Methanol interaction over Cu-Pt clusters supported on CeO2: Towards an understanding of adsorption sites

Abstract

For the first time, the present work shows a combined theoretical and experimental analysis of the preferred adsorption sites for both methanol and methoxy species over Cu, Pt, and bimetallic Cu-Pt clusters - up to 5 atoms in size - supported on CeO2(111) as model systems. DFT+U calculations compared to methanol adsorption experiments followed by DRIFT spectroscopy over Cu, Pt, and bimetallic Cu-Pt nanoparticles supported over CeO2 octahedra, exposing only {111} crystal planes. From our DFT+U calculations, we found that methanol adsorption over the CuPt/CeO2(111) system is slightly favored, compared to the pristine CeO2(111) surface. Similarly, methoxy species formed through methanol dehydrogenation (CH3O- + H+) adsorb preferably over the metallic clusters, compared to the pristine oxide surface. This promotion is explained by the higher coordination of the methoxy species over the cluster, especially for those containing a higher amount of Cu atoms. This results highlights the oxidized Cu sites' role in the catalytic process, followed by the surface cerium atoms reduction of the support. The interaction of methanol with the substrate oxygen weakens the O-H bond, which results in a reduction in the energy barrier when the reaction occurs on the surface and interface, compared when this occurs exclusively on the metal cluster. However, after this step, the potential energy surface's depth suggests that methoxy species prefer to be adsorbed over the metal clusters. Experimentally, methanol was adsorbed over Cu, Pt, and bimetallic Cu-Pt nanoparticles supported over CeO2 {111}-octahedral particles. Through in-situ DRIFTS, bands related to adsorbed methanol and methoxy species were identified. A blue shift in all the bands was observed, related to metals’ influence in adsorbed species' vibrational mode. In the experimental spectra, new contributions in Cu-containing catalysts appeared, which were identified as adsorbed species over the metallic cluster and the interface, rationalized by calculated spectra.