Heats of Adsorption of Linear and Bridged CO Species Adsorbed on a 3% Ag/Al2O3 Catalyst Using in situ FTIR Spectroscopy under Adsorption Equilibrium
Résumé
The heats of adsorption of linear and bridged CO species adsorbed on Ag0 sites of a reduced 3% Ag/Al2O3 catalyst and of the linear CO species on Ag+ sites of the oxidized catalyst are determined as the function of their respective coverages by using the adsorption equilibrium infrared spectroscopy method previously developed. The evolutions of the intensities of the IR bands of each adsorbed species: 2045 cm-1 and 2000 cm-1 at 300 K, for the linear and bridged CO species, respectively, on Ag0 sites of the reduced solid and 2168 cm-1 for a linear CO species on Ag+ sites of the oxidized solid are determined as a function of the adsorption temperature Ta, at a constant CO adsorption pressure PCO. This provides the evolutions of the coverages of each adsorbed CO species with Ta in isobar conditions that give the individual heats of adsorption of the three adsorbed CO species at several coverages according to an adsorption model. The heats of adsorption of the linear and bridged adsorbed CO species on Ag0 sites vary linearly with their coverages : from E0 = 76 kJ/mol to E1 = 58 kJ/mol for the linear CO species and from E0 = 88 kJ/mol to E1 = 84 kJ/mol for the bridged CO species at coverages 0 and 1, respectively. Volumetric measurements indicate that the total amount of the adsorbed CO species represents a small fraction of the superficial sites of the reduced Ag particles suggesting that they are adsorbed on defect sites. The heat of adsorption of the linear CO species on the Ag+ sites linearly varies with its coverage from E0 = 66 kJ/mol to E1 = 41 kJ/mol at coverages 0 and 1, respectively.