Topological Chemistry of PtPb4 Surfaces under Reconstruction and Hydrogen Adsorption

Topological materials have recently been proposed as promising catalysts owing to the presence of robust topological surface states (TSSs). However, the microscopic role of TSSs in catalytic processes remains elusive. In particular, it is unclear how TSSs can simultaneously remain robust against chemical perturbations while actively participating in surface reactions. Here, we address this question through a combined study of the bulk topology, surface structure, and hydrogen evolution reaction (HER) behavior of PtPb4, a valence analogue of a known topological catalyst PtSn4. We show that PtPb4 is a topological semimetal with non-trivial ℤ2 indices under spin-orbit coupling. This nontrivial band topology originates from a band inversion between two Pb-derived states near the Fermi level. Surface calculations reveal well-defined TSSs on the (100) surface, a surface that also exhibits potential HER properties. Hydrogen binding is primarily achieved by the deep H-1s/Pt-d bonding state located well below the Fermi level, while the TSSs play only a secondary, perturbative role in adsorption. By continuously tracking the evolution of the surface bands during adsorption, we demonstrate that the topological order is preserved despite substantial energy shifts, charge transfer, and hybridization in surface states. Our work provides a qualitative picture of how the TSSs of PtPb4 respond to hydrogen adsorption while the global topology of the surface remains inherently robust.