Spinel oxide

The electrical conductivity requirement for interconnect applications necessitates the use of chromia-forming (or Cr-rich spinel) oxidation-resistant alloys. One drawback of the chromia-forming alloys for this particular application, however, is the Cr volatility of the chromia or Cr-rich scale. As indicated by many studies [185-189], during high-temperature exposure Cr203 (s) reacts with 02 via the following reaction... 

C02 — 12.5 mol % space velocity 6400 h 1, the CO conversion rate was found to depend very strongly on the calcination temperature of the catalyst. For example, a catalyst calcined at 500 °C achieved only 25% conversion by 350 °C, while a catalyst calcined at 900 °C achieved 80% conversion by 225 °C. The authors suggested that the Cu-Mn spinel oxide was more easily reducible after high temperature calcination treatment based on a TPR study, allowing for a greater number of highly dispersed Cu species, as verified by XRD. The optimum Cu/Mn ratio was found to be 1/2. 

Mn,tCo3 t04) phase. This spinel oxide is broken up during reduction to make MnO t and a metallic surface. Due to the pre-existence of this Mn-M interaction, electronic promotion is much more easily achieved after reduction as well. It is worthwhile to mention that Mn,tCo3 t04 compounds are well studied in the literature, because they have important electrocatalytical properties. More specifically, spinel-type manganese oxides are widely used as precursors in the preparation of X-Mn02 ([ ]A[Mn2]B04], an oxide of technical interest due to its application as a cathode material for rechargeable cells. " ... 

Structural and textural properties of zinc(II)-chromium(III) spinel oxides prepared using a hydrotalcite-like compound... 

Spinel oxides are important industrial catalysts, possessing good thermal and chemical stability and an ability to maintain catalytic activity at high temperatures. The study reported here is concerned with the properties ofZn(II)-Cr(III) spinel oxides prepared from a carbonate containing LDH precursor. For this study the spinel oxide was separated, by treatment with dilute mineral acid, from the oxide mixture (ZnO and ZnC O, ) obtained by the thermal decomposition of LDH. For comparison, Zn(II)-Cr(lII) spinel oxides were also prepared by more standard synthesis methods. Structural and textural properties were evaluated, using PXRD, TG/MS, FT-IR and N2 adsorption. 

Spinel oxides with a general formula AB2O4 (i.e. the so-called normal spinels) are important materials in industrial catalysis. They are thermally stable and maintain enhanced and sustained activities for a variety of industrially important reactions including decomposition of nitrous oxide [1], oxidation and dehydrogenation of hydrocarbons [2], low temperature methanol synthesis [3], oxidation of carbon monoxide and hydrocarbon [4], and oxidative dehydrogenation of butanes [5]. A major problem in the applications of this class of compound as catalyst, however, lies in their usually low specific surface area [6]. 

Although the thermal decomposition of Zn-Cr-C03-LDHs has, therefore, been studied in detail, a specific characterisation of the actual spinel oxides obtained from the LDHs, separated from the mixed oxide phase, has not been reported either for Zn-Cr-LDHs or for other cation combinations. Although the structure of the spinel oxides formed from LDHs has been reported occasionally, the effect of separating the spinel phases from the whole thermal decomposition product in the properties of the spinel oxide has not been evaluated. Here we report on the properties of the spinel oxides produced by the thermal decomposition of a Zn-Cr-C03-LDH. For comparison, spinel oxides phases were synthesised by the two other methods described above, and treated in a similar way to those obtained by LDH decomposition. 

To prepare the spinel oxide by solid-state reaction of a mixture of the individual metal cation oxides, the following procedure was used [8] a mixture of powdered ZnO (Aldrich >99%) and C CL (Aldrich >99%) (0.5 Zn Cr ratio), was carefully grounded for 10 minutes in order to obtain a good dispersion of the compounds in the solid mixture. This mixture was then heated at 900 °C for 12 hours in a N2 atmosphere. 

The PXRD pattern for the spinel oxide prepared by solid-state reaction of a Zn0-Cr203 gave an a parameter for the cubic unit cell of 8.308 A (the average of all a parameters calculated by the formula a = <7(h2 + k2 + l2)112 for each observed reflection), close to the reported value of 8.3275 A [22]. The PXRD pattern for this compound showed intense and sharp peaks, indicating high crystallinity. Post-treatment with acid did not show any significant change in the PXRD pattern. 

The mixed hydroxide precursor (MH) was treated at 500, 900 and 1200 °C. Again a progressive increase in crystallinity was observed with the increase of the temperature. FT-IR spectra showed a band around 3400 cm 1 for the sample treated at 500 °C, which was absent in samples treated at 900 and 1200 °C. The explanation follows that given for the LDH system. As observed for the spinel oxide prepared by the solid-reaction method the treatment with mineral acid did not affect the PXRD pattern. 

Figure 4. Evolution of the a parameter of fee unit cell of spinel oxides with temperature, obtained by different methods. TA means post-treatment with mineral acid.

Analysis of the evolution of the a parameter of the spinel oxides indicated a variation as shown in Figure 4. 

Materials obtained from the LDH show a reduction in SSA with the temperature as reported in the literature. This reduction can be attributed to the crystallisation of the material [15,17]. However, post-treatment with mineral acid was an increase in the SSA for all temperatures. It is possible to attribute this increase to two combined effects, which can both increase the porosity of the materials, as well as yield more active adsorption sites (i) the elimination of ZnO and (ii) the elimination Zn(II) cations occupying octahedral sites in the spinel oxide structure. Even though the SSA had varied sensibly, the average pore size (APS) remained fairly constant with temperature. Acid treatment increases the APS value for all temperature tested, although the effect was very small (Figure 5b). Comparison of the materials obtained by the different synthesis methods showed that spinel oxides obtained from the LDH presented greater SSA values than those obtained by other methods, principally after the posttreatment with mineral acid. On other hand, the treatment with acid had little influence on the textural properties of the spinel oxides obtained by the other methods. 

Comparing the SSA for materials obtained at 900 °C, it is possible to note that the spinel oxide obtained from a LDH precursor (after elimination of the ZnO) showed the greatest surface area. The value obtained, 24.5 m2 g 1 was even greater than the reported data for a similar material prepared from an MH precursor treated at 800 °C, 12.94 m2 g 1 [9]. The difference become more significant when we utilise for comparison the value obtained for the LDH treated at 800 °C (after a treatment with acid), 32.7 m2 g 1. 

The results obtained in this work indicated that the use of a LDH as precursor to prepare spinel oxides is viable. Separation of the spinel from the oxide mixture can be achieved by a simple treatment with mineral acid. The materials obtained by this method present an excess of bivalent cations in the structure, due to the starting material possessing a Mn Min ratio... 

Spinel oxides obtained by LDHs showed greater specific surface area than those prepared by the classical methods at the same calcination temperature. The segregation of the Zn(II) from the structure, as well as the elimination of the ZnO increase both the SSA and the APS. Oxides obtained from an LDH or a MH precursor at relatively low temperatures (from 400 °C) present very high specific surface area for a spinel oxide (reaching more than 80 m2 g 1). Although these materials have low crystallinity, they present chemical stability (since these materials were not decomposed by the post-treatment with acid, pH = 0) and are thermally stable at least to the temperature of preparation. Therefore, these materials may be very usef >1 in catalytic applications. 

Structural and Textural Properties of Zinc (ll)-Chromium (III) Spinel Oxides 691... 

Mao, H.-K. Bell, P. M (1975a) Crystal-field effects in spinel oxidation states of iron and chromium. Geochim. Cosmochim. Acta, 39,865-74 [see also Ann. Rept. Geophys. Lab., Yearb. 73, 332—41 (1974)]. 

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