Kinetics of low-temperature methane activation on IrO2(1 1 0): Role of local surface hydroxide species

Kim, Minkyu; Franklin, A.D.; Martin, Rachel; Bian, Yingxue; Weaver, Jason F.; Asthagiri, Aravind

doi:10.1016/j.jcat.2020.01.027

Cited by 37 publications

(78 citation statements)

References 49 publications

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“…For comparison, prior DFT calculations predict barriers between 190 and 220 kJ/mol for gaseous H 2 O production from the recombination of HO br groups and barriers of about 220 and 165 kJ/mol for CO desorption and CO oxidation to gaseous CO 2 on IrO 2 (110), respectively. 7,9,18 The similarities in these intrinsic energy barriers predicted by DFT suggest that subsurface oxygen can replenish O br atoms at rates that are comparable to those for reactive O br abstraction during CH 4 oxidation on IrO 2 (110). Notably, the migration of a subsurface O atom into an isolated O br,v site is predicted to be endothermic by 48 kJ/mol so the reverse migration process (O br filling the subsurface vacancy) can also occur at high rates at reaction conditions.…”

Section: ■ Results and Discussionmentioning

confidence: 96%

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

et al. 2021

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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).

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Section: ■ Results and Discussionmentioning

confidence: 96%

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

et al. 2021

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“…Prior studies have shown that H 2 reacts facilely with IrO 2 (110) and that appreciable OH coverages are generated in several minutes during cooling (<∼400 K), even at total background pressures in the 10 −10 Torr range. 3,5,14,15,26 These studies show that HO ot groups are stable on IrO 2 (110) at temperatures near 300 K and that the rates at which these species recombinatively desorb as H 2 O become appreciable only above ∼425 K. Given that the base pressure was about 10 −8 Torr, it is reasonable to expect that the background H 2 exposure was sufficient to convert most of the O ot atoms to HO ot groups during the XPS measurements. Adsorption of H 2 O from the background could also generate a fraction of the HO ot groups identified with XPS.…”

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confidence: 99%

High-Resolution X-ray Photoelectron Spectroscopy of an IrO₂(110) Film on Ir(100)

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Kim

Lee

et al. 2020

J. Phys. Chem. Lett.

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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).

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“…In temperature programmed reaction (TPR) experiments the oxidized surface of Ir(100) was covered with methane (and oxygen) at temperatures below À 173°C and then the products were recorded with mass spectrometry while ramping the sample to 427-527°C. [16,17] The active phase has been assigned to a IrO 2 (110) layer, as previously predicted by Wang et al on the basis of density functional theory (DFT) calculations. [18] We should emphasize here that these TPR experiments are mere transient and not catalytic experiments; for catalytic experiments one needs to demonstrate steady state conversion under flow reaction conditions.…”

Section: Introductionmentioning

confidence: 72%

“…Recently, an oxidized Ir(100) single‐crystalline surface was reported to be surprisingly efficient in the low‐temperature methane activation. In temperature programmed reaction (TPR) experiments the oxidized surface of Ir(100) was covered with methane (and oxygen) at temperatures below −173 °C and then the products were recorded with mass spectrometry while ramping the sample to 427–527 °C [16,17] . The active phase has been assigned to a IrO 2 (110) layer, as previously predicted by Wang et al.…”

Section: Introductionmentioning

confidence: 90%

Mixed Ru_xIr_1−xO₂ Supported on Rutile TiO₂: Catalytic Methane Combustion, a Model Study

et al. 2021

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With a modified Pechini synthesis, mixed Ru x Ir 1À x O 2 is grown on rutile-TiO 2 with full control of the composition x, where the preformed TiO 2 particles serve as nucleation sites for the active component. Catalytic and kinetic data of the methane combustion over Ru x Ir 1À x O 2 @TiO 2 and unsupported Ru x Ir 1À x O 2 catalysts reveal that the least active catalyst is RuO 2 @TiO 2 (onset temperature: 270°C), while the most active catalyst is Ru 0.25 Ir 0.75 O 2 with an onset temperature below 220°C. Surprisingly, even Ru 0.75 Ir 0.25 O 2 @TiO 2 is remarkably active in methane combustion (onset temperature: 230°C), indicating that little iridium in the mixed Ru x Ir 1À x O 2 oxide component already improves the activity of the methane combustion considerably. We conclude that iridium in the mixed Ru x Ir 1À x O 2 oxide enables efficient methane activation, while ruthenium promotes the subsequent oxidation steps of the methyl group to produce CO 2 . Kinetic data provide a reaction order in O 2 of zero, while that of methane is close to one, indicating that the methane activation is rate limiting. The apparent activation energy varies among Ru x Ir 1À x O 2 from 110 (x = 0) to 80 kJ • mol À 1 (x = 1). This variation in the apparent activation energy may be explained by the variation in adsorption energy of oxygen. Under the given reaction conditions the catalyst's surface is saturated with adsorbed oxygen and only if oxygen desorbs, methane can be activated and the methyl group can be accommodated at the liberated surface metal sites.

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Kinetics of low-temperature methane activation on IrO2(1 1 0): Role of local surface hydroxide species

Cited by 37 publications

References 49 publications

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

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

High-Resolution X-ray Photoelectron Spectroscopy of an IrO₂(110) Film on Ir(100)

Mixed Ru_xIr_1−xO₂ Supported on Rutile TiO₂: Catalytic Methane Combustion, a Model Study

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Kinetics of low-temperature methane activation on IrO2(1 1 0): Role of local surface hydroxide species

Cited by 37 publications

References 49 publications

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

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

High-Resolution X-ray Photoelectron Spectroscopy of an IrO2(110) Film on Ir(100)

Mixed RuxIr1−xO2 Supported on Rutile TiO2: Catalytic Methane Combustion, a Model Study

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

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

High-Resolution X-ray Photoelectron Spectroscopy of an IrO₂(110) Film on Ir(100)

Mixed Ru_xIr_1−xO₂ Supported on Rutile TiO₂: Catalytic Methane Combustion, a Model Study