Carbon nitride based materials: more than just a support for single-atom catalysis.

Recently, the missing link between homogeneous and heterogeneous catalysis has been found and it was named single-atom catalysis (SAC). However, the SAC field still faces important challenges, one of which is controlling the bonding/coordination between the single atoms and the support in order to compensate for the increase in surface energy when the particle size is reduced due to atomic dispersion. Excellent candidates to meet this requirement are carbon nitride (CN)-based materials. Metal atoms can be firmly trapped in nitrogen-rich coordination sites in CN materials, which makes them a unique class of hosts for preparing single-atom catalysts (SACs). As one of the most promising two-dimensional supports to stabilize isolated metal atoms, CN materials have been increasingly employed for preparing SACs. Herein, we will cover the most recent advances in single-atoms supported by CN materials. In this review, the most important characterization techniques and the challenges faced in this topic will be discussed, and the commonly employed synthetic methods will be delineated for different CN materials. Finally, the catalytic performance of SACs based on carbon nitrides will be reviewed with a special focus on their photocatalytic applications. In particular, we will prove CN as a non-innocent support. The relationship between single-atoms and carbon nitride supports is two-way, where the single-atoms can change the electronic properties of the CN support, while the electronic features of the CN matrix can tune the catalytic activity of the single sites in photocatalytic reactions. Finally, we highlight the frontiers in the field, including analytical method development, truly controlled synthetic methods, allowing the fine control of loading and multi-element synthesis, and how understanding the two-way exchange behind single-atoms and CN supports can push this topic to the next level.

A Solar to Chemical Strategy: Green Hydrogen as a Means, Not an End.

Green hydrogen is the key to the chemical industry achieving net zero emissions. The chemical industry is responsible for almost 2% of all CO2 emissions, with half of it coming from the production of simple commodity chemicals, such as NH3, H2O2, methanol, and aniline. Despite electrolysis driven by renewable power sources emerging as the most promising way to supply all the green hydrogen required in the production chain of these chemicals, in this review, it is worth noting that the photocatalytic route may be underestimated and can hold a bright future for this topic. In fact, the production of H2 by photocatalysis still faces important challenges in terms of activity, engineering, and economic feasibility. However, photocatalytic systems can be tailored to directly convert sunlight and water (or other renewable proton sources) directly into chemicals, enabling a solar-to-chemical strategy. Here, a series of recent examples are presented, demonstrating that photocatalysis can be successfully employed to produce the most important commodity chemicals, especially on NH3, H2O2, and chemicals produced by reduction reactions. The replacement of fossil-derived H2 in the synthesis of these chemicals can be disruptive, essentially safeguarding the transition of the chemical industry to a low-carbon economy.

Single-Atoms on Crystalline Carbon Nitrides for Selective C─H Photooxidation: A Bridge to Achieve Homogeneous Pathways in Heterogeneous Materials.

Single-atom catalysis is a field of paramount importance in contemporary science due to its exceptional ability to combine the domains of homogeneous and heterogeneous catalysis. Iron and manganese metalloenzymes are known to be effective in C─H oxidation reactions in nature, inspiring scientists to mimic their active sites in artificial catalytic systems. Herein, a simple and versatile cation exchange method is successfully employed to stabilize low-cost iron and manganese single-atoms in poly(heptazine imides) (PHI). The resulting materials are employed as photocatalysts for toluene oxidation, demonstrating remarkable selectivity toward benzaldehyde. The protocol is then extended to the selective oxidation of different substrates, including (substituted) alkylaromatics, benzyl alcohols, and sulfides. Detailed mechanistic investigations revealed that iron- and manganese-containing photocatalysts work through a similar mechanism via the formation of high-valent M═O species. Operando X-ray absorption spectroscopy (XAS) is employed to confirm the formation of high-valent iron- and manganese-oxo species, typically found in metalloenzymes involved in highly selective C─H oxidations.