Revealing the ZrO₂ crystal effect of Pd/ZrO₂ catalyst for toluene combustion: A combined DRIFTS and DFT study

The effect of ZrO₂ crystalline phases on Pd/ZrO₂ for catalytic combustion of toluene was investigated. It was found that Pd/t-ZrO₂ showed superior activity when the toluene concentration was lower than 2000 ppm, while Pd/m-ZrO₂ performed better with toluene concentration higher than 3000 ppm. Kinetic studies showed that the reaction order of toluene on Pd/t-ZrO₂ and Pd/m-ZrO₂ is 0 and 2, respectively. The catalytic activity was controlled by combined effect of PdO activity and quantity, which was correlated with the interaction between palladium and zirconia. The t-ZrO₂ increased PdO species activity, while m-ZrO₂ could enhance the amount of PdO. DFT results showed that the p-band center of the O of PdO on t-ZrO₂ was higher, suggesting that PdO was more active. In situ-DRIFTS revealed that the reaction pathway of catalytic oxidation of toluene on Pd/ZrO₂ was as follows: toluene was oxidized stepwise to benzyl alcohol, benzaldehyde, benzoic acid, CO₂ and H₂O.