Heats of Adsorption of the Linear CO Species Adsorbed on a Ir/Al_2O_3 Catalyst Using in Situ FTIR Spectroscopy under Adsorption Equilibrium

摘要

The adsorption of CO on a reduced 1% Ir/Al_2O_3 catalyst has been studied by using Fourier transform infrared spectroscopy (FTIR) at two constant CO partial pressures P_a (200 and 2000 Pa) in the adsorption temperature (denoted by T_a) range of 300-740 K. At 300 K, it is observed a strong IR band at 2044 cm~(-1) ascribed to linear CO species (denoted by L) associated to weak shoulders at ≈2080 and ≈2000 cm~(-1). The FTIR spectra show how the coverage θ_L of the L CO species evolves with T_a for the two P_a values. At T_a > 580 K, the curves θ_L = f(T_a) are in very good agreement with an adsorption model based on Temkin's approximations indicating that the heat of adsorption linearly increases with the decrease in θ_L from E_1 to E_0 at θ_L = 1 and 0, respectively. This allows us to evaluate the ratio E_0/(E_0 = E_1) ≈ 2.1 and then E_0 - E_1 ≈ 115 kJ/mol. Moreover, it is shown that, in the T_a range of 300-740 K, the curves θ_L = f(T_a) for the two CO partial pressures are in very good agreement with an adsorption model considering the following: (a) an immobile adsorbed species and (b) a linear decrease in the heat of adsorption with the increase in θ_L from E_0 = 225 kJ/mol at θ_L = 0 to E_1 = 115 kJ/mol at θ_L = 1. Finally the two isobars are used to determine the isosteric heats of adsorption at several coverages in the range of 1-0.8 from the Clausius-Clapeyron equation. A good agreement is observed with the values determined using the adsorption model and this validates the various assumptions involved in the model. The heat of adsorption values are compared to the literature data on single crystals and supported Ir catalysts. Morever, it is shown that the heat of adsorption values allow us to quantify several quantitative data found in the literature on the stability of the L CO species on Ir-containing solids either in a vacuum or in an inert gas.