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

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 θ of the linear CO species with adsorption temperature Ta for the three adsorption pressures Pa. This cannot be achieved with the bridged CO species due to the low intensity of its IR band. The three curves θ = f(Ta) lead to the isosteric heat of adsorption ΔH of the linear CO species using the Clausius–Clapeyron equation (i.e., ΔH = 140 kJ mol−1 at θ = 0.9). In the coverage range 1–0.7, the curve ΔH = f(θ) 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 θ in the range θ = 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 E0 = 206 kJ mol−1 at θ = 0 to E1 = 115 kJ mol−1 at θ = 1. The good accord between the isosteric heat of adsorption and that determined using the adsorption model for θ > 0.7 validates the assumptions involved in the adsorption model.