Water-Gas-Shift Reaction

The significance of the WGS reaction (see Section 1.6.4) in homogeneous systems is twofold. First, it operates in both Monsanto and Cativa processes as a side reaction. Second, studies employing other metals as well as rhodium have provided useful information about the mechanism of heterogeneous WGS reaction. 

Reaction of 4.9 with HI produces and 4.46. Nucleophilic attack by water onto the coordinated CO of 4.46 produces 4.47. Decarboxylation of 4.47 gives 4.48, which picks up a CO and rearranges to 4.9 to complete the catalytic cycle. 

The WGS reaction has a useful function in stabilizing the rhodium complex in the absence of CHjI. In a situation where no CH3I is available, the acetic acid-forming catalytic cycle ceases to exist. However, since the WGS cycle continues to be operational, rhodium remains in solution and does not precipitate out. 

The removal of carbon dioxide from the product gases of methane steam reforming 

The use of an adsorptive reactor operation with reactant enrichment, in which adsorption is used to maintain a high level of excess steam within the catalytic reaction zone, has attracted comparatively little attention. 

The water-gas shift reaction (Eq. 11.58) is an industrially important equilibrium that controls the composition of hydrogen or CO in water-gas, syngas, or reformed 

This reaction is slightly exothermic, and commercial plants operate with heterogeneous catalysts at elevated temperatures (200-450 °C) [108]. Quite recently, heterogeneous catalysts with ruthenium have been intensively studied to remove CO from the reformed gases for fuel cells [109]. 

On the other hand, several homogeneous transition metal complexes such as Fe(CO)5, FeH(CO)4, Ru3(CO)i2, [Ru(bipy)2(CO)a]-, FeH2Ru3(CO)i3, K[Ru(H-EDTA)-(CO)[, [Rh(CO)2l2], and Pt[P(i-Pr)3]3, have been shown to catalyze the reaction at low temperature [108a[. Among them, ruthenium complexes are very efficient catalysts, and this reaction is used to reduce organic compounds without using molecular hydrogen. 

Alkyl (Eq. 11.59) [99] and aryl (Eq. 11.60) [99, 110, 111[ nitro compounds can be reduced to the corresponding amines in high yields under the water-gas shift reaction. 

The hydroformylation of alkenes such as 1-pentene can be achieved under water-gas shift reaction conditions with ruthenium catalysts. Although the catalytic activity is not satisfactory, the n/i ratio of the produced alcohols is very high [112], 

The water-gas shift reaction (wgsr) may be represented by the equation  

The experimental data show that, independent of the acidity of the solution, 

The reactivity of the complex [(OC)4FeCOOH] in the gaseous phase confirms the earlier postulated mechanism of its decomposition. According to this mechanism, the first dissociation of a proton occurs [scheme (13.244)]. Therefore, the presence of bases, which promote dissociation of the proton, should facilitate the conversion of carbon monoxide. 

The water-gas shift reaction is a key process for industrial production of hydrogen (CO + H2O - CO2 -H H2). Because of the rigorous reaction conditions in industry. 

Subsequently, the catalytic activity of different transition-metal complexes of Ru, 

Recently, the relationship between the IL film distribution and catalyst performance in water-gas shift reaction was studied by solid-state NMR [107]. The model catalyst was silica gel-supported [Ru(CO)3Cl2]2/[EMIm][NTf2] with variable loadings in the range of 0-40 vol%. The results suggested that, during the deposition of the IL phase, the IL first filled the micropores of silica gel prior to forming a film. 

The NMR characterization showed that complete surface coverage was obtained with above 10vol% IL loading. At this point, the surface silanol signals nearly disappeared. The peaks of the bulk IL became dominant if 20-40vol% IL was 

A famous example is the water-gas shift reaction [14]. Efficient catalysts are late transition metals such as iron, e.g. Fe(CO)5 [14]. 

If all the CH4 is converted and the water-gas shift (WGS) reaction concurring within the reformer is neglected, the product will contain 25% CO, which is too high for PEMFCs. Therefore, CO is converted to CO2 in the following WGS reactions according to Reaction 5.6. 

This is an exothermic reaction, and it releases -41.2 kJ moH of heat under standard conditions therefore, it should be able to proceed without the addition of external heat. In order to shift this exothermic reaction to the right side to minimize the CO concentration, the WGS temperature should be kept as low as possible. But at lower temperatures, the reaction rate will be slower. So the optimal temperature is a compromise between the two factors. For 

The overall enthalpy change is 164.7 kJ mol under the standard condition. It seems that these energy losses associated with converting 1 mole of CH4 were wasted. In reality, the energy is not lost but stored in the 4 moles of H2 produced. Under standard conditions, the enthalpy in 4 moles of H2 is -967.3 kJ, the enthalpy of burning 1 mole of CH4 is -802.6 kJ, and the difference is just -164.7 kJ. So, no energy is created or destroyed. 

After showing the high activity of gold for the activation of carbon monoxide oxidation, the water gas shift reaction is another of the most relevant reactions to be studied. 

The catalytic activity of gold for this reaction at low temperature was reported by Andreeva et al., who used Au/ Fe203 catalysts [260, 261]. Venugopal et al. also studied this system and showed that the combination Au-Ru/Fe203 or even gold supported on hydroxyapatite were more useful for the transformation [262]. 

The next improvement was reported by Hua et al. by means of the addition of metal oxides to Au/Fe203 [263]. 

Traces of water can enhance the rate of CO oxidation at low temperature but if the water concentration is too high, much higher temperatures are required in order to avoid the reduction of cationic gold to metallic gold [264]. Haruta et al. [265] and Idakiev et al. [266] studied Ti02 and mesoporous 2 and both demonstrated their 

Subsequent studies focused on Au/Ce02 catalysts [267]. One study showed that the addition of C ions increased the catalytic activity of Au/Ce02 by removing most of the gold from the catalyst, and it was proposed that the resulting cationic gold was the 

Several metal carbonyl dusters (e.g. [Ru3(CO)i2] and [Ir4(CO),2]) have been investigated in solution as possible catalysts for the water gas shift reaction. [120] A plausible common mechanism has been proposed Nudeophilic attack by H2O or OH on an electrophilic metal center of the duster to form an unstable carbo-hydroxy metal complex, which is then decarboxylated to give a metal hydride from which H2 is eliminated. Hiis reaction is one of the relatively few for which there is good evidence for catalysis by dusters themselves, rather than fragments or metal aggregates formed from them. 

Some zeolites containing metal carbonyl dusters also are catalytically active for the shift reaction. Iwamoto et al. [SO] investigated the catalytic activity of zeolite NaY incorporating [HFe3(CO)ii] at 60-180°C and 1 bar. The catalytic activity was comparable to that reported for the solution phase reaction which uses [Fe(CO)s] as the catalyst precursor at 180 °C and 40 bar. Infrared and ultraviolet spectra indicated that [HFe3(CO)ii] was stable under the reaction conditions. A plausible mechanism was proposed to exjdain the observations (Eqs. 4.9-4.12). 

Kinetic and spectroscopic results indicated that the reaction between [HFe3(CO)ii] and H2O was rate determining. 

Wang et al. [121] reported that the NaY zeolite containing Pt carbonyl dusters of the family [Pt3(CO)6] (n = 3, 4) is catalytically active for the shift reaction at 27-150°C and that the photocatalytic reaction was about 38 times faster than the catalytic reaction at 25 °C. Since the platinum carbonyls present in the used catalyst were identified only by infrared spectroscopy, there remains doubt about what they were as well as what the catalytically and photocatalytically active spedes might have been. 

In summary, it is difficult to determine the catalytically active spedes in any supported catalyst, and there are still no well documented examples of catalysis by metal carbonyl clusters themselves in zeolites. There is, however, substantial indirect evidence that metal carbonyl clusters in zeolite cages may be either the catalysts or the catalyst precursors for a number of reactions involving CO. In some cases, these dusters are the only detectable organometallic or metallic spedes, and they are stable under the conditions of the catalytic reactions. Some of the catalysts retain the colors and the infrared spectra of the metal carbonyl dusters even after weeks of catalytic operation. In the few instances when EXAFS data were available, the presence of metal carbonyl clusters within the zeolites was indicated however, evidence for other spedes that are plaudble catalyst precursors was also obtained. 

Two basically different reactor technologies are currently in operation low temperature and high temperature. The former operates at -220 °C and 25-45 bar, employing either a multitubular, fixed bed (i.e. trickle bed) reactor or a slurry bubble column reactor with the catalyst suspended in the liquid hydrocarbon wax product. 

In contrast, the high-temperature reactor operates at -350 °C and 25 bar, using a gas-fluidized bed reactor of either the circulating or the normal type. The high-tem-perature process is mainly used to produce gasoline and chemicals, such as alpha olefins, and the low temperature process to produce waxes. 

Proven, industrially used catalysts are mostly based on either iron or cobalt. Ruthenium is an active F-T catalyst but is too expensive for industrial use. Both Fe and Co are prepared by several techniques including both precipitation and impregnation of (e.g. alumina or silica) supports. The more noble Ni catalyst produces nearly exclusively methane and is used for the removal of trace of CO in H2. 

The increase in industrial and academic research on Fischer-Tropsch catalysis following the Second World War and the oil crises of the 1970s is set to continue as the process is expected to become increasingly important 

The low temperature water-gas shift reaction is well described by a micro-kinetic model [C.V. Ovesen, B.S. Clausen, B.S. Hammershoj, G. Sreffensen, T. Askgaard, I. Chorkendorffi J.K. Norskov, P.B. Rasmussen, P. Stoltze and P.J. Taylor,/. Catal. 158 (1996) 170] and follows to a large extent the scheme in Eqs. (23-31). The analysis revealed that formate may actually be present in nonvanishing amounts at high pressure (Fig. 8.18). 

Synthesis ofPt and Au Nanoparticle Arrays in Mesoporous Silica Films and their Electric/Magnetic Properties in Terms of the Quantum-Size Effect 

We have reviewed the state of the art of the synthesis and applications of metal/ alloy nanowires and nanoparticles by surface-mediated fabrication using different mesoporous silica templates. New nanoscale materials are appearing one after another by the use of many kinds of mesoporous materials as templates, and we are facing a rapid advance of this research field. So far, many studies have focused 

We have overviewed some strategies for the surface-mediated fabrication of metal and alloy nanoscale wires and particles in mesoporous space, and their structural characterization and catalytic performances. Extension of the present approaches for metal/alloy nanowires may lead to the realization of the prospechve tailored design of super active, selective and stable catalysts applicable in industrial processes. The organometallic clusters and nanowires offer exciting and prospechve opportunities for the creahon of new catalysts for industry. Various metal/ alloy nanowires and nanoparhcles in the anisotropic arrangement in porous supports would help in understanding the unexpected electronic and optic properties due to the quantum effect, which are relevant to the rational design of advanced electronic and optic devices. 

We thank Dr. Y. Sakamoto, Mr. N. Higasimoto, Mr. T. Higuchi, Mr. J. Kimura, Mr. T. Ohtake, Mr. T. Oshio and Dr. N. Shimomura at Hokkaido University, and Dr. S. Inagaki, Dr. Y. Fukusima and Mr. N. Sugjmoto at the Tokyota Central Laboratory for experimental assistance and useful discussion. This work was supported by a Grant-in-Aid for Scienhfic Research from the Ministry of Educahon, Science, Sports and Culture, Japan. 

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The Fischer-Tropsch reaction is essentially that of Eq. XVIII-54 and is of great importance partly by itself and also as part of a coupled set of processes whereby steam or oxygen plus coal or coke is transformed into methane, olefins, alcohols, and gasolines. The first step is to produce a mixture of CO and H2 (called water-gas or synthesis gas ) by the high-temperature treatment of coal or coke with steam. The water-gas shift reaction CO + H2O = CO2 + H2 is then used to adjust the CO/H2 ratio for the feed to the Fischer-Tropsch or synthesis reactor. This last process was disclosed in 1913 and was extensively developed around 1925 by Fischer and Tropsch [268]. 

Again with platinized Ti02, ultraviolet irradiation can lead to oxidation of aqueous CN [323] and to the water-gas shift reaction, CO + H2O = H2 + CO2 [324]. Some mechanistic aspects of the photooxidation of water (to O2) at the Ti02-aqueous interface are discussed by Bocarsly et al. [325]. 

Methanol (qv) is one of the 10 largest volume organic chemicals produced in the wodd, with over 18 x 10 t of production in 1990. The reactions for the synthesis of methanol from CO, CO2, and H2 are shown below. The water gas shift reaction also is important in methanol synthesis. 

Study of the mechanism of this complex reduction-Hquefaction suggests that part of the mechanism involves formate production from carbonate, dehydration of the vicinal hydroxyl groups in the ceUulosic feed to carbonyl compounds via enols, reduction of the carbonyl group to an alcohol by formate and water, and regeneration of formate (46). In view of the complex nature of the reactants and products, it is likely that a complete understanding of all of the chemical reactions that occur will not be developed. However, the Hquefaction mechanism probably involves catalytic hydrogenation because carbon monoxide would be expected to form at least some hydrogen by the water-gas shift reaction. 

The mixture of carbon monoxide and hydrogen is enriched with hydrogen from the water gas catalytic (Bosch) process, ie, water gas shift reaction, and passed over a cobalt—thoria catalyst to form straight-chain, ie, linear, paraffins, olefins, and alcohols in what is known as the Fisher-Tropsch synthesis. 

Prior to methanation, the gas product from the gasifier must be thoroughly purified, especially from sulfur compounds the precursors of which are widespread throughout coal (23) (see Sulfurremoval and recovery). Moreover, the composition of the gas must be adjusted, if required, to contain three parts hydrogen to one part carbon monoxide to fit the stoichiometry of methane production. This is accompHshed by appHcation of a catalytic water gas shift reaction. 

Synthesis Gas Chemicals. Hydrocarbons are used to generate synthesis gas, a mixture of carbon monoxide and hydrogen, for conversion to other chemicals. The primary chemical made from synthesis gas is methanol, though acetic acid and acetic anhydride are also made by this route. Carbon monoxide (qv) is produced by partial oxidation of hydrocarbons or by the catalytic steam reforming of natural gas. About 96% of synthesis gas is made by steam reforming, followed by the water gas shift reaction to give the desired H2 /CO ratio. 

Reactions of Synthesis Gas. The main hydrogen manufacturing processes produce synthesis gas, a mixture of H2 and CO. Synthesis gas can have a variety of H2-to-CO ratios, and the water gas shift reaction is used to reduce the CO level and produce additional hydrogen, or to adjust the H2 to-CO ratio to one more beneficial to subsequent processing (69) ... 

In the next step, the CO is converted to CO2 and hydrogen by the water gas shift reaction step ... 

This reaction is first conducted on a chromium-promoted iron oxide catalyst in the high temperature shift (HTS) reactor at about 370°C at the inlet. This catalyst is usually in the form of 6 x 6-mm or 9.5 x 9.5-mm tablets, SV about 4000 h . Converted gases are cooled outside of the HTS by producing steam or heating boiler feed water and are sent to the low temperature shift (LTS) converter at about 200—215°C to complete the water gas shift reaction. The LTS catalyst is a copper—zinc oxide catalyst supported on alumina. CO content of the effluent gas is usually 0.1—0.25% on a dry gas basis and has a 14°C approach to equihbrium, ie, an equihbrium temperature 14°C higher than actual, and SV about 4000 h . Operating at as low a temperature as possible is advantageous because of the more favorable equihbrium constants. The product gas from this section contains about 77% H2, 18% CO2, 0.30% CO, and 4.7% CH. ... 

This reaction is commonly known as the CO or water gas shift reaction. The conversion of CO by this reaction is slightly exothermic and favored by lower temperatures. The lower practical operating temperature range for this reaction is between 180 and 200°C (8). 

Subtracting reaction 2 from reaction 1 gives the familiar water gas shift reaction (eq. 3). 

These reactions show that the synthesis gas stoichiometry is dependent on both the nature of the feedstock as well as the generation process. Reactions 4 and 5, together with the water gas shift reaction 3, serve to independently determine the equiUbrium composition of the synthesis gas. 

Synthesis gas, a mixture of CO and o known as syngas, is produced for the oxo process by partial oxidation (eq. 2) or steam reforming (eq. 3) of a carbonaceous feedstock, typically methane or naphtha. The ratio of CO to may be adjusted by cofeeding carbon dioxide (qv), CO2, as illustrated in equation 4, the water gas shift reaction. 

The overall process for producing a 1 1 CO to ratio by partial methane oxidation and the water gas shift reaction is represented by equation 5. 

Synthesis gas preparation consists of three steps ( /) feedstock conversion, (2) carbon monoxide conversion, and (2) gas purification. Table 4 gives the main processes for each of the feedstocks (qv) used. In each case, except for water electrolysis, concommitant to the reactions shown, the water-gas shift reaction occurs. 

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

Tubular Fixed-Bed Reactors. Bundles of downflow reactor tubes filled with catalyst and surrounded by heat-transfer media are tubular fixed-bed reactors. Such reactors are used most notably in steam reforming and phthaUc anhydride manufacture. Steam reforming is the reaction of light hydrocarbons, preferably natural gas or naphthas, with steam over a nickel-supported catalyst to form synthesis gas, which is primarily and CO with some CO2 and CH. Additional conversion to the primary products can be obtained by iron oxide-catalyzed water gas shift reactions, but these are carried out ia large-diameter, fixed-bed reactors rather than ia small-diameter tubes (65). The physical arrangement of a multitubular steam reformer ia a box-shaped furnace has been described (1). 

This is the reverse of the water-gas shift reaction in the production of hydrogen and ammonia (qv). Carbon dioxide may also be reduced catalyticaHy with various hydrocarbons and with carbon itself at elevated temperatures. The latter reaction occurs in almost all cases of combustion of carbonaceous fuels and is generally employed as a method of producing carbon monoxide. 

Conversion to Hydrogen (Water Gas Shift Reaction). Carbon monoxide reacts with water over a catalyst to produce hydrogen and carbon monoxide (25). This reaction is used to prepare high purity hydrogen or synthesis gas with a higher hydrogen-to-carbon monoxide ratio than the feed (eq. 3). 

In addition to platinum and related metals, the principal active component ia the multiflmctioaal systems is cerium oxide. Each catalytic coaverter coataias 50—100 g of finely divided ceria dispersed within the washcoat. Elucidatioa of the detailed behavior of cerium is difficult and compHcated by the presence of other additives, eg, lanthanum oxide, that perform related functions. Ceria acts as a stabilizer for the high surface area alumina, as a promoter of the water gas shift reaction, as an oxygen storage component, and as an enhancer of the NO reduction capability of rhodium. 

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). 

Hydrogen and carbon monoxide are produced by the gasification reaction, and they react with each other and with carbon. The reaction of hydrogen with carbon as shown in reaction (27-15) is exothermic and can contribute heat energy. Similarly, the methanation reaction (27-19) can contribute heat energy to the gasification. These equations are interrelated by the water-gas-shift reaction (27-18), the equilibrium of which controls the extent of reactions (27-16) and (27-17). 

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