Catalytic activity of PtSn₄: Insights from surface-science spectroscopies

Bulk PtSn₄ has recently attracted the interest of the scientific community for the presence of electronic states exhibiting Dirac node arcs, enabling possible applications in nanoelectronics. Here, by means of surface-science experiments and density functional theory, we assess its suitability for catalysis by studying the chemical reactivity of the (0 1 0)-oriented PtSn₄ surface toward CO, H₂O, O₂ molecules at room temperature and, moreover, its stability in air. We demonstrate that the catalytic activity of PtSn₄ is determined by the composition of the outermost atomic layer. Specifically, we find that the surface termination for PtSn₄ crystals cleaved in vacuum is an atomic Sn layer, which is totally free from any CO poisoning. In oxygen-rich environment, as well as in ambient atmosphere, the surface termination is a SnO_x skin including SnO and SnO₂ in comparable amount. However, valence-band states, including those forming Dirac node arcs, are only slightly affected by surface modifications. The astonishingly beneficial influence of surface oxidation on catalytic activity has been demonstrated by electrocatalytic tests evidencing a reduction of the Tafel slope, from 442 down to 86 mV dec⁻¹, whose origin has been explained by our theoretical model. The use of surface-science tools to tune the chemical reactivity of PtSn₄ opens the way toward its effective use in catalysis, especially for hydrogen evolution reaction and oxygen evolution reaction.