Pure hydrogen streams from a reversible reactor

Making molecular hydrogen for industrial chemical and petroleum processes is costly. The typical approach mixes water and carbon monoxide (a by-product of fossil-fuel processing) at thermodynamic equilibrium in an industrial-sized reactor heated to about 800 °C. Besides producing about 50 million tons of H₂ each year, the method yields carbon dioxide, which engineers must then remove through an expensive separation process. Now Ian Metcalfe of Newcastle University and his colleagues have reported an alternative process. The reactor that Metcalfe devised exploits an approach known as chemical looping, which bypasses the need to mix the reactants.

Central to the chemical loop and the new reactor is an oxygen-carrying material (OCM)—in this case, a bed of solid-phase lanthanum ferrite perovskite. To avoid mixing the reactants, the loop comprises two halves, and inert argon is fed through the reactor to prevent contamination. Once the reactor is heated to about 800 °C, a feed stream of water vapor travels across the OCM, represented by the colored circles in the figure. The vapor then loses oxygen to yield molecular hydrogen; actuating a valve shuts off the first stream, and a separate stream of CO moves across the OCM, picking up oxygen along the way to form CO₂.

Crucially, the OCM is nonstoichiometric—the oxygen in its chemical formula cannot be expressed as an integer ratio with the other elements. As a result, the OCM remembers the oxidation potential of the reactants that flowed across it. As shown in the lower part of the figure, the oxygen content varies across the OCM’s surface. Unlike most other reactors that require energy input to convert products back to reactants, the one designed by Metcalfe is reversible. When the researchers sent a H₂ stream to the reduced end of the OCM bed and a CO₂ stream to the oxidized end, the reactor converted those products back to the H₂O and CO reactants. Though industrial hydrogen production doesn’t necessarily need a reversible reactor, Metcalfe says the new design may provide safer conditions in chemical plants that use carbon monoxide. (I. S. Metcalfe et al., Nat. Chem., 2019, doi:10.1038/s41557-019-0273-2 .)