Thermodynamic Prediction of Gas-Solid Reactions for the Synthesis of Transition Metal Carbides Catalysts

Transition metal carbides have garnered significant attention due to their extensively adjustable physicochemical characteristics based on their structure, phase, and polymorph, making them highly versatile in a broad spectrum of electrochemical applications owing to their unique d-band electronic structures. Nevertheless, the synthesis of metal carbides and the precise control of their phases and structures present formidable challenges. In comparison to other metal compounds like oxides, sulfides, and phosphides, metal carbides exhibit the highest Gibbs free energy of formation (ΔG^o), indicating the lowest inclination for synthesis. Additionally, the formation of metal carbides requires stringent processing conditions due to the preferential oxidation of metals in the presence of atmospheric oxygen. Typically, metal carbides are generated through carburization and carbothermic reduction using methane and graphite. However, the sluggish kinetics of carbon (C)-induced reactions demand elevated temperatures, leading to undesired sintering and aggregation. Moreover, an excessive supply of C sources inevitably results in coke encapsulation and precipitation.<br/>In this work, we describe an approach for the predictive synthesis of nanostructured Mo carbides, achieved by steering the equilibrium reactions involving oxocarbons (carbon monoxide (CO) and carbon dioxide (CO₂)) during the calcination process. Oxocarbons engage in two equilibrium reactions: the Boudouard reaction (2CO (g) ↔ CO₂ (g) + C (s)) and the CO/CO₂ redox reaction (2CO₂ (g) ↔ 2CO (g) + O₂ (g)). This phenomenon allows for concurrent C–O reactions (selective C combustion) and Mo–C interactions (Mo carbide formation) within the nanofibers, serving as a framework for the initiation and growth of Mo carbides. This process enables precise manipulation of the phases and structures of Mo compounds. Since inducing selective redox reactions in multi-atomic systems like Mo–C–O poses significant challenges, requiring considerable trial and error in experimentally determining process variables. Hence, the specific prediction of processing parameters becomes imperative.<br/>We successfully established processing windows for nanoscale carbide synthesis by thermodynamically forecasting oxocarbon-mediated gas-solid reactions. By computing ΔG^o for Mo compound formation, we present a method for discovering specific processing windows to control the phases of diverse Mo compounds. Concurrently, the extent of selective C combustion is anticipated through pO₂ calculations to regulate the porosity of C nanofibers and diffusion behavior of Mo. By utilizing the thermodynamically predicted processing window, we not only controlled a wide range of Mo phases (MoO₂, α-MoC_1-x, and β-Mo₂C) but also manipulated various nanostructures (nanoparticle, spike, stain, and core–shell) by steering diffusion during the phase synthesis process in the Mo compounds/C nanofibers.<br/>Moreover, to confirm the connections between material and properties in nanostructured Mo carbides, we examine their catalytic performance in the hydrogen evolution reaction (HER). Our results demonstrate a performance achievement, with an overpotential of 152 mV @ 10 mA cm^-1, accomplished through the formation of active nano-spikes of Mo₂C on C nanofibers. We expect that the predictions of element supply and spontaneous reactions will offer valuable insights to researchers involved in the synthesis and structural manipulation of diverse catalysts based on transition metal compounds.