Understanding the energetics of disordered transition metal carbides

Transition metal caribdes are an important class of materials that are of a particular interest for catalysis both as catalysts and support materials. Mo2C is a good support material for fuel cell ORR catalysts due to its durability and high electrical conductivity. Recent work has shown that ultra-low Pt loading in the form of Pt nanorafts can be obtained on Mo2C support due to the disordered nature of the C arrangement in Mo2C. Here, we used first-principles calculations to study the energetics of different C arrangements in disordered Mo2C. Despite the complex nature of bonding in Mo2C with contributions from metal-carbon, metal-metal and carbon-carbon interactiosn, we find that the energies of Mo2C arrangements can be represented by a simple model that accurately reproduces the density functional theory results. The model enables the study of very large cells of Mo2C and a prediction of realistic structure for the catalytically important Mo2C surface. This approach is then extended to the modeling of C arrangement energetics in non-stoichiometric titanium carbide.