Thermal Stability of Single-Crystalline IrO 2 (110) Layers: Spectroscopic and Adsorption Studies Marcel J. S. Abb, Tim Weber, Daniel Langsdorf, Volkmar Koller, Sabrina M. Gericke, Sebastian Pfaff, Michael Busch, Johan Zetterberg, Alexei Preobrajenski, Henrik Grö nbeck, Edvin Lundgren, and Herbert Over* Cite This: J. Phys. Chem. C 2020, 124, 15324-15336 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: The interaction of ultrathin single-crystalline IrO 2 (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 IrO 2 (110). We provide spectroscopic fingerprints of the active surface sites of IrO 2 (110). The freshly prepared IrO 2 (110) surface is virtually inactive toward gas-phase molecules. The IrO 2 (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 IrO 2 (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 IrO 2 (110) surface. Single-crystalline IrO 2 (110) is thermally not very stable although chemically stable. Chemical reduction of IrO 2 (110) by extensive CO exposure at 473 K is not observed, which is in contrast to the prototypical RuO 2 (110) system. 1. INTRODUCTION In chemical terms, iridium (Ir) and ruthenium (Ru) behave in many aspects quite similarly; 1,2 for instance, both form metallic conducting oxides with a rutile structure, RuO 2 and IrO 2 . 3 However, there are also distinct differences, for instance, in the oxidation behavior of Ir and Ru. While the closed packed surface of the Ru(0001) single crystal is quite easily oxidized by molecular oxygen at elevated temperatures 4 to form flat conforming RuO 2 (110) layers, Ir(111) is much less prone to getting oxidized. 5,6 The metallic oxides RuO 2 and IrO 2 are both intensively employed in electrocatalysis as active anode materials for the chlorine and oxygen evolution reaction (OER). 2 Under strongly anodic conditions, IrO 2 has shown to be less active but much more stable than RuO 2 . One would expect that RuO 2 and IrO 2 are efficient oxidation catalysts in heteroge- neous gas-phase catalysis as well. Indeed, RuO 2 is an excellent catalyst for the oxidation of CO, 7 while corresponding studies for IrO 2 are missing. However, both RuO 2 and IrO 2 are reported to be active catalysts for HCl oxidation (Deacon process), 8-12 albeit RuO 2 has been shown to be more efficient than IrO 2 . Recently, oxidized Ir(100) was found to be surprisingly active in the low-temperature activation of methane, 13 while methane activation is not observed for RuO 2 . 14 The active phase of oxidized Ir(100) has been assigned to an IrO 2 (110) layer as previously predicted by Wang et al. on the basis of density functional theory (DFT) calculations. 15 Electrochemically single-crystalline IrO 2 (110) films are shown to be extraordinarily stable. In the OER potential region, IrO 2 (110) films 16 are more stable compared to RuO 2 (110). 17 Under cathodic polarization conditions (HER potential region), RuO 2 (110) is easily reduced, 18 while IrO 2 seems to be more reluctant toward hydrogen insertion and electrochemical reduction. 19 However, the thermal stability of bulk IrO 2 is significantly lower than that of bulk RuO 2 : the decomposition temperature under UHV conditions is 800 K (IrO 2 ) versus 1000 K (RuO 2 ). 6,20 Upon annealing to 500-600 K under UHV conditions, single-crystalline IrO 2 (110) layers start reducing on the surface. 21-23 This problem with thermal stability makes the preparation of IrO 2 (110) films challenging and requires gradient cooling of the samples in the oxygen atmosphere after initial oxide formation at 700 K, to suppress thermal reduction. 23 Quite in contrast, RuO 2 (110) films can be Received: May 15, 2020 Revised: June 18, 2020 Published: June 22, 2020 Article pubs.acs.org/JPCC © 2020 American Chemical Society 15324 https://dx.doi.org/10.1021/acs.jpcc.0c04373 J. Phys. Chem. C 2020, 124, 15324-15336 Downloaded via CENTRAL MICHIGAN UNIV on August 17, 2020 at 08:21:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.