Metal Oxide Supports Tuning Activity of Palladium Catalysts for Methane Combustion: In Situ Spectroscopic Approach

Methane combustion over palladium-based catalysts is a critical process for reducing greenhouse gas emissions from lean-burn engines and natural gas installations, yet the role of oxide support in controlling both the population and the intrinsic reactivity of Pd active centres remains incompletely understood. In this work, Pd catalysts at two series of higher and lower loading were prepared on five oxide supports—Al2O3, CeO2, SiO2, TiO2, and ZrO2—and characterised by a complementary suite of techniques including SEM-EDX, XRD, BET, AAS, in situ CO-FTIR, DRIFTS with methanol as a probe molecule, and Raman spectroscopy. Catalytic activity testing revealed the order Pd/CeO2 > Pd/ZrO2 > Pd/Al2O3 > Pd/TiO2 > Pd/SiO2. In situ CO-FTIR site quantification showed that active site density spans nearly an order of magnitude across the series, with Pd/CeO2 reaching 105.44 µmol g−1 and Pd/Al2O3 only 11.63 µmol g−1. Turnover frequency analysis revealed a striking inversion: Pd/Al2O3 exhibited the highest TOF (0.1327 s−1), approximately six times greater than Pd/CeO2 (0.0226 s−1). DRIFTS/methanol profiling demonstrated that CeO2 and ZrO2 expose cooperative redox and basic centres that promote methane activation, while SiO2 supports only weakly bound methoxy species, consistent with its lowest activity. These results establish that the oxide support simultaneously governs Pd dispersion—and hence site density—and the electronic environment of each Pd centre, thereby modulating intrinsic reactivity. High specific surface area alone does not guarantee catalytic performance, and rational support selection is therefore the decisive lever for optimising methane combustion catalysts at ultra-low Pd loadings. In all, our findings provide a quantitative, molecular-level framework that disentangles support-controlled site density from intrinsic site reactivity under identical reaction conditions. By combining in situ CO-FTIR, DRIFTS, and Raman spectroscopy with kinetic analysis on well-defined, high-purity oxide supports, this work transforms previously qualitative “support effects” in Pd-catalysed methane combustion into predictive structure–activity relationships.