Development of reactive force field for Pd–Zn system and kinetic study of diffusion-driven alloy formation

PdZn alloys are catalysts for many reactions, and the 1:1 tetragonal PdZn intermetallic compound has been identified as the active phase for methanol steam reforming (MSR), a crucial process in hydrogen production. Yet, the mechanism and kinetics of the formation of PdZn alloys have not been addressed theoretically. In this work, reactive force field (ReaxFF) parameters for the Pd-Zn system were developed using density functional theory (DFT) training sets generated with the aid of structure prediction and DFT calculations, and the alloying process of Pd and Zn metals was investigated by molecular dynamics (MD) simulations. MD simulations of Pd@Zn and Zn@Pd core-shell models revealed a two-stage, diffusion-controlled alloying process with distinct diffusion coefficients and activation energies for Pd and Zn atoms, and a kinetic model was proposed for the fast alloying stage. It is found that with the advance of alloying, the alloying activation energy of the rapid alloying stage increases, and it is also proportional to the cube root of the total atom number of the system. Simulations of a Zn/Pd(111) slab model further reproduced the experimentally observed temperature dependence of the surface alloy structure: a multilayer PdZn alloy at lower temperatures and a bilayer alloy at higher temperatures. The results provide valuable insights into the fundamental mechanisms of PdZn alloy formation and offer a robust foundation for further research in catalytic applications.