Heats of Adsorption of the Linear CO Species Adsorbed on a Pt/Al2O3Catalyst 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/Al 2 O 3 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 θ of this adsorbed CO species with T a and (b) that the curve θ = f ( T a ) allows the determination of the heats of adsorption of the L species at several coverages (denoted by EL θ ) 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), CO 2 (1% CO/14% CO 2 /He), H 2 O (1% CO/ y % H 2 O/He with y = 0.5 and 3.5), and C 2 H 4 (1% CO/2% C 2 H 4 /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/Al 2 O 3 (CO/H 2, CO/H 2 O, 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.