Frank Abild-pedersen - Academia.edu (original) (raw)
Papers by Frank Abild-pedersen
The work function and stability of 570 alloyed alkali-earth oxide films on W(100) have been calcu... more The work function and stability of 570 alloyed alkali-earth oxide films on W(100) have been calculated within density functional theory. Computational screening of this large phase space was enabled by utilizing the virtual crystal approximation, where the degree of freedom in the chemical composition is modeled with virtual atoms of mixed Ca, Sr, and Ba character. Low work functions are achieved by doping the films with Sc or Li and alloys containing small amounts of Ca. In particular, Li-doped systems with small Ca content also show favorable stability. Identification of such alloys outperforming any of the constituents relies on careful sampling of the chemical composition. We have identified the interactions within the film that limit the reduction in work function given by the dipole normal to the surface. We have shown that control of the screening of the intrafilm interactions by O atoms is essential for the design of new low work function materials.
We introduce a model for the effect of cesium adsorbates on the work function of transition metal... more We introduce a model for the effect of cesium adsorbates on the work function of transition metal surfaces. The model builds on the classical point-dipole equation by adding exponential terms that characterize the degree of orbital overlap between the 6s states of neighboring cesium adsorbates and its effect on the strength and orientation of electric dipoles along the adsorbate–substrate interface. The new model improves upon earlier models in terms of agreement with the work function–coverage curves obtained via first-principles calculations based on density functional theory. All the cesiated metal surfaces have optimal coverages between 0.6 and 0.8 monolayers, in accordance with experimental data. Of all the cesiated metal surfaces that we have considered, tungsten has the lowest minimum work function, also in accordance with experiments.
Papers by Frank Abild-pedersen
Physical review letters, Jan 17, 2015
We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from... more We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5σ and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorp...
Physical Chemistry Chemical Physics, 2011
Nature materials, Jan 24, 2016
ACS nano, 2015
Amorphous MoSx is a highly active, earth abundant catalyst for the electrochemical hydrogen evolu... more Amorphous MoSx is a highly active, earth abundant catalyst for the electrochemical hydrogen evolution reaction (HER). Previous studies have revealed that this material initially has a composition of MoS3, but after electrochemical activation, the surface is reduced to form an active phase resembling MoS2 in composition and chemical state. However, structural changes in the MoSx catalyst and the mechanism of the activation process remain poorly understood. In this study, we employ transmission electron microscopy (TEM) to image amorphous MoSx catalysts activated under two hydrogen-rich conditions: ex situ in an electrochemical cell and in situ in an environmental TEM. For the first time, we directly observe the formation of crystalline domains in the MoSx catalyst after both activation procedures as well as spatially-localized changes in the chemical state detected via electron energy loss spectroscopy (EELS). Using density functional theory (DFT) calculations, we investigate the mec...
Nature Materials, 2015
As a promising non-precious catalyst for the hydrogen evolution reaction (HER; refs ,,,,), molybd... more As a promising non-precious catalyst for the hydrogen evolution reaction (HER; refs ,,,,), molybdenum disulphide (MoS2) is known to contain active edge sites and an inert basal plane. Activating the MoS2 basal plane could further enhance its HER activity but is not often a strategy for doing so. Herein, we report the first activation and optimization of the basal plane of monolayer 2H-MoS2 for HER by introducing sulphur (S) vacancies and strain. Our theoretical and experimental results show that the S-vacancies are new catalytic sites in the basal plane, where gap states around the Fermi level allow hydrogen to bind directly to exposed Mo atoms. The hydrogen adsorption free energy (ΔGH) can be further manipulated by straining the surface with S-vacancies, which fine-tunes the catalytic activity. Proper combinations of S-vacancy and strain yield the optimal ΔGH = 0 eV, which allows us to achieve the highest intrinsic HER activity among molybdenum-sulphide-based catalysts.
The Journal of Physical Chemistry Letters, 2015
In this Letter, we examine bond activation induced by nonmetal surface promoters in the context o... more In this Letter, we examine bond activation induced by nonmetal surface promoters in the context of dehydrogenation reactions. We use C-H bond activation in methane dehydrogenation on transition metals as an example to understand the origin of the promoting or poisoning effect of nonmetals. The electronic structure of the surface and the bond order of the promoter are found to establish all trends in bond activation. On the basis of these results, we develop a predictive model that successfully describes the energetics of C-H, O-H, and N-H bond activation across a range of reactions. For a given reaction step, a single data point determines whether a nonmetal will promote bond activation or poison the surface and by how much. We show how our model leads to general insights that can be directly used to predict bond activation energetics on transition metal sulfides and oxides, which can be perceived as promoted surfaces. These results can then be directly used in studies on full catalytic pathways.
Journal of Catalysis, 2015
The Journal of Physical Chemistry Letters, 2015
We present calculated adsorption energies of oxygen on gold and platinum clusters with up to 923 ... more We present calculated adsorption energies of oxygen on gold and platinum clusters with up to 923 atoms (3 nm diameter) using density functional theory. We find that surface tension of the clusters induces a compression, which weakens the bonding of adsorbates compared with the bonding on extended surfaces. The effect is largest for close-packed surfaces and almost nonexistent on the more reactive steps and edges. The effect is largest at low coverage and decreases, even changing sign, at higher coverages where the strain changes from compressive to tensile. Quantum size effects also influence adsorption energies but only below a critical size of 1.5 nm for platinum and 2.5 nm for gold. We develop a model to describe the strain-induced size effects on adsorption energies, which is able to describe the influence of surface structure, adsorbate, metal, and coverage.
Nanoscale, 2015
We report on the design and synthesis of high performance catalytic nanoparticles with a robust g... more We report on the design and synthesis of high performance catalytic nanoparticles with a robust geometry via magnetron-sputter inert-gas condensation. Sputtering of Pd and Mg from two independent neighbouring targets enabled heterogeneous condensation and growth of nanoparticles with controlled Pd core-MgO porous shell structure. The thickness of the shell and the number of cores within each nanoparticle could be tailored by adjusting the respective sputtering powers. The nanoparticles were directly deposited on glassy carbon electrodes, and their catalytic activity towards methanol oxidation was examined by cyclic voltammetry. The measurements indicated that the catalytic activity was superior to conventional bare Pd nanoparticles. As confirmed by electron microscopy imaging and supported by density-functional theory (DFT) calculations, we attribute the improved catalytic performance primarily to inhibition of Pd core sintering during the catalytic process by the metal-oxide shell.
A simple web-based application called "CatApp" has recently been made available to the ... more A simple web-based application called "CatApp" has recently been made available to the public, which provides access to the adsorption and transition state energies of different molecules on transition metal surfaces. This paper illustrates the utility of such a database for predicting trends in the activities and selectivities of materials for a given reaction. In this case, the anhydrous dehydrogenation of methanol to formaldehyde is used as a test reaction, but the methods presented here are widely applicable to a broad range of reactions. The DFT-calculated adsorption and transition state energies of the relevant intermediates were accessed via CatApp for the stepped (211) surfaces of Ag, Cu, Pd, Pt, and Rh. As described in the literature, the binding energies for these intermediates on a given surface can be scaled with the binding energies of carbon and oxygen. Likewise, the energies of a given transition state species can be scaled with the energies of the reaction ...
Transition-metal oxides are widely used materials in catalysis as substrates and promoters, but a... more Transition-metal oxides are widely used materials in catalysis as substrates and promoters, but also as the active catalyst materials themselves. We compare the reactivity of transition-metal oxides with the one of transition metals. The comparison is exemplified for the ammonia synthesis reaction. First we show that there exist characteristic Brønsted-Evans Polanyi (BEP) relations (linear relations between transition state and dissociation energies) for dissociation of molecules on transition-metal oxides in the rutile and perovskite structure. It is well-known that the (211) metal surface is several orders of magnitude more reactive than the (111) metal surface due to the lower BEP line for the 211 facet. We find that both rutiles and perovskites follow BEP relations that are lower than the one of the 211 facet. Second we utilize the established BEP relations together with calculated adsorption energetics in a micro-kinetic model to obtain a volcano plot for the catalytic activity...
Finding sustainable energy solutions for the future will rely heavily on the energy influx from t... more Finding sustainable energy solutions for the future will rely heavily on the energy influx from the sun. One convenient way of storing solar energy is by transforming that energy into a chemical form - like a fuel. The efficiency of such a transformation will require catalysts that are optimized for specific reactions, and we will need to find new catalysts for a number of processes, if we are to successfully synthesize fuels from sunlight. A fundamental insight into the way the catalysts work at the molecular level is an essential ingredient if one wants to speed up the discovery process. In this presentation I will discuss some of the challenges in catalyst discovery. In particular, I will focus on the conversion of syngas to methanol, an important sub-reaction in the biomass to fuels process.
The work function and stability of 570 alloyed alkali-earth oxide films on W(100) have been calcu... more The work function and stability of 570 alloyed alkali-earth oxide films on W(100) have been calculated within density functional theory. Computational screening of this large phase space was enabled by utilizing the virtual crystal approximation, where the degree of freedom in the chemical composition is modeled with virtual atoms of mixed Ca, Sr, and Ba character. Low work functions are achieved by doping the films with Sc or Li and alloys containing small amounts of Ca. In particular, Li-doped systems with small Ca content also show favorable stability. Identification of such alloys outperforming any of the constituents relies on careful sampling of the chemical composition. We have identified the interactions within the film that limit the reduction in work function given by the dipole normal to the surface. We have shown that control of the screening of the intrafilm interactions by O atoms is essential for the design of new low work function materials.
We introduce a model for the effect of cesium adsorbates on the work function of transition metal... more We introduce a model for the effect of cesium adsorbates on the work function of transition metal surfaces. The model builds on the classical point-dipole equation by adding exponential terms that characterize the degree of orbital overlap between the 6s states of neighboring cesium adsorbates and its effect on the strength and orientation of electric dipoles along the adsorbate–substrate interface. The new model improves upon earlier models in terms of agreement with the work function–coverage curves obtained via first-principles calculations based on density functional theory. All the cesiated metal surfaces have optimal coverages between 0.6 and 0.8 monolayers, in accordance with experimental data. Of all the cesiated metal surfaces that we have considered, tungsten has the lowest minimum work function, also in accordance with experiments.
Physical review letters, Jan 17, 2015
We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from... more We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5σ and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorp...
Physical Chemistry Chemical Physics, 2011
Nature materials, Jan 24, 2016
ACS nano, 2015
Amorphous MoSx is a highly active, earth abundant catalyst for the electrochemical hydrogen evolu... more Amorphous MoSx is a highly active, earth abundant catalyst for the electrochemical hydrogen evolution reaction (HER). Previous studies have revealed that this material initially has a composition of MoS3, but after electrochemical activation, the surface is reduced to form an active phase resembling MoS2 in composition and chemical state. However, structural changes in the MoSx catalyst and the mechanism of the activation process remain poorly understood. In this study, we employ transmission electron microscopy (TEM) to image amorphous MoSx catalysts activated under two hydrogen-rich conditions: ex situ in an electrochemical cell and in situ in an environmental TEM. For the first time, we directly observe the formation of crystalline domains in the MoSx catalyst after both activation procedures as well as spatially-localized changes in the chemical state detected via electron energy loss spectroscopy (EELS). Using density functional theory (DFT) calculations, we investigate the mec...
Nature Materials, 2015
As a promising non-precious catalyst for the hydrogen evolution reaction (HER; refs ,,,,), molybd... more As a promising non-precious catalyst for the hydrogen evolution reaction (HER; refs ,,,,), molybdenum disulphide (MoS2) is known to contain active edge sites and an inert basal plane. Activating the MoS2 basal plane could further enhance its HER activity but is not often a strategy for doing so. Herein, we report the first activation and optimization of the basal plane of monolayer 2H-MoS2 for HER by introducing sulphur (S) vacancies and strain. Our theoretical and experimental results show that the S-vacancies are new catalytic sites in the basal plane, where gap states around the Fermi level allow hydrogen to bind directly to exposed Mo atoms. The hydrogen adsorption free energy (ΔGH) can be further manipulated by straining the surface with S-vacancies, which fine-tunes the catalytic activity. Proper combinations of S-vacancy and strain yield the optimal ΔGH = 0 eV, which allows us to achieve the highest intrinsic HER activity among molybdenum-sulphide-based catalysts.
The Journal of Physical Chemistry Letters, 2015
In this Letter, we examine bond activation induced by nonmetal surface promoters in the context o... more In this Letter, we examine bond activation induced by nonmetal surface promoters in the context of dehydrogenation reactions. We use C-H bond activation in methane dehydrogenation on transition metals as an example to understand the origin of the promoting or poisoning effect of nonmetals. The electronic structure of the surface and the bond order of the promoter are found to establish all trends in bond activation. On the basis of these results, we develop a predictive model that successfully describes the energetics of C-H, O-H, and N-H bond activation across a range of reactions. For a given reaction step, a single data point determines whether a nonmetal will promote bond activation or poison the surface and by how much. We show how our model leads to general insights that can be directly used to predict bond activation energetics on transition metal sulfides and oxides, which can be perceived as promoted surfaces. These results can then be directly used in studies on full catalytic pathways.
Journal of Catalysis, 2015
The Journal of Physical Chemistry Letters, 2015
We present calculated adsorption energies of oxygen on gold and platinum clusters with up to 923 ... more We present calculated adsorption energies of oxygen on gold and platinum clusters with up to 923 atoms (3 nm diameter) using density functional theory. We find that surface tension of the clusters induces a compression, which weakens the bonding of adsorbates compared with the bonding on extended surfaces. The effect is largest for close-packed surfaces and almost nonexistent on the more reactive steps and edges. The effect is largest at low coverage and decreases, even changing sign, at higher coverages where the strain changes from compressive to tensile. Quantum size effects also influence adsorption energies but only below a critical size of 1.5 nm for platinum and 2.5 nm for gold. We develop a model to describe the strain-induced size effects on adsorption energies, which is able to describe the influence of surface structure, adsorbate, metal, and coverage.
Nanoscale, 2015
We report on the design and synthesis of high performance catalytic nanoparticles with a robust g... more We report on the design and synthesis of high performance catalytic nanoparticles with a robust geometry via magnetron-sputter inert-gas condensation. Sputtering of Pd and Mg from two independent neighbouring targets enabled heterogeneous condensation and growth of nanoparticles with controlled Pd core-MgO porous shell structure. The thickness of the shell and the number of cores within each nanoparticle could be tailored by adjusting the respective sputtering powers. The nanoparticles were directly deposited on glassy carbon electrodes, and their catalytic activity towards methanol oxidation was examined by cyclic voltammetry. The measurements indicated that the catalytic activity was superior to conventional bare Pd nanoparticles. As confirmed by electron microscopy imaging and supported by density-functional theory (DFT) calculations, we attribute the improved catalytic performance primarily to inhibition of Pd core sintering during the catalytic process by the metal-oxide shell.
A simple web-based application called "CatApp" has recently been made available to the ... more A simple web-based application called "CatApp" has recently been made available to the public, which provides access to the adsorption and transition state energies of different molecules on transition metal surfaces. This paper illustrates the utility of such a database for predicting trends in the activities and selectivities of materials for a given reaction. In this case, the anhydrous dehydrogenation of methanol to formaldehyde is used as a test reaction, but the methods presented here are widely applicable to a broad range of reactions. The DFT-calculated adsorption and transition state energies of the relevant intermediates were accessed via CatApp for the stepped (211) surfaces of Ag, Cu, Pd, Pt, and Rh. As described in the literature, the binding energies for these intermediates on a given surface can be scaled with the binding energies of carbon and oxygen. Likewise, the energies of a given transition state species can be scaled with the energies of the reaction ...
Transition-metal oxides are widely used materials in catalysis as substrates and promoters, but a... more Transition-metal oxides are widely used materials in catalysis as substrates and promoters, but also as the active catalyst materials themselves. We compare the reactivity of transition-metal oxides with the one of transition metals. The comparison is exemplified for the ammonia synthesis reaction. First we show that there exist characteristic Brønsted-Evans Polanyi (BEP) relations (linear relations between transition state and dissociation energies) for dissociation of molecules on transition-metal oxides in the rutile and perovskite structure. It is well-known that the (211) metal surface is several orders of magnitude more reactive than the (111) metal surface due to the lower BEP line for the 211 facet. We find that both rutiles and perovskites follow BEP relations that are lower than the one of the 211 facet. Second we utilize the established BEP relations together with calculated adsorption energetics in a micro-kinetic model to obtain a volcano plot for the catalytic activity...
Finding sustainable energy solutions for the future will rely heavily on the energy influx from t... more Finding sustainable energy solutions for the future will rely heavily on the energy influx from the sun. One convenient way of storing solar energy is by transforming that energy into a chemical form - like a fuel. The efficiency of such a transformation will require catalysts that are optimized for specific reactions, and we will need to find new catalysts for a number of processes, if we are to successfully synthesize fuels from sunlight. A fundamental insight into the way the catalysts work at the molecular level is an essential ingredient if one wants to speed up the discovery process. In this presentation I will discuss some of the challenges in catalyst discovery. In particular, I will focus on the conversion of syngas to methanol, an important sub-reaction in the biomass to fuels process.
The Journal of Physical Chemistry Letters, 2015
The underlying mechanisms for the nucleation of carbon nanotubes as well as their helicity, remai... more The underlying mechanisms for the nucleation of carbon nanotubes as well as their helicity, remain elusive. Here, using van der Waals dispersion force calculations implemented within density functional theory, we study the cap formation, believed to be responsible for the chirality of surface-catalyzed carbon nanotubes. We find the energetics associated with growth along different facets to be independent of the surface orientation and that the growth across an edge along the axis of the metal particle leads to a perfect honeycomb lattice in a curved geometry. The formation of defects in the graphene matrix, which bend the carbon plane, requires that two or more graphene embryos with significantly different growth axis merge. Such scenario is only possible at the front- or back-end of the metal particle where growth symmetry is broken. The graphene embryos reconstruct their hexagonal structure into pentagons, heptagons, and octagons counterpart to accommodate the tube curvature.
Using Random Matrix Theory we set out to compute the microscopic correlators of the Euclidean Dir... more Using Random Matrix Theory we set out to compute the microscopic correlators of the Euclidean Dirac operator in four dimensions. In particular we consider: the chiral Orthogonal Ensemble (chOE), corresponding to a Yang-Mills theory with two colors and fermions in the fundamental representation, and the chiral Symplectic Ensemble (chSE), corresponding to any number of colors and fermions in the adjoint
We present a density functional theory-based method for calculating thermionic emission currents ... more We present a density functional theory-based method for calculating thermionic emission currents from a cathode into vacuum using a non-equilibrium Green's function approach. It does not require semi-classical approximations or crude simplifications of the electronic structure used in previous methods and thus provides quantitative predictions of thermionic emission for adsorbate-coated surfaces. The obtained results match well with experimental measurements of temperature-dependent current densities. Our approach can thus enable computational design of composite electrode materials.
Based on a nonequilibrium Green’s-function approach to the calculation of emission currents from ... more Based on a nonequilibrium Green’s-function approach to the calculation of emission currents from first principles, we show that cathodes consisting of LaB6/BaB6 superlattices can yield an order-of-magnitude higher thermionic current densities than pure LaB6 cathodes. Because of a 0.46-eV lowering of the work function, such a heterostructure cathode could thus be operated at significantly lower temperatures. Neither the stability nor the magnitude of electronic tunneling coefficients is compromised in the superlattice system as compared to pure LaB6, which is in contrast to the generally reduced stability and large dipole barriers in the case of adsorbate-induced lowering of the work function. The heterostructure could thus be used as a cathode material that at the same time is stable and has emission properties superior to those of pure LaB6.
The d-band shape of a metal site, governed by the local geometry and composition of materials, pl... more The d-band shape of a metal site, governed by the local geometry and composition of materials, plays an important role in determining trends of the surface reactivity of transition-metal alloys. We discuss this phenomenon using the chemisorption of various adsorbates such as C, N, O, and their hydrogenated species on Pd bimetallic alloys as an example. For many alloys, the d-band center, even with consideration of the d-band width and sp electrons, can not describe variations in reactivity from one surface to another. We investigate the effect of the d-band shape, represented by higher moments of the d band, on the local electronic structure of adsorbates, e.g., energy and filling of adsorbate-metal antibonding states. The upper d-band edge ɛu, defined as the highest peak position of the Hilbert transform of the density of states projected onto d orbitals of an active metal site, is identified as an electronic descriptor for the surface reactivity of transition metals and their alloys, regardless of variations in the d-band shape. The utilization of the upper d-band edge with scaling relations enables a considerable reduction of the parameter space in search of improved alloy catalysts and further extends our understanding of the relationship between the electronic structure and chemical reactivity of metal surfaces.
We discuss three concepts that have made it possible to develop a quantitative understanding of t... more We discuss three concepts that have made it possible to develop a quantitative understanding of trends in transition-metal catalysis: scaling relations, activity maps, and the d-band model. Scaling relations are correlations between surface bond energies of different adsorbed species including transition states; they open the possibility of mapping the many parameters determining the rate of a full catalytic reaction onto a few descriptors. The resulting activity map can be viewed as a quantitative implementation of the classical Sabatier principle, which states that there is an optimum “bond strength” defining the best catalyst for a given reaction. In the modern version, the scaling relations determine the relevant “bond strengths” and the fact that these descriptors can be measured or calculated makes it a quantitative theory of catalysis that can be tested experimentally by making specific predictions of new catalysts. The quantitative aspect of the model therefore provides new possibilities in catalyst design. Finally, the d-band model provides an understanding of the scaling relations and variations in catalytic activity in terms of the electronic structure of the transition-metal surface.