Effects of Flame Conditions on Structure and Methane Oxidation Activity of Pd/CexZr1–xO₂ Catalysts

Solid solution catalysts doped with Pd can be used to enhance the low-temperature methane oxidation activity by increasing the oxygen storage capacity, metal–support interaction, and oxygen mobility. In this work, we systematically synthesized Pd-doped CexZr1–xO2 solid solution catalysts using flame spray pyrolysis (FSP) as a robust synthesis method to determine the synthesis–structure–activity relationship for methane oxidation. We examined the impact of precursor molarity, liquid feed rate, and oxygen flow rate on the oxygen vacancy percentage (OV%), Pd speciation, and metal–support interaction. Our results indicate that increasing the liquid feed rate or decreasing the oxygen flow rate in FSP results in higher OV% of the Pd/CexZr1–xO2 catalyst. The catalysts with higher OV% have weaker metal–support interaction and promote the formation of peroxide species (O22–). A lower precursor molarity leads to the formation of relatively larger PdO nanoparticles, which in turn decreases Pd dispersion. Of particular importance, Pd speciation can be controlled by the equivalence ratio (ER). Catalysts synthesized in ER < 1 favor the formation of incorporated Pd2+ species, while mostly finely dispersed PdO nanoparticles can be observed at ER > 1. Finely dispersed PdO nanoparticles in Pd/CexZr1–xO2 exhibit higher turnover frequencies (TOFs) for methane oxidation compared to those with more incorporated Pd2+ species. The TOFs at 400 °C of Pd/Ce0.5Zr0.5O2 samples can be adjusted from 0.10 to 0.21 s–1 by modifying the synthesis conditions in the FSP, which impacts the OV% and Pd speciation of the catalyst. The enhanced methane oxidation activity is attributed to the synergistic interaction among PdO nanoparticles, oxygen vacancies, and peroxide species.