Surface structure atomic scale model

Before the introduction of STM, high-resolution (HR-)TEM was the primary technique for determination of the surface structures of nanoparticle model catalysts (14,54,74,77,197,198,211,226-230). For technological catalysts, it is still the only method that provides a direct atomic-scale characterization of metal nanoparticles and of the oxide support (211,231-238). Although TEM is unable to detect adsorbed molecules (in contrast to the methods discussed above), it is briefly mentioned here because HR-TEM was sometimes employed to corroborate STM data characterizing model catalysts and, in particular, to demonstrate the internal... 

One approach to a better understanding of the properties of the solid surface is to model the electron structure with quantum mechanical methods, which is a useful complement to experimental techniques. It allows direct observation of atomic-scale phenomena in complete isolation, which cannot be achieved in current experimental studies. 

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. 

In comparison to most other methods in surface science, STM offers two important advantages (1) it provides local information on the atomic scale and (2) it does so in situ [50]. As STM operates best on flat surfaces, applications of the technique in catalysis relate to models for catalysts, with the emphasis on metal single crystals. Several reviews have provided excellent overviews of the possibilities [51-54], and many studies of particles on model supports have been reported, such as graphite-supported Pt [55] and Pd [56] model catalysts. In the latter case, Humbert et al. [56] were able to recognize surface facets with (111) structure on palladium particles of 1.5 nm diameter, on an STM image taken in air. The use of ultra-thin oxide films, such as AI2O3 on a NiAl alloy, has enabled STM studies of oxide-supported metal particles to be performed, as reviewed by Freund [57]. 

The catalytic properties of a surface are determined by its composition and structure on the atomic scale. Thus, it is not sufficient to know that a surface consists of a metal and a promoter, say iron and potassium, but it is essential to know the exact structure of the iron surface, including defects, steps, etc., as well as the exact location of the promotor atoms. Thus, from a fundamental point of view, the ultimate goal of catalyst characterization should be to look at the surface atom by atom, and under reaction conditions. At present, this is only occasionally possible in highly simplified model systems such as the well defined surfaces of single crystals, or the needle shaped tips used in field emission studies [8]. 

In principle, two approaches can be adopted for fundamental investigations of the relations between catalytic properties on one hand, and catalyst composition and structure on the other. The first is to model the catalytic surface, for example with that of a single crystal. By using the appropriate combination of surface spectroscopies, the desired characterization on the atomic scale is certainly possible in favourable cases. The disadvantage, however, is that although one may be able to study the catalytic properties of such samples under realistic conditions (pressures of 1 atm or higher), most of the characterization is necessarily carried out in ultra high vacuum, and not under reaction conditions. 

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