Tunable Surface Cation Arrangements in Ir x Ru 1– x O 2 (110) and Their Impact on Adsorbate Binding

Advancing methods to control and characterize the local cation arrangements on mixed metal oxide surfaces can afford opportunities for designing catalysts with tunable chemical properties. Here, we show that Ir x Ru 1– x O 2 (110) solid solutions with variable near-surface cation arrangements can be synthesized by annealing IrO 2 /RuO 2 layered structures under mild oxidizing conditions in ultrahigh vacuum. Temperature-programmed desorption (TPD) of adsorbed N 2 and atomic O provides a quantitative probe of the Ir and Ru populations in coordinatively unsaturated (cus) binding sites (M cus ) and their nearest-neighbor subsurface sites (M subcus ), due to the strong, highly localized influence of these cations on N 2 and O binding. The resulting M cus /M subcus configurations remain stable up to at least 650 K, a temperature range relevant for catalytic studies. Heating IrO 2 /RuO 2 layered structures to ∼800 K induces rapid Ru–Ir intermixing and drives Ru enrichment in the near-surface region. TPD measurements show that Ru strongly favors incorporation into cus-sites during our heating protocol and begins to occupy subcus-sites only after the Ru cus population approaches saturation. After extended heating, the surface is dominated by Ru cus over Ru subcus motifs (Ru/Ru 2 ) characteristic of pure RuO 2 (110), with small populations of Ru/Ir 2 and Ru/IrRu. Density functional theory (DFT) predicts binding-site distributions for O-terminated surfaces that closely match those determined from TPD. DFT reveals that enhanced O binding on Ru cus relative to Ir cus, along with stabilization by neighboring Ir subcus atoms, drives Ir–Ru interchange to selectively form Ru/Ir 2 motifs and promote Ru occupation of subcus sites after cus sites are filled. Together, these results provide insight into the synthesis and characterization of mixed Ir x Ru 1– x O 2 (110) surfaces and demonstrate that controlling near-surface cation arrangements offers a promising route to tuning the chemical properties of mixed-oxide catalysts.