| 1. |
- Delesma, Francisco A., et al.
(författare)
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A Chemical View on X-ray Photoelectron Spectroscopy : the ESCA Molecule and Surface-to-Bulk XPS Shifts
- 2018
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Ingår i: ChemPhysChem. - : Wiley. - 1439-4235 .- 1439-7641. ; 19:2, s. 169-174
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we remind the reader of a simple, intuitive picture of chemical shifts in X-ray photoelectron spectroscopy (XPS) as the difference in chemical bonding between the probed atom and its neighbor to the right in the periodic table, the so called Z + 1 approximation. We use the classical ESCA molecule, ethyl trifluoroacetate, and 4d-transition metals to explicitly demonstrate agreement between core-level shifts computed as differences between final core-hole states and the approach where each core-ionized atom is replaced by a Z + 1 atom. In this final state, or total energy picture, the XPS shift arises due to the more or less unfavorable chemical bonding of the effective nitrogen in the carbon geometry for the ESCA molecule. Surface core level shifts in metals are determined by whether the Z + 1 atom as an alloy segregates to the surface or is more soluble in the bulk. As further illustration of this more chemical picture, we compare the geometry of C 1s and O 1s core-ionized CO with that of, respectively, NO+ and CF+. The scope is not to propose a new method to compute XPS shifts but rather to stress the validity of this simple interpretation.
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| 2. |
- Fernandes, V. R., et al.
(författare)
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Reversed Hysteresis during CO Oxidation over Pd75Ag25(100)
- 2016
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 6:7, s. 4154-4161
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Tidskriftsartikel (refereegranskat)abstract
- CO oxidation over Pd(100) and Pd75Ag25(100) has been investigated by a combination of near-ambient pressure X-ray photoelectron spectroscopy, quadrupole mass spectrometry, density functional theory calculations, and microkinetic modeling. For both surfaces, hysteresis is observed in the CO2 formation during the heating and cooling cycles. Whereas normal hysteresis with light-off temperature higher than extinction temperature is present for Pd(100), reversed hysteresis is observed for Pd75Ag25(100). The reversed hysteresis can be explained by dynamic changes in the surface composition. At the beginning of the heating ramp, the surface is rich in palladium, which gives a CO coverage that poisons the surface until the desorption rate becomes sufficiently high. The thermodynamic preference for an Ag-rich surface in the absence of adsorbates promotes diffusion of Ag from the bulk to the surface as CO desorbs. During the cooling ramp, an appreciable surface coverage is reached at temperatures too low for efficient diffusion of Ag back into the bulk. The high concentration of Ag in the surface leads to a high extinction temperature and, consequently, the reversed hysteresis.
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| 3. |
- Florén, Carl-Robert, 1988, et al.
(författare)
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Modelling complete methane oxidation over palladium oxide in a porous catalyst using first-principles surface kinetics
- 2018
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Ingår i: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 8:2, s. 508-520
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Tidskriftsartikel (refereegranskat)abstract
- A comprehensive model is developed for complete methane oxidation over supported palladium. The model is based on first-principles microkinetics and accounts for mass and heat transport in a porous catalytic layer. The turnover frequency (TOF) is simulated for wet exhaust gas compositions, exploring the effects of temperature and total pressure on the TOF. Three different temperature regimes are identified each with different dependency on the total pressure. The regimes originate from temperature and pressure dependent coverages of carbon dioxide and water, which are the most abundant surface species hindering methane dissociation at low temperatures. The TOF is controlled by surface kinetics below 400 °C whereas above 500 °C and up to 8 atm, internal mass transport is controlling. A combination of kinetics, external and internal mass transport controls the TOF at other reaction conditions. The physically meaningful model paves the way for extrapolation and optimization of catalyst design parameters for high catalytic efficiency.
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| 4. |
- Martin, Natalia, et al.
(författare)
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Dissociative Adsorption of Hydrogen on PdO(101) Studied by HRCLS and DFT
- 2013
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:26, s. 13510-13519
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution core-level spectroscopy (HRCLS) and density functional theory (DFT) calculations have been used to investigate the adsorption and dissociation of hydrogen on a PdO(101) film grown on Pd(111). Energy-dependent measurements of the O 1s and Pd 3d(5/2) binding energies enable identification of surface components that correspond to undercoordinated Pd and O atoms. HRCLS data obtained at 110 K, after hydrogen exposure at the same temperature, reveal hydrogen adsorption and formation of Pd-H and O-H groups. Adsorption at room temperature results instead in complete reduction of the oxide. The experimental results are supported by the DFT calculations of core-level shifts and barriers for water formation.
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| 5. |
- Martin, Natalia, et al.
(författare)
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Intrinsic Ligand Effect Governing the Catalytic Activity of Pd Oxide Thin Films
- 2014
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Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 4:10, s. 3330-3334
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Tidskriftsartikel (refereegranskat)abstract
- High-pressure X-ray photoelectron spectroscopy, mass spectrometry, and density functional theory calculations have been combined to study methane oxidation over Pd(100). The measurements reveal a high activity when a two-layer PdO(101) oriented film is formed. Although a one-layer PdO(101) film exhibits a similar surface structure, no or very little activity is observed. The calculations show that the presence of an oxygen atom directly below the coordinatively unsaturated Pd atom in the two-layer PdO(101) film is crucial for efficient methane dissociation, demonstrating a ligand effect that may be broadly important in determining the catalytic properties of oxide thin films.
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| 6. |
- Martin, N. M., et al.
(författare)
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CO Adsorption on Clean and Oxidized Pd(111)
- 2014
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:2, s. 1118-1128
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of CO on clean and oxidized Pd(111) surfaces has been investigated using a combination of high-resolution core level spectroscopy (HRCLS), reflection absorption infrared spectroscopy (RAIRS), and density functional theory (DFT) calculations. The HRCLS and RAIRS measurements reveal that CO adsorbs on Pd(111), Pd(5)O4 and PdO(101) at 100 +/- 10 K and that the CO coverage decreases with increasing oxidation state of Pd for the same CO exposures of 10 Langmuirs. Based on the DFT calculations, the CO layer on clean Pd(111) was found to include molecular adsorption in both hollow and bridge sites, whereas CO occupies a combination of bridge and atop sites on the Pd5O4 and PdO(101) surfaces.
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| 7. |
- 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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| 8. |
- Nilsson, Viktor, 1985, et al.
(författare)
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Trends in adsorbate induced core level shifts
- 2015
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 640, s. 59-64
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Tidskriftsartikel (refereegranskat)abstract
- Photoelectron core level spectroscopy is commonly used to monitor atomic and molecular adsorption on metal surfaces. As changes in the electron binding energies are convoluted measures with different origins, calculations are often used to facilitate the decoding of experimental signatures. The interpretation could in this sense benefit from knowledge on trends in surface core level shifts for different metals and adsorbates. Here, density functional theory calculations have been used to systematically evaluate core level shifts for (111) and (100) surfaces of 3d, 4d, and 5d transition metals upon CO, H, O and S adsorption. The results reveal trends and several non-intuitive cases. Moreover, the difficulties correlating core level shifts with charging arid d-band shifts are underlined. (C) 2015 Elsevier B.V. All rights reserved.
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| 9. |
- van den Bossche, Maxime, 1989, et al.
(författare)
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Adsorbate Pairing on Oxide Surfaces: Influence on Reactivity and Dependence on Oxide, Adsorbate Pair, and Density Functional
- 2017
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 121:15, s. 8390-8398
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Tidskriftsartikel (refereegranskat)abstract
- Open-shell molecules on metal oxide surfaces frequently display cooperative adsorption mechanisms, where pairs of adsorbates are significantly more stable than the isolated species. In this work, density functional theory is used to investigate the cooperative adsorption of OH-H and NO2-NO on rocksalt BaO(100), rutile TiO2(110), fluorite CeO2(111), and tetragonal PdO(101) surfaces. The OH and NO2 adsorbates are considered to be located at the metal sites, whereas H and NO are situated on the oxygen sites. Despite differences in adsorption mechanisms, the pairing is found to be consistently exothermic. The pairing is most pronounced on BaO(100) and weakest on PdO(101). OH and H form more stable pairs than do NO2 and NO. In all cases except CeO2(111), the hybrid HSE06 functional predicts a stronger pairing than the semilocal PBE functional. The absolute pairing energy is a convoluted measure of charge transfer energies, electrostatic interactions, and ionic relaxations, which is analyzed in detail using a thermodynamic cycle. Adsorbate pairing has marked effects on the reactivity, which is exemplified by studying the reaction of CO with OH on CeO2(111).
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| 10. |
- van den Bossche, Maxime, 1989, et al.
(författare)
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Effects of non-local exchange on core level shifts for gas-phase and adsorbed molecules
- 2014
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 141:3
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory calculations are often used to interpret experimental shifts in core level binding energies. Calculations based on gradient-corrected (GC) exchange-correlation functionals are known to reproduce measured core level shifts (CLS) of isolated molecules and metal surfaces with reasonable accuracy. In the present study, we discuss a series of examples where the shifts calculated within a GC-functional significantly deviate from the experimental values, namely the CLS of C 1s in ethyl trifluoroacetate, Pd 3d in PdO and the O 1s shift for CO adsorbed on PdO(101). The deviations are traced to effects of the electronic self-interaction error with GC-functionals and substantially better agreements between calculated and measured CLS are obtained when a fraction of exact exchange is used in the exchange-correlation functional.
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