Heat of adsorption of carbon monoxide on a Ru/Al 2O 3 catalyst using adsorption equilibrium conditions at high temperatures

Abstract

The adsorption of CO on a reduced 3.5%Ru/Al 2O 3 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 θ 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 θ= 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 θ. This leads for the linear CO species to the heat of adsorption E θ , at various coverages θ: from E 0=175 kJ mol −1 at θ=0 to E 1=115 kJ mol −1 at θ=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 (≈±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).