No 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.

We have developed a microscope with a spatial resolution of 5 μm, which can be used to image the two-dimensional surface optical reflectance (2D-SOR) of polycrystalline samples in operando conditions. Within the field of surface science, operando tools that give information about the surface structure or chemistry of a sample under realistic experimental conditions have proven to be very valuable to understand the intrinsic reaction mechanisms in thermal catalysis, electrocatalysis, and corrosion science. To study heterogeneous surfaces in situ , the experimental technique must both have spatial resolution and be able to probe through gas or electrolyte. Traditional electron-based surface science techniques are difficult to use under high gas pressure conditions or in an electrolyte due to the short mean free path of electrons. Since it uses visible light, SOR can easily be used under high gas pressure conditions and in the presence of an electrolyte. In this work, we use SOR in combination with a light microscope to gain information about the surface under realistic experimental conditions. We demonstrate this by studying the different grains of three polycrystalline samples: Pd during CO oxidation, Au in electrocatalysis, and duplex stainless steel in corrosion. Optical light-based techniques such as SOR could prove to be a good alternative or addition to more complicated techniques in improving our understanding of complex polycrystalline surfaces with operando measurements.

The CO hydrogenation reaction over the Rh(111) and (211) surfaces has been investigated operando by X-ray photoelectron spectroscopy at a pressure of 150 mbar. Observations of the resting state of the catalyst give mechanistic insight into the selectivity of Rh for generating ethanol from CO hydrogenation. This study shows that the Rh(111) surface does not dissociate all CO molecules before hydrogenation of the O and C atoms, which allows methoxy and other both oxygenated and hydrogenated species to be visible in the photoelectron spectra.