| 1. |
- Boucly, Anthony, et al.
(författare)
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Soft X-ray Heterogeneous Radiolysis of Pyridine in the Presence of Hydrated Strontium-Hydroxyhectorite and its Monitoring by Near-Ambient Pressure Photoelectron Spectroscopy
- 2018
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Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- The heterogeneous radiolysis of organic molecules in clays is a matter of considerable interest in astrochemistry and environmental sciences. However, little is known about the effects of highly ionizing soft X-rays. By combining monochromatized synchrotron source irradiation with in situ Near Ambient Pressure X-ray Photoelectron Spectroscopy (in the mbar range), and using the synoptic view encompassing both the gas and condensed phases, we found the water and pyridine pressure conditions under which pyridine is decomposed in the presence of synthetic Sr2+-hydroxyhectorite. The formation of a pyridine/water/Sr2+ complex, detected from the Sr 3d and N 1s core-level binding energies, likely presents a favorable situation for the radiolytic breaking of the O-H bond of water molecules adsorbed in the clay and the subsequent decomposition of the molecule. However, decomposition stops when the pyridine pressure exceeds a critical value. This observation can be related to a change in the nature of the active radical species with the pyridine loading. This highlights the fact that the destruction of the molecule is not entirely determined by the properties of the host material, but also by the inserted organic species. The physical and chemical causes of the present observations are discussed.
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| 2. |
- Chaudhary, Shilpi, et al.
(författare)
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Real-Time Study of CVD Growth of Silicon Oxide on Rutile TiO2(110) Using Tetraethyl Orthosilicate
- 2015
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 119:33, s. 19149-19161
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Tidskriftsartikel (refereegranskat)abstract
- The interaction of the ruffle TiO2(110) surface with tetraethyl orthosilicate (TEOS) in the pressure range from UHV to 1 mbar as well as the TEOS-based chemical vapor deposition of SiO2 on the TiO2(110) surface were monitored in real time using near-ambient pressure X-ray photoelectron spectroscopy. The experimental data and density functional theory calculations confirm the dissociative adsorption of TEOS on the surface already at room temperature. At elevated pressure, the ethoxy species formed in the adsorption process undergoes further surface reactions toward a carboxyl species not observed in the absence of a TEOS gas phase reservoir. Annealing of the adsorption layer leads to the formation of SiO2, and an intermediate oxygen species assigned to a mixed titanium/silicon oxide is identified. Atomic force microscopy confirms the morphological changes after silicon oxide formation.
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| 3. |
- Tissot, Heloise, et al.
(författare)
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Acetic acid conversion to ketene on Cu2O(100) : Reaction mechanism deduced from experimental observations and theoretical computations
- 2021
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 402, s. 154-165
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Tidskriftsartikel (refereegranskat)abstract
- Ketene, a versatile reagent in production of fine and specialty chemicals, is produced from acetic acid. We investigate the synthesis of ketene from acetic acid over the (3,0;1,1) surface of Cu2O(1 0 0) through analysis of the adsorption and desorption characteristics of formic and acetic acids. The results allow us to establish a reaction mechanism for ketene formation. Observations from x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy, and temperature programmed desorption (TPD), supported by a comparison with formic acid results, suggest that acetic acid reacts with Cu2O through deprotonation to form acetate species coordinated to copper sites and hydroxylation of nearby surface oxygen sites. For formic acid the decomposition of adsorbed formate species results in desorption of CO2 and CO while, for acetic acid, high yields of ketene are observed at temperature >500 K. Modeling by density functional theory (DFT) confirms the strong interaction of acetic acid with the (3,0;1,1) surface and the spontaneous dissociation into adsorbed acetate and hydrogen atom species, the latter forming an OH-group. In an identified reaction intermediate ketene binds via all C and O atoms to Cu surface sites, in agreement with interpretations from XPS. In the vicinity of the adsorbate the surface experiences a local reorganization into a c(2 × 2) reconstruction. The total computed energy barrier for ketene formation is 1.81 eV in good agreement with the 1.74 eV obtained from TPD analysis. Our experimental observations and mechanistic DFT studies suggests that Cu2O can operate as an efficient catalyst for the green generation of ketene from acetic acid.
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| 4. |
- Tissot, Heloise, et al.
(författare)
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Interaction of Atomic Hydrogen with the Cu2O(100) and (111) Surfaces
- 2019
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 123:36, s. 22172-22180
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Tidskriftsartikel (refereegranskat)abstract
- Reduction of Cu2O by hydrogen is a common preparation step for heterogeneous catalysts; however, a detailed understanding of the atomic reaction pathways is still lacking. Here, we investigate the interaction of atomic hydrogen with the Cu2O(100):(3,0;1,1) and Cu2O(111):(root 3 x root 3)R30 degrees surfaces using scanning tunneling microscopy (STM), low-energy electron diffraction, temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The experimental results are compared to density functional theory simulations. At 300 K, we identify the most favorable adsorption site on the Cu2O(100) surface: hydrogen atoms bind to an oxygen site located at the base of the atomic rows intrinsic to the (3,0;1,1) surface. The resulting hydroxyl group subsequently migrates to a nearby Cu trimer site. TPD analysis identifies H-2 as the principal desorption product. These observations imply that H-2 is formed through a disproportionation reaction of surface hydroxyl groups. The interaction of H with the (111) surface is more complex, including coordination to both Cu+ and O-CUS sites. STM and XPS analyses reveal the formation of metallic copper clusters on the Cu2O surfaces after cycles of hydrogen exposure and annealing. The interaction of the Cu clusters with the substrate is notably different for the two surface terminations studied: after annealing, the Cu clusters coalesce on the (100) termination, and the (3,0;1,1) reconstruction is partially recovered. Clusters formed on the (111) surface are less prone to coalescence, and the (root 3 x root 3)R30 degrees reconstruction was not recovered by heat treatment, indicating a weaker Cu cluster to support interaction on the (100) surface.
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| 5. |
- Tissot, Heloise, et al.
(författare)
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The Surface Structure of Cu2O(100) : Nature of Defects
- 2019
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:13, s. 7696-7704
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Tidskriftsartikel (refereegranskat)abstract
- The Cu2O(100) surface is most favorably terminated by a (3,0;1,1) reconstruction under ultrahigh-vacuum conditions. As most oxide surfaces, it exhibit defects, and it is these sites that are focus of attention in this study. The surface defects are identified, their properties are investigated, and procedures to accurately control their coverage are demonstrated by a combination of scanning tunneling microscopy (STM) and simulations within the framework of density functional theory (DFT). The most prevalent surface defect was identified as an oxygen vacancy. By comparison of experimental results, formation energies, and simulated STM images, the location of the oxygen vacancies was identified as an oxygen vacancy in position B, located in the valley between the two rows of oxygen atoms terminating the unperturbed surface. The coverage of defects is influenced by the surface preparation parameters and the history of the sample. Furthermore, using low-energy electron beam bombardment, we show that the oxygen vacancy coverage can be accurately controlled and reach a complete surface coverage (1 per unit cell or 1.8 defects per nm(2)) without modification to the periodicity of the surface, highlighting the importance of using local probes when investigating oxide surfaces.
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| 6. |
- Wang, Chunlei, et al.
(författare)
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High-Density Isolated Fe1O3 Sites on a Single-Crystal Cu2O(100) Surface
- 2019
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 10:23, s. 7318-7323
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Tidskriftsartikel (refereegranskat)abstract
- Single-atom catalysts have recently been subject to considerable attention within applied catalysis. However, complications in the preparation of well-defined single-atom model systems have hampered efforts to determine the reaction mechanisms underpinning the reported activity. By means of an atomic layer deposition method utilizing the steric hindrance of the ligands, isolated Fe1O3 motifs were grown on a single-crystal Cu2O(100) surface at densities up to 0.21 sites per surface unit cell. Ambient pressure X-ray photoelectron spectroscopy shows a strong metal-support interaction with Fe in a chemical state close to 3+. Results from scanning tunneling microscopy and density functional calculations demonstrate that isolated Fe1O3 is exclusively formed and occupies a single site per surface unit cell, coordinating to two oxygen atoms from the Cu2O lattice and another through abstraction from O-2. The isolated Fe1O3 motif is active for CO oxidation at 473 K. The growth method holds promise for extension to other catalytic systems.
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| 7. |
- Wang, Chunlei, et al.
(författare)
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Inverse single-site Fe1(OH)X/Pt(111) model catalyst for preferential oxidation of CO in H2
- 2022
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Ingår i: Nano Reseach. - : Springer Nature. - 1998-0124 .- 1998-0000. ; 15:1, s. 709-715
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Tidskriftsartikel (refereegranskat)abstract
- Inverse oxide/metal model systems are frequently used to investigate catalytic structure-function relationships at an atomic level. By means of a novel atomic layer deposition process, growth of single-site Fe1Ox on a Pt(111) single crystal surface was achieved, as confirmed by scanning tunneling microscopy (STM). The redox properties of the catalyst were characterized by synchrotron radiation based ambient pressure X-ray photoelectron spectroscopy (AP-XPS). After calcination treatment at 373 K in 1 mbar O2 the chemical state of the catalyst was determined as Fe3+. Reduction in 1 mbar H2 at 373 K demonstrates a facile reduction to Fe2+ and complete hydroxylation at significantly lower temperatures than what has been reported for iron oxide nanoparticles. At reaction conditions relevant for preferential oxidation of CO in H2 (PROX), the catalyst exhibits a Fe3+ state (ferric hydroxide) at 298 K while re-oxidation of iron oxide clusters does not occur under the same condition. CO oxidation proceeds on the single-site Fe1(OH)3 through a mechanism including the loss of hydroxyl groups in the temperature range of 373 to 473 K, but no reaction is observed on iron oxide clusters. The results highlight the high flexibility of the single iron atom catalyst in switching oxidation states, not observed for iron oxide nanoparticles under similar reaction conditions, which may indicate a higher intrinsic activity of such single interfacial sites than the conventional metal-oxide interfaces. In summary, our findings of the redox properties on inverse single-site iron oxide model catalyst may provide new insights into applied Fe-Pt catalysis. [Figure not available: see fulltext.]
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| 8. |
- Wang, Chunlei, et al.
(författare)
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Redox Properties of Cu2O(100) and (111) Surfaces
- 2018
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 122:50, s. 28684-28691
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Tidskriftsartikel (refereegranskat)abstract
- Intense research efforts are directed toward Cu and Cu2O based catalysts as they are viewed as potential replacements for noble metal catalysts. However, applications are hampered by deactivation, e.g., through facile complete oxidation to CuO. Despite the importance of the redox processes for Cu2O catalysts, a molecular level understanding of the deactivation process is still lacking. Here we study the initial stages of oxidization of well-defined Cu2O bulk single crystals of (100) and (111) termination by means of synchrotron radiation X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM). Exposure of the (100) surface to 1 mbar O-2 at 25 degrees C results in the formation of a 1.0 monolayer (ML) CuO surface oxide. The surface is covered by 0.7 ML OH groups from trace moisture in the reaction gas. In contrast, neither hydroxylation nor oxidation was observed on the (111) surface under similar mild exposure conditions. On Cu2O(111) the initial formation of CuO requires annealing to similar to 400 degrees C in 1 mbar 02, highlighting the markedly different reactivity of the two Cu2O surfaces. Annealing of the (100) surface, under ultrahigh vacuum conditions, to temperatures up to similar to 225 degrees C resulted in removal of the OH groups (0.46 ML decrease) at a rate similar to a detected increase in CuO coverage (0.45 ML increase), suggesting the reaction path 2OH(adsorbed) + CU2Osolid -> H2Ogas + 2CuO(solid). STM was used to correlate the observed changes in surface chemistry with surface morphology, confirming the surface hydroxylation and CuO formation. The STM analysis showed dramatic changes in surface morphology demonstrating a high mobility of the active species under reaction conditions.
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| 9. |
- Wang, Chunlei, et al.
(författare)
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Stabilization of Cu2O through Site-Selective Formation of a Co1Cu Hybrid Single-Atom Catalyst
- 2022
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 34:5, s. 2313-2320
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Tidskriftsartikel (refereegranskat)abstract
- Single-atom catalysts (SACs) consist of a low coverage of isolated metal atoms dispersed on a metal substrate, called single-atom alloys (SAAs), or alternatively single metal atoms coordinated to oxygen atoms on an oxide support. We present the synthesis of a new type of Co1Cu SAC centers on a Cu2O(111) support by means of a site-selective atomic layer deposition technique. Isolated metallic Co atoms selectively coordinate to the native oxygen vacancy sites (Cu sites) of the reconstructed Cu2O(111) surface, forming a Co1Cu SAA with no direct Co- Ox bonds. The centers, here referred to as Co1Cu hybrid SACs, are found to stabilize the active Cu+ sites of the low-cost Cu2O catalyst that otherwise is prone to deactivation under reaction conditions. The stability of the Cu2O(111) surface was investigated by synchrotron radiation-based ambient-pressure X-ray photoelectron spectroscopy under reducing CO environment. The structure and reduction reaction are modeled by density functional theory calculations, in good agreement with experimental results.
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| 10. |
- Zhu, Suyun, et al.
(författare)
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HIPPIE : a new platform for ambient-pressure X-ray photoelectron spectroscopy at the MAX IV Laboratory
- 2021
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Ingår i: Journal of Synchrotron Radiation. - : INT UNION CRYSTALLOGRAPHY. - 1600-5775 .- 0909-0495. ; 28, s. 624-636
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Tidskriftsartikel (refereegranskat)abstract
- HIPPIE is a soft X-ray beamline on the 3 GeV electron storage ring of the MAX IV Laboratory, equipped with a novel ambient-pressure X-ray photoelectron spectroscopy (APXPS) instrument. The endstation is dedicated to performing in situ and operando X-ray photoelectron spectroscopy experiments in the presence of a controlled gaseous atmosphere at pressures up to 30 mbar [1 mbar = 100 Pa] as well as under ultra-high-vacuum conditions. The photon energy range is 250 to 2200 eV in planar polarization and with photon fluxes >1012 photons s-1 (500 mA ring current) at a resolving power of greater than 10000 and up to a maximum of 32000. The endstation currently provides two sample environments: a catalysis cell and an electrochemical/liquid cell. The former allows APXPS measurements of solid samples in the presence of a gaseous atmosphere (with a mixture of up to eight gases and a vapour of a liquid) and simultaneous analysis of the inlet/outlet gas composition by online mass spectrometry. The latter is a more versatile setup primarily designed for APXPS at the solid-liquid (dip-and-pull setup) or liquid-gas (liquid microjet) interfaces under full electrochemical control, and it can also be used as an open port for ad hoc-designed non-standard APXPS experiments with different sample environments. The catalysis cell can be further equipped with an IR reflection-absorption spectrometer, allowing for simultaneous APXPS and IR spectroscopy of the samples. The endstation is set up to easily accommodate further sample environments.
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