In Situ Studies of Methanol Decomposition Over Cu(111) and Cu2O/Cu(111): Effects of Reactant Pressure, Surface Morphology, and Hot Spots of Active Sites

The dissociative adsorption of methanol was investigated on Cu(111) and ultrathin Cu2O films. We employed synchrotron-based Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS) and Scanning Tunneling Microscopy (STM) to study the dynamics of gas-solid interactions, and calculations based on Density Functional Theory (DFT) were used to examine the reaction path. C 1s XPS spectra revealed that methanol underwent dissociative adsorption on plain Cu(111) to form methoxy (CH3O), formaldehyde (H2CO), and formate (HCOO) at a pressure range of 0.5-10 mTorr, with these species remaining on the surface after evacuation. This was accompanied by the appearance of a low coverage (similar to 0.05 ML) of O-ads in the O 1s which can be considered a highly active site for methanol activation. The high activity is apparent by a coverage of 0.8 ML of methoxy at room temperature. STM was unable to image these species at room temperature as they were highly mobile on metallic copper. In contrast, for CH3OH on Cu2O/Cu(111), STM showed clear hot spots for reaction and a complex array of adsorption structures. On the oxide substrate, there was decomposition of methanol to H2CO, CH3O, HCOO, and hydrocarbon species (CHx) due to the subsequent interactions of methanol with lattice oxygen. Cu(111) remained entirely saturated with decomposition products under 10 mTorr of methanol (theta approximate to 0.97 ML), whereas the Cu2O overlayer was saturated at a much lower coverage (theta approximate to 0.30 ML). STM revealed rows and step edges of Cu2O decorated with decomposition products and metallic Cu islands similar to 5 nm in size. The difference in activity between Cu(111) and Cu2O/Cu(111) is attributed to the significant amount of O present on the oxide surface. Density Functional theory (DFT) calculations described the XPS measurements well, showing a likely methanol dissociation to *CH3O and therefore a surface reduction. More importantly, the DFT results revealed that it was the chemisorbed oxygen on Cu2O/Cu(111) which oxidized the dissociated *CH3O to *HCOO and eventually CO2, while the reaction only involving upper oxygen on the Cu2O hexagonal ring led to the formation of H2CO.

Automated summary

The dissociative adsorption of methanol on Cu(111) and ultrathin Cu2O films was investigated using a combination of AP-XPS, STM, and DFT calculations. It was found that methanol adsorbed dissociatively on Cu(111) forming methoxy, formaldehyde, and formate, while on Cu2O/Cu(111) it decomposed into a wider variety of species due to interactions with lattice oxygen. The difference in reactivity was attributed to the presence of oxygen on the oxide surface.