transfer from Ce to C, compared to H to C, at the alkyl end. This interpretation of the C 1s spectrum is also consistent with the carbanion depicted in Scheme 2. Enolate species have been reported for aldehydes and ketones adsorbed on surfaces11,30,31 or in organometallic complexes.32 These have most often been depicted as pure enolates, that is, possessing a C-O single bond and a CdC double bond. Gutierrez-Sosa et al proposed a carbanion species for acetone on Zn(0001)-Zn; however, the C K-edge NEXAFS spectrum had the π* (CdO) resonance but not the π* (CdC) resonance.33 We could not find any reports of a carbanion/ enolate species with the electron delocalized over the O-C-C bridge. Such a delocalized structure is more common in carboxylates where the O-C-O bridge binds to surfaces11,34-36 and in complexes.37,38 In the organometallic complexes, the distance between the metal centers is 0.355 nm. This is comparable to the 0.383 nm distance between the Ce cations on CeOX(111). The adsorption and reaction of the carbonyl-functionalized oxygenates acetaldehyde and acetone are distinctly different from their alcohol analogs, ethanol and 2-propanol. Whereas the aldehyde and ketone only weakly adsorb on oxidized CeO2(111), the alcohols form intermediates that are stable to above 500 K, where they decompose into aldehydes and alkenes.2 This is likely due to the ability of the alcohols to dissociate upon adsorption into alkoxys and surface hydroxyls. The alkoxys may be further stabilized by a reaction between the hydroxyls at low temperature

Scheme 2. Adsorption and Reaction of Acetaldehyde on Reduced CeOX(111)

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The Journal of Physical Chemistry C that produces water and an O vacancy. The carbonyl on the aldehyde/ketone interacts much more weakly through the lonepair electrons on the O and the under-coordinated Ce cation. Hydroxyls are not formed upon adsorption, so there is no opportunity to create O vacancies on the surface through water desorption. On reduced CeOX(111), the carbonyl moiety bonds strongly to the surface in the pre-existing O vacancy sites. The surface intermediate is stabilized through the interaction of the methyl group with the surface, forming the carbanion. The alcohols show no tendency to form carbanions and produce only alkoxy intermediates even on the reduced surface.2 This may be related to the relative stabilities of a carbanion versus an alkoxy, a structural difference between aldehyde/ketone and the alkoxy that might inhibit the methyl interaction with the surface, or an inability to break a C-H bond on the R-C in the alkoxy, which is necessary to form the carbanion. The carbanion intermediate apparently reacts with surface hydroxyls on reduced CeOX(111). The desorption of acetaldehyde above 500 K requires that the deprotonated methyl groups in the carbanion recombine with surface hydroxyls. In addition, the formation of ethylene requires the addition of H to the R-C when the C-O bond cleaves. It has been previously shown that hydroxyls react to produce H2 on reduced CeOX(111) between 500 and 600 K.1,2,39 The product formation shown by the dotted lines in Figure 2 demonstrate the competition between the reaction of the carbanion intermediate with the surface hydroxyls and the reaction between hydroxyls to desorb as H2. Below 600 K, the hydroxyls are still available on the surface and the H-rich products, acetaldehyde and ethylene, are produced. Above 600 K, the surface is depleted of H so that the H-poor product, acetylene, is observed along with the H2 from the decomposition of the carbanion. There are some broad similarities between the decomposition of acetaldehyde and ethanol2 on CeOX(111). Reduction/oxygen vacancies shift the selectivity of the ethanol reaction toward dehydration as ethylene formation increases relative to acetaldehyde. There are no C1 products produced by either acetaldehyde or ethanol, indicating that the C-C bond remains intact throughout the reaction. The formation of ethylene from both oxygenates is somewhat coincidental, however, as they appear to be produced by different surface intermediates. Ethylene is produced from ethanol through the decomposition of ethoxy. This requires C-O bond cleavage at one end and C-H cleavage at the other. Acetaldehyde proceeds to ethylene through the carbanion intermediate. This requires C-O cleavage accompanied by C-H formation at the same C. The production of propene and propyne during the decomposition of acetone on reduced CeOX(111) (Figure 3) highlights the similarities and differences between acetone and acetaldehyde. Both carbonyl-functionalized reactants produce a carbanion intermediate on reduced CeOX(111).8 However, whereas acetaldehyde produces more ethylene than acetylene, acetone produces almost exclusively propyne. The alkene results from a nucleophilic attack by surface H at the carbonyl. This process appears to be less favorable with the acetonederived intermediate. The selectivity for alkene versus alkyne may also be closely coupled to the availability of H on the surface. It has been shown that surface H recombines and desorbs as H2 between 500 and 600 K.1,2,39 As the temperature increases, the H2 desorption process competes with the alkene formation. For acetaldehyde, the ethylene formation reaction may successfully compete, at least initially, then acetylene is

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formed at higher temperatures where the surface H concentration is lower. The adsorption and reaction of acetaldehyde on other singlecrystal oxide surfaces, ZnO(0001),13,14 UO2(111),19 and TiO2(001),15-17 are significantly different compared with those on CeOX(111). On the Zn-terminated ZnO(0001) surface, acetaldehyde readily oxidized via nucleophilic attack by surface O to produce carboxylate intermediates. Both acetate and formate were observed, resulting from hydride or methyl elimination, respectively. The carboxylate species decomposed, producing CO and CO2. On nonstoichiometric Ar-sputtered UO2-X(111), CO, CO2, and ketene were observed, which also suggests reaction through a carboxylate intermediate.19 Surface carboxylates are not formed following acetaldehyde adsorption on CeO2(111) except as the result of X-ray damage. Alternatively, the {011}-faceted surface of TiO2(001) showed considerable reduction of acetaldehyde to ethanol.16 Idriss et al suggested that this occurred through total decomposition of some of the acetaldehyde to produce C and H on the surface, followed by hydride transfer from the surface to the remaining acetaldehyde to produce ethoxy. Total decomposition of acetaldehyde and ethoxy formation at low temperatures are not indicated on oxidized or reduced CeOX(111), and no ethanol was produced. These oxidative and reductive pathways were also observed for acetaldehyde on reduced CeO2-X powders.12 Acetaldehyde adsorbed at room temperature produced ethoxide and acetate. Most of the ethoxide reoxidized to produce acetaldehyde, and a small amount desorbed as ethanol. The acetate decomposed to produce CO, CO2, and methane. As noted above, ethoxide and acetate were not observed during the acetaldehyde reaction on CeO2-X(111). One obvious difference between the present study and that of Idriss et al is that the surface structure of the powders is not known. It is possible that other ceria faces or the presence of defects not present on CeO2-X(111) may promote different reaction pathways. In addition, the reduced ceria powders had a substantial coverage of hydroxyls, as indicated by O 1s XPS.12 The surfaces used in our study always had a negligible amount of OH on the surface prior to acetaldehyde exposure, as indicated by O 1s sXPS. On both the {011}- and the {114}-faceted surfaces of TiO2(001),15,17 Idriss et al observed aldol condensation leading to crotonaldehyde and crotyl alcohol. Similar products were also observed on reduced CeO2-X powders. Crotonaldehyde and a trace amount of benzene were also observed on nonstoichiometric UO2-X(111).19 It was proposed that aldol condensation on TiO2(001) proceeded through a reaction between acetaldehyde and an enolate (-CH2CHO-). This enolate is the same species we have referred to as a carbanion (see Scheme 2). The C 1s sXPS and especially the NEXAFS indicate that the aldehyde and the carbanion are both present at 300 K on reduced CeO2X(111) (Figures 6 and 8). These species apparently do not react with each other, however, as neither crotonaldehyde nor crotyl alcohol are observed. Acetaldehyde desorbs at 400 K, leaving only the carbanion on the surface. It is not clear why aldol condensation does not occur on CeOX(111). Perhaps there is a spatial restriction that prevents reaction between the adsorbates; that is, the adsorption sites are too far apart. Alternatively, the interaction between the carbanion and the surface may have been stronger than the nucleophilic attraction to the aldehyde. Finally, on highly reduced TiO2(001), produced by Arþ ion sputtering, reductive coupling produced 2-butene.16 The proposed 3391

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The Journal of Physical Chemistry C mechanism involved the reductive coupling of two aldehydes to form a diolate or pinocolate surface species. The scission of both of the C-O bonds and the formation of a CdC bond produced 2-butene. This product is also not observed from CeOX(111). There is also no evidence in the C 1s or NEXAFS of the diolate species. This is further evidence of the inability of adsorbates to react with each other on CeOX(111), perhaps due to spatial limitations.

5. CONCLUSION Acetaldehyde only weakly binds to oxidized CeO2(111) and desorbs at low temperature without further reaction. On the reduced CeO2-X(111) surface, it first chemisorbs in an η1acetaldehyde configuration. Some of this acetaldehyde desorbs near 400 K while the remainder forms a carbanion species through the interaction of the methyl group with a Ce cation. The carbanion intermediates react along three competing pathways: recombination with the surface hydroxyls to produce acetaldehyde; nucleophilic attack by surface H, resulting in ethylene through deoxygenation; and dehydration to produce acetylene. The products produced by these three processes are observed at progressively greater temperatures. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 865-574-2796. Fax: 865576-5235.

’ ACKNOWLEDGMENT The research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. This paper has been authored by a contractor of the U.S. Government under Contract No. DE-AC0S-00OR22725. ’ REFERENCES

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