Surface active sites of vanadium pentoxide-molybdenum trioxide catalysts

Abstract

The promoting effect of MoO{sub 3} on the surface active sites of V{sub 2}O{sub 5} catalyst was investigated by using various methods, and the catalytic activity of V{sub 2}O{sub 5}-MoO{sub 3} catalysts was discussed in the light of the surface active sites and the bulk structure. The surface concentration of the redox sites, measured by the NARP (NO-NH{sub 3} rectangular pulse) technique, was increased by the addition of MoO{sub 3} to V{sub 2}O{sub 5} both on the solid solution and on the intermediate compounds, i.e., Mo{sub 4}V{sub 6}O{sub 25} and Mo{sub 6}V{sub 9}O{sub 40}. On the solid solution, the increase in the surface redox sites is attributed to the formation of the surface V = O species on various crystal planes of vanadium oxide containing solved MoO{sub 3}, although V{sub 2}O{sub 5} crystal has the surface V = O species only on the (010) plane. The increase in the surface V = O species on solid solution may cause the high activity of V{sub 2}O{sub 5}-MoO{sub 3} catalysts on various reactions, such as benzene oxidation. On the intermediate compounds, it was proved by NARP and SIMS that the surface Mo = O species act as the redox sites for themore » reduction of NO with NH{sub 3} in a manner similar to that of the surface V = O species on V{sub 2}O{sub 5} catalysts, although MoO{sub 3} itself is inactive at the temperature range examined. The activation of the surface Mo = O species was attributed to the atomic mixing of the surface vanadium and molybdenum ions on the exposed crystal plane of the intermediate compounds.« less