(760e) Dry Reforming of Methane Using Two-Dimensional Metal Carbides

The upgrading of natural gas has come to the forefront of the chemical industry as a challenge of the 21^st century, and the primary technology to address these challenges is using more active, selective, and stable catalysts. The recent shale gas revolution has lowered the price of natural gas, and thus made it as an attractive feedstock for the production of light olefins, syngas, or other valuable chemicals. The conversion of the most abundant natural gas component, methane, in-to valuable chemicals is highly desirable. Focus is being stressed upon the utilization of CO₂ along with the natural gas in catalytic processes to manufacture valuable chemicals and fuel. As conversion is more desirable than sequestration, considering the net amount of CO₂ mitigated by conversion being 20-40 times greater than sequestration.¹ Dry reforming of methane (DRM) is an attractive route to convert CH₄ and CO₂ into syngas in a 1:1 ratio of H₂ and CO, which is the optimal ratio to produce liquids hydrocarbons through the Fischer-Tropsch process.^{2, 3} However, due to multiple thermodynamic equilibrium processes, DRM is adversely affected by the undesired secondary gas-phase reactions such as³:

CH₄ → C + 2H₂ methane thermal degradation

2CO → CO₂ + C carbon monoxide disproportionation

CO + H₂O → H₂ + CO₂ water gas shift

These gas phase reactions severely deteriorate the catalyst performance and affects the H₂/CO ratio. Several studies have been done to develop high-performance catalysts. ^{2, 3}However, despite considerable environmental benefits, DRM is not an industrially mature process due to the high endothermic nature together with rapid catalyst deactivation, that hindered its industrialization.

Novel parameters for the preparation of the carbide catalyst are introduced and a series of 2D Vanadium carbide (V₂C) catalysts are prepared to study their effect on the structural and catalytic properties. Characterization techniques such as x-ray diffraction (XRD), x-ray photoelectron microscopy (XPS), scanning electron microscopy (SEM) and Raman spectroscopy are employed to analyze the catalyst. Kinetic studies are performed in the stainless-steel reactor and the final gas analysis is done using Agilent GC equipped with flame ionization detector (FID) and thermal conductivity detector (TCD). Steady-state kinetics of bulk and two-dimensional vanadium carbide shows a stable catalytic activity and selectivity towards syngas for 2D V₂C as compared to the bulk VC. Insights into the kinetic data and catalyst characterization results co-relates the kinetic data with the structure of the catalyst. The 2D V₂C with hexagonal closed packing (hcp) structure exhibits remarkable catalytic/stable performance as compared to the bulk VC, which exhibits face centered cubic (fcc) structure. The difference in the structure of the carbides plays an important role in facilitating re-carburization of the catalysts. The spent catalyst is further re-carburized and similar catalytic performance is observed. It is apparent from our study that the catalytic cycle goes through the redox mechanism, where the rate for oxidation of catalyst by CO₂ are in equilibrium with rate of re-carburization by CH₄.

In this presentation we will demonstrate our steady-state kinetic studies in DRM reaction using 2D vanadium carbides and the bulk 3D vanadium carbides with an attempt to uncover the relationship between the structures of catalysts and its catalytic activity. Stress is being focused upon maximizing activity, selectivity, and stability towards syngas. A fundamental relationship between catalyst structural morphology and its catalytic performance is established. Redox mechanism is proposed as a driving phenomenon for enhanced and stable catalytic performance for 2D vanadium carbide.

References

1. M. D. Porosoff, B. Yan and J. G. Chen, Energy & Environmental Science, 2016, 9, 62-73.
2. M. Mohamedali, A. Henni and H. Ibrahim, ChemEngineering, 2018, 2, 9.
3. N. A. K. Aramouni, J. G. Touma, B. A. Tarboush, J. Zeaiter and M. N. Ahmad, Renewable and Sustainable Energy Reviews, 2017.