Direct Conversion of Methane to Hydrogen and Elemental Carbon
In a recent paper in Science in November of 2017, Upham et. al., describe a catalytic process that employs a molten metal catalyst system to convert methane directly into hydrogen and elemental carbon. The reaction is very clean and occurs in 1 step with little or no product gas cleanup required, Equation 1, below.
For certain applications this process should favorably compete with the standard hydrogen production process of steam methane reforming (SMR). In the SMR process methane is reacted with water to produce carbon monoxide and hydrogen, Equation 2. The next step, the water gas reaction, is used to convert the carbon monoxide through reaction with water to hydrogen and carbon dioxide, Equation 3. The article also provides a good historical background to the SMR process.
The direct reaction:
The direct conversion of methane to hydrogen and carbon
CH4 (g) -à C(s) + 2H2 ΔH⁰ (298) + 74 kJ/mol (1)
The reaction is endothermic so external heat has to be applied to get any reasonable conversion. In fact, the reaction is conducted at 1000C and involves the use of a combination of Ni and a low melting metal (relative to 1000C) as the Ni carrying medium. Supporting metals tested included Bi, Au, Pb, Ga, In & Sn, all as melts.
The molten metal reactor system for the direct conversion of methane
The authors found that a 27% Ni – 73% Bi alloy exhibited the best reactivity for the process, equation 1. When operated at a temperature of 1065C a 95% conversion of the methane was observed. The reaction is very clean with the gaseous hydrogen easily separating from the reactor. The solid carbon product floated on the top of the metal melt. The initial carbon product contained a small amount of Ni + Bi (<4atom%) which could be removed by submersion of the carbon in the molten metal bath.
The reactor system is shown in Figure 1, in the PDF link. The figure shows Pt as the active metal catalyst but most of the work was done with Ni as it was more active than Pt. The rate data for all the systems studied are given in Figure S-12. The boiling point for Bi is 1564C with a low vapor pressure at 1000C.
Hydrogen from the Direct Methane to Hydrogen Process
The hydrogen from the subject process is extremely clean but may have some unreacted methane (a who cares?). (If the hydrogen is used for either ammonia syntheses or as a PEM fuel, the presence of methane is a “who cares”.) Another virtue of this process is that it is a single “pot” process which should scale with the volume of the reactor, a cubic function. Where could this technology be of use? See Appendix for clues.
 Another, hydrocarbon free process for hydrogen production is water electrolysis which produces both hydrogen and oxygen. The best electrolyzers run with about a 68% efficiency and scale with electrode area, a square function.
Is There a Market Potential for the Direct Methane to Hydrogen Process?
(Important background information is contained in Appendix A, see PDF link.) The US already has 2 extensive energy infrastructure networks in place, the electric grid and the natural gas pipeline grid. So, what if we set suitably sized (market determined) direct methane to hydrogen processing units along the major transportation corridors to produce the fuel hydrogen onsite. What could these service centers look like?
The hydrogen coming off the reactors would be fairly clean and dry but at low pressure (not too much above atmosphere pressure). The working pressure for hydrogen fueling stations for vehicle usage is typically 70 MPa (10,000 psi). The product hydrogen would have to be compressed to this level with, most likely, 2 stage compressors, not inexpensive. It would have to be stored in (very) high pressure storage tanks again not cheap. Finally, the byproduct carbon would have to be stored before pickup. This approach would obviate the need to develop a 3rd infrastructure of hydrogen distribution pipelines, probably a pipe dream, pun intended.