Composite Catalytic Phases

Using the sol-gel strategy for the preparation of multimetallic crystalline catalytic phases with well-organized solid surfaces is based on its potential for mixing the cations at the precursor s molecular scale and thus on the potential for direct crystallization of composite phases. The principles of the sol-gel protocol for the preparation of multimetallic phases are discussed by Narendar and Messing in a comprehensive review (27). 

When the activation of metal cations is done by their chelation with polydentate carboxylic acids the reasons for cation segregation in multimetallic systems are incomplete chelation, so that part of the metal precipitates as nitrates or hydroxides during the dehydration (condensation) step, and selective precipitation of some metal carboxylates occurs at corresponding pH 

Perovskites are mixed oxide compounds of the general formula ABO3 containing a transition metal, such as Ti, Nb, Ta, in B position, Ca, Sr, 

The pure PbMgo.33Nbo.67O3 perovskite phase was crystallized at 1253 K from xerogel obtained by cogelation of Pb-acetate, Mg- and Nb-ethoxides in methoxyethanol [69]. Its purity was controlled by addition of excess Pb and Mg components that prevented formation of the lead niobate pyrochlore phase. 

A pure perovskite phase with the composition SrBi2Ta209 and a crystal size 50 nm was formed at 1073 K from a xerogel precursor, obtained by gelation from Ta(EtO)5 and Sr-acetate and BiO-nitrate solution in methoxyethanol using an acetic acid catalyst [70]. Acetate ions act as bidentate ligands in this solution, equalizing the hydrolysis/condensation rates of Ta and other ions. 

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Depending on the nature and the final state of entrapped guests this can produce composite catalytic phases appearing as xero- or aerogels (i.e. 

The nucleophilic nature of the alkanes is also shown by the influence of the acidity level on their solubility. Torek and co-workers90 have investigated the composition of the catalytic phase obtained when w-pentane or w-hexane is thoroughly mixed with HF-SbF5 in an autoclave under hydrogen pressure [Eq. (5.41)]. The total amount of hydrocarbon in the catalytic phase (dissolved ions and neutrals) was obtained by extraction with excess of methylcyclopentane. The amount of physically dissolved... 

Figure 5.11. Variation of the composition of the catalytic phase as a function of the SbF5 concentration in the n-pentane isomerization in HF—SbF5.90 T — 15°C, pm = 5 bars, volume of the catalytic phase = 57 ml. , Mass of C5+, o, mass of C5H (Freon-113 extract) a, % weight of Cs+ + C5H (methylcyclopentane extract).

Raney nickel electrocatalysts have also found useful applications as active electrodes for the HER (179, 180). The activity of Raney Ni catalysts is established after leaching out the base metal, Al or Zn. Choquette et al. (181) have examined the changes in morphology and composition of Raney-Ni composite catalytic electrodes accompanying dissolution of the base metal in concentrated NaOH. The depletion of Al from the Raney particles is, of course, accompanied by a major increase in real area with time of leaching and also, interestingly, with possible phase transformations (181). The electro-catalytic activity is, however, surprisingly, practically independent of time. 

Our work is a tentative to rationalize the influence of the preparation parameters over the composition, active phase and catalytic activity. Modifying the pH of precipitation between 6.0 and 8.5, the surface area, the bulk, the surface composition and the active phases of the catalysts were modified significantly. Raman spectroscopy has revealed the presence of M0O3 phase only in the catalyst prepared at pH 6.0. From the literature data, we know that the precipitation of the Ni(OH)2 and Co(OH)2 occurs at pH 7 and 7.5, respectively. At pH higher than 7.5 our calculations suggest the presence of nickel or/and cobalt oxides but these phases were not detected with any of the characterization techniques used. 

Catalyst composition Crystaline phase Fresh Crystaline phase after catalytic reaction... 

The nucleophilic nature of the alkanes is also shown by the influence of the acidity level on their solubility. Torek and coworkers have investigated the variation of the composition of the catalytic phase as a function of SbFs concentration in isomerization of pentane in HFiSbFs. 

When a remote control operates, spill-over oxygen must flow easily from one phase to the other. The number of contacts between the small domains of different chemical composition should thus be large and the quality of the contacts excellent for permitting an easy "jump" of spill over species from one phase to the other. The formation of the spill-ovo species depends on the surface area developed by the phase dissociating oxygen (e.g. Sb204). The number of sites to create on the catalytic phase (e.g. M0O3) depends on the surface area of that last phase. A second very important parameter is thus the phase A hase B surface area ratio. 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

Naphthaleneamine. 1-Naphthylamine or a-naphth5iamine/7i5 -i2- can be made from 1-nitronaphthalene by reduction with iron—dilute HCl, or by catalytic hydrogenation it is purified by distillation and the content of 2-naphthylamine can be reduced as low as 8—10 ppm. Electroreduction of 1-nitronaphthalene to 1-naphthylamine using titania—titanium composite electrode has been described (43). Photoinduced reduction of 1-nitronaphthalene on semiconductor (eg, anatase) particles produces 1-naphthylamine in 77% yield (44). 1-Naphthylamine/7J4-J2-. can also be prepared by treating 1-naphthol with NH in the presence of a catalyst at elevated temperature. The sanitary working conditions are improved by gas-phase reaction at... 

In these particular experiments it proved impossible to investigate the effect of copper concentration on the catalytic activity of alloys free of the hydride phase. Figure 10 69, 64a, 65) illustrates the changing values of the recombination coefficient on nickel-copper alloys related to the composition of the alloy at room temperature. The small amount of copper introduced into the nickel already distinctly decreased the catalytic ac-... 

The ordinary burning of sulfur produces SO2. This is the hrst step in the manufacture of sulfuric acid. The second step oxidizes SO2 to SO3 in a gas-solid catalytic reactor. The catalyst increases the reaction rate but does not change the equilibrium compositions in the gas phase. 

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