Heterogeneous water-gas shift reaction

York, 1963] that there are three fundamental gasification reactions the Boudouard reaction (24-17), the heterogeneous water-gas-shift reaction (24-18), and the hydrogasification reaction (24-15). The equilibrium constants for these reactions are sufficient to calculate equi-... 

The relevance of the water-gas shift reaction in the petrochemical industry has already been discussed (see Section 1.1). The significance of the water-gas shift reaction in homogeneous systems is twofold. First, it plays a crucial role in stabilizing the rhodium catalyst in the Monsanto process. Second, studies carried out in homogeneous systems employing metals other than rhodium have provided useful mechanistic insights into the heterogeneous water-gas shift reaction. We first discuss the catalytic cycle with 4.1 as one of the catalytic intermediates, and then mechanistic results that are available from an iron-based catalytic system. 

Metal coordination compounds may also provide alternatives to the heterogeneous catalysts used for the water gas shift reaction. In fact, Ru, Rh, Ir, and Pt coordination compounds have all shown some promise (27). 

Additional utilization of the water gas shift reaction also allows ethylene or methanol to be produced in a second synthesis step, which was developed around 1925 by Fischer and Tropsch [2], The catalyst for this heterogeneous process consists of Co-Th02-MgO mixtures supported on kieselgur. 

Several studies have reported the catalysis of the liquid-phase water gas shift reaction (WGSR). Actually, homogeneous catalysis of the WGSR is not competitive with its heterogeneous counterpart due to the limited rate, instability of the catalysts, and high costs. Scheme 64 shows the most important steps. 

The data available for heterogeneous Fischer-Tropsch catalysts indicate that with cobalt-based catalysts the rate of the water gas-shift reaction is very slow under the synthesis conditions (5). Thus, water is formed together with the hydrocarbon products [Eq. (14)]. The iron-based catalysts show some shift activity, but even with these catalysts, considerable quantities of water are produced. 

Heterogeneous catalysts increase the rate at which a chemical reaction reaches equilibrium, but cannot shift this equilibrium. Mention some of the possible ways to increase the CO conversion over the equilibrium value in the case of the water-gas shift reaction. 

As an application of Pt nanowires in heterogeneous catalysis, we performed preferential oxidation (PROX) of CO as a test reaction [32]. The PROX reaction is useful for PEM fuel cells for the selective removal of contaminating CO from hydrogen gas, because CO works as a strong catalyst poison for Pt electrode catalysts (Figure 15.24). H2 produced in steam-reforming and the water-gas shift reaction needs further to be purified in the PROX reaction to selectively oxidize a few% CO towards inert CO2 in a H 2-rich atmosphere, to reduce the CO content to <10ppm. Under the PROX conditions, the facile oxidation of H2 to H2O may also occur, thus the catalyst selectivity for CO oxidation over H2 oxidation is an... 

The water gas shift reaction occurs on or within the iron oxide particle and therefore a heterogeneous model is employed using separate balances for the pellets and the reactor. 

This is called the water gas shift reaction. We discuss this reaction in some detail in Chapter 4 (see Section 4.3). The heterogeneous catalysts used for the water gas shift reaction are of two types. The high-temperature shift catalyst is a mixture of Fe304 and Cr203 and operates at about 500°C. The low-temperature shift catalyst contains copper and zinc oxide on alumina, operates at about 230°C, and is more widely used in industry. 

The water gas shift reaction is considered to be in equilibrium. However, the heterogeneous reactions are influenced by both chemical kinetics and diffusive transport of reactants. Further, in the case of the carbon-steam reaction, the inhibition by both carbon monoxide and hydrogen is also included. 

In general, these processes, when performed using heterogeneous catalysts, require significantly elevated temperatures and pressures. Consequently, as in the case of the water gas shift reaction, there has been great interest in developing homogeneous catalysts that can perform the same functions under much milder conditions. 

Three different versions of the basic one dimensional model were developed, two heterogeneous models and a pseudo-homogeneous model. The difference in the two heterogeneous models is the way the sorbent was installed in the reactor bed. One model version considers that Li2Zr03 and the reforming catalyst are coated on two different particles, while in the other case there is one particle with both catalytic and capture properties. The steam methane reforming and the water-gas shift reaction kinetics are taken from Xu and Proment [14], but corrected for different properties of the catalyst. 

As an example for heterogeneous catalysis we can consider the water-gas shift reaction... 

Much of the justification for the extensive study of transition metal cluster chemistry is embedded in the assumption that reactions of metal clusters are realistic structural models for reactions at metal surfaces in such processes as heterogeneous catalysis (9,10,11). For example, the metal carbonyl clusters, Ir4(CO)i2 and Os3(CO)i2, were demonstrated to be effective homogeneous catalysts for methanation (12). Additionally, Demitras and Muetterties (13) have found Ir4(CO)i2 to be a homogeneous catalyst in the Fischer-Tropsch synthesis of aliphatic hydrocarbons. Homogeneous catalysis of the water gas shift reaction by metal carbonyl clusters (e.g., Ru3(CO)i2) in alkaline solution has been reported by Laine, Rinker, and Ford (14), and more recently by Pettit s group (15). Nevertheless, mononuclear metal carbonyls (e.g., Fe(CO)s and the group VIb metal hexacarbonyls) have been demonstrated to have considerable activity above 120°C as soluble catalysts for Reaction 2 (16),... 

Although the water gas shift reaction is accomplished with heterogeneous catalysts, useful informahon regarding the reaction mechanism was obtained in organometallic chemistry [103],... 

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