Showing papers by "Jens K. Nørskov published in 2013"

Theoretical Investigation of the Activity of Cobalt Oxides for the Electrochemical Oxidation of Water

Abstract: The presence of layered cobalt oxides has been identified experimentally in Co-based anodes under oxygen-evolving conditions. In this work, we report the results of theoretical investigations of the relative stability of layered and spinel bulk phases of Co oxides, as well as the stability of selected surfaces as a function of applied potential and pH. We then study the oxygen evolution reaction (OER) on these surfaces and obtain activity trends at experimentally relevant electro-chemical conditions. Our calculated volume Pourbaix diagram shows that β-CoOOH is the active phase where the OER occurs in alkaline media. We calculate relative surface stabilities and adsorbate coverages of the most stable low-index surfaces of β-CoOOH: (0001), (0112), and (1014). We find that at low applied potentials, the (1014) surface is the most stable, while the (0112) surface is the more stable at higher potentials. Next, we compare the theoretical overpotentials for all three surfaces and find that the (1014) surface is the most active one as characterized by an overpotential of η = 0.48 V. The high activity of the (1014) surface can be attributed to the observation that the resting state of Co in the active site is Co(3+) during the OER, whereas Co is in the Co(4+) state in the less active surfaces. Lastly, we demonstrate that the overpotential of the (1014) surface can be lowered further by surface substitution of Co by Ni. This finding could explain the experimentally observed enhancement in the OER activity of Ni(y)Co(1-y)O(x) thin films with increasing Ni content. All energetics in this work were obtained from density functional theory using the Hubbard-U correction.

Understanding Trends in the Electrocatalytic Activity of Metals and Enzymes for CO2 Reduction to CO.

Abstract: We develop a model based on density functional theory calculations to describe trends in catalytic activity for CO2 electroreduction to CO in terms of the adsorption energy of the reaction intermediates, CO and COOH The model is applied to metal surfaces as well as the active site in the CODH enzymes and shows that the strong scaling between adsorbed CO and adsorbed COOH on metal surfaces is responsible for the persistent overpotential The active site of the CODH enzyme is not subject to these scaling relations and optimizes the relative binding energies of these adsorbates, allowing for an essentially reversible process with a low overpotential

Insights into C ? C Coupling in CO2 Electroreduction on Copper Electrodes

Abstract: We present a first‐principles theoretical study of carbon–carbon coupling in CO2 electroreduction on the copper 2 1 1 surface. Using DFT, we have determined kinetic barriers to the formation of a CC bond between adsorbates derived from CO. The results of our nudged elastic band calculations demonstrate that kinetic barriers to CC coupling decrease significantly with the degree of hydrogenation of reacting adsorbates. We also show that this trend is not affected by the electrical fields present at the solid‐electrolyte interface during electrocatalysis. Our results explain how copper can catalyze the production of higher hydrocarbons and oxygenates in the electrochemical environment, despite producing only single carbon atom products in gas‐phase catalysis, and how CC bonds can be formed at room temperature in the electrochemical environment, whereas substantially higher temperatures are needed in the Fischer–Tropsch catalysis. The unique feature of the electrochemical environment is that the chemical potential of hydrogen (electrons and protons) can be varied through the applied potential. This allows a variation of the degree of hydrogenation of the reactants and thus the activation barrier for CC coupling.

Direct observation of the oxygenated species during oxygen reduction on a platinum fuel cell cathode

Abstract: Understanding the oxygen reduction reaction at fuel cell cathodes requires information on adsorbed oxygenated species. Sanchez Casalongue et al. report in situ identification of oxygenated intermediates at cathodes and establish a correlation between the cathode potential and the surface speciation.

Investigation of Catalytic Finite-Size-Effects of Platinum Metal Clusters.

Abstract: In this paper, we use density functional theory (DFT) calculations on highly parallel computing resources to study size-dependent changes in the chemical and electronic properties of platinum (Pt) for a number of fixed freestanding clusters ranging from 13 to 1415 atoms, or 0.7-3.5 nm in diameter. We find that the surface catalytic properties of the clusters converge to the single crystal limit for clusters with as few as 147 atoms (1.6 nm). Recently published results for gold (Au) clusters showed analogous convergence with size. However, this convergence happened at larger sizes, because the Au d-states do not contribute to the density of states around the Fermi-level, and the observed level fluctuations were not significantly damped until the cluster reached ca. 560 atoms (2.7 nm) in size.

Theoretical evidence for low kinetic overpotentials in Li-O2 electrochemistry

Abstract: We develop a density functional theory model for the electrochemical growth and dissolution of Li2O2 on various facets, terminations, and sites (terrace, steps, and kinks) of a Li2O2 surface. We argue that this is a reasonable model to describe discharge and charge of Li-O2 batteries over most of the discharge-charge cycle. Because non-stoichiometric surfaces are potential dependent and since the potential varies during discharge and charge, we study the thermodynamic stability of facets, terminations, and steps as a function of potential. This suggests that different facets, terminations, and sites may dominate in charge relative to those for discharge. We find very low thermodynamic overpotentials (<0.2 V) for both discharge and charge at many sites on the facets studied. These low thermodynamic overpotentials for both discharge and charge are in very good agreement with the low kinetic overpotentials observed in recent experiments. However, there are other predicted paths for discharge/charge that have higher overpotentials, so the phase space available for the electrochemistry opens up with overpotential.

Real-Time Observation of Surface Bond Breaking with an X-ray Laser

Abstract: We used the Linac Coherent Light Source free-electron x-ray laser to probe the electronic structure of CO molecules as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate but without notable desorption. A large fraction of the molecules (30%) was trapped in a transient precursor state that would precede desorption. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells—chemisorbed and precursor state separated by an entropy barrier—explain the anomalously high prefactors often observed in desorption of molecules from metals.

Li–O2 Kinetic Overpotentials: Tafel Plots from Experiment and First-Principles Theory

Abstract: We report the current dependence of the fundamental kinetic overpotentials for Li–O2 discharge and charge (Tafel plots) that define the optimal cycle efficiency in a Li-air battery. Comparison of the unusual experimental Tafel plots obtained in a bulk electrolysis cell with those obtained by first-principles theory is semiquantitative. The kinetic overpotentials for any practical current density are very small, considerably less than polarization losses due to iR drops from the cell impedance in Li–O2 batteries. If only the kinetic overpotentials were present, then a discharge–charge voltaic cycle efficiency of ∼85% should be possible at ∼10 mA/cm2 superficial current density in a battery of ∼0.1 m2 total cathode area. We therefore suggest that minimizing the cell impedance is a more important problem than minimizing the kinetic overpotentials to develop higher current Li-air batteries.

Tunneling and Polaron Charge Transport through Li2O2 in Li–O2 Batteries

Abstract: We describe Li–O2 discharge experiments in a bulk electrolysis cell as a function of current density and temperature. In combination with a simple model, these imply that charge transport through Li2O2 in Li–O2 batteries at practical current densities is based principally on hole tunneling, with hole polaron conductivity playing a significant role near the end of very low current discharges and at temperatures greater than 30 °C. We also show that charge-transport limitations are much less significant during charging than those in discharge. A key element of the model that qualitatively explains all results is the alignment of the Li2O2 valence band maximum close to the electrochemical Fermi energy and how this alignment varies with overpotentials during discharge and charge. In fact, comparison of the model with the experiments allows determination of the alignment of the bands relative to the electrochemical Fermi level.

CO and CO2 Hydrogenation to Methanol Calculated Using the BEEF-vdW Functional

Abstract: Hydrogenation of CO and CO2 to methanol on a stepped copper surface has been calculated using the BEEF-vdW functional and is compared to values derived with RPBE. It is found that the inclusion of vdW forces in the BEEF-vdW functional yields a better description of CO2 hydrogenation as compared to RPBE. These differences are significant for a qualitative description of the overall methanol synthesis kinetics and it is suggested that the selectivity with respect to CO and CO2 is only described correctly with BEEF-vdW.

The catalyst genome.

Abstract: The quest for the materials genome— the properties of a material that define its functional properties—has started. This signifies a transition to a new era of materials research where large amounts of materials data become available. The expectation is that this will significantly speed up the discovery of new materials. This is particularly true in the area of catalytic materials, where there is an urgent need for new catalysts and processes to enable the sustainable production of fuels and chemicals.

Electronic origin of the surface reactivity of transition-metal-doped TiO2(110)

Abstract: We investigate the surface reactivity of doped rutile M-TiO2(110) (M = V, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, and Ni) using density functional theory (DFT) and Hubbard-U corrected DFT calculations (DFT+U method). The oxygen adsorption bond, used as the surface reactivity measure, is stronger on the doped TiO2 surfaces as compared with that on the undoped TiO2 surface. We relate this increase in reactivity of the doped TiO2 surfaces to the presence of localized surface resonances and surface states in the vicinity of the Fermi level. We find that the center of these localized states on doped TiO2 is a good descriptor for the oxygen adsorption energy. The inclusion of the Hubbard-U correction to DFT barely modifies the oxygen adsorption energy on undoped TiO2, whereas it destabilizes the oxygen adsorption energies on doped TiO2 when compared with results from standard DFT. Nevertheless, we find that the oxygen adsorption energy trends predicted by a standard GGA-DFT functional are reproduced when the Hubbard-U c...

Modeling CO2 reduction on Pt(111)

Abstract: Density functional theory was used to model the electrochemical reduction of CO2 on Pt(111) with an explicit solvation layer and the presence of extra hydrogen atoms to represent a negatively charged electrode. We focused on the electronic energy barriers for the first four lowest energy proton–electron transfer steps for reducing CO2 on Pt(111) beginning with adsorbed *CO2 and continuing with *COOH, *CO + H2O, *COH, and ending with *C + H2O. We find that simple elementary steps in which a proton is transferred to an adsorbate (such as the protonation of *CO to *COH) have small barriers on the order of 0.1 eV. Elementary steps in which a proton is transferred and a C–O bond is simultaneously cleaved show barriers on the order of 0.5 eV. All barriers calculated for these steps show no sign of being insurmountable at room temperature. To explain why these barriers are so small, we analyze the charge density and the density of states plots to see that first, the electron transfer is decoupled from the proton transfer so that in the initial state, the surface and adsorbate are already charged up and can easily accept the proton from solution. Also, we see that in the cases where barriers are on the order of 0.1 eV, electron density in the initial state localizes on the oxygen end of the adsorbate, while electron density is more spread out on the surface for initial states of the C–O bond cleaving elementary steps.

In silico search for novel methane steam reforming catalysts

Abstract: This paper demonstrates a method for screening transition metal and metal alloy catalysts based on their predicted rates and stabilities for a given catalytic reaction. This method involves combining reaction and activation energies (available to the public via a web-based application ‘CatApp’) with a microkinetic modeling technique to predict the rates and selectivities of a prospective material. This paper illustrates this screening technique using the steam reforming of methane to carbon monoxide and hydrogen as a test reaction. While catalysts are already commercially available for this process, the method demonstrated in this paper is very general and could be applied to a wide range of catalytic reactions. Following the steps outlined herein, such an analysis could potentially enable researchers to understand reaction mechanisms on a fundamental level and, on this basis, develop leads for new metal alloy catalysts.

Methanol to Dimethyl Ether over ZSM-22: A Periodic Density Functional Theory Study

Abstract: Methanol-to-DME conversion over ZSM-22 Bronsted acid sites is investigated on the basis of periodic density functional theory calculations. DME formation has been speculated to take place via the dissociative or associative pathway. It is shown that the dissociative pathway is the predominant pathway. We find that water lowers the activation energies of key reactions but that the lowering of the activation energies is insufficient to increase the rate because of the entropy loss associated with water adsorption. The consequence of acid strength on the methanol-to-DME conversion pathways is investigated on the basis of Al-, Ga-, or In-induced Bronsted acid sites. We show that linear correlations between activation energies and acid strength exist. It is found that weaker acidity leads to higher activation energies. We find that changes in acidity will not change the conclusion that the dissociative pathway is the predominant pathway.

Ni–Fe–S Cubanes in CO2 Reduction Electrocatalysis: A DFT Study

Abstract: In this work, we perform extensive mechanistic studies of CO2 (electro)reduction by analogs to the active sites of carbon monoxide dehydrogenase (CODH) enzymes. We explore structure–property relationships for different cluster compositions and interpret the results with a model for CO2 electroreduction we recently developed and applied to transition metal catalysts. Our results validate the effectiveness of the CODH in catalyzing this important reaction and give insight into why specific cluster compositions were adopted by nature.

Selective ultrafast probing of transient hot chemisorbed and precursor states of CO on Ru(0001).

Abstract: We have studied the femtosecond dynamics following optical laser excitation of CO adsorbed on a Ru surface by monitoring changes in the occupied and unoccupied electronic structure using ultrafast soft x-ray absorption and emission. We recently reported [M. Dell'Angela et al. Science 339, 1302 (2013)] a phonon-mediated transition into a weakly adsorbed precursor state occurring on a time scale of >2 ps prior to desorption. Here we focus on processes within the first picosecond after laser excitation and show that the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, we probe each of these states selectively and determine the respective transient populations depending on optical laser fluence. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process.

First principles investigation of zinc-anode dissolution in zinc–air batteries

Abstract: With surging interest in high energy density batteries, much attention has recently been devoted to metal–air batteries. The zinc–air battery has been known for more than a hundred years and is commercially available as a primary battery, but recharging has remained elusive, in part because the fundamental mechanisms still remain to be fully understood. Here, we present a density functional theory investigation of the zinc dissolution (oxidation) on the anode side in the zinc–air battery. Two models are envisaged, the most stable (0001) surface and a kink surface. The kink model proves to be more accurate as it brings about some important features of bulk dissolution and yields results in good agreement with experiments. From the adsorption energies of hydroxyl species and experimental values, we construct a free energy diagram and confirm that there is a small overpotential associated with the reaction. The applied methodology provides new insight into computational modelling and design of secondary metal–air batteries.

Electroreduction of Methanediol on Copper

Abstract: We have used density functional theory calculations to study intermediates in the electroreduction of methanediol on copper. We find that methanediol, which is the hydrated form of formaldehyde, may be reduced to methanol with a limiting potential close to the experimental onset for reduction of aqueous formaldehyde.

Ammonia Synthesis: State of the Bellwether Reaction

Abstract: Catalytic ammonia synthesis has been judged to be one of mankind's greatest scientific achievements during the twentieth century. The socioeconomic implications of producing ammonia industrially have been a strong driving force, and this development has spurred a range of new discoveries within physics, chemistry, and chemical engineering. In this chapter, we describe how it has been possible in recent years to provide a full understanding of the catalytic ammonia synthesis reaction at the atomic level through the combined use of experiments and quantum mechanical electronic structure calculations, thus clearly showing many of the reasons why ammonia synthesis has been, and still is, the bellwether reaction in heterogeneous catalysis.

First-Principles Calculations of Fischer-Tropsch Processes Catalyzed by Nitrogenase Enzymes

Abstract: Nitrogenases are bacterial enzymes that provide reduced nitrogen to the biogeochemical nitrogen cycle, in which they account for more than half of the supply in nature. The fact that nitrogenases are capable of fixing N2 under ambient conditions has made them extremely intriguing catalysts, as the industrial Haber–Bosch process for ammonia synthesis requires high temperatures and pressures. Although the reduction of N2 to ammonia has been the most extensively studied aspect of nitrogenases, these metalloenzymes are capable of reducing a variety of other substrates including N2H4, C2H2, and CN. [3–5] Most recently, Ribbe and co-workers discovered that nitrogenases can also reduce CO to produce higher-order hydrocarbons. This makes nitrogenases interesting catalysts in connection with the production of liquid fuels from CO and CO2. An understanding of the exact mechanism(s) behind CO, CN, and N2 reduction by nitrogenases remains elusive and could offer great insight into the future design and development of other versatile and efficient catalysts. Here we address these mechanisms through first-principles calculations by identifying relevant chemical pathways and intermediate species involved in the reduction of CO and CN by the isolated cofactors of molybdenum and vanadium nitrogenases. With the calculated free energies for a large number of intermediates, we are able to estimate the potential needed to make each elementary step exergonic. There is ample evidence from studies of other electrochemical reactions that this so-called limiting potential correlates well with the potential at which experimental observations show the onset of reaction. 10] The reaction step with the most negative limiting potential defines the potential limiting step, which is the last step to become downhill in free energy as the potential is decreased. The difference between this limiting potential and the equilibrium potential defines the theoretical overpotential for the reaction. The complexity inherent to enzymes has made it extremely difficult to explain how nitrogenase functions. Many pioneering efforts have identified that the members of the nitrogenase family use a reductant (Fe protein) to donate electrons to the cofactor site (FeMoco and FeVco) active during catalysis. 5] The cofactor sites are part of another protein (the MoFe and VFe protein), and the ultimate reduction of various substrates at the active site requires a concerted transfer of charge and energy that is facilitated by the hydrolysis of adenosine-5’-triphosphate (ATP). The difficulty in controlling and understanding the details of the charge and energy transfer in the full enzyme system has led to many efforts to try to reduce the substrates without ATP. This has been achieved through the inclusion of genetic and chemical modifications to the proteins, such as the addition of photosensitizers to drive the electron transfer with light or the extraction of the isolated cofactors in organic solvents. 13, 14] This last method is extremely promising, as the isolated cofactors may be subjected to a controlled potential for charge transfer either by standard electrochemical techniques or by combining with a suitable reductant in an ATP-free buffer system. Indeed, Lee et al. recently successfully reduced CO and CN to a number of higher-order hydrocarbons with isolated FeMoco and FeVco by using the reductant europium(II) diethylenetriaminepentaacetate [EuDTPA], which corresponds to a potential of 1.14 V at pH 8 relative to the standard hydrogen electrode (SHE) or a potential of 0.67 V on the reversible hydrogen electrode (RHE) scale. 15] These findings can help shed new light on the reduction mechanisms of CO and CN by the nitrogenase active sites and enable a more direct comparison with theoretical models that can focus explicitly on the cofactor cluster. To describe and identify the relevant chemical steps, we employed a method of modeling electrochemical reactions within the computational hydrogen electrode (CHE) model. As an input, the CHE model requires the free energies of a number of adsorbates, which can be obtained from DFT calculations by using the GPAW code (see the Supporting Information for computational details). From the calculated adsorption energies, a voltage-dependent chemical pathway can be constructed for a given reaction. The CHE model has been successful in describing both relevant intermediates in electrochemical reactions and the potential regimes at which new chemical species emerge. 10] We will show that our results within the CHE model are in excellent agreement with the recent experiments of Lee et al. and that the results also offer explanations as to why the isolated cofactors appear equally active and why CN reduction is more facile than CO reduction. Specifically, we find that the availability of the active bridging S adsorption sites limits the cofactors’ reduction of CO but not that of CN at potentials comparable to those achieved with Eu–DTPA. For our model of the cofactor, we assume the structure of the Fe-S metal cluster deduced by Einsle et al. , in which C is the light central atom, as confirmed recently by X-ray emission spectroscopy and X-ray diffraction. We include our model [a] J. B. Varley, Prof. J. K. Nørskov Department of Chemical Engineering Stanford University Stanford, CA 94305-5025 (USA) E-mail : norskov@stanford.edu [b] Prof. J. K. Nørskov SUNCAT Center for Interface Science and Catalysis SLAC National Accelerator Laboratory Menlo Park, CA 94025 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201200635.

Platinum and palladium alloys suitable as fuel cell electrodes

Abstract: The present invention concerns electrode catalysts used in fuel cells, such as proton exchange membrane (PEM) fuel cells. The invention is related to the reduction of the noble metal content and the improvement of the catalytic efficiency by low level substitution of the noble metal to provide new and innovative catalyst compositions in fuel cell electrodes. The novel electrode catalysts of the invention comprise a noble metal selected from Pt, Pd and mixtures thereof alloyed with a further element selected from Sc, Y and La as well as any mixtures thereof, wherein said alloy is supported on a conductive support material.

Universal Transition State Scaling Relations for Hydrogenation and Dehydrogenation Reactions over Transition Metals

Abstract: This journal is © The Royal Society of Chemistry 2010 Physical Chemistry Chemical Physics, 2010, vol, 00–00 | 1 Universal transition state scaling relations for hydrogenation and dehydrogenation reactions over transition metals Shengguang Wang, Vivien Petzold, Vladimir Tripkovic, Jesper Kleis, Jakob Geelmuyden Howalt, Egill Skúlason, Eva M. Fernández, Britt Hvolbæk, Glenn Jones, Anja Toftelund, Hanne Falsig, a Mårten Björketun, Felix Studt, Frank Abild-Pedersen, Jan Rossmeisl, Jens Kehlet Nørskov, and 5 Thomas Bligaard*