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

The physicochemical properties of palladium supported on alumina solids 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 after reaction at 600-degrees-C under the O2-CH4 mixture under reducing as well as under oxidizing conditions, whereas the dispersion of the metallic phase decreased. Fresh Pd/Al2O3 samples exhibited mainly Pd (111) surface crystal planes and an epitaxy with the Al2O3 (222) planes may be considered. After reaction with the two oxygen-methane mixtures, the Pd (200) surface crystal planes developed at the expense of the Pd (111) ones, the formation of the Pd (200) crystal planes was evidenced by nanodiffraction as well as from the FT-IR spectra of adsorbed carbon monoxide. The increase in catalytic activity was connected with the presence of the Pd (200) surface planes. It was proposed that the presence of these latter planes allows the reversible transition from surface Pd0 to surface PdO without drastic changes of lattice parameters.