Catalysts for Sustainable Hydrogen Production: Preparation, Applications and Process Integration, 2nd Edition

The effects of climate change are now evident all over the world. The news tells us about extreme heat, drought, floods, and loss of biodiversity. The above phenomena can be attributed to global warming, which is strongly influenced by human activities and mainly induced by the combustion of fossil fuels. To limit global warming, the Intergovernmental Panel on Climate Change (IPCC) suggests achieving carbon neutrality by the mid-21st century [1]. Through the REPowerEU plan [2], the European Union supports the uptake of renewable hydrogen, obtained with low carbon emissions, to help decarbonize in a cost-effective way, reducing dependence on fossil fuels [3].

To create a hydrogen supply chain, however, a series of challenges relating to production, transport, storage, distribution, and logistics will be necessary to address. The second edition of the “Catalysts for Sustainable Hydrogen Production: Preparation, Applications and Process Integration” Special Issue offers a broad overview of the latest research in hydrogen production.

Hydrogen is usually classified by assigning a color based on the CO₂ emissions of the production process. Gray hydrogen production generates high CO₂, e.g., coal gasification or reforming; blue hydrogen low CO₂, e.g., coal gasification with carbon capture (CCUS); turquoise hydrogen no CO₂, e.g., methane pyrolysis; and green hydrogen no carbon is involved, e.g., electrolysis [4]. Green hydrogen is the main road to decarbonization; however, the total replacement of fossil fuels with renewables in the short term does not appear to be immediately achievable. Many hydrogen carriers and hydrogen production technologies are currently under investigation.

Water is an ideal carrier of hydrogen, is abundant, is non-toxic, and is carbon-free; however, electrolysis of water, as well as thermochemical water splitting, still faces several challenges that could drive future research to improve these technologies. The hydrogasification of biomass to produce syngas offers an interesting alternative to fossil sources (contribution 1), allowing the use of production plants and infrastructure currently in use. However, downstream of the gasification process, the presence of tar, particulates, and contaminants requires cleaning processes. In this context, tar physical adsorption coupled with pure hydrogen production via a chemical looping process can be considered a valuable proposal (contribution 2). On the other hand, the selection of suitable catalysts is mandatory to achieve the desired performance, and the exploitation of the redox properties of transition metals was reported as a very good choice.

As already mentioned, reforming is a gray hydrogen production process due to the large quantities of CO₂ obtained downstream of the process. A possible solution is substitution with bi-reforming, in which both steam and carbon dioxide are reacted, thus converting CO₂ into higher-value products (contribution 3). On the other hand, the choice of highly dispersed catalysts (achieved, for example, by employing mesoporous SiO₂ as catalytic support or by preparing mixed oxides with proper morphological as well as redox properties) can help modulate catalyst selectivity, reduce byproduct formation, and limit deactivation phenomena.

In addition to natural gas, the characteristics of a multitude of hydrogen carriers have been investigated. Liquid organic hydrogen carriers (LOHCs), including formic acid, methanol, etc., free of toxicological problems, are also attractive for their compatibility with the existing energy infrastructure. Oxidative steam reforming of acetic acid has been proposed as a valuable method for obtaining hydrogen (contribution 4).

Excellent prospects come from the use of ammonia, whose hydrogen content is about 17.7%, making it a promising carbon-free hydrogen carrier. However, further decomposition studies are needed for widespread deployment (contribution 5) as toxicity represents a significant limit in the use of liquefied ammonia.

Photocatalysis processes are cutting-edge. Interesting are the visible-light-driven photodegradation techniques of organic pollutants, which allow the production of hydrogen from hazardous organic materials in wastewater (contribution 6). Visible light-activated photocatalyst materials, such as the CeO₂/MoS₂ composite, have been successfully synthesized and tested in the evolution of hydrogen from an aqueous solution containing 0.3 M Na₂SO₃/Na₂S (contribution 7). A ternary Cu₂O/CuS/ZnS nanocomposite has been proposed for blue LED-light-induced photocatalytic hydrogen production from 0.1 M of a solution with a sacrificial reagent such as sodium sulfide, sodium sulfite, methanol, and ethanol (contribution 8).

Really attractive is the use of perovskite nanosheets, such as HB₂Nb₃O₁₀, in relation to light-driven hydrogen production from aqueous methanol as well as pure water under near-ultraviolet irradiation (contribution 9).

The proposed studies demonstrate that the researchers’ efforts are accompanying our world through the transition to clean energy in the hydrogen economy. However, further efforts are necessary to make the production processes more efficient and to solve the big storage problem. In conclusion, the energy transition is a transformation process that has just begun and will involve future generations.