Reverse Water Gas-Shift Reaction

Research is also being conducted in Japan to aromatize propane in presence of carhon dioxide using a Zn-loaded HZSM-5 catalyst/ The effect of CO2 is thought to improve the equilibrium formation of aromatics by the consumption of product hydrogen (from dehydrogenation of propane) through the reverse water gas shift reaction. 

Figure 8.56. Effect of catalyst potential and work function on the rate of CO2 hydrogenation on Pd/YSZ (reverse water-gas shift reaction). pC02=22.5 kPa pH2=73 kPa , T= 546°C , T= 559°C , T= 573°C.59 Open symbols correspond to open-circuit.

C02 hydrogenation on Pd was investigated29 under atmospheric pressure and at temperatures 540°C to 605°C. The CO formation rate (reverse water-gas shift reaction) exhibits purely electrophilic behaviour with a rate increase by up to 600% with increasing sodium coverage (Fig. 9.20). This purely electrophilic behaviour is consistent with low reactant coverages and enhanced electron acceptor C02 adsorption on the Pd surface with increasing sodium coverage (Rule G2). 

When methanol is produced from a mixture of CO2, CO and H2, the reverse water-gas shift reaction complicates the system, since it competes with the methanol synthesis. 

The mole fractions of the reverse water-gas shift reaction are given in Tab. 8.5. Table 8.5. Mole fractions in the water-gas shift reaction. 

Goguet, A., Shekhtman, S.O., Burch, R., Hardacre, C., Meunier, F.C., and Yablonsky, G.S. 2006. Pulse-response TAP studies of the reverse water-gas shift reaction over a Pt/Ce02 catalyst../. Catal. 237 102-10. 

The hydrogenation of C02 to CO is the reverse water-gas shift reaction, which has been reviewed elsewhere [110, 111]. 

Saito and coworkers134 reported on the homogeneous reverse water-gas shift reaction catalyzed by Ru3(CO)i2. Conditions employed were 20 ml of N-methyl-2-pyrrolidone solution 0.2 mmol Ru3(CO)i2 1 mmol bis(triphenylphosphine)immi-nium chloride and C02-H2 1 3 under 80 kg/cm2 at 160 °C. The major products were CO (15.1 mmol), H20 (21.6 mmol), and methanol (0.8 mmol). As no formic acid was detected, and because the authors only detected Ru cluster anion species H3Ru4(CO)i2, H2Ru4(CO)i22, and HRu3(CO)n, they concluded that the mechanism did not involve formic acid as an intermediate. Rather, they proposed that the mechanism proceeds by dehydrogenation of a metal hydride, C02 addition, and electrophilic attack from the proton to yield H20, as outlined in Scheme 48. 

In 1997, the authors304 successfully modeled the kinetics of the reverse water-gas shift reaction over Cu0/Zn0/Al203 catalysts by applying the redox process to... 

Reverse transcriptase, 21 281 Reverse water-gas shift reactions, 5 14-15 Reversible addition-fragmentation chain transfer (RAFT), 7 621, 623 Reversible addition-fragmentation chain transfer (RAFT) polymerization,... 

The effect of CO2 poisoning has been well understood. Gut et al. showed that CO is produced by a Pt anode when fed with CO2/H2 mixtures, indicating that the reverse water-gas shift reaction is active at cell operating temperatures of 70°C. ° As previously observed, this has low performance over and above that expected for dilution. Gut et al.. Ball et al., and de Bruijn et al. showed that PtRu anodes were less poisoned by CO2. Using in situ IR and mass spectroscopy measurements, Smolinka et al. showed that CO2... 

In addition to DME steam reforming, the reverse water gas shift reaction (r-WGSR), Equation 6.20, generally takes place over such metal catalysts during the reforming process ... 

Because of the pure performance of traditional Cu catalysts in the hydrogenation of C02, efforts have been made to find new, more effective catalysts for direct C02 hydrogenation. The problem is to improve selectivity, specifically, to find catalysts that display high selectivity toward methanol formation and, at the same time, show low selectivity in the reverse water-gas shift reaction, that is, in the formation of CO. It appears that copper is the metal of choice for methanol synthesis from C02 provided suitable promoters may be added. Special synthesis methods have also been described for the preparation of traditional three-component Cu catalysts (Cu-ZnO-A1203 and Cu-Zn0-Cr203) to improve catalytic performance for C02 reduction. 

Further carbonylation can occur, adversely affecting the selectivities obtained. Where it is used, the effect of hydrogen pressure on selectivity is also significant, as not all of the reactions require hydrogen. The production of water and the presence of alcohols lead to esterification-hydrolysis equilibria and water can affect the hydrogen pressure via the reversible water-gas shift reaction (equation 70). 

It is important to note that the formation of CO from the reverse water gas shift reaction (C02 + H2 = CO + H20) has been detected at low temperature with the commonly used Pt group metals [49, 50]. Under the operational conditions used to carry out these reactions, the CO coverage is relatively low and concentrated on specific sites such as the step and kink. This has been considered a possible explanation for the fact the catalytic performances are generally unaffected unless demanding reactions that require the catalytic action of specific sites on the catalyst surface are considered, as is the case with enantioselective hydrogenation reactions [49]. 

This was ascribed to the short residence times applied (50-100 ms). Under these conditions, assuming the reaction mechanism proposed by Takahashi et al. shown above, carbon monoxide could only be formed by the reverse water-gas shift reaction, which is known to be slower than the reforming reaction. This is the case especially for catalyst systems with low activity towards water-gas shift. Holladay et al. [19] compared the performance of the same proprietary catalyst with that of a Cu/Zn catalyst which produced a higher carbon monoxide concentration of 3.1% in the reformate. 

The experiments were carried out at residence times between 200 and 100 ms and total flow rates between 500 and 900 Ncm3 min-1 using five coated plates. A 15% decrease in conversion was found when the flow rate was increased from 500 to 800 Ncm3 min-1, leading to a 36% relative decrease in the hydrogen content of the product. This was attributed to the slow kinetics of the reverse water-gas shift reaction. Increasing the temperature from 200 to 275 °C increased the conversion from 37 to 65%. At temperatures exceeding 250 °C, carbon monoxide formation started [26]. 

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] ... 

Ring-opening metathesis polymerization Residence time distribution Reverse water-gas shift reaction... 

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