Structural Evolution and Underlying Mechanism of Single-Atom Centers on Mo2C(100) Support during Oxygen Reduction Reaction

Electrostatic Interaction in Amino Protonated Chitosan-Metal Complex Anion Hydrogels: A Simple Approach to Porous Metal Carbides/N-Doped Carbon Aerogels for Energy Conversion

In the face of the increasingly serious rapid depletion of fossil fuels, exploring alternative energy conversion technologies may be a promising choice to alleviate this crisis. Transition metal carbides (TMCs)/carbon composites are considered as prospective electrocatalysts due to their high catalytic activities and structural stability. In this work, we report the simple synthesis of TMCs/N-doping carbon aerogels (TMCs/NCAs, including Fe₃C/NCA, Mo₃C₂/NCA, and Fe₃C-Mo₂C/NCA) for the oxygen reduction reaction (ORR) using protonated chitosan/metal complex anion-chelated aerogels. Among them, the Fe₃C/NCA composite possesses efficient ORR activity (similar to Pt/C), and the Fe₃C/NCA-assembled Zn-air battery exhibits high power densities of about 250 mW cm^-2. The density functional theory calculation reveals that the presence of graphite-N, pyridine-N, and carbon defects in the carbon framework effectively reduces the free energy of ORR occurring in Fe₃C. This work provides a simple and extensible strategy for the preparation of TMCs from chitosan, which is expected to be extended to other metal carbides.

Highly Durable Heterogeneous Atomic Catalysts

ConspectusSingle-atom catalysts (SACs), in which surface metal atoms are isolated on the surface of a support, have received a tremendous amount of attention recently because this structure would utilize precious metals fully, without occluding atoms inside nanoparticles, and enable unique surface reactions which typical nanoparticle catalysts cannot induce. Various synthesis methods and characterization techniques have been reported that yield enhanced activity and selectivity. The single-atom structures were realized on various supports such as metal oxide/carbide/nitride, porous materials derived from zeolite or metal-organic frameworks, and carbon-based materials. Additionally, when the metal atoms are isolated on other metal nanoparticles, this material is denoted as a single-atom alloy (SAA). The single-atom structure, however, cannot catalyze the surface reaction that necessitates ensemble sites, where several metal atoms are located nearby. Very recently, ensemble catalysts, in which all of the metal atoms are exposed at the surface with neighboring metal atoms, have been reported, overcoming the limitation of single-atom catalysts. We call all of these materials (SACs, SAAs, and ensemble catalyst) heterogeneous atomic catalysts, indicating that the surface metal atomic structure is intentionally controlled. To use these atomic catalysts for practical applications, high durability should be guaranteed, which has received relatively less attention.In this Account, we discuss recent examples of heterogeneous atomic catalysts with high durability. Structural stability, indicating whether the surface atomic structure is thermodynamically stable, should be carefully considered. Typically, metal atoms are immobilized on a highly defective support, stabilizing both the metal atom and the support. The surface metal atoms might become destabilized upon the adsorption of chemical intermediates. This transient behavior should be carefully monitored; density functional theory (DFT) calculations are particularly useful in estimating this stability. Aside from structural stability, the catalyst performance can be degraded significantly by poisoning with impurities. If the single-atom sites are susceptible to impurities with stronger adsorption, the surface reaction would not occur efficiently, leading to a decrease in activity without structure degradation. A long-term durability test should be performed for target reactions. Heterogeneous atomic catalysts have been used for various electrochemical, photocatalytic, and thermal reactions. Although electricity, light, and heat are just different forms of energy, the specific conditions which the catalyst should satisfy are different. Whereas precious metal atoms are mostly used as surface-active sites, the properties of the support are different depending on the type of reaction. For example, the support should have high conductivity for electrochemical reactions, it should be able to absorb light for photocatalytic reactions, and it should be durable at high temperature in the presence of steam for thermal reactions. Highly durable heterogeneous atomic catalysts are certainly possible with a great potential for practical applications. These new catalysts can accelerate the current paradigm shift toward more sustainable chemical production.