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| 2. |
- Andersson, M, et al.
(author)
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Preparation of nanosize anatase and rutile TiO2 by hydrothermal treatment of microemulsions and their activity for photocatalytic wet oxidation of phenol
- 2002
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 106:41, s. 10674-10679
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Journal article (peer-reviewed)abstract
- Titanium dioxide (TiO2) nanoparticles of both anatase and rutile phases were synthesized by hydrothermal treatment of microemulsions, and their photocatalytic activity for wet oxidation of phenol was studied. The only difference between the two syntheses used was that different acids were added to the microemulsions, making direct comparison of the catalytic activity of the two polymorphs possible. If hydrochloric acid was used, the rutile structure formed, and if nitric acid was used, anatase formed. The phase stability of the microemulsion was studied and according to conductivity and turbidity measurements the idea of a direct template effect could be discarded during the hydrothermal treatment. However, an initial size-templating phenomenon is possible during the mixing step. The particles, which were in the size range of a few nanometers were characterized with N-2-adsorption; XRD, SEM, and XPS. The activity of the two polymorphs for the photocatalytic oxidation of phenol in water was examined. It was shown that the rutile phase initially decomposed phenol much faster and follows a first-order process reasonably well (k = 4 x 10(-5) s(-1)). The photodecomposition process using the anatase phase led, however, to a much more rapid overall degradation following an initial slower rate of phenol oxidation. The results indicate that the observed difference of the photodecomposition process for the two TiO2 phases is due to the formation of different intermediates.
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| 4. |
- Carlsson, Per-Anders, 1972, et al.
(author)
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A transient in situ FTIR and XANES study of CO oxidation over Pt/Al2O3 catalysts
- 2004
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In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 226:2, s. 422-434
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Journal article (peer-reviewed)abstract
- We report experimental results for the oxidation of CO over supported Pt/Al2O3 catalysts operating in oxygen excess at atmospheric pressure. To study the reaction kinetics under transient conditions we have employed step changes of the O2 concentration by intermittently switching off the O2 supply at various temperatures ranging from 523 to 623 K. Detailed in situ FTIR and XANES data for CO coverage and the chemical state of Pt, respectively, are presented together with the CO conversion, which in both cases was monitored by mass spectrometry. A red-shift of the vibrational frequency of linearly bonded CO which correlates with a blue-shift of the Pt LIII binding energy indicates that the Pt catalyst initially is partially oxidised and gradually reduced when the O2 supply is switched off. Control experiments with a NO2 oxidised Pt/Al2O3 catalyst support these findings. A hysteresis in the catalytic activity due to the different rates whereby Pt is oxidised and reduced as a function of gas-phase composition is observed. The activation energy for the Pt oxide reduction (decomposition) process is estimated to be about 50 kJ/mol. The results further emphasise that the conventional three-step Langmuir-Hinshelwood (LH) scheme used to interpret CO oxidation on Pt surfaces must be complemented by a Pt oxidation and reduction mechanism during transient conditions. Moreover, FTIR data suggest that during the extinction, the partially oxidised platinum surface is reduced by chemisorbed CO which should be explicitly accounted for in the modeling of the reaction mechanism.
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| 6. |
- Jansson, Jonas, 1973, et al.
(author)
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On the catalytic activity of Co3O4 in low-temperature CO oxidation
- 2002
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In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 211:2, s. 387-397
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Journal article (peer-reviewed)abstract
- Oxidation of CO over Co3O4 at ambient temperature was studied with flow reactor experiments, and in-situ spectroscopic and structural methods. The catalyst deactivates during the reaction. The rate of deactivation increased with increasing CO or CO2 gas-phase concentration but decreased with increased 02 concentration or increased temperature. Regeneration of the catalyst in 10% O-2/Ar was more efficient than regeneration in Ar alone. The presence of carbonates and surface carbon on the deactivated catalyst was concluded from TPO experiments. None of these species could, however, be correlated with the deactivation of the catalyst. In-situ FTIR showed the presence of surface carbonates, carbonyl, and oxygen species. The change in structure and oxidation state of the catalyst was studied by in-situ XRD, in-situ XANES, XPS, and flow reactor experiments. One possible explanation for the deactivation of the catalyst is a surface reconstruction hindering the redox cycle of the reaction. (C) 2002 Elsevier Science (USA).
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| 7. |
- Johansson, S, et al.
(author)
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CO oxidation bistability diagrams for Pt/CeOx and Pt/SiO2 model catalysts prepared by electron-beam lithography
- 2001
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In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 201:2, s. 275-285
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Journal article (peer-reviewed)abstract
- The CO oxidation bistability diagrams (i.e., the interval of gas mixing ratio beta drop P-CO/(P-CO + P-O2) vs temperature for which a bistability exists) have been determined for different electron beam lithography (EBL) fabricated supported model catalysts. Three different samples were studied, one Pt/SiO2 sample and two Pt/CeOx samples, where the support type, particle size, and interface length were varied independently. The kinetic bistability is an inherent property of the CO oxidation on Pt at certain gas mixtures and temperatures, and we find that the actual position of the bistable region is a very sensitive tool to measure differences in reaction kinetics on different samples. The bistable region is shifted considerably along the gas mixing ratio axis, beta, between the three samples (at constant temperature). Simulations show that the experimental results can be understood by introducing an oxygen reactant supply via the CeOx, support (spillover), which does not exist for SiO2. This extra supply of oxygen will maintain a high CO conversion rate up to higher relative CO partial pressures on ceria samples (by suppressing the CO poisoning effect). Energy barriers for O diffusion on CeOx, and the attachment across the Pt/CeOx interface have been estimated. The formation of a less reactive Pt state under oxidizing conditions is discussed. (C) 2001 Academic Press.
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| 8. |
- Laegsgaard, E, et al.
(author)
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A high-pressure scanning tunneling microscope
- 2001
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In: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 72:9, s. 3537-3542
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Journal article (peer-reviewed)abstract
- We present the design and performance of a high-pressure scanning tunneling microscope (HP-STM), which allows atom-resolved imaging of metal surfaces at pressures ranging from ultrahigh vacuum (UHV) to atmospheric pressures (1 x 10(-10)-1000 mbar) on a routine basis. The HP-STM is integrated in a gold-plated high-pressure cell with a volume of only similar to0.5 l, which is attached directly to an UHV preparation/analysis chamber. The latter facilitates quick sample transfer between the UHV chamber and the high-pressure cell, and allows for in situ chemical and structural analysis by a number of analytical UHV techniques incorporated in the UHV chamber. Reactant gases are admitted to the high-pressure cell via a dedicated gas handling system, which includes several stages of gas purification. The use of ultrapure gasses is essential when working at high pressures in order to achieve well-defined experimental conditions. The latter is demonstrated in the case of H/Cu(110) at atmospheric H-2 pressures where impurity-related structures were observed. (C) 2001 American Institute of Physics.
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| 9. |
- Li, W X, et al.
(author)
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Oxidation of Pt(110)
- 2004
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 93:14, s. 146104-
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Journal article (peer-reviewed)abstract
- Using scanning tunneling microscopy and temperature programed desorption we investigate the Pt(110) surface under strongly oxidizing conditions involving either high-pressure O-2 or atomic oxygen exposure. At low temperatures, only disordered Pt oxide structures are observed. After annealing ordered surface oxide islands are observed to coexist with a highly stable reconstructed (12x2)-O chemisorption structure. From density functional theory calculations a model for the surface oxide phase is revealed. The phase is found to be metastable, and its presence is explained in terms of stabilizing defects in the chemisorption layer and reduced Pt mobility.
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| 10. |
- Lou, L, et al.
(author)
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Electronic structure and kinetics of K on graphite
- 2000
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 112:10, s. 4788-4796
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Journal article (peer-reviewed)abstract
- The K/graphite adsorption system is studied in a cluster model using ab initio density-functional methods. From the investigation of the potential energy surface a lower bound for the potassium atom binding energy 1.5 eV is obtained, and a surface diffusion barrier of 0.2 eV. To simulate experimentally reported thermal desorption spectra, a two-phase kinetic model is investigated and a desorption energy of 1 eV is found. The thermally activated surface diffusion of K atoms leads to intercalation at defects or steps, which is followed by desorption when further heating the sample. A normal mode analysis yields a K-graphite in-phase and out-of-phase vibrational mode with an energy split of 8 meV, which indicates a relatively strong dynamical coupling between the adsorbed K atom and the graphite substrate. The calculated electron density distribution is verified by an accurate reproduction of the measured dipole moment. From a projected density of state analysis we find a K 4s and an antibonding K 4p resonance located slightly above and 2.6 eV above the Fermi level, respectively. The location of the K 4s resonance, with a lower occupied tail, is consistent with an incomplete charge transfer, and the location of the K 4p resonance is consistent with a proposed hot-electron model to explain recent photodesorption data. The new assignment of the K-induced states near the Fermi level resolves previous apparent discrepancies of the charge state of the dispersed K atom. (C) 2000 American Institute of Physics. [S0021-9606(00)70310-2].
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