Showing papers by "Jens K. Nørskov published in 2008"

Identification of non-precious metal alloy catalysts for selective hydrogenation of acetylene

Abstract: The removal of trace acetylene from ethylene is performed industrially by palladium hydrogenation catalysts (often modified with silver) that avoid the hydrogenation of ethylene to ethane. In an effort to identify catalysts based on less expensive and more available metals, density functional calculations were performed that identified relations in heats of adsorption of hydrocarbon molecules and fragments on metal surfaces. This analysis not only verified the facility of known catalysts but identified nickel-zinc alloys as alternatives. Experimental studies demonstrated that these alloys dispersed on an oxide support were selective for acetylene hydrogenation at low pressures.

Ammonia for hydrogen storage: challenges and opportunities

Abstract: The possibility of using ammonia as a hydrogen carrier is discussed. Compared to other hydrogen storage materials, ammonia has the advantages of a high hydrogen density, a well-developed technology for synthesis and distribution, and easy catalytic decomposition. Compared to hydrocarbons and alcohols, it has the advantage that there is no CO2 emission at the end user. The drawbacks are mainly the toxicity of liquid ammonia and the problems related to trace amounts of ammonia in the hydrogen after decomposition. Storage of ammonia in metal ammine salts is discussed, and it is shown that this maintains the high volumetric hydrogen density while alleviating the problems of handling the ammonia. Some of the remaining challenges for research in ammonia as a hydrogen carrier are outlined.

The nature of the active site in heterogeneous metal catalysis.

Abstract: This tutorial review, of relevance for the surface science and heterogeneous catalysis communities, provides a molecular-level discussion of the nature of the active sites in metal catalysis. Fundamental concepts such as “Bronsted–Evans–Polanyi relations” and “volcano curves” are introduced, and are used to establish a strict partitioning between the so-called “electronic” and “geometrical” effects. This partitioning is subsequently employed as the basis for defining the concept “degree of structure sensitivity” which can be used when analyzing the structure sensitivity of catalytic reactions.

Oxidation and Photo-Oxidation of Water on TiO2 Surface

Abstract: The oxidation and photo-oxidation of water on the rutile TiO2(110) surface is investigated using density functional theory (DFT) calculations. We investigate the relative stability of different surface terminations of TiO2 interacting with H2O and analyze the overpotential needed for the electrolysis and photoelectrolysis of water. We found that the most difficult step in the splitting of water process is the reaction of a H2O molecule with a vacancy in the surface to form an adsorbed hydroxyl group (OH*). Comparison to experiment shows that the computed overpotential for O2 evolution (0.78 V) is available under the experimental conditions required for both oxygen and hydrogen evolution.

Surface Pourbaix diagrams and oxygen reduction activity of Pt, Ag and Ni(111) surfaces studied by DFT

Abstract: Based on density functional theory calculations we investigate the electrochemically most stable surface structures as a function of pH and electrostatic potential for Pt(111), Ag(111) and Ni(111), and we construct surface Pourbaix diagrams. We study the oxygen reduction reaction (ORR) on the different surface structures and calculate the free energy of the intermediates. We estimate their catalytic activity for ORR by determining the highest potential at which all ORR reaction steps reduce the free energy. We obtain self-consistency in the sense that the surface is stable under the potential at which that particular surface can perform ORR. Using the self consistent surfaces, the activity of the very reactive Ni surface changes dramatically, whereas the activity of the more noble catalysts Pt and Ag remains unchanged. The reason for this difference is the oxidation of the reactive surface. Oxygen absorbed on the surface shifts the reactivity towards the weak binding region, which in turn increases the activity. The oxidation state of the surface and the ORR potential are constant versus the reversible hydrogen electrode (RHE). The dissolution potential in acidic solution, on the other hand, is constant vs. the standard hydrogen electrode (SHE). For Ag, this means that where the potential for dissolution and ORR are about the same at pH = 0, Ag becomes more stable relative to RHE as pH is increased. Hence the pH dependent stability offers an explanation for the possible use of Ag in alkaline fuel cell cathodes.

Trends in the Catalytic CO Oxidation Activity of Nanoparticles

Abstract: Introduction While extended gold surfaces are generally considered chemically inert [1], nanosized (<5 nm) gold particles can be very effective catalysts for a number of oxidation reactions [2-5]. There are reports of similar size effects for silver catalysts [6]. The origin of the nano-effects in the catalytic properties of these metals is widely debated [5], and no consensus has been reached yet. Based on a set of density functional theory calculations we compare the catalytic activity for the CO oxidation reaction over extended surfaces and small nano-particles of a number of metals.

Scaling Relationships for Adsorption Energies on Transition Metal Oxide, Sulfide, and Nitride Surfaces

Abstract: There has been substantial progress in the description of adsorption and chemical reactions of simple molecules on transition-metal surfaces. Adsorption energies and activation energies have been obtained for a number of systems, and complete catalytic reactions have been described in some detail. Considerable progress has also been made in the theoretical description of the interaction of molecules with transition-metal oxides, sulfides, and nitrides, but it is considerably more complicated to describe such complex systems theoretically. Complications arise from difficulties in describing the stoichiometry and structure of such surfaces, and from possible shortcomings in the use of ordinary generalized gradient approximation (GGA) type density functional theory (DFT). Herein we introduce a method that may facilitate the description of the bonding of gas molecules to transitionmetal oxides, sulfides, and nitrides. It was recently found that there are a set of scaling relationhips between the adsorption energies of different partially hydrogenated intermediates on transition-metal surfaces. We will show that similar scaling relationships exist for adsorption on transition metal oxide, sulfide, and nitride surfaces. This means that knowing the adsorption energy for one transition-metal complex will make it possible to quite easily generate data for a number of other complexes, and in this way obtain reactivity trends. The results presented herein have been calculated using self-consistent DFT. Exchange and correlation effects are described using the revised Perdew–Burke–Ernzerhof (RPBE) GGA functional. It is known that GGA functionals give adsorption energies with reasonable accuracy for transition metals. It is not clear, however, whether a similar accuracy can be expected for the oxides, sulfides, and nitrides, although there are examples of excellent agreement betweenDFT calculations and experiments, for example, with RuO2 surfaces. [9] In our study we focused entirely on variations in the adsorption energies from one system to another, and we expected that such results would be less dependent than the absolute adsorption energies on the description of exchange and correlation. For the nitrides, a clean surface and a surface with a nitrogen vacancy were studied. For MX2-type oxides or sulfides, an oxygenor sulfur-covered surface with an oxygen or sulfur vacancy was studied. The structures of the clean surface considered in the present work and their unit cells are shown in Figure 1. The adsorption energies given below are for the adsorbed species in the most stable adsorption site on the surface. By performing calculations for a large number of transition-metal surfaces of different orientations, it was found that the adsorption energy of intermediates of the type AHx is linearly correlated with the adsorption energy of atom A (N, O, S) according to Equation (1):

Chemical bonding at surfaces and interfaces

Abstract: Molecular surface science has made enormous progress in the past 30 years. The development can be characterized by a revolution in fundamental knowledge obtained from simple model systems and by an explosion in the number of experimental techniques. The last 10 years has seen an equally rapid development of quantum mechanical modeling of surface processes using Density Functional Theory (DFT). Chemical Bonding at Surfaces and Interfaces focuses on phenomena and concepts rather than on experimental or theoretical techniques. The aim is to provide the common basis for describing the interaction of atoms and molecules with surfaces and this to be used very broadly in science and technology. The book begins with an overview of structural information on surface adsorbates and discusses the structure of a number of important chemisorption systems. Chapter 2 describes in detail the chemical bond between atoms or molecules and a metal surface in the observed surface structures. A detailed description of experimental information on the dynamics of bond-formation and bond-breaking at surfaces make up Chapter 3. Followed by an in-depth analysis of aspects of heterogeneous catalysis based on the d-band model. In Chapter 5 adsorption and chemistry on the enormously important Si and Ge semiconductor surfaces are covered. In the remaining two Chapters the book moves on from solid-gas interfaces and looks at solid-liquid interface processes. In the final chapter an overview is given of the environmentally important chemical processes occurring on mineral and oxide surfaces in contact with water and electrolytes. * Gives examples of how modern theoretical DFT techniques can be used to design heterogeneous catalysts* This book suits the rapid introduction of methods and concepts from surface science into a broad range of scientific disciplines where the interaction between a solid and the surrounding gas or liquid phase is an essential component* Shows how insight into chemical bonding at surfaces can be applied to a range of scientific problems in heterogeneous catalysis, electrochemistry, environmental science and semiconductor processing* Provides both the fundamental perspective and an overview of chemical bonding in terms of structure, electronic structure and dynamics of bond rearrangements at surfaces

Indirect, reversible high-density hydrogen storage in compact metal ammine salts.

Abstract: The indirect hydrogen storage capabilities of Mg(NH3)6Cl2, Ca(NH3)8Cl2, Mn(NH3)6Cl2, and Ni(NH3)6Cl2 are investigated. All four metal ammine chlorides can be compacted to solid tablets with densities of at least 95% of the crystal density. This gives very high indirect hydrogen densities both gravimetrically and volumetrically. Upon heating, NH3 is released from the salts, and by employing an appropriate catalyst, H2 can be released corresponding to up to 9.78 wt % H and 0.116 kg H/L for the Ca(NH3)8Cl2 salt. The NH3 release from all four salts is investigated using temperature-programmed desorption employing different heating rates. The desorption is found mainly to be limited by heat transfer, indicating that the desorption kinetics are extremely fast for all steps. During desorption from solid tablets of Mg(NH3)6Cl2, Mn(NH3)6Cl2, and Ni(NH3)6Cl2, nanoporous structures develop, which facilitates desorption from the interior of large, compact tablets. Density functional theory calculations reproduce tren...

A molecular view of heterogeneous catalysis.

Abstract: The establishment of a molecular view of heterogeneous catalysis has been hampered for a number of reasons. There are, however, recent developments, which show that we are now on the way towards reaching a molecular-scale picture of the way solids work as catalysts. By a combination of new theoretical methods, detailed experiments on model systems, and synthesis and in situ characterization of nano-structured catalysts, we are witnessing the first examples of complete atomic-scale insight into the structure and mechanism of surface-catalyzed reactions. This insight has already proven its value by enabling a rational design of new catalysts. We illustrate this important development in heterogeneous catalysis by highlighting recent examples of catalyst systems for which it has been possible to achieve such a detailed understanding. In particular, we emphasize examples where this progress has made it possible to propose entirely new catalysts, which have then been proven experimentally to exhibit improved performance in terms of catalytic activity or selectivity.

Using scaling relations to understand trends in the catalytic activity of transition metals.

Abstract: A method is developed to estimate the potential energy diagram for a full catalytic reaction for a range of late transition metals on the basis of a calculation (or an experimental determination) for a single metal. The method, which employs scaling relations between adsorption energies, is illustrated by calculating the potential energy diagram for the methanation reaction and ammonia synthesis for 11 different metals on the basis of results calculated for Ru. It is also shown that considering the free energy diagram for the reactions, under typical industrial conditions, provides additional insight into reactivity trends.

BEP relations for N2 dissociation over stepped transition metal and alloy surfaces.

Abstract: We present density functional theory (DFT) calculations for N2 dissociation on stepped face-centred cubic (211) surface slabs. By using the same crystal structure, the same adsorption site for atomic nitrogen, and the same transition-state bond length of N2 over a range of pure metal surfaces, a perfectly linear Bronsted–Evans–Polanyi (BEP) relation between the transition-state potential energy and the dissociative chemisorption energy is obtained. The perfect BEP relation, which extends over 12 eV in chemisorption energy, suggests that the manifestation of BEP relations for surface reactions is a general electronic structure effect, and that geometric effects are responsible for the scatter which is normally observed around the BEP line. The BEP relation is also shown to be valid for both surface and bulk alloys. The scatter is, however, larger than for the pure elements. This can be understood as a larger geometrical variance. To analyze the accuracy of the DFT calculations a detailed convergence study is performed for several adsorbates on stepped hexagonal close-packed and face-centred cubic Ru slabs.

Indirect hydrogen storage in metal ammines

Abstract: Publisher Summary The indirect storage of hydrogen in ammonia is a promising concept for storage and transportation of hydrogen. This chapter focuses on the potential of indirect hydrogen storage by the use of ammonia stored in metal ammines and describes ammonia, as a potential hydrogen storage medium; ammonia production and infrastructure; safety concerns; and the energy costs involved in indirect storage. It also discusses the storage of ammonia in solid form using MgCl2 as the model carrier material and covers the improved safety of ammonia stored in metal ammines, methods for preparation and powder compaction, low materials cost, and easy scale-up. The chapter describes the thermodynamic properties of different metal ammines, for example, van't Hoff plots, desorption properties, reversibility and reloading, and selection of specific ammines by weighting parameters, such as safety and desorption temperature. It considers the development of novel metal ammine systems, focusing on the design of superior metal ammines by closely integrating experimental and calculational work. From a detailed understanding of structure and stability, porosity and particle size, desorption and diffusion, and alloy formation, it is possible to engineer these materials on the nano-, micro-, and macro-scales. The chapter further discusses the commercial potential and perspectives of using metal ammines in connection with, for example, Polymer Electrolyte Membrane (PEM) and Solid Oxide Fuel Cells (SOFCs), as well as Selective Catalytic Reduction (SCR)-DeNOx (NOx removal) in the transport sector.

Recent density functional studies of hydrodesulfurization catalysts: insight into structure and mechanism

Abstract: The present article will highlight some recent density functional theory (DFT) studies of hydrodesulfurization (HDS) catalysts. It will be summarized how DFT in combination with experimental studies can give a detailed picture of the structure of the active phase. Furthermore, we have used DFT to investigate the reaction pathway for thiophene HDS, and we find that the reaction entails a complex interplay of different active sites, depending on reaction conditions. An investigation of pyridine inhibition confirmed some of these results. These fundamental insights constitute a basis for rational improvement of HDS catalysts, as they have provided important structure-activity relationships.

Understanding Heterogeneous Catalysis from the Fundamentals

Abstract: Catalysis describes the acceleration of a chemical reaction by means of a substance that is itself not consumed by the overall reaction. It is not only important for numerous human activities, but it has also always been a major spur for the development of surface science. Today there is an extensive surface-science heritage of understanding, and there are examples of catalysis phenomena that are now understood from the fundamentals. For instance, modern-day theory is able to predict the turnover frequency of an industrially relevant catalytic reaction in a semi-quantitative way. A major part of this chapter sums up such a successful surface-scientific development, pointing out descriptors for metal catalysts and identifying trends in adsorption energies and activation energies for surface reactions on transition metal surfaces by extensive computations. This is done using the density-functional theory (DFT), whose accuracy in this context is secured, and analyzed in electron-structural terms, in particular the d-band model. Via correlations determined from DFT calculations, universal relationships in heterogeneous catalysis are built up, including variations in catalytic rates, volcano relations. The optimization and design of catalysts through modeling is within reach. For instance, experimental verification for pure CO methanation, for CO2 methanation, and for simultaneous CO and CO2 methanation means that a technical methanation catalyst is discovered on the basis of computational screening. To further detail the surface-science approach to catalysis it is natural to supplement this presentation with some other examples of recent work on some catalytic reactions from the fundamentals. Oxidation of some monoxides illustrates the use of kinetic Monte Carlo simulations. The successful prediction of the outcome of the ammonia synthesis from first-principles supports the view that in the future theory will be a fully integrated tool in the search for the next generation of catalysts. The hydrogen evolution reaction on MoS2 is given as an example of successful interplay between theory and experiment. It is concluded that, thanks to the strong development in surface science, the understanding of heterogeneous catalysis from the fundamentals is approaching an advanced stage. Design of new catalyst on the basis of computational screening is today a realistic perspective. The list of issues that need further considerations includes nonadiabaticity, complex reactions, and other classes of catalyst materials than transition metals.

Theoretical Modelling of Catalytic Reactions

Abstract: The sections in this article are Introduction The Hierarchy of Models The Full Reaction Dynamics The Kinetic Monte Carlo Approach Langmuir–Hinshelwood Description Power-Law Kinetics Microkinetic Modeling Equilibrium Statistical Thermodynamics Sticking Temperature-Programmed Desorption Catalytic Reaction at Steady State Catalytic Reaction at Quasi-Equilibrium Parameters Test Applications Ab Initio Kinetics Applications Keywords: microkinetics; reaction dynamics; reaction kinetics; ab initio kinetics

Transition metal sulfide catalysts: - A DFT study of structure and reactivity

Abstract: In this thesis, density functional theory (DFT) is applied in a study of topics related to hydrodesulfurization catalysis. A series of calculated adsorption energies of hydrogen containing molecules on transition metals are presented. From the data set of adsorption energies linear relations between adsorption energies of the central atom and the hydrogenated central atom are derived. Insight into the underlying physics dictating the linear correlations is obtained by the development of a model based on the d-band model and effective medium theory. The study is extended to sulfides, nitrides, and oxides where similar linear relations are observed. The structure of Ni and Co promoted MoS2 catalysts is investigated in a combined DFT and scanning tunneling microscopy study. This study reveals that promotion with Co and Ni changes the shape and electronic structure of the nanoparticles. Two different kinds of morphology are observed, type A which is hexagons with promoters positioned at the (1̄010) edge, these are formed both for Ni and Co promoted particles. The second morphology termed type B is only formed by Ni promotion and has the shape of truncated hexagons, with the (1̄010) fully promoted with Ni and the (101̄0) edge partially promoted with Ni. All structures have bright brims near and on the edge which are found to be the results of metallic edge states. The hydrodesulfurization of thiophene is investigated over MoS2 and Co promoted MoS2 (CoMoS). The active sites are found to be vacancy sites at the (1̄010) edge of MoS2 and so-called brim sites at the CoMoS (1̄010) edge and the (101̄0) edge of MoS2. The hydrogenation pathway (HYD) and the direct desulfurization (DDS) pathway are investigated on all sites. For the non promoted catalyst it is found that interaction between the (1̄010) and the (101̄0) edge is important. The reason being that hydrogenation is facile at the (101̄0) edge while the (1̄010) has the highest activity for SC scission. The HYD pathway is found to be more important than the DDS pathway on non promoted MoS2. The DDS pathway is proposed to be slow since adsorption and hydrogenation of thiophene at the non promoted (1̄010) edge vacancy is unlikely. Co promotion increases the importance of the DDS pathway. This is because Co promotion increases the thiophene adsorption energy and at the same time the hydrogenation activity of the