Elementary steps of the catalytic NO_x reduction with NH _3: Cluster studies on adsorbate diffusion and dehydrogenation at vanadium oxide substrate

摘要

We discuss the details of important steps of the selective catalytic reduction (SCR) of NOx at model V_2O_5(010) substrate. First, diffusion processes at the substrate surface are considered where hydrogen and ammonium, NH4, are used as examples. Hydrogen diffusion, a prerequisite for water formation involving substrate oxygen, is described by diffusion paths between adjacent surface oxygen sites. Corresponding energy barriers are determined mainly by the flexibility and the amount of distortion of the oxygen atoms which participate in the O-H-O bridge formation at the transition state. Further, diffusion of sub-surface oxygen to fill surface oxygen vacancies of the V_2O_5(010) substrate has been considered and results in reactive surface sites which have not been discussed so far. NH_4 diffusion at the V_2O _5(010) surface can be described as a combined tumbling and rotation process characterized by quite low diffusion barriers which make the adsorbate rather mobile. Finally, hydrogenation and dehydrogenation of different NH x species at the V_2O_5(010) substrate surface are studied where special emphasis is given to the influence of surface reduction simulated locally by oxygen vacancies. The results confirm experimental findings of the presence of both NH_2 and NH_4 species after ammonia adsorption at the V_2O_5(010) surface.