Surface structure and electronic

Careful characterization of the oxide-electrolyte interface is needed electrochemical area, surface structure, and electronic properties (potential distribution and density of electrical carriers). Chemical and electrochemic-ally induced transformations of the oxide surface in contact with electrolyte can substantially modify the behavior of oxide electrodes. Extrapolation of gas/solid oxide results to oxide electrodes is not always valid since the oxide-electrolyte interface can strongly depend on electrolyte type and applied potential. 

Studies of Classy Metals in As-quenched State a) Surface Structure and Electronic Properties... 

We have hinted above that by alloying, the surface structure and electronic density of a given surface can be modified. Accordingly, the interaction with adsorbates, and hence the catalytic performance, can be engineered. A great deal of effort has been devoted to the preparation, characterization, and study of alloys of the composition PtgX (X = Fe, Co, Ni, Cr, Mn). In a seminal work, Jalan and Taylor identified carbon supported PtCr alloy as the most active alloy for the ORR in phosphoric acid fuel cells. They also proposed PtNi and PtCo as the next best alloys. This line of research was further explored by other groups Mukeijee et... 

As part of an overall study of the electrode/electrolyte interphase of the electronically conducting polymer, polypyrrole, the surface structure and electronic properties have been investigated. 

A widespread interest for the electrochemical oxygen reduction reaction (ORR) has two aspects. The reaction attracts considerable attention from fundamental point of view, as well as it is the most important reaction for application in electrochemical energy conversion devices. It has been in the focus of theoretical considerations as four-electron reaction, very sensitive to the electrode surface structural and electronic properties. It may include a number of elementary reactions, involving electron transfer steps and chemical steps that can form various parallel-consecutive pathways [1-3]. 

Key structural characteristics determining these processes are atomistic surface structure and electronic structure of the catalyst, morphology of the pore network, surface structure and wettability of the support, catalyst nanoparticle shape and size, ionomer structure, mixed wettability of the composite layer, and, last but not least, the electrode thickness, Icl-... 

We shall first review the basic principles of VASP and than describe exemplary applications to alloys and compounds (a) the calculation of the elastic and dynamic properties of a metallic compound (CoSi2), (b) the surface reconstruction of a semiconducting compound (SiC), and (c) the calculation of the structural and electronic properties of K Sbi-j, Zintl-phases in the licpiid state. 

Scanning Electron Microscopy in the Study of Solid Propellant Combustion. Part 111. The Surface Structure and Profile Characteristics of Burning Composite Solid Propellants , NavWeps-Cent r TP 5142-Part 3 (1971) 48) B.T. 

Two working modes are used for the STM first, the constant height-mode, in which the recorded signal is the tunneling current versus the position of the tip over the sample, and the initial height of the STM tip with respect to the sample surface is kept constant (Fig. 22(a)). In the constant currentmode, a controller keeps the measured tunneling current constant. In order to do that, the distance between tip and sample must be adjusted to the surface structure and to the local electron density of the probed sample via a feedback loop (Fig. 22(b)). 

Figure 1.6 Structures and electron density changes of dissociating CO on Ru(OOOl) surface (a) adsorbed CO, (b) transition state for dissociation, and (c) dissociated state.

In this contribution it is shown that local density functional (LDF) theory accurately predicts structural and electronic properties of metallic systems (such as W and its (001) surface) and covalently bonded systems (such as graphite and the ethylene and fluorine molecules). Furthermore, electron density related quantities such as the spin density compare excellently with experiment as illustrated for the di-phenyl-picryl-hydrazyl (DPPH) radical. Finally, the capabilities of this approach are demonstrated for the bonding of Cu and Ag on a Si(lll) surface as related to their catalytic activities. Thus, LDF theory provides a unified approach to the electronic structures of metals, covalendy bonded molecules, as well as semiconductor surfaces. 

This prompted us [111 to try to represent CeoMu by clusters of carbon atoms, CisHuMu and C30H12MU, the external atoms being constrained to lie on a part of a spherical surface with the same radius as Ceo- The results were very similar to the CeoMu calculations with partial geometry optimisation to suggest that this adduct did not depend on the full structure but corresponded to a locdised defect , both structurally and electronically. 

These results show that the electrochemical measurements can, via ab initio simulations, be linked to phenomena at the atomic level, such as structural and electronic effects and, in this case, binding energies on the surfaces. 

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

Local surface structure and coordination numbers of neighbouring atoms can be extracted from the analysis of extended X-ray absorption fine structures (EXAFS). The essential feature of the method22 is the excitation of a core-hole by monoenergetic photons modulation of the absorption cross-section with energy above the excitation threshold provides information on the distances between neighbouring atoms. A more surface-sensitive version (SEXAFS) monitors the photoemitted or Auger electrons, where the electron escape depth is small ( 1 nm) and discriminates in favour of surface atoms over those within the bulk solid. Model compounds, where bond distances and atomic environments are known, are required as standards. 

The late 1980s saw the introduction into electrochemistry of a major new technique, scanning tunnelling microscopy (STM), which allows real-space (atomic) imaging of the structural and electronic properties of both bare and adsorbate-covered surfaces. The technique had originally been exploited at the gas/so id interface, but it was later realised that it could be employed in liquids. As a result, it has rapidly found application in electrochemistry. 

Here, we discuss in some detail the DFT implementation in our computer program, since only in the last few years DFT is becoming more familiar to the chemists community, as opposite to the physicists community, where it was used routinely for the last thirty years for obtaining structural and electronic properties of bulk solids and surfaces [19],... 

Molecular design of the active site on catalyst surfaces regarded as a reaction intermediate for a target catalytic reaction is also a way to provide efficient catalysis [22], In order to achieve the catalyst design, the reaction mechanism including the structural and electronic change of active metal sites must be known at a molecular level. 

Section I reviews the new concepts and applications of nanotechnology for catalysis. Chapter 1 provides an overview on how nanotechnology impacts catalyst preparation with more control of active sites, phases, and environment of actives sites. The values of catalysis in advancing development of nanotechnology where catalysts are used to facilitate the production of carbon nanotubes, and catalytic reactions to provide the driving force for motions in nano-machines are also reviewed. Chapter 2 investigates the role of oxide support materials in modifying the electronic stmcture at the surface of a metal, and discusses how metal surface structure and properties influence the reactivity at molecular level. Chapter 3 describes a nanomotor driven by catalysis of chemical reactions. 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

The second most apparent limitation on studies of surface reactivity, at least as they relate to catalysis, is the pressure range in which such studies are conducted. The 10 to 10 Torr pressure region commonly used is imposed by the need to prevent the adsorption of undesired molecules onto the surface and by the techniques employed to determine surface structure and composition, which require relatively long mean free paths for electrons in the vacuum. For reasons that are detailed later, however, this so-called pressure gap may not be as severe a problem as it first appears. There are many reaction systems for which the surface concentration of reactants and intermediates found on catalysts can be duplicated in surface reactivity studies by adjusting the reaction temperature. For such reactions the mechanism can be quite pressure insensitive, and surface reactivity studies will prove very useful for greater understanding of the catalytic process. 

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