Wide range temperature stability of palladium on ceria-praseodymia catalysts for complete methane oxidation

Catalytic oxidation is the most effective technology to control methane emissions from both mobile and stationary sources. Palladium-based materials are widely viewed as the most active catalysts for the methane abatement reaction, even though the high-temperature PdO/Pd transition is linked to a decrease in catalytic activity. Aimed at minimizing this phenomenon, this work compares the catalytic activity and the thermal stability of different Pd-impregnated cerium-praseodymium mixed oxides prepared via Solution Combustion Synthesis. Although the palladium deposition on pure ceria allows obtaining a highly active system, the introduction of praseodymium enhances the thermal stability of the catalyst in an extended temperature range. X-ray photoelectron spectra show that the presence of praseodymium retains Pd in a more oxidized form, thus stabilizing the high-temperature active phase. This effect, as evident from X-ray diffractograms and Raman analyses, was attributed to a strong interaction of palladium particles with praseodymium, thereby hindering their reduction to the metallic form. Moreover, Pr doping played a significant role during methane oxidation in the presence of 5% H2O, improving both activity and stability compared to Pd on pure ceria. On the whole, Pd/Ce90Pr10 (2 wt% palladium supported on a mixed oxide with a praseodymium content of 10% on a cerium-praseodymium molar basis) was found to be the most promising catalyst amongst the studied materials in both dry and wet conditions, benefitting from the synergistic effect of ceria and praseodymia in improving the Pd activity and stability.