Adsorption isobars for CO on a Pt/Al2O3 catalyst at high temperatures using FTIR spectroscopy:: isosteric heat of adsorption and adsorption model

The adsorption of CO on a reduced 2.9%Pt/Al2O3 catalyst in a temperature range 298-740 K and at three CO pressures (1000, 100 and 10 Pa) is studied using Fourier transform infrared (FTIR) spectroscopy. A linear and a bridged CO adsorbed species are detected at 300 K on Pt sites characterized by IR bands at 2073 and 1845 cm(-1), respectively. The recorded FTIR spectra lead to the determination of the evolution of the coverage theta of the linear CO species with adsorption temperature T-a for the three adsorption pressures P-a. This cannot be achieved with the bridged CO species due to the low intensity of its IR band. The three curves theta =f(T-a) lead to the isosteric heat of adsorption Delta H of the linear CO species using the Clausius-Clapeyron equation (i.e., Delta H = 140 kT mol(-1) at theta = 0.9). In the coverage range 1-0.7, the curve Delta H =f(theta) is a straight line. This permits comparison of the profiles of the three isobars with those obtained from an adsorption model which considers (a) a linear decrease in the heat of adsorption as a function of theta in the range theta = 0-1; and (b) an immobile adsorbed species. This model can be approximated to Temkin's model in a certain adsorption temperature range. The adsorption model leads to the conclusion that the heat of adsorption of the linear CO species varies with coverage from E-0 = 206 kJ mol(-1) at theta = 0 to E-1 = 115 kT mol(-1) at theta = 1. The good accord between the isosteric heat of adsorption and that determined using the adsorption model for theta > 0.7 validates the assumptions involved in the adsorption model. (C) 2000 Elsevier Science B.V. All rights reserved.