Size-Selective CO2 Activation at Rhodium Cluster Anions.

Carbon dioxide (CO2) fixation and subsequent activation represent grand challenges in materials science and chemical catalysis, with the aim of mitigating the worst impacts of global warming and associated climate change. Vibrational spectroscopy can provide essential structural insights into the binding and activation of CO2 on potential reduction catalysts. The nature of carbon dioxide adsorption on rhodium cluster anions, Rhn- (n = 3-12), has been investigated using a combination of infrared free-electron laser spectroscopy and quantum chemical calculations. A clear cluster-size dependence to the nature of the binding is observed. On the smallest clusters, Rhn- (n ≤ 4), CO2 is dissociatively adsorbed, as indicated by a carbonyl stretch at 1880 cm-1. By contrast, larger clusters, Rhn- (n ≥ 7), exhibit highly activated molecular binding, but both motifs are observed on intermediate cluster sizes (n = 5, 6). The extent of chemical activation is clearly discernible spectroscopically and arises from the propensity of CO2 to attach across Rh-Rh bridge sites, coupled with significant electron transfer from the metal cluster.