Nonstoichiometric catalyst

The dehydration of alcohols on stoichiometric and nonstoichiometric (calcium-deficient) hydroxyapatite (series 8 and 9 in Table II) gave results consistent with the above findings. Although there is a difference in the reaction temperature, it is evident that with the nonstoichiometric catalyst, which must be more acidic, the slope found is more negative than that with the stoichiometric calcium phosphate. 

With regard to the methanol synthesis from CO and Hg in the presence of ZnO, we believe that the small activation effects observed under irradiation to be attributable to the contribution of a new reaction mechanism, more precisely to the possibility of chemisorbing CO, in the form CO+, through capture of a positive hole generated by irradiation. In this respect, let us note that, as already pointed out by Romero-Rossi and Stone (75), the interstitial excess zinc atoms compete with CO molecules for the capture of these positive holes therefore, when a nonstoichiometric catalyst... 

Further infonnatioii on the catalytic properties of stoichiometric and nonstoichiometric CaHAp may be obtained from studies on the adsorption and dehydrogenation of methanol. WiA stoichiometric CaHAp methanol decomposes at 600°C to produce predominantly carbon monoxide (Table 1) whose selectivity diminishes as the Ca/P ratio decreases while those to formaldehyde and dimethyl ether increase. Infrared spectra show that methoxy groups are formed on the surface of both the stoichiometric and nonstoichiometric catalysts. The results from temperature-programmed desorption experiments together with those from infrared spectroscopy suggest that the acidic sites found on the nonstoichiometric CaHAp catalyze the dissociative adsorption whereas the basic sites on the stoichiometric analogue catalyze the C-H bond scission and formation of CO and H. ... 

With calcium as the cation the oxidation of methane on stoichiometric CaHAp produces predominantly COj and Hj while with the nonstoichiometric catalyst the principal products are CO and H. ... 

Nonstoichiometric Catalyst. Beneficial results in optimizing selectivity to low-molecular-weight telomer can be obtained by deviating from the 1 1 stoichiometry of organosodium compounds to chelating agent. 

The oxides often are nonstoichiometric (with an excess or dehcit of oxygen). Many oxides are semiconducting, and their conductivity can be altered by adding various electron donors or acceptors. Relative to metals, the applications of oxide catalysts in electrochemistry are somewhat limited. Cathodic reactions might induce a partial or complete reduction of an oxide. For this reason, oxide catalysts are used predominantly (although not exclusively) for anodic reactions. In acidic solutions, many base-metal oxides are unstable and dissolve. Their main area of use, therefore, is in alkaline or neutral solutions. 

It is interesting to note that cobalt cobaltite, C03O4, is a good catalyst, too, for anodic chlorine evolution. In this case, too, a correlation is observed between the reaction rate and the spinel s defect concentration (amount of nonstoichiometric oxygen). 

Figure 2. These high-resolution micrographs show how a so-called x-ray amorphous, nonstoichiometric molybdenum sulfide catalyst exhibits structural (as well as compositional) heterogeneity. Amorphous, quasi-crystalline, and crystalline regions coexist at the ultramicro level (18,).

Nonstoichiometric metal oxides are effective catalysts for a variety of oxidation-reduction reactions (as might be expected) since the variable valence of the constituent ions enables the oxide to act as a sort of electron bank. Nonstoichiometric metal oxides resemble metals in that they can also catalyze hydrogenation and alkene isomerization reactions. However, on zinc oxide, for instance, these two processes are independent, whereas hydrogen must be present for isomerization to occur on metals. 

No particular significance is attached to the fact that the molar amounts of glyme and diglyme in these two catalysts are almost the same. Extensive vacuum drying of the cobalt catalyst, which reduced the water level from 4.29 to less than 1.0 mole per mole of the zinc salt (and, presumably, reduced the glyme to a similar or larger extent), gave no appreciable effect on catalytic activity. This result emphasizes the nonstoichiometric nature of the catalytic forms of the hexacyanometalate salt complexes. 

The basis of most of these methods is comparative in nature, that is, results are compared to known pure compounds. The inference is that if the catalyst resembles some property of the known compound, then that compound is present in the catalyst. However, caution must be exercised in this interpretation because we may be dealing with indefinite and nonstoichiometric surface complexes which may have some properties in... 

It is known that fresh manganese dioxide, Mn02 xnH20, a nonstoichiometric mixed-valent (+IV, +III) system, acts as a good heterogeneous catalyst for the decomposition of H202 to 02 and water. 

The Pd-catalyzed nonstoichiometric polycondensations of other monomers, in which an olefin-Pd(O) complex after the first allylic substitution exchanges the olefin ligand with the remote double bond, was also reported [258]. The dependence of nonstoichiometric polycondensation behavior on a catalyst is interesting in comparison to the polycondensation mentioned above, where that behavior only stems from the structure of the monomers. 

Notably, SVO can display a variety of phases, both stoichiometric and nonstoichiometric. Thus, variations in reaction conditions, starting materials, and reagent stoichiometries for the preparation of SVO can result in a wealth of products that display different structures and different properties. In addition, the variety of oxidation states available to the silver and especially the vanadium components of SVO, plus the open structure of some of the SVO materials, suggest that these materials are well suited for electron transfer applications. It is thus logical and not surprising that reports of SVO battery applications and SVO redox catalyst applications appear within similar time frames. Some reports involving the structure of SVO solids and the catalysis of organic substrate oxidation by SVO-based catalysts will be described in Section 13.2, due to their possible relevance to the SVO battery chemistry described in Section 13.3. 

Ying, X Y., and Tschope, A., Gas phase synthesis of nonstoichiometric nanocrystalline catalysts, in Advanced Catalysts and Nanostructured Materials Modern Synthetic Methods (W. R. Moser, Ed.), p. 231, Academic Press, San Diego (1996). 

Mixed oxides have a widespread application as magnets, catalysts, and ceramics. Often, nonstoichiometric mixtures with unusual properties can be prepared for example, Fe203 and ZnO have been milled for the production of zinc ferrite [40], while mixed oxides of Ca(OH)2 and Si02 were described by Kosova et al. [77]. Piezoceramic material such as BaTi03 from BaO and anatase Ti02 has been prepared [78], while ZnO and Cr203 have been treated by Marinkovic et al. [79] and calcium silicate hydrates from calcium hydroxide and silica gel by Saito et al. [80]. The thermal dehy-droxylation of Ni(OH)2 to NiO or NiO-Ni(OH)2 nanocomposites has also been investigated [81]. 

Methanol is produced from a nonstoichiometric syngas mixture (CC>2 CO H2 = 5 5 90) at 50-100 bar pressure and a temperature between 225°C and 275°C over a Cu/Zn0/Al203 catalyst. The predominant reaction is... 

In the elementary reactions of the pyrolysis, the atomic carbon is formed first. Then it transforms into the final product, whether it be soot, graphite, carbon nanofibers, or so forth. Why does the presence of catalysts make it possible to grow carbon nanofibers or nanotubes instead of soot In many cases, this is the so called carbide cycle that is characteristic of the catalytic process of hydrocarbon pyrolysis that is responsible for the growth of the elongated structures but not soot particles. The primary car bon atoms produced by pyrolytic decomposition of the hydrocarbon molecules are dissolved in the metal particle of the active catalyst compo nent to form a nonstoichiometric carbide (the carbon solution in the... 

The specific role of pyrite (FeS2) as a catalyst has been under investigation since pyrite was identified as the most active inherent mineral for coal liquefaction. Under liquefaction conditions, FeS2 is transformed into a nonstoichiometric iron sulfide, Fei-x (0 X 0.125). Thomas et al. (15) studied the kinetics of this decomposition under coal liquefaction conditions, and concluded that the catalytic activity of FeS2 is associated with radical initiation resulting from the... 

Venuto et al. (75) prepared a synthetic partially deuterated decation-ated Y catalyst by thermal decomposition (in He) of a deuteroammo-nium Y sample prepared by a reconstitution procedure. This catalyst (designated nonstoichiometrically as DHY) showed vqh/i od ratios of 1.365-1.365, a range consistent with the ratios of corresponding groups in other decationated zeolites (71), SiOa (77) and H2SO4 (75). When this catalyst was contacted with 2,3-dimethylbutene-l or 1-hexene at 0-25° in the liquid phase, the transfer of small amounts of deuterium to organic reactant accompanied the isomerization and polymerization reactions (79). 

Semiconductors are classified as p type if they tend to attract electrons from the chemisorbed species, or as n type if they donate electrons to this species. The p type are normally the compounds, such as NiO. The n type are substances which contain small amounts of impurities, or the oxide is present in nonstoichiometric amounts (as, for example, when some of the zinc in ZnO has been reduced). Reviews of semiconductors as catalysts are given by P. H. Emmett ( New Approaches to the Study of Catalysis, 36th Annual Priestly Lectures, Pennsylvania State University, April 9-13, 1962) and by P. G. Ashmore ( Catalysis and Inhibition of Chemical Reactions, Butterworths Co. (Publishers), London, 1963. 

Unfortunately, at present, while the bulk structure of magnetite is well understood, little is known about the surface structure of magnetite-based catalysts. In general, the surface may be nonstoichiometric (FeoO ) with iron cations in both octahedral and tetrahedral sites ( ). 

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