A TRANSIENT KINETIC-STUDY OF THE CO/H2 REACTION ON RH/AL2O3 USING FTIR AND MASS-SPECTROSCOPY

The evolution of surface species (their chemical composition and surface coverage) formed during the CO/H-2 reaction at 220-degrees-C on 1 wt% Rh/Al2O3 catalyst was studied by transient isotopic and various hydrogen titration techniques using both in situ mass spectrometry and FTIR. There is a very small amount of active carbon, CH(x) (theta(CHx) < 0.03), and a large amount of surface linear and bridged CO species (theta(CO) = 0.93) which participate in the formation of CH4 during reaction. Their amounts stay practically constant with reaction time in CO/H-2 even after 1 h on stream. Also present on the Rh surface are some C(x)H(y) (alkyl chains) species which largely grow with reaction time (during the first hour on stream) but do not participate in the reaction mechanism of CH4 formation (spectator species). In addition, formate and carbonate (spectator species) build slowly on the alumina support surface even after 1 h of reaction. The surface coverage of hydrogen, theta(H), is found to be very small, a result consistent with the coverages of CO, CH(x), and C(x)H(y) species. CO dissociation over the present 1 wt% Rh/Al2O3 (1.5-nm Rh particles) appears to largely control the overall rate of methane formation, a result similar to that previously reported over the 5 wt% Rh Al2O3 (9.0-nm Rh particles) catalyst. (C) 1994 Academic Press, Inc,