Thermal Decomposition of Thin Methanol Films on Deoxygenated Vanadium

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.

Decomposition of Formaldehyde Multilayer Films on Deoxygenated, Oxygenated, and Water-Adsorbed Vanadium Substrates at Cryogenic Temperatures

“…In this respect, the TPD H 2 yield from formaldehyde presented in Figure 2b is less than 5% of that from 8 ML water during decomposition on the deoxygenated V substrate, suggesting that H 2 is descended mainly from the hydroxyl group. 16 In this context, the interaction of oxygen with the V substrate is instructive for elucidating the reaction mechanism of formaldehyde. The O 2 molecule is adsorbed dissociatively onto the deoxygenated V substrate at 70 K. The surface is saturated with O 2 exposure of approximately 2 langmuirs, as determined from evolution of the V + intensity in TOF-SIMS (not shown).…”

“…The contaminant level of oxygen can be reduced satisfactorily, as confirmed using TPD and TOF-SIMS. 15,16 The deoxygenated V sample was further modified by adsorption of extra dry oxygen (5 N) and water molecules. Formaldehyde was obtained by heating a solid paraformaldehyde sample (Kanto Chemical) that was outgassed thoroughly before use.…”

Interactions of multilayer ethanol, acetone, and diethyl ether films with clean and oxygenated vanadium substrates

“…This possibility is inferred from the experimental fact that HD molecules are released considerably relative to H 2 during decomposition of CD 3 OH lms on the deoxygenated V substrate. 29 In the case of ethanol decomposition, ethylene can be formed via the hydrogen abstraction from the ethyl species. However, the ethylene yield is so small that most of the ethyl species is thought to be dissociated into methyl and methylene species via the C-C bond cleavage.…”

“…The hydrogen abstraction can occur at this stage from the methyl species as well. The dehydrogenation is ineffective from the methylene species; 29,30 it can be incorporated in the subsurface layers without further dissociation.…”

“…In contrast to molecules in the submonolayer regime, those in multilayer lms are thought to desorb without decomposition because physisorbed molecules are not inuenced by chemical properties of metal substrates. It was recently demonstrated that this conventional picture does not hold for a clean V substrate: most of water, 28 methanol, 29 and formaldehyde 30 molecules in thin lms are decomposed. However, the reaction mechanism of multilayer lms at the interface is not fully understood.…”