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

Author： Bourane A. Dulaurent O. Bianchi D.

Publisher： Academic Press

ISSN： 0021-9517

Source： Journal of Catalysis, Vol.196, Iss.1, 2000-11, pp. : 115-125

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Abstract

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 &thgr; of each adsorbed CO species with T_a. The curves &thgr;=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_&thgr; as a function of &thgr;; 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.