Model catalysts catalysis investigation

In this chapter, we will illustrate with a few selected examples how well-defined, ordered Pt-Sn surface alloys have been used to elucidate the overall chemical reactivity of Pt-Sn alloys, clarify the role of Sn in altering this chemistry and catalysis, and develop general principles for understanding the reactivity and selectivity of bimetallic alloy catalysts. Most studies have involved chemisorption under UUV conditions, but the use of these alloys as model catalysts for investigating catalysis at pressures up to one atmosphere will also be discussed. 

In this chapter we discuss how the STM may be applied in fundamental catalysis research. The examples are numerous, and it is beyond the scope of this review to present an exhaustive review of the field several reviews have already appeared (50-52). Instead we mainly focus on three illustrative examples in which STM investigations have played an important role, not only for a better fundamental understanding of the geometrical and electronic structure of model catalysts, but also for the design and development of new and improved catalysts to operate under technologically relevant conditions. First, however, we summarize the working concepts of the STM. 

Investigation of model catalysts can play a decisive role in a rational approach to understand heterogeneous catalysis, and this is the topic of this chapter [3-6],... 

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. 

The typical industrial catalyst has both microscopic and macroscopic regions with different compositions and stmctures the surfaces of industrial catalysts are much more complex than those of the single crystals of metal investigated in ultrahigh vacuum experiments. Because surfaces of industrial catalysts are very difficult to characterize precisely and catalytic properties are sensitive to small stmctural details, it is usually not possible to identify the specific combinations of atoms on a surface, called catalytic sites or active sites, that are responsible for catalysis. Experiments with catalyst poisons, substances that bond strongly with catalyst surfaces and deactivate them, have shown that the catalytic sites are usually a small fraction of the catalyst surface. Most models of catalytic sites rest on rather shaky foundations. 

The dilemma is thus investigations of real catalysts under relevant conditions by in situ techniques give little information on the surface of the catalyst, while techniques that are surface sensitive can often only be applied on model surfaces under vacuum. Bridging the gap between UHV and high pressures and between the surfaces of single crystals and of real catalysts is therefore an important issue in catalysis. 

In comparison to most other methods in surface science, STM offers two important advantages STM gives local information on the atomic scale and it can do so in situ [51]. As STM works best on flat surfaces, applications of the technique in catalysis concern models for catalysts, with the emphasis on metal single crystals. A review by Besenbacher gives an excellent overview of the possibilities [52], Nevertheless, a few investigations on real catalysts have been reported also, for example on the iron ammonia synthesis catalyst, on which... 

---