Nanostructured Pr-Rich Ce_xPr_1-xO_2-δ Mixed Oxides for Diesel Soot Combustion: Importance of Oxygen Lability

Soot combustion experiments with 5%O₂/He were conducted using model soot, and four distinct compositions of Ce_xPr_1-xO_2-δ oxides of varying nominal cerium compositions (x = 0, 0.2, 0.3, and 1) were prepared. The catalyst samples were comprehensively characterized using techniques such as XRD, Raman spectroscopy, HR-TEM, N₂ adsorption at -196 °C, XPS, O₂-TPD, H₂-TPR, and work function measurements. The Pr-rich compositions, ranging from Ce_0.3Pr_0.7O_2-δ to PrO_2-δ, resulted in a significant increase in the total evolved O₂ amounts and enhanced catalyst reducibility. However, a decrease in the textural properties of the catalysts was noted, which was particularly important for the pure praseodymia under the synthesis route conducted. The catalytic activity was investigated under the two following contact modes of mixing between soot and catalyst: loose and tight. The results revealed that the catalytic performance is associated with the surface contact in tight contact mode and with the combination of surface/subsurface/bulk oxygen mobility and the BET surface area in loose contact mode. Notably, the temperatures estimated at 10% and 50% of the conversion (T₁₀ and T₅₀) parameters were achieved at much lower temperatures than the uncatalyzed soot combustion, even under loose contact conditions. Specifically, the 50% conversion was achieved at 511 °C and 538 °C for Ce_0.3Pr_0.7O₂ and Ce_0.2Pr_0.8O₂, respectively. While no direct correlation between catalytic activity and work function was observed, a significant relationship emerges between work function values and the formation of oxygen vacancies, whatever the conditions used for these measurements. On the other hand, the ability to generate a high population of oxygen vacancies at low temperatures, rather than the direct activation of gas-phase O₂, influences the catalytic performance of Pr-doped ceria catalysts, highlighting the importance of surface/subsurface oxygen vacancy generation, which was the parameter that showed a better correlation with the catalytic activity, whatever the soot conversion value or the mode of contact considered.