Experimental microkinetic approach of the CO/H2 reaction on Pt/Al2O3 using the Temkin formalism. 1. Competitive chemisorption between adsorbed CO and hydrogen species in the absence of reaction

Abstract

The present study is dedicated to the development of a Temkin model for competitive chemisorption (denoted Temkin-C) which can be applied in large range of experimental conditions (partial pressures and temperatures). It is based on experimental data from the adsorption/reaction of x% CO/H2 gas mixtures (x=1, 10−2 and 10−3, total pressure 1atm.) on a reduced 2.9% Pt/Al2O3 catalyst for two platinum dispersions (D≈0.6 and 0.26) in the temperature range 300–740K. FTIR spectroscopy (a) shows that three adsorbed CO species: linear, bridged and threefold coordinated CO species (denoted L, B and 3FC respectively) are formed at 300K and (b) provides the evolution of the coverage of the dominant L CO species (IR band at 2080cm−1 at 300K) for the three CO/H2 gas mixtures in the 300–740K temperature range. These data support the rigorous development of a Temkin-C model providing the theoretical coverage of the L CO and hydrogen species in the absence of the CH4 production. The comparison of the theoretical and experimental evolutions of the coverage of the L CO species shows that (a) the L CO species dominates the competitive chemisorption with the hydrogen species and (b) the heats of adsorption of the two species are not significantly modified by their co-adsorption. Moreover, in line with the debate dedicated to the paradox of kinetics on heterogeneous surface (briefly the catalytic activity of a heterogeneous surface can be well represented by assuming an homogeneous surface), the Temkin-C model is compared to models based on the Langmuir formalism currently used in the literature dedicated to kinetic studies. This reveals the clear advantage of the Temkin-C model for the representation of experimental data in large range of experimental conditions. In Part 2, the Temkin-C model is extended by considering that the L CO species is the adsorbed intermediate species of the CH4 formation from the CO/H2 reaction at T>500K.