Scientists from the HySA Infrastructure Centre of Competence at North-West University in South Africa have conducted a comprehensive analysis of modern technologies that make it possible to improve the efficiency of hydrogen production and use. They summarized the experience of developing catalytic reactors based on metal foams, materials whose porous structure can accelerate chemical reactions and improve heat dissipation. This work is important because hydrogen energy, which is meant to replace fossil fuels, is currently hampered by inadequate equipment: reactors often operate inefficiently, overheat or require too much energy to pump gases.
The metal foams in question look like sponges with open pores. There are various ways to produce them: melting with the use of polymer templates, sintering of metal powders with subsequent filler leaching and even 3D printing. The resulting material has a porosity of up to 98%, an enormous specific surface area and the ability to create turbulence in a gas flow, which is especially important for chemists. This means that the reactants are better mixed and react faster, while the pressure drop is less pronounced than in conventional reactors with catalyst pellets.
However, a simple metal foam cannot work as a catalyst; it needs to be activated. The scientists have thoroughly analyzed the methods for applying active substances to the pore surface, including the use of suspensions of premade catalyst, sol-gel technology, electrochemical deposition and even deposition of metals from the gas phase. Each of these methods has its intricacies: in some cases, one has to calibrate the acidity of the solution to prevent particles from sticking together, while in others, it is important to heat the workpiece to ensure the layer holds fast. The scientists emphasize that the quality of the coating is absolutely crucial: if the catalyst crumbles, the reactor will become a useless piece of metal.
The researchers reviewed the specific reactors in which these foams are already being used. These include simple fixed-bed tubular reactors, more complex membrane units where hydrogen is removed right during the reaction, shifting the equilibrium in the desired direction, and even microreactors with channels that are millimeters thick. In some cases, foam is used not as a carrier, but as a filler: the pores get filled with catalyst pellets, resulting in a design that combines the high thermal conductivity of the metal with a large mass of the active substance. Experiments show that these hybrids make it possible to avoid overheating even in very fast and hot reactions such as steam methane reforming.
Comparisons with conventional reactors often favor foam. For instance, in Fischer-Tropsch synthesis where liquid fuel is produced from carbon monoxide and hydrogen, the temperature is distributed much more evenly across the reactor cross-section with foam than in a bed of pellets. This means that fewer byproducts are formed, and the catalyst lasts longer. In stirred tank reactors where gas, liquid and solid catalyst must be in particularly close contact, porous metal blades have also proven superior to conventional baskets filled with pellets, as mass transfer accelerates exponentially.
Overall, the researchers’ analysis shows that reactors based on metal foams can significantly improve the efficiency of many hydrogen energy processes. Foams are especially useful where the process is limited not by the chemical reaction rate but by heat input or the delivery of reactants to the active sites.
