| 1. |
- Busch, Michael, 1983, et al.
(författare)
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Adsorption of NO on Fe3O4(111)
- 2018
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 693, s. 84-87
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Tidskriftsartikel (refereegranskat)abstract
- Adsorption of NO on Fe 3 O 4 (111) is studied by density functional theory (DFT) calculations. NO is preferably adsorbed atop the octahedral site which has a clearly higher adsorption energy than the tetrahedral site. The difference in adsorption energy correlates with differences in adsorption geometries and N–O stretch vibrations. The results are in good agreement with temperature programmed desorption (TPD) and reflection-adsorption IR spectroscopy (RAIRS) measurements and provide an explanation of the observation of only one vibrational mode despite two distinct TPD features.
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| 2. |
- Martin, Rachel, et al.
(författare)
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Isothermal Reduction of IrO2 (110) Films by Methane Investigated Using In Situ X-ray Photoelectron Spectroscopy
- 2021
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 11:9, s. 5004-5016
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Tidskriftsartikel (refereegranskat)abstract
- Continuous exposure to methane causes IrO2 (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 CH4 oxidation on IrO2 (110) converts so-called bridging oxygen atoms (O-br) at the surface to HObr groups while concurrently removing oxygen from the oxide film. Reduction of the IrO2 (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 IrO2 (110) surface during CH4 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 IrO2 (110) reduction by methane decreases continuously as metallic Ir replaces surface IrO2 (110) domains, demonstrating that IrO2 (110) is the active phase for CH4 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 IrO2 (110) domains at the surface and enable CH4 to reduce the similar to 10-layer IrO2 (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 CH4 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 IrO2 (110).
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| 3. |
- Martin, Rachel, et al.
(författare)
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Methane oxidation on an IrO2(110) film
- 2019
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Ingår i: Abstracts of Papers of the American Chemical Society. - : American Chemical Society (ACS). - 0065-7727. ; 258
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 4. |
- Mehar, Vikram, et al.
(författare)
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Adsorption of NO on FeOx films grown on Ag(111)
- 2016
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:17, s. 9282-9291
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Tidskriftsartikel (refereegranskat)abstract
- We used temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) to characterize the adsorption of NO on crystalline iron oxide films grown on Ag(111), including a Fe3O4(111) layer, an FeO(111) monolayer, and an intermediate FeOx multilayer structure. TPD shows that the NO binding energies vary significantly among the Fe cation sites present on these FeOx surfaces, and provides evidence that NO binds more strongly on Fe2+ sites than Fe3+ sites. The NO TPD spectra obtained from the Fe3O4(111) layer exhibit a dominant peak at 380 K, attributed to NO bound on Fe2+ sites, as well as a broad feature centered at ∼250 K that is consistent with NO bound on Fe3+ sites of Fe3O4(111) as well as NO adsorbed on a minority FeO structure. The NO TPD spectra obtained from the monolayer FeO(111) film exhibits a prominent peak at 269 K. After growing FeOx multilayer islands within the FeO(111) monolayer, we observe a new NO TPD feature at ∼200 K as well as diminution of the sharp TPD peak at 269 K. We speculate that these changes occur because the multilayer FeOx islands expose Fe3+ sites that bind NO more weakly than the Fe2+ sites of the FeO monolayer. RAIR spectra obtained from the NO-covered FeOx surfaces exhibit an N-O stretch band that blueshifts over a range from about 1800 to 1840 cm-1 with increasing NO coverage. The measured N-O stretching frequency is only slightly red-shifted from the gas-phase value, and lies in a range that is consistent with atop, linearly bound NO on the Fe surface sites. In contrast to the NO binding energy, we find that the N-O stretch band is relatively insensitive to the NO binding site on the FeOx surfaces. This behavior suggests that π-backbonding occurs to similar extents among the adsorbed NO species, irrespective of the oxidation state and local structural environment of the Fe surface site.
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| 5. |
- Mehar, Vikram, et al.
(författare)
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Formation of Epitaxial PdO(100) During the Oxidation of Pd(100)
- 2023
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 14:38, s. 8493-8499
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The catalytic oxidation of CO and CH4 can be strongly influenced by the structures of oxide phases that form on metallic catalysts during reaction. Here, we show that an epitaxial PdO(100) structure forms at temperatures above 600 K during the oxidation of Pd(100) by gaseous O atoms as well as exposure to O2-rich mixtures at millibar partial pressures. The oxidation of Pd(100) by gaseous O atoms preferentially generates an epitaxial, multilayer PdO(101) structure at 500 K, but initiating Pd(100) oxidation above 600 K causes an epitaxial PdO(100) structure to grow concurrently with PdO(101) and produces a thicker and rougher oxide. We present evidence that this change in the oxidation behavior is caused by a temperature-induced change in the stability of small PdO domains that initiate oxidation. Our discovery of the epitaxial PdO(100) structure may be significant for developing relationships among oxide structure, catalytic activity, and reaction conditions for applications of oxidation catalysis.
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| 6. |
- Mehar, Vikram, et al.
(författare)
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Understanding the Intrinsic Surface Reactivity of Single-Layer and Multilayer PdO(101) on Pd(100)
- 2018
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 8:9, s. 8553-8567
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the intrinsic reactivity of CO on single-layer and multilayer PdO(101) grown on Pd(100) using temperature-programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) experiments, as well as density functional theory (DFT) calculations. We find that CO binds more strongly on multilayer than single-layer PdO(101) (∼119 kJ/mol vs 43 kJ/mol), and that CO oxidizes negligibly on single-layer PdO(101), whereas nearly 90% of a saturated layer of CO oxidizes on multilayer PdO(101) during TPRS experiments. RAIRS further shows that CO molecules adsorb on both bridge-Pdcusand atop-Pdcussites (coordinatively unsaturated Pd sites) of single-layer PdO(101)/Pd(100), while CO binds exclusively on atop-Pdcussites of multilayer PdO(101). The DFT calculations reproduce the much stronger binding of CO on multilayer PdO(101), as well as the observed binding site preferences, and reveal that the stronger binding is entirely responsible for the higher CO oxidation activity of multilayer PdO(101)/Pd(100). We show that the O atom below the Pdcussite, present only on multilayer PdO(101), modifies the electronic states of the Pdcusatom in a way that enhances the CO-Pdcusbonding. Lastly, we show that a precursor-mediated kinetic model, with energetics determined from the present study, predicts that the intrinsic CO oxidation rates achieved on both single-layer and multilayer PdO(101)/Pd(100) can be expected to exceed the gaseous CO diffusion rate to the surface during steady-state CO oxidation at elevated pressures, even though the intrinsic reaction rates are 4-5 orders of magnitude lower on single-layer PdO(101)/Pd(100) than on multilayer PdO(101)/Pd(100).
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| 7. |
- Shipilin, M., et al.
(författare)
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Fe Oxides on Ag Surfaces : Structure and Reactivity
- 2017
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Ingår i: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1022-5528 .- 1572-9028. ; 60:6-7
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Tidskriftsartikel (refereegranskat)abstract
- One layer thick iron oxide films are attractive from both applied and fundamental science perspectives. The structural and chemical properties of these systems can be tuned by changing the substrate, making them promising materials for heterogeneous catalysis. In the present work, we investigate the structure of FeO(111) monolayer films grown on Ag(100) and Ag(111) substrates by means of microscopy and diffraction techniques and compare it with the structure of FeO(111) grown on other substrates reported in literature. We also study the NO adsorption properties of FeO(111)/Ag(100) and FeO(111)/Ag(111) systems utilizing different spectroscopic techniques. We discuss similarities and differences in the data obtained from adsorption experiments and compare it with previous results for FeO(111)/Pt(111).
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