CH4 conversion

Thus indeed CH4 oxidation in a SOFC with a Ni/YSZ anode results into partial oxidation and the production of synthesis gas, instead of generation of C02 and H20 as originally believed. The latter happens only at near-complete CH4 conversion. However the partial oxidation overall reaction (3.12) is not the result of a partial oxidation electrocatalyst but rather the result of the catalytic reactions (3.9) to (3.11) coupled with the electrocatalytic reaction (3.8). From a thermodynamic viewpoint the partial oxidation reaction (3.12) is at least as attractive as complete oxidation to C02 and H20. 

It is known32 reported that the solid electrolyte itself, i.e. Y203-doped-Zr02, is a reasonably selective catalyst for CH4 conversion to C2 hydrocarbons, i.e., ethane and ethylene32 and this should be taken into account in studies employing stabilized Zr02 cells. At the same time it was found54 that the use of Ag catalyst films leads to C2 selectivities above 0.6 for low methane conversions. 

As shown on Fig. 8.49 one can influence dramatically both the total CH4 conversion as well as product selectivity by varying the Ag catalyst potential. Thus under open-circuit conditions (Uwr=U r ) the CH4 conversion is near 0.02 with a C2 selectivity (methane molecules reacting to form C2H4 and C2H6 per total reacting CH4 molecules) near 0.5. Increasing Uwr increases the methane conversion to 0.3 and decreases the selectivity to 0.23, while decreasing Uwr decreases the conversion to 0.01 and increases the... 

Figure 8,49. Effect of Ag/YSZ catalyst potential on CH4 conversion and on selectivity to C2 hydrocarbons. T=800°C, pO2=0.25 kPa, pCH4=lO.I3 kPa, U R=-0.45 V.2,54 Open symbols correspond to open-circuit. Reprinted from ref. 2 with permission from Elsevier Science.

The effect of CH4 conversion on the total C2, C2H4, C2He hydrocarbon selectivity and yield is shown in detail on Figure 4 for the case of I=5mA. 

Figure 6a shows the effect of F02 on the C2 selectivity and yield. The C2 yield is up to 53%. Figure 6b refers to the same experiments and shows the corresponding elBfect of CH4 conversion on the selectivity and yield of ethylene and ethane. The ethylene yield is up to 50% (65% ethylene selectivity at 76% methane conversion). To the best of our knowledge this is the maximum ethylene yield obtained for the OCM reaction under continuous-flow steady-state conditions. 

Effects of Li content on the catalytic behaviors and structures of LiNiLaOx catalysts The dpendence of performance of LiNiLaOx catalysts on Li content at 1073K was shown in Fig.l. When D/Ni mole ratio was 0, the relatively acidic LaNiOx had the highest CH4 conversion(92.0%), but no C2 yielded. The products were CO, CO2 and H2, and CO selectivity was 98.3%. It is not an OCM catalyst but a good catalyst for partial oxidation of methane(POM). With Li content and the baric property of LiNiLaOx catalysts increasing, CH4 conversion and CO selectivity decreased, but there was still no C2 formed imtil Li/Ni mole ratio was 0.4. There was a tumpoint of catalytic behavior between 0.2 and 0.4 (Li/Ni mole... 

Supported palladium oxide is the most effective catalyst used in total methane oxidation and in catalytic oxidation of VOCs [1-5]. However, the activity of the conventional catalysts is not sufficient [5-6]. Recently, the Pd-zeolite catalysts have attracted considerable attention due to their high and stable CH4 conversion efficiency [4-8]. In this work, the effect of the preparation method, the nature of the charge-balancing cations, the palladium loading and the pre-treatment gas nature on the texture, structure and catalytic activity of the Pd-ZSM-5 solids are investigated. 

Consequently, in the early 1990s, interest in the direct processes decreased markedly, and the emphasis in research on CH4 conversion returned to the indirect processes giving synthesis gas (13). In 1990, Ashcroft et al. (13) reported some effective noble metal catalysts for the reaction about 90% conversion of methane and more than 90% selectivity to CO and H2 were achieved with a lanthanide ruthenium oxide catalyst (L2Ru207, where L = Pr, Eu, Gd, Dy, Yb or Lu) at a temperature of about 1048 K, atmospheric pressure, and a GHSV of 4 X 104 mL (mL catalyst)-1 h-1. This space velocity is much higher than that employed by Prettre et al. (3). Schmidt et al. (14-16) and Choudhary et al. (17) used even higher space velocities (with reactor residence times close to 10-3 s). 

Fig. 8. CH4 conversion as a function of the number of CH4/O2 pulses for partial oxidation of CH4 catalyzed by Ni/La203. Reaction conditions temperature, 873 K catalyst, 20 mg of 20 wt% Ni/La203 loaded in a fixed-bed flow reactor feed gas, 0.9 mL CH4/02 (molar ratio 2/1) in each pulse carrier gas, helium (flow rate, 100 mL min-1) (134).

Fig. 13. CH4 conversion in the C02 reforming of CH4 catalyzed by Ni/Zr02. Before reaction, the catalyst was reduced in flowing H2/N2 (1/9, molar ratio) at 973 K for 3 h. Reaction conditions pressure, 1 atm temperature, 1030 K feed gas molar ratio, CH4/C02=1/1 GHSV, 24,000 mL (g catalyst)-1 h-1 (237).

In 1989, Gadalla and Sommer (252) reported that a solid-solution NiO/MgO (1 1.35) catalyst prepared by precipitation can inhibit the carbon deposition in the CO2 reforming of methane however, they obtained a low CO2 conversion (66%), a low H2 selectivity (79%), and a low CO selectivity (77%), even at the very low WHSV of 3714 cm3 (g catalyst)-1 h-1 with a CH4/CO2 (1/1, molar) feed gas and the high temperature of 1200 K. Their relatively high CH4 conversion was partly a consequence of homogeneous gas-phase reactions that occurred under their conditions. Indeed, the authors found extensive carbon deposits plugging the reactor upstream and downstream of the reaction zone. 

In summary, the basicity and the strong NiO-MgO interactions in binary NiO/MgO solid solution catalysts, which inhibit carbon deposition and catalyst sintering, result in an excellent catalytic performance for C02 reforming. The characteristics of MgO play an important role in the performance of a highly efficient NiO/MgO solid-solution catalyst. Moreover, the NiO/MgO catalyst performance is sensitive to the NiO content a too-small amount of NiO in the solid solution leads to a low activity, and a too-high amount of NiO to a low stability. CoO/MgO solid solutions have catalytic performances similar to those of NiO/MgO solid solutions, but require higher reaction temperatures. So far, no experimental information is available regarding the use of a FeO/MgO solid solution for CH4 conversion to synthesis gas. 

For WGS, commercial catalysts are only operated up to 550 °C and no catalysts are available for higher temperatures, because adverse equilibrium conversion makes the process impractical in the absence of a CO2 sorbent. Han and Harrison [38] have shown that, at 550 °C, dolomite and limestone have a sufficiently high WGS activity. For SMR a conventional Ni SMR catalyst is used in a 1 1 ratio with CaO [30]. Meyer et al. [32] have also used a Ni-based catalyst in combination with limestone and dolomite, and achieved CH4 conversions of 95% at 675 °C while the CH4 conversion at equilibrium was 75%. 

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