Spinel-type Catalyst

Ferrites are spinels with a general formula AFe204. But, bulk Fe304 has an inverse spinel structure, where Fe3+ is located in the tetrahedral (T) sites and Fc2 and Fe3+ are located in the octahedral (O) sites. The inverse spinel structure consists of a FCC oxygen anionic framework (CCP), with the I c31 ions filling 1/8 of the tetrahedral A sites, and equal amounts of Fc2 and Fe3+ ions filling 1/2 of the octahedral B sites. 

The Physical Chemistry of Materials Energy and Environmental Applications 

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Cu/Cr or Cu/M/Cr (M= cadmium, zinc or magnesium ions) cubic nonstoichiometric spinel-type catalysts, containing an excess of bivalent cations,... 

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). 

Takahashi, M Nakatani, T Iwamoto, S Watanabe, T Inoue, M. Effect of the composition of spinel-type Ga203-Al203-Zn0 catalysts on activity for the CH4-SCR of NO and optimization of catalyst composition, Ind. Eng. Chem. Res., 2006, Volume 45, Issue 10, 3678-3683. 

These effects have been observed by Margolis and co-workers in their detailed kinetic study of the catalytic oxidation of hydrocarbons (219-222). The extensive oxidation of hydrocarbons to carbon dioxide over catalysts of the spinel type has been studied by a number of investigators. It was possible for Margolis and co-workers to establish the effect of additives upon the basic kinetic constants, namely the activation energy of oxidation and the frequency factor for this reaction. 

The present article deals primarily with the elucidation of the surface nature of common metallic and oxidic catalysts, and with statistical-mechanical investigations of the chemisorption equilibrium on these catalysts. The surface areas of these catalysts as determined by the Brunauer-Emmett-Teller method have been taken into consideration. It was shown that a number of certain metallic catalysts such as nickel, cobalt, and platinum and also oxide catalysts of the spinel type act as an array of homogeneous active sites. There is no reason to believe that a few limited regions of the surfaces of these catalysts, such as corners, edges, lattice defects, etc. are particularly important for their catalytic activity. This conclusion is in accordance with the poisoning experiments of Maxted et al. There is some evidence that the surfaces of these catalysts... 

We make clear the following facts in this study. The K/Cu-Zn-Fe oxides catalyst has a spinel type structure. The K/Cu-Zn-Fe oxides catalyst is deactivated by the segregation of catalyst components to FeCOj, ZnO, and Cu during the reaction. The segregation is prevented by the addition of Cr component to the catalyst. The long life of K/Cu-Zn-Fe-Cr oxides catalyst can be explained by its slow segregation rate. It is considered that the Cr component stabilizes thermally the spinel type structure of the catalyst. 

Tanaka, Y., Utaka, T., Kikuchi, R., Takeguchi, T., Sasaki, K., and Eguchi, K. Water gas shift reaction for the reformed fuels over Cu/MnO catalysts prepared via spinel-type oxide. Journal of Catalysis, 2003, 215, 271. 

In line with the XRD results, the DSC/FTIR study of CR also suggests the formation of hydrotalcite-like phase [13]. High Mg-containing CS is more thermally stable than CR. With increase in calcination temperatures, the FTIR spectrum evolution reveals the formation of mixed metal oxides, i.e. spinel-type phase [40]. The decomposition of catalyst precursors is also reflected in the XRD study which shows that the resultant oxides are largely amorphous although weak spinel-type feature can be observed [41]. 

Evidence of spinel structure within some of the alumina and titania catalysts was established by x-ray powder techniques. Samples were removed from the reaction bed of some of the runs where high yields of sodium sulfate had been obtained using either alumina or titanium dioxide as catalyst. The presence of compounds having spinel type structure was established by x-ray powder diffraction. The amount of spinel was variable and of small quantity because neither the proportions of necessary ions nor the reaction conditions were ideal for spinel formation. 

It seems possible, from the results obtained with magnesium aluminate and from x-ray evidence, that a satisfactory catalyst for the Hargreaves reaction will be any compound possessing a spinel-type structure. 

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... 

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. 

The best starting materials are active oxides, possibly produced in situ, preferably from hydroxides, hydrated oxides or carbonates. In some cases it may be necessary to start with a very well-defined oxide modification in order to obtain an active catalyst [29]. Occasionally, the required starting oxides are produced by thermal decomposition of nitrates. However, the activity of such products is not very high and their maximum specific surface does not exceed a few m. /g. For this reason, nitrate decomposition Is Important only for the production of supported catalysts (see below). Oxides calcined at a high temperature, as well as spinel-type materials, should be avoided, since their reduction times tend to be extremely long [2]. 

In all cases, the evolution of the spinel-type phases towards the smichiometric forms, which takes place with a consistent segregation of oxide phases, destroys diis system and leads to considerable modifications in the properties of the oxides obtained. In particular, for the Ni-rich samples an increase in the reducibility of the main fraction of die Ni " ions was observed, with a behaviour similar to that of free NiO, while for Mg-rich samples the reduction of the Ni " " ions is further hindered by die formation of NiO/MgO solid solutions, with reduction temperatures very similar to those reprated for nickel-magnesia catalysts. 

With increase in the Co/Al atomic ratio (compare CatA and Cat C), an increase in the activity is observed which could be explained by the fact that the number of adsorption centers increases on the surface. Our XPS results also confirmed tiiat the Co/Al surface composition increases with increase in Uie bulk composition althouf the former is lower in value. The activity of the present samples are compared witii some of the most active catal3rsts reported in the literature [28]. Under our experimental conditions, Co-Al-HTs showed appreciable conversion even at 150 C and Cat C showed 100% conversion at 250°C which is 100°C less than the most active catalyst reported so far. Analysis of the spent catalysts by XRD showed non-stoichiometric spinel type oxides. The high activity for Co-Al-HTs can be explained based on the specific interaction between Co and Co ion with the support responsible for the generation of the active sites. However, a detailed study is required in understanding the nature of active centers. 

Supported copper-chromium oxide catalysts. The non-modified support after its thermal treatment at 773 K, if coated with a 7% (Cu+Cr) mixture, seems to contain a series of spinel-type phases on the base of the support and copper chromite structures as well. Then diffractogramms of the sample are characterized by distorted lines of the support only. After thermal treatment at 1273 K, there coexist a-Al203 with the increased cell parameter and aluminium-copper-chromium spinel with a = 8.098 A, which is typical for Cu(Ali 8Cro.2)04 composition. No lines of copper (+1), i.e. Cu2Cr204 and CU2AI2O4, are observed. 

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