(363f) Reverse Microemulsion Synthesized Molybdenum and Cobalt Carbides for Thermocatalytic Conversion of CO2 to Renewable Natural Gas and Syngas

Introduction: Recently, CO₂ capture & utilization (CCU) has attracted significant interest as an alternative approach to CO₂ capture & storage (CCS) [1]. Converting CO₂, which is contained in many industrial and biogenic waste streams (e.g., landfill gas, biogas, fermentation off-gas), into synthetic fuels is an attractive pathway to reduce our dependence on fossil fuels. Thermocatalytic hydrogenation (e.g., reverse water gas shift (RWGS) and Sabatier reaction) provides advantages of fast reaction rates and high conversion efficiencies, thus allowing for compact, high-throughput operation [1-7]. The required H₂ can be generated via water electrolysis using renewable electricity (hydro, wind, and solar), nuclear power or surplus electricity. One of the key elements in the developing technologies for thermocatalytic CO₂ conversion is the catalyst.

Transition metal carbides have recently attracted attention as promising catalytic materials due to their electronic structure similar to that of platinum group metals, low cost, and excellent stability at elevated temperatures [8]. However, synthesizing supported transition metal carbide nanoparticles is challenging as the preparation procedure requires high temperature thermochemical treatment that leads to low specific surface area. In this study, high surface are, supported transition metal carbides were synthesized via the reverse microemulsion (RME) method followed by thermochemical treatment, characterized by a variety of analytical techniques, and evaluated for catalytic performance in high temperature CO₂ hydrogenation. The reaction mechanisms were investigated by in situ FTIR.

Materials and Methods: MoO₃/γ-Al₂O₃ and CoO/γ-Al₂O₃ nanoparticles were prepared in reverse microemulsions of water/cyclohexane/Triton X-100/2-propanol. These materials were converted into carbides by reduction in the CH₄/H₂ atmosphere. The synthesized catalysts were characterized by XRD, BET, ICP-OES, HRSEM, TEM, TPR-FTIR, TGA-FTIR, and in situ FTIR. Catalytic activity, selectivity, and stability were analyzed over the range of temperatures, pressures, and space velocities.

Results and Discussion: Catalyst characterization confirmed the composition (ICP-OES) and indicated a high specific surface area (BET), 200-300 m²/g. Crystalline phases were identified by XRD. Both the MoO₃/γ-Al₂O₃ and Mo₂C/γ-Al₂O₃ were 100% selective to CO formation, attaining nearly equilibrium RWGS CO₂ conversion. The Co₂C/γ-Al₂O₃ catalyst, on the other hand, was highly selective to CH₄ formation with CO₂ conversions exceeding 90% under optimal conditions. Both the Mo₂C/γ-Al₂O₃ and Co₂C/γ-Al₂O₃ catalysts were stable over 100 h on stream.

Significance: We report a versatile method for synthesis of high surface area, supported transition metal carbides. Depending on the selection of a transition metal, these catalysts can be used for various reaction pathways, which can be utilized for chemical synthesis allowing resource management and economic gain.

References

[1] Simakov, D.S.A. “Renewable Synthetic Fuels and Chemicals from Carbon Dioxide”. Springer, 2017.

[2] Tathod, A.P.; Hayek, N.; Shpasser, D.; Simakov, D.S.A.; Gazit, O.M. Appl. Catal. B: Environ. 2019, 249, 106.

[3] Zhuang, Y.; Currie, R.; McAuley, K.B.; Simakov, D.S.A. Appl. Catal. A: Gen. 2019, 575, 74.

[4] Currie, R.; Nicolic, D.; Petkovska, M.; Simakov, D.S.A. Isr. J. Chem. 2018, 58, 762.

[5] Walker, S.B.; Sun, D.; Kidon, D.; Siddiqui, A.; Kuner, A.; Fowler, M.; Simakov, D.S.A. Int. J. Energ. Res. 2018, 42, 1714.

[6] Sun, D.; Khan, F.M.; Simakov, D.S.A. Chem. Eng. J. 2017, 329, 165.

[7] Sun, D.; Simakov, D.S.A. J. CO. Util. 2017, 21, 368.

[8] Simakov, D.S.A.; Wright, M.M.; Shakeel, A.; Mokheimer, E.M.A.; Roman-Leshkov, Y. Catal. Sci. Technol. 2015, 5, 1991.

Acknowledgements

The authors acknowledge funding support from the Natural Science and Engineering Research Council (NSERC) of Canada through the NSERC Discovery Grant program.