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

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.