CO₂ activation by copper oxide clusters: size, composition, and charge state dependence

The interaction of CO₂ with copper oxide clusters of different size, composition, and charge is investigated via infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations. Laser ablation of a copper target in the presence of an O₂/He mixture leads to the preferred formation of oxygen-rich copper oxide cluster cations, CuO⁺ (y > x; x ≤ 8), while the anionic cluster distribution is dominated by stoichiometric (x = y) and oxygen-deficient (y < x; x ≤ 8) species. Subsequent reaction of the clusters with CO₂ in a flow tube reactor results in the preferred formation of near-stoichiometric CuO(CO₂)^+/- complexes. IR-MPD spectroscopy of the formed complexes reveals the non-activated binding of CO₂ to all cations while CO₂ is activated by all anions. The great resemblance of spectra for all sizes investigated demonstrates that CO₂ activation is largely independent of cluster size and Cu/O ratio but mainly determined by the cluster charge state. Comparison of the IR-MPD spectra with DFT calculations of the model systems Cu₂O₄(CO₂)^- and Cu₃O₄(CO₂)^- shows that CO₂ activation exclusively results in the formation of a CO₃ unit. Subsequent CO₂ dissociation to CO appears to be unfavorable due to the instability of CO on the copper oxide clusters indicating that potential hydrogenation reactions will most likely proceed via formate or bicarbonate intermediates.