(415d) Thermo-Catalytic Decomposition of Decarbonization of Natural Gas

Hydrogen is envisioned as the energy carrier (fuel) of the future and is a crucial feedstock for various manufacturing industries. Thermo-catalytic decomposition (TCD) of methane can produce CO_X-free hydrogen for PEM fuel cells, oil refineries, ammonia and methanol production [1]. Recent research has focused on enhancing the production of hydrogen by the direct thermo-catalytic decomposition of methane to form elemental carbon and hydrogen as an attractive alternative to the conventional steam reforming process [2].

Carbon as a catalyst has many advantages compared to other catalytic materials: a) fuel flexibility, b) insensitivity to sulfur poisoning and c) high temperature resistance. TCD offers 100% carbon capture with the solid, high purity carbon useful as electrode material for energy storage [3]. Despite these advantages, carbon as a catalyst also problematically deactivates. Ideally the deposited carbon would be autocatalytic but all studies with methane find that the deposited carbon is not as active a catalyst as the original carbon.

The overall project goal is to connect carbon catalyst nanostructure, active sites and kinetic rates. The approach consists of direct measures of these parameters, resolved by species, parametric with temperature and differentiated by nanostructure of nascent carbon. Project objectives are as follows: 1) Quantified nanostructure, time-resolved and correlated with TCD rate will test the dependence of rate on initial nanostructure and its time-variation. 2) Similarly, the TCD dependence on active sites will test the correlation between active sites and kinetic rates. 3) Correlation between nanostructure and active sites throughout TCD will test continuity of dependence. Similarly, under regeneration, active sites will be related to nanostructure and experimental kinetic rate measurements and nanostructure metrics. Notably, the CO generated by the Boudouard regeneration reaction could be combined with the H₂ for FT synthesis of liquid fuels.

A hot-wall CVD reactor is used for carbon deposition with quartz substrates using natural gas mixtures as feed. Reaction kinetics are evaluated based deposited carbon. Carbon deposition is determined by measuring film thickness via SEM image analysis. The test matrix consists of reaction duration (0.5 – 3 hrs.) and temperature (700 – 1,100 ) for each gas mixture. Active sites are determined by a two-step procedure consisting of activated O₂ chemisorption at 300 followed by XPS analysis for surface oxygen concentration and resolution by bonding type. Nanostructure is characterized by HRTEM and fringe image analysis applied to deposited carbon films, extracted and deposited upon TEM grids. Using a packed bed reactor, gas chromatography resolves both reactant conversion and selectivity to H₂. Experiments are establishing the relation between rates and active sites at systematic intervals during TCD and regeneration, coupled with nanostructure quantification.

If nanostructure can be correlated to active sites, a surrogate metric will be established by which to gauge carbon structure for reactivity under TCD and PO conditions. Nanostructure provides a more straightforward method of assessing carbon reactivity based on structure than active site measurement – the latter being very dependent upon sample preparation and chemisorption procedure.

References

1. 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.
2. Al-Hassani, A. A., Abbas, H. F., & Daud, W. W. (2014). Production of COx-free hydrogen by the thermal decomposition of methane over activated carbon: Catalyst deactivation. International journal of hydrogen energy, 39(27), 14783-14791.
3. Vander Wal, R., & Makiesse Nkiawete, M. (2020). Carbons as catalysts in thermo-catalytic hydrocarbon decomposition: A review. C, 6(2), 23.