IR spectroscopic characterization of the co-adsorption of CO and H onto cationic Cu clusters.

To understand elementary reaction steps in the hydrogenation of CO over copper-based catalysts, we experimentally study the adsorption of CO and H onto cationic Cu clusters. For this, we react Cu clusters formed by laser ablation with a mixture of H and CO in a flow tube-type reaction channel and characterize the products formed by IR multiple-photon dissociation spectroscopy employing the IR free-electron laser FELICE. We analyze the spectra by comparing them to literature spectra of Cu clusters reacted with H and with new spectra of Cu clusters reacted with CO. The latter indicate that CO is physisorbed in an end-on configuration when reacted with the clusters alone. Although the spectra for the co-adsorption products evidence H dissociation, no signs for CO activation or reduction are observed. This lack of reactivity for CO is rationalized by density functional theory calculations, which indicate that CO dissociation is hindered by a large reaction barrier. CO reduction to formate should energetically be possible, but the lack of formate observation is attributed to kinetic hindering.