Predictive Synthesis of Transition Metal Carbides Catalysts by Thermochemical CO-CO₂ Equilibrium

Transition metal carbides (TMCs) have attracted considerable interest because their structure, phase, and polymorph provide highly tunable physicochemical properties. Their unique d-band electronic structures make them suitable for a wide range of electrochemical applications. For example, molybdenum (Mo) carbides are promising candidates for electrocatalysis. Interstitial C in Mo carbides interacts with Mo orbitals and induces lattice distortion, thereby modifying their d-band electronic structures to resemble those of noble metals, resulting in outstanding catalytic activities.<br/>Despite extensive efforts to synthesize nanoscale transition metal carbides, the process remains a significant challenge. Among various transition metal compounds, such as oxides, nitrides, sulfides, and phosphides, metal carbides require the highest formation energy, thus exhibiting the lowest tendency for synthesis. In general, metal carbides are fabricated by carburization with gaseous carbon sources (e.g. methane (CH₄)) or by carbothermic reduction with graphite, inevitably leading to an excessive carbon supply during carbide formation. This can induce the precipitation of graphitic carbon, limiting the processing window for the synthesis and structural control of carbides. Therefore, it is necessary to develop techniques that can synthesize transition metal carbides with various phases and structures in nanoscale.<br/>In this study, we developed a technique for synthesizing various nanostructured transition metal carbides. By introducing a CO-CO₂ gas mixture and utilizing the Boudouard reaction (2CO (g) ↔ CO₂ (g) + C (s)), we supplied carbon to the metal to synthesize carbides. The process was designed by thermodynamically predicting the CO/CO₂ ratio and processing temperature, enabling the synthesis of various transition metal carbides such as molybdenum carbides (α-MoC_1-x, β-Mo₂C), tungsten carbides (W₂C, WC), and a niobium carbide (NbC_1-x). Additionally, we controlled metal diffusion during the carbide synthesis process to form various surface and internal nanostructures. We used carbon nanofibers as templates to control metal diffusion. We controlled the oxygen partial pressure (pO₂) by predicting the CO/CO₂ redox reaction (2CO₂ (g) ↔ 2CO (g) + O₂ (g)), which is an equilibrium reaction of CO-CO₂. By controlling the pO₂ during the calcination, we selectively induced the combustion of carbon around the metal, thereby regulating the diffusion of the metal. We induced diffusion to the surface during the carbide formation process through stress-relaxation diffusion, resulting in the formation of various surface structures (nanospike, stain, particle, nanoflake). Furthermore, we elucidated the formation mechanisms of the nanostructured carbides, enabling us to control the size, aspect ratio, and amount of nanospikes and nanoflakes. Additionally, by creating pores, we were able to diffuse the metal inward through CO-CO₂ calcination, forming core-shell carbide/C structures.<br/>We analyzed the hydrogen evolution reaction (HER) catalytic performance of nanostructured molybdenum carbides/C nanofibers. The nanospike-Mo₂C/C nanofiber achieved excellent HER performance with 142.6 mV at 10 mA cm^-2 and 186.0 mV at 50 mA cm^-2.<br/>We hope that our predictive synthesis will provide crucial guidance to researchers involved in the synthesis and structural engineering of a wide range of catalysts based on transition metal compounds.