Unraveling the role of structure, electronic property and charge state on the catalytic activity of bi-metallic clusters at realistic conditions
Shikha Saini* and Saswata Bhattacharya
Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016
*Shikha.Saini[at]physics.iitd.ac.in
We study the structure, thermodynamic stability, electronic properties and catalytic activity of bi-metallic clusters with a combination of density-functional theory and many-body perturbation theory. In a real application, the catalytic materials operate under the operational environment (e.g., temperature (T) and pressure (p) in an atmosphere of reactive molecules). Therefore, we have studied a large data set of [TMxMgyOz]+/0/- clusters (TM =Cr, Fe, Co, Ni; x + y ≤ 5) in an oxygen (O)-environment. The optimum cluster structures are identified by employing a massively parallel cascade genetic algorithm (cGA). Further, the stability of the clusters with minimum energy is determined by evaluating the formation free energy via ab initio atomistic thermodynamics at realistic conditions of T and p. Our results show that the neutral and anionic clusters are the most stable phases in the wide range of T and p. We have conveyed [and validate by forming a huge data-set of TMxMgyOz (TM=Co, Ni, Fe, Cr and x+y ≤ 3) clusters] one central message: that catalytic reactivity of this type of bi-metallic oxide system is expected to be correlated more strongly with oxygen rich environment than the choice of any specific TM atoms[1]. The latter is, however, conventionally believed to play the lead role in catalysis. Following this finding, it's therefore interesting to understand the explicit role of TM (if any) despite one should aim for O-rich environmental condition for synthesis of these catalysts. As per the structural analysis, we have noticed higher molecular adsorption of oxygen in Co and Ni-based clusters as compared to Cr and Fe-based bi-metallic oxide clusters. In Cr and Fe-based clusters, dissociative adsorption is preferable. Metal oxide clusters having high molecular adsorption possess the more active sites (O2- and O22-) for the oxidative coupling of methane, and therefore, Co and Ni-based clusters are expected to have a better catalytic performance amongst various TMxMgyOz clusters. However, Cr and Fe-based clusters may be good choice for the reduction of CO2.
References:
1. S. Saini, D. Sarker, P. Basera, S. V. Levchenko, L. M. Ghiringhelli, and S. Bhattacharya, J. Phys. Chem. C 122, 16788-16794 (2018).