Heat of adsorption of carbon monoxide on various Pd-containing solids using in situ infrared spectroscopy at high temperatures

The FTIR spectra of the adsorbed CO species (linear and bridged species) formed on five Pd-supported solids are recorded at various adsorption temperatures, Ta, in the range 30000 K. These spectra are used to determine for each solid the individual coverage ϑ of the two adsorbed CO species as a function of Ta. The curves ϑ =f(Ta) observed on four solids are in good agreement with an adsorption model assuming a linear decrease in the heat of adsorption with an increase in coverage. This permits the determination of the heat of adsorption of each species at various coverages. The results observed on the various solids allowus to correlate the variation of the heats of adsorption with some parameters involved in the preparation of the solids, such as (a) the composition of the support and (b) the nature of the salt used as a precursor of the metallic phase (with and without chlorine). 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 independent of the parameters studied. The heat of adsorption of the bridged CO species is significantly modified by the nature of the salt precursor of the metallic phase. The presence of chlorine leads to a clear decrease in the heat of adsorption at high coverages (ϑ > 0.5) compared with the solids without chlorine, but at low coverages this difference disappears (i.e., on two 1.4% Pd/20% CeO2/Al2O3 solids, the heat of adsorption of the bridged CO species varies from ≈100 kJ/mol at ϑ = 1 to ≈170 kJ/mol at ϑ = 0 on the Cl-free solid and from 80 kJ/mol to 170 kJ/mol on the Cl-containing solid).