The active (110) surface of the benchmark oxygen evolution catalyst RuO2 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 H2, O2, 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 Obr pz, the FBSS disperses along ⟨001⟩ oriented chains of bridging Rubr 4dz2 orbitals, collapses upon Obr removal, yet remains surprisingly unaffected by the oxidation state of the undercoordinated 1f-cus-Ru species. This directly reflects in the ability of RuO2(110) to oxidize CO and H2 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 reactantsa route toward a microscopic understanding of heterogeneous catalysis
