Engineering low Pd content catalysts for soot combustion through tuning the Pd-SnO2 interface interaction: Disclosing the critical role of dual Pd valence states for the activity

To manufacture efficient and cost-effective catalysts for soot combustion, SnO2 nano-rod (SnO2-Rod), nano-cube (SnO2-NP-Cube) and regular nanoparticles (SnO2-NP) have been designed as supports for Pd. The soot combustion activity ranks as 0.5 % Pd/SnO2-NP-Cube > 0.5 % Pd/SnO2-Rod > 0.5 % Pd/SnO2-NP. The active oxygen amount of the supports increases as SnO2-Rod < SnO2-NP < SnO2-NP-Cube, which affects significantly Pd valence state distributions. On SnO2-NP-Cube, Pd is highly oxidized with 30.3 % Pd4+ and 69.7 % Pd2+. On SnO2-NP, nearly 100 % Pd2+ is present. On SnO2-Rod, Pd is more reduced with 73.4 % Pd0 and 26.6 % Pd2+. The concerted action of surface active oxygen and dual Pd valence states determines the activity. In detail, the active oxygen sites devote directly to soot oxidation via a Mars-van-Krevelen mechanism with a redox-cycle. The presence of dual Pd valence states on 0.5 % SnO2-NP-Cube and 0.5 % Pd/SnO2-Rod can promote this redox-cycle, thereby improving the soot combustion activity evidently.