Self-sustained oscillatory behavior of NO+CH4+O-2 reaction over titania-supported Pd catalysts

Self-sustained, regular oscillations were observed in NO + CH4 + O-2 reaction over Pd/TiO2 catalysts at specific temperatures and oxygen concentrations. The oscillatory behavior was investigated combining NO reduction and methane combustion reaction studies with additional catalyst characterization experiments performed under controlled atmospheres. The catalyst was prepared using a wet impregnation technique with Pd-acetate as the precursor for palladium. A fixed-bed, flow reactor system was used to perform the NO + CH4 + O-2 and CH4 + O-2 reaction experiments. Feed and product analyses were done on-line using gas chromatography-mass spectrometry, chemiluminescence, and wet chemistry techniques. Additional catalyst characterization was performed using thermal gravimetric analysis and high-temperature, controlled-atmosphere X-ray diffraction techniques. Detailed analysis of the oscillatory behavior has indicated that oscillations in the product and reactant profiles are coupled with temperature oscillations in the catalyst. When combined with controlled-atmosphere characterization experiments, these results, which can be reproduced in both NO + CH4 + O-2 and CH4 + O-2 systems, suggested that the oscillations are the result of periodic phase change of palladium on the surface. These cyclic phase transformations, in turn, are the result of temperature variations that are caused by the varying levels of exothermicity of the two major reactions, namely NO reduction and CH4 combustion, that are favored by the metallic and the oxidic sites, respectively. (C) 1997 Academic Press.