DFT-ONIOM study of ammonia activation by MM'APO-5 (M,M' = Ti, Zr, Co) bimetallic molecular sieves. The influence of the neighboring metal from a theoretical point of view

ONIOM-DFT calculations have been carried out in order to determine how the neighboring metal influences the activation free energies (Delta G(not equal)) and the mechanisms of NH3 activation in a series of mono and disubstituted aluminophosphates MM'APO-5, (M, M' = Co, Ti Zr). The results revealed that metal clustering is thermodynamically favorable (Delta G < 0) for all MAPO-5s and MM'APO-5s studied here. The aluminophosphates TiAPO-5 and ZrAPO-5 are unable to activate NH3 due to their inability to stabilize the key intermediate MNH2. The calculated activation free energy values (Delta G(not equal)) suggest that CoZrAPO-5 is better suited for NH3 activation, as its values are lower than those of CoTiAPO-5 in the mechanism studied.