Adsorption and dehydrogenation of ammonia at the V2O5(010) surface: DFT cluster studies

Transition metal oxide catalysts are widely used for selective oxidation reactions. However, in many cases details of the catalytic reaction mechanisms are still under discussion. One prominent example is the ammoxidation of propylene to acrylonitrile at transition metal oxide surfaces (SOHIO process). This catalytic reaction includes, amongst other steps, the adsorption and dehydrogenation of NHx, x<4, at the catalyst surface. We have performed theoretical studies on these reaction steps where the catalyst is simulated by a finite section of the V2O5(010) surface. The calculations use density-functional theory combined with clusters modeling the surface and adsorbate system. Calculations for the clean V2O5(010) surface show that binding energies of the H atom are always significantly larger than of the NHx species. Further, the substrate is found to lower corresponding dehydrogenation energies compared with values for the gas phase reaction. However, the lowering is too small to make dehydrogenation likely to happen under ammoxidation reaction conditions. This suggests that surface defects such as oxygen vacancies become important for the reaction. Therefore, the role of oxygen vacancies for the dehydrogenation of NHx is discussed in detail