Reverse Water Gas Shift Reaction RWGS

As already stated, Syngas is widely used to produce methanol or other long-chain hydrocarbons. CO, which is considered to be the initial step of CO2 hydrogenation on metal catalysts [200], can be formed in the RWGS reaction. Such a reaction is crucial for the production of CO, which is an important precursor for various other chemical productions [201]. 

This endothermic reaction, which occurs at high temperatures ( 973 K), is shown in (7.101). 

However, several undesired parallel and side reactions tend to occur as well  

All such reactions have been encountered in the processes discussed above and occur simultaneously any time H2O, CO, CO2, H2, C are present in the reaction medium. 

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Another problem that occurrs in PrOx reactors at low load levels is the reproduction of carbon monoxide at the catalyst by the reverse water-gas shift reaction (RWGS) in an oxygen-deficient atmosphere [85] ... 

Formation of carbon monoxide over the catalyst by the reverse water-gas shift reaction (RWGS) in an oxygen-deficient atmosphere is frequently observed especially under conditions of partial load, because most catalysts for preferential oxidation of carbon monoxide have some activity for WGS and its reverse reaction. Therefore oversizing the reactor bears the danger of impaired conversion and the same applies for partial load of the reactor unfortunately. Because the concentration of carbon monoxide that is tolerated by low-temperature fuel cells is usually in the range below 100 ppm or less, even low catalytic activity for reverse shift becomes an issue. 

Another problem occurring in preferential oxidation reactors at low concentration of carbon monoxide is the re-formation of carbon monoxide over the catalyst by the Reverse Water-Gas Shift Reaction (RWGS) in an oxygen defident atmosphere ... 

The kinetics of the preferential oxidation reaction of carbon monoxide (PrOx), for the hydrogen oxidation reaction (H2OX) and for the reverse water-gas shift reaction (RWGS) were provided, which had been determined in the relevant parameter space [398] ... 

Carbon dioxide (COj) Reformed fuels, biogas 1 % levels Production of CO from the COj on the Pt surface due to the well-known reverse water gas shift reaction (RWGS) or from electro-reduction As above... 

The carbon dioxide reforming of methane has attracted academic and industrial interest, since it produces synthesis gas with a H2/CO ratio closed to 1, which is more suitable for methanol and Fischer-Tropsch synthesis compared to the steam reforming of methane providing synthesis gas with a H2/CO ratio = 3. This reaction is of particular importance for the valorization of C02-rich fossil natural gas but also for the transformation of biogas extensively produced by various anaerobic waste treatments [1]. The production of synthesis gas from CH4 and CO2 is highly endothermic (Eq. (22.1)), the reaction equilibrium is influenced by the simultaneous occurrence of the reverse water-gas shift reaction (RWGS) (Eq. (22.2)). 

Methanol synthesis from C02 (Equation [1]) and CO (Equation [2]) is mildly exothermic and results in volumetric contraction. Methanol steam reforming (MSR) refers to the inverse of reaction (1), and the inverse of reaction (2) is conventionally referred to as methanol decomposition - an undesired side reaction to MSR. The slightly endothermic reverse water-gas shift (rWGS) reaction (Equation [3]) occurs as a side reaction to methanol synthesis and MSR. According to Le Chatelier s principle, high pressures and low temperatures would favor methanol synthesis, whereas the opposite set of conditions would favor MSR and methanol decomposition. It should be noted that any two of the three reactions are linearly independent and therefore sufficient in describing the compositions of equilibrated mixtures. 

WGS), reverse water-gas shift (RWGS), CO disproportionation (Boudouard reaction), and methane decomposition reactions as described in Equations 22-2.5 ... 

We have previously shown that the kinetics of the reverse water gas shift (RWGS) reaction can be modelled on the basis of a simple redox mechanism (1). This result contradicted an earlier assertion deriving from a kinetic analysis of the reaction which claimed that the reaction proceeded through a formate intermediate (2). The individual elementary reactions of the redox mechanism, which in combination constitute the overall reaction, are listed in table 1, together... 

CO2 is generally considered as a diluent to H2 in the reformate with PtRu as the anode catalyst. But if the fuel cell condition leads to the reversed water gas shift (RWGS) to occur (please refer to Section 5.2.1), trace amounts of CO may be able to form according to Reactions 4.11 and 4.12 ... 

Reaction 5.9 shows no CO formation in the reforming process. However, some CO will form through the reversed water-gas shift (RWGS) reaction according to Reaction 5.10. 

Another alternative to the in situ generation of syngas is the reaction of carbon dioxide with hydrogen (reversed water gas shift = RWGS). A particular activity in this respect displays homogeneous ruthenium catalysts (see also Section 1.6). 

Scheme 3.11 Mechanism of the Ru-catalyzed reversed water gas shift (RWGS) reaction.

The second way to obtain ammonia during the NSR regeneration is the isocyanate route [23]. This intermediate reaction is observed when a carbon source is present in the reaction mixture, especially CO [11]. However, CO can also be produced in situ, for instance by the reverse water gas shift (RWGS) reaction between H2 and CO2, the later being always present in large amounts in a real exhaust gas (see also the influence of the WGS equilibrium Sect. 19.2.3). 

The Pt-CO bond created by the reduction of carbon dioxide affects hydrogen electrooxidation in the same manner as the Pt-CO formed via CO2 reduction. Although, the reduction of carbon dioxide polarizes hydrogen electrooxidation, more studies are tailored to understanding the reverse water-gas shift (RWGS) reaction and its effects. The WGSR is ... 

Two major roles are proposed for CO2 in the CO2-ODH reaction, i.e., (1) CO2 oxidizes the reduced catalytic sites during the dehydrogenation process, hence participating in a redox process and (2) CO2 removes hydrogen formed in the dehydrogenation process via the reverse water gas shift (RWGS) reaction, thus shifting the reaction equilibrium towards product formation. 

Pekridis G, Kalimeri K, Kaklidis N, Vakouftsi E, Iliopoulou EF, Athanasiou C, Mamellos GE (2007) Study of the reverse water gas shift (RWGS) reaction over Pt in a solid oxide fuel cell (SOFC) operating under open and closed-circuit conditions. Catal Today 127 337-346... 

CO can be converted into either hydrocarbon products and water (via FTS) or C02 and Fl2 via the water-gas shift (WGS) reaction. The reversible WGS reaction accompanies FTS over the iron-based catalyst only at high temperature conditions. The individual rates of FTS (rFTS) and the WGS reaction (rWGS) can be calculated from experimental results as rWGS = r(,and rFTS = rco-rc02, where rCo2 is the rate of C02 formation and rco is the rate of CO conversion. 

As previously mentioned, the presence of H2O and CO2 in the gas mixture directly impacts ammonia formation mechanism. According to reaction (19.14), the Water Gas Shift (WGS) and the reverse reaction (Reverse WGS, RWGS) can... 

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