Heats of adsorption of the linear CO species adsorbed on a Pt/Al2O3 catalyst in the presence of coadsorbed species using FTIR spectroscopy

The adsorption of CO (1% CO/He mixture, 1 atm. total pressure) in the presence of several gases is studied on a reduced 2.9% Pt/Al2O3 catalyst in the 300-740 K temperature range by using Fourier transform infrared spectroscopy. A strong IR band is observed at 2075 cm(-1) in the absence of a second gas associated to a weak and broad IR band around 1850 cm(-1) ascribed to linear and multibound CO species, respectively. We have previously shown (a) that the evolution of the intensity of the IR band of the L species with the adsorption temperature T-a leads to the determination of the change in the coverage theta of this adsorbed CO species with T-a and (b) that the curve theta = f(T-a) allows the determination of the heats of adsorption of the L species at several coverages (denoted by ELtheta) using an adsorption model. In the present study, the modifications of the IR band of the L species in the presence of a second gas (H-2 (1% CO/H-2), O-2 (1% CO/x% O-2/He with x < 0.25), CO2 (1% CO/14% CO2/He), H2O (1% CO/y% H2O/He with y = 0.5 and 3.5), and C2H4 (1% CO/2% C2H4/10% H-2/He)) are studied in the temperature range 300-740 K. It is shown that the coverage of the L species is not significantly modified for T-a > 450 K by the presence of the coadsorbed species even when catalytic reactions are detected. One may conclude (a) that the heat of adsorption of the L species is not significantly modified in the presence of coadsorbed species and (b) that the coverage of the L species during several catalytic reactions on Pt/Al2O3 (CO/H-2, CO/H2O, CO/O-2 with a CO/O-2 ratio > 2) is not significantly different from the coverage determined at the adsorption equilibrium. However, this last conclusion is not true for CO/O-2 ratios of < 2.