Experimental microkinetic approach of the CO/H2 reaction on Pt/Al2O3 using the Temkin formalism. 2. Coverages of the adsorbed CO and hydrogen species during the reaction and rate of the CH4 production

- Experimental microkinetic approach of the CO/H2 reaction on 2.9% Pt/Al2O3.
- Experimental conditions: PCO = 1, 10 and 1000 Pa, PH2 ≈ 105 Pa, T = 300-720 K.
- Coverage of the linear CO and hydrogen species from FTIR spectroscopy.
- Modeling of the CO/H2 reaction using the Temkin formalism.
- Comparison between Temkin and Langmuir formalism.

The present experimental microkinetic approach is dedicated to the interpretation using the Temkin formalism of the experimental evolutions of (a) the coverage of adsorbed CO and hydrogen species and (b) the rate of the CH4 production during the increase in the temperature for x% CO/H2 gas mixtures (x = 1, 10−2 and 10−3, PT = 1 atm) on a reduced 2.9% Pt/Al2O3 catalyst (Pt dispersion D ≈ 0.26). 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 300 K and (b) provides the evolution of the coverage of the dominant L CO species (IR band at 2080 cm−1 at 300 K) for the three CO/H2 gas mixtures in the 300-740 K temperature range. These data support the development of a rigorous (without approximations) competitive adsorption model associated with a reaction based on the Temkin formalism (denoted Temkin-C.R model). This model provides (a) the theoretical coverages of the L CO and hydrogen species and (b) the theoretical rate of the CH4 production during the CO/H2 reaction as a function of the partial pressures of the reactants and the reaction temperature which are compared to the experimental data. This shows that the L CO species which dominates the CO adsorption is not the active adsorbed species at the beginning of the CH4 production (T ≈ 475 K for 1% CO/H2). It is shown that the active species is the 3FC CO species which represents ≈ 1/7 of the total amount of adsorbed CO species. These conclusions are consistent which literature data on Pt particles indicating that a small number of sites/adsorbed CO species are involved at the beginning of the CO/H2 reaction. Moreover, as a contribution to the debate on the paradox of kinetics on heterogeneous surfaces, the Temkin-C.R model is compared to models based on the Langmuir formalism mainly used in kinetic studies. This reveals the clear advantage of the Temkin-C.R model for the representation of experimental data in large ranges of experimental conditions.

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