Unraveling the thermodynamic stability, catalytic activity and electronic structure of bimetallic clusters at realistic conditions*

Cluster is considered as the prototypical system to gain a better understanding of active sites and the elementary steps of reaction mechanism at the atomistic level. Aiming toward catalytic applications, a large data set is generated on [TM_xMg_yO_z]^+/0/− clusters (TM = Cr, Fe, Co, Ni, x + y ≤ 5) using a massively parallel cascade genetic algorithm (cGA) at the hybrid density functional level of theory. The low-energy isomers are further analyzed via ab initio atomistic thermodynamics to estimate their free energy of formation at a realistic temperature T and partial pressure of oxygen p_O. A thermodynamic phase diagram is drawn by minimizing the free energy of formation to identify the stable phases of [TM_xMg_yO_z]^+/0/− clusters. From this analysis, we notice that neutral and negatively charged clusters are stable in the wide range of (T, p_O). The negatively charged clusters are more effective as a catalyst to lower the C−H bond activation barrier of methane. We find that the nature of TM atoms toward controlling the activation barrier is less important. However, TM gives rise to different structural motifs in the cluster, which may act as active centers for catalysis.