Bulk oxide catalysts

The surface acid-base properties of bulk oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. Acidic and basic sites are often present simultaneously on solid surfaces. The two centers may work independently or in a concerted way, and the occurrence of bifunctional reaction pathways requiring a cooperative action of acidic and basic centers has also received considerable attention [39]. The acid-base properties of numerous amorphous metal oxides investigated by mrcrocalorime-try have been summarized in an extensive review by Cardona-Martinez and Dumesic [11]. 

The influence of the pre-treatment temperature on the acidic properties is a very important factor. For Bronsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density towards an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [40]. 

Increasing the pre-treatment temperature modifies the surface acidity of the solids. For y-alumina, there are numerous surface models, and various acid sites having different strengths are formed on the surface during dehydration. The influence of the pre-treatment temperature, between 573 and 1073 K, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry. The number and strength of the strong sites, which should be 

Figu re 9.5 Variation with the activation temperature of the differential heats of adsorption versus ammonia coverage. 

As can be seen, a regular decrease in the adsorption heats occurs when the evacuation temperature increases from 423 to 823 K. The number of strong sites is less affected by dehydroxylation of the surface than that of weak sites, and tends to a limit. 

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This chapter addresses recent results typical of highly dispersed and bulk oxide catalysts in the working state. 

Hence, the challenge remains to fully analyze the surface layers of bulk oxide catalysts. Recently, Zhao et al. succeeded for the first time in detecting surface MoO and VOv species on molybdenum-niobium and vanadium-niobium mixed metal oxides by Raman spectroscopy (Zhao et al., 2003). 

Eckert, H. (1994) NMR spectroscopy of bulk oxide catalysts, in NMR Techniques in Catalysis (eds A.T. Bell and A. Pines), Marcel Dekker, New York, p. 195. 

B. Structural Issues and Mechanistic Aspects of Bulk Oxide Catalysts... 

Many bulk oxide catalysts are multiphase and compositionally rather complex materials. Therefore, an important part of academic catalysis research is concerned with simple model systems. The goals of such studies are to identify key... 

Since the solid-state NMR approach is relatively new, most applications to bulk oxide catalysts so far have focused on introducing a new experimental technique to an existing catalytic system, rather than actually maximizing knowl-... 

One example of MAS applications to catalytic materials includes the precise measurement of H chemical shifts in the solid state. These data differentiate sensitively among B-OH, Al-OH, Si-OH, and P-OH groups and can also be correlated with the Bronsted acidity. Several applications of this idea to bulk oxide catalysts have been published and reviewed in the literature 5,14-18). 

The oxidation of SOi to sulfuric acid, SOt + H2O + 0.5 O2 H2SO4, is catalyzed by potassium vanadium(V) oxide compounds. A typical catalyst preparation sequence involves impregnation of a silica support with a solution containing potassium vanadate (K/V = 3), followed by drying and subsequent calcination at 500°C in air. Under typical operating conditions in SO2/O2/SO3 atmospheres at 400-500°C, the catalytically active species is molten and forms a thin liquid film on the silica. support. As such the. system functions like a bulk oxide catalyst under operating conditions, and the silica mostly serves as a mechanical support medium. 

The purpose of this review has been to highlight the innate power of solid-state NMR spectroscopy for investigating detailed questions concerning the solid state and surface chemistry of bulk oxide catalysts. While MAS-NMR has been the mainstay of most applications, more sophisticated selective averaging experi-... 

Monolayer Supported versus Bulk Oxide Catalysts Which is... 

The major technical contribution of their work was the determination of the number of surface active sites through the quantification of the methanol molecules adsorbed at room temperature. A decade later, this pioneering work inspired the development of a novel chemisorption technique to determine the density of surface active sites of supported and bulk oxide catalysts that will be discussed in the following sections. 

The catalytic activity per surface active site (TOP) toward a specific reaction is the right parameter to obtain reliable surface structure-activity correlations. The knowledge of the TOFs values dismissed the believes that the catalytic activity is influenced by bulk properties and that monolayer supported oxide catalysts are more active than bulk oxide catalysts. There is no doubt that the specific activity would contribute to design more active and selective catalytic materials at a molecular level. 

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