IR spectroscopic characterization of the co-adsorption of CO(2) and H(2) onto cationic Cu(n)(+) clusters

To understand elementary reaction steps in the hydrogenation of CO(2) over copper-based catalysts, we experimentally study the adsorption of CO(2) and H(2) onto cationic Cu(n)(+) clusters. For this, we react Cu(n)(+) clusters formed by laser ablation with a mixture of H(2) and CO(2) 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(n)(+) clusters reacted with H(2) and with new spectra of Cu(n)(+) clusters reacted with CO(2). The latter indicate that CO(2) is physisorbed in an end-on configuration when reacted with the clusters alone. Although the spectra for the co-adsorption products evidence H(2) dissociation, no signs for CO(2) activation or reduction are observed. This lack of reactivity for CO(2) is rationalized by density functional theory calculations, which indicate that CO(2) dissociation is hindered by a large reaction barrier. CO(2) reduction to formate should energetically be possible, but the lack of formate observation is attributed to kinetic hindering.