Model catalyst systems

About 800 000 tons of solid catalysts are prepared and used every year worldwide, but this does not mean that we understand how they work. Catalyst synthesis and treatment recipes are often based on empirical studies, handed down the generations like ancient medicines. Since chemists dislike ignorance even more than nature abhors a vacuum, much research is done to find out how solid catalysts work. And, since the real industrial catalysts are usually multicomponent, nonuniform solids, much of this research is done on simplified model systems [28], 

This difference between the model catalysts and the real ones is a major debating point in the catalysis community. Industrial catalysts are usually porous, 

Other common model catalyst systems include thin metal and oxide films [38,39], glassy metals [40], supported catalysts based on chemical vapor deposition [41], and 

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The development of modern surface characterization techniques has provided means to study the relationship between the chemical activity and the physical or structural properties of a catalyst surface. Experimental work to understand this reactivity/structure relationship has been of two types fundamental studies on model catalyst systems (1,2) and postmortem analyses of catalysts which have been removed from reactors (3,4). Experimental apparatus for these studies have Involved small volume reactors mounted within (1) or appended to (5) vacuum chambers containing analysis Instrumentation. Alternately, catalyst samples have been removed from remote reactors via transferable sample mounts (6) or an Inert gas glove box (3,4). 

Heterogeneously catalyzed hydrogenation of alkenes is generally considered to be a structure-insensitive reaction, as was deduced from numerous studies on more or less complex model catalyst systems [40-54]. However, the following sections will give examples of the opposite case. 

A similar study was performed by the same group on a model catalyst system i.e., a Mn/Ru (0001) surface, prepared by sputtering Mn on a Ru (0001) surface at room temperature using a sputtered ion gun. The techniques of choice were SSIMS, EELS and TPD. It was concluded that Mn reduces the coverage of CO adsorbed on the surface by physically blocking the adsorption sites. The adsorbed CO molecule is predominantly linearly bonded, whereas EELS indicated a possible existence of an electronic interaction between the deposited Mn and Ru. This was reflected by the change in the CO stretching frequency to lower wavenumbers once Mn was deposited on the Ru surface. 

The present chapter will primarily focus on oxidation reactions over supported vanadia catalysts because of the widespread applications of these interesting catalytic materials.5 6,22 24 Although this article is limited to well-defined supported vanadia catalysts, the supported vanadia catalysts are model catalyst systems that are also representative of other supported metal oxide catalysts employed in oxidation reactions (e.g., Mo, Cr, Re, etc.).25 26 The key chemical probe reaction to be employed in this chapter will be methanol oxidation to formaldehyde, but other oxidation reactions will also be discussed (methane oxidation to formaldehyde, propane oxidation to propylene, butane oxidation to maleic anhydride, CO oxidation to C02, S02 oxidation to S03 and the selective catalytic reduction of NOx with NH3 to N2 and H20). This chapter will combine the molecular structural and reactivity information of well-defined supported vanadia catalysts in order to develop the molecular structure-reactivity relationships for these oxidation catalysts. The molecular structure-reactivity relationships represent the molecular ingredients required for the molecular engineering of supported metal oxide catalysts. 

There is, nevertheless, some evidence (35, 36), based in NiNaY and NiMo/ alumina/Y model catalyst systems, that the amount of coke formed is reduced with increasing intimacy of mixing of the two functions at the submicron level. This concept is further supported by the reported relatively high performance of NiW/ASA (amorphous silica-alumina) cogel HC catalysts which, it is claimed, exhibit an excellent distribution of the NiW hydrogenation function throughout the catalyst particles (37). 

A model catalyst system Idealized to permit meaningful TPD and ISS studies of pt/TiO2 was prepared as shown schematically in Fig. 2. 

Greater structural resolution is certainly available, with point resolution at 500kV or greater being of the order of 1.8A, but even at this level anion positions remain undetermined. The real value of the HREM technique, however, lies in its ability to characterise grossly defective structural systems. If the data obtained in this way can be used as an aid to the interpretation of other, indirect evidence such as x-ray photoelectron-spectroscopic data, then the results may be extrapolated to deal with materials where defects are only present at or near the surface, such as real, rather than model catalyst systems. 

Finally, the all-important effect of the presence of the bifunctional structural promoters will be demonstrated. They promote both formation and stabilization of the metastable, active form of a-iron. This form has been found to be exactly the same in the technical catalyst and in a model catalyst system prepared from amorphous alloy precursors.In Fig. 2.48 conversion measurements are shown for a set of catalysts made from the same iron oxide starting material, but which contained different promoter additives. All three catalysts were activated in the same way, according to the manufacturer s specification. 

Most of the RAIRS applications have been concerned with basic work on model catalyst systems, since many of the metals listed above as being suitable reflecting substrates are also by a fortuitous coincidence catalytically active. Also, many of the molecules of interest in catalytic processes possess large absorption cross-sections in the infrared. As a result much valuable insight has been gained into the mechanisms of surface interactions, particularly in catalysis but also in other areas. 

Niesz, K., Koebel, M.M. and Somorjai, G.A. (2006) Fabrication of two- and three-dimensional model catalyst systems with monodispersed platinum nanopartides as active metal building blocks. Inorganica Chimica Acta, 359, 2583. 

The use of sihca gel-supported IL-phase catalyst in fixed-bed reactor might solve the leaciing problem partially [86]. Here, the model catalyst system is Rh(acac)(CO)2-ligand 1 or 2/[BMIm][PF5]/silica gel (Rh-1 and Rh-2), Scieme 2.20, and the model reaction is hydroformylation of l-octene. With a Hquid hourly space velocity of 16h i, the TOF of aldehyde formation was maintained at about 40 mol Rh h and the ratio of hnear and branch aldehyde was >2. Noteworthy, inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis of outlet samples taken at steady-state conversion demonstrated rhochum metal leaching to be neghgible (<0.7%, detection limit). 

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