Doping SnO 2 with metal ions of varying valence states: discerning the importance of active surface oxygen species vs. acid sites for C 3 H 8 and CO oxidation.

To elucidate the valence state effect of doping cations, Li + , Mg 2+ , Cr 3+ , Zr 4+ and Nb 5+ with radii similar to Sn 4+ (CN = 6) were chosen to dope tetragonal SnO 2 . Cr 3+ , Zr 4+ and Nb 5+ can enter the SnO 2 lattice to produce solid solutions, thus creating more surface defects. However, Li + and Mg 2+ can only stay on the SnO 2 surface as nitrates, thus suppressing the surface defects. The rich surface defects facilitate the generation of active O 2 - /O δ - and acid sites on the solid solution catalysts, hence improving the reactivity. On the solid solution catalysts active for propane combustion, several reactive intermediates can be formed, but are negligible on those with low activity. It is confirmed that for propane combustion, surface acid sites play a more vital role than active oxygen sites. Nevertheless, for CO oxidation, the active oxygen sites play a more vital role than the acid sites.