Dr Paola Quaino
Institute of Theoretical Chemistry
Ulm University

German Soldano
Institute of Theoretical Chemistry
Ulm University

Dr Elisbeth Santos
Institute of Theoretical Chemistry
Ulm University

Dr Wolfgang Schmickler
Institute of Theoretical Chemistry
Ulm University

The physical and electronic properties of several platinum nanostructures have been investigated by density functional calculations. Particular attention has been paid to the structure of the d band. Our preliminary results predict, that nanowires and small platinum clusters supported on Au(111) should be excellent catalysts for the hydrogen evolution reaction; a monolayer of platinum on Au(111) should also be better than pure platinum.

• Some properties of electrochemical nanostructures. E. Santos, G. Soldano, P. Quaino, W. Schmickler. J. Chem. Sciences (India), special issue to honour the late Prof. S.K. Rangarajan, (2009), in press.
• Hydrogen adsorption and fractional conductance of electrochemical nanowires. E. Santos, G. Soldano, P. Quaino, W. Schmickler. Electrochem. Comm., 11 (2009) 1764.
• On the electrocatalysis of nanostructures: monolayers of a foreign atom (Pd) on different susbtrates M(111). E. Santos, P. Quaino, W. Schmickler. Electrochim. Acta., in press.
• (26/07/2009 - 31/07/2009) Poster presentation in the 11th International Fischer-Symposium on Microscopy in Electrochemistry (Benedikbeuern, Germany). Topic: Chemical Adsorption at Nanowires. Authors:G. Soldano, P. Quaino, E. Santos and W. Schmickler.

Dr Paola Quaino
Institute of Theoretical Chemistry
Ulm University

Dr Elisbeth Santos
Institute of Theoretical Chemistry
Ulm University

Dr Wolfgang Schmickler
Institute of Theoretical Chemistry
Ulm University

Electrochemical nanostructures have gained a great interest in the last decade because of the large variation of their reactivity in comparison with bulk material. A number of groups worldwide investigate diverse nanostructures in order to improve the electrocatalytic properties of electrode materials for several technological applications such as energy conversion. An interesting aspect is the possibility to design at the nanoscale materials with specific properties. Particularly in surface science, for reactions at the gas / solid interface, an enormous amount of research, both experimental and theoretical, has been carried out. However, the implementation of these nano-materials is difficult because of the lack of understanding of the fundamental aspects and mechanisms which determine their stability, reactivity and dynamics. Analysis of the present level of studies suggests that although many interesting achievements have been accomplished, most of them concern empirical correlations between the geometry at the nano-scale, such as the density of defects and width of the terraces, with thermodynamic properties, such as the work function, potential of zero charge, adsorption enthalpy.

In these project, we have applied our theory of electrocatalysis to investigate the reactivity of various nanostructures. Particularly in this contribution, we shall show the results obtained for monolayers of Pd on different substrates. A good agreement with experimental results has been obtained for the systems under study. Although the strain induced by the mismatch in the lattice constants of substrate and adsorbate is observed; we show that it is not the only effect for changing the electrocatalytic properties; also the specific chemical interaction between the monolayer and the substrate plays an important role. The catalytic properties of the nanostructures can be different for oxidation and reduction reactions.=q

• Trends in the Electrocatalytic Activity of Nanostructures of Pd on Au(111). K. Pötting, P. Quaino, E. Santos, W. Schmickler. 7th Spring Meeting of the International Society of Electrochemistry (ISE), Szczyrk (Poland), march 2009. Poster presentation
• On the electrocatalysis of nanostructures. E. Santos, P. Quaino, W. Schmickler. 2nd International Symposium on Surface Imaging/Spectroscopy at the Solid/Liquid Interface Krakow (Poland), june 2009. Oral Presentation
• Electrocat´alisis de nanoestructuras. E. Santos, P. Quaino, W. Schmickler. FYQS-IV-2009, La Plata (Argentina), october 2009, Poster Presentation.
• On the electrocatalysis of nanostructures: monolayers of a foreign atom (Pd) on different susbtrates M(111). E. Santos, P. Quaino, W. Schmickler. Electrochim. Acta., in press.{

Dr Paola Quaino
Institute of Theoretical Chemistry
Ulm University

Dr Angelica Lundin
Institute of Theoretical Chemistry
Ulm University

Dr Kay Pötting
Institute of Theoretical Chemistry
Ulm University

Dr Elisbeth Santos
Institute of Theoretical Chemistry
Ulm University

Dr Wolfgang Schmickler
Institute of Theoretical Chemistry
Ulm University

Hydrogen evolution is the best investigated electrochemical reaction, and is considered as a prototype for electrocatalysis. However, in spite of decades of efforts, there was no real understanding of hydrogen electrocatalysis, or of any other electrochemical reaction, until few years ago. All that existed were a few correlations, the volcano plot between the reaction rate and the energy of hydrogen adsorption being the best known.

A proper understanding cannot be based on correlations, nor can it be obtained by performing DFT calculation for a large number of cases - it requires a theory. Therefore, we have linked a theory for electrocatalysis proposed by Santos and Schmckler with DFT calculations and applied it to the hydrogen evolution reaction, focusing on the adsorption of the proton and the adsorbed hydrogen atom, separated by a saddle point, from which we determined the energy of activation. Explicit calculations have been performed for five metals: Pt, Au, Ag, Cu and Cd. In accord with experimental findings we find a high activation energy for Cd, medium values for the coin metals, and on Pt the transfer occurs with little activation. These results are explained in terms of the position of the d band of these metals and their interaction with the hydrogen 1s orbital as the latter passes the Fermi level in the presence of the solvent.

• Hydrogen oxidation and evolution on platinum. P.M. Quaino, Electrocatalysis at the Nanoscale - Theory and Modeling, Workshop. (october 2009) Reisensburg, Germany. Oral Presentation
• A theory for the electrocatalysis of hydrogen evolution. E. Santos, A. Lundin, K. Pötting, P. Quaino, W. Schmickler. Phys. Rev. B, 79 (2009) 235436.
• Hydrogen Electrode Reaction: Electrocatalytic Activity of Metal Surfaces. P. Quaino, K. Pötting, E. Santos, W. Schmickler. 7th Spring Meeting of the International Society of Electrochemistry (ISE), Szczyrk (Poland), march 2009. Oral presentation
• Reaccion del electrodo de hidrogeno: actividad electrocatalitica de superficies metalicas. E. Santos, P. Quaino, W. Schmickler. FYQS-IV-2009, La Plata (Argentina), october 2009.
• Hydrogen evolution and oxidation- a prototype for a electrocatalytic reaction. E. Santos, A. Lundin, K. Pötting, P. Quaino, W. Schmickler. J. Solid State Electrochem. 13 (2009) 1101.
• Hydrogen Electrode Reaction (HER) on different metals: Evaluation of the Electrocatalytic Activity. P. Quaino, K. Pötting, E. Santos, W. Schmickler. Gesellschaft Deutscher Chemiker (GDCh), Electrochemistry: Crossing boundaries, Giessen (Alemania), octubre de 2008.Poster Presentation
• Electrocatalysis@nanoscale: techniques and applications. Lorentz Center - International Center for workshops in the Sciences. Netherlands. November 2008.
• Theory of Hydrogen reaction on metals. P. Quaino. Seminar. Institute of Theoretical Chemistry. Ulm University.

Dr Paola Quaino
Institute of Theoretical Chemistry
Ulm University

Dr Angelica Lundin
Institute of Theoretical Chemistry
Ulm University

Dr Kay Pötting
Institute of Theoretical Chemistry
Ulm University

Dr Elisbeth Santos
Institute of Theoretical Chemistry
Ulm University

Dr Wolfgang Schmickler
Institute of Theoretical Chemistry
Ulm University

Hydrogen evolution on single crystal copper and silver has been investigated by a combination of density functional theory and a theory developed in our own group by Santos and Schmickler. At short times, the reaction rate is determined by the transfer of the first proton to the electrode surface. In accord with experiment, we find for both metals that this reaction proceeds faster on the (111) surfaces than on the (100). The main cause is the lower, i.e. more favourable, adsorption energy on the former surfaces. On both silver surfaces, the second step is electrochemical desorption. The same mechanism is likely to operate on copper.

• Hydrogen evolution on single crystal copper and silver - a theoretical study. E. Santos, A. Lundin, K. Pötting P. Quaino, W. Schmickler. Phys. Chem. Chem. Phys., submitted.
• Hydrogen Electrode Reaction on Cu and Silver electrodes. P. Quaino. Seminar. Programa de Electroquimica Aplicada e Ingenieria Electroquimica - Universidad Nacional del Litoral.

Dr, Payam Kaghazchi
Institute of Electrochemistry
Ulm University

Jochen Bandlow
Institute of Electrochemistry
Ulm University

Fedir Strygunov
Institute of Electrochemistry
Ulm University

PD Dr. Timo Jacob
Institute of Electrochemistry
Ulm University

Surface faceting can be understood as a morphology change from a flat bulk-truncated surface to a hill-and-valley structure. While clean surfaces rarely facet, adsorbate-induced faceting of surfaces, driven by the anisotropy of surface free energy, is a general phenomenon observed in many systems. Since high-index clean metal surfaces typically have lower surface atom densities and higher surface free energies compared to the close-packed surfaces of the same metal they can be used as the basis for surface reconstruction and facet formation studies. Furthermore, it allows us to deepen the understanding of the stability of surfaces in contact with a reactive gas environment, which is essential for selecting and controlling a desired surface morphology. Faceted surfaces have also been used as model systems to study structural sensitivity in catalytic reactions.

So far experimental studies of adsorbate-induced faceting of metal surfaces focused mainly on body-centered cubic or face-centered cubic metals, such as W(111), Mo(111), Ni(210), Pt(210), Ir(210), Rh(553), and vicinal Cu surfaces. Although the enhancement of the anisotropy in surface free energy is the thermodynamic driving force for facet formation, in most cases this process is hindered by kinetic limitations. Therefore not only a critical adsorbate coverage is required but also a minimum annealing temperature allowing the system to overcome all kinetic barriers in the process of facet formation.

With this project we aim to understand the driving force behind the faceting by means of density functional theory and thermodynamic considerations. In our theoretical method we assumed the size of the facets to be large enough such that the overall contributions of step edges, kinks and strain to the formation energy are rather small. In this case we can restrict ourselves to the thermodynamic (Herring-)condition for facet formation, where the Gibbs-energy of facet formation can be expressed by the surface contributions only.

Since in our case facets showing different faces are formed on the initially planar surface after adsorption of oxygen, the faceting conditions is fulfilled in case the overall surface energy of the faceted surface is lower than this of the planar unfaceted surface orientation. The presence of the oxygen environment and its influence on the surface stabilities can then be treated by the ab initio atomistic thermodynamics approach (similar for nitrogen atmosphere).

In order to understand the oxygen-induced faceting of Ir(210) we performed DFT calculations on clean and oxygen-covered surfaces of Ir(210), Ir(311) and Ir(110), which are involved in facet formation. Using these DFT calculations together with thermodynamics consideration (explained above) we were able to generate the corresponding O/Ir(210)-surface phase diagram and found that three-sided pyramids with (311) and (110) faces are indeed stabilized under temperature and pressure conditions applied in typical experiments.

• W. Chen, A. L. Stottlemyer, J. G. Chen, P. Kaghazchi, T. Jacob, T. E. Madey, R. A. Bartynski, Adsorption and Decomposition of NO on O-covered Planar and Faceted Ir(210), Surf. Sci., 603, 3136 (2009).
• T. Jacob, Electrochemical Surface Faceting of Re(11-21), Electrochim. Acta (special issue), 54, 5023 (2009).
• P. Kaghazchi, W. Chen, H. Wang, I. Ermanoski, T. E. Madey, T. Jacob, First Principle Studies on Oxygen-Induced Faceting of Ir(210), ACS Nano, 2, 1280 (2008).

Donato Fantauzzi
Institute of Electrochemistry
Ulm University

Dr. John A. Keith
Institute of Electrochemistry
Ulm University

Dr. Josef Anton
Institute of Electrochemistry
Ulm University

PD Dr. Timo Jacob
Institute of Electrochemistry
Ulm University

Platinum-based catalysts supported on carbon still serve as the most used electrocatalyst for the oxygen reduction reaction (ORR), which is one of the most important reactions in basic electrochemistry but also in various applications such as low-temperature polymer electrolyte fuel cells. In order to find a more active catalyst than pure Pt, in the past years various platinum alloys such as PtNi, PtCo, etc. with different atom ratios were synthesized and investigated. Since Pt is limited and rather expensive, these multi-metallic catalysts are not only saving precious metal but also show enhanced reactivity and selectivity.

Nanoparticles of these alloys even showed a higher tolerance against impurities. For example, the increased tolerance against methanol that could be of use in the polymer electrolyte membrane fuel cell (PEMFC) with methanol as hydrogen source.

Since not only the morphology and composition of nanoparticle catalysts but also their reactivity are strongly correlated to their size and shape, comparing Pt-based alloys with pure platinum catalysts requires a careful analysis of the aforementioned parameters: particle size, shape, and surface or bulk composition, respectively. These factors are directly correlated to the electronic and magnetic properties of each individual catalyst particle and are therefore determining their reactivity, selectivity, etc.

The present work is motivated by recent experiments on bi-metallic nanoparticles Pt-M (M= Ni, Co) supported on carbon XC-72. The catalyst particles, whose composition was varied from 1:1 over 2:1 to 3:1, were synthesized by a carbonyl chemical route. High dispersion of the catalyst particles was shown by transmission electron microscopy (TEM). The nanoparticles mostly had an average size of about 2.40±1.00nm with a Gauss-type size distribution. The bulk composition of the different particles was characterized by energy dispersive X-ray spectroscopy (EDX), while the corresponding surface composition was analyzed electrochemically using cyclic voltammetry and comparing the hydrogen adsorption behavior of the catalyst to that of pure platinum.

In this project we investigated the bulk and surface structure as well as the segregation behavior of these alloy nanoparticles using molecular dynamics (MD) with a self-optimized reactive forcefield. By applying subsequent simulated annealing cycles to particles of variable size and bulk composition direct comparison to the experimental results was possible.

• E. Favry, D. Wang, D. S. Su, N. Alonso-Vante, D. Fantauzzi, T. Jacob, in preparation.

Dr. Josef Anton
Institute of Electrochemistry
Ulm University

PD Dr. Timo Jacob
Institute of Electrochemistry
Ulm University

Dr. Valeria Pershina
GSI Helmholtzzentrum Darmstadt

Investigations of the chemical properties of the heaviest elements are among the most fundamental. They seek to probe the uppermost reaches of the Periodic Table of the elements, where the nuclei become extremely unstable and relativistic effects on electron shells are rather significant. Due to developments in experimental techniques, elements as heavy as 108 and 112 have been studied, while experiments on elements beyond 112 are planned.

In this context, the aim of this project is to evaluate the properties as well as the adsorption behavior of these superheavy elements on the detector surface. Due to the weakness of these interactions, predictions of physisorption of molecular species still remain a challenge for theoretical chemistry. Using our fully relativistic (four-component) density functional theory (4c-DFT) program in combination with adsorption models, we aim to predict adsorption energies Hads and temperatures Tads of group 4-8 halides and oxyhalides of the heaviest elements.

For instance, with calculated properties and the model of physisorption, we recently could determine adsorption enthalpies of different MO₄ (M=Ru, Os, and Hs) tetra-oxides on quartz. The obtained value for the adsorption of HsO₄ of -H_ads(HsO₄)=45.1 kJ/mol is in excellent agreement with experimental measurements (at the GSI in Darmstadt) of 46 kJ/mol. Furthermore, we find the following trend in the volatility of the group 8 tetroxides: RuO₄ < OsO₄ > HsO₄. This inversion in the trend was also observed for other properties, which is also in agreement with corresponding experimental results.

• V. Pershina, J. Anton, T. Jacob, Theoretical Predictions of Adsorption Behavior of Elements 112 and 114 and their Homologs Hg and Pb, J. Chem. Phys., 131, 084713 (2009).
• V. Pershina, J. Anton, T. Jacob, Electronic Structures and Properties of MAu and MOH, where M=Tl and Element 113, Chem. Phys. Lett., 480, 157 (2009).

I. Respondek
Nachwuchsgruppe Theorie - SFB 569
Ulm University

D. M. Benoit
Nachwuchsgruppe Theorie - SFB 569
Ulm University

A. Tschetschetkin
Institute of Solid State Physics
Ulm University

B. Koslowski
Institute of Solid State Physics
Ulm University

A. Groß
Institute of Theoretical Chemistry
Ulm University

Thiols, such as 4-mercaptopyridine (4-MPy) adsorb readily on a gold surface and form well-ordered structures, so-called self-assembled monolayers (SAMs). SAMs offer a way of tailoring the interfacial properties of metals, metal oxides and semiconductors for molecular electronics applications. Also, SAMs themselves have very useful properties which can be tuned, such as their thickness, insulating or conducting properties, their ability to fabricate them into patterns, and their easy preparation. It has been shown that multilayer structures can be formed using 4-MPy as a building block, where the SAM acts as a connecting agent between two different kinds of metals.

It is important to understand the surface-molecule and molecule-molecule interactions in order to tailor the properties of the SAM. Here, the type of bonding between metal and surface, and the influence the surface has on the molecular properties is a fundamental aspect for an understanding of the stability of the system.

Our theoretical approach is based on the VSCF (vibrational self-consistent field) method. In this method, vibrational anharmonicity is explicitly included via the potential energy surface (PES). However, the construction of the PES for such large systems as studied in this project is computationally demanding. In our VSCF implementation, the PES is represented on a grid, where each grid point corresponds to a single-point energy (SPE) calculation of a quantum chemistry program. For 4-MPy on Au(111), about 36,464 SPE calculations were required for each of the three adsorption sites studied. However, each of these SPE calculations is completely independent from the other calculations, so that the jobs can be run simultaneously on different machines. This kind of computations are ideal for grid applications, since they run simultaneously and each job takes about one to two hours.

With the help of these calculations, the differences between the adsorbates could be resolved. At the harmonic level, the vibrational signatures of the molecule at different adsorption sites are identical; in order to distinguish the different structures, an anharmonic treatment is indispensable. The experimental spectrum of 4-MPy on Au(111) could be assigned, and we proposed an experimental adsorption site.

• I. Respondek, D. M. Benoit, A. Tschetschetkin, B. Koslowski, A. Groß, Vibrationally excited states of a chemisorbed molecule on a metal surface, 45th Symposium on Theoretical Chemistry, September 8-12, Neuss am Rhein, Germany
• I. Respondek, D. M. Benoit, A. Tschetschetkin, B. Koslowski, A. Groß, Anharmonic vibrational calculations and the IET spectrum of an adsorbed species on a metal surface, Molecular Properties '09, June 18-21, 2009, Oslo, Norway
• I. Respondek, D. M. Benoit, A. Tschetschetkin, B. Koslowski, A. Groß, Anharmonic vibrational calculations and the IET spectrum of 4-mercapto-pyridine on Au(111), Symposium Trends in Nanoscience, February 28 -- March 4, 2009, Kloster Irsee, Germany

I. Respondek
Nachwuchsgruppe Theorie - SFB 569
Ulm University

D. M. Benoit
Nachwuchsgruppe Theorie - SFB 569
Ulm University

A. Groß
Institute of Theoretical Chemistry
Ulm University

Acetylene is involved in many catalytic processes, e.g. its trimerization to form benzene. The structure of the free molecule and the adsorbate is shown in Fig. 1. In the gas phase, the acetylene molecule is linear, with a triple bond between the carbon atoms. On surfaces, acetylene adsorbs via both of the carbon atoms, which leads to a rehybridization and distortion of the molecule.

This project focuses on the computation of anharmonic vibrational properties of the system. Our theoretical approach is based on the VSCF (vibrational self-consistent field) method. The anharmonic vibrational frequencies give valuable insights into the accuracy, strengths and weaknesses of electronic-structure methods: The potential energy surface (PES) necessary for the computation of the anharmonic vibrational frequencies is explicitly calculated with the desired quantum-chemistry method. The vibrational Schrödinger equation is then solved using this PES. The deviation of the computed frequencies with respect to the experimental ones indicates characteristic features of the electronic-structure method. Our computations have shown for example, that standard GGA functionals fail to describe the strength of chemical bonds. Including the kinetic-energy density (meta-GGA), or using hybrid functionals yields improved results.

The challenging aspect here is that the electronic-structure method has to describe a system with both a fairly localized electron density -- the adsorbate -- and a very homogeneous electron density -- the metal. Most density-functionals describe only one part of the system well, and fail to predict the properties of the other part correctly.

In the future, other density-functional approximations will be tested. Furthermore, the adsorption of acetylene on Pd surfaces, where the binding between molecule and surface differs from the one on Cu surfaces will be investigated.

German Soldano
Institute of Theoretical Chemistry
Ulm University

Dr Paola Quaino
Institute of Theoretical Chemistry
Ulm University

Dr Kay Pötting
Institute of Theoretical Chemistry
Ulm University

Dr Elisbeth Santos
Institute of Theoretical Chemistry
Ulm University

Dr Wolfgang Schmickler
Institute of Theoretical Chemistry
Ulm University

The physico-chemical properties of monoatomic nanowires composed of Cu, Ag, Au and Pt have been investigated by density functional theory, using DACAPO code. On the three coin metals, the d band is shifted to higher energies in comparison with fcc(111) surfaces. Moreover, on Cu and Au it extends right to the Fermi level. This particular position of the d band entails a much stronger adsorption of the hydrogen atom on Cu and Au than on surfaces. Our calculations explain recent experiments, which shown fractional conductance on electrochemical Cu and Au nanowires in the hydrogen evolution region, while no such effect was found on Ag wires. Fractional conductance occurs only if hydrogen is stably adsorbed on such wires. Since the position of the d band is not favourable for Ag nanowires (it lies well below the Fermi level), the adsorption is endergonic, and therefore fractional conductance is not observed.

Since wires promise to be good catalysts for hydrogen evolution, we calculated the free energy surface for the adsorption and discharge of a proton. We obtained an activation energies of 0.1 eV, 7 times lower than that of fcc(111)platinum surfaces.

• Hydrogen adsorption and fractional conductance of electrochemical nanowires. E. Santos, G. Soldano, P. Quaino, W. Schmickler. Electrochem. Comm., 11 (2009) 1764.
• Some properties of electrochemical nanostructures. E. Santos, G. Soldano, P. Quaino, W. Schmickler. J. Chem. Sciences (India), special issue to honour the late Prof. S.K. Rangarajan, (2009), in press.
• (05/10/2009 - 08/10/2009) Tutorial talk presentation in the workshop Electrocatalysis at the nanoscale: Theory and Modeling). Topic: Concepts for electronic structure, bands in solids, densities of states, d-band model, Wannier orbitals. German Soldano, P. Quaino, E. Santos, W. Schmickler.