Water gas shift reaction
The water-gas shift reaction (WGS) is a chemical reaction in which carbon monoxide reacts with water vapor to form carbon dioxide and hydrogen:
- CO(g) + H2O(v) → CO2(g) + H2(g)
The water-gas shift reaction is an important industrial reaction. It is often used in conjunction with steam reforming of methane or other hydrocarbons,[1] 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.[1]
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[edit] Applications
This reaction has been used as a CO removal method from the reformate for fuel cell applications.
The reverse water-gas shift reaction has recently found a possible application in In-Situ Resource Utilization on Mars to provide oxygen for fuel for the Mars Direct mission concept.
[edit] Reaction conditions
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).[2] 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.[3]
[edit] Catalysts
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.[4]
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.[5] 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.
[edit] See also
[edit] References
- ^ a b "HFCIT Hydrogen Production: Natural Gas Reforming". United States Department of Energy. 2006-11-08. http://www1.eere.energy.gov/hydrogenandfuelcells/production/natural_gas.html. Retrieved 2008-01-07.
- ^ Stages
- ^ Schumacher, N.; et, al.; Dahl, S; Gokhale, A; Kandoi, S; Grabow, L; Dumesic, J; Mavrikakis, M et al (2005). "Trends in low-temperature water–gas shift reactivity on transition metals". Journal of Catalysis 229 (2): 265–275. doi:10.1016/j.jcat.2004.10.025
- ^ Mellor, JR et al. (2 January 1997). "Raney copper catalysts for the water gas shift reaction – II. Initial catalyst optimisation". Applied Catalysis A-General 164: 185–195. doi:10.1016/S0926-860X(97)00168-3. hdl:10204/776.
- ^ Crabtree, Robert H. (2005). "12. Applications of Organometallic Chemistry". The Organometallic Chemistry of the Transition Metals (4th ed.). pp. 360–361. doi:10.1002/0471718769.ch12.
