High-resolution X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) were used to characterize IrO2(110) films on Ir(100) with stoichiometric as well as OH-rich terminations. Core-level Ir 4f and O 1s peaks were identified for the undercoordinated Ir and O atoms and bridging and on-top OH groups at the IrO2(110) surfaces. Peak assignments were validated by comparison of the core-level shifts determined experimentally with those computed using DFT, quantitative analysis of the concentrations of surface species, and the measured variation of the Ir 4f peak intensities with photoelectron kinetic energy. We show that exposure of the IrO2(110) surface to O2 near room temperature produces a large quantity of on-top OH groups because of reaction of background H2 with the surface. The peak assignments made in this study can serve as a foundation for future experiments designed to utilize XPS to uncover atomic-level details of the surface chemistry of IrO2(110).

Continuous exposure to methane causes IrO 2 (110) films on Ir(100) to undergo extensive reduction at temperatures from 500 to 650 K. Measurements using in situ X-ray photoelectron spectroscopy (XPS) confirm that CH 4 oxidation on IrO 2 (110) converts so-called bridging oxygen atoms (O br ) at the surface to HO br groups while concurrently removing oxygen from the oxide film. Reduction of the IrO 2 (110) film by methane is mildly activated as evidenced by an increase in the initial reduction rate as the temperature is increased from 500 to 650 K. The XPS results show that subsurface oxygen efficiently replaces O br atoms at the IrO 2 (110) surface during CH 4 oxidation, even after the reduction of multiple layers of the oxide film, and that metallic Ir gradually forms at the surface as well. The isothermal rate of IrO 2 (110) reduction by methane decreases continuously as metallic Ir replaces surface IrO 2 (110) domains, demonstrating that IrO 2 (110) is the active phase for CH 4 oxidation under the conditions studied. A key finding is that the replacement of O br atoms with oxygen from the subsurface is efficient enough to preserve IrO 2 (110) domains at the surface and enable CH 4 to reduce the ∼10-layer IrO 2 (110) films nearly to completion. In agreement with these observations, density functional theory calculations predict that oxygen atoms in the subsurface layer can replace O br atoms at rates that are comparable to or higher than the rates at which O br atoms are abstracted during CH 4 oxidation. The efficacy with which oxygen in the bulk reservoir replenishes surface oxygen atoms has implications for understanding and modeling catalytic oxidation processes promoted by IrO 2 (110).

We have combined three techniques, High Energy Surface X-Ray Diffraction (HESXRD), Surface Optical Reflectance, and Planar Laser Induced Fluorescence in an operando study of CO oxidation over a Pd(100) catalyst. We show that these techniques provide useful new insights such as the ability to verify that the finite region being probed by techniques such as HESXRD is representative of the sample surface as a whole. The combination is also suitable to determine when changes in gas composition or surface structure and/or morphology occur and to subsequently correlate them with high temporal resolution. In the study, we confirm previous results which show that the Pd(100) surface reaches high activity before an oxide can be detected. Furthermore, we show that the single crystal catalyst surface does not behave homogeneously, which we attribute to the surface being exposed to inhomogeneous gas conditions in mass transfer limited scenarios.

Using grazing incidence X-rays and X-ray photoelectron spectroscopy during the mass transfer limited catalytic oxidation of CO, the long-range surface structure of Pd(100) was investigated. Under the reaction conditions of 50:4 O 2 to CO, 300 mbar pressure, and temperatures between 200 and 450 °C, the surface structure resulting from oxidation and the subsequent oxide reduction was elucidated. The reduction cycle was halted, and while under reaction conditions, angle-dependent X-ray photoelectron spectroscopy close to the critical angle of Pd and modeling of the data was performed. Two proposed models for the system were compared. The suggestion with the metallic islands formed on top of the oxide island was shown to be consistent with the data.

High energy surface x-ray diffraction (HESXRD), x-ray reflectivity (XRR), mass spectrometry (MS) and surface optical reflectance (SOR) have been combined to simultaneously obtain sub-second information on the surface structure and morphology from a Pd(100) model catalyst during in situ oxidation at elevated temperatures and pressures resulting in Pd bulk oxide formation. The results show a strong correlation between the HESXRD and SOR signal intensities during the experiment, enabling phase determination and a time-resolved thickness estimation of the oxide by HESXRD, complemented by XRR measurements. The experiments show a remarkable sensitivity of the SOR to changes in the surface phase and morphology, in particular to the initial stages of oxidation/reduction. The data imply that SOR can detect the formation of an ultrathin PdO surface oxide layer of only 2–3 Å thickness.