Abstract
The active (110) surface of the benchmark oxygen evolution catalyst RuO 2 spans a flat-band surface state (FBSS) between the surface projections of its Dirac nodal lines (DNLs) that define the electronic properties of this functional semimetal. Monitoring well-known surface adsorption processes of H 2, O 2, NO, and CO by in operando angle-resolved photoemission spectroscopy, we selectively modify the oxidation state of individual Ru surface sites and identify the electronic nature of the FBSS: stabilized by bridging oxygen O br p z, the FBSS disperses along ⟨001⟩ oriented chains of bridging Ru br 4d z 2 orbitals, collapses upon O br removal, yet remains surprisingly unaffected by the oxidation state of the undercoordinated 1f-cus-Ru species. This directly reflects in the ability of RuO 2 (110) to oxidize CO and H 2 along with its inability to oxidize NO, demonstrating the FBSS’s active role in catalytic charge transfer processes at the oxygen bridge sites. Our synergetic approach provides momentum-resolved insights to the interplay of a catalyst’s delocalized electronic band structure and the localized orbitals of its surface reactants—a route toward a microscopic understanding of heterogeneous catalysis.