In situ EXAFS study of Rh/Al2O3 catalysts for catalytic partial oxidation of methane

Alumina-supported rhodium particles, active for the catalytic partial oxida tion (CPO) of methane, have been investigated by in situ X-ray absorption f ine structure (XAFS), monitoring the gas composition simultaneously by mass spectrometry, The catalysts were prepared by anchoring of Rh-4(CO)(12) ont o the support and activated by treatments in He and H-2, EXAFS analyses sho wed that the Rh carbonyl precursor maintained its structure during fixation , but decomposed to small metallic Rh particles upon heating in He. However , the particle size increased significantly during treatment in hydrogen, I nvestigations of the catalyst in the CH4/O-2 reaction mixture showed that t he CPO reaction ignited at about 330 degreesC and, at the same time, the ca talyst changed its structure. The metal particles disaggregated upon heatin g in the reaction mixture and some Rh clusters containing oxidic or carbony lic species were formed. This process was reversible with respect to temper ature, The selectivity of the catalyst in the CH4/O-2 reaction mixture was significantly dependent on the residence time, However, no significant stru ctural differences of the Rh particles have been found at different residen ce times. Additionally, catalyst oxidation and reduction were investigated upon O-2 and CH4 treatment. (C) 2001 Academic Press.