Showing papers by "Jens K. Nørskov published in 2005"
TL;DR: The ability of different metal surfaces and of the enzymes nitrogenase and hydrogenase to catalyze the hydrogen evolution reaction is analyzed and a necessary criterion for high catalytic activity is found: that the binding free energy of atomic hydrogen to the catalyst is close to zero.
Abstract: The electrochemical hydrogen evolution reaction is catalyzed most effectively by the Pt group metals. As H2 is considered as a future energy carrier, the need for these catalysts will increase and alternatives to the scarce and expensive Pt group catalysts will be needed. We analyze the ability of different metal surfaces and of the enzymes nitrogenase and hydrogenase to catalyze the hydrogen evolution reaction and find a necessary criterion for high catalytic activity. The necessary criterion is that the binding free energy of atomic hydrogen to the catalyst is close to zero. The criterion enables us to search for new catalysts, and inspired by the nitrogenase active site, we find that MoS2 nanoparticles supported on graphite are a promising catalyst. They catalyze electrochemical hydrogen evolution at a moderate overpotential of 0.1−0.2 V.
3,302 citations
TL;DR: A density functional theory database of hydrogen chemisorption energies on close packed surfaces of a number of transition andnoble metals is presented in this article, where the bond energies are used to understand the trends in the exchange current for hydrogen evolution.
Abstract: Department of Physics, Technical University Munich, D-85748 Garching, GermanyA density functional theory database of hydrogen chemisorption energies on close packed surfaces of a number of transition andnoble metals is presented. The bond energies are used to understand the trends in the exchange current for hydrogen evolution. Avolcano curve is obtained when measured exchange currents are plotted as a function of the calculated hydrogen adsorptionenergies and a simple kinetic model is developed to understand the origin of the volcano. The volcano curve is also consistent withPt being the most efficient electrocatalyst for hydrogen evolution.© 2005 The Electrochemical Society. @DOI: 10.1149/1.1856988# All rights reserved.Manuscript submitted May 10, 2004; revised manuscript received August 12, 2004. Available electronically January 24, 2005.
2,623 citations
TL;DR: In this paper, a method for obtaining the thermochemistry of the electrochemical water splitting process as a function of the bias directly from the electronic structure calculations was developed, and the binding energies of the different intermediates were linearly correlated for a number of metals.
1,298 citations
TL;DR: The rate of ammonia synthesis over a nanoparticle ruthenium catalyst can be calculated directly on the basis of a quantum chemical treatment of the problem using density functional theory, and offers hope for computer-based methods in the search for catalysts.
Abstract: The rate of ammonia synthesis over a nanoparticle ruthenium catalyst can be calculated directly on the basis of a quantum chemical treatment of the problem using density functional theory. We compared the results to measured rates over a ruthenium catalyst supported on magnesium aluminum spinel. When the size distribution of ruthenium particles measured by transmission electron microscopy was used as the link between the catalyst material and the theoretical treatment, the calculated rate was within a factor of 3 to 20 of the experimental rate. This offers hope for computer-based methods in the search for catalysts.
1,028 citations
374 citations
TL;DR: Using a combination of density functional theory calculations and X-ray emission and absorption spectroscopy for nitrogen on Cu and Ni surfaces, a detailed picture of the chemisorption bond was given in this paper.
Abstract: Using a combination of density functional theory calculations and X-ray emission and absorption spectroscopy for nitrogen on Cu and Ni surfaces, a detailed picture is given of the chemisorption bond. It is suggested that the adsorption bond strength and hence the activity of transition metal surfaces as catalysts for chemical reactions can be related to certain characteristics of the surface electronic structure.
327 citations
TL;DR: In this paper, the physical, chemical and electronic structure properties of several close-packed surfaces of early transition metal carbides, including β-Mo2C(0, 0, 0), and the (1, 1, 1) surfaces of TiC, VC, NbC, and TaC were investigated.
285 citations
TL;DR: It is shown, from an interplay between scanning tunnelling microscopy experiments and density functional theory calculations, that the activation of ethylene on Ni(111) follows the trend of higher reactivity for decomposition at step edges as compared with the higher-coordinated terrace sites.
Abstract: The reactivity of catalytic surfaces is often dominated by very reactive low-coordinated atoms such as step-edge sites1,2,3,4,5,6,7,8,9,10,11. However, very little knowledge exists concerning the influence of step edges on the selectivity in reactions involving multiple reaction pathways. Such detailed information could be very valuable in rational design of new catalysts with improved selectivity. Here we show, from an interplay between scanning tunnelling microscopy experiments and density functional theory calculations, that the activation of ethylene on Ni(111) follows the trend of higher reactivity for decomposition at step edges as compared with the higher-coordinated terrace sites. The step-edge effect is considerably more pronounced for the C–C bond breaking than for the C–H bond breaking, and thus steps play an important role in the bond-breaking selectivity. Furthermore, we demonstrate how the number of reactive step sites can be controlled by blocking the steps with Ag. This approach to nanoscale design of catalysts is exploited in the synthesis of a new high-surface-area AgNi alloy catalyst, which is tested in hydrogenolysis experiments.
279 citations
TL;DR: In this article, the authors investigated the dissociation of methane on the terraces and steps of a Ni(1/1) surface and found that the steps exhibit a higher activation energy than terraces, in complete agreement with their calculated difference in activation energy of 17kJ/mol.
233 citations
TL;DR: In this article, it has been found that the optimal catalyst for ammonia synthesis is never the optimal catalysts for ammonia decomposition, which is almost counter-intuitive in catalysis research.
203 citations
TL;DR: In this article, the authors studied the adsorption of molecules and the oxidation reaction of CO on gold clusters using density functional theory and showed that low-coordinated sites on the gold nanoparticles can adsorb small inorganic molecules such as O2 and CO, and the presence of these sites is the key factor for the catalytic properties of supported gold nanoclusters.
Abstract: We present a summary of our theoretical results regarding CO oxidation on both oxide-supported and isolated gold nanoparticles. Using Density Functional Theory we have studied the adsorption of molecules and the oxidation reaction of CO on gold clusters. Low-coordinated sites on the gold nanoparticles can adsorb small inorganic molecules such as O2 and CO, and the presence of these sites is the key factor for the catalytic properties of supported gold nanoclusters. Other contributions, induced by the presence of the support, can provide parallel channels for the reaction and modulate the final efficiency of Au-based catalysts. Finally, our theoretical simulations allow us to discuss the selectivity of supported Au nanoparticles.
TL;DR: It is demonstrated that the error bars on energy differences may vary by orders of magnitude for different systems in good agreement with existing experience.
Abstract: We present a practical scheme for performing error estimates for density-functional theory calculations. The approach, which is based on ideas from Bayesian statistics, involves creating an ensemble of exchange-correlation functionals by comparing with an experimental database of binding energies for molecules and solids. Fluctuations within the ensemble can then be used to estimate errors relative to experiment on calculated quantities such as binding energies, bond lengths, and vibrational frequencies. It is demonstrated that the error bars on energy differences may vary by orders of magnitude for different systems in good agreement with existing experience.
TL;DR: In this article, a simple scheme for the estimation of oxygen binding energies on transition metal surface alloys is presented, based on a simple d-band center model of the alloy surfaces.
TL;DR: In this article, the authors investigated the possibility of using metal ammine complexes as a solid form of hydrogen storage using Mg(NH3)6Cl2 as the example, and showed that it can store 91% hydrogen by weight in the form of ammonia.
Abstract: The hopes of using hydrogen as an energy carrier are severely dampened by the fact that there is still no safe, high-density method available for storing hydrogen We investigate the possibility of using metal ammine complexes as a solid form of hydrogen storage Using Mg(NH3)6Cl2 as the example, we show that it can store 91% hydrogen by weight in the form of ammonia The storage is completely reversible, and by combining it with an ammonia decomposition catalyst, hydrogen can be delivered at temperatures below 620 K
TL;DR: The present results show that the differences in activities for MoS2 and Co-Mo-S structures can be attributed to the electronic and bonding differences introduced by the bridging O bonds, and find that the Mo-O linkages are most probably located on the (1010) S edge.
Abstract: Density functional theory is used to investigate the origin of the activity differences between Type I and Type II MoS2-based structures in hydrotreating catalysts. It is well known that the Type II structures, where only weak interactions with the support exist, have a higher catalytic activity than Type I structures, where Mo−O linkages to the alumina are present. The present results show that the differences in activities for MoS2 and Co−Mo−S structures can be attributed to the electronic and bonding differences introduced by the bridging O bonds. We find that the Mo−O linkages are most probably located on the (1010) S edge. The presence of oxygen linkages increases the energy required to form sulfur vacancies significantly so that almost no vacancies can be formed at these and neighboring sites. In this way, the reactivity of the S edge is reduced. In addition, the studies also show that the linkages introduce changes in the one-dimensional metallic-like brim states. Furthermore, the presence of oxyg...
TL;DR: In this paper, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) studies show that the high activity Type II structures may be present as single sulfide sheets.
TL;DR: In this paper, density functional theory was used to investigate the hydride formation in magnesium-3d transition metal (TM) alloys of potential interest as hydrogen storage media.
Patent•
03 Aug 2005
TL;DR: A solid ammonia storage and delivery material is defined in this article as an ionic salt of the general formula: Ma(NH3)nXz, where M is one or more cations selected from alkaline earth metals, and/or one of transition metals, such as Mn, Fe, Co, Ni, Cu, and or Zn, X is the number of cations per salt molecule, and ri is the coordination number.
Abstract: A solid ammonia storage and delivery material A solid ammonia storage material comprising: an ammonia absorbing salt, wherein the ammonia absorbing salt is an ionic salt of the general formula: Ma(NH3)nXz, wherein M is one or more cations selected from alkaline earth metals, and/or one or more transition metals, such as Mn, Fe, Co, Ni, Cu, and/or Zn, X is one or more anions, a is the number of cations per salt molecule, z is the number of anions per salt molecule, and ri is the coordination number of 2 to 12, wherein M is Mg provides a safe, light-weight and cheap compact storage for ammonia to be used in the automotive industry.
TL;DR: In this article, a single parameter characterizing the electronic structure of a transition metal surface, the d-band center (ed), was used to provide a unified description of a range of phenomena in heterogeneous catalysis.
Abstract: It is shown that a single parameter characterizing the electronic structure of a transition metal surface, the d-band center (ed), can be used to provide a unified description of a range of phenomena in heterogeneous catalysis. Using methane activation on Ni surfaces as an example, we show that variations in ed can be used to quantitatively describe variations in the activation energy when the surface structure is changed, when the coverage of carbon is changed, when the surface is strained, when the surface is alloyed, and when the surface is poisoned by sulfur. The d-band center is, therefore, a very general descriptor of the reactivity of a surface.
TL;DR: In this article, the adsorption energy of CO for a platinum overlayer on Ru(0001) has been analyzed and a significant reduction in the binding energy was observed compared to that obtained on Pt(111).
Abstract: Density Functional Theory (DFT) calculations of the adsorption energy of CO, for a platinum overlayer on Ru(0001), have been performed. For all coverages a significant reduction in the binding energy of up to 0.5 eV has been observed compared to that obtained on Pt(111). In addition, a Steady-State Isotopic Transient Kinetic Analysis (SSITKA) study has been performed to determine the desorption rate dependence on the partial pressure of CO over commercial Pt/Ru electrocatalysts. As expected, no significant difference in the rate of exchange of CO at any given pressure is observed on going from Pt to Pt/Ru electrocatalysts when the diluted gas used was argon since the CO states will be filled to the same desorption energy for the two catalysts. However on changing the diluent gas to hydrogen, a reduction in the exchange rate for CO is observed clearly reflecting the lower CO binding energy and the increased competition for sites at the surface of the catalyst. The reduction efficiency of the Pt/Ru electrocatalyst was also studied and found to be highly dependent on whether CO or hydrogen was used. These results will be discussed with reference to the anode catalysis of the Polymer Electrolyte Membrane Fuel Cell (PEMFC).
TL;DR: Quantum-mechanical calculations of the reaction rate for dissociative adsorption of N2 on stepped Ru(0001) are presented and the tunneling effect is found to be small.
Abstract: Quantum-mechanical calculations of the reaction rate for dissociative adsorption of N2 on stepped Ru(0001) are presented. Converged six-dimensional quantum calculations for this heavy-atom reaction have been performed using the multiconfiguration time-dependent Hartree method. A potential-energy surface for the transition-state region is constructed from density-functional theory calculations using Shepard interpolation. The quantum results are in very good agreement with the results of the harmonic transition-state theory. In contrast to the findings of previous model calculations on similar systems, the tunneling effect is found to be small.
Patent•
22 Mar 2005
TL;DR: An electric power generating unit comprising of an ammonia storage device in the form of a container (2) comprising an ammonia absorbing and releasing salt of the general formula: M a (NH 3 ) n X z, wherein M is one or more cations selected from alkali metals, alkaline earth metals, and transition metals such as Li, K, Mg, Ca, V, Cr, Mn, Fe, Co, Ni, Cu or Zn.
Abstract: An electric power generating unit comprising (i) an ammonia storage device in the form of a container (2) comprising an ammonia absorbing and releasing salt of the general formula: M a (NH 3 ) n X z , wherein M is one or more cations selected from alkali metals, alkaline earth metals, and transition metals such as Li, K, Mg, Ca, V, Cr, Mn, Fe, Co, Ni, Cu or Zn, X is one or more anions selected from fluoride, chloride, bromide, iodide, nitrate, thiocyanate, sulphate, molybdate, phosphate, and chlorate ions, a is the number of cations per salt molecule, z is the number of anions per salt molecule, and n is the coordination number of 2 to 12. (ii) means (3a) for heating said container (2) and ammonia absorbing and releasing salt for releasing ammonia gas and (iiia) a fuel cell for converting ammonia directly into electric power; or (iiib1) a reactor (6) for dissociating ammonia into hydrogen and nitrogen and (iiib2) a fuel cell (4) for converting hydrogen into electric power.
Journal Article•