Showing papers by "Jens K. Nørskov published in 2004"
TL;DR: In this paper, the stability of reaction intermediates of electrochemical processes on the basis of electronic structure calculations was analyzed and a detailed description of the free energy landscape of the electrochemical oxygen reduction reaction over Pt(111) as a function of applied bias was presented.
Abstract: We present a method for calculating the stability of reaction intermediates of electrochemical processes on the basis of electronic structure calculations. We used that method in combination with detailed density functional calculations to develop a detailed description of the free-energy landscape of the electrochemical oxygen reduction reaction over Pt(111) as a function of applied bias. This allowed us to identify the origin of the overpotential found for this reaction. Adsorbed oxygen and hydroxyl are found to be very stable intermediates at potentials close to equilibrium, and the calculated rate constant for the activated proton/electron transfer to adsorbed oxygen or hydroxyl can account quantitatively for the observed kinetics. On the basis of a database of calculated oxygen and hydroxyl adsorption energies, the trends in the oxygen reduction rate for a large number of different transition and noble metals can be accounted for. Alternative reaction mechanisms involving proton/electron transfer to ...
7,711 citations
TL;DR: Time-resolved, high-resolution in situ transmission electron microscope observations of the formation of carbon nanofibres from methane decomposition over supported nickel nanocrystals show that metallic step edges act as spatiotemporal dynamic growth sites and may be important for understanding other types of catalytic reactions and nanomaterial syntheses.
Abstract: The synthesis of carbon nanotubes with predefined structure and functionality plays a central role in the field of nanotechnology1,2, whereas the inhibition of carbon growth is needed to prevent a breakdown of industrial catalysts for hydrogen and synthesis gas production3. The growth of carbon nanotubes and nanofibres has therefore been widely studied4,5,6,7,8,9,10. Recent advances in in situ techniques now open up the possibility of studying gas–solid interactions at the atomic level11,12. Here we present time-resolved, high-resolution in situ transmission electron microscope observations of the formation of carbon nanofibres from methane decomposition over supported nickel nanocrystals. Carbon nanofibres are observed to develop through a reaction-induced reshaping of the nickel nanocrystals. Specifically, the nucleation and growth of graphene layers are found to be assisted by a dynamic formation and restructuring of mono-atomic step edges at the nickel surface. Density-functional theory calculations indicate that the observations are consistent with a growth mechanism involving surface diffusion of carbon and nickel atoms. The finding that metallic step edges act as spatiotemporal dynamic growth sites may be important for understanding other types of catalytic reactions and nanomaterial syntheses.
1,357 citations
TL;DR: In this paper, it was shown that the reaction rate under given reaction conditions shows a maximum as a function of dissociative adsorption energy of the key reactant, and that for most conditions this maximum is in the same range of reaction energies.
1,218 citations
TL;DR: Periodic density functional calculations are used to illustrate how the combination of strain and ligand effects modify the electronic and surface chemical properties of Ni, Pd, and Pt monolayers supported on other transition metals.
Abstract: Periodic density functional calculations are used to illustrate how the combination of strain and ligand effects modify the electronic and surface chemical properties of Ni, Pd, and Pt monolayers supported on other transition metals. Strain and the ligand effects are shown to change the width of the surface d band, which subsequently moves up or down in energy to maintain a constant band filling. Chemical properties such as the dissociative adsorption energy of hydrogen are controlled by changes induced in the average energy of the d band by modification of the d-band width.
1,169 citations
TL;DR: In this paper, it is suggested that there may be several effects contributing to the catalytic properties of supported nanosized gold particles, and that it is useful to order them in a hierarchy.
1,109 citations
TL;DR: The surfaces investigated in this study had no lateral strain in them, demonstrating that strain is not a necessary factor in the modification of bimetallic surface properties.
Abstract: The modification of the electronic and chemical properties of Pt(111) surfaces by subsurface 3d transition metals was studied using density-functional theory. In each case investigated, the Pt surface d-band was broadened and lowered in energy by interactions with the subsurface 3d metals, resulting in weaker dissociative adsorption energies of hydrogen and oxygen on these surfaces. The magnitude of the decrease in adsorption energy was largest for the early 3d transition metals and smallest for the late 3d transition metals. In some cases, dissociative adsorption was calculated to be endothermic. The surfaces investigated in this study had no lateral strain in them, demonstrating that strain is not a necessary factor in the modification of bimetallic surface properties. The implications of these findings are discussed in the context of catalyst design, particularly for fuel cell electrocatalysts.
1,081 citations
TL;DR: In this article, high-resolution scanning tunneling microscopy (STM) is used in combination with density functional theory (DFT) to provide new insight into the morphology and atomic-scale structure of MoS2 nanoclusters in hydrodesulfurization (HDS) catalysts.
378 citations
TL;DR: In this paper, a single-layer MoS2 nanoclusters were synthesized on a Au substrate as a model system for the hydrotreating catalyst and studied by atomically resolved scanning tunneling microscopy (STM) to achieve atomic-scale insight into the interactions with hydrogen and thiophene (C4H4S).
315 citations
TL;DR: In this article, Sehested et al. studied the effects of atmosphere and temperature on the rate of sintering of nickel steam-reforming catalysts and concluded that in steam/hydrogen mixtures, OH-bonded nickel dimers dominated the surface transport on nickel particles and consequently sintered via particle migration and coalescence.
271 citations
TL;DR: In this article, the adhesion, shape, and electronic structure of gold particles supported on TiO 2 (110) have been determined by density-functional theory calculations, to gain more insight into the catalytic activity of small supported gold particles.
240 citations
TL;DR: New experimental evidence based on infrared spectroscopy, in conjunction with X-ray diffraction and surface-area measurements, is presented that θ-alumina has indeed very stable surface OH groups at high temperatures, and that this form of alumina does not sinter even at temperatures up to 1,300 K.
Abstract: The surface energy of a solid measures the energy cost of increasing the surface area. All normal solids therefore have a positive surface energy-if it had been negative, the solid would disintegrate. For this reason it is also generally believed that when certain ceramics can be found in a highly porous form, this is a metastable state, which will eventually sinter into the bulk solid at high temperatures. We present theoretical evidence suggesting that for theta-alumina, the surface energy is strongly dependent on the size of the crystallites, and that for some facets it is negative for thicknesses larger than approximately 1 nm. This suggests a completely new picture of porous alumina in which the high-surface-area, nanocrystalline form is the thermodynamic ground state. The negative surface energy is found to be related to a particularly strongly adsorbed state of dissociated water on some alumina surfaces. We also present new experimental evidence based on infrared spectroscopy, in conjunction with X-ray diffraction and surface-area measurements, that theta-alumina has indeed very stable surface OH groups at high temperatures, and that this form of alumina does not sinter even at temperatures up to 1,300 K.
TL;DR: In this article, the authors report combined scanning tunneling microscopy and electrochemical reactivity measurements on individual palladium nanoparticles supported on a gold surface and show that the catalytic activity towards electrochemical proton reduction is enhanced by more than two orders of magnitude as the diameter of the palladium particles parallel to the support surface decreases from 200 to 6 nm.
TL;DR: This work investigates several possible routes for insertion of the central nitrogen ligand and concludes that all routes involve barriers and intermediate states, which are inaccessible at ambient conditions.
Abstract: We investigate the chemical consequences of a central ligand in the nitrogenase FeMo cofactor using density functional calculations. Several studies have shown that the central ligand most probably is a nitrogen atom, but the consequences for the chemical reactivity of the cofactor are unknown. We investigate several possible routes for insertion of the central nitrogen ligand and conclude that all routes involve barriers and intermediate states, which are inaccessible at ambient conditions. On this basis we suggest that the central nitrogen ligand is present at all times during the reaction. Furthermore, we investigate how the FeMoco with the central ligand can interact with N2 and reduce it.
TL;DR: In this article, the authors present a density-functional-theory study of formaldehyde and methanol synthesis from CO and H2 on a Ni catalyst and calculate the reaction enthalpy and energy barrier for each elementary reaction.
Abstract: We present a density-functional-theory study of formaldehyde and methanol synthesis from CO and H2 on a Ni catalyst We investigate the intermediate products of the reaction and calculate the reaction enthalpy and energy barrier for each elementary reaction, taking into account several different adsorption geometries and the presence of isomers of the intermediate products Hydrogenation of CO is favored over desorption or dissociation of CO on flat Ni(111), to form the formyl radical (HCO) or its isomer, COH Subsequent hydrogenation leads to formaldehyde (CH2O), methoxy (CH3O), and, finally, methanol (CH3OH) The overall reaction barrier for formaldehyde and methanol formation is 20 eV
TL;DR: In this paper, a linear dependence of the exchange rate of CO under different partial pressures of CO (p co) in argon was observed and a change in surface coverage of CO with different CO pressures and a subsequent reduction in the CO binding energy as demonstrated by Density Functional Theory (DFT) calculations.
Abstract: Carbon monoxide adsorption on high area platinum fuel cell catalysts was investigated. Isotopic exchange experiments were performed to determine the exchange rate (k) of CO under different partial pressures of CO (p co) in argon. A linear dependence of In(k) with In(pco) was observed. This pressure dependence of the rate of exchange is explained by considering a change in surface coverage of CO with different CO pressures and a subsequent reduction in the CO binding energy as demonstrated by Density Functional Theory (DFT) calculations. High Pressure Scanning Tunneling Microscopy (HP STM) studies on the Pt(111) surface have also displayed a pressure dependency of the coverage consistent with this data. The relevance of these observations to the Polymer Electrolyte Membrane Fuel Cell (PEMFC) anode reaction is discussed. (Less)
TL;DR: Theoretical calculations rationalize the results for transition metal surfaces in terms of the energy shift of the d-band center of mass and this proves that adsorbate-induced SCL shifts provide a spectroscopic measure of local surface reactivity.
Abstract: In this Letter we show that sequences of adsorbate-induced shifts of surface core level (SCL) x-ray photoelectron spectra contain profound information on surface changes of electronic structure and reactivity. Energy shifts and intensity changes of time-lapsed spectral components follow simple rules, from which adsorption sites are directly determined. Theoretical calculations rationalize the results for transition metal surfaces in terms of the energy shift of the d-band center of mass and this proves that adsorbate-induced SCL shifts provide a spectroscopic measure of local surface reactivity.
TL;DR: In this article, the ground state geometical and magnetic structure of the FeFe-cofactor of the iron-only nitrogenase was determined and the results were in agreement with experimental observations.
Abstract: We use density functional calculations to model the FeFe-cofactor of the iron-only nitrogenase. We determine the ground state geometical and magnetic structure and show that our results are in agreement with experimental observations. We model the reactivity of the FeFeco towards hydrogen and nitrogen and compare to the FeMoco. Our results for the difference in reactivity can be linked to experimental observations.
TL;DR: In this paper, the authors present extensive density functional theory calculations of the bonding between strands in β-sheets and identify a significant cooperative effect whereby the interaction increases in strength with the number of strands.
Abstract: We present extensive density functional theory calculations of the bonding between strands in β-sheets. We identify a significant cooperative effect whereby the interaction increases in strength with the number of strands. We show that the effect is related to a coupling between interstrand bonding and intrastrand elastic properties. It is found that a direct consequence of this coupling is that the pitch of β-sheets should contract with increasing number of strands, and we show that the effect can be observed directly in experimental data from the Protein Data Bank.
Patent•
10 Aug 2004
TL;DR: In this article, a supported catalyst consisting of nickel as the active catalytic component promoted with silver or gold was presented in an amount between 0.001% to 30% by weight calculated on the amount of nickel in the catalyst.
Abstract: Catalytic process for oxidation or reduction of organic and inorganic compounds comprising contacting the compound under oxidation or reduction conditions with a supported catalyst consisting of nickel as the active catalytic component promoted with silver or gold, the silver or gold being present in an amount between 0.001% to 30% by weight calculated on the amount of nickel in the catalyst.
Patent•
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25 May 2004
TL;DR: In this paper, a process for suppressing the formation of solid carbon in a fuel cell comprising contacting a hydrocarbon feedstock with a promoted nickel-comprising anode, the promoter including gold or silver in an amount of 0.001 to 30% by weight calculated on the amount of nickel in the anode.
Abstract: Process for suppressing the formation of solid carbon in a fuel cell comprising contacting a hydrocarbon feedstock with a promoted nickel-comprising anode, the promoter including gold or silver in an amount of 0.001 to 30% by weight calculated on the amount of nickel in the anode.