A critical analysis of the experimental evidence for and against a formate mechanism for high activity water-gas shift catalysts

An in-depth analysis of the evidence for and against a formate-based mechanism for the water-gas shift reaction has shown that with very few exceptions the published results cannot be used to provide any mechanistic information either for or against a formate model. Where reliable quantitative data are available, these show unequivocally that for all high activity catalysts the contribution of IR-observable formate to the production of CO2 is only of minor importance. It is found that the formates seen by IR typically account for less than about 10–15% of the total WGS reaction products. The formates observed by IR were potentially the main reaction intermediates only in the case of very low activity catalysts. For the high activity catalysts that are of interest for current fuel cell applications it is clear that the major contribution of a formate mechanism is yet to be proven. The current published in situ/operando FTIR data relating to formates cannot be used to elucidate the main reaction mechanism nor for deriving improved catalytic formulations.

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► The contribution of IR-observable formates to the WGS is minor.
► IR-seen formates are main reaction intermediates only for very low activity catalysts.
► Published FTIR data cannot be used to elucidate the main reaction mechanism.
► Operando FTIR work must be analysed using accurate quantitative methods.