Heats of Adsorption of CO on a Cu/Al2O3Catalyst Using FTIR Spectroscopy at High Temperatures and under Adsorption Equilibrium Conditions

The adsorption of CO on a 4.7% Cu/Al 2 O 3 catalyst either reduced or oxidized is studied using the FTIR spectra recorded at adsorption temperatures T a in the range 298−740 K and with two constant partial pressures P a (10 3 and 10 4 Pa). On the reduced solid and at T a = 300 K a single IR band is detected at 2092 cm -1 ascribed to a linear CO species (denoted by L 0 ) on Cu° particles. The FTIR spectra lead to the determination of the evolution of the coverage θ of the L 0 species with T a for the two CO pressures. In the temperature range 380 K > T a > 480 K, the curves θ = f ( T a ) for the two CO pressures are in very good agreement with Temkin's adsorption model. Moreover, it is shown that, in the range 300−600 K, the curves are in agreement with an adsorption model considering (a) an immobile adsorbed species and (b) a linear decrease in the heat of adsorption with the coverage θ in the range 0−1. This leads to the determination of the heat of adsorption E θ, at various coverages θ from E 0 = 82 kJ/mol at θ = 0 to E 1 = 57 kJ/mol at θ = 1. It is shown that these values are similar to those obtained using the Clausius−Clapeyron equation. On the oxidized Cu/Al 2 O 3 catalyst, an IR band is detected at 2120 cm -1 at 300 K. Its intensity increases either with time on stream in 1% CO/He or with increase in T a . This IR band is ascribed to a linear CO species (denoted by L 1 ) on Cu + sites and the increase in its intensity is assigned to the reduction of the copper oxide surface. The maximum in the superficial concentration of the Cu + sites is obtained after reduction in 1% CO/He at 473 K. This allows the determination of the heats of adsorption of L 1 species at various coverages which linearly vary with θ from E ‘ 0 = 115 kJ/mol at θ = 0 to E ‘ 1 = 58 kJ/mol at θ = 1. The two L species are present simultaneously on the surface for an incomplete reduction of the solid, and it is shown that this does not significantly affect the heats of adsorption of the two adsorbed species. Moreover, the aging of the reduced catalyst has no effect on the heats of adsorption of the L 0 and L 1 species but leads to the detection of a new IR band at 2003 cm -1 ascribed to a bridged CO species on Cu° sites. The heat of adsorption of this species is strongly higher than those of the two L species varying with the coverage from 130 kJ/mol at θ = 0 to 78 kJ/mol at θ = 1. Finally, it is shown that the heats of adsorption of the L 0 and L 1 species are in accord with the literature data on the stability of the two adsorbed species in the course of a desorption at room temperature.