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The water-gas shift reaction is an important industrial reaction. It is often used in conjunction with steam reforming of methane or other hydrocarbons, which is important for the production of high purity hydrogen for use in ammonia synthesis. The water-gas shift reaction was discovered by Italian physicist Felice Fontana in 1780. The reaction is slightly exothermic, yielding 41.1 kJ (10 kcal) per mole.
The water gas shift reaction is sensitive to temperature, with the tendency to shift towards reactants as temperature increases due to Le Chatelier's principle. In fuel-rich hydrocarbon combustion processes, the water gas reaction at equilibrium state is often employed as a means to provide estimates for molar concentrations of burnt gas constituents.
The process is often used in two stages, stage one a high-temperature shift (HTS) at 350 °C (662 °F) and stage two a low-temperature shift (LTS) at 190–210 °C (374–410 °F). Standard industrial catalysts for this process are iron oxide promoted with chromium oxide for the HTS step and copper on a mixed support composed of zinc oxide and aluminum oxide for the LTS step.
Attempts to lower the reaction temperature of this reaction have been done primarily with a catalyst such as Fe3O4 (magnetite), or other transition metals and transition metal oxides. Another catalyst is the Raney copper catalyst.
The mechanism for the transition metal-catalyzed reaction can be generally understood as follows: a metal carbonyl complex ([M]-CO) reacts with hydroxide to give a metallacarboxylic acid ([M]-COOH−), which decarboxylates to give a metal hydride ([M]-H−). Reaction with hydronium from water and carbon monoxide regenerates the metal carbonyl complex. The mechanism of decarboxylation is debated; it may involve β-hydride elimination, or it may require the action of an external base.
The water-gas shift reaction may be an undesired side reaction in processes involving water and carbon monoxide, e.g. the rhodium-based Monsanto process. The iridium-based Cativa process uses less water, which suppresses this reaction.