Low-temperature (500-650 ºC) catalytic MD has been only used in industry to produce solid carbon structures because, after short operation times, these completely deactivate the catalyst and shut-down the reactor. This has been hindering continuous H2 production that is much cheaper than electrolysis and that can replace the reforming of hydrocarbons, which requires expensive carbon capture and sequestration systems.
Competing institutions/companies are developing high temperatures (> 1000 ºC) MD processes, involving either metal liquid reactors or using carbon catalysts; however, these approaches are energy-intensive, dangerous to operate, and display low catalytic activity. Though the promising advantages of fast reaction kinetics, high power density and lower costs, the low-temperature catalytic MD process has never been successfully demonstrated for periods longer than 200 h. EU Project 112CO2 overcame the challenges by developing a unique cyclic regeneration strategy (WO/2020/121287), which combined with a disruptive reactor design and a nickel-based supported catalyst, made the catalytic MD process to be fully stable for thousands of hours. The reaction in these conditions is 100 % selective, allowing the hydrogenation of the catalyst/carbon interface, making the produced carbon to detach periodically from the catalyst surface; this hydrogenation process uses ca. 5 % of the H2 produced. At the end, the consortium proved >3000 h of continuous operation at 550 °C with a constant catalyst productivity of 0.64 gH2 gcat-1 h-1. The carbon by-product is metal-free graphitic nanofilaments, which is a quite valorized carbon (predicted to cost ca. 3 €/kg) and especially suitable for making electric conductive carbon paints and electrodes for electrochemical devices, among a variety of other high-value applications.
This reaction is equilibrium limited, where at 600 °C and 1 bar the equilibrium conversion is ca. 60 %, but it reaches ca. 88 % between 700-750 ºC and the H2 concentration 94 %. Therefore, the 112CO2 team is continuing to improve the catalyst aiming at displaying higher catalytic activities; very recently, new catalysts/catalytic substrates were engineered. UPORTO and CSIC filed three patents disclosing this new process at intermediate-temperature. Also, the reactor design is being optimized to reach even higher power densities – a target power density of 1.5 kW L-1.
112CO2 consortium had an ambitious program for delivering a demonstrative system by the end of 2024, reaching TRL 4.
The other direction to look is when the MD reaction is applied to the biomethane from biogas, i.e. CO2-negative hydrogen is produced along with graphitic carbon, which has a high market price and supports carbon credits; this makes the produced bright-hydrogen – the proposed color-code for H2 produced from a renewable source, with negative-CO2 emissions and low-cost. The CO2 from the biogas can then be hydrogenated to produce very low-cost green methanol with an estimated cost of ca. 200 € t-1, which compares quite nicely with the present price from Methanex.
