Water gas shift reaction catalysis

Newsome, D.S. The water-gas shift reaction. Catalysis Reviews—Science and Engineering, 1980, 21, 275. 

Bickford, E.S., Velu, S., and Song, C. Nano-structured Ce02 supported Cu-Pd bimetallic catalysts for the oxygen-assisted water-gas-shift reaction. Catalysis Today, 2005, 99, 347. 

Zhao, S. and Gorte, R.J. The activity of Fe-Pd alloys for the water-gas shift reaction. Catalysis Letters, 2004, 92, 75. 

Fig. 2.70. Details of the addition of carbon monoxide to [HRu3(CO)j J in water-gas shift reaction catalysis [Payne MW, Lenssing DC, Shore SG, (1987), J. Am. Chem. Soc. 109 618]...

Oxidation of carbon monoxide by metal ions and homogeneous catalysis of the water gas shift reaction and related processes. J. Halpern, Comments Inorg. Chem., 1981,1, 3-15 (42). 

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. 

Grenoble, D.C., Estadt, M.M., and Ollis, D.F. 1981. The chemistry and catalysis of the water gas shift reaction. 1. The kinetics over supported metal catalysts. J. Catal. 67 90-102. 

The attack on coordinated carbon monoxide by nucleophiles was first extensively developed in synthetic organometallic chemistry by E. 0. Fischer and his students (6) as discussed by others in this volume, this reaction provides one route to the reduction of coordinated CO and to catalysis of the water gas shift reaction. Those carbonyl groups which are susceptible to attack by nucleophiles are electron deficient, as judged by their high CO stretcing frequencies (7). 

Homogeneous Catalysis of the Water Gas Shift Reaction Using Simple Mononuclear Carbonyls... 

In situ dynamic surface structural changes of catalyst particles in response to variations in gas environments were examined by ETEM by Gai et al. (78,97). In studies of copper catalysts on alumina, which are of interest for the water gas shift reaction, bulk diffusion of metal particles through the support in oxygen atmospheres was shown (78). The discovery of this new catalyst diffusion process required a radical revision of the understanding of regeneration processes in catalysis. 

Skeletal catalysts are usually employed in slurry-phase reactors or fixed-bed reactors. Hydrogenation of cottonseed oil, oxidative dehydrogenation of alcohols, and several other reactions are performed in sluny phase, where the catalysts are charged into the liquid and optionally stirred (often by action of the gases involved) to achieve intimate mixing. Fixed-bed designs suit methanol synthesis from syngas and catalysis of the water gas shift reaction, and are usually preferred because they obviate the need to separate product from catalyst and are simple in terms of a continuous process. 

R. Stobbe, and P. D. Cobden, Applied Catalysis A General, 306, 17 (2006). Discovery of New Catalytic Materials for the Water-Gas Shift Reaction by High Throughput Experimentation. 

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... 

Figure 5 Survey of possible synergistic promotion effects occurring in Co-based Fischer-Tropsch catalysis (A) water-gas shift reaction, (B) hydrogenationjdehydrogenation reactions and (C) H2S adsorption...

The action nl catalysis cun he illustrated hy an cxample-the water gas shift reaction catalyzed by iron and chromium oxides. 

It is currently used in ammonia synthesis as a means of removing CO (89), and its employment as a hydrogen source is expected to increase in the future (67). We shall discuss catalysis of the water gas shift reaction more extensively later in this section. 

Two homogeneous metal complex water-gas shift catalyst systems have recently appeared 98, 99). The more active of these comes from our Rochester laboratory (99, 99a). It is composed of rhodium carbonyl iodide under CO in an acetic acid solution of hydriodic acid and water. The catalyst system is active at less than 95°C and less than 1 atm CO pressure. Catalysis of the water-gas shift reaction has been unequivocally established by monitoring the CO reactant and the H2 and C02 products by gas chromatography The amount of CO consumed matches closely with the amounts of H2 and C02 product evolved throughout the reaction (99). Mass spectrometry confirms the identity of the C02 and H2 products. The reaction conditions have not yet been optimized, but efficiencies of 9 cycles/day have been recorded at 90°C under 0.5 atm of CO. Appropriate control experiments have been carried out, and have established the necessity of both strong acid and iodide. In addition, a reaction carried out with labeled 13CO yielded the same amount of label in the C02 product, ruling out any possible contribution of acetic acid decomposition to C02 production (99). 

A group at Monsanto has also studied the catalysis of the water-gas shift reaction by rhodium carbonyl iodide (103b). The main difference between their work and our own is the choice of reaction conditions. Their study was conducted at 185°C under 200-400 psig carbon monoxide. Despite this drastic difference in reaction conditions, the studies are surprisingly consistent. In particular, the Monsanto group also finds evidence for two rate-limiting reactions. They did not find this by temperature variation, but instead, consistent with our own work, find that at low acid and iodide... 

Ford and co-workers have also recently developed a homogeneous catalyst system for the water-gas shift reaction (95). Their system consists of ruthenium carbonyl, Ru3(CO)12, in an ethoxyethanol solvent containing KOH and H20 under a CD atmosphere. Experiments have been conducted from 100-120°C. The identity of the H2 and CD2 products has been confirmed, and catalysis by both metal complex and base has been verified since the total amount of H2 and COz produced exceeds the initial amounts of both ruthenium carbonyl and KOH. The authors point out that catalysis by base in this system depends on the instability of KHC03 in ethoxyethanol solution under the reaction conditions (95). Normally the hydroxide is consumed stoichiometrically to produce carbonate, and this represents a major reason why a water-gas shift catalyst system has not been developed previously under basic conditions. As has been noted above, coordinated carbonyl does not have to be greatly activated in order for it to undergo attack by the strongly nucleophilic hydroxide ion. Because of the instability of KHC03... 

This section has dealt with the oxidation of CO to C02, especially as it enters into the water-gas shift reaction (26a). A reasonable view of the homogeneous catalysis of this reaction, whether in basic or acidic media, is emerging in which CO formation proceeds from nucleophilic attack of water or OH" on an activated carbonyl followed by either reductive decarboxylation or hetero-atom -elimination yielding, respectively, a reduced metal or a metal hydride species. 

Scheme 3 forms a catalytic cycle for the water-gas shift reaction (63) employing [Rh2(/i-CO)(CO)2(dpm)2] in the presence of acid as a catalyst (62). It should be reiterated that alternative cycles might be written which do not involve formate intermediates. For example, a possible mechanism for catalysis of the water-gas shift reaction involving the binuclear metal species, [Pt2H2( -HXdpm)2]+, is outlined below (Scheme 4) (64). We have critically discussed the role of formate versus carboxylic acid intermediates in homogeneous catalysis of the water-gas shift reaction by mononuclear metal catalysts elsewhere (34).

Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenyl-phosphino)methane ligands, has been prepared and structurally characterized. Included are diplati-num(I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered. 

Since several cationic carbonyls react with water to give carbon dioxide and a metal hydride (18), it is predicted that Complex XV or XXII should be catalysts for the water gas shift reaction. Preliminary results indicate that this is correct and catalysis occurs slowly at 100°C in aqueous methanol, with the slow step in the catalytic cycle being the reaction of Complex XXII with water to regenerate Complex XV with the evolution of C02. 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

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