(570a) Carbon Catalyzed Decarbonization of Natural Gas for Clean Hydrogen and Solid Carbon

Hydrogen is envisioned as the energy carrier (fuel) of the future and is a crucial feedstock for various manufacturing industries. Today ~ 10 million metric tons (MMT) are produced in the U.S, ~ 95% of this is from steam reforming of methane (SMR), for use in oil refineries, methanol and ammonia production [1]. SMR produces 13.7 kg CO₂ (equiv.)/kg of net hydrogen [2] and consumes 19.8 liters of water per kg of hydrogen [3]. In contrast, thermo-catalytic decomposition (TCD) of natural gas does not produce CO₂, nor does it consume water resources [4]. The energy requirement for methane cracking process (37.8 kJ/mole of H₂) is substantially less than that for steam reforming (74.8 kJ/mole of CH₄) [5-7]. Finally, the carbon stripped from the natural gas is in solid form – ideal for carbon sequestration [8,9].

Decarburization of natural gas (NG) represents a path towards the hydrogen economy till renewable H₂ generation reaches scale. The solid (elemental) carbon produced not only presents a tremendous advantage in carbon capture and storage [8,10-13] but has potential commercial value [14], as additive for composites, structural applications and even for soil amendment [15,16]. Given its purity, it is useable as electrode materials in energy storage applications [1,17] or metallurgical processing [18].

The barrier to TCD is maintaining catalytic activity [1-6]. TCD requires a catalyst to overcome the high thermodynamic barrier for NG decomposition to H₂ and solid carbon [5]. Carbon itself is an excellent catalyst, cheap, and tolerant of NG impurities that can poison (deactivate) a metal catalyst [2,4,7,8]. Thus, the depositing carbon could potentially serve as catalyst [19].

In this study, a synthetic natural gas blend diluted in an argon carrier served as the initial feed. The test matrix encompassed a series of temperatures (900 – 1,100 ℃) and reaction durations (1 – 12 hrs.). TCD measurements were performed using a hot wall reactor and flat substrates of silicon and quartz. After deposition for varied durations, substrates were removed for film thickness determination. TCD rates were measured by deposit thickness, using scanning electron microscopy (SEM). As kinetic rates are mapped, they are related to active site number for each of the two processes, TCD and regeneration, in order to provide a common connection

The nanostructure of the deposit was accessed using transmission electron microscopy (TEM) and Raman spectroscopy. Nanostructure was assessed to identify correlations with kinetic rates. Nanostructure is an easier metric with which to gauge carbon catalyst activity and deposition rate, with comparison to chemisorption followed by XPS, an approach more definitive than TPD. For reference there is considerable literature precedence using nanostructure for interpretation of soot oxidative reactivity [20-22]. Here nanostructure is also used to gauge the efficacy of initial carbon catalysts prior to TCD testing. At selected stages during TCD, samples were subjected to activated chemisorption in preparation of active site measurement. Active sites were quantified by X-ray photoelectron spectroscopy (XPS) with deconvolution to identify oxygen functional groups. The differences in oxygen content between nascent and partially oxidized samples quantifies the number of active sites created while the difference in oxygen content across the two carbons can be attributed to the nanostructure differences (and corresponding active site number) between the two carbon samples.

References

1. Dagle, R. A., Dagle, V., Bearden, M. D., Holladay, J. D., Krause, T. R., & Ahmed, S. (2017). An Overview of Natural Gas Conversion Technologies for Co-Production of Hydrogen and Value-Added Solid Carbon Products (No. PNNL-26726; ANL-17/11). Pacific Northwest National Lab. (PNNL), Richland, WA (United States); Argonne National Lab. (ANL), Argonne, IL (United States).
2. Abbas, H. F., Daud, W. W. (2010). Hydrogen production by methane decomposition: a review. International Journal of Hydrogen Energy, 35(3), 1160-1190.
3. Spath, P. L., and Mann, M. K., Life cycle assessment of hydrogen production via natural gas steam reforming. (2001). Technical Report NREL/TP-570-27637.
4. Muradov N. Thermocatalytic CO2-free production of hydrogen from hydrocarbon fuels. U.S. DOE Hydrogen Program Review. U.S.: Department of Energy (DOE); 2002. NREL/CP-610-32405.
5. Amin, A. M., Croiset, E., Epling, W. (2011). Review of methane catalytic cracking for hydrogen production. international journal of hydrogen energy, 36(4), 2904-2935.
6. Torres, D., Pinilla, J., & Suelves, I. (2018). Co-, Cu-and Fe-Doped Ni/Al2O3 Catalysts for the Catalytic Decomposition of Methane into Hydrogen and Carbon Nanofibers. Catalysts, 8(8), 300.
7. Muradov, N. (2001). Hydrogen via methane decomposition: an application for decarbonization of fossil fuels. International Journal of Hydrogen Energy, 26(11), 1165-1175.
8. Ashik, U. P. M., Daud, W. W., Abbas, H. F. (2015). Production of greenhouse gas free hydrogen by thermocatalytic decomposition of methane–a review. Renewable and Sustainable Energy Reviews, 44, 221-256.
9. Gaudernack, B., Lynum, S. (1998). Hydrogen from natural gas without release of CO2 to the atmosphere. International journal of hydrogen energy, 23(12), 1087-1093.
10. Muradov, N. (2001). Hydrogen via methane decomposition: an application for decarbonization of fossil fuels. International Journal of Hydrogen Energy, 26(11), 1165-1175.
11. Muradov, N. (2000, May). Thermocatalytic CO2-free production of hydrogen from hydrocarbon fuels. In Proceedings of the 2000 Hydrogen Program Review, NREL/CP-570-28890.
12. Muradov, N., Chen, Z., & Smith, F. (2005). Fossil hydrogen with reduced CO2 emission: modeling thermocatalytic decomposition of methane in a fluidized bed of carbon particles. International journal of hydrogen energy, 30(10), 1149-1158.
13. Muradov, N. Z. (1998). CO2-free production of hydrogen by catalytic pyrolysis of hydrocarbon fuel. Energy & Fuels, 12(1), 41-48.
14. Parkinson, B., Tabatabaei, M., Upham, D. C., Ballinger, B., Greig, C., Smart, S., & McFarland, E. (2018). Hydrogen production using methane: Techno-economics of decarbonizing fuels and chemicals. International Journal of Hydrogen Energy, 43(5), 2540-2555.
15. De Falco, M., Basile, A. (Eds.). (2015). Enriched Methane: The First Step Towards the Hydrogen Economy. Springer.
16. Muradov, N. Z., & Veziroǧlu, T. N. (2005). From hydrocarbon to hydrogen–carbon to hydrogen economy. International Journal of Hydrogen Energy, 30(3), 225-237.
17. Lane, J. M., & Spath, P. L. (2001). Technoeconomic analysis of the thermocatalytic decomposition of natural gas. Colorado USA: Department of Energy (DOE) US.
18. Parkinson, B., Tabatabaei, M., Upham, D. C., Ballinger, B., Greig, C., Smart, S., & McFarland, E. (2018). Hydrogen production using methane: Techno-economics of decarbonizing fuels and chemicals. International Journal of Hydrogen Energy, 43(5), 2540-2555.
19. Vander Wal, R., & Nkiawete, M. (2020). Carbons as catalysts in thermo-catalytic hydrocarbon decomposition: a review. C, 6(2), 23.
20. Vander Wal, R. L., & Tomasek, A. J. Soot nanostructure: dependence upon synthesis conditions. Combustion and Flame, 2004, 136(1-2), 129-140.
21. Yehliu, K., Vander Wal, R. L., Armas, O., & Boehman, A. L. Impact of fuel formulation on the nanostructure and reactivity of diesel soot. Combustion and Flame, 2012, 159(12), 3597-3606.
22. Gaddam, C. K., Vander Wal, R. L., Chen, X., Yezerets, A., & Kamasamudram, K. Reconciliation of carbon oxidation rates and activation energies based on changing nanostructure. Carbon, 2016, 98, 545-556.