Showing papers by "Jens K. Nørskov published in 2016"
TL;DR: This work reports the first activation and optimization of the basal plane of monolayer 2H-MoS2 for HER by introducing sulphur (S) vacancies and strain, which allows it to achieve the highest intrinsic HER activity among molybdenum-sulphide-based catalysts.
Abstract: Molybdenum disulphide is a promising non-precious catalyst for hydrogen evolution because it contains active edge sites and an inert basal plane. Introducing sulphur vacancies and strain now leads to activation and optimization of the basal plane. As a promising non-precious catalyst for the hydrogen evolution reaction (HER; refs 1,2,3,4,5), molybdenum disulphide (MoS2) is known to contain active edge sites and an inert basal plane1,6,7,8. 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.
1,938 citations
TL;DR: The IrOx/SrIrO3 catalyst outperforms known IrOx and ruthenium oxide (RuOx) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte.
Abstract: Oxygen electrochemistry plays a key role in renewable energy technologies such as fuel cells and electrolyzers, but the slow kinetics of the oxygen evolution reaction (OER) limit the performance and commercialization of such devices. Here we report an iridium oxide/strontium iridium oxide (IrOx/SrIrO3) catalyst formed during electrochemical testing by strontium leaching from surface layers of thin films of SrIrO3 This catalyst has demonstrated specific activity at 10 milliamps per square centimeter of oxide catalyst (OER current normalized to catalyst surface area), with only 270 to 290 millivolts of overpotential for 30 hours of continuous testing in acidic electrolyte. Density functional theory calculations suggest the formation of highly active surface layers during strontium leaching with IrO3 or anatase IrO2 motifs. The IrOx/SrIrO3 catalyst outperforms known IrOx and ruthenium oxide (RuOx) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte.
1,420 citations
TL;DR: A method for using battery electrode materials to directly and continuously control the lattice strain of platinum (Pt) catalyst and thus tune its catalytic activity for the oxygen reduction reaction (ORR) is reported.
Abstract: We report a method for using battery electrode materials to directly and continuously control the lattice strain of platinum (Pt) catalyst and thus tune its catalytic activity for the oxygen reduction reaction (ORR). Whereas the common approach of using metal overlayers introduces ligand effects in addition to strain, by electrochemically switching between the charging and discharging status of battery electrodes the change in volume can be precisely controlled to induce either compressive or tensile strain on supported catalysts. Lattice compression and tension induced by the lithium cobalt oxide substrate of ~5% were directly observed in individual Pt nanoparticles with aberration-corrected transmission electron microscopy. We observed 90% enhancement or 40% suppression in Pt ORR activity under compression or tension, respectively, which is consistent with theoretical predictions.
457 citations
TL;DR: In this paper, the authors examined the CO2 → CO electrocatalytic pathway on Ag(111) using density functional theory (DFT) calculations and an explicit model of the electrochemical interface.
Abstract: Electrochemical reduction of CO2 has the potential to reduce greenhouse gas emissions while providing energy storage and producing chemical feedstocks. A mechanistic understanding of the process is crucial to the discovery of efficient catalysts, and an atomistic description of the electrochemical interface is a major challenge due to its complexity. Here, we examine the CO2 → CO electrocatalytic pathway on Ag(111) using density functional theory (DFT) calculations and an explicit model of the electrochemical interface. We show that the electric field from solvated cations in the double layer and their corresponding image charges on the metal surface significantly stabilizes key intermediates—*CO2 and *COOH. At the field-stabilized sites, the formation of *CO is rate-determining. We present a microkinetic model that incorporates field effects and electrochemical barriers from ab initio calculations. The computed polarization curves show reasonable agreement with experiment without fitting any parameters.
340 citations
TL;DR: Lead and silver surfaces are found to be the most promising monometallic catalysts showing high faradaic efficiencies for the electroreduction of CO2 to HCOOH with small overpotentials.
Abstract: The electrochemical reduction (electroreduction) of CO2 to formic acid (HCOOH) and its competing reactions, that is, the electroreduction of CO2 to CO and the hydrogen evolution reaction (HER), on twenty-seven different metal surfaces have been investigated using density functional theory (DFT) calculations. Owing to a strong linear correlation between the free energies of COOH* and H*, it seems highly unlikely that the electroreduction of CO2 to HCOOH via the COOH* intermediate occurs without a large fraction of the current going to HER. On the other hand, the selective electroreduction of CO2 to HCOOH seems plausible if the reaction occurs via the HCOO* intermediate, as there is little correlation between the free energies of HCOO* and H*. Lead and silver surfaces are found to be the most promising monometallic catalysts showing high faradaic efficiencies for the electroreduction of CO2 to HCOOH with small overpotentials. Our methodology is widely applicable, not only to metal surfaces, but also to other classes of materials enabling the computational search for electrocatalysts for CO2 reduction to HCOOH.
285 citations
TL;DR: In this paper, the doped sulfur edge of MoS2 satisfies this requirement by binding CO* significantly weaker than COOH*, CHO*, and COH*, relative to transition-metal surfaces.
Abstract: Linear scaling relationships between the adsorption energies of CO2 reduction intermediates pose a fundamental limitation to the catalytic efficiency of transition-metal catalysts. Significant improvements in CO2 reduction activity beyond transition metals require the stabilization of key intermediates, COOH* and CHO* or COH*, independent of CO*. Using density functional theory (DFT) calculations, we show that the doped sulfur edge of MoS2 satisfies this requirement by binding CO* significantly weaker than COOH*, CHO*, and COH*, relative to transition-metal surfaces. The structural basis for the scaling of doped sulfur edge of MoS2 is due to CO* binding on the metallic site (doping metal) and COOH*, CHO*, and COH* on the covalent site (sulfur). Linear scaling relations still exist if all the intermediates bind to the same site, but the combined effect of the two binding sites results in an overall deviation from transition-metal scaling lines. This principle can be applied to other metal/p-block materials...
222 citations
TL;DR: In this article, the effect of coverage, strain, and electric field on CO-CO coupling energy on Cu (100), (111), and (211) was investigated with a Pourbaix analysis.
197 citations
TL;DR: A simple and computationally efficient method is presented to determine the potential dependence of the activation energies for proton-electron transfer from a single ab initio barrier calculation that shows that the transfer coefficient is given by the charge transferred from the initial to transition state.
Abstract: We present a simple and computationally efficient method to determine the potential dependence of the activation energies for proton–electron transfer from a single ab initio barrier calculation. We show that the potential dependence of the activation energy is given by the partial charge transferred at the transition state. The method is evaluated against the potential dependence determined explicitly through multiple calculations at varying potential. We show that the transfer coefficient is given by the charge transferred from the initial to transition state, which has significant implications for electrochemical kinetics.
185 citations
TL;DR: In this article, a static headspace-gas chromatography for liquid phase analysis was used to identify acetaldehyde as a minor product and key intermediate in the electroreduction of CO to ethanol on OD-Cu electrodes.
Abstract: Oxide-derived copper (OD-Cu) electrodes exhibit unprecedented CO reduction performance towards liquid fuels, producing ethanol and acetate with >50 % Faradaic efficiency at −0.3 V (vs. RHE). By using static headspace-gas chromatography for liquid phase analysis, we identify acetaldehyde as a minor product and key intermediate in the electroreduction of CO to ethanol on OD-Cu electrodes. Acetaldehyde is produced with a Faradaic efficiency of ≈5 % at −0.33 V (vs. RHE). We show that acetaldehyde forms at low steady-state concentrations, and that free acetaldehyde is difficult to detect in alkaline solutions using NMR spectroscopy, requiring alternative methods for detection and quantification. Our results represent an important step towards understanding the CO reduction mechanism on OD-Cu electrodes.
171 citations
TL;DR: Improved atomic-scale understanding and new insight is provided into the mechanism, active site, and intrinsic selectivity of syngas conversion over rhodium catalysts and may also guide rational design of alloy catalysts made from more abundant elements.
Abstract: Synthesis gas (CO + H2) conversion is a promising route to converting coal, natural gas, or biomass into synthetic liquid fuels. Rhodium has long been studied as it is the only elemental catalyst that has demonstrated selectivity to ethanol and other C2+ oxygenates. However, the fundamentals of syngas conversion over rhodium are still debated. In this work a microkinetic model is developed for conversion of CO and H2 into methane, ethanol, and acetaldehyde on the Rh (211) and (111) surfaces, chosen to describe steps and close-packed facets on catalyst particles. The model is based on DFT calculations using the BEEF-vdW functional. The mean-field kinetic model includes lateral adsorbate–adsorbate interactions, and the BEEF-vdW error estimation ensemble is used to propagate error from the DFT calculations to the predicted rates. The model shows the Rh(211) surface to be ∼6 orders of magnitude more active than the Rh(111) surface, but highly selective toward methane, while the Rh(111) surface is intrinsicall...
163 citations
TL;DR: In this article, the authors used density functional theory (DFT) calculations to systematically evaluate monocopper species as active sites for the partial methane oxidation reaction in Cu-exchanged SSZ-13.
Abstract: Direct conversion of methane to methanol using oxygen is experiencing renewed interest owing to the availability of new natural gas resources. Copper-exchanged zeolites such as mordenite and ZSM-5 have shown encouraging results, and di- and tri-copper species have been suggested as active sites. Recently, small eight-membered ring (8MR) zeolites including SSZ-13, -16, and -39 have been shown to be active for methane oxidation, but the active sites and reaction mechanisms in these 8MR zeolites are not known. In this work, we use density functional theory (DFT) calculations to systematically evaluate monocopper species as active sites for the partial methane oxidation reaction in Cu-exchanged SSZ-13. On the basis of kinetic and thermodynamic arguments, we suggest that [CuIIOH]+ species in the 8MR are responsible for the experimentally observed activity. Our results successfully explain the available spectroscopic data and experimental observations including (i) the necessity of water for methanol extraction...
TL;DR: In this paper, periodic density functional theory (DFT) methods were employed to explore the molecular features of the selective oxidation of methane to methanol catalyzed by Cu-exchanged mordenite (Cu-MOR).
Abstract: Selective oxidation of methane to methanol is one of the most difficult chemical processes to perform. A potential group of catalysts to achieve CH4 partial oxidation are Cu-exchanged zeolites mimicking the active structure of the enzyme methane monooxygenase. However, the details of this conversion, including the structure of the active site, are still under debate. In this contribution, periodic density functional theory (DFT) methods were employed to explore the molecular features of the selective oxidation of methane to methanol catalyzed by Cu-exchanged mordenite (Cu-MOR). We focused on two types of previously suggested active species, CuOCu and CuOOCu. Our calculations indicate that the formation of CuOCu is more feasible than that of CuOOCu. In addition, a much lower C–H dissociation barrier is located on the former active site, indicating that C–H bond activation is easily achieved with CuOCu. We calculated the energy barriers of all elementary steps for the entire process, including catalyst acti...
TL;DR: In this paper, the authors use density functional theory with an explicit solvent model of the electrochemical interface to calculate activation energy barriers for various proton-electron transfer elementary reactions steps for CO2 reduction on Au, Cu, and Pt surfaces.
TL;DR: In this article, the authors explore bifunctional catalysts using Au-Cd-based alloys inspired by theoretical modelling and find that the catalytic performance is most consistent with Au-terminated step surface on a bulk alloy, possibly formed through adsorbate-induced restructuring.
TL;DR: It is found that several alloys may reduce the thermodynamic overpotential for CO reduction by more than 0.2 V compared to a copper step, however, these alloys are most often rather unstable in aqueous environment or have low alloy formation energies and may be susceptible to segregation destroying the active sites.
Abstract: We use density functional theory to study the reduction of CO2 and CO to hydrocarbons through a formyl pathway on (111) and (211) facets of L12 alloys with an A3B composition. We find that several alloys may reduce the thermodynamic overpotential for CO reduction by more than 0.2 V compared to a copper step, however, these alloys are most often rather unstable in aqueous environment or have low alloy formation energies and may be susceptible to segregation destroying the active sites. Strategies to improve alloy stability against corrosion or segregation would likely be needed in order to realize the full potential of these alloys.
TL;DR: In this paper, an improved version of the energy scaling relation between OOH and OOH intermediates in the ORR and oxygen evolution reaction (OER) has been determined using density functional theory and shown to dictate a minimum thermodynamic overpotential.
Abstract: A widely used adsorption energy scaling relation between OH* and OOH* intermediates in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), has previously been determined using density functional theory and shown to dictate a minimum thermodynamic overpotential for both reactions. Here, we show that the oxygen–oxygen bond in the OOH* intermediate is, however, not well described with the previously used class of exchange-correlation functionals. By quantifying and correcting the systematic error, an improved description of gaseous peroxide species versus experimental data and a reduction in calculational uncertainty is obtained. For adsorbates, we find that the systematic error largely cancels the vdW interaction missing in the original determination of the scaling relation. An improved scaling relation, which is fully independent of the applied exchange–correlation functional, is obtained and found to differ by 0.1 eV from the original. This largely confirms that, although obtained wit...
TL;DR: Using a Gaussian process regression model, the free energy of all possible adsorbate coverages for surfaces is predicted for a finite number of adsorption sites, demonstrating a rational, simple, and systematic approach for generating accurate free-energy diagrams with reduced computational resources.
Abstract: Surface phase diagrams are necessary for understanding surface chemistry in electrochemical catalysis, where a range of adsorbates and coverages exist at varying applied potentials. These diagrams are typically constructed using intuition, which risks missing complex coverages and configurations at potentials of interest. More accurate cluster expansion methods are often difficult to implement quickly for new surfaces. We adopt a machine learning approach to rectify both issues. Using a Gaussian process regression model, the free energy of all possible adsorbate coverages for surfaces is predicted for a finite number of adsorption sites. Our result demonstrates a rational, simple, and systematic approach for generating accurate free-energy diagrams with reduced computational resources. The Pourbaix diagram for the IrO2(110) surface (with nine coverages from fully hydrogenated to fully oxygenated surfaces) is reconstructed using just 20 electronic structure relaxations, compared to approximately 90 using t...
TL;DR: In this paper, the enthalpy of dissociative adsorption of water is measured on NiO(111)-2 × 2 at 300 K using single-crystal adaption calorimetry.
Abstract: The energetics of the reactions of water with metal oxide surfaces are of tremendous interest for catalysis, electrocatalysis, and geochemistry, yet the energy for the dissociative adsorption of water was only previously measured on one well-defined oxide surface, iron oxide. In the present paper, the enthalpy of the dissociative adsorption of water is measured on NiO(111)-2 × 2 at 300 K using single-crystal adsorption calorimetry. The differential heat of dissociative adsorption decreases with coverage from 170 to 117 kJ/mol in the first 0.25 ML of coverage. Water adsorbs molecularly on top of that, with a heat of ∼92 kJ/mol. Density functional theory (DFT) calculations reproduce the measured energies well (all within 17 kJ/mol) and provide insight into the atomic-level structure of the surfaces studied experimentally. They show that the oxygen-terminated O-octo(2 × 2) structure is the most stable NiO(111)-2 × 2 termination and gives reaction energies with water that are more consistent with the calorime...
TL;DR: This work scrutinizes claims of bifunctional coupling of two different catalytic site types with generic scaling-relation-based microkinetic models that allow exploration of the theoretical limits for such a bifunctionsal gain for several model reactions.
Abstract: Bifunctional coupling of two different catalytic site types has often been invoked to explain experimentally observed enhanced catalytic activities. We scrutinize such claims with generic scaling-relation-based microkinetic models that allow exploration of the theoretical limits for such a bifunctional gain for several model reactions. For sites at transition-metal surfaces, the universality of the scaling relations between adsorption energies largely prevents any improvements through bifunctionality. Only the consideration of systems that involve the combination of different materials, such as metal particles on oxide supports, offers hope for significant bifunctional gains.
TL;DR: In this article, the authors employ density functional theory to examine a variety of two-dimensional materials, including, carbon based materials, hexagonal boron nitride (h-BN), transition metal dichalcogenides (e.g. MoS2, MoSe2) and layered oxides, to give an overview of the trends in adsorption energies.
Abstract: Although large efforts have been dedicated to studying two-dimensional materials for catalysis, a rationalization of the associated trends in their intrinsic activity has so far been elusive. In the present work we employ density functional theory to examine a variety of two-dimensional materials, including, carbon based materials, hexagonal boron nitride (h-BN), transition metal dichalcogenides (e.g. MoS2, MoSe2) and layered oxides, to give an overview of the trends in adsorption energies. By examining key reaction intermediates relevant to the oxygen reduction, and oxygen evolution reactions we find that binding energies largely follow the linear scaling relationships observed for pure metals. This observation is very important as it suggests that the same simplifying assumptions made to correlate descriptors with reaction rates in transition metal catalysts are also valid for the studied two-dimensional materials. By means of these scaling relations, for each reaction we also identify several promising candidates that are predicted to exhibit a comparable activity to the state-of-the-art catalysts. Scaling relationship for the chemisorption energies of OH* and OOH* on various 2D materials.
01 Apr 2016
TL;DR: It is shown that acetaldehyde forms at low steady‐state concentrations, and that free acetaldehyde is difficult to detect in alkaline solutions using NMR spectroscopy, requiring alternative methods for detection and quantification.
Abstract: Abstract Oxide‐derived copper (OD‐Cu) electrodes exhibit unprecedented CO reduction performance towards liquid fuels, producing ethanol and acetate with >50 % Faradaic efficiency at −0.3 V (vs. RHE). By using static headspace‐gas chromatography for liquid phase analysis, we identify acetaldehyde as a minor product and key intermediate in the electroreduction of CO to ethanol on OD‐Cu electrodes. Acetaldehyde is produced with a Faradaic efficiency of ≈5 % at −0.33 V (vs. RHE). We show that acetaldehyde forms at low steady‐state concentrations, and that free acetaldehyde is difficult to detect in alkaline solutions using NMR spectroscopy, requiring alternative methods for detection and quantification. Our results represent an important step towards understanding the CO reduction mechanism on OD‐Cu electrodes.
TL;DR: Time-resolved soft X-ray spectroscopy at a free-electron laser is applied to directly observe the decreased bonding-antibonding splitting following bond-activation using an ultrashort optical laser pulse.
Abstract: The concept of bonding and antibonding orbitals is fundamental in chemistry. The population of those orbitals and the energetic difference between the two reflect the strength of the bonding interaction. Weakening the bond is expected to reduce this energetic splitting, but the transient character of bond-activation has so far prohibited direct experimental access. Here we apply time-resolved soft X-ray spectroscopy at a free-electron laser to directly observe the decreased bonding–antibonding splitting following bond-activation using an ultrashort optical laser pulse.
TL;DR: In this paper, the trends in binding energies of different reaction intermediates on transition metal complexes for homogeneous catalysis were investigated, and linear scaling relationships were established for linear scaling relationship.
Abstract: In this work, we investigate the trends in binding energies of different reaction intermediates on transition metal complexes for homogeneous catalysis. We find that linear scaling relationships ex ...
TL;DR: In this article, the chemisorption energies of oxygen, carbon-, and nitrogen-containing species on different transition metals exchanged in ZSM-22 are investigated, and scaling relations are presented for the corresponding species.
Abstract: Copper exchange on all the different T sites of ZSM-22 and ZSM-5 is considered and the chemisorption energies of dioxygen, OH, and O species are studied. We show that for different T sites the adsorption energies vary significantly. The oxygen adsorption energy on copper-exchanged zeolites is quite similar to those of the most selective catalysts for oxidation reactions, that is, Ag and Au surfaces. The chemisorption energies of oxygen, carbon-, and nitrogen-containing species on different transition metals exchanged in ZSM-22 are also investigated. The study covers three different oxidation states, that is, 1+, 2+, and 3+ for the transition-metal exchanges. Scaling relations are presented for the corresponding species. Chemisorption of O scales with chemisorption of OH for all three considered exchanges, whereas there are essentially rough correlations for NH2 and N as well as CH3 and C.
TL;DR: In this article, a descriptor-based approach was used to determine that the (211) facet leads to the highest possible rates, and a range of 96 binary alloys were screened for their potential activity and a rate control analysis was performed to assess how the overall rate could be improved.
Abstract: Density functional theory calculations are used to investigate thermal water decomposition over the close-packed (111), stepped (211), and open (100) facets of transition metal surfaces. A descriptor-based approach is used to determine that the (211) facet leads to the highest possible rates. A range of 96 binary alloys were screened for their potential activity and a rate control analysis was performed to assess how the overall rate could be improved.
TL;DR: In this article, the authors examined the thermochemistry and kinetics of the partial oxidation of methanol to formaldehyde on silver surfaces using periodic density functional theory calculations employing the BEEF-vdW functional.
Abstract: In this work, we examine the thermochemistry and kinetics of the partial oxidation of methanol to formaldehyde on silver surfaces Periodic density functional theory calculations employing the BEEF-vdW functional are used to identify the most stable phases of the silver surface under relevant reaction conditions and the reaction energetics are obtained on these surfaces The calculated binding energies and transition state energies are used as input in a mean-field microkinetic model providing the reaction kinetics on silver surfaces under different reaction conditions Our results show that, under conditions pertaining to methanol partial oxidation, oxygen is present at low concentrations and it plays a critical role in the catalytic reaction Surface oxygen promotes the reaction by activating the OH bond in methanol, thus forming a methoxy intermediate, which can react further to form formaldehyde The dissociation of molecular oxygen is identified as the most critical step
Ames Laboratory1, Argonne National Laboratory2, Brookhaven National Laboratory3, Lawrence Berkeley National Laboratory4, Lawrence Livermore National Laboratory5, National Renewable Energy Laboratory6, Oak Ridge National Laboratory7, Pacific Northwest National Laboratory8, Sandia National Laboratories9, Fermilab10
TL;DR: The work at the U.S. Department of Energy's (DOE's) National Laboratories covers a wide range of research topics in heterogeneous catalysis, homogeneous/molecular catalysis and biocatalysis, electrocatalysis as discussed by the authors.
Abstract: Here the catalysis research at the U.S. Department of Energy's (DOE's) National Laboratories covers a wide range of research topics in heterogeneous catalysis, homogeneous/molecular catalysis, biocatalysis, electrocatalysis, and surface science. Since much of the work at National Laboratories is funded by DOE, the research is largely focused on addressing DOE's mission to ensure America's security and prosperity by addressing its energy, environmental, and nuclear challenges through transformative science and technology solutions.
01 Sep 2016