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]

Contents

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.

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]

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 which use water and carbon monoxide are present, e.g. the rhodium-based Monsanto process. The iridium-based Cativa process uses less water, which suppresses this reaction.

See also

References

  1. ^ 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. 
  2. ^ Stages
  3. ^ 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 
  4. ^ 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. 
  5. ^ 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.