Dissociative chemisorption of O2 on Agn and Agn−1Ir (n = 3–26) clusters: a first-principle study

Abstract

Understanding the interactions between O₂ and small metal clusters is of great importance in exploring heterogeneous catalysis particularly involving an oxidation reaction. We herein present the dissociative chemisorption of O₂ on Ag_n and Ag_n−1Ir clusters (n = 3–26) by using density functional theory calculations. Combining a particle swarm optimization algorithm and a minima hopping method, we have optimized and obtained stable geometric structures of Ag_n and Ag_n−1Ir clusters without and with O₂ adsorption. Some important physical parameters, including bond length, adsorption energy, dissociation barriers and bader charge, have been systematically calculated for appraising the stability and reactivity of Ag_n and Ag_n−1Ir clusters. It is found that the dopant Ir atom can largely enhance the stability and promote the O₂ dissociation, especially on small Ag_n−1Ir clusters (n = 3–10). It is mainly attributed to the dopant Ir atom being completely exposed outside the Ag atoms. For O₂ adsorption and dissociation on large Ag_n−1Ir clusters (n = 11–26), the dissociation barriers are much higher due to the dopant Ir emerging into the core of Ag_n−1Ir clusters, which is very similar to those on large Ag_n (n = 11–26). Microkinetic simulation results provide direct evidence for high reaction temperature and pressure effects on improving O₂ dissociation on Ag_n and Ag_n−1Ir clusters especially for small clusters (n < 10). It is found that the Ag₅Ir cluster is the most suitable nanocluster for promoting O₂ dissociation at the given reaction temperatures and pressures. Our theoretical work is helpful for the rational design of doped silver nanocluster catalysts in future experiments.