The water gas shift reaction: Catalysts and reaction mechanism

The water gas shift reaction (WGSR) is a moderately exothermic reaction between carbon monoxide and steam to form carbon dioxide and hydrogen. In typical industrial applications, the WGSR is conducted as a two stage process. The high temperature stage, conducted over an iron based catalyst in the temperature range 320 – 450 °C. The low temperature stage, conducted over copper-based catalysts in the temperature range 150 – 250 °C. There is no universally accepted reaction mechanism for the WGSR. The accepted mechanism depends on whether it is being studied for HT or LT as well as on the catalyst type. The redox mechanism usually accepted for the HT-WGSR and, depending on the active metal, also for the LT-WGSR as well as the mechanism involving formate and/or carboxyl species for the LT-WGSR are discussed. Catalyst deactivation presents a limitation on the utilization of different catalysts for the WGSR. The main causes of catalysts deactivation are (a) thermal sintering, (b) sulfur poisoning, (c) chloride poisoning. In addition to the traditionally used Fe-based catalysts for the HT-WGSR and Cu-based catalysts for the LT-WGSR, other catalysts such as nickel, cobalt, molybdenum, platinum, gold, rhodium, and ruthenium are active for the WGSR. Catalyst preparation and pre-treatment steps play a crucial role in catalyst activity.