Role of geometry, charge and fluxionality of clusters in CO2 activation on supported sub-nanometer metal clusters: The case of Cu tetramers on pristine and O-terminated MXene

Abstract

Reduction of CO2 to useful chemicals using supported few atom copper clusters has been an active area of research. The first step in this process is chemisorption and reduction of CO2 to CO. Previous studies have shown that the ease of chemisorption depends on the cluster geometry and charge. In an effort to elucidate the role of cluster-support interactions and thereby cluster geometry, charge on cluster on CO2 chemisorption, in this work, using density functional theory based calculations, we have studied the physisorption and chemisorbtion of CO2 on Cu tetramers supported on pristine and O-terminated Ti2C MXene. Our calculations show that for CO2 to exhibit exothermic adsorption, the cluster should (a) wet the support thereby exposing all the Cu atoms to the approaching CO2 molecule, (b) be preferably positively charged with Cu atoms present in more than one oxidation state and (c) be fluxional on the support. Our nudged elastic band based calculations show that the transition from physisorbed to chemisorbed CO2 is an activated process. Amongst the systems considered in this study, the activation barrier is usually low except on the tetrahedral cluster on the oxygen terminated Ti2C support.