Supported metals stepped single crystals

The catalytic properties of a surface are determined by its composition and structure on the atomic scale. Hence, it is not sufficient to know that a surface consists of a metal and a promoter - perhaps iron and potassium - but it is essential to know the exact structure of the iron surface, including any defects and steps, as well as the exact location of the promoter atoms. Thus, from a fundamental point of view, the ultimate goal of catalyst characterization should be to examine the surface atom by atom, and under reaction conditions. The well-defined surfaces of single crystals offer the best perspectives for atom-by-atom characterization, although occasionally atomic scale information can also be obtained from real catalysts under in-situ conditions (for some examples, see Chapter 9). The many aspects that we need to study in order to properly understand supported catalysts on a fundamental level are shown schematically in Figure 1.4 [13]. 

Support for this step was obtained by us in experiments where M was shown to add directly to bis(arene)metal complexes to give M2L2 (45, 46). Maximum nuclearity and stability of these species depends on the element, atom deposition rate, temperature and the type of arene. Mono-alkyl and -silyl substituted arenes give compounds that are unstable above 0 C. On the other hand, single crystals of a related triple-decker sandwich of mesitylene and chromium have been obtained (47),... 

These difficulties have stimulated the development of defined model catalysts better suited for fundamental studies (Fig. 15.2). Single crystals are the most well-defined model systems, and studies of their structure and interaction with gas molecules have explained the elementary steps of catalytic reactions, including surface relaxation/reconstruction, adsorbate bonding, structure sensitivity, defect reactivity, surface dynamics, etc. [2, 5-7]. Single crystals were also modified by overlayers of oxides ( inverse catalysts ) [8], metals, alkali, and carbon (Fig. 15.2). However, macroscopic (cm size) single crystals cannot mimic catalyst properties that are related to nanosized metal particles. The structural difference between a single-crystal surface and supported metal nanoparticles ( 1-10 nm in diameter) is typically referred to as a materials gap. Provided that nanoparticles exhibit only low Miller index facets (such as the cuboctahedral particles in Fig. 15.1 and 15.2), and assuming that the support material is inert, one could assume that the catalytic properties of a... 

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