The performance and mechanism of HCHO oxidation on Pd/SiO2 catalysts with different Pd particle sizes

In general, for Pd-based catalysts, the smaller the Pd particles on the catalysts are, the better the activity for HCHO oxidation; this has solely been ascribed to the high Pd dispersion on the catalysts’ surface. However, the Pd site effect on HCHO oxidation is still not clear. In this work, we regulated the proportions of different Pd sites on Pd/SiO2 catalysts by high-temperature treatment of the carriers. With the increase in treatment temperature, the dominant active sites on Pd particles changed from terraces to steps and finally to corner sites. The corner sites were found to be important for oxygen activation, which led to the HCHO catalytic oxidation performance increasing in the order Pd/SiO2, Pd/SiO2(500) and Pd/SiO2(800). It is worth noting that in the reaction pathway for HCHO oxidation, formate was the primary intermediate on the Pd/SiO2 catalyst. However, in addition to formate, CO species were also detected on the Pd/SiO2(500) and Pd/SiO2(800) catalysts. DFT results confirmed that the corner sites were more favorable for the HCHO → CHO → CO pathway, compared to the terrace sites. This work elucidates the effect of Pd sites on the HCHO oxidation mechanism over Pd/SiO2 catalysts, providing valuable insights for the design of highly active catalysts.