Heat of adsorption of carbon monoxide on a Ru/Al2O3 catalyst using adsorption equilibrium conditions at high temperatures

The adsorption of CO on a reduced 3.5%Ru/Al2O3 catalyst has been studied by means of FTIR spectroscopy between 298 and 740 K at a constant partial pressure P-a=1000 Pa. At 300 K, three adsorbed species were detected on the Ru particles: a linear CO species (strong IR-band at 2047 cm(-1)), a bridged CO species (weak IR band at 1750 cm(-1)) and a gem-dicarbonyl species (weak IR bands at 2135 and 2080 cm(-1)). The FTIR spectra show how the coverage theta of the linear CO species evolves with the adsorption temperature T-a. This cannot be achieved for the bridged CO species or for the gem-dicarbonyl species due to the weak intensities of their IR bands. At T-a>600 K, the curve theta=f(T-a) is in very good agreement with Temkin's adsorption model. Moreover, it is shown that, in the range 300-720 K, the curve agrees well with an adsorption model considering (a) an immobile adsorbed species; and (b) a linear decrease in the heat of adsorption with the increase in theta. This leads for the linear CO species to the heat of adsorption E-theta, at various coverages theta: from E-0=175 kJ mol(-1) at theta=0 to E-1=115 kJ mol(-1) at theta=1. This procedure for the determination of the heat of adsorption of CO on Ru particles is simple (it requires a single isobar) and accurate (approximate to+/-5 kJ mol(-1)). It constitutes a well adapted tool to study the modifications of this parameter with the method of preparation of the catalyst (precursor, support, particle size). (C) 2000 Elsevier Science B.V. All rights reserved.