Heats of adsorption of CO on a Cu/Al2O3 catalyst using FTIR spectroscopy at high temperatures and under adsorption equilibrium conditions

The adsorption of CO on a 4.7% Cu/Al2O3 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) a nd 10(4) Pa). On the reduced solid and at T-a = 300 K a single IR band is d etected at 2092 cm(-1) ascribed to a linear CO species (denoted by L-0) on Cu degrees particles. The FTIR spectra lead to the determination of the evo lution of the coverage theta of the L-0 species with T-a for the two CO pre ssures. In the temperature range 380 K > T-a > 480 K, the curves theta = f( T-a) for the two CO pressures are in very good agreement with Temkin's adso rption model. Moreover, it is shown that, in the range 300-600 K, the curve s are in agreement with an adsorption model considering (a) an immobile ads orbed species and (b) a linear decrease in the heat of adsorption with the coverage theta in the range 0-1. This leads to the determination of the hea t of adsorption E-theta, at various coverages theta from E-0 = 82 kJ/mol at theta = 0 to E-1 = 57 kJ/mol at theta = 1. It is shown that these values a re similar to those obtained using the Clausius-Clapeyron equation. On the oxidized Cu/Al2O3 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 inc rease 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 reduc tion of the copper oxide surface. The maximum in the superficial concentrat ion 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 L1 species at vario us coverages which linearly vary with 8 from E-0' = 115 kJ/mol at theta = 0 to E-1' = 58 kJ/mol at theta = 1. The two L species are present simultaneo usly on the surface for an incomplete reduction of the solid, and it is sho wn that this does not significantly affect the heats of adsorption of the t wo adsorbed species. Moreover, the aging of the reduced catalyst has no eff ect on the heats of adsorption of the Lo and L-1 species but leads to the d etection of a new IR band at 2003 cm(-1) ascribed to a bridged CO species o n Cu degrees sites. The heat of adsorption of this species is strongly high er than those of the two L species varying with the coverage from 130 kJ/mo l at theta = 0 to 78 kJ/mol at theta = 1. Finally, it is shown that the hea ts of adsorption of the L-0 and L-1 species are in accord with the literatu re data on the stability of the two adsorbed species in the course of a des orption at room temperature.