Surface active sites of V₂O₅-SnO₂ catalysts

The surface active sites on V₂O₅-SnO₂ catalysts, i.e., redox sites and acid sites, were investigated by using two series of catalysts prepared from different starting materials: Sn(OH)₂ and SnO. By use of the NO-NH₃ rectangular pulse technique, it was found that the surface concentration of the redox sites increases with the addition of SnO₂, depending strongly on the starting materials at lower SnO₂ content. Based on XRD, IR, ESR, XPS, and SIMS data, the increase in the redox sites is attributed to the reduction of vanadium oxide caused by atomic mixing of vanadium and tin oxides. The difference between the two series of catalysts is attributed to the degree of mixing, which may result from the difference in chemical processes during the calcination of precursor of the catalysts. The strengths, surface concentrations, and types of acid sites were determined by using NH₃ TPD and IR of adsorbed NH₃ and pyridine. The properties of the acid sites on the V₂O₅-SnO₂ catalysts appear to be due just to those of pure V₂O₅ and SnO₂. Finally, the effects of various promoters, i.e., P₂O₅, WO₃, MoO₃, and SnO₂, on the redox sites are summarized, and two important factors responsible for the increase in the concentration of the redox sites are discussed: (1) the mixing of vanadium and promoter ions at the surface rather than the formation of intermediate compounds or solid solution in the bulk and (2) a redox function of the promoter oxides.