Decun Luo,Ziyu Tang,Xiyang Yu,Tao Zhang,Chun-Ran Chang,Zhun Hu
The effect of ZrO 2 crystalline phases on Pd/ZrO 2 for catalytic combustion of toluene was investigated. It was found that Pd/t-ZrO 2 showed superior activity when the toluene concentration was lower than 2000 ppm, while Pd/m-ZrO 2 performed better with toluene concentration higher than 3000 ppm. Kinetic studies showed that the reaction order of toluene on Pd/t-ZrO 2 and Pd/m-ZrO 2 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-ZrO 2 increased PdO species activity, while m-ZrO 2 could enhance the amount of PdO. DFT results showed that the p-band center of the O of PdO on t-ZrO 2 was higher, suggesting that PdO was more active. In situ -DRIFTS revealed that the reaction pathway of catalytic oxidation of toluene on Pd/ZrO 2 was as follows: toluene was oxidized stepwise to benzyl alcohol, benzaldehyde, benzoic acid , CO 2 and H 2 O.
The selectivity and stability of catalysts for methanol synthesis from CO 2 still remain to be enhanced. In this work, we synthesized a series of Pd/ZrO 2 catalysts inversely loaded with various metal oxide promoters (ZnO, In 2 O 3 , and CeO 2 ), those would partially cover Pd metal surface and form some new metal-promoter interface, to explore the nature in the performance of CO 2 hydrogenation to methanol. The catalyst synthesized by this approach could limit the growth of Pd particles during the reduction and form new metal–metal oxide interfaces with different functions of CO 2 hydrogenation, improving the reaction performance of Pd-ZrO 2 catalysts. As a result, the ZnO-Pd/ZrO 2 catalyst exhibited the highest CO 2 conversion (12.0 %), methanol selectivity (95.6 %), and STY of methanol (472 g MeOH kg cat -1 h −1 ), with excellent stability. The results of HRTEM, H 2 -TPR, XPS, and XAFS showed that the ZnO-Pd/ZrO 2 catalyst had the typical Pd-ZnO interface with Pd 2+ species after the H 2 reduction, which could enhance the adsorption and activation of CO 2 . In addition, the in situ DRIFTS and NAP-XPS results implied that the Pd-ZnO interface significantly improved the formation of formate (HCOO*) and methoxy (H 3 CO*) species, which were considered to be the key intermediates of methanol generation, and thus greatly enhanced the selectivity of methanol.
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