Methanol Adsorption on Vanadium Oxide Surfaces Observed by Ambient Pressure X-ray Photoelectron Spectroscopy

Abstract

Ambient pressure X-ray photoelectron spectroscopy (APXPS) has been used to study the initial stages of methanol adsorption on vanadium oxide surfaces. V 2p, O 1s, C 1s, and K 2p XPS spectra were collected as a function of relative methanol pressure in a series of isotherm and isobar experiments on two VO 2 /TiO 2 (100) films with different surface vanadium oxidation states. The binding energies and O 1s/C 1s peak area ratios for adsorbates were consistent with a mixture of molecular methanol, methoxide, hydroxide, and water, indicating that both molecular and dissociative methanol adsorption occur. In contrast to water adsorption experiments on similar films, an adsorption onset was observed at a consistent temperature, rather than a consistent relative pressure, indicating that a more complex reaction mechanism is at play. Vanadium oxidation state, C 1s peak position, and the area of carbon and oxygen adsorbate peaks were correlated, suggesting that reduced surface sites play a critical role in enhancing both the dissociative and molecular adsorption of methanol. The two fairly similar VO 2 /TiO 2 (100) films showed quite different behavior, with the more reduced surface showing greater reactivity toward methanol. The difference in reactivity could be linked to different levels of potassium in the two films, which appears to play an important role in determining the vanadium oxidation state and has important practical consequences for the design of catalytic systems.