Heat of adsorption of carbon monoxide on a Pd/Rh three-way catalyst and on a Rh/Al 2O 3 solid

Abstract

In previous studies it has been shown that the change of the FTIR spectra of the adsorbed CO species formed on supported metal (Pt and Pd) catalysts with the adsorption temperature Ta (range 300–800 K) can lead to the determination of the heat of adsorption of each adsorbed CO species (linear CO species on Pt-containing solids, and linear and bridged CO species on Pd-containing solids). In the present study, this procedure is used for the determination 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/Rhthree-way catalyst. 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 formation of linear and bridged CO species on Rhsites. It is shown that the heats of adsorption of the two species vary with the coverage according to linear relationships: from 195 kJ/mol at ϑ = 0 to 103 kJ/mol at ϑ = 1 for the linear CO species and from 125 kJ/mol at ϑ =0 to 75 kJ/mol at ϑ = 1 for the bridged CO species. On palladium-containing solids with and without Rh, the adsorption of CO at T > 300 Kleads to IR bands attributed mainly to linear and bridged CO species on Pdsites. It is shown that the heat of adsorption of the linear CO species (varying from 54 kJ/mol at ϑ = 1 to 92 kJ/mol at ϑ = 0) is not modified by the presence of Rh. For the bridged CO species, the heat of adsorption is slightly decreased, at high coverages, in the presence of Rh but is not modified at low coverages (i.e., at ϑ = 0.95 the heats of adsorption of the bridged CO species are 113 and 100 kJ/mol on the monometallic and the bimetallic solids respectively whereas at ϑ = 0.52 the heat of adsorption is 135 kJ/mol on the two solids).