Langmuir 1989, 5, 965-972 unusual diamagnetic dimer form can be produced under the micropore fields of the ACF. The investigation on the enhancement of the NO dimerization by micropores that can adsorb much more NO molecules is in progress. An FT-IR study will also be necessary for clarifying the molecular structure of the NO dimer in the micropores.
965
Acknowledgment. This work was supported by both a Grant in Aid for Fundamental Scientific Research from the Japanese Government and a grant from Osaka Gas Corp. Registry No. NO, 10102-43-9.
Formation of Stable Alkyl and Carboxylate Intermediates in the Reactions of Aldehydes on the ZnO(0001) Surface J. M. Vohs and M. A. Barteau* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received November 21, 1988. I n Final Form: March 17, 1989 Stable alkyl and carboxylate intermediates were formed by oxidation of higher aldehydes on the zinc polar surface of zinc oxide at low temperatures. Adsorbed acetaldehyde and propionaldehyde underwent nucleophilic attack by lattice oxygen to form dioxyalkylidene species (RCH2CH02). Decomposition proceeded via two separate pathways: hydride elimination to form the corresponding surface carboxylate and alkyl elimination to form surface formate. The alkyl elimination pathway also resulted in the formation of stable surface alkyl species. The selectivity of these two competing elimination reactions was found to be a strong function of the surface temperature, with low temperatures favoring alkyl elimination. TPD and XPS provided the clearest evidence for alkyl elimination and the corresponding formation of surface formates; UPS and XPS provided spectroscopic evidence for the existence of stable surface alkyls. The observation of alkyls as stable surface intermediates in the decomposition of aldehydes on zinc oxide has implications for a variety of catalytic processes, including the synthesis of higher alcohols and oxygenates on oxide surfaces. Introduction Lattice oxygen ions at the (0001)-Znpolar surface of zinc oxide react as nucleophiles with adsorbed carbonyl compounds.l+ For simple reactants such as formaldehyde and methyl formate, these nucleophilic reactions are rather straightforward: nucleophilic attack by lattice oxygen followed by elimination results in the formation of surface formate species.lp2 By analogy with these reactions, one might expect higher aldehydes (RCHO) to react similarly on this surface to produce the corresponding higher carboxylates. However, as we have recently reported: reactions of higher aldehydes on the (0001)-Zn polar surface result in the formation of not only the corresponding higher carboxylate (RCOO) species but surface formate (HCOO) species as well. This result suggests that nucleophilic attack by lattice oxygen at the carbonyl carbon of aldehydes can be followed by either hydride elimination to produce the higher carboxylates or alkyl elimination to form formates. Alkyl elimination in the course of aldehyde oxidation is quite surprising: this reaction would not be expected on the basis of analogous chemistry in basic solution, where hydride elimination from aliphatic aldehydes is always preferred over alkyl eliminati~n.~ It appears that this alkyl elimination pathway is a novel property of the surface chemistry of metal oxides,6 perhaps reflecting the dual acid-base functions provided by the cation-anion site pairs of these materials. Several important questions regarding alkyl and hydride elimination pathways in the reaction of aldehydes on zinc oxide remain unanswered. The factors which control the selectivity of these competing pathways are not understood. The fate of the eliminated alkyl groups has also not
* Author to whom correspondence
should be addressed.
0743-7463 I89 12405-0965$01.50,IO ,
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been elucidated. It was not determined previously whether the eliminated alkyl groups form gaseous products, adsorb as stable complexes, or rapidly decompose to deposit surface carbon. If alkyls are stable intermediates in the decomposition of aldehydes, the reactions of alkyls with C1 species such as CO, COz, or surface formates may provide a route for the synthesis of higher oxygenates on metal oxides. Therefore, in this investigation we have extended our previous studies6 of the reactivity of higher aldehydes on the (0001)-Zn polar surface of zinc oxide in an effort to examine the mechanism of alkyl elimination from aldehydes and to determine the identity of the surface intermediates formed via this reaction. As with other aliphatic oxygenates,1v2,6the (0001)-0 polar surface is inactive for the decomposition of aldehydes.6 Experimental Section All experiments were conducted in a stainless steel ultrahigh-vacuum chamber equipped with a quadrupole mass spectrometer, a double-pass cylindrical mirror analyzer with integral electron gun, and X-ray and ultraviolet photon sources, allowing the collection of TPD, AES, XPS, and UPS spectra. The chamber has previously been described in detail.' The zinc oxide single crystal was obtained from Litton Airtron and was approximately 6 mm X 6 mm X 2 mm. The crystal was aligned to within h O . 5 O of the normal to the c-axis by using the Laue method, and the oxygen and zinc polar surfaces were identified by etching the (1)Vohs, J. M.;Barteau, M. A. Surf. Sci. 1986, 176, 91. (2) Vohs, J. M.; Barteau, M. A. Surf. Sci. 1988, 197, 109. (3) Akhter, S.;Lui, K.; Kung, H. H. J . Phys. Chem. 1984, 89, 1958. (4) Cheng, W.H.; Akhter, S.; Kung, H. H. J . Catal. 1983,82, 341. (5)Akhter, S.; Cheng, W. H.; Lui, K.; Kung, H. H. J. Catal. 1984,85,
437. (6) Vohs, J. M.;Barteau, M. A. J . Catal. 1988, 113, 497. (7) Schubert, W.M.; Kinter, R. R. In The Chemistry of the Carbonyl Group; Patai, S., Ed.; Wiley Interscience: London, 1966; p 695.
0 1989 American Chemical Society
966 Langmuir, Vol. 5, No. 4, 1989
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crystal in nitric acid according to the procedure described by Mariano and Hannemam8 All crystal polishing, mounting, and co cleaning procedures were identical with those used p r e v i o ~ s l y . ~ ~ ~ ~ Acetaldehyde (Aldrich, 99%) and propionaldehyde (Alfa, 99%) were purified by repeated freeze-pump-thaw cycles prior to use. The reactants were admitted into the vacuum chamber via a stainless steel dosing needle directed toward the front face of the crystal. Saturation exposures were used in all experiments; a 50-langmuir exposure was found to be sufficient to saturate the surface with either of the aldehydes. During the collection of X P S spectra, a n A1 Ka X-ray source 220K (hu = 1486.6 eV) was used. All X P S spectra were referenced to 180K the Zn 2p3/, core level, which has a binding energy relative to the 1 I I I I Fermi level of 1021.7 eV on the clean (0001)-Zn surface. UPS rl spectra were collected by using He(I1) photons and were aligned with respect to the Zn 3d peak a t 8.9 eV below E (The valence band maximum was shown previously to be 1.1 eV below the Fermi level,2 based on the measured separation between the Zn 2p,/, and 3d peaks of 1011.7 eV in XPS experiments.) The Zn 3d peak was chosen as the reference point since its position can be easily and unambiguously determined, unlike EFor EmM, which are difficult to measure accurately for oxide surfaces. Further, since the Fermi level is within the band gap region for semiconductors and insulators, the valence band maximum, EWM, is usually taken as the zero of energy for such materials. In order to account for the attenuation of the substrate bands due to the presence of adsorbed species, during the generation of UPS difference spectra the clean surface spectrum was multiplied by TEMPERATURE ( K ) a scaling factor before subtraction from the spectrum of the adsorbate-covered surface? Scaling factors were chosen such that Figure 1. Temperature-programmed desorption spectra for CO the area of negative peaks was minimized in the resulting difand COz as a function of the adsorption temperature of CH&ference spectrum. HzCHO on the (0001)-Zn surface. Mass to charge ratios of 28 and 44 were monitored for CO and COz, respectively. The curves represent the raw data and were not corrected for the mass Results spectrometer sensitivity.
Temperature-Programmed Desorption. Similar trends in the reactivity of acetaldehyde and propiononly the high-temperature CO and C 0 2 peaks were dealdehyde with temperature were observed. Following adtected. Experiments were also conducted in which lesssorption at 300 K, the primary decomposition pathway for than-saturation exposures of the aldehyde were used. The both aldehydes was complete oxidation, resulting in CO, results of these experiments were qualitatively similar to COz, H20, and Zn peaks between 600 and 850 K. The those in Figure 1; regardless of the coverage, increasing the desorption of zinc metal is typical of reactions which readsorption temperature shifted the selectivity to favor the move lattice oxygen from the (0001)-Zns u r f a ~ e . ~For ~ ~ ~ ~ Jhigher ~ temperature product peaks. acetaldehyde, in addition to the complete oxidation The effect of the adsorption temperature on the TPD products, a small amount of the dehydrogenation product, spectra obtained following acetaldehyde adsorption on the ketene, was observed at 590 K.6 In both cases, the TPD (0001)-Zn surface was less pronounced than that for prospectra were similar to those for the decomposition of the pionaldehyde, as suggested previ~usly.~ For acetaldehyde corresponding carboxylic acids'l (i.e., acetic acid and decomposition, as for propionaldehyde, carbon oxides propionic acids), suggesting that the aldehydes, like the desorbed at both 560 and 800 K, with the former pathway carboxylic acids, decompose via carboxylate species on the favored by lower adsorption temperatures. However, se(0001)-Zn polar surface. lectivity differences between the two aldehydes were obFollowing adsorption at 180 K, the TPD spectra for the served. For example, the relative yield of the low-temaldehyde-dosed surfaces were significantly different from perature (560 K) products derived from acetaldehyde was those obtained following adsorption at room temperature: always significantly less than that of the high-temperature in addition to the high-temperature carbon oxide products, (800 K) products, in contrast to the behavior of propionCO, C 0 2 ,and H 2 0 peaks centered at 560 K were observed. aldehyde. This effect of the adsorption temperature on the TPD As noted above, the carbon oxide peaks near 800 K for product distributions can be seen clearly in Figure 1,where the aldehyde-dosed (0001)-Zn surfaces were similar to the CO and C 0 2 desorption peaks following adsorption of those for the decomposition of the corresponding surface propionaldehyde to saturation coverage at different temcarboxylate species, indicating that the aldehydes reacted peratures are displayed. Following adsorption at 180 K, to form higher carboxylates on this surface. The CO and the major CO and COz peaks occurred at 560 K; a small COz peaks centered a t 560 K were characteristic of the C 0 2 peak centered at 800 K was also observed. For addecomposition of surface formate s p e c i e ~ . ~ - ~ Thus, J~-~~ sorption a t 220 K, roughly equal amounts of CO and C 0 2 the TPD results indicated that reaction of the higher aldesorbed a t 560 and 800 K. Increasing the adsorption dehydes a t low temperatures produced both higher cartemperature above 220 K continued to shift the selectivity boxylates and formates. On the basis of these results, one toward the high-temperature products, and by ca. 260 K can postulate the following mechanism for the reaction of aldehydes on the (0001)-Zn surface. Adsorbed aldehydes (8)Mariano, A. N.;Hanneman, R. E. J. Appl. Phys. 1964, 34, 384. Vohs, J. M.; Barteau, M. A. J.Phys. Chem. 1987,91, 4766. (10)Lui, K.;Vest, M.; Berlowitz, P.; Akhter, S.; Kung, H. H. J.Phys. Chem. 1986,90, 3138. (11)Vohs, J. M.;Barteau, M. A. Surf. Sci. 1988, 201, 481. (9)
(12)Bowker, M.;Houghton, H.; Waugh, K. C.; Giddings, T.; Green, M. J. Catal. 1983, 84, 252. (13)Bowker, M. Vacuum 1982,33,669. (14)Chan, L.;Griffin, G. L. Surf. Sci. 1985, 155, 400.
Formation of Alkyl and Carboxylate Intermediates I
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289 285 28 I BINDING ENERGY,eV Figure 2. C 1s XPS spectra of the CH3CH2CHO-dosed(0001)-Zn surface as a function of the adsorption temperature. For each spectrum, the surface was dosed at the specified temperature and heated to 450 K. The spectrum was then collected. undergo nucleophilic attack by lattice oxygen to initially produce surface intermediates of the form R\C/H
?' ?' I
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We have previously referred to these species as dioxyalkylidenes, by analogy with their C1 homolog H2C02, commonly referred to as dioxymethylene16J6or a variant thereof.13J7 As with the surface hydrocarbon ligands RCH2CH and RCH2C,commonly referred to as alkylidenes and alkylidynes, respectively, the a-carbon is assumed to be sp3-hybridized and attached in a bridging configuration. The term gem-dioxyalkane, describing species of the form RCH02, might avoid confusion over this point; however, it does not appear to be in common usage. Spectroscopic evidence for the formation of these species on zinc oxide has been presented previously.6 Subsequent hydride elimination from dioxyalkylidene species produces the corresponding higher carboxylate, while alkyl group elimination produces formate. The selectivity for these two elimination pathways is highly temperature dependent, with lower temperatures favoring alkyl elimination. The fate of the alkyl groups in the low-temperature alkyl elimination reaction is not clear from the TPD results alone. No gaseous products directly attributable to the alkyl groups (e.g., alkanes, alkenes, etc.) were detected in TPD experiments. It is, therefore, likely that the alkyl groups remained on the surface and were oxidized to CO and C 0 2 at higher temperatures, along with the carbon produced by decomposition of surface carboxylate species. More insight into the identity of the surface species derived from these alkyl groups is provided by XPS and UPS spectra of the stable surface intermediates. XPS. C 1s spectra of the stable surface species formed via reaction of propionaldehyde on the (0001)-Zn surface at various adsorption temperatures are displayed in Figure 2. For each spectrum, the surface was exposed to pro(15) Lavalley, J. C.; Lamotte; J.; Busca, G.; Lorenzelli, V. J. Chem.
Soc., Chem. Commun. 1985, 1006.
(16) Idriss, H.; Hindermann, J. P.; Kieffer, R.; Kiennemann, A.; Vallet, A.; Chauvin, C.; Lavalley, J. C.: Chaumette, P. J. Mol. Catal. 1987, 42, 205.
(17) He,M. Y.;Ekert, J. G.J. Catal. 1984,90, 17.
Langmuir, Vol. 5 , No. 4, 1989 967
pionaldehyde at the specified temperature and then heated to 450 K to desorb molecular species. The sample was then allowed to cool to room temperature, and the spectrum was collected. It is evident from Figure 2 that increasing the adsorption temperature resulted in significant changes in the relative intensities of the C 1s peaks of propionaldehyde-derived adsorbates. This observation, like the TPD results, demonstrates that the identity of the surface intermediates formed depends upon the adsorption temperature. The C 1s spectra obtained for an adsorption temperature of 300 K (Figure 2) consisted of two peaks centered at 285.4 and 289.0 eV. By analogy with the C 1s spectra of carboxylate species produced on the (0001)-Zn surface by reaction of carboxylic acids" and C1 oxygenates,'V2 these two peaks can be assigned in the following manner: the high binding energy peak corresponds to the carboxyl carbon of surface carboxylate species and the low binding energy peak to the alkyl group carbons of the carboxylate. Comparison of this spectrum to that of surface propionates produced via reaction of propionic acid on the (0001)-Zn surface" reveals that the peak positions and peak area ratios of the two spectra are nearly identical. This result is consistent with the TPD results and indicates that CH3CH2CH0reacts on the (0001)-Zn surface a t 300 K to produce primarily surface propionate intermediates. Reduction of the propionaldehyde adsorption temperature to 240 K resulted in several significant changes in the C 1s spectrum. For this adsorption temperature, the spectrum (Figure 2) again consisted of carboxyl and alkyl peaks; however, the position of the envelope of peaks at lower binding energy was shifted by 0.5 eV to lower energy relative to that for adsorption at 300 K. The peak at 284.9 eV in Figure 2 was also significantly broader than that a t 285.4 eV in the previous spectrum and was slightly asymmetric, with extra area on the low binding energy side of the peak. These observations indicate that the lower binding energy peak was due to the combined contributions of two overlapping C 1s signals. Curve resolution of the spectrum revealed that it could be fit to two peaks centered at 285.4 and 284.0 eV. As was the case for adsorption at 300 K, the peak at 285.4 eV can be attributed to the alkyl group of surface carboxylate species. The lower binding energy peak indicates the presence of surface species other than carboxylates. The ratio of the area of the carboxyl group peak to that of the alkyl group peak at 285.4 eV was near unity. This is more than 3 times the value of this ratio (0.3) observed for propionate species on this surface." Formates adsorbed on the (OOOl)-Znsurface have a C 1s binding energy of 289.1 eV;'I2 therefore, the production of formates in addition to propionates would tend to decrease the area of the alkyl peak of surface carboxylate species without affecting the area of the carboxyl peak. Thus, the C 1s spectrum is in accord with the TPD results and is consistent with the formation of both surface propionate and formate intermediates. The peak centered at 284.0 eV can be assigned to the surface alkyl species eliminated in the course of formate formation from higher aldehydes, as will be shown below. Further reduction of the propionaldehyde adsorption temperature to 170 K had little effect on the carboxyl peak but produced additional changes in the shape and position of the lower binding energy envelope. For adsorption a t 170 K, this envelope was centered at 284.3 eV. Curve resolution again revealed that this peak could be fit to two peaks centered a t 285.4 and 284.0 eV. These peaks can be assigned in a similar fashion as those for adsorption at 240 K. The ratio of the contributions from the carboxyl
968 Langmuir, Vol. 5, No. 4 , 1989 I
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Figure 3. C Is XPS spectra of the CH3CHO-dosed (0001)-Zn surface for adsorption temperatures of 170 and 450 K. Following adsorption at 170 K the sample was heated to 450 K before the spectrum was collected.
and alkyl functions of surface carboxylates was slightly greater than that for adsorption at 240 K and further illustrates the shift in selectivity to favor alkyl elimination as the aldehyde adsorption temperature is decreased. C 1s spectra of the surface species formed by reaction of acetaldehyde on the (0001)-Zn surface were also collected. The C 1s spectra of the stable surface intermediates following CHBCHOadsorption at 170 and 450 K are displayed in Figure 3. Although the differences in these spectra are not as pronounced as those for the propionaldehyde-dosed surface, they exhibit similar trends. For adsorption at 450 K, the spectrum was essentially identical with that of surface acetate species produced from acetic acid." Decreasing the adsorption temperature produced a broadening of the alkyl peak toward lower binding energy. The fwhm of the 285.5-eV peak in the 450 K spectrum was 2.0 eV and of the 285.2-eV peak in the 170 K spectrum was 3.0 eV. A small increase in the carboxyl/ alkyl peak area ratio of the surface carboxylate species was also observed. It is clear that the peak centered at 284.0 eV in the spectra from the propionaldehyde-dosed surface is due to a product of the alkyl elimination reaction. Caution is required, however, in assigning this peak to a stable surface alkyl species. Atomic carbon adsorbed on the (0001)-Zn surface has a C 1s binding energy of 284.2 *O.l eV,9J1 and therefore it may be possible to attribute this low binding energy peak to atomic carbon. Atomic carbon would be produced if the alkyl groups decomposed upon elimination from the aldehyde. Several experimental observations suggest, however, that this is not the case. The decomposition of a surface alkyl species would likely proceed via sequential dehydrogenation to form vinyl (CH,=CH) and acetylide (HC=C) surface intermediates. Indeed, the latter species are known to be stable on the (0001)-Zn surface. For example, acetylene adsorbs dissociatively on this surface to form adsorbed a~etylides.~ These acetylides are quite stable and decompose to surface carbon at temperatures in excess of 650 K. Thus, it is doubtful that the low-energy peak in the C Is spectra from the propionaldehyde-dosed surface is due to atomic carbon, since this would require that the eliminated ethyl groups decompose at, a temperature more than 200 K below that of the acetylide, a likely intermediate in the decomposition process. Recent results from a study of the adsorption of diethylzinc on the (0001)-Zn surface of zinc oxidela also
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BINDING ENERGY, eV Figure 4. Composite of XPS spectra comparing the products of propionaldehyde adsorption with adsorbed formate, alkyl, and propionate species. (a) CH3CH2CH0adsorbed at 170 K and heated at 450 K. (b) (C2H&Zn adsorbed at 150 K. (c) HCOO from HCOOH adsorption at 300 K. (d) CH3CH2CO0from CH3CH2COOHadsorption at 300 K.
support the assignment of the peak of 284.2 eV to a surface alkyl species. In that study, the XPS and UPS spectra of (C2H5),Znadsorbed on the (0001)-Znsurface at 170 K were collected. The C 1s spectrum of (C2H,),Zn adsorbed on the (0001)-Znsurface consisted of a broad peak resolvable by curve fitting into two peaks of equal intensity centered at 284.6 and 283.4 eV. These peaks were assigned to the methyl group carbons and the zinc-bound carbons of the C2H5groups, respectively. The unusually low C 1s binding energy of the zinc-bound carbon demonstrates the electron-rich character of the alkyl ligand and thus the high polarity of the zinc-alkyl bond. On the basis of this result, one might expect the bond of alkyls to surface zinc cations to be highly polar and to result in a low C 1s binding energy for the zinc-bound carbon of a surface zinc-alkyl complex. However, the removal of electron density from the surface zinc atoms due to charge transfer to the oxygens of the lattice would likely decrease the polarity of a surface zinc-alkyl bond relative to that in the free molecule; thus, the C 1s binding energy of a zinc-bound carbon of a surface alkyl complex would be expected to be equal to or greater than that of molecularly adsorbed diethylzinc. The assignment of the peak at 284.0 eV to an alkyl carbon bound to a surface zinc cation is consistent with these observations. The methyl carbon of a surface ethyl species would be indistinguishable from that of the alkyl carbons of surface carboxylates and would contribute to the peak at 285.4 eV. The correspondence of the binding energies of the C 1s peaks of diethylzinc with the low binding energy peaks derived from propionaldehyde is shown most clearly in Figure 4. This figure is a composite of the C 1s spectra of propionate, formate, and alkyl species and of the surface intermediates obtained by reaction of propionaldehyde. The spectrum following propionaldehyde adsorption at 170 ~~~
(18) Vohs, J M ; Barteau, M A. J Electron Spectros. Relat. Phenom., in press.
Langmuir, Vol. 5, No. 4 , 1989 969
Formation of Alkyl and Carboxylate Intermediates
Table I. Peak Positions for CH3CH0/(0001)-ZnHe(I1) UPS Spectra CH3CH0/(0001)-Zn Tab = 180 K
gaseous CHSCHOa energyb
orbital loa'
3.6
2.8
28''
6.5 7.5
6.8
9a' 8a', la" 7a' 6a' 5a' a
T = 180 K
8.7 9.8 12.7 17.5
CH,COO(ad) from CH&OOH/ (OOO1)-Zne
CH3CH0/(0001)-Zn Tada= 300 K T = 300 K T = 450 K
T=300K
T = 450 K
2.1
1.7
3.8
6.7
3.6 6.8
3.7 6.6
3.7 6.6
3.7 6.7
9.2
8.5
8.6
8.6
9.0
10.2 11.9 16.9
9.8 11.9 16.5
11.9 16.6
11.7 16.6
11.8 16.9
9.8 13.3 16.5
From ref 19, shifted to lower energy by 6.68 eV to align with spectra of surface species. All values in eV. From ref 11. I
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15 12 9 6 3 ,E BINDING ENERGY, eV Figure 5. He(I1) UPS spectra of the CH&HO-dosed (0001)-Zn surface for an adsorption temperature of 300 K. (a) Clean (0001)-Znsurface. (b) CH&HO-dosed (0001)-Znsurface at 300 K. (c) Sample in b heated to 450 K. (d) (b - a) X 2.9, difference spectrum. (e) (c - a) X 2.9, difference spectrum. I8
K and annealing to 450 K is virtually a linear combination of the peaks observed for HCOO and (C2H5)2Znadsorbed on the surface. Although it is likely that some propionate species are also present on the surface following the reaction of propionaldehyde, the separation of the two peaks in curve a of Figure 4 is clearly too large (and the ratio of their intensities too near unity) for propionates to account for the majority of the carbon-containing species on the surface. UPS. Although the XPS results provide evidence for the formation of stable surface alkyl species during the reaction of aldehydes on the (0001)-Zn surface, the data are by no means conclusive. Therefore, in an effort to further verify the formation of stable surface alkyl species, UPS spectra of the surface species were collected. He(I1) UPS spectra of the clean (0001)-Zn surface and that following acetaldehyde adsorption at 300 K are displayed in Figure 5. The peak positions for the difference spectra are listed in Table I. Following acetaldehyde adsorption at 300 K and heating to 450 K, the difference spectra were similar and consisted of five peaks centered at 3.7, 6.6,8.6, 11.7, and 16.6 eV below EmM. As shown in Table I, these peak positions are nearly identical with those obtained for surface acetate species produced by reaction of acetic acid on this surface.'l This result, in conjunction with the XPS
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15
(0001)-Znsurface. (b) CH3CHO-dosed(0001)-Zn surface at 180
K. (c) Sample in b heated to 300 K. (d) Sample in cheated to 450 K. (e) (b - a) X 8, difference spectrum. (f) (c - a) X 8,
difference spectrum. (g) (d - a) X 8, difference spectrum.
and TPD results, clearly demonstrates that acetaldehyde reacts on the (0001)-Zn polar surface at 300 K to form surface acetates. The UPS spectra following CH3CH0 adsorption at lower temperatures were more congested than those obtained following adsorption at room temperature. The He(I1) spectra for an acetaldehyde adsorption temperature of 180 K are displayed in Figure 6. The difference spectra and the spectrum of gaseous acetaldehydelg are also displayed in this figure, and their peak positions are listed in Table I. At 180 K, the spectrum of the adsorbed species was similar to that of gaseous CH3CH0, indicating that significant amounts of molecular CH,CHO were present on the surface. Heating the surface to 300 and then 450 K to desorb the molecular species resulted in significant changes in the difference spectra. After the surface was heated to 300 K, the spectrum could be resolved into seven (19)Bieri, G.; Asbrink, L.; Von Niessen, W. J. Electron Spectros. Relat. Phenom. 1982,27, 129.
970 Langmuir, Vol. 5, No. 4, 1989
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Table 11. Peak Positions for CH8CH&H0/(0001)-Zn He(I1) U P S Spectra
gaseous CH3CHZCHO' orbital energyb 14a'
3.7
2a" 13a' la" 12a' lla' loa' 9a'
6.5 7.2 7.6 8.3 9.3 9.8 10.5
CH3CHzCH0/(0001)-Zn Tab = 175 K T = 175 K T=300K T = 450 K 1.2 3.7 3.7 3.4 5.2 5.8 5.7 6.9 6.6 6.6
9.3
12.4 14.6 17.5
CH3CHzCHO/(0001)-Zn Tsh= 300 K T = 300 K T = 450 K
CH3CHzC00(ad) from CH&HzCOOH/ (OOO1)-Znc
3.9 6.1 6.9
3.9 6.0 6.6
3.7 6.0 6.8
8.9
8.6 9.6
9.2
9.2
8.7
11.6
11.4
11.1
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14.5 17.8
14.2 17.2
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14.1 17.5
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'From ref 20, shifted to lower energy by 6.1 eV to align with spectra of surface species. *All values in eV. cFrom ref 11. I
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BINDING ENERGY, eV Figure 7. He(I1) UPS spectra of the CH3CHzCHO-dosed (0001)-Zn surface for an adsorption temperature of 300 K. (a) Clean (0001)-Zn surface. (b) CH&HzCHO-dosed (0001)-Zn surface a t 300 K. (c) Sample in b heated to 450 K. (d) (b - a) X
2.5, difference spectrum. (e) (c - a) X 2.5, difference spectrum.
peaks centered between 2 and 17 eV below EVBM (the individual peak positions are listed in Table I). The TPD and XPS results indicated that for adsorption a t 180 K surface acetate and a small amount of surface formate species were producede6 Since the extent of formate production was quite small, one would expect the UPS spectrum of the adsorbed species to most closely resemble that of surface acetate. This appears to be the case; the peaks centered a t 3.8, 6.7, 9.2, 11.9, and 16.9 eV are all within 0.2 eV of the peak positions for surface acetate produced from acetic acid.'l It would, therefore, seem reasonable to assign these peaks to surface acetate species. Since the majority of the peaks for surface formate species adsorbed on the (0001)-Zn surface occur at energies similar to those of the surface acetates (peaks at 3.8,7.4, 10.5, and 12.7 eV were observed following adsorption of HCOOH a t 300 K),2 it would be difficult to resolve the spectrum of a small amount of formates from that of the more abundant acetates. It is possible, however, that the peak cen-
Figure 8. He(I1) UPS spectra of the CH3CH2CHO-dosed (0001)-Zn surface for an adsorption temperature of 175 K. (a) Clean (0001)-Zn surface. (b) CH3CHzCHO-dosed (0001)-Zn surface a t 175 K. (c) Sample in b heated to 300 K. (d) Sample in c heated to 450 K. (e) (b - a) x 3, difference spectrum. (f) (c - a) X 3, difference spectrum. (g) (d - a) X 3, difference spectrum.
tered at 9.8 eV in the higher temperature difference spectra of Figure 6 corresponds to the combined signals of the 3b2 and 5al orbitals of surface formate species. These orbitals give rise to a peak centered a t ca. 10.5 eV for formates produced via reaction of formic acid on the (0001)-Zn surface.* This leaves the peak centered a t 1.7 eV as the only peak in the spectrum which cannot be accounted for. Neither surface acetates nor formates give rise to peaks with binding energies less than 3 eV; therefore, the peak at 1.7 eV cannot be assigned to surface carboxylate species. The UPS spectra of the adsorbed species produced by reaction of propionaldehyde on the (0001)-Zn surface displayed trends similar to those observed for acetaldehyde. The UPS spectra of the CH,CH,CHO-dosed (0001)-Znsurface for an adsorption temperature of 300 K
Formation of Alkyl and Carboxylate Intermediates RCHzCHO ( g )
CO(g)+COz(g)r
Hzo(g)
C (ad)+ nzo(9) R = H, CH,
7I
co(g)+co2 ( 9 )
Figure 9. Reaction pathways for the decomposition of higher aldehydes on the (0001)-Zn polar surface of zinc oxide.
are displayed in Figure 7, and the peak positions are listed in Table 11. As shown in the table, the spectrum of the adsorbed species was essentially identical with that of surface propionate species produced from propionic acid and thus is consistent with the T P D and XPS results. The UPS spectra of the CH3CH2CHO-dosed(0001)-Zn surface following adsorption a t 175 K, along with the spectrum of gaseous CH3CH2CH0,20are displayed in Figure 8, and the peak positions are listed in Table 11. At 175 K, the spectrum of the adsorbed species could be resolved into seven peaks centered between 3 and 18 eV below E v B ~This . spectrum exhibited reasonable agreement with that of gaseous propionaldehyde.20Heating the surface to 450 K to desorb any molecularly adsorbed aldehyde resulted in the appearance of several new peaks in the spectrum. Unfortunately, due to the congested nature of the spectrum and the numerous overlapping peaks, it would be somewhat risky to attempt to deconvolute the spectrum and assign individual peaks. It does appear, however, that the spectrum exhibits general features indicative of surface carboxylate species, with one major exception: the lowest binding energy peak at 1.2 eV cannot be attributed to either surface propionate or surface formate species. This peak is in a position near that observed for the acetaldehyde-dosed surface for similar temperatures and heating sequences and appears to be indicative of the surface species formed via alkyl elimination. As was the case for the XPS results, the spectrum of adsorbed (C2H5)2Zncan be used as a guide in assigning the low-energy peaks in the UPS spectra. The He(I1) UPS spectrum of (C2H5)2Znadsorbed on the (0001)-Znsurface has been reported previouslyl8 and consists of six peaks centered at 1.3,4.9,6.8, 8.8, 12.2, and 15.9 eV below E ~ M The lowest energy feature at 1.3 eV has been assigned to an orbital with Zn-C u character and is therefore characteristic of a zinc-alkyl bond. The low energy of this peak again reflects the polar nature of this bond. The presence of similar low-energy peaks in the spectra of the aldehyde-dosed (0001)-Zn surfaces under conditions where alkyl elimination occurs provides further evidence for the formation of stable surface zinc-alkyl complexes. (20) Kimaru, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules; Japan Scientific Societies Press: Tokyo, 1981.
Langmuir, Vol. 5, No. 4, 1989 971
Discussion The combination of the TPD, XPS, and UPS results provides a detailed description of the reactions of aldehydes on the (0001)-Zn surface of ZnO. The reaction network for the conversion of higher aldehydes on the (0001)-Zn surface is summarized in Figure 9. Following adsorption, the aldehyde undergoes nucleophilic attack by surface lattice oxygen to form a dioxyalkylidene intermediate (step 1in Figure 9). The subsequent reaction of this species is a strong function of the surface temperature: at temperatures above ca. 260 K, hydride elimination results in the formation of higher carboxylate species (step 3 in Figure 9), while at temperatures below 260 K, in addition to hydride elimination, alkyl elimination (step 2 in Figure 9) is possible, resulting in the formation of surface formate and alkyl species. The higher carboxylates decompose at temperatures between 600 and 750 K to produce adsorbed atomic carbon and liberate HzO and a small amount of C02 into the gas phase (step 6 in Figure 9). In the case of acetates produced from acetaldehyde, a small amount of ketene is also produced during carboxylate decomposition! In contrast to the higher carboxylates, the formates decompose directly to carbon oxides, hydrogen, and water at 560 K (step 4 in Figure 9). The surface alkyl species undergo dehydrogenation between 600 and 750 K to produce atomic carbon (step 5 in Figure 9). The adsorbed carbon produced from both alkyl and carboxylate decomposition is oxidized by lattice oxygen at temperatures in excess of 750 K to carbon oxides. Reduction of surface zinc ions to zinc metal accompanies these oxidation reactions. This zinc metal is also detected as a gaseous product. The difference between the reactivity of aldehydes in solution and those adsorbed on zinc oxide is quite surprising in light of recent studies comparing the reactivity of organic molecules in solution and adsorbed on oxide surfaces. For example, the relative acidities of organic acids adsorbed on several metal oxides have been found to track those measured in solution and to be substantially different than those in the gas phase.21,n Thus, one might expect that the reactivity of organic species on oxide surfaces and in solution would be similar. Indeed, it has been suggested that the role of the surface in stabilizing adsorbed species is much the same as that of solvent molecules in solution.21r22In both cases, stabilization is thought to be brought about by the delocalization of charge on ionic species. In solution, this delocalization arises from interactions with surrounding polar solvent molecules, while on an oxide surface it may occur via interaction with the surface cations, anions, or hydroxide groups. Although this model is able to rationalize the similarity in the relative acidities of hydrocarbons in solution and on oxide surfaces, it appears to be inadequate to explain the observations in the present investigation. The selectivity for additionelimination reactions of aldehydes on the (0001)-Znsurface is clearly different from that in solution, where selective formation of higher carboxylates would be expected. . In addition to comparisons with reactions in solutions, it is interesting to compare the decomposition behavior of aldehydes on the (0001)-Znsurface with similar reactions occurring on noble metal surfaces. Adsorbed oxygen atoms on both ~ o p p e r and ~~-~~ surfaces act as nucleophiles and react with carbonyl compounds in much the same manner observed for lattice oxygen on the (0001)-Zn surface of zinc oxide. The chemistry of adsorbed species both on Ag(l10)28129 and C U ( ~ ~has O )also ~ ~been found to (21) Garrone, E.; Stone, F. S. Proc. Int. Congr. Catal. 8th 1984,3,441. (22) Spitz, R. N.; Barton, J. E.; Barteau, M. A.; Staley, R. H.; Sleight, A. W . J. Phys. Chem. 1986, 90, 4067.
Vohs and Barteau
972 Langmuir, Vol. 5, No. 4, 1989
correlate well with similar ion-molecule reactions occurring in the gas phase. Adsorbates on these surfaces have been shown to be quite ionic in Thus, one might expect reactions on oxygen-covered copper or silver surfaces to be similar to those on the surfaces of metal oxides. Reaction of acetaldehyde on oxygen-dosed Ag(l10)27at 160 K proceeds in a similar fashion to that reported here for reaction on ZnO(0001). On silver, acetaldehyde undergoes nucleophilic attack by adsorbed atomic oxygen to initially form a CH3CH02species. This species is analogous to the dioxyalkylidene species formed from the aldehydes or ZnO(0001). A t 250 K, this surface CH3CH02 species undergoes hydride elimination to form surface acetate. The reaction of CH3CH0 with adsorbed oxygen atoms on the Ag(ll0) surface at temperatures above 250 K results in the direct formation of surface acetate speci e ~ Thus . ~ ~on silver, unlike zinc oxide, only hydride and not alkyl elimination pathways are observed. Similar results have also been reported for the reaction of acetaldehyde on C U ( ~ ~ OOn ) . ~this ~ surface as well, acetaldehyde reacts with adsorbed atomic oxygen to form only acetate species; alkyl elimination is not observed. These selectivity differences most likely reflect the role of the zinc oxide surface in stabilizing the eliminated ligands. Zinc alkyls are stable molecules; alkyls of the l b metals are not stable, nor are alkyl anions in aqueous solution. The stabilization of the alkyl ligands by interaction with a surface zinc cation lowers the activation barrier for cleavage of the carbon-carbon bond of the dioxyalkylidene sufficiently that alkyl elimination is preferred over hydride elimination at low temperatures. Thus, both the nucleophilicity of the oxygen anions and the Lewis acidity of the zinc cations are required in order for this reaction to occur. The shift in selectivity toward hydride elimination with increasing adsorption temperature on zinc oxide can be understood in terms of the relative bond strengths of the cy C-H and cy C-C bonds.6 Thermochemical calculations predict that the cy C-H bond energy is approximately 13 kcal/mol greater than the cy C-C bond energy.6p31p32Thus, hydride elimination is more highly activated than alkyl elimination. It is, therefore, not surprising that hydride elimination is favored at higher temperatures. The observation of alkyl elimination from aldehydes to form stable alkyls on the zinc polar surface of zinc oxide has several important implications in the area of catalysis by metal oxides. In particular, alkyl elimination reactions may limit the selectivity of oxide-catalyzed selective oxidation reactions, since these reactions provide a low-temperature pathway to complete oxidation products. The relevance of these reactions to allylic oxidations has been the subject of a previous paper.6 The demonstration of the stability of the surface alkyl species resulting from alkyl elimination in this work also has important implications for other reactions catalyzed by metal oxides. For example, Zhang et al.33have recently shown that lithium-promoted Sexton, B. A.; Hughes, A. E.; Avery, N. R. Surf. Sci. 1985, 155, Rnwker. M.: Madix. R. cJ. Sci. .. . .. ., . . ., . __ , -. . Surf. ..
1981. 102. 542. Bowker, M.; Madix, R. J. A p j l . Surf. %I. 1981, 8, 299. Barteau, M. A.; Bowker, M.; Madix, R. J. Surf. Sci. 1980, 94, 303. Barteau, M. A.; Bowker, M.; Madix, R. J. J . Catal. 1981,6, 118. Barteau, M. A,; Madix, R. J. Surf. Sci. 1982, 120, 262. Vohs, J. M.; Carney, B. A,; Barteau, M. A. J . Am. Chem. Sco.
1985, 107,7841.
(30) Jorgensen, S. W.; Madix, R. J. Surf. Sci. 1983, 130, L291. (31) Sanderson, R. T. Chemical Bonds and Bonds Energies; Academic Press: New York, 1971. (32) Benson, S. W. Thermochemical Kinetics; Wiley: New York, 1968.
zinc oxide is an effective catalyst for oxidative dimerization of methane. Since the selectivity of this reaction is postulated to be limited by the combustion of methyl groups on the ZnO the stability of alkyls on ZnO undoubtedly contributes to the performance of this catalyst. Zinc oxide is also a major component in methanol synthesis catalysts, and, although the mechanism for the synthesis of methanol over zinc oxide based catalysts is becoming increasingly well u n d e r s t ~ o d ,the ~ ~mechanisms ,~~ for the synthesis of higher alcohols and oxygenates with modified versions of these catalysts are still largely unknown. The alkyl elimination reactions observed in this investigation suggest a possible mechanism for the synthesis of higher alcohols and oxygenates on metal oxides. Since surface carboxylate and alkyl species are both stable and isolable on the (0001)-Zn surface, by the concept of microscopic reversibility it is likely that the reverse of alkyl elimination (Le., alkyl addition) may also occur on zinc oxide. The addition of alkyl groups to surface formate species would result in the production of primary alcohols and aldehydes, while addition to higher carboxylates would produce secondary alcohols and ketones. This mechanism for higher alcohol synthesis is in contrast to that for metal surfaces, on which it is typically suggested that higher alcohols are produced via acyl (RC=O) intermediate^.^^^ On oxides, higher oxygenates may be produced via carboxylate species; on metals, via acyl species; and on oxide-promoted metals, by a combination of these two routes. Conclusions
The higher aldehydes, CH3CH0 and CH3CH2CH0,react with lattice oxygen on the (0001)-Zn polar surface of zinc oxide to form surface carboxylate and alkyl species. These reactions proceed via nucleophilic attack of lattice oxygen at the carbonyl carbon of the aldehyde followed by either hydride elimination to produce the corresponding surface carboxylate or alkyl elimination to produce formate and stable surface alkyl species. The selectivity of these two competing decomposition pathways is a strong function of temperature, with lower temperatures favoring alkyl elimination. The observation of alkyl elimination from aldehydes on the (0001)-Zn surface and the absence of analogous reactions in the gas phase, in solution, and on noble metal surfaces demonstrate the important role of surface-adsorbate interactions in determining the reactivity of adsorbed species on oxide surfaces. Finally, the observation of stable surface carboxylate, alkyl, and dioxyalkylidene species on the (0001)-Znsurface suggests alkyl addition to adsorbed carboxylates as a possible mechanism for the synthesis of higher alcohols and other oxygenates on oxide surfaces. Acknowledgment. We gratefully acknowledge the National Science Foundation (Grants CBT 8311912 and CBT 8451055) for support of this research. Registry No. CH3CH2CH0,123-38-6;CH,CHO, 75-07-0; ZnO, 1314-13-2.
(33) Zhang, H.-S.; Wang, J.-X.; Driscoll, D. J.; Lunsford, J. H. J . Catal. 1988. 112. ____,.. ~, 366. ~~
(34) Kunp, H. H. Catal. Reu.-Sci. Eng. 1980, 22, 235. (35) Klie;, K. Adu. Catal. 1982, 31, 243. (36) van der Lee, G.; Ponec, V. Cat. Reu.-Sci. Eng. 1987, 29, 183. (37) Sachtler, W. M. H.; Ichikawa, M. J . Phy. Chem. 1986, 90, 4753. (38) Orta, H.; Naito, S.; Tamaru, K. J . Catal. 1984, 90, 183.
