Heat of adsorption of carbon monoxide on a Pd/Rh three-way catalyst and ona Rh/Al2O3 solid

In previous studies it has been shown that the change of the FTIR spectra o f the adsorbed CO species formed on supported metal (Pt and Pd) catalysts w ith the adsorption temperature T-a (range 300-800 K) can lead to the determ ination of the heat of adsorption of each adsorbed CO species (linear CO sp ecies on Pt-containing solids, and linear and bridged CO species on Pd-cont aining solids). In the present study, this procedure is used for the determ ination of the heats of adsorption of the two adsorbed CO species formed on Pd sites in the presence of a second metal to form a Pd/Rh three-way catal yst. In a first step the heats of adsorption of CO on a reduced 3% Rh/Al2O3 solid are determined. On this solid, the adsorption of CO leads to the for mation of linear and bridged CO species on Rh sites. It is shown that the h eats of adsorption of the two species vary with the coverage according to l inear relationships: from 195 kJ/mol at theta =0 to 103 kJ/mol at theta = 1 for the linear CO species and from 125 kJ/mol at theta = 0 to 75 kJ/mol at theta = 1 for the bridged CO species. On palladium-containing solids with and without Rh, the adsorption of CO at T > 300 K leads to IR bands attribu ted mainly to linear and bridged CO species on Pd sites. It is shown that t he heat of adsorption of the linear CO species (varying from 54 kJ/mol at t heta = 1 to 92 kJ/mol at theta = 0) is not modified by the presence of Rh. For the bridged CO species, the heat of adsorption is slightly decreased, a t high coverages, in the presence of Rh but is not modified at low coverage s (i.e., at theta = 0.95 the heats of adsorption of the bridged CO species are 113 and 100 kJ/mol on the monometallic and the bimetallic solids respec tively whereas at theta = 0.52 the heat of adsorption is 135 kJ/mol on the two solids). (C) 2000 Academic Press.