Selective CO methanation

Zyryanova, M., Snytnikov, R, Gulyaev, R. et al. (2013) Performance of Ni/Ce02 catalysts for selective CO methanation in hydrogen-rich gas. Chemical Engineering Journal, 238, 189-197. 

For the practical use of this CO removal reactor, the microchannel reactor should be operated carefully to maintain operating temperature ranges because the reaction temperature is critical for the microchannel reactor performance such as CO conversion, selectivity and methanation as disclosed in the above results. It also seems that the present microchannel reactor is promising as a compact and high efficient CO remover for PEMFC systems. 

On-line GC analysis (Shimadzu GC 14A) was used to measure product selectivity and methane conversion. Details on the analysis procedure used for batch and continuous-flow operation are given elsewhere [12]. The molecular sieve trap was found to trap practically all ethylene, COj and HjO produced a significant, and controllable via the adsorbent mass, percentage of ethane and practically no methane, oxygen or CO, for temperatures 50-70 C. The trap was heated to -300°C in order to release all trapped products into the recirculating gas phase (in the case of batch operation), or in a slow He stream (in the case of continuous flow operation). 

The observed effects of syngas composition on both the CO conversion and the product selectivity, again, are in agreement with data already reported.1112141819 For example, Schulz and Claeys18 observed that when increasing CO partial pressure by decreasing the H2/CO inlet ratio, both the rate of CO consumption (and thus CO conversion) and the selectivity to methane decrease, while the selectivity to the heavier products increases. 

Catalysts were prepared by impregnation using cobalt (ii) nitrate. Co/A1203 was the most the active and selective catalyst. Suppresses the ethanol decomposition and CO methanation reactions... 

The ability of bimetallic systems to enhance various reactions, by increasing the activity, selectivity, or both, has produced a great deal of interest in understanding the different roles and relative importance of ensemble and electronic effects. Deposition of one metal onto the single-crystal face of another provides an advantage by which the electronic and chemical properties of a well-defined bimetallic surface can be correlated with the atomic structure.5 22 23 Besenbacher et al.24 used this method to study steam reforming (the reverse of the CO methanation process) on Ni(l 11) surfaces... 

They reported that the catalyst exhibits very high selectivity to hydrogen and carbon dioxide. The CO methanation and ethanol decomposition are considerably reduced. In addition, coke formation is strongly depressed because of the benefits induced by the use of the basic support, which modify positively the electronic properties of Ni. 

Early in the nineties Ruiz et al. reported enhanced catalyst activities and increased selectivities to alkenes and higher hydrocarbons upon addition of V, Mg, and Ce oxides to Co-based F-T catalysts.These variations were attributed to electronic effects induced by the transition metal oxide. Similar results were obtained by Bessel et al. using a Cr promoter in Co/ZSM-5 catalysts.This group observed that the addition of Cr improved the catalyst activity, and shifted the selectivity from methane to higher, generally more olefinic, hydrocarbons. Based on H2 and CO chemisorption, as well as TPR and TPD results, they suggested that the promotion was caused by an interaction between the transition metal oxide and the cobalt oxide, which inhibits... 

Recently, Martinez et al. reported the use of mesoporous Co/SBA-15 catalysts promoted with Mn for the F-T synthesis. They observed that Mn favored the formation of long-chain n-paraffins (Cio+), while decreasing the selectivity towards methane. The Mn-promoted catalysts, however, turned out to be less active than the unpromoted ones. 

Figure 4.10 Temperature dependencies of reaction product yields and selectivity at methane oxidation molar ratio CH4 25% H202 = 1 1, t= 1.2s (1 methanol 2 CO + C02 3 formaldehyde 4 selectivity by formaldehyde and 5 total methane conversion).

Directly supported clusters of type Os3H(CO)10(O—metal oxide) break down at quite low temperatures to give species which have a high selectivity to methane from CO and H2 (381,400). Similar behavior has been reported for Os3(CO)12 itself (401), but it is difficult to rule out metal as the catalyst. Os3(CO)12 also leads to methanol, methyl and ethyl formate, and acetone by reaction with CO and H 2 (190° C, 180 atm) in glyme solvents (402). The water-gas-shift reaction is catalyzed by Os3(CO)12, using KOH or even sodium sulfide in methanol as the base (403), although ruthenium catalysts are better (404). 

Conventional catalysts are based on iron/iron oxide mixtures (256). Recently, Lunsford and co-workers (257) have shown that reduced RhNaY is also an active catalyst for this reaction at 329 C. A high selectivity to methane was observed and there was evidence of a two-step reaction scheme as shown above. Unfortunately, no comparative data were presented under similar conditions for a conventional catalyst. 

With increasing CO2 concentration in the reaction gas, the selectivity to methane remained nearly unchanged in the case of Fe. For Co, methane became more and more the main product and for the hydrogenation of pure CO2 an almost exclusive conversion to methane was obtained. When starting the experiment on Fe with H2/CO2, yield data were slightly different due to irreversible changes of the catalyst [4]. 

For the long-term durability of PEMFC, the acceptable CO concentration appears to be 10-100 ppm. To meet the requirement, three possible reactions can be considered preferential (or selective) oxidation, methanation, and Pd (or Pd alloy) membrane processes. Preferential oxidation (PrOx) of CO can convert CO to CO2, without excessive hydrogen oxidation (to water), to acceptable levels of CO using multi-stage reactors... 

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