Isothermal Reduction of IrO₂(110) Films by Methane Investigated Using In Situ X-ray Photoelectron Spectroscopy

Abstract

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).

Activity of Ir(100) and IrO2(110) for the Catalytic Oxidation of Methane

“…Photon energies of 720 and 490 eV were selected to generate photoelectrons with ∼190 eV kinetic energy for the O 1s and C 1s spectra, respectively, while Ir 4f spectra were collected at a photon energy of 720 eV. Peak deconvolution methods have been discussed in prior work 17,22,27 and are outlined in the Supporting Information.…”

“…Measurements using AP-XPS were performed at the National Synchrotron Light Source II at Brookhaven National Laboratory. Details of the experimental procedures have been reported previously and are summarized in the Supporting Information. ,, Briefly, Ir 4f, O 1s, and C 1s XPS spectra were acquired after stabilizing a gas mixture with 0.1 Torr of O 2 and 0.8 Torr of CH 4 at a sample temperature of ∼300 K and then heating in a stepwise manner from 500 to 650 K. Photon energies of 720 and 490 eV were selected to generate photoelectrons with ∼190 eV kinetic energy for the O 1s and C 1s spectra, respectively, while Ir 4f spectra were collected at a photon energy of 720 eV. Peak deconvolution methods have been discussed in prior work ,, and are outlined in the Supporting Information.…”

“…Details of the experimental procedures have been reported previously and are summarized in the Supporting Information. 17,22,27 Briefly, Ir 4f, O 1s, and C 1s XPS spectra were acquired after stabilizing a gas mixture with 0.1 Torr of O 2 and 0.8 Torr of CH 4 at a sample temperature of ∼300 K and then heating in a stepwise manner from 500 to 650 K.…”

Synergetic effect between Pd2+ and Ir4+ species promoting direct ethane dehydrogenation into ethylene over bimetallic PdIr/AC catalysts

“…The binding energies obtained for Pd 3d 5/2 were characteristic of Pd 0 (335.2 eV) and Pd 2+ (336.9 eV) in fresh Pd7Ir2/AC-B, and Ir 4+ species (Ir 4+ 4f 7/2 62.1 eV) and Ir metal (Ir 0 4f 7/2 60.8 eV) were both detected over fresh Pd7Ir2/AC-B. [50][51][52][53] Nevertheless, the Pd 0 signal was higher than the Pd 2+ signal on the surface of the fresh sample, and the Ir 0 signal was much weaker than that of Ir 4+ species. In addition, the Pd 3d and Ir 4f signals on spent Pd7Ir2/AC-B were greatly weakened or even vanished, which was induced by severe carbon deposition on the sample surface and is discussed later. )…”

Predicting X-ray Photoelectron Peak Shapes: the Effect of Electronic Structure

“…Analysis of peak intensities and binding energies can be used to quantitatively determine the concentration of species and the presence of different chemical environments; however, the accuracy of the fits depends crucially on fitting with an appropriate spectral line profile. , Asymmetry in background-corrected XPS is commonly observed due to the overlap of peaks from multiple chemical environments, − vibrational excitations, − and secondary electronic excitations . Accurately predicting XPS line shapes that arise from secondary electronic excitations for each material component is critical for data interpretation when the line shapes are species dependent or are distinct from bulk references, such as for adsorbates, thin films, and alloy materials. − Currently, a method does not exist that can predict differences in XPS line shapes, and instead different line shapes are justified by yielding better fits. The increased use of XPS to investigate dynamic materials systems has demonstrated that XPS line shapes can change during chemical and thermal treatments, which highlights the need to predict the effect of electronic structure on XPS peaks.…”