Other Supported Metals

The use of Ce02 in catalytic SO2 removal, and the possible effects of this gas on the operation of TWCs, has led to studies on S02-Ce02 interaction. Early XPS data on 

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A 5 wt.% CoOx/Ti02 catalyst was prepared via an incipient wetness technique in which an aqueous solution of Co(N03)2 6H20 (Aldrich, 99.999%) was impregnated onto a shaped Ti02 (Milleimium Chemicals, commercially designated as DT51D, 30/40 mesh), as described in detail elsewhere [6]. Other supported metal oxide catalysts, such as FeOx, CuO, and NiOx, were obtained in a fashion similar to that used for preparing the CoO, catalyst. 

In addition, the catalyst appeared very stable under the reaction conditions little carbon was deposited on the spent catalyst. Other supported metals were less active. The activity order, Ru Rh > Ni > Ir > Co > Pt > Pd > Fe, is very comparable to that measured for the steam reforming of methane. Of all the supports tested, Y203 and Zr02 gave the best results for the Ru-catalyzed steam reforming of glycerol. 

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. 

The emphasis here is principally on metals rather than the other materials, because metals are simpler, more widely investigated, and better understood than the others. Supported metal nanoparticles are considered here in only a few summary statements, and nanolayers are beyond the scope of the chapter. 

Supported metal carbonyl clusters such as [Os5C(CO)i4] , Ir4(CO)i2, and Rh6(CO)i6 are relatively stable and are often used as precursors of other supported metal clusters. Most attempts to investigate the reactivities of supported metal carbonyl clusters have led to the loss of imiformity of the supported species. Treatments intended to remove the CO hgands from supported metal carbonyl clusters typically lead to aggregation to form nonuniform mixtures of clusters and/or fragmentation to form cationic complexes of the metal (support hydroxyl groups may facilitate oxidative fragmentation) [12,14]. 

While the use of a support or carrier stabilizes small metal crystallites against growth and surface area loss, reactions of support and inetal may occur, especially if the catalyst is subjected to elevated temperatures The main factors affecting these phenomena are temperature, time of exposure and atmosphere. Most previously reported sintering studies were for supported platinum and particularly for Pt/Al20 catalysts (1,2). Considerably less information is available on the sintering of other supported metal catalysts (3,4), and thus there is very little information on the behavior of biiDetallic catalyst systems such as supported platinum-rhodium. 

The work of Tauster and coworkers (1,2) showed that hydrogen chemisorption is suppressed on group VIII metals supported on a series of oxides after these samples have been reduced at high temperatures. The term strong metal-support interactions (SMSI) was introduced to describe this behavior. A similar suppression in hydrogen chemisorption has since been reported for many other supported metal systems 0-5). However, the use of other chemical probes (4, 5) demonstrated that different mechanisms of metal-support interactions could exist for different types of oxides. Furthermore, even for a so-called SMSI oxide, the degree of interaction could be influenced by many parameters such as crystallite size and reduction temperature. It would thus be desirable to find an approach to systematically compare catalytic behavior of different systems. 

One unfortunate observation for all the supported metal chloride catalysts is that they all deactivate with time on stream when used in a standard fixed bed reactor [18,248,249], However, supported An catalysts deactivate much less rapidly than other supported metal catalysts and if high loadings of gold are used (>1 wt%) then deactivation is minimised [18,248,249],... 

Although deactivation was a problem with these Au catalysts, in common with other supported metal chloride catalysts, it was found that Au catalysts could be reactivated by treatment off-line with HCl or CI2 [259], However, perhaps the most important observation was that NO could significantly enhance and restore the catalyst activity when co-fed with the reactants. Indeed, CO-feeding NO with the reactants from the start of the reaction showed that deactivation could be virtually eliminated. Most important of all, no effect on vinyl chloride selectivity was observed during the NO co-feeding experiment [259]. This represents an important example of on-line reactivation of a heterogeneous catalyst. 

Enantioselective hydrogenation over other supported metals... 

Pd on C or other support (slurry reactor, batch or continuous) (other supported metals may also be used)... 

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