Water–gas shift reaction

The water–gas shift reaction (WGSR) describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen:

: CO + H2O CO2 + H2

The water gas shift reaction was discovered by Italian physicist Felice Fontana in 1780. It was not until much later that the industrial value of this reaction was realized. Before the early 20th century, hydrogen was obtained by reacting steam under high pressure with iron to produce iron oxide and hydrogen. With the development of industrial processes that required hydrogen, such as the Haber–Bosch ammonia synthesis, a less expensive and more efficient method of hydrogen production was needed. As a resolution to this problem, the WGSR was combined with the gasification of coal to produce hydrogen.

Applications

The WGSR is a highly valuable industrial reaction that is used in the manufacture of ammonia, hydrocarbons, methanol, and hydrogen. Its most important application is in conjunction with the conversion of carbon monoxide from steam reforming of methane or other hydrocarbons in the production of hydrogen. In the Fischer–Tropsch process, the WGSR is one of the most important reactions used to balance the H2/CO ratio. It provides a source of hydrogen at the expense of carbon monoxide, which is important for the production of high purity hydrogen for use in ammonia synthesis.

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.

Fuel cells

The WGSR can aid in the efficiency of fuel cells by increasing hydrogen production. The WGSR is considered a critical component in the reduction of carbon monoxide concentrations in cells that are susceptible to carbon monoxide poisoning such as the proton-exchange membrane (PEM) fuel cell. The commercial LTS catalyst used in large scale industrial plants is also pyrophoric in its inactive state and therefore presents safety concerns for consumer applications.

Reaction conditions

The equilibrium of this reaction shows a significant temperature dependence and the equilibrium constant decreases with an increase in temperature, that is, higher hydrogen formation is observed at lower temperatures.

Temperature dependence

400px|thumbnail|right|Temperature dependence of the free molar (Gibbs) enthalpy and equilibrium constant of the water-gas shift reaction.

With increasing temperature, the reaction rate increases, but hydrogen production becomes less favorable thermodynamically since the water gas shift reaction is moderately exothermic; this shift in chemical equilibrium can be explained according to Le Chatelier's principle. Over the temperature range of 600–2000 K, the equilibrium constant for the WGSR has the following relationship: can be used:

<div align="center"><math> K_\mathrm{p} = e^{-4.33 + \frac{4577.8}{T</math></div>

Alternatively, the equilibrium constant for the WGSR directly derived from thermodynamic quantities leads to:

<div align="center"><math> ln(K_\mathrm{p}) = -13.148 + 1.077~~ln(T) + 5.44~~10^{-4}~~T - 1.125~~10^{-7}~~T^2 + \frac{5693.5}{T} - \frac{49170}{T^2}</math></div>

Practical concerns

In order to take advantage of both the thermodynamics and kinetics of the reaction, the industrial scale water gas shift reaction is conducted in multiple adiabatic stages consisting of a high temperature shift (HTS) followed by a low temperature shift (LTS) with intersystem cooling. The chromium acts to stabilize the iron oxide and prevents sintering. The operation of HTS catalysts occurs within the temperature range of 310 °C to 450 °C. The temperature increases along the length of the reactor due to the exothermic nature of the reaction. As such, the inlet temperature is maintained at 350 °C to prevent the exit temperature from exceeding 550 °C. Industrial reactors operate at a range from atmospheric pressure to 8375 kPa (82.7 atm). with values as low as Ea = 34 kJ/mol reported relative to hydrogen generation.

Low temperature shift catalysis

Catalysts for the lower temperature WGS reaction are commonly based on copper or copper oxide loaded ceramic phases, While the most common supports include alumina or alumina with zinc oxide, other supports may include rare earth oxides, spinels or perovskites. A typical composition of a commercial LTS catalyst has been reported as 32-33% CuO, 34-53% ZnO, 15-33% Al2O3. The active catalytic species is CuO. The function of ZnO is to provide structural support as well as prevent the poisoning of copper by sulfur. The Al2O3 prevents dispersion and pellet shrinkage. The LTS shift reactor operates at a range of 200–250 °C. The upper temperature limit is due to the susceptibility of copper to thermal sintering. These lower temperatures also reduce the occurrence of side reactions that are observed in the case of the HTS. Noble metals such as platinum, supported on ceria, have also been used for LTS.

Mechanism

[[File:WGS mechanism.png|550px|thumb|right|Proposed associative and redox mechanisms of the water gas shift reaction Two mechanisms have been proposed: an associative Langmuir–Hinshelwood mechanism and a redox mechanism. The redox mechanism is generally regarded as kinetically relevant during the high-temperature WGSR (> 350 °C) over the industrial iron-chromia catalyst.

Associative mechanism

In 1920 Armstrong and Hilditch first proposed the associative mechanism. In this mechanism CO and H2O are adsorbed onto the surface of the catalyst, followed by formation of an intermediate and the desorption of H2 and CO2. In general, H2O dissociates onto the catalyst to yield adsorbed OH and H. The dissociated water reacts with CO to form a carboxyl or formate intermediate. The intermediate subsequently dehydrogenates to yield CO2 and adsorbed H. Two adsorbed H atoms recombine to form H2.

There has been significant controversy surrounding the kinetically relevant intermediate during the associative mechanism. Experimental studies indicate that both intermediates contribute to the reaction rate over metal oxide supported transition metal catalysts.

Redox mechanism

The redox mechanism involves a change in the oxidation state of the catalytic material. In this mechanism, CO is oxidized by an O-atom intrinsically belonging to the catalytic material to form CO2. A water molecule undergoes dissociative adsorption at the newly formed O-vacancy to yield two hydroxyls. The hydroxyls disproportionate to yield H2 and return the catalytic surface back to its pre-reaction state.

Homogeneous models

The mechanism entails nucleophilic attack of water or hydroxide on a M-CO center, generating a metallacarboxylic acid.

Thermodynamics

The WGSR is exergonic, with the following thermodynamic parameters at room temperature (298 K)

:{|class=wikitable

!Free energy

|ΔG⊖ = –28.6 kJ/mol

!Enthalpy

|ΔH⊖ = –41.2 kJ/mol

!Entropy

|ΔS⊖ = –41.84 J/K.mol

In aqueous solution, the reaction is less exergonic.

Reverse water–gas shift

In the conversion of carbon dioxide to useful chemicals, fuels, and materials, the water–gas shift reaction is used to produce carbon monoxide from hydrogen and carbon dioxide. This is often called the reverse water–gas shift reaction (RWGS), and is generally differentiated from the water-gas shift process by the use-cases. Catalyst development for the RWGS reaction are researched in academia, with companies such as Dimensional Energy leading catalyst commercial efforts in a process that converts carbon dioxide to products such as sustainable aviation fuel via RWGS in tandem with the Fischer Tropsch reaction.

Water gas is defined as a fuel gas consisting mainly of carbon monoxide (CO) and hydrogen (H2). The term 'shift' in water–gas shift means changing the water gas composition (CO:H2) ratio. The ratio can be increased by adding CO2 or reduced by adding steam to the reactor.