In situ XPS and MS study of methanol decomposition and oxidation on Pd(111) under millibar pressure range

Abstract

The methanol decomposition and oxidation on a Pd(111) single crystal have been investigated in situ using ambient-pressure X-ray photoelectron spectroscopy (XPS) and mass-spectrometry (MS) in the temperature range of 300–600K. It was found that even in the oxygen presence the methanol decomposition on palladium proceeds through two competitive routes: fast dehydrogenation to CO and H2, and slow decomposition of methanol via the C–O bond scission. The rate of the second route is significant even in the millibar pressure range, which leads to a blocking of the palladium surface by carbon and to a prevention of the further methanol conversion. As a result, no gas phase products of methanol decomposition were detected by mass-spectrometry at 0.1mbar CH3OH in the whole temperature range. The methanol C–O bond scission produces CHx species, which fast dehydrogenate to atomic carbon even at room temperature and further partially dissolve in the palladium bulk at 400K with the formation of the PdCx phase. According to in situ XPS data, the PdCx phase forms even in the oxygen excess. The application of an in situ XPS–MS technique unambiguously shows a good correlation between a decrease in the surface concentration of all carbon-containing species and the rate of methanol conversion. Since these carbon species have a high reactivity towards oxygen, heating of Pd(111) above 450K in a methanol–oxygen mixture yields CO, CO2, and water. The product distribution indicates that the main route of methanol conversion is the dehydrogenation of methanol to CO and hydrogen. However, under the experimental conditions used, hydrogen is completely oxidized to water, while CO is partially oxidized to CO2. No palladium oxide was detected by XPS in these conditions.