Spinels high-surface-area

Troia A, Pavese M, Geobaldo F (2009) Sonochemical preparation of high surface area MgAl204 spinel. Ultrason Sonochem 16(1) 136—140... 

A clear advantage of alkaline electrolysers is the use of nickel-based electrodes, thus avoiding the use of precious metals. Catalytic research is aimed at the development of more active anodes and cathodes, primarily the development of high surface area, stable structures. Nickel-cobalt spinel electrodes for oxygen evolution and high surface area nickel and nickel cobalt electrodes for hydrogen evolution have been shown at the laboratory scale to lead to a decrease in electrolyzer cell voltage [47]. More active electrodes can lead to more compact electrolysers with lower overall systems cost. 

O2 and H2 dissociation kinetics are better at higher temperatures (>400 °C), low-cost electrode structures of high surface area Ni and oxides such as spinels or perovskites to replace the very effective, but costly, Pt catalysts have been sought. 

Various metal and metal oxide nanoparticles have been prepared on polymer (sacrificial) templates, with the polymers subsequently removed. Synthesis of nanoparticles inside mesoporus materials such as MCM-41 is an illustrative template synthesis route. In this method, ions adsorbed into the pores can subsequently be oxidized or reduced to nanoparticulate materials (oxides or metals). Such composite materials are particularly attractive as supported catalysts. A classical example of the technique is deposition of 10 nm particles of NiO inside the pore structure of MCM-41 by impregnating the mesoporus material with an aqueous solution of nickel citrate followed by calicination of the composite at 450°C in air [68]. Successful synthesis of nanosized perovskites (ABO3) and spinels (AB2O4), such as LaMnOs and CuMn204, of high surface area have been demonstrated using a porous silica template [69]. 

The difference patterns for deuterium absorbed on Co304 at room temperature (Figure 1) show axially symmetric 200 and 420 peaks which indicate that the deuterium atoms adsorb at Co3+ ions located in tetrahedral sites. The presence of such ions was unexpected in a normal spinel and the authors suggested that this may be a product of the special preparation required of Co304 to obtain a high surface area specimen. 

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). 

The physical properties of the spinels prepared by this method (Table I) are very different from those prepared by the co-condensation method. The spinels prepared by this method have a very high surface area (-150 m2/g). 

Co-gel formation. In addition to the co-precipitation of the two hydroxides we have found that a very homogeneous mixture of Mg2+ and Al3+ species can be obtained by co-gel formation (8). This co-gel, usually prepared by combining aqueous slurries of psuedoboehmite alumina, high surface area MgO, and an acid, is dried and calcined at 700 to 800°C to produce both stoichiometric and high magnesium spinels (reaction 4). 

High surface area oxides are attractive materials for numerous applications in catalysis and sorption [1], There are many techniques to manually prepare these materials, such as precipitation, sol-gel pathways, templating routes and so on [2,3,4,5]. We have developed a novel versatile route which offers a simple and straightforward manner to prepare a great variety of different oxides with even higher surface areas. This method avoids filtering steps and handling of suspensions which enables simple pipette robotic systems to prepare these materials. The method is suitable for the preparation of defined phases, such as spinels or perowskites, but also for the synthesis of amorphous or multiphase mixed metal oxides and can easily be parallelized. 

Hydrotalcite-like compounds (HTlcs) have attracted much attention in recent years as catalyst precursors and catalyst support. This is due to (i) their ability to accommodate a large variety of bivalent and trivalent cations (ii) the homogeneous mixture of the cations on an atomic scale and (iii) the formation of thermostable mixed oxides, often denoted as ex-HTlcs, with high surface area upon decomposition. The first two properties are a result of the precursor while the last property appears to be related to the decomposition mechanism. The transitions in the structural properties of Co-based hydrotalcites upon high temperature treatments have been extensively studied in our group.15,16 In the first decomposition step, water is removed from the structure. This transition is followed by dehy-droxylation and decarbonation, as well as carbonate reorganization in the interlayer. Thermal treatment in air finally leads to a solid solution of cobalt spinels (Co(Co, A1)2C>4). Mixtures of CoO and C0AI2O4 are formed upon treatment in inert. 

Eleven spinels were calcined in air at 800° and 900°C in order to reduce their surface areas. Two classes of spinels were defined (Tables I and II). Class I spinels generally had a high catalytic activity for oxidation of carbon monoxide and hydrocarbons, but they did not have or maintain a large surface area upon calcination. Class I spinels all had a surface area of less than 1 m2/g after firing at 900°C for 16 hrs. Class II spinels, on the other hand, had high surface area and stability, but they were very poor oxidation catalysts. 

A series of hydrotalcites of general formula Co -M -COg-HT (M " " = Al,Fe and Cr) are prepared by coprecipitation technique. The influence of parameters such as preparation method, atomic ratio, supersaturation levels, aging and hydrothermal treatments are investigated to study their effect on the structure and texture of these materials. The obtained materials are characterised by X-ray diffraction, FT-IR studies, thermogravimetry-differential scanning calorimetry, transmission electron microscopy and BET surface area measurements. Thermal calcination of these materials resulted in the formation of high surface area non-stoichiometric spinels whose catalytic activity is studied using N2O decomposition reaction as the test reaction. The order of activity observed is Co-Al-C03-HT>Co-Fe-C03-HT>Co-Cr-C03-HT. 

Ruhler et al. considered that Ba-Ru/MgO catalysts have high activities and stabilities. The researchers in Tops Company, Denmark developed series of ruthenium based ammonia synthesis catalysts support on different supports, with electronic and structural promoters. In these studies, the ruthenium catalysts supported on the Mg Al spinel and high surface area graphite show promising activity (see Table 6.6). However, the stability has some problems under industrial conditions. ... 

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