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Harnessing photocatalysis: Hydrogen generation and organic synthesis explored through new database

Generation of hydrogen (Hâ‚‚) by means of photocatalysis has been at the forefront of research since the 1970s because it can potentially fulfill the demand for this green fuel by employing abundant solar light as the only energy source. It encompasses mainly two approaches: overall water splitting and selective dehydrogenation of organic compounds.

Research in overall water splitting focuses on the design and preparation of more effective photocatalysts capable of utilizing a wider range of the electromagnetic spectrum and operating at lower overpotentials for water oxidation and proton reduction half-reactions. In recent years, the field has made considerable progress in advancing experiments beyond the laboratory stage.

On the other hand, selective dehydrogenation of organic compounds is a more complex and multifaceted area of research. There are nearly an infinite number of organic compounds capable of serving as Hâ‚‚ sources, while the products of their dehydrogenation are valuable chemicals.

However, the dehydrogenation of most organic molecules could proceed to different extents and yield a range of products. Therefore, selectivity towards a specific product is a central parameter in this area of research, which could be modulated by selecting the right photocatalysts and adjusting process parameters.

Despite its importance for fundamental and applied research, an overarching summary of the progress that has been achieved by the community in selective dehydrogenation of organic compounds in the past 40 years was missing. To fill this gap, recently, Prof. Oleksandr Savateev (the Chinese University of Hong Kong) created a "."

The database contains 236 entries, which were extracted from 216 articles published between 1982–2023 and more than 100 descriptors that are associated with each entry. For example, these descriptors are:

• Reaction classification according to the type of formed bond, such as C–B, C–C, C–H, C–N, C–O, C–P, C–S, C–Si, S–S, Si–O, N–N.
• Type of the photocatalyst (homogeneous, heterogeneous), its chemical structure and performance, such as yield rate of H₂ and the organic product.
• Quantum yield of the reaction, and others.

These data were analyzed by the team led by Prof. Oleksandr Savateev (the Chinese University of Hong Kong), in cooperation with Prof. Junwang Tang (Tsinghua University) and Prof. Shaowen Cao (Wuhan University of Technology), and the results of the analysis are published in the review article in the .

Among the findings that outline future directions of the field development are:

• More efficient utilization of the solar spectrum. Dehydrogenation of organic compounds is thermodynamically less challenging—the Gibbs free energy change is less positive than that of water splitting. Therefore, in principle, photocatalysts with an energy gap narrower than 1 eV and photons in the near IR region of the electromagnetic spectrum are sufficient to drive these reactions. However, there are only a few examples of photocatalysts and reactions performed at longer wavelengths, around 500 nm. The vast majority of reactions are enabled by photocatalysts having the band gap in the range 2.8–3.5 eV, requiring photons at the edge of the visible spectrum and near UV.
• Development of new organic reactions. There are many examples of photocatalytic dehydrogenation of organic compounds that proceed together with the formation of C–C, C–O, and C–N bonds. However, there are only a few examples of constructing more complex organic molecules via C–S, C–Si, and N–N coupling of molecular fragments accompanied by Hâ‚‚ evolution. This outlines opportunities for making organic synthesis more atom-efficient—Hâ‚‚ is a light molecule and the only byproduct in acceptorless dehydrogenative cross-coupling.

There are numerous areas where the database and the results of its analysis may be applied immediately. They allow researchers to accurately rank the performance of their newly developed photocatalytic systems in a selected dehydrogenation reaction under given conditions with respect to other reported photocatalysts.

On the other hand, they allow the identification of the most promising photocatalysts and photocatalytic dehydrogenation reactions, those with higher quantum yields and yield rates of Hâ‚‚ and organic products, to develop at a higher technology readiness level.