Relation of Activity to Surface Electronic Structure

Activation methods can be divided into two groups. Activation by addition of selected metals (a few wt%), mainly transition metals, e.g., fine powders of Fe, Ni, Co, Cr, Pt, Pd, etc. ", or chlorides of these metals when these are reducible to the metal by hydrogen during presintering. The mechanism of activation is not understood (surface tension, surface diffusion, etc.) but is related to the electronic structure of the metal additive. Activation by carbon is also effective. Alternatively, activation utilizes powders in a specially activated state, e.g., very fine (submicronic) powders. ... 

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. 

A key to achieving improvements in electrocatalytic activity in fuel cell electrodes lies in understanding how the structure of the catalyst determines the rates of relevant surface processes (Bayati et al., 2008 Eikerling et al., 2007a Maillard et al., 2004 Mulla et al., 2006 Vielstich et al., 2003). On nanoparticles, the proportions of the different surface sites, located at facets, edges, and corners, and the surface electronic structure, are closely related to the particle size (Bradley, 2007 Mayrhofer et al., 2005 Mukerjee, 1990 Mukerjee and McBreen, 1998 Somorjai, 1994). These particle size effects exhibit varying trends for different reactions (Ahmadi et al., 1996 ... 

CO (and CO2) with ZnO in some detail with the aim of establishing the geometric and the electronic requirements of the catalytically relevant surface species. In Section 18.3 and Section 18.4, the nature of the metal oxide promotion of theCO c will be explored to define electronic structure differences relative to CO binding to pure ZnO, which are related to differences in efficiency in hydrogenation reactions. It is pointed out that the traditional concept of the individual active site involved in the hydrogenation reactions of carbon oxides must be replaced by one that involves a metal-metal oxide interface, since any CO bond responds to an oxide promoter and exhibits very accelerated reaction rates. Section 18.5 describes the performance of metal oxide and metal-metal oxide systems for hydrogenation of CO, CO-CO2 mixtures, and CO2. 

Activity loss can also be accessed directly using potentiodynamic cycling. In this case electroactive species (reaetants) for specific electrocatalytic reaction under consideration should be present in the solution, so the decay of Faradaic current can be measured directly. These types of measurements are erucial for solving the problems related to catalyst stability, and this is a field of active research, with different solutions offered. In the case of Pt supported cathode catalysts for PEMFCs enhanced stability can be achieved in different ways. For example, this can be done by suppressing dissolution processes through alteration of surface electronic structure by the designing well defined monolayer catalysts [41] or by stabilization of the surface by gold clusters [42]. In addition, suitably chosen support can also increase catalyst stability, for example different carbon nanoarchitectures with or without heteroatom [43]. 

Although the majority of authors who have investigated CNTs as supports for Pt and PtRu particles claim higher activity or performance compared to conventional catalysts, it is not clear why these enhancement arise. It seems unlikely that the CNTs provide any electronic enhancement to Pt(Ru) reactivity, so it is likely that CNTs provide benefits for catalyst layer structure. Part of this may be related to surface area because CNTs can have relatively high surface areas and are often compared to XC72 supported catalysts that have only a moderate surface area ( 250 m g ). Given the current high expense of these materials ( 10 kgr ), further benefits of their use need to be identified before fhey can be practically considered as candidates for fuel cell catalyst supports. 

As described above, XAS measurements can provide a wealth of information regarding the local structure and electronic state of the dispersed metal particles that form the active sites in low temperature fuel cell catalysts. The catalysts most widely studied using XAS have been Pt nanoparticles supported on high surface area carbon powders,2 -27,29,so,32,33,38-52 represented as Pt/C. The XAS literature related to Pt/C has been reviewed previ-ously. In this section of the review presented here, the Pt/C system will be used to illustrate the use of XAS in characterizing fuel cell catalysts. 

Two are the main factors governing the activity of materials (i) electronic factors, related to chemical composition and structure of materials influencing primarily the M-H bond strength and the reaction mechanism, and (ii) geometric factors, related to the extension of the real surface area influencing primarily the reaction rate at constant electronic factors. Only the former result in true electrocatalytic effects, whereas the latter give rise to apparent electrocatalysis. 

From a description of the geometric structure of electrified interfaces we moved to a description of models for electrochemical electron transfer across an electrode interface. The science of atomic scale electrochemistry was presented with an emphasis on the bonding of water molecules and anions on electrode surfaces. Subsequently, we presented an in-depth description of the role of surface bonding in a number of important electrocatalytic processes for energy conversion. We have attempted to illustrate how closely surface bonding and catalytic activity are related. 

Both stereoisomers were formed, implying a loss of stereochemical integrity during the formation of the second carbon-carbon bond. When the reaction was conducted on ZnO, surface-related processes affected both the rate and stereochemistry. The effect of various quenchers could be explained as competitive adsorption at active sites, with or without interference with electron transfer. A reaction scheme involving formation of dimer, both in the adsorbed state and in solution, was proposed, the former route being the more important On CdS, the reaction could sometimes be induced in the dark as well because of the presence of acceptor-iike surface states. Neither particle size, surface area, nor crystal structure appeared to significantly influence the dimerization observations parallel to those found in the CdS photoinduced dimerization of N-vinylcarbazole... 

The catalytic activity of TMS catalysts is related to defects in their crystal lattice that occur at the surface. However, the properties and stability of these defects are determined by the bulk atomic and electronic structure. It is therefore particularly important to know which phases are stable in a catalytic environment and what are their crystal structures. This knowledge was incomplete when the Massoth article was written (1). The subsequent development of this knowledge formed the basis on which new understanding of the fundamental origins of the catalytic effects in the TMS was developed. 

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