Conversion of methanol at copper clusters on TiO2(110) and SiOx: direct dehydrogenation vs. partial oxidation and influence of cluster size and substrate.

Copper is known to catalyze the conversion of methanol to formaldehyde in single crystal experiments. Here, we present a systematic study of methanol reactions at different-sized nanoparticulate copper clusters on rutile TiO2(110) as well as a native silicon oxide film on a Si(111) wafer. By temperature-programmed reaction spectroscopy (TPRS), we have identified two different pathways, namely the direct dehydrogenation and in the presence of oxygen the partial oxidation to formaldehyde for large copper clusters. While the silica substrate is nonreactive, for rutile TiO2(110) the competing conversion of methanol to methane dominates the formaldehyde formation, depending on the titania reduction degree. At the same time, the low-temperature formaldehyde formation at the highly reduced TiO2(110) was not observed, suggesting the suppression of the dioxomethylene-like intermediate of this species at the surface. Concurrent with these reactions the high-temperature desorption of CO2 was observed as a side-product on all substrates, which can be correlated with the formation of a formate intermediate decomposing into CO2 and H2 at elevated temperatures.