Thermal Stability of Single-Crystalline IrO2(110) Layers: Spectroscopic and Adsorption Studies

The interaction of ultrathin single-crystalline IrO2(110) films with the gas phase proceeds via the coordinatively unsaturated sites (cus), in particular Ir-cus the undercoordinated oxygen species on-top O (O-ot) that are coordinated to Ir-cus, and bridging O (O-br). With the combination of different experimental techniques, such as thermal desorption spectroscopy, scanning tunneling microscopy (STM), high-resolution core-level spectroscopy (HRCLS), infrared spectroscopy, and first-principles studies employing density functional theory calculations, we are able to elucidate surface properties of single-crystalline IrO2(110). We provide spectroscopic fingerprints of the active surface sites of IrO2(110). The freshly prepared IrO2(110) surface is virtually inactive toward gas-phase molecules. The IrO2(110) surface needs to be activated by annealing to 500-600 K under ultrahigh vacuum (UHV) conditions. In the activation step, Ir cus sites are liberated from on-top oxygen (O-ot) and monoatomic Ir metal islands are formed on the surface, leading to the formation of a bifunctional model catalyst. Vacant Ir-cus sites of IrO2(110) allow for strong interaction and accommodation of molecules from the gas phase. For instance, CO can adsorb atop on Ir-cus and water forms a strongly bound water layer on the activated IrO2(110) surface. Single-crystalline IrO2(110) is thermally not very stable although chemically stable. Chemical reduction of IrO2(110) by extensive CO exposure at 473 K is not observed, which is in contrast to the prototypical RuO2(110) system.