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 surfa ce coverage) formed during the CO/H-2 reaction at 220-degrees-C on 1 w t% Rh/Al2O3 catalyst was studied by transient isotopic and various hyd rogen titration techniques using both in situ mass spectrometry and FT IR. 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 (t heta(CO) = 0.93) which participate in the formation of CH4 during reac tion. 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 react ion mechanism of CH4 formation (spectator species). In addition, forma te and carbonate (spectator species) build slowly on the alumina suppo rt surface even after 1 h of reaction. The surface coverage of hydroge n, theta(H), is found to be very small, a result consistent with the c overages 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 contro l the overall rate of methane formation, a result similar to that prev iously reported over the 5 wt% Rh Al2O3 (9.0-nm Rh particles) catalyst . (C) 1994 Academic Press, Inc,