Spinel oxide preparation

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. 

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. 

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. " ... 

The low-pressure methanol synthesis process utilizes ternary catalysts based on copper, zinc oxide, and another oxide, such as alumina or chromia, prepared by coprecipitation. Cu-Zn0-Al203 and Cu-Zn0-Cr203 are usually the most important industrial catalysts. A significant advance was made when a two-stage precipitation was suggested in which ZnAl2C>4, a crystalline zinc aluminate spinel, was prepared prior to the main precipitation of copper-zinc species.372 This alteration resulted in an increase in catalyst stability for long-term performance with respect to deactivation. Catalyst lifetimes industrially are typically about 2 years. 

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. 

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. 

Three different Cr-Co spinels were prepared and tested as catalysts for the oxidation of methane in the presence of SO2, a typical catalyst poison. The spinels were prepared from nitrate precursors using a co-precipitation method, followed by calcining at three different temperatures, (400, 600 and 800 °C) for 5 hours. Characterisation results indicate that the catalyst calcined at 800 C presents a structure of pure spinel, whereas the presence of single oxides is observed in the catalyst calcined at 600 C, and the catalysts calcined at 400 C presents a very complex structure (probably a mixture of several single and binary oxides). Experiments show an important influence of calcining temperature on the catalyst performance. In absence of SO2, catalysts calcined at 400"C and 600 C performs similarly, whereas the activity of the catalysts calcined at 800 C is worse. When sulphur compounds were added to the feed, catalyst calcined at 600"C deactivated faster than the other two catalysts. 

Similar behavior has been observed with different modifications of Mn(IV) oxides. The cryptomelane type of hydrous manganese dioxide prepared by the reaction of Mn(II) and KMn04 in sulfuric acid solution exhibits an extremely high selectivity for potassium ions [120]. The spinel type of manganese oxide prepared by the introduction of lithium ion has exhibited an extremely high selectivity for lithium ion [121, 122]. These aspects are considered later. 

Metal oxide nanostructures have been fabricated using different methods and preparation conditions. The most promising technique is sol-gel processing in combination with dipcoating technique.This method enables us to prepare spinel oxide thin film electrodes at ambient temperature with high level of doping and large surface area [117,118], The physical and chemical vapor deposition is another technique for metal oxide preparation [119,120],... 

Klissurski et al. [87] have examined the combustion of acetone, toluene and styrene by zinc-cobalt spinel oxides supported on alumina. Catalysts were prepared by co-precipitation with sodium carbonate from a mixed zinc/cobalt nitrate solution at pH 9. The supported catalyst was prepared by deposition of the precursor on Y-AI2O3 from a suspension in dimethylformamide and water. The supported precursor was dried at 150°C and calcined at 300°C to produce the catalyst. The bulk and supported catalysts both showed the formation of zinc cobaltite spinel structures which were thermally stable. Microreactor studies at 15,000 h- space velocity showed that the components of a mix of acetone, toluene and styrene were destroyed at 225°C, 280°C and 350°C respectively. The VOC concentrations were not specifically expressed but it is assumed that they... 

Zeolite catalyst performances were also checked in a direct manner in the methane oxidation reaction, a model reaction which tested the spinel oxide type catalysts prepared by us for hydrocarbons oxidation to the purpose of purifying engines exhaust gases. The results are presented in Table 2. From among the... 

T. Tabakova, V. Idakiev, G. Avgouropoulos, C. J. Papavasiliou, M. ManzoU, F. Boccuzzi, T. loannides. Highly active copper catalyst for low-temperature water-gas shift reaction prepared via a Cu-Mn spinel oxide precursor, AppL CataL A Gen. 451 (2013) 184—191. 

Restovic A, Rios E, Barbato S, Ortiz J, Gautier JL (2002) Oxygen reduction in alkaline medium at thin MnxCo3 x04 (0 < x < 1) spinel films prepared by spray pyrolysis. Effect of oxide cation composition on the reaction kinetics. J Electroanal Chem 522(2) 141-151... 

The spinel-type catalyst is a complex oxide that has attracted attention owing to its wide applications as a catalyst [18-19]. The basic state of Cu-Fe-O and Cu-Cr-O spinels is prepared by the coprecipitation method and is measured in air by the DTA-EGD coupled technique [20] (Figures 5.7 and 5.8). 

Oxides commonly studied as catalytic materials belong to the structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure. These structures are commonly reported for oxides prepared by normal methods under mild conditions [1,5]. Many transition metal ions possess multiple stable oxidation states. The easy oxidation and reduction (redox property), and the existence of cations of different oxidation states in the intermediate oxides have been thought to be important factors for these oxides to possess desirable properties in selective oxidation and related reactions. In general terms, metal oxides are made up of metallic cations and oxygen anions. The ionicity of the lattice, which is often less than that predicted by formal oxidation states, results in the presence of charged adsorbate species and the common heterolytic dissociative adsorption of molecules (i.e., a molecule AB is adsorbed as A+ and B ). Surface exposed cations and anions form acidic and basic sites as well as acid-base pair sites [1]. The fact that the cations often have a number of commonly obtainable oxidation states has resulted in the ability of the oxides to undergo oxidation and reduction, and the possibility of the presence of rather high densities of cationic and anionic vacancies. Some of these aspects are discussed in this chapter. In particular, the participation of redox sites in oxidation and ammoxidation reactions and the role of redox sites in various oxides that are currently pursued in the literature are presented with relevant references. 

By way of an example, the spinel oxides will be briefly considered. As indicated above, they can be discussed in terms of two extreme cases, normal and inverse spinels, and they can exist with intermediate ion distributions, and ionic size, electron configurations, stoichiometry (method of preparation), and heat treatment will influence the distribution of the transition metal ions. ... 

Chromates III). Mixed oxides, e.g. FeCr204, having spinel structures and prepared by solid state reactions. 

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