Carbon nanohorns (CNHs) are conical nanostructures composed of sp²-bonded carbon, resembling single-walled carbon nanotubes but with a distinctive horn-like morphology. Their unique structural and electronic properties make them promising candidates for catalytic applications, either as standalone catalysts or as supports for metal nanoparticles. Unlike other carbon nanomaterials, CNHs exhibit high surface area, abundant edge sites, and intrinsic defects, which contribute to their catalytic activity. This article explores their role in key reactions such as oxidation, hydrogenation, and electrocatalysis, highlighting their mechanisms and advantages.

The structure of CNHs consists of aggregated conical tubes with diameters ranging from 2 to 5 nm and lengths up to 50 nm. These assemblies form spherical aggregates with diameters between 50 and 100 nm, creating a highly porous network. The presence of pentagonal and heptagonal carbon rings at the cone tips introduces curvature and strain, leading to localized electronic states that enhance catalytic activity. Additionally, the open ends and sidewalls of CNHs provide accessible sites for reactant adsorption and metal nanoparticle anchoring.

In oxidation reactions, CNHs serve as effective catalysts or supports due to their ability to activate oxygen molecules. For instance, CNHs doped with nitrogen or sulfur exhibit improved activity for the oxidation of organic compounds such as alcohols and alkenes. The defect sites and heteroatom incorporation create electron-deficient regions that facilitate the transfer of electrons to oxygen, generating reactive oxygen species. When used as supports for platinum or palladium nanoparticles, CNHs enhance the dispersion and stability of the metal particles, preventing aggregation during reactions. The metal-CNH interface further promotes oxygen activation, leading to higher turnover frequencies compared to conventional carbon supports.

Hydrogenation reactions benefit from the synergistic effects between CNHs and metal nanoparticles. The curved geometry of CNHs induces strain in supported metal clusters, altering their electronic structure and improving their affinity for hydrogen adsorption. For example, ruthenium nanoparticles supported on CNHs show higher activity for the hydrogenation of carbonyl compounds compared to those on flat graphene or activated carbon. The confinement effect within CNH aggregates also restricts reactant diffusion, increasing the residence time and selectivity for desired products. Furthermore, the hydrophobic nature of CNHs minimizes competitive adsorption of water in aqueous-phase hydrogenation, enhancing catalytic efficiency.

Electrocatalysis is another area where CNHs excel, particularly in oxygen reduction reactions (ORR) and hydrogen evolution reactions (HER). The intrinsic defects and edge sites of CNHs act as active centers for ORR, reducing the overpotential required for oxygen reduction. Nitrogen-doped CNHs demonstrate comparable activity to platinum-based catalysts in alkaline media, with the pyridinic nitrogen sites facilitating the four-electron transfer pathway. In HER, CNHs loaded with molybdenum disulfide or cobalt phosphide exhibit low overpotentials and high stability, attributed to the efficient charge transfer between the catalyst and the conductive CNH framework. The mesoporous structure of CNH aggregates ensures rapid mass transport of reactants and products, critical for high-current-density applications.

The catalytic mechanisms involving CNHs often revolve around their ability to modulate electron density and stabilize transition states. In oxidation reactions, the electron-rich regions near defects donate electrons to adsorbed oxygen, forming superoxide or peroxide intermediates. For hydrogenation, the spillover of hydrogen atoms from metal nanoparticles to the CNH surface creates active hydrogen species that react with unsaturated bonds. In electrocatalysis, the interplay between CNH conductivity and catalyst redox properties ensures efficient electron transfer during reaction cycles.

Compared to other carbon nanomaterials, CNHs offer distinct advantages such as higher defect density, better metal-support interactions, and superior thermal stability. Their aggregated form provides mechanical robustness, making them suitable for industrial-scale applications. However, challenges remain in controlling the uniformity of CNH aggregates and optimizing their surface chemistry for specific reactions. Future research may focus on tailoring CNH morphology and composition to further enhance their catalytic performance.

In summary, carbon nanohorns represent a versatile platform for catalysis, leveraging their unique structural features to drive oxidation, hydrogenation, and electrocatalytic processes. Their role as supports for metal nanoparticles amplifies catalytic activity while maintaining stability under harsh conditions. As understanding of their structure-property relationships deepens, CNHs are poised to play a pivotal role in advancing sustainable catalytic technologies.