CATALYTIC COMBUSTION OF METHANE OVER PALLADIUM SUPPORTED ON ALUMINA CATALYSTS - EVIDENCE FOR RECONSTRUCTION OF PARTICLES

The physicochemical properties of palladium supported on alumina solid s were studied before and after the catalytic combustion of methane by combining nanodiffraction, electron microscopy and Fourier transform IR (FT-IR) analysis. The catalytic activity was strongly increased aft er reaction at 600-degrees-C under the O2-CH4 mixture under reducing a s well as under oxidizing conditions, whereas the dispersion of the me tallic phase decreased. Fresh Pd/Al2O3 samples exhibited mainly Pd (11 1) surface crystal planes and an epitaxy with the Al2O3 (222) planes m ay be considered. After reaction with the two oxygen-methane mixtures, the Pd (200) surface crystal planes developed at the expense of the P d (111) ones, the formation of the Pd (200) crystal planes was evidenc ed by nanodiffraction as well as from the FT-IR spectra of adsorbed ca rbon monoxide. The increase in catalytic activity was connected with t he presence of the Pd (200) surface planes. It was proposed that the p resence of these latter planes allows the reversible transition from s urface Pd0 to surface PdO without drastic changes of lattice parameter s.