Template model catalyst

A more detailed picture of the temperature dependence of the growth is given in Figure 2.4, where the island density is plotted as a function of temperature. It can be seen that only in the temperature range from 207 to 288 K the growth is perfectly template controlled and the number of islands matches the number of available nucleation sites. This illustrates the importance of kinetic control for the creation of ordered model catalysts by a template-controlled process. Obviously, there has to be a subtle balance between the adatom mobility on the surface and the density of template sites (traps) to allow a template-controlled growth. We will show more examples of this phenomenon below. 

Before we discuss the template-controlled growth of model catalysts in more detail, we will have to consider a few aspects of STM imaging of these systems. This will be crucial for the characterization of the model catalyst surfaces. 

The results for Pd and V lead to the conclusion that the high-symmetry sites of the alumina film on Ni3Al(l 1 1) can act as template for the growth of nanostructured model catalysts. They also prove that kinetic control of the growth is of utmost importance. 

Oxide surfaces, and in particular oxide films, are versatile substrates for the preparation of model catalysts. Quite a few of these systems show nanoscale reconstructions, which can be employed as templates for the growth of ordered model catalysts of reduced complexity. In order to efficiently control the growth of nanostructured metal particle arrays, two conditions have to be met. First, the template must provide sites of high interaction energy that trap the deposited metals. Second, the kinetics of the growth process must be carefully controlled by choosing... 

Self-assembly of molecules and nanoparticles to build well-defined structures, constitutes another approach to make model catalysts [33,34]. Here, nano-structured surfaces are made from nanoscale building blocks that are synthesized from atoms and molecules by chemical means. There has been a tremendous development in this field during the past decade, which includes a number of different strategies, including microemulsions [33], (micellar) block copolymers [35,36], and template CVD growth [37]. Relatively little work has, however, so far been directed toward heterogeneous catalysis in the sense described in this chapter, i.e., to make supported catalysts [38]. There are many reports on preparations but relatively much fewer on evaluations of catal3dic activity, trends, or reactivity versus particle size, etc. A main issue for model catalysts prepared by self-assembly is whether they maintain the well-defined character after, e.g., template removal and calcinations and other pretreatment steps, before they can be used as model catalysts. 

In this section the use of oxide surfaces as templates will be discussed. These surfaces are particularly interesting because of their potential use in industrial applications in which insulating or inert substrates are required. In this context one has to refer to nanocatalysts or electronic devices, which nowadays rely on an active patterning of the surface. Oxidic templates can be used for the fabrication of well-ordered model systems in these fields, fii fact the search for more powerful catalysts is often hampered by the fact that the complexity of the real world catalyst does not allow an in-depth investigation. Ordered nanostructured model catalysts prepared in a template-controlled process can provide a pathway to systems of lower complexity, which opens a route to the investigation of basic steps in heterogeneous catalysis. 

Of course other alumina films have been used for the preparation of model catalysts and some of them as in the case of Co on 6>-Al2O3/CoAl(100) [188] and Cr on Al2O3/NiAl(001) [189] show a certain template effect, but the resulting layers are in general not as nicely ordered as on Al302/Ni3Al(l 11). 

Margitfalvi, J.L., Hegedus, M. (1995) Cinchona-modified Pt as a catalyst for enantioselective hydrogenation criticism of the "Template model", J. Catal. 156, 175-179. 

A recent discovery that RNA will act as a self-catalyst, called a ribozyme, leads to a simple three-step model for self-replication - this might include a surface. In the model (Figure 8.18), the template molecule T is self-complementary and is able to act as an autocatalyst. In the first step, it reversibly binds with its constituents A and B, forming the termolecular complex M. The termolecular complex undergoes irreversible polymerisation and becomes the duplex molecule D. Reversible dissociation of D gives two template molecules T, which can initiate new replication. The model preserves the order of the moieties on the template (the direction of the arrow) and the backbone, which may be on the surface... 

C. Hofstetter, P. S. Wilkinson, T. C. Pochapsky, NMR Structure Determination of Ion Pairs Derived from Quinine A Model for Templating in Asymmetric Phase-Transfer Reductions by BH4" with Implications for Rational Design of Phase-Transfer Catalysts , J. Org. Chem 1999, 64, 8794-8800. 

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