Adsorption Dynamics of CO on Silica Supported CuO_x Clusters: Utilizing Electron Beam Lithography To Study Methanol Synthesis Model Systems

Electron beam lithography was used to nanofabricate 12 and 63 nm Cu clusters supported on silica (model nanoarray catalysts). The Cu clusters could reversibly be oxidized and reduced at ultrahigh vacuum conditions. The chemical activity of these clusters was probed by Auger and X-ray photoelectron spectroscopy, thermal desorption spectroscopy, and molecular beam scattering. CO was used as the probe molecule. Scanning electron microscopy was used to obtain cluster size distributions. CO adsorption is molecular and nonactivated. CO binding energies on the oxidic clusters are larger than for the metallic clusters. Adsorption transients, recorded as a function of surface temperature and CO impact energy, are consistent with precursor models, as expected from the so-called capture zone model (CZM). Cluster size effects are evident, as predicted by the CZM. However, unexpectedly, the CO saturation coverage does not simply scale with the cluster area but depends also on the rim length of the deposits. Metallic Cu clusters are more reactive than oxidic clusters, in part not only due to the cluster size effect but apparently also because of the electronic effect.