14.4.2: Water-Gas Shift Reaction

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The water-gas shift reaction (WGSR) describes the reaction of carbon monoxide and water vapor (steam) at very high temperatures to form carbon dioxide and hydrogen:

\[\ce{CO + H2O <=> CO2 + H2}\]

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 the conversion of carbon monoxide or other hydrocarbons to produce hydrogen gas, an important source of fuel for a modern hydrogen economy. If hydrogen could be produced at low cost and using a renewable resource, it could significantly advance the movement to sustainable energy.

Most of the catalysts that are used for this process are heterogeneous catalysts. Unfortunately, the process using heterogeneous catalysts occurs only at extremely high temperatures and pressures. The high energy cost of performing the reaction limits its industrial utility, and there are efforts to produce homogeneous catalysts that catalyze the reaction at more mild conditions. Another limitation is that the reaction requires sufficient quantities of \(\ce{CO}\), which are primarily obtained from non-renewable coal or petroleum sources. However, there are processes that can convert methane gas, which is an abundant greenhouse gas, to \(\ce{}\).

The WGSR has been extensively studied for over a hundred years. The kinetically relevant mechanism depends on the catalyst composition and the temperature. 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 Fe-Cr catalyst. Historically, there has been much more controversy surrounding the mechanism at low temperatures. Recent experimental studies confirm that the associative carboxyl mechanism is the predominant low temperature pathway on metal-oxide-supported transition metal catalysts.

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. However, the carboxyl pathway accounts for about 90% of the total rate owing to the thermodynamic stability of adsorbed formate on the oxide support. The active site for carboxyl formation consists of a metal atom adjacent to an adsorbed hydroxyl. This ensemble is readily formed at the metal-oxide interface and explains the much higher activity of oxide-supported transition metals relative to extended metal surfaces. The turn-over-frequency for the WGSR is proportional to the equilibrium constant of hydroxyl formation, which rationalizes why reducible oxide supports (e.g. CeO₂) are more active than irreducible supports (e.g. SiO₂) and extended metal surfaces (e.g. Pt). In contrast to the active site for carboxyl formation, formate formation occurs on extended metal surfaces. The formate intermediate can be eliminated during the WGSR by using oxide-supported atomically dispersed transition metal catalysts, further confirming the kinetic dominance of the carboxyl pathway.

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.

Source: https://en.Wikipedia.org/wiki/Water%...shift_reaction