Us. Ozkan et al., SELF-SUSTAINED OSCILLATORY BEHAVIOR OF NO-2 REACTION OVER TITANIA-SUPPORTED PD CATALYSTS(CH4+O), Journal of catalysis, 171(1), 1997, pp. 67-76
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.