In situ Raman and in situ XRD analysis of PdO reduction and Pd degrees oxidation supported on gamma-Al2O3 catalyst under different atmospheres

Reduction of Pd degrees and decomposition of palladium oxide supported on g-alumina were studied at atmospheric pressure under different atmospheres (H-2, CH4, He) over a 4 wt% Pd/Al2O3 catalyst (mean palladium particle size: 5 nm with 50% of small particles of size below 5 nm). During temperature programmed tests (reduction, decomposition and oxidation) the crystal domain behaviour of the PdO/Pd degrees phase was evaluated by in situ Raman spectroscopy and in situ XRD analysis. Under H-2/N-2, the reduction of small PdO particles (<5 nm) occurs at room temperature, whereas reduction of larger particles (>5 nm) starts at 100 degrees C and is achieved at 150 degrees C. Subsequent oxidation in O-2/N-2 leads to reoxidation of small crystal domain at ambient temperature while oxidation of large particles starts at 300 degrees C. Under CH4/N-2, the small particle reduction occurs between 240 and 250 degrees C while large particle reduction is fast and occurs between 280 and 290 degrees C. Subsequent reoxidation of the catalyst reduced in CH4/N-2 shows that small and large particle oxidation of Pd degrees starts also at 300 degrees C. Under He, no small particle decomposition is observed probably due to strong interactions between particles and support whereas large particle reduction occurs between 700 and 750 degrees C. After thermal decomposition under He, the oxidation starts at 300 degrees C. Thus, the reduction phenomenon (small and large crystal domain) depends on the nature of the reducing agent (H-2, CH4, He). However, whatever the reduction or decomposition treatment or the crystal domain, Pd degrees oxidation starts at 300 degrees C and is completed only at temperatures higher than 550 degrees C. Under lean conditions, with or without water, the palladium consists of reduced sites of palladium (Pd degrees, Pd delta+ with delta < 2 or PdOx with x < 1) randomly distributed on palladium particles.