Stoichiometric spinel

It is possible that all three phenomena contribute to the capacity loss of 4V Lix[Mn2]04 electrodes. Nonetheless, all three can be at least partly circumvented by slightly modifying the composition of the spinel electrode. For example, cell performance can be improved by increasing the amount of lithium in the spinel structure [107, 123, 130] and, in particular, by substituting a small amount of manganese on the B sites with lithium [107], which drives the composition a small way down the stoichiometric spinel tie-line, towards... 

This discussion has focused on stoichiometric spinel structure, but nonstoichiometric spinels can exist as well. In the case of spinels that are oxygen deficient there could be significant concentrations of Mn + remaining at full charge. The results of this study suggest that such spinels may be susceptible to cation rearrangement if tliey are energetically metastable. 

The minimum temperature required to obtain a fully dense product increases with A1203 content from the stoichiometric spinel (71.68 wt% A1203) to 85 wt% A1203, requiring from 1650°C to 1680°C. The sinterability of spinel thus depends markedly on stoichiometry it then decreases steadily to 1650°C with further increased alumina content (93 wt%).38... 

Thermomechanical behaviour of stoichiometric spinel at high temperatures... 

Co-condensation of oxides. The stoichiometric spinel MgAl204 can be prepared (4) by reacting boehmite type reactive alumina with high surface area MgO at a temperature higher than 1200°C (equation 1). 

When impregnated with Ce(N03)3 solution and calcined the resulting catalysts are extremely active towards SOx abatement (Table I, Figure 4). The solid solution spinel catalyst is nearly 25% more active than the stoichiometric spinel catalyst. This can be explained by assuming that -MgO- structural fragments of the spinel are the chemisorption active sites (1). There are more active sites in Mg2Al20c than in MgA1 04. We also see that the catalyst prepared by co-precipitation method is 4 times more active than the catalyst prepared by the thermal co-condensation method. In Zone D we see that the sulfated catalyst is very effectively reduced by H. Nearly 70% of the absorbed species is reduced within 2 min. of reduction. 

It is very apparent from this work that coprecipitation and co-gel formation methods for the preparation of spinels provide very active catalysts for FCC SOx reduction whereas the co-condensation method produces very dense ceramic type spinels with low activity. The high magnesium spinel catalyst is more active than the stoichiometric spinel catalyst. 

In the intermediate case, the ions of divalent metal do not form their own structural layer but are localized in the vicinity of vacant octahedrons [ Al2(OH)6] 2oo using OH" groups of this layer, as well as OH" groups and H2O molecules of the freshly formed third network of layered packing. In this case, interplanar distance is equal to 0.76 nm. Hence, the crystal chemical formula for this compound is [Me(0H)2-H20][Al2(0H) ]. It was obtained in Mg(OH)2 - A1(0H)3 system. The decomposition of such compounds at moderate temperatures (about 800°C) can result in the formation of stoichiometric spinels, which was demonstrated in [15]. 

According to Fig. 6.26, spinels, prepared by mechanical activation followed by heat treatment, are characterized by very low lattice parameter a, which increases as temperature rises. It is well known that the radius of Mu ions is lower than that of Mn . Mn ions are more stable at lower temperatures. Thus, low values of a parameter for the samples with molar ratio Li/Mn=l/2 point to the increased amount of Mn ions comparing with that of stoichiometric spinel [93]. One can see that in the samples prepared from Li2C0 by mechanical activation and heating at 450°C, the value of a and the amount of Mn ions are higher than in the case of LiOH samples because of reducing character of CO2, eliminating in the course of interaction [98]. 

Sintering studies on the Al203-rich side of stoichiometric spinel were carried out by Bailey and Russell [6]. Transparent ceramics were not the objective of this study, but several findings were noteworthy. Compositions from 71.67 wt % A1203 (stoichiometric spinel) to 93 wt % A1203 were... 

Previously we have studied such catalysts in hydrocarbon dehydrogenation and oxidative dehydrogenation reactions [6,7]. Instrumental methods such as XRD, X-ray, photoelectron spectroscopy, DTA, UV-spectroscopy, EM were used. It has been found that activity of the Zn-Cr catalysts is determined by the stoichiometric spinel ZnCr204 [8]. In the case of the vanadium-magnesium system the activity and selectivity depend upon the presence of ions V and V grouped on the catalyst surface into clusters of 2-3 vanadium ions [9]. This was taken as a principal for the purposeful synthesis of the catalytic systems mentioned. In this work an attempt was made to spread the obtained experience on the dehydrogenation of alcohol groups. 

The binary phase diagram for MgO-Al203 is simpler than that for the Ca0-Al203 system (Fig. 2). There is only one stable intermediate compound that of the spinel phase (Mg2A104) [60]. Spinel melts at 2,105°C, but there is a eutectic at 1,995°C and a limited solid solution between stoichiometric spinel and MgO (periclase), up to 6wt% MgO, can be dissolved into the spinel structure without exsolution. This limited solid solution is an important property that is utilized in manufacture of spinels for use in reducing conditions [70]. 

Starting with stoichiometric spinel, write the incorporation reactions for both alumina and magnesia. Why do you think that spinel has such a wide range of solubility for its end members as compared to, say, Na20 Si02 ... 

If without additives, a carrier such as A3 consists of a mixture of 8-, 0-, and a-aluminas between 900° and 1000°C. The presence of the metal oxides, introduced by impregnation, effectively maintains a cubic type structure at a calcining temperature of 900°-1000°C. With alumina, these oxides form spinel-structured compounds which are more or less well crystallized. Because of the insertion of alumina, the lattice parameter of these compounds is expanded with respect to that of the stoichiometric spinel. The properties of the carrier are thus maintained up to a temperature which depends on the considered mixed oxide. At this temperature—about 1000 °C with magnesium aluminate and 900°C with zinc and copper aluminates—the stoichiometric spinel recrystallizes while a-alumina is rejected (6). The carrier then suddenly loses its mechanical and structural properties. None of the mentioned additives could improve the stability of the Cl carrier above 1000°C. 

With the exception of Cat 1 (Cu/Cr= 33 67, as atomic ratio, i.e. with a composition corresponding to the stoichiometric spinel), all samples contain 50% chromium. In samples 3, 4, and 5, 20% of the copper ions was replaced by 2anc cadmium and magnesiinn ions, respectively. All catalysts were prepared by coprecipitation at pH 8.0 0.1 from a solution of nitrates of the elements with a shght excess of NaHCOs, washed until the sodium concentration was lower than... 

However, for samples 2-5, the amounts of CuO detected by quantitative XRD analysis (18) approach a maximum of 50% of the value calculated on the basis of a phase composition CuO + stoichiometric spinels, showing the existence of a consistent fraction of copper ions which escape XRD detection, probably present inside the spinel-type phase or strongly interacting with it (19). 

I2 Picmonicse, M., Trillro, F., Vaccari, A., Forcsti, E. and Gazzano. M. (1991). Synthesis of non-stoichiometric spinel-type phases - A key tool for the preparation of tailored catalysts with specific activity. In G. Poncelet, P.A. Jacobs. P. Grange and B. Dcimon (Eds.). Preparation of Catalyts V (pp. 49-58). Amsterdam, Elsevier. 

Thus, considering the results obtained by TPR and the high interaction existing between Ni and Co in the non-stoichiometric spinel matrix, the formation would be expected, to a certain extent, of a Ni-Co alloy in the Ni-Co-Zn-Al system after reduction in H2 at 500°C. This type of compound would lead to an important reduction in the number of three nickel atom arrangements on the cluster surface, responsible for the production of ethane. The consequence of this, once the hydrogenol)4 ic sites generated by addition of Co are poisoned, would be the suppression of the reaction pathway shown in Scheme 1. Therefore, ethylene... 

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