IR spectroscopic characterization of the co-adsorption of CO2 and H2 onto cationic Cun+ clusters

Abstract

To understand elementary reaction steps in the hydrogenation of CO₂ over copper-based catalysts, we experimentally study the adsorption of CO₂ and H₂ onto cationic Cu_n⁺ clusters. For this, we react Cu_n⁺ clusters formed by laser ablation with a mixture of H₂ and CO₂ 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 H₂ and with new spectra of Cu_n⁺ clusters reacted with CO₂. The latter indicate that CO₂ is physisorbed in an end-on configuration when reacted with the clusters alone. Although the spectra for the co-adsorption products evidence H₂ dissociation, no signs for CO₂ activation or reduction are observed. This lack of reactivity for CO₂ is rationalized by density functional theory calculations, which indicate that CO₂ dissociation is hindered by a large reaction barrier. CO₂ reduction to formate should energetically be possible, but the lack of formate observation is attributed to kinetic hindering.