Carbon mitigation in the dry reforming of methane

Dry reforming of methane is a technique to produce syngas from biogas or CO2-rich natural gas at high temperatures, generally over a Ni or Co catalyst. Syngas produced by dry reforming has a H2/CO ratio around 1, which makes it CO-rich, and therefore suitable for the production of pure CO or Gas-to-Liquid processes. Up to date, large-scale application of dry reforming has been limited, with the main barriers to industrial deployment being the highly endothermic reaction pathway that requires high operating temperatures to reach acceptable conversion levels in the presence of alumina-supported nickel catalysts and the formation of high-strength carbon whiskers catalysed by nickel crystallites, which are destructive to catalyst pellets. A thermodynamic analysis of the reaction pathway is first performed while relaxing the conventional assumption that graphite is the phase of carbon that forms. The effect of catalyst dispersion and the precursors to coking are identified, and the effect of sintering on carbon deposition is therefore better understood. Optimized temperature-pressure-time trajectories for the reactor operation show that pressure must be gradually increased with time on stream to avoid the carbon limits as the catalyst sinters. In parallel, two catalyst systems are developed and tested: Supported molybdenum and nickel-molybdenum nitrides are synthesized and characterized. The nitrides are observed to perform well in terms of carbon resistance due to enhanced CO2 adsorption by the support, but to deactivate within 7 hours on stream, with a phase transition to an oxide/carbide phase that provides terminal activity. In comparison, the tested trimetallic Ni-Co-Ru catalysts have both a higher activity (>90% conversion) and an excellent stability, but exhibit a slightly higher carbon formation rate. Synergetic effects in the NiCo system stabilize the active phase by a hydrogen spill-over effect and coking is reduced by the oxophilicity of Co. Higher activity is exhibited by Ni-rich catalysts, and Ru is shown to improve the reducibility and coke resistance by 51% at the expense of activity. Experimental work in catalysis confirms the identified trade-off between activity, stability and ease of activation. Eventually, a combination of catalyst design and operating conditions optimization brings the process one step closer to industrial application by resolving this important limitation associated with dry reforming.