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ABSTRACT: IntroductionHaldor Topsøe was born in Copenhagen, Denmark, in 1913. He studied chemical engineering at the Technical University of Denmark (DTU). In 1940, he founded the company Haldor Topsøe, which has become one of the most successful companies in the field of heterogeneous catalysis. The company develops and markets catalysts and processes for petroleum refining, air-pollution control, synthesis gas production, ammonia and methanol production and solid oxide fuel cells. The company Haldor Topsøe is also a leader in the production and state-of-the-art scientific characterization of new catalysts. Dr. Topsøe is currently the sole owner of the company. He has written numerous articles on ammonia synthesis, catalysis, and energy questions. He is the author of several books on economics. Dr. Topsøe has received numerous awards, including honorary doctorates from Aarhus University, DTU, and the Chalmers University in Sweden; the Gold Medal from the Royal Academy of Sciences; the Hoover Medal for his technica...
06/2012; 3:1-10.
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Lindsay R Merte,
Guowen Peng,
Ralf Bechstein,
Felix Rieboldt,
Carrie A Farberow,
Lars C Grabow,
Wilhelmine Kudernatsch,
Stefan Wendt,
Erik Lægsgaard, Manos Mavrikakis,
Flemming Besenbacher
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ABSTRACT: The diffusion of hydrogen atoms across solid oxide surfaces is often assumed to be accelerated by the presence of water molecules. Here we present a high-resolution, high-speed scanning tunneling microscopy (STM) study of the diffusion of H atoms on an FeO thin film. STM movies directly reveal a water-mediated hydrogen diffusion mechanism on the oxide surface at temperatures between 100 and 300 kelvin. Density functional theory calculations and isotope-exchange experiments confirm the STM observations, and a proton-transfer mechanism that proceeds via an H(3)O(+)-like transition state is revealed. This mechanism differs from that observed previously for rutile TiO(2)(110), where water dissociation is a key step in proton diffusion.
Science 05/2012; 336(6083):889-93. · 31.20 Impact Factor
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ABSTRACT: The kinetics of oxygen reduction was investigated in acid solutions on Pt monolayers deposited on modified carbon-supported
PdIr nanoparticles using the rotating disk-electrode technique. Iridium is introduced into the Pd substrate in order to fine-tune
the Pt–Pd interactions and to improve Pd stability under operating conditions of the fuel cell. The kinetics of the oxygen
reduction reaction shows enhancement with the Pt monolayer on the PdIr nanoparticle surfaces in comparison with the reaction
on Pt/C and Pt monolayer on Pd/C nanoparticles. The electrochemical measurements suggest that reduced oxidation of Pt monolayer
on PdIr/C compared to Pt/C and Pt monolayer on Pd/C is the cause of enhanced activity. Besides a ligand effect induced to
the Pt surface by the presence of PdIr in the second sublayer of the nanoparticle, as suggested by our density functional
theory analysis, Ir also leads to a Pd skin contraction, so the Pt monolayer on PdIr/C is compressed more than on Pd/C. Both
effects lead to further weakening of the Pt–OH interaction, thus increasing the ORR activity. The Pt-specific activity for
PtMLPdIr/C is three times and 25% higher than that of Pt/C and PtMLPd/C respectively; the Pt-mass activity of PtMLPdIr/C is more than 20 times and 25% higher than that of Pt/C and PtMLPd/C, respectively.
KeywordsCore-shell-PEM fuel cell-ORR-Copper UPD
04/2012; 1(4):213-223.
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ABSTRACT: The interaction of hydrogen with the close-packed facets of seventeen transition metals overlaid with 1ML of five transition
metals (Au, Ag, Cu, Pt, and Pd) has been studied using periodic self-consistent (GGA-PW91) density functional theory (DFT)
calculations. For noble metal overlayers (Au, Ag, and Cu), hydrogen at the host-metal/overlayer interface (subsurface hydrogen)
is more stable than subsurface hydrogen in the pure host. For certain Au and Ag overlayers, subsurface hydrogen is more stable
than surface hydrogen in the same system. The presence of subsurface hydrogen was found to have a significant effect on the
electronic structure of the overlayer, resulting in its modified surface reactivity.
KeywordsDFT-Hydrogen-Alloys
Topics in Catalysis 04/2012; 53(5):384-392. · 2.62 Impact Factor
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ABSTRACT: Using a combination of density functional theory (DFT) calculations and an array of experimental techniques including in situ
X-ray absorption spectroscopy, we identified, synthesized, and tested successfully a new class of electrocatalysts for the
oxygen reduction reaction (ORR) that were based on monolayers of Pt deposited on different late transition metals (Au, Pd,
Ir, Rh, or Ru), of which the Pd-supported Pt monolayer had the highest ORR activity. The amount of Pt used was further decreased
by replacing part of the Pt monolayer with a third late transition metal (Au, Pd, Ir, Rh, Ru, Re, or Os). Several of these
mixed Pt monolayers deposited on Pd single crystal or on carbon-supported Pd nanoparticles exhibited up to a 20-fold increase
in ORR activity on a Pt-mass basis when compared with conventional all-Pt electrocatalysts. DFT calculations showed that their
superior activity originated from the interaction between the Pt monolayer and the Pd substrate and from a reduced OH coverage
on Pt sites, the result of enhanced destabilization of Pt–OH induced by the oxygenated third metal. This new class of electrocatalysts
promises to alleviate the major problems of existing fuel cell technology by simultaneously decreasing materials cost and
enhancing performance.
Topics in Catalysis 04/2012; 46(3):276-284. · 2.62 Impact Factor
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ABSTRACT: A new type of electrocatalyst with a core–shell structure that consists of a platinum monolayer shell placed on an iridium–rhenium nanoparticle core or platinum and palladium bilayer shell deposited on that core has been prepared and tested for electrocatalytic activity for the oxygen reduction reaction. Carbon-supported iridium–rhenium alloy nanoparticles with several different molar ratios of Ir to Re were prepared by reducing metal chlorides dispersed on Vulcan carbon with hydrogen gas at 400 °C for 1 h. These catalysts showed specific electrocatalytic activity for oxygen reduction reaction comparable to that of platinum. The activities of PtML/PdML/Ir2Re1, PtML/Pd2layers/Ir2Re1, and PtML/Pd2layers/Ir7Re3 catalysts were, in fact, better than that of conventional platinum electrocatalysts, and their mass activities exceeded the 2015 DOE target. Our density functional theory calculations revealed that the molar ratio of Ir to Re affects the binding strength of adsorbed OH and, thereby, the O2 reduction activity of the catalysts. The maximum specific activity was found for an intermediate OH binding energy with the corresponding catalyst on the top of the volcano plot. The monolayer concept facilitates the use of much less platinum than in other approaches. The results with the PtML/PdML/Ir2Re electrocatalyst indicate that it is a promising alternative to conventional Pt electrocatalysts in low-temperature fuel cells.
ACS Catalysis 03/2012; 2(5):817.
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ABSTRACT: Using a combination of periodic, self-consistent, density functional theory (DFT) calculations and CO-stripping voltammetry experiments, we have designed a new class of Pt-M bimetallic monolayer catalysts supported on a non-Pt metal, which exhibit improved stability against CO poisoning and might be suitable for proton-exchange membrane fuel cell anodes. These surfaces help in reducing the overpotential associated with anodic CO oxidation and minimize the amount of Pt used, thereby reducing materials cost. DFT calculations predict highly repulsive interactions between adsorbed CO molecules on these surfaces, leading to weaker binding and lower coverage of CO than on pure Pt, which in turn facilitates oxidative removal of CO from these catalytic surfaces.
Journal of the American Chemical Society 11/2011; 133(46):18574-6. · 9.91 Impact Factor
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Lindsay R. Merte,
Jan Knudsen,
Falk M. Eichhorn,
Soeren Porsgaard,
Helene Zeuthen,
Lars C. Grabow,
Erik Lægsgaard,
Hendrik Bluhm,
Miquel Salmeron, Manos Mavrikakis,
Flemming Besenbacher
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ABSTRACT: The reduction of a single-layer FeO film grown on Pt(111) by CO at elevated pressures and temperatures has been studied through an interplay of scanning tunneling microscopy, ambient-pressure X-ray photoelectron spectroscopy, and density functional theory calculations. Exposure of the FeO thin film to CO at pressures between 1 and 30 Torr and temperatures between 500 and 530 K leads to formation of a honeycomb-structured Fe3O2 film with hollow sites occupied by single Pt atoms extracted from the substrate surface. The formation of these adatoms is driven by an increase in CO adsorption energy. In addition, the structure incorporates undercoordinated Fe centers, which are proposed to have substantial effects on the catalytic properties of the surface.
07/2011;
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Lindsay R Merte,
Jan Knudsen,
Falk M Eichhorn,
Soeren Porsgaard,
Helene Zeuthen,
Lars C Grabow,
Erik Lægsgaard,
Hendrik Bluhm,
Miquel Salmeron, Manos Mavrikakis,
Flemming Besenbacher
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ABSTRACT: The reduction of a single-layer FeO film grown on Pt(111) by CO at elevated pressures and temperatures has been studied through an interplay of scanning tunneling microscopy, ambient-pressure X-ray photoelectron spectroscopy, and density functional theory calculations. Exposure of the FeO thin film to CO at pressures between 1 and 30 Torr and temperatures between 500 and 530 K leads to formation of a honeycomb-structured Fe(3)O(2) film with hollow sites occupied by single Pt atoms extracted from the substrate surface. The formation of these adatoms is driven by an increase in CO adsorption energy. In addition, the structure incorporates undercoordinated Fe centers, which are proposed to have substantial effects on the catalytic properties of the surface.
Journal of the American Chemical Society 06/2011; 133(28):10692-5. · 9.91 Impact Factor
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ABSTRACT: High-resolution scanning tunneling microscope (STM) images of moiré-structured FeO films on Pt(111) were obtained in a number of different tip-dependent imaging modes. For the first time, the STM images are distinguished and interpreted unambiguously with the help of distinct oxygen-vacancy dislocation loops in the FeO moiré structure. The experimental STM results are compared with the results of electronic structure calculations within the DFT+U scheme for a realistic (√91 × √91)R5.2° moiré FeO unit cell supported on Pt(111) as well as with the results from previous studies. We find that one type of STM imaging mode, showing both Fe and O atoms, agrees well with simulated STM images, indicating that the simple Tersoff−Hamann theory is partially valid for this imaging mode. In addition, we identify other distinct, element-specific imaging modes which reveal a strong dependence on the specific tip apex state and likely result from specific tip−sample chemical interactions. From the present STM results we show that several of the previously published conclusions for the FeO system have to be revisited.
01/2011;
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ABSTRACT: Through an interplay between density functional theory and scanning tunneling microscopy, we investigated the mechanism of low-temperature CO oxidation on Ni(111) and NiO(111) surfaces. We systematically examined CO oxidation on different possible active sites. We find that sub- and full monolayers of O chemisorbed on Ni(111) surfaces play no significant role in low-temperature CO oxidation. We further show that CO oxidation at the perimeter of O islands on Ni(111) cannot occur at low temperatures. In contrast, we suggest that oxidized Ni(111) surfaces, i.e., NiO(111), can catalyze low-temperature CO oxidation when NiO(111) is saturated by O2. Our findings can rationalize low-temperature CO oxidation on Ni(111) surfaces that have been predosed with large amounts of oxygen, as observed in recent experiments.
11/2010;
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Yanping Zhai,
Danny Pierre,
Rui Si,
Weiling Deng,
Peter Ferrin,
Anand U Nilekar,
Guowen Peng,
Jeffrey A Herron,
David C Bell,
Howard Saltsburg, Manos Mavrikakis,
Maria Flytzani-Stephanopoulos
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ABSTRACT: We report that alkali ions (sodium or potassium) added in small amounts activate platinum adsorbed on alumina or silica for the low-temperature water-gas shift (WGS) reaction (H(2)O + CO → H(2) + CO(2)) used for producing H(2). The alkali ion-associated surface OH groups are activated by CO at low temperatures (~100°C) in the presence of atomically dispersed platinum. Both experimental evidence and density functional theory calculations suggest that a partially oxidized Pt-alkali-O(x)(OH)(y) species is the active site for the low-temperature Pt-catalyzed WGS reaction. These findings are useful for the design of highly active and stable WGS catalysts that contain only trace amounts of a precious metal without the need for a reducible oxide support such as ceria.
Science 09/2010; 329(5999):1633-6. · 31.20 Impact Factor
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ABSTRACT: From an interplay between scanning tunneling microscopy, temperature programmed desorption, X-ray photoelectron spectroscopy, and density functional theory calculations we have studied low-temperature CO oxidation on Au/Ni(111) surface alloys and on Ni(111). We show that an oxide is formed on both the Ni(111) and the Au/Ni(111) surfaces when oxygen is dosed at 100 K, and that CO can be oxidized at 100 K on both of these surfaces in the presence of weakly bound oxygen. We suggest that low-temperature CO oxidation can be rationalized by CO oxidation on O(2)-saturated NiO(111) surfaces, and show that the main effect of Au in the Au/Ni(111) surface alloy is to block the formation of carbonate and thereby increase the low-temperature CO(2) production.
ACS Nano 08/2010; 4(8):4380-7. · 10.77 Impact Factor
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ABSTRACT: First-principles, periodic, density functional theory (DFT) calculations are carried out on Pt(111) to investigate the structure and energetics of dehydrogenated ethylene glycol species and transition states for the cleavage of C−H/O−H and C−C bonds. Additionally, reaction kinetics studies are carried out for the vapor phase reforming of ethylene glycol (C2H6O2) over Pt/Al2O3 at various temperatures, pressures, and feed concentrations. These results are compared to data for aqueous phase reforming of ethylene glycol on this Pt catalyst, as reported in a previous publication (Shabaker, J. W.; et al. J. Catal. 2003, 215, 344). Microkinetic models were developed to describe the reaction kinetics data obtained for both the vapor-phase and aqueous-phase reforming processes. The results suggest that C−C bond scission in ethylene glycol occurs at an intermediate value of x (3 or 4) in C2HxO2. It is also found that similar values of kinetic parameters can be used to describe the vapor and aqueous phase reforming data, suggesting that the vapor phase chemistry of this reaction over platinum is similar to that in the aqueous phase over platinum.
08/2010;
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ABSTRACT: We report on the first-principles-guided design, synthesis, and characterization of core-shell nanoparticle (NP) catalysts made of a transition metal core (M = Ru, Rh, Ir, Pd, or Au) covered with a approximately 1-2 monolayer thick shell of Pt atoms (i.e., a M@Pt core-shell NP). An array of experimental techniques, including X-ray diffraction, Fourier transform infrared spectroscopy, high resolution transmission electron microscopy, and temperature-programmed reaction, are employed to establish the composition of the synthesized NPs. Subsequent studies of these NPs' catalytic properties for preferential CO oxidation in hydrogen-rich environments (PROX), combined with Density Functional Theory (DFT)-based mechanistic studies, elucidate important trends and provide fundamental understanding of the reactivity of Pt shells as a function of the core metal. Both the PROX activity and selectivity of several of these M@Pt core-shell NPs are significantly improved compared to monometallic and bulk nonsegregated bimetallic nanoalloys. Among the systems studied, Ru@Pt core-shell NPs exhibit the highest PROX activity, where the CO oxidation is complete by 30 degrees C (1000 ppm CO in H(2)). Therefore, despite their reduced Pt content, M@Pt core-shell NPs afford the design of more active PROX catalysts. DFT studies suggest that the relative differences in the catalytic activities for the various core-shell NPs originate from a combination of (i) the relative availability of CO-free Pt surface sites on the M@Pt NPs, which are necessary for O(2) activation, and (ii) a hydrogen-mediated low-temperature CO oxidation process that is clearly distinct from the traditional bifunctional CO oxidation mechanism.
Journal of the American Chemical Society 06/2010; 132(21):7418-28. · 9.91 Impact Factor
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ABSTRACT: The conductance of π-conjugated molecular wires bonded to gold electrodes at zero bias is studied using density functional theory combined with nonequilibrium Green’s function method. For all systems considered, we find that the conductance length dependence follows the simple exponential law characteristic of tunneling through a barrier, G = Gc exp(−βL). For thiophene, pyrrole, and phenyl wires with thiol end-groups, we calculate decay constants (β) of 0.211, 0.257, and 0.264 Å−1, respectively, and contact conductances (Gc) of 1.25, 2.90, and 1.22G0, where G0 = 2e2/h is the conductance quantum. In comparison, the corresponding values for amine-terminated thiophene are calculated to be β = 0.160 Å−1 and Gc = 0.038G0. These results show that (1) the contact resistance is mainly determined by the anchoring group and (2) the decay constant, which determines the conductance in the long wire limit, is not solely determined by the intrinsic band gap of the molecular wire but also depends on the anchoring group. This is because the alignment of the metal Fermi level with respect to the molecular levels is controlled by charge transfer and interface dipoles which in turn are determined by the local chemistry at the interface. Analysis of the charge transfer at the interface shows that the thiol-bonded molecules receive electrons from the Au electrodes while the amine-bonded molecules donate electrons to the Au electrodes.
12/2009;
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ABSTRACT: We improved the effectiveness of Pt monolayer electrocatalysts for the oxygen-reduction reaction (ORR) using a novel approach to fine-tuning the Pt monolayer interaction with its support, exemplified by an annealed Pd(3)Fe(111) single-crystal alloy support having a segregated Pd layer. Low-energy ion scattering and low-energy electron diffraction studies revealed that a segregated Pd layer, with the same structure as Pd (111), is formed on the surface of high-temperature-annealed Pd(3)Fe(111). This Pd layer is considerably more active than Pd(111); its ORR kinetics is comparable to that of a Pt(111) surface. The enhanced catalytic activity of the segregated Pd layer compared to that of bulk Pd apparently reflects the modification of Pd surface's electronic properties by underlying Fe. The Pd(3)Fe(111) suffers a large loss in ORR activity when the subsurface Fe is depleted by potential cycling (i.e., repeated excursions to high potentials in acid solutions). The Pd(3)Fe(111) surface is an excellent substrate for a Pt monolayer ORR catalyst, as verified by its enhanced ORR kinetics on PT(ML)/Pd/Pd(3)Fe(111). Our density functional theory studies suggest that the observed enhancement of ORR activity originates mainly from the destabilization of OH binding and the decreased Pt-OH coverage on the Pt/Pd/Pd(3)Fe(111) surface. The activity of Pt(ML)/Pd(111) and Pt(111) is limited by OH removal, whereas the activity of Pt(ML)/Pd/Pd(3)Fe(111) is limited by the O-O bond scission, which places these two surfaces on the two sides of the volcano plot.
Journal of the American Chemical Society 10/2009; 131(35):12755-62. · 9.91 Impact Factor
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ABSTRACT: We have investigated the structure sensitivity of methanol electrooxidation on eight transition metals (Au, Ag, Cu, Pt, Pd, Ir, Rh, and Ni) using periodic, self-consistent density functional theory (DFT-GGA). Using the adsorption energies of 16 intermediates on two different facets of these eight face-centered-cubic transition metals, combined with a simple electrochemical model, we address the differences in the reaction mechanism between the (111) and (100) facets of these metals. We investigate two separate mechanisms for methanol electrooxidation: one going through a CO* intermediate (the indirect pathway) and another that oxidizes methanol directly to CO(2) without CO* as an intermediate (the direct pathway). A comparison of our results for the (111) and (100) surfaces explains the origin of methanol electrooxidation's experimentally-established structure sensitivity on Pt surfaces. For most metals studied, on both the (111) and (100) facets, we predict that the indirect mechanism has a higher onset potential than the direct mechanism. Ni(111), Au(100), and Au(111) are the cases where the direct and indirect mechanisms have the same onset potential. For the direct mechanism, Rh, Ir, and Ni show a lower onset potential on the (111) facet, whereas Pt, Cu, Ag, and Au possess lower onset potential on the (100) facet. Pd(100) and Pd(111) have the same onset potential for the direct mechanism. These results can be rationalized by the stronger binding energy of adsorbates on the (100) facet versus the (111) facet. Using linear scaling relations, we establish reactivity descriptors for the (100) surface similar to those recently developed for the (111) surface; the free energies of adsorbed CO* and OH* can describe methanol electrooxidation trends on various metal surfaces reasonably well.
Journal of the American Chemical Society 09/2009; 131(40):14381-9. · 9.91 Impact Factor
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ABSTRACT: Using a combination of density functional theory and experimental electrochemical methods, we have investigated Au−Ir near-surface alloys and their ability to take up hydrogen (H). Despite the relative instability of H in the subsurface of pure Au(111) and Ir(111), H in the subsurface of a near-surface alloy made of a monolayer of Au over Ir(111) (Au*/Ir) is stabilized compared to surface H. While dissociative adsorption of H2 is activated on this alloy surface, the presence of subsurface H stabilizes the transition state for further H2 dissociation. This is explained by the upshift in the d-band center of the surface Au atoms induced by the presence of subsurface H in Au*/Ir. Submonolayers of Au on Ir(111) stabilize H at the Au−Ir interface while allowing for nonactivated H2 dissociation at the exposed Ir atoms. Synthesis of submonolayer alloys of Au on Ir(111) shows that Au does not block hydrogen adsorption on the Ir surface, despite the inability of pure Au to take up hydrogen under these conditions. The possible existence of subsurface hydrogen in these alloys may explain this unexpected behavior.
01/2009;
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10/2008: pages 271 - 315; , ISBN: 9780470463772
