Structure and Reactivity of Cu2O Nanocubes in Ethanol Dehydrogenation.

We investigated the structure and reactivity of cubic Cu2O nanoparticles (nanocubes) in the ethanol dehydrogenation (EDH) reaction, which is considered as an ecofriendly process for production of "green" hydrogen and valuable chemicals such as acetaldehyde. High-loaded catalysts prepared by physical mixing of nc-Cu2O and SiO2 demonstrated activity considerably higher than that of those prepared by conventional impregnation/calcination. Reactivity tests revealed the catalytic performance (conversion and selectivity) to be independent of the initial state of the catalyst, i.e., oxidized or reduced, due to the facile reduction of the Cu-(I) oxide to metallic Cu in the ethanol atmosphere, as observed by operando XRD, DRIFTS, and near ambient pressure (NAP) XPS. The reduction of nc-Cu2O is accompanied by strong morphological changes, i.e., the transformation of the nanocubes into roundish nanoparticles and their sintering as shown by TEM and SEM. The Cu-(I) oxide catalyst is initially active in EDH, but the Cu(0) phase formed in situ is considerably more active, and the Cu(0) phase is the only one that exists at the steady state. Analysis of the Cu surface by NAP XPS and CO DRIFTS revealed only the metallic state, with no indication for surface Cu oxide formation under the reaction conditions. The catalysts are stable at relatively low temperatures (∼170 °C) but deactivate, most notably at temperatures above 230 °C, due to coke (mostly amorphous carbon) formation. The results suggest that nc-Cu2O can be used as a well-defined precursor for the synthesis of high-loaded Cu catalysts for alcohol dehydrogenation at moderate temperatures.