The Decomposition Pathways of Methanol on Clean Ru(0001

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J. Phys. Chem. B 2001, 105, 11186-11193

The Decomposition Pathways of Methanol on Clean Ru(0001), Studied by Reflection-Absorption Infrared Spectroscopy (RAIRS) Ricardo B. Barros, Ana Rosa Garcia, and Laura M. Ilharco* Centro de Quı´mica-Fı´sica Molecular, Complexo I, Instituto Superior Te´ cnico, AV. RoVisco Pais, 1049-001 Lisboa, Portugal ReceiVed: May 9, 2001; In Final Form: August 20, 2001

The adsorption and decomposition of methanol on clean Ru(0001) were investigated by reflection-absorption infrared spectroscopy (RAIRS). At low temperature (90 K) and coverage (0.1 L), it was confirmed that methanol adsorbs dissociatively as methoxide (CH3O-). No experimental evidence was obtained of an alternative decomposition for high coverage. Different bonding sites and geometries, depending on temperature and coverage, were proposed for methoxide and correlated with the corresponding CO stretching wavenumbers. Methoxide may either undergo complete dehydrogenation into CO(ads) and H(ads), if annealed in small temperature steps (in the range between 110 and 320 K), or partial dehydrogenation into very stable η2-formaldehyde, by a one-step thermal activation (from 130 K to at least 190 K), in the presence of previously formed products (CO and atomic species). At high temperatures (g190 K), methanol undergoes O-H, C-H, and C-O bond scission, leaving surface fragments undetectable by RAIRS. However, in a sequential dosing, the fragments from the first methanol molecules that hit the surface seem to have a passivating effect on Ru(0001). Subsequent doses undergo only partial dehydrogenation, yielding η2-formaldehyde, which was isolated on the surface in two bonding configurations: bridging [µ2-η2(C,O)-H2CO] and chelating [µ1-η2(C,O)-H2CO], characterized by the νCO mode at 1262 and 1277 cm-1, respectively. This assignment was confirmed by adsorbing CD3OH. The bridging form is favored at lower coverage. Formaldehyde prepared by sequential dosing is stable on the surface up to at least 290 K, although some dehydrogenates to CO(ads) above 190 K.

1. Introduction The smallest aliphatic alcohol, methanol, is one of the most important industrial synthetic raw materials (e.g., formaldehyde is obtained by partial oxidation of methanol over silver or copper catalysts1), and methanol fuel cells present distinct advantages over combustion engines and hydrogen fuel cells.2,3 Such a small molecule has, nonetheless, C-O, C-H, and O-H bonds, being an excellent model for the selective activation of different chemical bonds in heterogeneous catalysis. This has stimulated and continues to stimulate extensive theoretical4-6 and experimental7-9 research on the interaction of methanol with clean and modified metal surfaces. In the late 1970s, early 1980s, it was shown that, on most surfaces, methanol readily decomposes to methoxide (H3CO-), generally requiring no more than a little thermal agitation to induce metal-activated O-H bond scission.10 This process can be promoted by coadsorbed or preadsorbed oxygen, which is essential in the case of Ag(111),11 Au(110),12 and Cu(100) at low temperature.13 The methoxide species has been identified on a large number of clean transition metal surfaces, such as Fe(110),14 Ni(110),15 Mo(100),16 Rh(111),17 Pd(111),18 and Pt(111),19 mainly by High-Resolution Electron Energy Loss Spectroscopy (HREELS), X-ray Photoelectron Spectroscopy (XPS), UV-Photoelectron Spectroscopy (UPS), and Reflection-Absorption Infrared Spectroscopy (RAIRS). In what concerns the bonding geometry of methoxide, there are different proposals. On some compact surfaces, such as Al(111), Cu(111), Ag(111), Au(111), and Pt(111), the problem * Corresponding Author. Tel: 351-21-8419220. Fax: 351-21-8464455. E-mail: [email protected].

has been addressed by theoretical methods. According to Hartree-Fock (HF) cluster calculations on Al(111),20 an isolated methoxide adsorbs preferentially on face-centered cubic (fcc) 3-fold hollow sites, with the CO bond normal to the surface. The computed vibrational frequencies were consistent with the experimental results obtained by HREELS.21 Those are also the favored sites for isolated methoxide on Cu(111), Ag(111), and Au(111), according to density functional theory (DFT) calculations.6 These results compare well with RAIRS spectra on Ag(111)11 and on Cu(111).22 Quasi-relativistic DFT calculations on Pt(111) pointed to identical results.5 Hybrid HF/DFT methods were performed for methoxide adsorbed on a less compact surface, Al(100), indicating a preference toward adsorption on 2-fold bridge sites.23 While the adsorption energies on bridge and on hollow sites may be comparable, on-top sites are clearly the least favorable for methoxide on most surfaces. On clean Ru(0001), the adsorption energies for molecular methanol ontop and 3-fold hollow sites were computed by quasi-relativistic DFT methods, but no calculations were performed for adsorbed methoxide.5 EELS results suggest that the CO bond of adsorbed methoxide is oriented perpendicularly to the Ru(0001) surface,24 but no conclusions about bonding geometries and sites have been drawn. Methoxide undergoes different decomposition pathways, depending on the metal selectivity. Using a coarse classification of metal surfaces, the results may be summarized as follows: on the more reactive Group VIII transition metal surfaces, such as Ru(0001),24,25 Rh(111),17 Ni(111),26 Pd(111),18 and Pt(111),9,19 methoxide typically decomposes directly to CO(ads) and H(ads), whereas on noble (relatively more inert) metal surfaces, such as Cu(110) and Ag(110), only a partial dehy-

10.1021/jp011780g CCC: $20.00 © 2001 American Chemical Society Published on Web 10/19/2001

Decomposition Pathways of Methanol on Clean Ru(0001)

J. Phys. Chem. B, Vol. 105, No. 45, 2001 11187

drogenation to formaldehyde (H2CO-) occurs.27,28 Other intermediate species include formate (HCOO-), which has been identified on several surfaces, such as oxygen pre-covered Pt(111).9,29 Since we are particularly interested on the interaction of methanol with Ru(0001), a more detailed review of the previous works on this surface will follow. On clean Ru(0001), Hrbek et al.24,25 observed, by EELS, that the coverage is decisive on the decomposition of methanol at low temperatures (85 K): for a low coverage, it undergoes O-H bond breaking, leaving adsorbed methoxide (surface reaction 1): Ru(0001)

CH3OH 98 CH3O(ads) + H(ads)

(1)

At saturation coverage, only ∼80% of the methanol molecules yield methoxide, adsorbed with the C-O bond oriented perpendicular to the surface, while the remaining ∼20% undergo C-O and C-H bond breaking, producing water (identified by TPD), hydrogen, and carbon (surface reaction 2): Ru(0001)

CH3OH 98 H2O(ads) + C(ads) + 2H(ads)

(2)

Regarding the kinetics of the decomposition of adsorbed methoxide to CO (surface reaction 3), Ru(0001)

CH3O(ads) 98 CO(ads) + 3H(ads)

(3)

isothermal LITD (laser-induced thermal desorption) measurements revealed that, for low coverage, the initial decomposition rate follows a first-order kinetics, whereas at later times it cannot be fitted by such a simple relation, suggesting a self-poisoned reaction.30 For higher coverage, it was shown that methoxide recombines with surface hydrogen and desorbs as methanol between 220 and 250 K, following a second-order kinetics (reaction 4), Ru(0001)

CH3O(ads) + H(ads) 98 CH3OH

(4)

or further decomposes to carbon monoxide and hydrogen, the reaction being completed at 300 K.24 No other intermediate species were observed on clean Ru(0001). On the basis of ESDIAD (electron-stimulated desorption ion angular distribution) images, Sasaki et al.31 have recently suggested that the methoxide layer formed at 100 K on clean Ru(0001) is disordered, yielding species with a large variety of C-O bond angles toward the surface. Once again, the ESDIAD images acquired during temperature-programmed dehydrogenation of the methoxide species did not contain contributions from other intermediates. Despite all the effort to understand the chemistry of methanol on Ru(0001), the geometry and bonding of the intermediate methoxide remain unclear. It is the purpose of this work to contribute to the understanding of the decomposition of methanol on clean Ru(0001), at different coverages and temperatures, taking advantage of the excellent resolution of reflectionabsorption infrared spectroscopy (RAIRS), associated to its good sensitivity and strict obedience to the metal surface selection rule. Special emphasis was given to the exposure effect on the geometry of methoxide and to the conditions that favor the formation and stabilization of other surface intermediate species, such as formaldehyde. When necessary, confirmation was obtained by adsorbing partially and fully deuterated methanol. 2. Experimental Section The experiments were performed in a Kratos Analytical bakeable ultrahigh-vacuum (UHV) chamber, operating at a base

pressure of 10-10 Torr. Briefly, the reflection-absorption infrared (RAIRS) system consists of a Mattson Research Series 1 FTIR spectrometer coupled, via ancillary optics, to a UHV chamber, which is equipped with a disk-shaped ruthenium single crystal, 1 mm thick and 10 mm diameter. The crystal was cut parallel to the (0001) surface, diamond polished, and mounted in a sample holder with electron-beam heating and liquid-nitrogen cooling facilities. The details concerning the experimental apparatus were fully described elsewhere.32 The Ru(0001) surface was cleaned by sputtering cycles with 2500 eV Ar+ ions, for 30 min, followed by a quick anneal to 1200 K. The cleanliness and smoothness of the surface were tested by the characteristic RAIR spectrum of CO at saturation coverage and at 100 K.33 The RAIR spectra were recorded as the ratio of 1000 co-added scans to the same number of background single beams for the clean surface, with 4 cm-1 resolution. A narrow-band mercury cadmium telluride (MCT) detector, from EG&G Judson, with a wire-grid polarizer, was used. Methanol (CH3OH 99.8%+, from Fluka), and deuterated and partially deuterated methanol (methanol-d4, CD3OD, 99.8%+ and methanol-d3, CD3OH, 99%+, from Aldrich) were purified by distillation under vacuum (10-7 Torr). The crystal surface was exposed to these alcohols, at the required temperature, by back-filling the chamber. Pressures of ∼1 × 10-8 Torr were used for low exposures and ∼5 × 10-7 Torr for high exposures. Exposures are quoted in units of Langmuir (1 L ) 10-6 Torr s) and the corresponding coverages are not calibrated in absolute values. The RAIR spectra were scanned at 90 K, unless stated differently. 3. Results and Discussion 3.1. Formation of Methoxide. The RAIR spectra of methanol and deuterated methanol (methanol-d4) adsorbed on Ru(0001) at 90 K are shown in Figures 1A and 1B, respectively, for increasing exposures (from 0.1 to 10 L). In Figure 1A, the spectrum corresponding to an exposure of 10 L exhibits the characteristic OH stretching mode of methanol, centered at 3285 cm-1. The strong and narrow band at 1045 cm-1 is assigned to the νCO mode and the remaining bands are characteristic of the methyl group. The symmetric stretching mode is visible at 2832 cm-1 and the antisymmetric stretches (combined with deformation overtones) appear at 2955 cm-1, with a shoulder at 2984 cm-1. The antisymmetric and symmetric deformation modes yield weaker, overlapped, bands, with maxima at 1472 and 1446 cm-1, respectively. The weak band at 1130 cm-1 is assigned to the methyl rocking mode, with some contribution of the in-plane C-O-H deformation.11 This is a typical spectrum of a methanol multilayer, although another fingerprint of molecular methanol, the out-of-plane OH bending mode, could not be detected, since it lies below the cutoff of the detector used. In Figure 1B, the spectrum corresponding to 10 L of methanol-d4 shows the strong OD stretching mode at 2442 cm-1. The isotopic substitution has just a slight shifting effect on the CO stretch, which appears as a strong band at 982 cm-1. The antisymmetric and symmetric stretching modes of the CD3 group are shifted to 2218/2247 and 2072 cm-1, respectively. The CD3 antisymmetric deformation is observed at 1067 cm-1 and the symmetric deformation (A′ in the Cs symmetry point group) at 1125 cm-1, intensified by Fermi resonance with the νCO mode (also A′).34 The complete assignments of these spectra and the symmetry species of each vibrational mode in the Cs symmetry point group are given in Table 1. For a very low exposure of CH3OH (0.1 L) the RAIR spectrum is weak, but it shows clear differences when compared

11188 J. Phys. Chem. B, Vol. 105, No. 45, 2001

Barros et al.

Figure 1. RAIR spectra of methanol (A) and methanol-d4 (B) adsorbed on Ru(0001) at 90 K. Exposures as indicated.

TABLE 1: Vibrational Wavenumbers and Mode Assignments of Methanol, Methanol-d4, and Corresponding Methoxide on Ru(0001)a CH3OH

CD3OD

CH3O-

CD3O-

assignment11, 13, 34-36

ν(OH) νas(CH3) (A′) νas(CH3) (A′′) + 2δas(CH3) (A′+A′′) 2832 (m) 2816 (w) νs(CH3) (A′) 2442 (vs) ν(OD) (A′) 2247 (m) 2247 (w) νas(CD3) (A′) 2218 (m) 2228 (m) νas(CD3) (A′′) + 2δas(CD3) (A′+A′′) 2072 (s) 2073 (s) νs(CD3) (A′)35 1472 (w) δas(CH3) (A′+A′′) 1446 (w) δs(CH3) (A′) 1139 (w) F(CH3) (A′+A′′) 1130 (w) F(CH3) (A′+A′′) + γ(OH) (A′) 1125 (s) 1111 (w) δs(CD3) (A′)35 1067 (w) δas(CD3) (A′+A′′) 1045 (vs) 1005 (s) ν(CO) (A′) 982 (vs) 941 (s) ν(CO) (A′) 897 (vw) F(CD3) (A′+A′′)

3285 (vs) 2984 (sh) 2955 (m)

2949 (w) 2926 (m)

a vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh, shoulder.

to the methanol spectrum. The OH stretching band, characteristic of molecularly adsorbed methanol, is absent. The main band in this spectrum is at 1005 cm-1. Also observed are those at 2949, 2926, 2816, and 1139 cm-1. In a similar way, the RAIR spectrum of adsorbed CD3OD for the same exposure (Figure 1B) does not exhibit the OD stretching mode, the dominant band being centered at 941 cm-1, followed by those at 2073, 2228 (with a higher wavenumber shoulder), and the very weak feature at 1111 cm-1. In both cases, the spectra would be compatible with molecular adsorption of the alcohol only if all the molecules were oriented with the O-H (O-D) bond parallel to the surface. This orientation, although not probable, has been admitted on weakly interacting substrates such as clean

Ag(111),11 Cu(110),37 and polycrystalline Pt.38 Besides, the νCO mode is largely shifted to lower wavenumbers when compared to the corresponding multilayer (from 1045 to 1005 cm-1 in CH3OH and from 982 to 941 cm-1 in CD3OD), suggesting that the oxygen is bonded to the metal. In fact, a strong metal-oxygen coordination would be responsible for weakening the C-O bond, with the consequent shift of the νCO mode to lower wavenumbers.39 These observations, associated with different relative intensities of the CH3 (CD3) bands and nonnegligible shifts observed in the νCH3 (νCD3) stretching modes reinforce the hypothesis of dehydrogenation of the alcohol upon adsorption at very low temperature and exposure. The main bands in the 0.1 L spectra are, therefore, assignable to adsorbed methoxide (deuterated methoxide). These observations are consistent with the EELS results obtained by Hrebck et al.,25 of a prompt dissociation of methanol to adsorbed methoxide and hydrogen on Ru(0001), at low temperature and coverage. The orientation of the C-O bond toward the surface in adsorbed methoxide has been argued by different authors.4,11,22,24,31 The methyl deformation region is of no use to this matter, since the modes are too weak to be observed. This has also been the case in other RAIRS studies.11,22,40 The CH stretching region, by the contrary, may be very informative. The assignment of the 2816 cm-1 peak to the symmetric stretch is unambiguous, and its presence is compatible with both a tilted and an upright form. In the later (methoxide in a C3V local symmetry), this was expected to be the strongest band in this region,11 which is not the case in the bottom spectrum of Figure 1A. The two other bands in this region (at 2926 and 2949 cm-1) could be assigned to overtones of the methyl deformations in Fermi resonance with the symmetric stretch.22 If so, the antisymmetric mode would be absent from the spectrum, indicating that the C-O bond was perpendicular to the surface. However, the band at 2949 cm-1 is at a somewhat high wavenumber in comparison to other observations and can more reasonably be assigned to the νasCH3 A′ component of a slightly tilted methoxide (Cs


Figure 2. RAIR spectra of methanol (20 L) adsorbed on Ru(0001) at 100 K and subsequently annealed in 10 K steps for ∼5 min. Only the spectra corresponding to relevant temperatures are shown.

symmetry). The relatively low intensity of the symmetric mode is more consistent with this geometry. In the equivalent spectrum of deuterated methoxide (Figure 1B), the unresolved doublet at 2228 and 2247 cm-1 is assigned to a combination of 2δasCD3 with νasCD3 and to νasCD3 modes of a slightly tilted CD3Oon Ru(0001). Although this conclusion is far from unanimous, the more recent work on methoxide on Ru(0001) at low temperature also suggests a nonupright orientation of the C-O bond.31 By increasing exposure to 0.5 L and subsequently to 2 L, the RAIR spectra in Figure 1A become more similar to that of the multilayer, indicating that methanol over-layers are successively building. Similar conclusions may be drawn from the spectra in Figure 1B, for methanol-d4. In the spectra corresponding to 0.5 L, the νCO and the νOH (νOD) bands are broad and the latter significantly shifted from the characteristic wavenumber for methanol. Associated with small shifts observed for other bands, this suggests some disorder in the first physically adsorbed layers. The hypothesis of a mixture with adsorbed water (resulting from the C-O bond cleavage at saturation coverage, as proposed by Hrbek et al.24) would be speculative and was not confirmed by intermediate exposures between 0.1 and 0.5 L (not shown). We must conclude that no experimental evidence was obtained in this work for this second decomposition path of methanol at low temperature. 3.2. Methoxide Geometries and Thermal Decomposition. To obtain a more packed methoxide layer on Ru(0001), a thick multilayer of methanol was deposited at low temperature and annealed to desorb the molecular methanol ad-layers. The RAIR spectrum obtained by exposing the surface to 20 L of methanol at 100 K and at a high pressure (5 × 10-7 Torr), shown in Figure 2, suggests a more crystalline solid phase than the one obtained by increasing exposures at 90 K (Figure 1). In fact, the νCO and νOH bands are very narrow and the corresponding wavenumbers compare extremely well with those of a crystalline sample at low temperature.35

J. Phys. Chem. B, Vol. 105, No. 45, 2001 11189 Upon annealing the multilayer to 110 K, the spectrum becomes much weaker, the OH stretching band vanishes, and a low-wavenumber component of the νasCH3 band appears at 2926 cm-1. The similarities between this spectrum and that observed in Figure 1A for 0.1 L are enough to assign it to adsorbed methoxide, with the C-O bond tilted toward the surface (as both symmetric and antisymmetric methyl stretching modes are observed). However, the spectrum in Figure 2 is more intense than the corresponding one in Figure 1 (the integrated intensity of the νCO band is approximately three times larger), reflecting a higher coverage. The large shift of the νCO mode to a higher wavenumber (1026 cm-1) may accordingly be explained by dipole coupling between neighbor C-O groups.11,41 It is accompanied by a shift of the νsCH3 band to 2834 cm-1. Both may be associated with a less bound and more disordered methoxide layer, possible taking into account the exposing conditions in this experiment (namely the high introduction pressure). This disorder is in good agreement with the findings by Sasaki et al.31 Therefore, the possibility of different bonding sites and geometries of the methoxide species as a function of coverage must be considered. The DFT calculations referred to in Section 1 have shown that, on clean Cu(111), Ag(111), and Au(111), an isolated methoxide adsorbs preferentially on fcc 3-fold hollow sites, with the C-O bond nearly perpendicular to the surface. These are followed in stability by hexagonal close packed (hcp) 3-fold cavities, with the C-O bond perpendicular to the surface, and by a bridge geometry, with a higher tilting angle (∼30° for the C-O-Cu and C-O-Ag angles, and ∼46° for the C-O-Au angle).6 The less stable geometry on these metal surfaces corresponds to adsorption on-top sites. The wavenumber of the νCO mode depends on the adsorption site, on the tilting angle and, above all, on the surface. As a general trend, it decreases as the interaction with the surface gets stronger, e.g., νCO is at ∼1036 cm-1 for Cu(111)22 (where the calculated adsorption energy for fcc cavities is -241.4 kJ mol-1) and at ∼1048 cm-1 for Ag(111)11 (where the calculated adsorption energy is -178.5 kJ mol-1). A greater adsorption energy implies a more extensive charge transfer from the oxygen to the metal, weakening the C-O bond, and consequently shifting the νCO mode to a lower wavenumber. In the absence of calculations for methoxide on Ru(0001), and by analogy with other metal surfaces (Pt(111),5 Group IB,6 and Al(111)20), we assume that isolated methoxide species will adsorb preferentially on 3-fold hollow sites (the more stable being those on fcc cavities). This would be the case for low methoxide coverage at 90 K, which corresponds to a νCO band maximum of 1005 cm-1 (Figure 1A). In this geometry, the νsCH3 mode would also be affected by the proximity of the surface, which explains the lower wavenumber of this band (2816 cm-1). Even in this case, the RAIR spectra show that the C-O-Ru angle is somewhat different from 180°. For nonisolated adsorbed methoxide (at higher coverage) or for higher temperatures, less stable geometries, such as 3-fold on hcp hollow sites and 2-fold tilted bridge, will become more probable. The broad νCO band at 1026 cm-1 (Figure 2, at 110 K) may result from different bonding sites, not resolved. More revealing of the methoxide geometries are the modifications of the RAIR spectrum after annealing the previous surface to 130 K for 5 min. The band at 1962 cm-1, assigned to adsorbed carbon monoxide, shows that some methoxide has already dehydrogenated, following the expected decomposition route. Meanwhile, the νsCH3 band shifts to 2822 cm-1, and the νCO band of methoxide splits in two components, at 1045 and 1015 cm-1. Apparently, as the CO layer builds up, there is a


Barros et al.

Figure 3. Schematic representation of the proposed adsorption sites and geometries of methoxide on Ru(0001) and respective wavenumbers of the C-O stretching mode. Distances between Ru atoms are exaggerated for clarity.

reorganization of the remaining methoxide in two different geometries. The predominant, to which we assign the 1015 cm-1 and the 2822 cm-1 bands, may result from 3-fold methoxide bonded on hcp cavities. The other, responsible for the 1045 cm-1 component of the νCO band, corresponds to a less bonded methoxide, possibly a 2-fold tilted bridge. As the surface becomes more crowded, the adsorbed methoxide tends to become less bonded to less metal atoms. This change in bonding implies a re-hybridization of the oxygen atom, from the initial sp3 (in methanol and in methoxide adsorbed on 3-fold hollow sites) to a near sp2 in the bridge geometry.42 The shift of the νCO band of methoxide for lower wavenumbers as the interaction with the surface gets stronger is also observed on other surfaces.43 A schematic picture of the proposed methoxide geometries on Ru(0001), as well as the corresponding wavenumber of the νCO mode, are represented in Figure 3. Further annealing the previous layer to 150 K, in small temperature steps, results in additional dehydrogenation of methoxide to CO, as shown by the strong band at 1996 cm-1 (Figure 2). This wavenumber is an indication that the CO coverage at 150 K is significant.33 The weak band at 1045 cm-1 suggests that some methoxide is still stable at this temperature, the preferred adsorption geometry being 2-fold, on tilted bridge, given the high degree of occupation of the surface. An increase in the CO coverage is observed by annealing the surface to higher temperatures in 10 K steps. The presence of H(ads) and the desorption limited formation of H2 are predictable,30 but could not be detected by RAIRS. At 320 K (spectrum not shown), the decomposition is complete, which is in good agreement with previous findings.24 However, our results do not evidence a different decomposition pathway. 3.3. Stabilization of Other Decomposition Intermediates. The series of experiments summarized in Figure 4 allowed stabilizing surface intermediate species other than methoxide. A high dose of methanol (20 L) was adsorbed at a sufficiently high temperature to avoid the multilayer (110 K) and subsequently annealed. The first RAIR spectrum is clearly of a disordered methoxide layer. Given the high exposure used, some C-O bond scission was expectable,24 yielding H2O(ads) or OH(ads). However, we were not able to identify any of these species. The RAIR spectrum obtained after annealing the previous surface to 130 K is similar to the equivalent one in Figure 2, with small band shifts within the spectral resolution. Apparently, a mixed layer of methoxide (adsorbed on cavities and on bridge sites) and surface CO has formed. This layer was annealed to a high temperature (190 K) in one step, without allowing the decomposition of methoxide in successive equilibrium stages. At 190 K, not only the carbon monoxide coverage increases significantly as the amount of

Figure 4. RAIR spectra of methanol (20 L) adsorbed on Ru(0001) at 110 K and subsequently annealed to the indicated temperatures for ∼5 min. Spectra recorded at 110 K.

methoxide decreases, but a surprising feature develops at ∼1270 cm-1, which must be assigned to a different species. Once blown up (inset in Figure 4), it appears to be a composed band. By annealing to 220 K, this band gains intensity and clearly resolves in two components, at 1275 and 1262 cm-1, respectively. Simultaneously, the νCO bands of methoxide and carbon monoxide decrease (the previous spectrum was multiplied by 0.5) and the latter shifts to a lower wavenumber (2013 cm-1). At this temperature, methoxide may be decomposing to the new species or recombining with H(ads), desorbing as methanol. These effects are enhanced upon annealing to higher temperatures. At 420 K, the methoxide decomposition is complete. Apparently, CO is partially displaced from the surface by the new species. Whatever their nature, they are stable up to at least 420 K. In an attempt to form and stabilize these species directly from methanol, two types of experiments were performed at 190 K (the lowest temperature at which the 1275/1262 cm-1 bands were observed). In the first one, the clean surface was exposed to a very high dose of methanol (20 L) and no observable surface species were found (spectra not shown). Probably, the cleavage of O-H, C-O, and C-H bonds occurs upon adsorption. The


J. Phys. Chem. B, Vol. 105, No. 45, 2001 11191

Figure 5. RAIR spectra of methanol (A) and methanol-d3 (B) in a sequential dosing experiment at 190 K (exposures as indicated) and after a subsequent anneal to 290 K. Spectra recorded at 190 K.

second one was a sequential dosing experiment, starting at a very low value (0.05 L). For exposures below 0.1 L, no identifiable species were detected on the surface. For 0.1 L, the RAIR spectrum presents only a small band at 1262 cm-1 (first spectrum in Figure 5A). It gains intensity and a second one develops at 1277 cm-1 for increasing exposures (5 and 20 L). These bands do not shift upon deuteration, as shown in Figure 5B, for the adsorption of methanol-d3 under the same experimental conditions. Consequently, they must be assigned to a CO mode of one or more intermediate species. In Figure 5B, these bands are accompanied by a broad one at ∼1030 cm-1. All of them gain intensity upon increasing exposure successively to 1 and 20 L. Taking into account the chemistry of Group VIII transition metals, and given the consistency with the HREELS results obtained by Mitchell et al.44 on the hydrogenation of CO over-layers on Ru(0001), the band at 1262 cm-1 is tentatively assigned to the νCO mode of bidentate bridging formaldehyde [µ2-η2(C,O)-H2CO], formed upon adsorption. This implies a different decomposition pathway for methanol on Ru(0001) at this temperature, consisting on a partial dehydrogenation rather than a complete fragmentation, due to a decrease in surface reactivity. This “poisoning” effect may be caused by surface carbon (resulting from the first dose), known to reduce the reactivity of other surfaces toward dehydrogenation.45 The band that appears at ∼1275 cm-1 for higher exposures is assignable to the CO stretching mode of the same species, but in a different bonding geometry, i.e., bidentate chelating [µ1-η2(C,O)-H2CO]. In either geometry, the C-O bond would not be oriented parallel to the surface. The proposed bonding structures for the two η2-formaldehyde geometries are schematized in Figure 6. The lower wavenumber observed for the CO stretching mode of η2(C,O)-H2CO in a previous study of formaldehyde adsorption on clean Ru(0001) by HREELS46 can be explained by a lower coverage of η2(C,O)-H2CO in that work, implying less depletion of the local density of electronic states at the metal Fermi level. This leads to more extensive back-donation into

Figure 6. Schematic representation of the two proposed geometries of η2-formaldehyde: bridging and chelating.

the π* orbital of adsorbed H2CO, resulting in a weaker CO bond. In our experiments, these species are always observed for surfaces highly covered with passivating coadsorbates. The ωCH2 mode, expected for this species at ∼1280 cm-1,44 may be overlapped with the νCO band in the spectra in Figure 5A. In Figure 5B, the broad band at ∼1030 cm-1 may be assigned to the ωCD2 mode and its shape modification with increasing exposure is possibly due to the coexistence of the two formaldehyde geometries. These results are comparable with those obtained by Henderson et al. for the adsorption of acetaldehyde and ketene hydrogenation on Ru(0001).47,48 In both cases, there are intense electron energy loss bands between 1260 and 1290 cm-1, assigned to the νCO mode of η2(C,O)CH3CHO and its deuterated derivatives. In summary, when methanol is adsorbed on clean Ru(0001) at 190 K (both at high and very low exposures), O-H, C-H, and C-O bond scission occurs, leaving no RAIRS detectable surface species. To form and stabilize η2-formaldehyde, it is necessary that adsorption occurs by sequential dosing. Thus, the preformed fragments may have a passivating effect on Ru(0001), which becomes less active toward C-H bond scission, thus behaving more like Group IB metal surfaces. The relative amounts of the two proposed η2-formaldehyde configurations depend on coverage: apparently, the bridging is more stable at lower coverage, since it requires a larger number of sites per adsorbed species. An identical decomposition path is


Figure 7. RAIR spectra of methanol-d4 (20 L) adsorbed on Ru(0001) at 110 K and subsequently annealed to 270 K.

followed by methanol-d3. There is no evidence of η1-formaldehyde, since this species would originate a characteristic carbonyl band, at ∼1700 cm-1.

Barros et al. Upon heating the surface to 290 K (top spectra in Figures 5A and B), the η2-formaldehyde is still stable, in contrast to what happens on clean Ru(0001), where complete decomposition of this species occurs at 300 K.46 However, there is a partial decomposition into CO(ads), identified by the narrow band at 1989 cm-1 (1991 cm-1 in methanol-d3). On the contrary, on Ru(0001) with a high pre-coverage of CO(ads) (spectra in Figure 4), this dehydrogenation process is completely inhibited by a blocking effect. Similar observation was made on Fe(100), where an overlayer c(2 × 2) of carbon completely blocks the pathway for dehydrogenation of H2CO.45 In an attempt to identify surface water or hydroxyl groups by decomposition of a high exposure of methanol on Ru(0001), at low temperature, the experiments related to the spectra in Figure 4 were repeated using fully deuterated methanol. Two representative RAIR spectra are shown in Figure 7. The bottom spectrum was obtained after exposing the surface to 20 L of methanol-d4, at 110 K. The decomposition product is almost exclusively carbon monoxide plus residual methoxided3 (identified by the weak νCO band at 976 cm-1). The high CO coverage suggested by the band at 2043 cm-1 shows that the thermal activation at 110 K is already enough for significant C-D bond breaking. Despite the greater dissociation energy of the C-D bond versus C-H, the formation of CO at such a low temperature may be explained by a cooperative surface effect. In fact, there is a higher driving force toward the

Figure 8. Decomposition pathways of methanol on Ru(0001), indicating the proposed surface species. Those not actually identified by RAIRS are between brackets; (a) -adsorbed.

Decomposition Pathways of Methanol on Clean Ru(0001) formation of the Ru-D bond relative to Ru-H, as shown by the corresponding adsorption energies of 80 and 69 kJ mol-1, respectively.49 By annealing to 270 K, the methoxide fingerprint disappears without yielding carbon monoxide. In fact, the νCO band decreases and shifts to a lower wavenumber. Therefore, recombination [CD3O(ads) + D(ads) f CD3OD(g)] seems to be the more efficient process, similarly to the proposed for CH3OH above 290 K. One of the purposes of adsorbing methanol-d4 (to confirm whether part of the methanol undergoes C-O bond breaking at low temperature and high coverage, yielding D2O or OD groups) could not be fulfilled. The spectrum obtained after adsorbing 20 L at 110 K shows a very small band around 3000 cm-1, but this is a rather high wavenumber to allow a reasonable assignment to the νOD mode of OD(ads) or D2O(ads),39 even very weakly bonded to an almost CO saturated surface. 4. Conclusions By using RAIRS, we believe that an input to the understanding of methanol decomposition on clean Ru(0001) was brought. A schematic representation of the proposed surface species formed by different decomposition pathways is presented in Figure 8. At a low temperature (90 K), it was confirmed that a low exposure of methanol (0.1 L) readily dehydrogenates to methoxide (CH3O-). It is proposed that, for such a low coverage, this species adsorbs on face-centered cubic (fcc) 3-fold hollow sites (νCO mode at 1005 cm-1) with the CO bond slightly tilted toward the surface (both νasCH3 and νsCH3 are observed). It is shown that the adsorption site and binding geometry of methoxide depend on coverage and on temperature and a characteristic wavenumber of the νCO mode is proposed for each geometry (1015 and 1045 cm-1, respectively, for hcp 3-fold hollow sites and bridged sites). For high exposures of methanol at low temperature, no clear evidence of C-O bond scission was obtained. Methanol multilayers build at low temperature, but desorb below 110 K. Methoxide further decomposes to yield the usual products CO(ads) and H(ads), if the surface is annealed to high temperatures in small steps. A different intermediate is formed and stabilized when annealing a mixed layer of CH3Oand CO in a single large temperature step, by partial dehydrogenation of methoxide: η2-formaldehyde in two bidentate configurations, bridging [µ2-η2(C,O)-H2CO] and chelating [µ1-η2(C,O)-H2CO], with the characteristic νCO mode at 1262 and 1277 cm-1, respectively. These species are extremely stable in the presence of CO and other surface fragments (up to at least 420 K). Evidence of methoxide recombination with H(ads) below 420 K was obtained. By direct adsorption of methanol at high temperature (190 K), no products detectable by RAIRS are obtained, either for very low (

The Decomposition Pathways of Methanol on Clean Ru(0001

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