Real catalysis

By using thermosensitive poly-acrylamides, it is possible to prepare cubic Pt nanocrystals (with predominant (1 0 0) facets) and tetrahedral Pt nanocrystals (rich in (111) facets). These Pt nanocrystals can be supported on oxide (alumina) and used as a catalyst in structure-sensitive reaction, NO reduction by CH4. The results proved that morphologically controlled metal nanoparticles supported on adequate support give us a novel tool to connect the worlds of surface science with that of real catalysis. 

In real catalysis the actual situation will even be far more complex. Energetic heterogeneity due to the participation of various structural elements of the surface and interactions between adsorbed species are just a few of the complicating factors coming into play. Nevertheless it is concluded that adequate description of the kinetics may be achieved on the basis of the outlined strategy as long as the analysis is restricted to a limited range of parameters, which condition will frequently be full-filled with practical reaction situations. 

The results discussed in this article were mostly obtained with ultrahigh vacuum systems at total pressures not exceeding 10"4 Torr, whereas real catalysis is performed in the atmospheric pressure regime. This general pressure gap raises the serious question to which extent experiments of the type described using the spectroscopic techniques of "surface science are relevant at all for real-life catalysis. A general answer to this problem can certainly not yet be offered. However, a rather favorable situation is found in the present case, as long as the discussion is confined to temperatures below 7 ax at which the reaction rate reaches is maximum rmax (cf., for example, Fig. 35). This situation has been discussed in detail in Section IV for palladium and holds as well for the other platinum metals since the shape of the r(T) curve is always quite similar. It has been shown that the kinetics may then approximately be described by... 

As far as the kinetic data obtained under conditions of "real catalysis (catalysts are polycrystaline metals or supported samples and pressures... 

Surface science offers many opportunities in catalysis research because a variety of techniques are available to characterize in detail the composition and structure of the catalyst surface and to identify the adsorbed species. A frequent criticism of the surface science approach is that it is far removed from real catalysis since most of the surface science techniques can only be applied at low pressures and with model catalysts, often single-crystal surfaces. The so-called pressure gap has been bridged by combining, in the same apparatus, the techniques needed for surface analysis and characterization with the ability to measure reaction rates at elevated pressures. In addition, many techniques can also be apphed in situ at elevated pressures. 

Both examples revealed unique and size-dependent catalytic properties of very small metal clusters. As mentioned in the introduction, gas phase clusters will probably never be important in real catalysis, however, such small clusters can be stabilized on support materials so they may become relevant for future applications. 

Several reviews have been published on CO oxidation by gold catalysts [3, 9, 13-18]. Readers can also refer to the special issue of Applied Catalysis A 291 (2005), and to a recent book entitled Catalysis by Gold [19]. The objective of the present chapter is to try to summarise the state of art of this reachon with gold catalysts, from the point of view of a researcher involved in real catalysis (in contrast with model catalysis). The objective is also to try to show that the contributions of surface science and quantum chemical calculations are very useful in providing a better... 

Since the latter in most cases cannot be operated at the high-pressure conditions of "real" catalysis, this causes the appearance of a "pressure gap." And since the properties of well-defined single-crystal surfaces will generally be quite different from the surface properties of "real" catalysts, this gives rise to the so-called "materials gap." That these gaps can indeed be overcome will be demonstrated by some of the examples to be presented. 

The use of single-crystal surfaces under ultrahigh vacuum conditions as model systems will necessarily differ substantially from the conditions of real catalysis. These materials and pressure gaps can be bridged along various ways ... 

Similar microkinetic models developed by other groups [37-39] revealed similar close agreement between the experimental data from single crystal studies and "real" catalysis, and at a symposium in honor of H. Topsoe and A. Nielsen, two of the leaders in industrial ammonia synthesis, general agreement was reached that the main aspects of catalytic ammonia synthesis are now essentially understood [40]. 

In general, these modified probes give more accurate data on real catalysis as they are not closed systems, and constant partial pressures of reactive gases can be maintained. However, they are not so easy to use, require a wide bore magnet, and also usually a dedicated NMR. 

Effects of this type are by no means restricted to uniform and well-defined single crystal planes, but they are also observed with polycrystalline foils as well as with the fine tips used for field ion microscopy (FIM) [23]. There the individual crystal planes have diameters of only a few tens of nm, so that these systems may be regarded as good models for the small particles applied in real catalysis. 

The second case implies that the pores in the catalyst pellets are not interconnected and that the fraction of internal wetting corresponds directly to external wetting. This in general is not the case when dealing with real catalysis and hydrocarbon feeds which readily wet internal pore structures (22). 

These final statements provide a means to answer some of the questions of the re ider. Furthermore, they illustrate that the combination of a modem, analytical, multimethod approach with catalytic testing can indeed help to understand the complexity of real catalysis, such as is used in the example of ammonia synthesis. 

FROM REAL CATALYSIS TO APPROPRIATE MODEL SYSTEMS... 

During the last few years, a tremendous effort has been devoted to bridging the gap between the surface-science approach to catalytic phenomena and real catalysis under practical conditions. A significant proportion of this effort has been applied to ammonia synthesis. 

Further theoretical smdy to clarify alkyne insertion into various M-P and M-H bonds generated as intermediate in H-P bond addition reactions has suggested the following general trends [15] (1) insertion into M-H is more facile and (2) the relative reactivity decreases in the orders of Ni > Pd > Rh > Pt and PR2 > P(0)R2 > P(0)(0R)2. These conclusions appear to agree with most of the experimental results, although the detailed mechanism in real catalysis can be variant, depending on the specific cases. 

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