UC Santa Barbara team develops catalytic molten metals for direct conversion of methane to hydrogen without forming CO2
Researchers at the University of California Santa Barbara have developed catalytic molten metals to pyrolize methane to release hydrogen and to form solid carbon. The insoluble carbon floats to the surface of the melt, where it can be removed and stored or incorporated into composite materials. This method also avoids carbon formation on steam-reforming catalysts, which usually deactivates the catalysts.
In a paper in the journal Science, the team reported that a 27% Ni–73% Bi alloy (Ni0.27Bi0.73) achieved 95% methane conversion at 1065°C in a 1.1-meter bubble column and produced pure hydrogen without CO2 or other by-products. Under these conditions, the equilibrium conversion is 98%. When the temperature was reduced to 1040 °C, the CH4 conversion decreased to 86%.
Steam methane (CH4) reforming (SMR) followed by the water-gas shift reaction is the most common process for large-scale hydrogen production today. Although commercially optimized for decades, the endothermic SMR process is ex- pensive; high capital costs and high energy consumption are unavoidable. Furthermore, the process produces stoichiometric CO2, which may impose additional costs because of the need for sequestration or because of a possible carbon tax. Despite the fundamental economic and environmental limitations of SMR, none of the presently deployed renewable power sources, including hydrogen from electrolysis, can compete with the SMR process for large-scale H2 production.
Alternatively, H2 can be produced by pyrolysis of CH4 without producing CO2 … only half as much H2 is produced per mole of ch4 compared to SMR; however, considerably less energy input is required and solid carbon is coproduced rather than CO2.
… Metallic catalysts (e.g., Ni, Pd, Pt) achieve high conversion and selectivity to H2 at moderate temperatures; however, their melting temperatures are extremely high and as solids, they are rapidly deactivated by solid carbon (coke). The only report of the use of a molten metal as a catalyst for CH4 pyrolysis described pure liquid magnesium (Mg), which was used to achieve ~30% of the equilibrium conversion, at 700 °C. Higher conversions, at higher temperatures, were not possible because of Mg evaporation.—Upham et al.
In their study, the UCSB researchers prepared liquid alloys of active metals in low–melting-temperature metal “solvents” (Sn, Pb, Bi, In, and Ga) using known equilibrium phase behavior to produce catalysts that melt at <1000°C. The melts are used in molten-metal bubble columns, where carbon continuously floats to the surface where it can be removed.
The carbon produced—mostly graphite—accumulated as a fine powder at the top surface of the melt.
D. Chester Upham, Vishal Agarwal, Alexander Khechfe, Zachary R. Snodgrass, Michael J. Gordon, Horia Metiu, Eric W. McFarland (2017) “Catalytic molten metals for the direct conversion of methane to hydrogen and separable carbon” Science 917-921 doi: 10.1126/science.aao5023