Doping SnO2 with metal ions of varying valence states: discerning the importance of active surface oxygen species vs. acid sites for C3H8 and CO oxidation

Abstract

To elucidate the valence state effect of doping cations, Li⁺, Mg²⁺, Cr³⁺, Zr⁴⁺ and Nb⁵⁺ with radii similar to Sn⁴⁺ (CN = 6) were chosen to dope tetragonal SnO₂. Cr³⁺, Zr⁴⁺ and Nb⁵⁺ can enter the SnO₂ lattice to produce solid solutions, thus creating more surface defects. However, Li⁺ and Mg²⁺ can only stay on the SnO₂ surface as nitrates, thus suppressing the surface defects. The rich surface defects facilitate the generation of active O₂⁻/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.