Design principles of heteroepitaxial bimetallic catalysts

Heteroepitaxial bimetallic catalytic materials have been attracting considerable interest because of their unique properties and applications in energy, chemicals, and fuel sectors. Optimizing the activity of bimetallic catalysts requires the ability to understand and precisely manipulate the nanostructure (loading and dispersion of an admetal on a host metal). Ammonia decomposition catalyzed by Ni on Pt(111) [Ni/Pt(111)] is a prototype structure sensitive reaction for this purpose, in which both N-H bond breaking and N-N bond formation can be rate-limiting steps. Here, a first-principles kinetic Monte Carlo simulation framework reveals the bifunctional role of Ni clusters on Pt(111) in NH₃ decomposition: dehydrogenation on Ni terrace and N₂ desorption near Ni steps on Pt. We show that the dynamic interplay between surface nitrogen diffusion and association near the steps of Ni clusters and Pt terraces is responsible for the lower than expected nitrogen desorption temperature found in experiments. Our results illustrate the significance of metal loading and microstructure on macroscopic performance. A model is introduced with the aim of optimizing the bimetallic surface at nanoscale for improved reactivity. This simulation framework can be employed to understand and eventually tune bimetallic catalytic materials for arbitrarily complex chemistries.