(564b) Nanoparticle Size Effects on Phase Stability for Molybdenum and Tungsten Carbides

Transition metal carbides (TMC) have catalytic properties similar to Pt-group metals and are important materials for a variety of catalytic applications (hydrodenitrogenation, Fischer-Tropsch synthesis, hydrocarbon isomerization) and industrial processes, in part due to their variable crystal structures and surfaces. However, synthesis of TMCs often proceeds through metastable phases during particle growth, the appearance of which cannot be described by traditional phase diagrams. Several emerging syntheses have demonstrated precise control over TMC nanoparticle size, however the effects of particle size on the crystallization pathway followed during synthesis and the resulting product phase are largely unknown.

At small particle sizes, nanoparticles have large surface-area-to-volume ratios, causing contributions from the surface energy to dominate over bulk energy. As a result, high bulk energy metastable phases with low surface energy can be metastable at small nanoparticle sizes. Hence, combining the influence of surface energies with bulk energies is crucial for predicting phase-selection. Here, we use Density Functional Theory calculations, neural-network assisted calculations of surface energies, and thermodynamic analyses to construct particle size-dependent phase diagrams for Mo- and W-carbides and reveal the relationships between phase stability at different synthesis conditions (composition and temperature) and TMC nanoparticle size.

We compute size-dependent phase diagrams for a wide range of Mo- and W-carbide phases, determine predicted crystallization pathways during synthesis, and compare model results with a large body of available experimental data. We find that reported particle sizes for different phases are generally consistent with the trends predicted by our model, which suggests that particle size is an important factor when determining the product phase resulting from a large variety of synthesis protocols. Our results yield predictive insights for the influence of nanoparticle size on TMC nucleation and growth during synthesis and provide a computationally guided road map for navigating the synthesis of target TMC surfaces and phases.