Understanding Stability of the Transition Metal Carbides and Nitrides By Combining Experimental and Computed Thermochemical Properties

Abstract

Thermochemistry plays a key role in determining durability of the catalysts and catalytic supports, but is also critical in guiding the rational design of synthesis processes that lead to materials with advanced properties [1, 2]. In the last decade computations based on density functional theory have been used as an important tool in the calculation of thermodynamic properties, such as reaction enthalpies, free energies, and heat capacities. In this work we combine experimental thermochemical data available in the HSC Chemistry software and the data obtained using density functional theory calculations to study the stability of various transition metal carbides and nitrides in aqueous media as a function of pH, cell potential, and temperature. Earth abundant transition metal carbides and nitrides have drawn considerable interest due to their low cost, unique chemical and physical properties, and high catalytic activity for variety of reactions, such as nitrogen electroreduction to ammonia, ammonia decomposition, alcohol electrooxidation, and biomass conversion. We will discuss the thermodynamics directed towards understanding oxidation of transition metal carbides and nitrides at different cell potentials, pH and temperatures. Namely, oxidation of the material will lead to the change in the geometric and electronic structure of the material, which can cause degradation and decrease in the catalytic activity. More specifically, transition metal carbides and nitrides based on molybdenum, iron, vanadium, and nickel will be considered to illustrate the importance of mapping out their stable phase as a function of pH, potential, and temperature. These diagrams can then be used to determine the stability of the materials in the aqueous media and guide the reaction conditions that will ensure highest durability for a targeted application. [1] S. K. Nayak, A. D. Benavidez, I. Matanovic, and F. H. Garzon, Thermochemical analysis of Mo-C-H systems for synthesis of molybdenum carbides, Thermochim. Acta, 676, 27-32 (2019). [2] S. K. Nayak, A. D. Benavidez, F. H. Garzon, Facile synthesis of high surface area molybdenum carbide nanoparticles, J. Am. Ceram. Soc., 00: 1– 7 (2018).