SELF-SUSTAINED OSCILLATORY BEHAVIOR OF NO-2 REACTION OVER TITANIA-SUPPORTED PD CATALYSTS(CH4+O)

Self-sustained, regular oscillations were observed in NO + CH4 + O-2 r eaction over Pd/TiO2 catalysts at specific temperatures and oxygen con centrations. The oscillatory behavior was investigated combining NO re duction and methane combustion reaction studies with additional cataly st 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 sys tem was used to perform the NO + CH4 + O-2 and CH4 + O-2 reaction expe riments. Feed and product analyses were done on-line using gas chromat ography-mass spectrometry, chemiluminescence, and wet chemistry techni ques. Additional catalyst characterization was performed using thermal gravimetric analysis and high-temperature, controlled-atmosphere X-ra y diffraction techniques. Detailed analysis of the oscillatory behavio r has indicated that oscillations in the product and reactant profiles are coupled with temperature oscillations in the catalyst. When combi ned with controlled-atmosphere characterization experiments, these res ults, which can be reproduced in both NO + CH4 + O-2 and CH4 + O-2 sys tems, suggested that the oscillations are the result of periodic phase change of palladium on the surface. These cyclic phase transformation s, in turn, are the result of temperature variations that are caused b y the varying levels of exothermicity of the two major reactions, name ly NO reduction and CH4 combustion, that are favored by the metallic a nd the oxidic sites, respectively. (C) 1997 Academic Press.