Adsorption and oxidation of propane and cyclopropane on IrO<sub>2</sub>(110)

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

doi:10.1039/c8cp06125d

Cited by 28 publications

(37 citation statements)

References 26 publications

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“…Rutile IrO 2 has emerged as a promising material for enabling the catalytic conversion of light alkanes to value-added products, and IrO 2 is of wide interest for applications of electrocatalysis. Recent studies demonstrate that IrO 2 (110) efficiently promotes the initial C–H activation of light alkanes (C1–C3) at temperatures as low as 100 K. − The resulting C x H y intermediates remain stable on the surface up to ∼400 K in ultrahigh vacuum (UHV) and thereafter tend to oxidize to gaseous CO x and H 2 O. The ability of the IrO 2 (110) surface to promote alkane activation at low temperatures and stabilize the resulting alkyl intermediates over a wide temperature range are characteristics that could enable selective conversions of alkanes.…”

Section: Introductionmentioning

confidence: 99%

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: Introductionmentioning

confidence: 99%

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

et al. 2021

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“…In several past studies, we have demonstrated that alkane activation can occur facilely on late transition-metal-oxide surfaces (PdO, RuO 2 , IrO 2 ) that expose pairs of coordinatively unsaturated (cus) metal and oxygen atoms and have advanced the mechanistic understanding of this reaction. − We find that alkanes form strongly bound σ-complexes on the oxide surfaces by coordinating with cus-metal atoms, and that dative bonding weakens the metal-coordinated C–H bonds and thereby facilitates C–H bond cleavage via H-transfer to a neighboring cus-O-atom. Researchers have recently shown that the IrO 2 (110) surface is especially reactive with this surface promoting the C–H bond cleavage of methane, ethane, and propane at temperatures as low as 100 K. − These discoveries suggest possibilities for developing IrO 2 -based catalysts that can efficiently convert light alkanes to value-added products and, thus, motivate further investigations of IrO 2 (110) surface chemistry.…”

Section: Introductionmentioning

confidence: 99%

Adsorption and Oxidation of CH₄ on Oxygen-Rich IrO₂(110)

et al. 2019

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We investigated the oxidation of CH4 on oxygen-pre-covered IrO2(110) surfaces using temperature-programmed reaction spectroscopy (TPRS) and density functional theory (DFT). Our TPRS results show that on-top oxygen (Oot) species hinder CH4 adsorption, providing evidence that CH4 adsorbs on coordinatively unsaturated Ir atoms. We also find that the fractional yield of adsorbed CH4 that reacts during TPRS remains constant at ∼70% as the Oot-coverage increases to about 0.5 monolayer for saturation CH4 coverage, demonstrating that O-rich IrO2(110) surfaces are highly active in promoting CH4 C–H bond cleavage. Our results show that Oot atoms promote CH4 oxidation to CO2 as well as H2O formation while suppressing CO and recombinative CH4 desorption, as evidenced by an increase in the fractional yield of CO2 produced during TPRS and a downshift of CO2 and H2O TPRS peak maxima with increasing Oot-coverage. DFT predicts that initial CH4 bond cleavage is highly facile on both stoichiometric and O-rich IrO2(110) and can occur by either H-transfer to an Oot or a bridging O-atom of the surface. Our calculations also predict that oxidation of the CH x species that result from CH4 activation is more facile on O-rich compared with stoichiometric IrO2(110), and that complete oxidation is strongly favored on the O-rich surface, in good agreement with our experimental findings. According to the calculations, key steps in the CH4 oxidation pathway have significantly lower-energy barriers on O-rich vs stoichiometric IrO2(110) because these steps involve reaction with Oot atoms initially present on the surface rather than the abstraction of more strongly bound Obr species. High coverages of O-atoms also enable adsorbed intermediates to oxidize extensively on O-rich IrO2(110), without the intermediates needing to overcome diffusion barriers to access reactive O-atoms. Our results provide insights for understanding CH4 oxidation on IrO2(110) surfaces under reaction conditions at which Oot atoms and adsorbed CH4 can co-exist.

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“…Prior studies show that IrO 2 exhibits high activity towards the electrocatalytic conversion of water to hydrogen and oxygen [1][2][3][4], and thus motivate fundamental investigations of the chemical properties of well-defined IrO 2 surfaces towards species that are involved in water-splitting chemistry (e.g., H 2 , O 2 , H 2 O). Recent studies also demonstrate that the IrO 2 (110) surface efficiently promotes the C-H bond cleavage of methane, ethane and propane at low temperatures (<150 K) [5][6][7], and that the resulting alkyl groups react at higher temperature during temperature-programmed reaction spectroscopy (TPRS). Our prior results show that surface H-atoms can have a strong influence on the chemistry of alkyl groups on IrO 2 (110), and thus further motivate efforts to clarify the fundamental interactions between hydrogen and IrO 2 surfaces.…”

Section: Introductionmentioning

confidence: 99%

Hydrogen oxidation on oxygen-rich IrO₂(110)

Kim

Liang

et al. 2018

Catalysis, Structure & Reactivity

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We investigated the adsorption and oxidation of H 2 on O-rich IrO 2 (110) using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. Our results show that H 2 dissociation occurs efficiently on O-rich IrO 2 (110) at low temperature and initiates from an adsorbed H 2 σ-complex on the coordinatively-unsaturated Ir atoms (Ir cus ). We find that on-top oxygen atoms (O ot ), adsorbed on the Ir cus sites, promote the desorption-limited evolution of H 2 O during subsequent oxidation of the adsorbed hydrogen on IrO 2 ( 110) while suppressing reaction-limited production of H 2 O via the recombination of bridging HO groups (HO br ) (~500 to 750 K) during TPRS. The desorption-limited TPRS peak of H 2 O shifts from~490 to 550 K with increasing O ot coverage, demonstrating that O ot atoms stabilize adsorbed OH and H 2 O species. DFT predicts that molecularly-adsorbed H 2 dissociates on O-rich IrO 2 (110) at low temperature and that the resulting H-atoms redistribute to produce a mixture of HO br and HO ot groups, with equilibrium favouring HO ot groups. Our calculations further predict that subsequent H 2 O evolution occurs through the recombination of HO br /HO ot and HO ot /HO ot pairs, and that these reactions represent desorption-limited pathways because the dissociative chemisorption of H 2 O is favoured over molecular adsorption on IrO 2 (110). The higher stability of HO ot groups and their preferred formation causes the higher-barrier HO ot /HO ot recombination reaction to become the dominant pathway for H 2 O formation with increasing O ot coverage, consistent with the experimentally-observed upshift in the H 2 O TPRS peak temperature.

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Adsorption and oxidation of propane and cyclopropane on IrO₂(110)

Abstract: Initial activation by ring-opening enables cyclopropane to achieve higher reaction yields than n-propane on IrO2(110).

Cited by 28 publications

References 26 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

Adsorption and Oxidation of CH₄ on Oxygen-Rich IrO₂(110)

Hydrogen oxidation on oxygen-rich IrO₂(110)

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Adsorption and oxidation of propane and cyclopropane on IrO2(110)

Abstract: Initial activation by ring-opening enables cyclopropane to achieve higher reaction yields than n-propane on IrO2(110).

Cited by 28 publications

References 26 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

Adsorption and Oxidation of CH4 on Oxygen-Rich IrO2(110)

Hydrogen oxidation on oxygen-rich IrO2(110)

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Adsorption and oxidation of propane and cyclopropane on IrO₂(110)

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

Adsorption and Oxidation of CH₄ on Oxygen-Rich IrO₂(110)

Hydrogen oxidation on oxygen-rich IrO₂(110)