Thermal Decomposition of Thin Methanol Films on Deoxygenated

May 15, 2014 - By heating multilayer films at T > 120. K, hydrogen liberation occurs along with a strong depression of methanol desorption because met...
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Thermal Decomposition of Thin Methanol Films on Deoxygenated Vanadium Ryutaro Souda* International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ABSTRACT: A polycrystalline V sheet was deoxygenated via high-temperature annealing. Then, interactions with methanol films deposited on it were investigated at T < 200 K using temperature-programmed desorption and time-of-flight secondary ion mass spectrometry. The methanol decomposes at the interface when the film is deposited at 70 K, as inferred from modifications of sputtered ion intensities. By heating multilayer films at T > 120 K, hydrogen liberation occurs along with a strong depression of methanol desorption because methanol is decomposed sequentially at the interface. No release of reacted species including oxygen and carbon was identified, except for a trace of water and methane. The hydrogen is abstracted not only from the O−H bond but also from the C−H bond, as evidenced by isotope scrambling of liberated hydrogen. The results differ significantly from those of previous studies that examined single-crystal V substrates. The reduction of oxygen content in the bulk of the V substrate is necessary for multilayer methanol decomposition because reacted species including oxygen must be incorporated in the subsurface layers. surface.43 The reaction of methanol on clean and oxygencovered V(100) surfaces was also studied using temperatureprogrammed desorption (TPD),45 X-ray photoelectron spectroscopy (XPS),45 and first-principles calculation.46 Shen and Zaera45 reported that the O−H bond of CH3OH is broken to form CH3O at 260 K, which is followed by C−O bond scission at 320 K to yield CH3 species. Methane desorption is observed in two states. Hydrogen also desorbs in two peaks at 280 and 520 K, which are attributed to thermal decomposition of the hydroxyl and methyl moieties, respectively. First-principles calculation revealed that CH3O is the major intermediate in the decomposition process on the V(100) surface and that the C− O bond scission to form CH3 is much easier than the C−H bond scission to form formaldehyde.46 To date, most studies of molecular dissociation on metal surfaces have been conducted to examine (sub)monolayer coverage because it is presumed intuitively that physisorbed molecules simply desorb with no reactions with metal substrates upon heating. In fact, the evaporation temperature of methanol multilayers (approximately 150 K) appear to be too low to induce thermal decomposition as described above. However, it was demonstrated recently that water molecules in multilayer films were decomposed preferentially at the interface of a deoxygenated V surface at temperatures higher than approximately 140 K, as evidenced by hydrogen liberation and strong depression of water desorption.47 The uptake of oxygen

1. INTRODUCTION The interaction of alcohols with catalytically active surfaces has attracted considerable attention because alcohols are important precursors for the production of many chemicals. The liberation of hydrogen from small alcohols is a promising method for fuel cells not only because of the high hydrogen-tocarbon ratio but also because the problems of hydrogen storage can be circumvented.1−5 Much interest has arisen surrounding the catalytic decomposition of methanol on metal substrates.6−42 Hydrogen abstraction from the OH group and the formation of surface methoxide species are thought to be the first step for methanol dissociation. Two pathways are possible for additional reactions: (i) C−H bond scission to form formaldehyde (CH2O), which in turn decomposes to CO and H2, and (ii) C−O bond scission, which results in stable CH3 adspecies or surface carbon as a byproduct. The activity of adsorbed methanol is strongly dependent on the nature of metal surfaces. Total dehydrogenation of methanol into CO and H2 is the main reaction pathway on late transition metals, whereas the C−O bond scission is usually favored on early transition metals. Recently, the reaction of methanol on clean and oxidemodified and carbide-modified vanadium surfaces has been investigated because they are important industrial catalysts.43−46 Zellner et al.44 reported that methanol undergoes complete dissociative adsorption on V(110) and carbidemodified V(110) at 100 K using high-resolution electron energy loss spectroscopy (HREELS). The resulting methoxy species undergo further decomposition at higher temperatures (T > 300 K). Similar results were obtained using a VC(100) © 2014 American Chemical Society

Received: January 31, 2014 Revised: May 7, 2014 Published: May 15, 2014 11333

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exposing the substrate to the gas by direct backfilling of the chamber via high precision leak valves.

into the V substrate was suggested to be responsible for this reaction. As described in this paper, thermal decomposition of methanol multilayers on the deoxygenated V surface was investigated at temperatures below 200 K based on TPD and time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements. Results show that the reaction pathways of multilayers differ dramatically from those of submonolayer methanol on the V(100) and V(110) substrates. Hydrogen molecules descended from the hydroxyl moiety are identified as the main reaction product. A smaller amount of hydrogen from the methyl moiety is also observed. They are released in a narrow temperature range of 120−150 K together with a much smaller amount of nonreacted methanol.

3. RESULTS Figure 1 presents typical TOF-SIMS spectra of (a) positive and (b) negative ions sputtered from the V surface before and after

2. EXPERIMENTAL SECTION Experiments were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of less than 1 × 10−10 Torr. The procedure for TOF-SIMS measurements was described elsewhere.47−49 Briefly, a primary beam of 2 keV He+ ions was generated in an electron-impact-type ion source and was chopped into pulses using electrostatic deflectors. Positive and negative secondary ions ejected perpendicularly to the surface were detected using a microchannel plate after traveling a fieldfree TOF tube. To extract low-energy secondary ions efficiently, a bias voltage (±500 V) was applied to the sample surface. The fluence of He+ was kept below 1 × 1012 ions cm−2 to ensure the static condition for SIMS measurements. TPD measurements were made using a quadrupole mass spectrometer (QMS) placed in a differentially pumped housing. The desorbing species were sampled with the substrate positioned approximately 3 mm from the front aperture of the housing. The temperature was ramped at a rate of 5 K min−1 for both TPD and TOF-SIMS measurements. A polycrystalline sheet of V (99.7%, 0.1 mm thick) was used as a substrate. It was spot-welded to a Ta holder. The sample holder was transferred into the UHV chamber via a load-lock system. A Ni(111) substrate was used as a reference surface, which was cleaned several times by flash heating (approximately 1200 K) in UHV by electron bombardment from behind. Oxygen contaminants were removed by this treatment as revealed from the TOF-SIMS spectra. However, the V surface cannot be cleaned satisfactorily via sputter annealing (approximately 1300 K) because stable oxides are formed in the bulk. Oxygen atoms tend to segregate to the surface upon heating.50,51 It might be necessary to heat the sample at higher temperatures closer to the melting point for removal of oxygen and other contaminants from reactive metals such as V. In this study, a thin foil of V was used to attain better deoxygenation from the bulk. It was heated many times to approximately 1700 K until no traces of oxygen contaminants were detected using TOF-SIMS. The samples of CH3OH (99.8%; Kanto Chemical Co. Inc.) and CD3OH (99.8%; Aldrich Chemical Co.) were degassed by repeated freeze−pump−thaw cycles before use. Before each dosing, the gas line to the UHV chamber was evacuated, and old vapor was replaced by a fresh one. The sample was mounted on the end of a Cu coldfinger extended from a closed-cycle helium refrigerator cooled to 20 K. Temperature was controlled using a digital temperature controller by monitoring the temperature of the coldfinger close to the sample position using Au(Fe)−chromel thermocouples. Thin films of methanol were deposited by

Figure 1. TOF-SIMS spectra of (a) positive and (b) negative ions sputtered from the deoxygenated V surface before and after exposure to 5 L CH3OH at 70 K.

exposure to 5 L (langmuir; 1 L = 1 × 10−6 Torr s) methanol at 70 K. Spectra from the clean surface include a peak at the leading edge, which originates from backscattered He0 atoms. Small amounts of contaminants are recognizable in Figure 1a at m/q = 51 (V+) and 83 (V+(CH3OH)). They come from the background methanol adsorbed during a sample cooling time (approximately 10 min); no such ions were detected immediately after heating. The small peak at m/q = 19 in Figure 1b is assignable to F−. We believe that these TOF-SIMS spectra ensure cleanliness of the V surface (i.e., the absence of oxygen contaminants). Upon methanol deposition, a spectrum of cations consists of protonated methanol, H+(CH3OH), and fragment ions, such as H+, CH3+, CHO+, and CH3O+, together with V+ and its adducts such as V+(CH3OH) and VOH+. Regarding anions, the methanol adspecies yields fragment ions such as H−, CH−, O−, and OH−, together with CH3O− and its adducts. These ions are created during atomic collision processes. The intensity of backscattered He0 is used to normalize the primary He+ current (approximately 1 pA) when beam fluctuation occurs during the temperature scan. Mainly for this reason, we have used He+ instead of heavier ions such as Ar+ as a primary beam of TOF-SIMS measurements.48,49 Figure 2 presents intensities of typical (a) cations and (b) anions sputtered from the deoxygenated V substrate as a function of exposure to the CH3OH molecules. The initial evolutions of cations are gradual compared to those of anions. No saturation behavior is clearly recognizable in intensities of the cation fragments and protonated methanol, although the V+ 11334

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adspecies tend to be saturated at around 2.5 L. The Ni+ and Ni+(CH3OH) ions form a peak at around the saturation point of the fragment ions, which is followed by an exponential decay in intensity with increasing exposure. On the basis of this result, we have inferred that 1 monolayer (ML) of methanol is formed at exposure of approximately 2.5 L. Similar behaviors (i.e., the saturation of fragment ions and the peak of Ni+) have been observed for other adspecies.48,49 Consequently, the results obtained using the V substrate are exceptional. The cation (anion) intensities from the V substrate are suppressed (enhanced) at exposures less than 10 L (corresponding to 4 ML), indicating that interfacial reaction influences the ionization probability of sputtered species from the thin-film surface. The VOH+ intensity is considerably high relative to that of V+(CH3OH), which contrasts sharply with the relative intensities of NiOH+ to Ni+(CH3OH). The VOH+ species can be formed during reactions of the sputtered V atoms and the CH3OH molecule on the way out from the surface because VOH+ is emitted efficiently even after formation of the multilayer film. Effects of the substrate are obscured when the physisorbed molecular layer is formed, but such is not the case for the secondary ion emission from methanol on the V substrate. Ejection of the H+(CH3OH) ion is indicative of the formation of the physisorbed methanol layer; its intensity is suppressed considerably on the V substrate, especially at smaller coverage. Modifications of the cation and anion intensities suggest strongly that the ionization probability is controlled by the interfacial properties. Dissociative adsorption of methanol to yield a methoxy species is observed on V(110) at 100 K.44 Therefore, it is possible that the decomposed (or chemisorbed) species at the interface play a role in injection of the metal valence electron into the sputtered species across a few physisorbed layers. Figure 4a displays intensities of typical anions sputtered from the 2 ML methanol film deposited on deoxygenated V at 70 K. The intensities are almost constant up to 135 K. Those except for H− increase. At this temperature, a physisorbed methanol layer is thought to disappear. In fact, the changes in ion intensities during desorption of the physisorbed species appear to be an inverse of those observed during the deposition of methanol (see Figure 2b). The ions emitted from the surface at higher temperature arise from the chemisorbed methanol or its fragments. The assignment of such residues is difficult based on the TOF-SIMS spectra, but the chemisorbed methanol or methoxide is present on the surface, as revealed from the emission of the CH3O− ion. They undergo further dissociation at higher temperatures (T > 200 K), as discussed in previous studies.44−46 The molecular species desorbed at around 135 K are dominated by hydrogen rather than methanol, as evidenced by the TPD spectra portrayed in Figure 4b. This result illustrates clearly that the physisorbed methanol molecules on the deoxygenated V surface undergo significant decomposition rather than simple desorption. From the CH3OH film (2 ML) deposited on the Ni(111) substrate, no desorption of hydrogen is detectable at T < 200 K relative to the background level (not shown). The integrated methanol peak intensity (m/q = 32) from V is at most 2% of that from the Ni(111) substrate. Consequently, desorption of physisorbed species is detected only slightly from the deoxygenated V substrate despite the fact that the methanol multilayer can be deposited initially on the surface, as evidenced by TOF-SIMS measurements (Figures 2 and 4a). That result is explainable as that the physisorbed

Figure 2. Intensities of typical (a) cations and (b) anions sputtered from the deoxygenated V surface as a function of exposure to CH3OH at 70 K.

and V+(CH3OH) intensities tend to form a broad peak at around 5 L. The intensities of anions in Figure 2b saturate or form a peak at around 2.5 L. The disagreement of the saturation point between anions and cations is characteristic of the V substrate. For comparison, results obtained using the Ni(111) substrate are depicted in Figure 3. The intensities of both (a) cations and (b) anions sputtered from the methanol

Figure 3. Intensities of typical (a) cations and (b) anions sputtered from the Ni(111) surface as a function of exposure to CH3OH at 70 K. 11335

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Figure 5. TPD spectra of some molecular species desorbed from a thin methanol film prepared by deposition of 20 L CH3OH molecules on the deoxygenated V substrate at 70 K.

desorption of CH4, strongly suggests that the C−O bond scission occurs preferentially. The hydrogen abstraction from the O−H and C−H bonds and its roles in the formation of hydrogen, water, and methane are further investigated using a CD3OH film (20 L) deposited onto the deoxygenated V substrate. The TPD spectra of related species are displayed in Figure 6. Decomposition of the methyl moieties at 140 K is evidenced by the release of the DH (m/q = 3) and D2 (m/q = 4) molecules, but their intensities are less than the intensity of the H2 molecule. The formation of

Figure 4. (a) Temperature-programmed TOF-SIMS intensities and (b) TPD spectra of methanol and hydrogen from the deoxygenated V surface after exposure to 5 L CH3OH at 70 K.

methanol is consumed preferentially via the interfacial reaction. The reacted species other than hydrogen is below the detection limit at temperatures lower than 200 K. No desorption of molecules including hydrogen is observed at lesser exposure (1 L; not shown), which is consistent with previous studies that used the V(100) surface at submonolayer coverage.46 The methanol multilayer tends to be incorporated in the V substrate after fragmentation. Otherwise, an extremely small desorption yield of methanol cannot be explained. Consequently, decomposition of the multilayer methanol film is extremely efficient for the deoxygenated V substrate at around 130−140 K, but this reaction channel has not been identified to date. The mechanism of molecular dissociation at low temperature is explored further using a thicker CH3OH film (20 L or 8 ML) based on TPD. The temperature evolutions of some QMS signals are depicted in Figure 5. Hydrogen desorption is still dominant over methanol at this thickness. The integrated TPD peak area of CH3OH (m/q = 32) is at most 5% of that observed using the Ni(111) substrate (not shown), indicating that intake of the reacted species occurs even for the thicker methanol film. To confirm the possibility of desorption of other molecular species such as methane, water, carbon monoxide (or ethylene), and formaldehyde, the respective TPD spectra of m/ q = 16, 18, 28, and 30 species are also displayed. Care must be taken in their assignment because they can be created via electron impact of desorbed methanol in the QMS ionizer. The relative intensities of m/q = 2, 16, 18, 28, and 30 signals descended from CH3OH is approximately 0.16, 0.04, 0.06, 0.1, and 0.08, respectively, relative to the m/q = 32 signal. The methane and water molecules released from the surface can contribute to the m/q = 16 and 18 signals, whereas the m/q = 28 and 30 signals are explainable as methanol fragmentation. The absence of CO and H2CO molecules, together with

Figure 6. TPD spectra of some molecular species desorbed from a thin methanol film prepared by deposition of 20 L CD3OH molecules on the deoxygenated V substrate at 70 K. 11336

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reaction route at higher temperature (approximately 300 K) because methane is the main product of irreversible methanol decomposition. The TPD experiments using deuterium-labeled molecules (CD3OH) revealed that the hydrogen desorption takes place at around 280 K as a result of the thermal decomposition of the hydroxyl moieties. Decomposition of the methyl moieties is identified at much higher temperature (500 K). No significant scrambling is apparent in either hydrogen desorption state. However, the HREELS study revealed that complete scission of the O−H bond occurs on the V(110) surface, even at 100 K.44 Vibrational frequencies of only the methoxy intermediate were observed up to 300−400 K. On the basis of these observations, a consensus arises that methyl moieties remain intact on the single-crystal substrates at temperatures higher than 300 K. On the deoxygenated V surface, the O−H bond scission is likely to occur at the film deposition temperature of 70 K. The presence of methoxide at the interface is inferred from the significant modification of the cation and anion yields from the as-deposited methanol films (Figure 2), as well as the presence of the CH 3 O − ions in TOF-SIMS (Figure 4) after disappearance of the physisorbed layer. No hydrogen release is observed at 140 K during decomposition of (sub)monolayer methanol in present or earlier studies, suggesting that a smaller amount of hydrogen can remain on the surface together with the reacted (methoxy) species. It is also possible that a small amount of hydrogen can be incorporated in the substrate. For the methanol multilayer on the deoxygenated V surface, the formation of methoxide and cleavage of the C−O bond are likely to occur sequentially at the interface in a narrow temperature range around 140 K. The uptake of oxygen in the V substrate to form vanadium oxides is probably the ratedetermining step for the latter, as inferred from the fact that no decomposition of methanol is observed at this temperature when using an oxygen-contaminated V(100) substrate.45 The interfacial properties might not affect the thermal desorption rate of molecules from the solid surface, but the surface and interface phenomena can be mutually correlated if mobile molecules (i.e., liquids) play a role. A supercooled liquid of methanol is created at the glass transition temperature (Tg) of 103 K,49 but neither interfacial reaction nor thermal desorption is induced at this temperature. Thermal desorption occurs after methanol crystallizes at 120 K.49 A liquidlike layer is present on the surface or grain boundaries of crystallites, as inferred from the film morphology change after crystallization.49 This behavior is explainable in terms of premelting. A quasi-liquid layer can be exhausted preferentially via the interfacial reaction, leading to the consumption of crystallites. Consequently, most mobile molecules formed in the crystalline methanol films react with the V substrate without desorption from the surface. The onset of thermal desorption of methanol (120 K) is lower than the temperature at which the uptake of oxygen adspecies becomes evident (140 K).47 In the case of water, however, the thermal desorption and decomposition of molecules commence at around this temperature, which corresponds to the Tg value (136 K) of water.47 The low onset temperature of methanol decomposition may suggest that oxygen abstraction from the C−O bond of the methoxide is facilitated. The resulting methyl species is bound to V, which accelerates partial dehydrogenation from the C−H bond and scrambling with the pre-existing hydrogen descended from the O−H bond.

methoxy species is likely to precede the decomposition of the methyl moiety,44 so that the DH molecule is created via recombination of D atoms with preexisting H atoms. This fact also explains why the intensity of D2 is so small. Consequently, the abstraction of deuterium from the methyl moiety is not efficient relative to that of hydrogen from the hydroxyl moiety during the low-temperature chemical reaction. The H atoms originated from the hydroxyl moiety also react with the surface CD3 species, thereby forming the m/q = 19 species (CD3H).

4. DISCUSSION Hydrogen is released predominantly relative to other molecular species at temperatures lower than 200 K. Therefore, the oxygen and carbon atoms tend to be accumulated on the surface as a result of methanol decomposition. The V surface can be inactivated by deposition of oxygen because available vanadium binding sites are reduced by the development of vanadium oxides. Indeed, a V2O3 and VO2 mixed oxide is formed in subsurface layers at T > 170 K, as revealed by XPS.50 For catalytic decomposition of multilayer methanol to occur, it is necessary that oxygen atoms be removed from the near surface region of V. In this respect, chemisorbed oxygen is known to be incorporated in the bulk of deoxygenated V at temperatures higher than 140 K.47 The decomposition of water multilayers also commences at this temperature. Probably, sequential uptake of oxygen occurs at the interface to form vanadium oxides in subsurface layers, thereby maintaining chemical reactivity of the deoxygenated V substrate against the multilayer films. However, the fact that the TPD yield of deuterium from CD3OH is lower than that of hydrogen suggests that the abstraction of deuterium from the methyl moiety is incomplete. The resulting CDx species should also be removed from the interface to ensure decomposition of the methanol multilayer. The reaction route of these species to form carbides and hydrides on V is unknown. It is possible that the CD2 species is incorporated directly in V without further decomposition, which explains why hydrogen liberation from the methyl group is inefficient relative to that from the hydroxyl group. This assumption is also supported by the experimentally proven fact that hydrogen liberation is marginal during decomposition of the formaldehyde multilayer films on the deoxygenated V substrate.52 The effect of carbon species on the surface reactivity of V is expected to be milder than that of oxygen because a smaller amount of electron is transferred from V to carbon adspecies. This point is also inferred from the fact that the surface chemistry of methanol on the carbide-modified V(110) and VC(100) surfaces resembles that of the clean V surface at submonolayer coverage.43,44 Deoxygenation of the V substrate appears to be necessary for opening the reaction channel of methanol multilayer decomposition. In fact, the surface chemistry of the single crystals of V44,45 is distinct from that described here. Probably, this distinction arises because the sputter−annealed surface is insufficient to reduce the oxygen content in the bulk or subsurface layers to a sufficiently low level for sorption of additional oxygen and carbon species. The experimental results obtained using single-crystal substrates are summarized below for comparison with the present result. The methane and hydrogen desorb from the V(100) surface at temperatures higher than 280 and 320 K, respectively.45 Their desorption yields level off at methanol exposure of 0.5 L, indicating that the physisorbed methanol layer has nothing to do with the surface chemistry. The C−O bond scission is assigned as the dominant 11337

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(9) Fukui, K.; Aruga, T.; Iwasawa, Y. Novel Reaction Path Induced by Selective Blocking of Surface Atoms: Methanol Dehydrogenation on Mo(112)-(1 × 2)-O. Surf. Sci. 1993, 295, 160−168. (10) Lu, J. P.; Albert, M.; Bernasek, S. L.; Dwyer, D. S. Decomposition of Methanol on Oxygen-Modified Fe(100) Surfaces 0.1. The Effect of High-Temperature Oxygen Modification. Surf. Sci. 1989, 218, 1−18. (11) Rufael, T. S.; Batteas, J. D.; Friend, C. M. The Influence of Surface Oxidation on the Reactions of Methanol on Fe(110). Surf. Sci. 1997, 384, 156−167. (12) Barros, R. B.; Garcia, A. R.; Ilharco, L. M. Effect of Oxygen Precoverage on the Reactivity of Methanol on Ru(001) Surfaces. J. Phys. Chem. B 2004, 108, 4831−4839. (13) Habermehl-Cwirzen, K.; Lahtinen, J.; Hautojarvi, P. Methanol on Co(0001): XPS, TDS, WF and LEED Results. Surf. Sci. 2005, 598, 128−135. (14) Parmeter, J. E.; Jiang, X. D.; Goodman, D. W. The Adsorption and Decomposition of Methanol on the Rh(100) Surface. Surf. Sci. 1990, 240, 85−100. (15) Houtman, C.; Barteau, M. A. Reactions of Methanol on Rh(111) and Rh(111)-(2 × 2)O Surfaces: Spectroscopic Identification of Adsorbed Methoxide and η1-Formaldehyde. Langmuir 1990, 6, 1558−1566. (16) Solymosi, F.; Berko, A.; Tarnoczi, T. I. Adsorption and Decomposition of Methanol on Rh(111) Studied by Electron-Energy Loss and Thermal-Desorption Spectroscopy. Surf. Sci. 1984, 141, 533−548. (17) Bare, S. R.; Stroscia, J. A.; Ho, W. Characterization of the Adsorption and Decomposition of Methanol on Ni(110). Surf. Sci. 1985, 150, 399−418. (18) Jorgensen, S. W.; Madix, R. J. Hydrogen Transfer Pathways in the Oxidation of Methanol on Pd(100). Surf. Sci. 1987, 183, 27−43. (19) Pratt, S. J.; Escott, D. K.; King, D. A. Multilayer Growth and Chemisorbate Reactivity of Methanol on Pd{110}. J. Chem. Phys. 2003, 119, 10867−10878. (20) Rebholz, M.; Matolin, V.; Prins, R.; Kruse, N. Methanol Decomposition on Oxygen Precovered and Atomically Clean Pd(111) Single-Crystal Surfaces. Surf. Sci. 1991, 251, 1117−1122. (21) Chen, J. J.; Jiang, Z. C.; Zhou, Y.; Chakraborty, B. R.; Winograd, N. Spectroscopic Studies of Methanol Decomposition on Pd(111). Surf. Sci. 1995, 328, 248−262. (22) Zhang, C. J.; Hu, P. A First Principles Study of Methanol Decomposition on Pd(111): Mechanisms for O−H Bond Scission and C−O Bond Scission. J. Chem. Phys. 2001, 115, 7182−7186. (23) Kizhakevariam, N.; Stuve, E. M. Promotion and Poisoning of the Reaction of Methanol on Clean and Modified Platinum (100). Surf. Sci. 1993, 286, 246−260. (24) Lu, C.; Thomas, F. S.; Masel, R. I. Chemistry of Methoxonium on (2 × 1)Pt(110). J. Phys. Chem. B 2001, 105, 8583−8590. (25) Sexton, B. A. Methanol Decomposition on Platinum (111). Surf. Sci. 1981, 102, 271−281. (26) Ryberg, R. The Oxidation of Methanol on Cu(100). J. Chem. Phys. 1985, 82, 567−573. (27) Wachs, I. H.; Madix, R. J. Selective Oxidation of CH3OH to H2CO on a Copper(110) Catalyst. J. Catal. 1978, 53, 208−227. (28) Francis, S. M.; Leibsle, F. M.; Haq, S.; Xiang, N.; Bowker, M. Methanol Oxidation on Cu(110). Surf. Sci. 1994, 315, 284−292. (29) Davies, P. R.; Mariotti, G. G. Oxidation of Methanol at Cu(110) Surfaces: New TPD Studies. J. Phys. Chem. 1996, 100, 19975−19980. (30) Chen, A. K.; Masel, R. Direct Conversion of Methanol to Formaldehyde in the Absence of Oxygen on Cu(210). Surf. Sci. 1995, 343, 17−23. (31) Dai, Q.; Gellman, A. J. A HREELS Study of C1-C5 Straight Chain Alcohols on Clean and Pre-oxidized Ag(110) Surfaces. Surf. Sci. 1991, 257, 103−112. (32) Schwaner, A. L.; Fieberg, J. E.; White, J. M. Methyl Formate on Ag(111). 1. Thermal Adsorption-Desorption Characteristics and Alignment in Monolayers. J. Phys. Chem. B 1997, 101, 11112−11118.

5. CONCLUSION The interaction of methanol with the deoxygenated V substrate was investigated in comparison with those with the Ni(111) substrate using TOF-SIMS and TPD. The sputtering yields of cations and anions from methanol deposited on the V substrate differ significantly from those on the Ni(111) substrate because methanol decomposes preferentially at the interface with V. The ionization probability is influenced by the reacted species formed at the interface of deoxygenated V. The multilayer film of methanol grows on the deoxygenated V substrate at 70 K after the surface is passivated by the reacted species (probably methoxide). The multilayer decomposition commences at T > 120 K, as evidenced by the hydrogen liberation and depression of methanol desorption. For ensuring high interfacial reactivity against methanol multilayers, not only the C−O and C−H bond scission of methoxide but also intake of the reacted species including oxygen and carbon is prerequisite. The CD3 group further decomposes to form deuterium, as evidenced by scrambling with hydrogen from the hydroxyl group. The onset temperature of multilayer decomposition agrees with the crystallization temperature of methanol, suggesting that the mobile molecules formed on the boundaries of crystallites (i.e., quasi-liquid) play a role in both thermal desorption and interfacial reaction. Consequently, the scheme of chemical reaction of the methanol multilayer is markedly distinct from that of the methanol adspecies in the submonolayer regime. The sequential decomposition of the methanol multilayer is obscured when deoxygenation of the substrate is insufficient.

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp501098v | J. Phys. Chem. C 2014, 118, 11333−11339