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 T-a allows the determination of the change of the coverage a of each adsorbed CO species with T-a. The curves theta = f(T-a) 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 E-theta as a function of theta; 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, E3FC(0) = 135 kJ/mol and E3FC(1) = 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. (C) 2000 Academic Press.