Model supported catalysts, support

The mechanism of the synthesis reaction remains unclear. Both a molecular mechanism and an atomic mechanism have been proposed. Strong support has been gathered for the atomic mechanism through measurements of adsorbed nitrogen atom concentrations on the surface of model working catalysts where dissociative N2 chemisorption is the rate-determining step (17). The likely mechanism, where (ad) indicates surface-adsorbed species, is as follows ... 

Figure 3.3. Schematic representation of the adsorption, surface diffusion, and surface reaction steps identified by surface-science experiments on model supported-palladium catalysts [28]. Important conclusions from this work include the preferential dissociation of NO at the edges and defects of the Pd particles, the limited mobility of the resulting Nads and Oads species at low temperatures, and the enhancement in NO dissociation promoted by strongly-bonded nitrogen atoms in the vicinity of edge and defect sites at high adsorbate coverages. (Figure provided by Professor Libuda and reproduced with permission from the American Chemical Society, Copyright 2004).

In addition to performing experiments under pressures similar to those encountered in real processes to bridge the pressure gap , surface scientists have also been increasing the level of complexity of the model surfaces they use to better mimic real supported catalysts, thus bridging the materials gap . A few groups, including those of Professors Freund and Henry, have extended this approach to address the catalytic reduction of NO. The former has published a fairly comprehensive review on the subject [23], Here we will just highlight the information obtained on the reactivity of NO + CO mixtures on these model supported catalysts. 

Rainer, D. R., Vesecky, S. M., Koranne, M. et al. (1997) The CO + NO reaction over Pd A combined study using single-crystal, planar-model-supported, and high-surface-area Pd/Al203 catalysts , J. Catal., 167, 234. 

One can go a step further and use the /P//s ratio for a quantitative estimate of the dispersion. Through the years, several methods have been proposed to predict XPS intensity ratios for supported catalysts. Angevine et al. [29] modeled their catalyst with crystallites on top of a semi-infinite support, as sketched in Fig. 3.9a. However, as the inelastic mean free path of, for example, Si02 is 3.7 nm, photoelectrons coming from particles inside pores as deep as 10 nm below the surface still contribute to the XPS signal and the assumption of a semi-infinite support is probably too simple. Indeed, the model predicts /P//s ratios that may be a factor of 3 too high [30],... 

Techniques based on the interaction of ions with solids, such as SIMS and LEIS, have undoubtedly been accepted in catalyst characterization, but are by no means as widely applied as, for example, XPS or XRD. Nevertheless, SIMS, with its unsurpassed sensitivity for many elements, may yield unique information on whether or not elements on a surface are in contact with each other. LEIS is a surface technique with true outer layer sensitivity and is highly useful for determining to what extent a support is covered by the catalytic material. RBS is less suitable for studying catalysts but is indispensable for determining concentrations in model systems, where the catalytically active material is present in monolayer-like quantities on the surface of a flat model support. 

Figure 4.16 RBS spectra of an MoOt model catalyst supported on a flat SiO2/Si(100) model support, before and after sulfidation in a mixture of H2S and H2 at room temperature, and at 300 °C (courtesy of LJ. van IJzendoom, Eindhoven [21J).

Figure 9.1 Positive (left) and negative (right) static SIMS spectra of RhCI3 x H20 on tantalum (top) and of a model catalyst prepared by adsorbing Rh complexes derived from RhCI( x H20 in water on an AljOj/Al model support (from Borg etal. [4]).

Realistic model systems. Some techniques become much more informative if suitable model systems are used. Examples are the thin-film oxides used as conducting model supports, which offer much better opportunities for surface analysis than do technical catalysts. Another example is provided by the non-porous, spherical supports that have successfully been employed in electron microscopy. It is important that the model systems exhibit the same chemistry as the catalyst they represent. 

C. Zhao and l.E. Wachs, Selective oxidation of propylene over model supported V2O5 catalysts Influence of surface vanadia coverage and oxide support, J. Catal, 257, 181-189 (2008). 

Duchateau and coworkers have employed the hydroxysilsesquioxane (c-C5H9)7Si80i2(0H) and triphenylsilanol as model supports for silica-grafted olefin polymerization catalysts [5]. Treatment of [Cp MCfi] (M=Ti, Zr Cp"=C5H3(SiMe3)2)... 

Duchateau, R. (2003) Silsesquioxanes advanced model supports in developing silica-immobilized polymerization catalysts, in Nanostructured Catalysts, Kluwer Academic/Plenum Publishers, New York, pp. 57-83. 

Model metal catalysts can be prepared by vacuum evaporation of the metal on supports and this method offers a simple and convenient way of investigating surface reactions between metals and gases. Some selected examples of support preparations are as follows. 

In using a similar approach, Thiine et al. [29] applied static SIMS to show that Cr/SiC>2 model catalysts which are active for ethylene polymerization contain only monochromates. Secondary ions with more than one Cr ion in the cluster, such as Cr2C>4 and Cr203, disappeared from the spectra after the catalysts had been calcined only CrSiOx ions remained. Aubriet et al. [30] studied the anchoring of chromium acetyl acetonate, Cr(acac)3 to a planar SiO2/Si(100) model support with static SIMS. Chromium polymerization catalysts are discussed further in Chapter 9. 

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