Rim Effects in the Adsorption of CO2 on Silica-Supported Copper Oxide Clusters: Identifying Active Sites Utilizing Electron Beam Lithography

Electron beam lithography (EBL) was used to nanofabricate so-called model-nanoarray catalysts, which consist of a predetermined lattice of clusters. In doing so, 12, 35, and 63 nm Cu clusters, supported on silica, were studied by experimental surface science techniques. The Cu EBL clusters were subsequently oxidized to CuOx, as characterized by X-ray photoelectron and Auger electron spectroscopy, respectively. The catalytic activity of these clusters was probed by thermal desorption spectroscopy and supersonic molecular beam scattering. CO2 was used as the probe molecule because it is part of the syngas for the industrial methanol synthesis that utilizes copper-based catalysts. CO2 adsorption is molecular and nonactivated. Adsorption transients were recorded as a function of surface temperature and CO2 impact energy. The transients are consistent with precursor models, as expected from the so-called capture zone (CZ) model. Cluster size effects are evident, in agreement with the CZ model. Unexpectedly, the CO2 saturation coverage does not simply scale with the clusters' area but rather with the rim length of the clusters. Therefore, the active sites for CO2 adsorption (rim sites) were identified using a simple kinetics technique and EBL-fabricated model catalysts.