Catalytic Oxidation of Methane on IrO₂(110) Films Investigated Using Ambient-Pressure X-ray Photoelectron Spectroscopy

Abstract

The catalytic oxidation of CH4 over IrO2(110) films grown on Ir(100) was investigated using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at total pressures near 1 Torr. The IrO2(110) films undergo negligible reduction during catalytic CH4 oxidation in reactant mixtures with as much as 95% CH4 and temperatures from ca. 500–650 K, demonstrating that IrO2(110) can catalyze the oxidation of CH4 over a wide range of temperatures and mixture compositions. High coverages of OH groups and oxidized C-containing species formed on the IrO2(110) surfaces during CH4 oxidation, including excess OH groups bound directly to the initially, coordinatively unsaturated Ir atoms. The formation of excess OH groups demonstrates that O-rich IrO2(110) surfaces were maintained even under highly CH4-rich conditions and provides evidence that the dissociative adsorption of O2 is more facile than CH4 activation and conversion to adsorbed intermediates on IrO2(110). Extensively oxidized surface species with a CH y O2 stoichiometry preferentially formed under all reaction conditions studied. The conversion of CH4 to the CH y O2 surface species became optimal at an intermediate composition of the reactant mixture (∼90% CH4), consistent with a site competition between CH4 and O2 during their initial adsorption as well as a high oxidation activity of chemisorbed O atoms on IrO2(110). These results provide quantitative information about the identities and coverages of adsorbed species that form during the catalytic oxidation of CH4 on IrO2(110). Such knowledge is essential for validating first-principles models of the reaction kinetics for this system and ultimately gaining insights needed to optimize the performance of IrO2 catalysts for the oxidation of light alkanes.

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

“…Surface Ir atoms of adsorbate-free regions of Ir(100) produce a peak at 60.3 eV 22,27 which becomes more evident in the Ir 4f spectrum acquired at 650 K. These spectra resemble that obtained from a reference Ir(100) surface covered with chemisorbed O atoms (Figure S6). For comparison, the Ir 4f 17 Representative O 1s and C 1s spectra acquired from IrO 2 (110) during CH 4 oxidation are shown in Figure 3, mainly for comparison with similar results obtained from Ir(100) (shown below). The O 1s spectra collected from IrO 2 (110) exhibit prominent peaks from surface HO br , HO ot , and CO x species (Figure 3a), with the O 1s peak from CO x surface species decreasing as the temperature is increased from 500 to 650 K while that for HO ot groups concurrently increases.…”

“…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.…”

“…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.…”

“…13 Consistent with DFT calculations, 14,15 the IrO 2 (110) surface efficiently promotes CH 4 C−H cleavage at temperatures as low as 100 K, with CO 2 , H 2 O, and CO evolving as oxidation products above ∼400 K during temperature-programmed reaction spectroscopy. 16 AP-XPS measurements provide evidence that IrO 2 (110) films are also active for the catalytic combustion of methane at moderate temperatures (∼500−650 K), 17 though quantification of the catalytic activity has not yet been reported. Analogous to IrO 2 (110), metallic Ir exhibits the highest CH 4 C−H activity among all single-crystal metal surfaces that have been investigated, 10,18,19 suggesting that Ir catalysts could be effective for methane combustion.…”

The remarkable performance of a single iridium atom supported on hematite for methane activation: a density functional theory study

“…74,75 But methane is completely oxidized to CO 2 and H 2 O. 76,77 In this study, we only considered a single metal doping and a single oxygen vacancy which is very active for methane adsorption. Cheng et al studied the adsorption and dissociation of methane on ferric oxide oxygen carriers and they found that the activation barrier of C-H decreased when the surface oxygen vacancy concentration was increased from 0%-2.67%.…”