6984
J. Phys. Chem. B 1999, 103, 6984-6997
Interaction of Methanol with Well-Defined Ceria Surfaces: Reflection/Absorption Infrared Spectroscopy, X-ray Photoelectron Spectroscopy, and Temperature-Programmed Desorption Study A. Siokou and R. M. Nix* Department of Chemistry, Queen Mary and Westfield College, UniVersity of London, London E1 4NS, U.K. ReceiVed: April 2, 1999; In Final Form: June 21, 1999
The adsorption of methanol on ordered epitaxial layers of cerium oxides grown on a Cu(111) substrate has been studied using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), temperature-programmed desorption (TPD), and Fourier transform reflection/absorption infrared spectroscopy (FT-RAIRS) measurements. The oxide films exhibit a LEED pattern characteristic of a CeO2(111)-like structure, but the Ce/O stoichiometry achieved is strongly dependent on the exact pretreatment and film history. Grazing emission XPS also indicates that some Ce3+ ions are still present in the surface layers at 300 K after oxidation treatments. Methanol adsorbs dissociatively at 300 K, with a relatively high sticking probability, to yield surface methoxy species. The IR spectra of the methoxy species, in particular the CsO stretch frequency, provide information about their coordination to the oxide surface, the presence of surface oxygen vacancies, and the general level of oxidation of the film. The methoxy species are stable on the (111)-type terraces of thicker (>5 ML) oxide films to temperatures in excess of 550 K but then decompose at about 585 K to yield predominantly H2 and CO with some simultaneous evolution of formaldehyde and water. A substantial number of more coordinatively unsaturated cerium ions exist at and near the periphery of oxide islands on films of a submonolayer oxide coverage and on aggregated films of higher oxide coverage (between 1 and 5 ML). When the substrate is well-oxidized, then some of the methoxy species adsorbed at such sites are readily oxidized to the formate species while the decomposition temperature of the remaining methoxy groups in this peripheral region is lowered to about 560 K and their decomposition yields a higher proportion of formaldehyde than is seen for the (111) terrace sites.
Introduction Oxides of the lanthanide metals have featured prominently in the search for new and improved catalysts for a range of chemical and environmental processes. Ceria, in particular, is now a major component of three-way auto exhaust catalysts, in which context it has been attributed roles which are related to its oxygen storage capacity,1 its contribution to the stabilization of the surface area of the alumina washcoat,2 the stabilization of the precious metal dispersion,3 and the promotion of the water gas shift reaction.4 Another active area of research into catalysis by oxides is the production of functionalized hydrocarbons from common feedstocks; this research extends to processes such as the direct conversion of methane and synthesis gas (CO-CO2-H2) chemistry5,6 in which many of the reactions involve the production and interconversion of simple C1 molecules such as methane, methanol, formaldehyde, and formic acid. Cerium-containing catalysts have once again attracted attention in this area; examples of reactions for which they have been the subject of study range from methanol synthesis (over catalysts derived from Cu-Ce intermetallic compounds7) to the oxidation of methane to CO (over CeCu-O composite catalysts8). For these and other reasons, there is much current interest in the fundamental surface properties of the lanthanide oxides and in particular in the surface chemistry of ceria. An increasingly common approach to the study of these oxides, which cannot * To whom correspondence should be addressed. Phone: +44 171 775 3273. Fax: +44 181 981 8745. E-mail:
[email protected].
readily be produced in a single crystal form, is through the growth of the oxide as thin films on metallic substrates. In such systems, submonolayer coverages and thin films of the oxide on the metal may act as models for the highly dispersed oxide in mixed metal/oxide catalyst formulations, while thicker films are used as model substrates for fundamental studies of the surface chemistry of the bulk oxide. In this work, well-defined ceria overlayers were prepared on a Cu(111) substrate by the low-pressure oxidation of cerium predeposited by vacuum evaporation. The growth and characteristics of the ceria films, as well as their behavior during thermal treatment, have been investigated using surface-sensitive techniques such as low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). The adsorption of methanol has been further studied using a combination of temperature-programmed desorption (TPD) and reflection/ absorption infrared spectroscopy (RAIRS) measurements; as we will demonstrate, the study of methanol serves two purposes in that (i) it enables the surface chemistry of methanol itself to be elucidated (particularly with respect to its dissociation and oxidation pathways) and (ii) the methanol may be used as a probe molecule to study the reactivity and surface structure of the ceria surfaces. Experimental Section The experiments were carried out in an ultrahigh vacuum (UHV) system that has been described in detail elsewhere.9 The main analytical chamber is equipped with a 100 mm hemi-
10.1021/jp991127h CCC: $18.00 © 1999 American Chemical Society Published on Web 07/30/1999
Interaction of Methanol with Ceria spherical analyzer, electron gun, and Mg KR X-ray source for Auger electron spectroscopy (AES) and XPS studies; a quadrupole mass spectrometer for residual gas analysis and TPD studies; and rear-view LEED optics. A second vessel forms a small cell for RAIRS studies and dosing gases at pressures up to 0.1 Torr. The two interconnected vacuum vessels are independently pumped by a combination of ion, diffusion, and titanium sublimation pumps, and both have a base pressure of ca. 1 × 10-10 Torr. The Cu(111) crystal was initially cleaned using extensive Ar+ bombardment (700 eV, 12-14 µA), at both room temperature and 773 K, followed by annealing at 973 K for about 5 min to restore the surface order. This latter requirement was confirmed by LEED, with a sharp Cu(111)-(1 × 1) pattern evident after the cleaning procedure. The only impurity detected by XPS after cleaning was trace amounts of surface carbon. The cerium dosing source was purpose-built for this work and consisted of 99.9% Ce held in a resistively heated tungsten wire basket. The source was enclosed in a molybdenum cage that could be separately outgassed. The cerium atom flux was monitored using a mass spectrometer with direct line-of-sight to the evaporator and controlled by regulating the heating current. The amount of evaporated cerium was also quantitatively estimated using XPS, from an analysis of the attenuation of the Cu 2p3/2 signal, and assuming uniform oxide layer growth. The interpretation of Ce 3d XPS data has been a long-running controversy and only relatively recently has a general consensus emerged about the peak assignments. The data of this work are in very good agreement with the data of these more recent studies.10-12 Briefly, each spin-orbit component of the Ce 3d XP spectrum is dominated by three features in the case of CeO2 but contains only two main peaks for Ce2O3; the interpretation of the complete Ce 3d spectrum of a partially reduced ceria sample will therefore imply the complete analysis of 10 peaks in total when all spin-orbit components are taken into account. The value of the stoichiometric coefficient (x of CeOx) can be estimated quantitatively either from an analysis of the relative intensities of the Ce 3d and O 1s spectra or from a detailed deconvolution of the Ce 3d emission profile, although it needs to be recognized that both methods yield an aVerage stoichiometry (albeit a surface-weighted average) for the XPS-sampled region. Both approaches were used in this work and reasonable agreement (better than (0.05) obtained for the majority of samples. The deconvolution method involved the initial optimization of fits for samples in which cerium was present almost exclusively in one or another of the two oxidation states in order to generate photoemission profiles characteristic of the individual oxidation states. Other spectra were then analyzed by fitting the experimental data using linear combinations of the two profiles, with only fractional changes to the relative positions and variation of the complete profile intensities being permitted. Only the values of oxide stoichiometry obtained using this latter deconvolution method are quoted in subsequent sections. Methanol was purified by drying over a molecular sieve and dissolved permanent gases removed by freeze/pump/thaw cycles. Unless specifically mentioned, all methanol exposures were carried out using background pressures in the range 5 × 10-8 to 5 × 10-7 Torr and with the substrate at a temperature of 300-340 K. Results Deposition and Oxidation of Ce on Cu(111) at Ambient Temperatures. The main purpose of this study was to characterize the structure and reactivity of the cerium oxide
J. Phys. Chem. B, Vol. 103, No. 33, 1999 6985
Figure 1. (A) Ce 3d and (B) O 1s XP spectra recorded during the initial oxidation of freshly deposited cerium at 300 K and after annealing of the 10 ML of ceria film: (a) 2 L of O2; (b) 170 L of O2; (c) annealed at 820 K for 3 min.
overlayers after the oxidation of the deposited cerium, and little attention was given to the interface properties of the bimetallic Ce/Cu system. Previous reports13 indicate that extensive alloying can occur during the deposition of Ce onto polycrystalline Cu substrates, and while this was not immediately evident in this work on Cu(111), the copper substrate was always maintained at ambient temperatures during the initial deposition of Ce, and this deposition was immediately followed by a rapid oxidation of the deposited layers to minimize any possible effects arising from intermixing of the two metals. All the deposited Ce films exhibited a fast uptake of oxygen from molecular O2 at ambient temperatures, leading to a rapid transformation of Ce from the zerovalent metallic state to Ce3+ (Ce2O3) throughout the XPS sampling depth, even for relatively low oxygen exposures (spectrum a of Figure 1A). At this stage of oxidation, the O 1s peak is usually composed of two peaks, one at ca. 529.8 eV and another smaller peak at ca. 532 eV (spectrum a of Figure 1B). The latter peak is believed to
6986 J. Phys. Chem. B, Vol. 103, No. 33, 1999 correspond to hydroxide or carbonate species formed by adventitious adsorption; the evidence that this peak is in part due to carbonate comes from the observed concurrent reduction in intensity of this O 1s shoulder and a weak C 1s peak (286 eV) following heating to 473 K and above and from the observation in control TPD experiments of some CO2 desorption giving rise to a 44 amu desorption peak with a maximum above 500 K. The surface localization of the species responsible for the higher binding energy O 1s peak has been confirmed by comparison of spectra acquired at near-normal and grazing emission angles that show this peak to be much more prominent at grazing emission angles. Further oxidation of freshly evaporated Ce films at 300 K does not readily lead to complete conversion to Ce4+; instead the Ce 3d spectrum apparently contains a combination of Ce3+ and Ce4+ characteristics even after high oxygen doses (see, for example, spectrum b of Figure 1A). This coexistence of cerium in both Ce3+ and Ce4+ oxidation states indicates that the overall stoichiometry of the limiting oxide (CeOx) achieved at ambient temperatures must have an x value in the range 1.50-2.00. In the specific case of the film from which spectrum b in Figure 1A was obtained, the final stoichiometry is CeO1.84. In respect of the mixed valence exhibited by films obtained at ambient temperatures, these data are consistent with those obtained in previous studies of the low-pressure oxidation of Ce films evaporated on Pd(111),10 Rh(111),11 and Pt(111)14 and the oxidation of polycrystalline Ce foil.15 In agreement with our previous work, we propose that the film is rapidly oxidized to Ce2O3 and with further oxidation at ambient temperatures a thin capping surface layer that is rather closer in stoichiometry to CeO2 is formed.10 During extended dosing of a freshly evaporated film with oxygen, the main O 1s peak shifts continuously to lower binding energy; this behavior is illustrated in Figure 1B for a 10 ML Ce film, for which the O 1s binding energy attains a value of ca. 529.8 eV for a Ce2O3 stoichiometry (as noted above) and then continues to shift and ultimately attains a value of ca. 529.4 eV for exposures over 100 L of O2. The higher binding energy O 1s feature “tracks” the main peak and, in this particular case, contributes about 30% of the total O 1s integrated intensity for the highest exposures. The oxidized Ce layers formed at ambient temperatures, in the manner described above, showed no long-range order with only a faint Cu(111) substrate LEED pattern visible at low Ce coverages and a diffuse background evident at higher Ce coverages. Stepwise Annealing of the Oxidized Films and Reoxidation Characteristics. Annealing of films prepared by oxidation of the freshly deposited Ce at ambient temperatures has several consequences; this is again illustrated for a 10 ML film in spectrum c of Figure 1A and spectrum c of Figure 1B. First, the higher binding energy O 1s feature begins to lose intensity after annealing to 473 K and is completely removed by heating to 673 K. Above 473 K the main O 1s peak maximum shifts back to 529.6 eV. Heating above 673 K also causes changes in the Ce 3d spectrum; there is an increase in the relative intensity of the features characteristic of Ce3+, indicating that the cerium in the surface layers is being reduced (in this particular case the average stoichiometry falls from CeO1.84 to CeO1.80). These latter changes are attributed mainly to the diffusion of oxygen into subsurface regions of the film, as previously reported for cerium oxide films grown on Pd(111).10 In the case of films subjected to milder oxygen exposures than the film of Figure 1, the initial heating can apparently result in a complete reversion
Siokou and Nix of the cerium oxidation state to Ce3+, as evidenced from XPS. TPD experiments confirm that there is no desorption of cerium or molecular oxygen over the temperature range studied, although, as noted earlier, control experiments on freshly oxidized films did reveal some CO2 desorption in a peak above 500 K and a very small amount of H2O desorption at ca. 450 K (by contrast, virtually no CO2 desorption was seen from preannealed oxide films, although some H2O desorption was still evident at ca. 450 K). Reoxidation of annealed films, by reexposure to O2 at ambient temperatures, reverses the effects seen during the initial annealing with an increase in oxygen concentration and a concomitant conversion of Ce3+ to Ce4+; furthermore, the final oxidation level attained by reoxidation generally exceeded that obtained during the very initial oxidation procedure. For thicker oxide films (>5 ML), it was found that repeating this annealing/ reoxidation treatment several times proved to be an effective method for preparing films with a stoichiometry very close to that of CeO2. Figure 2 illustrates the reoxidation process for a thick oxide film that has been built up using a number of cycles of Ce deposition and oxidation. The Ce 3d spectrum recorded after the initial oxidation of the Ce deposited in the final cycle (trace a) clearly shows features due to Ce3+ in addition to those attributable to Ce4+ (and the overall stoichiometry is estimated as CeO1.88). Reoxidation by exposure to 30 L of O2 at 300 K transforms the Ce 3d profile to one that is much more consistent with the presence only of Ce4+ (spectrum b of Figure 2A), and there is a significant increase in the O 1s peak intensity(spectrum b of Figure 2B), the overall stoichiometry of the resultant oxide being CeO1.98. Such reoxidized films also exhibit greater stability with respect to any tendency to reduce during temperature treatments; spectra c and d of Figure 2A indicate an oxide stoichiometry of CeO1.92 and CeO1.83 after annealing at 690 and 930 K, respectively. By contrast, for thinner films we did not manage to attain a stoichiometry close to the dioxide even after repeating the same reoxidation procedure. Once again, this behavior is very similar to that observed for cerium oxides on palladium where the anomalous behavior of the thinner films was attributed to the immediate influence of the metal/oxide interface.10 Even with the thicker oxide films, however, we would note that there is invariably some Ce3+ closely localized to the vacuum interface (i.e., the surface is partially reduced, a condition that is also apparent from the RAIRS data presented below). In XPS this is demonstrated by a direct comparison of Ce 3d spectra recorded at near-normal and grazing emission angles (Figure 3). In addition to the effects mentioned above, annealing the oxide films to high temperatures can result in other more substantial changes. The onset of these changes is marked by the intensity of the Cu 2p signal starting to increase, while that of the Ce 3d and O 1s peaks begins to decrease. These changes suggest that the oxide film begins to aggregate on the Cu(111) surface. This phenomenon is shown clearly in Figure 4 where the variation of the Cu 2p and the Ce 3d signal intensities have been plotted as a function of annealing temperature for 4 and 15 ML of oxide films. From these data it is clear that the thicker 15 ML oxide film is stable to temperatures in excess of 773 K, whereas annealing the thinner (4 ML) oxide layer results in a decrease in Ce 3d intensity starting at about 650 K, indicating that this film is less stable to the heating treatment compared to the thicker oxide film. After annealing multilayer oxide films at 700 K (for ca. 5 min), we were able to detect a new LEED pattern. The pattern
Interaction of Methanol with Ceria
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Figure 3. Ce 3d XP spectra for normal (dashed line) and grazing (solid line) emission for a well-oxidized (preoxidized) film.
Figure 2. (A) Ce 3d and (B) O 1s XP spectra of a 9 ML thick oxide film prepared in steps: (a) after the last cycle of Ce evaporation and oxidation (30 L of O2) at 300 K; (b) annealed at 670 K and then reoxidized at 300 K (30 L of O2); (c) annealed at 690 K; (d) annealed at 930 K.
shown in Figure 5A was obtained by annealing a 1.3 ML of oxide film, and the diffraction spots from the Cu(111) substrate are still visible, whereas Figure 5B shows the LEED pattern from a much thicker 20 ML oxide layer. This new pattern, which is characteristic of the cerium oxide, corresponds to a (1.5 × 1.5) ( 0.05 overlayer with respect to the underlying Cu(111) surface and hence to a structure with the same hexagonal symmetry and a lattice parameter of a ) 3.84 ( 0.11 Å. For thick (>10 ML) oxide films this structure persists even after annealing up to 1000 K, although at higher temperatures spots attributable to the Cu(111) substrate can appear on the pattern, indicating the extended aggregation of the oxide layer. It appears therefore that even after the film has started to aggregate, the oxide still forms long-range ordered islands that retain a distinct epitaxial relationship with the substrate. For thicker films (e.g., 10 ML and above), the quality of the LEED pattern from the ordered oxide overlayer was very much
Figure 4. Variation of Cu 2p (a, b) and Ce 3d (c, d) XPS peak intensity with annealing temperature for well-oxidized (preoxidized) films of 4 ML (9) and 15 ML (b) coverage.
improved by preparing the oxide in steps. More specifically, well-ordered thick films could best be prepared by successive cycles in which oxide layers of ca. 2 ML thickness were deposited by Ce deposition and ambient oxidation, and these were then annealed and again reoxidized at 300 K before the next layer was deposited. Adsorption and Surface Reactions of CH3OH on Ceria. The adsorption of methanol has been used both as a probe of the surface structure and as a means of studying the reactivity of the ordered cerium oxide films. To permit comparison between the properties of films of various thickness, studies have been carried out on high (>10 ML of CeOx), intermediate (∼3ML of CeOx), and submonolayer (
