Heats of Adsorption of Linear and Multibound Adsorbed CO Species on a Pt/Al2O3 Catalyst Using in Situ Infrared Spectroscopy under Adsorption Equilibrium

The adsorption of CO (1 and 10% CO/He mixtures, 1 atm total pressure) on a reduced 2.9% Pt/Al2O3 catalyst in the temperature range 298–740 K is studied using Fourier transform infrared spectroscopy (FTIR). Several adsorbed CO species on Pt sites are detected at 300 K: linear (IR band at 2075 cm−1), bridged (three species; IR bands at 1878, 1835, and 1824 cm−1), and threefold coordinated CO species (IR band at 1800 cm−1) which are denoted by L, B, and 3FC, respectively. The evolution of the intensities of the IR bands with the adsorption temperature Ta allows the determination of the change of the coverage θ of each adsorbed CO species with Ta. The curves θ=f(Ta) permit the determination of the heats of adsorption of the three adsorbed species using the same adsorption model which considers: (a) a linear decrease in the heats of adsorption Eθ as a function of θ; and (b) immobile adsorbed species. The following heat of adsorption values were found (the subscript indicates the value of the coverage): for the L species, EL0=206 kJ/mol and EL1=115 kJ/mol; for the B species, EB0=94 kJ/mol and EB1=45 kJ/mol; for the 3FC species, E3FC0=135 kJ/mol and E3FC1=104 kJ/mol. In order to validate the assumptions of the adsorption model, the heats of adsorption obtained with the present procedure are compared to the isosteric heat of adsorption for the L and B species.