Theoretical insights into the nature of synergistic enhancement in bimetallic CoTiAlPO-5 catalysts for ammonia activation

Abstract

Bimetallic catalytic synergy, the concurrent action of two different metal ions in the same material, has resulted in improved efficiency in many catalytic systems and for a range of chemical processes. Via a computational mechanistic study, we investigate the catalytic benefits of the bimetallic CoTiAlPO-5 material in comparison to the monometallic CoAlPO-5 system, on the activation of NH₃. The presence of Ti in a framework site adjacent to Co stabilises the Co(II) oxidation state, and increases the Co(III)/Co(II) reduction potential. We show that this change lowers the activation barrier for the homolytic H extraction from NH₃ by Co(III), from 162 kJ mol⁻¹ in the monometallic CoAlPO-5 catalyst to 140 kJ mol⁻¹ in the bimetallic CoTiAlPO-5 (175 and 111 kJ mol⁻¹ respectively when considering both dispersion and free energy corrections). Elucidation of mechanistic details through computational studies can make significant contributions to the rational design of catalytic materials.