Activation process of Pd/Al2O3 catalysts for CH4 combustion by reduction/oxidation cycles in CH4-containing atmosphere

In this work, the effect of a conditioning process, consisting of several alternated CH4-lean combustion/CH4-reducing pulses at 350 °C, was investigated over Pd/Al2O3 catalysts with different Pd loadings (1, 2 and 4 w/w%) prepared from Cl-free precursors. Such a treatment resulted in a one order of magnitude activity enhancement and in the achievement of stable catalytic performances in all the investigated systems. The activity enhancement was associated with an increase in PdO reducibility, as revealed by the comparison of CH4-TPR profiles before and after the conditioning process. Characterization studies showed that negligible or minor (for the 1% w/w Pd sample) modifications of Pd dispersion occurred upon conditioning, thus ruling out a significant effect of particle size variations on the observed activity enhancement. TPR, FT-IR and XRD measurements showed that palladium is completely oxidized upon conditioning, PdO is less crystalline than in the fresh catalyst likely due to the low oxidation temperature in the conditioning cycles but high activity was retained upon re-crystallization of PdO at 600 °C. FT-IR characterization revealed that conditioning results in a weakening of Pd–support interactions, which possibly originated from the interaction of the acidic Pd precursor with basic hydroxyls of the alumina surface, which acts as germination center during the impregnation step. Such a weakening of Pd–support interactions is likely responsible for the observed activity/reducibility enhancement.