Microkinetic analysis of water-promoted CO oxidation, water-gas shift, and preferential oxidation of CO on Pt for hydrogen generation

Hydrogen based proton exchange membranes (PEM) fuel cells show good potential for production of electricity. However, widespread commercialization of PEM fuel cells will depend on cheap and environmentally benign production of hydrogen. Currently, hydrogen is mainly produced from syngas by the water-gas shift (WGS) reaction (CO+H2O↔CO2+H2) followed by preferential oxidation (PROX) of CO. Extensive work has been devoted to the catalytic oxidation of H2 and CO on transition metals, and several microkinetic models, consisting of elementary reaction steps, have been proposed, especially on Pt. Yet, microkinetic models that could be used in the design of WGS and PROX are still lacking. There are several generic shortcomings of the existing microkinetic models. In particular, thermodynamic inconsistency of kinetic parameters is common in nearly all POX models of syngas. Another specific problem to these chemistries is the synergetic effects, arising from using mixtures of CO and H2, which have not been exploited yet as much. An example of such coupling is the production of the carboxyl (COOH), via the recombination reaction (CO+OH→COOH), and its subsequent decomposition (COOH+OH→CO2+H2O or COOH→CO2+H). Recent temperature programmed reaction (TPR) results (Bergald et al., Surface Science, 495, 2001, L815) have clearly shown that even minute fractions of H2O have an autocatalytic effect on the CO oxidation by oxygen, an example of the aforementioned coupling.