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

Abstract

Interactions of thin formaldehyde films with a polycrystalline V substrate were investigated using temperatureprogrammed desorption and time-of-flight secondary ion mass spectrometry. A multilayer formaldehyde film grows on a deoxygenated V substrate at 70 K after the formation of a reacted layer at the interface. Most molecules in the film disappear at 100 K via interfacial reaction rather than thermal desorption. However, the liberation of possible reaction products such as H 2 and CO is depressed at T < 200 K. The reaction of formaldehyde with the deoxygenated V substrate is characterized by C−O bond scission and uptake of the reaction products in subsurface layers. The formaldehyde multilayer decomposition occurs on oxygenated V substrates as well, where methane and ethylene are observed as reaction products at 100 K. Their relative yields depend on the degrees of substrate oxidation. When a multilayer water film is present on the deoxygenated V substrate, hydrogen is liberated together with methane during the decomposition of formaldehyde (100 K) and water (160 K).

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

“…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.…”

Decomposition of multilayer benzene and n-hexane films on vanadium

“…This behavior is a characteristic of n-hexane decomposition because thermal desorption and decomposition of molecules occur simultaneously for other molecular films, including benzene. [20][21][22][23] The decomposition process of the n-hexane film was examined further using temperature-programmed TOF-SIMS. The intensities of typical cations sputtered from the n-hexane film (formed by exposure of 20 L) are shown in Fig.…”

“…However, physisorbed molecules in multilayer films generally desorb with no reactions except for those in direct contact with metal substrates. Recently, it was demonstrated that this conventional pattern does not hold for multilayers of water, 20 methanol, 21 formaldehyde, 22 ethanol, 23 acetone, 23 and diethyl ether 23 deposited onto a clean V substrate. It was proposed that the deoxygenation reaction to form oxides in the V substrate induces the decomposition of these molecules.…”

“…Therefore, we used a polycrystalline sheet of V as a substrate to attain better deoxygenation from the bulk. [20][21][22][23] The effects of V oxidation on the decomposition of benzene multilayer films are also explored in this respect. The TPD spectra of hydrocarbons from highly oriented pyrolytic graphite (HOPG) were also taken for comparison purposes to estimate the desorption and decomposition rates of the physisorbed molecules on a nonreactive substrate.…”