Magnesium alumina spinel

Soot precursors and residual methane are converted by steam reforming and shift reactions in the catalyst bed. These reactions are in equilibrium in the gas leaving the catalyst bed and the ATR reactor. The catalyst size and shape is optimised to have sufficient activity and low pressure drop to achieve a compact reactor design. The catalyst must be able to withstand the high temperature without excessive sintering or weakening and it should not contain components volatile at the extreme conditions. A catalyst with nickel on magnesium alumina spinel has proven to fulfil these requirements [107] [111]. 

Catalysts used for steam reforming are oftenbased on Ni/NiO or Co formulations supported on materials such as magnesium alumina spinel (Rostrap-Nielsen, 1993). These compositions can be combined with alkah or alkali-earth compounds to reduce methanation and facilitate coke gasification to avoid carbon deposition. 

Hetero-phase additions can influence both the transition alumina sequence and the temperature at which alpha alumina forms. For example, magnesium oxide additions introduced via addition of a magnesium nitrate salt to the boehmite precursor sol induces the formation of eta alumina after heating to 500°C rather than gamma alumina. This eta phase has a very diffuse X-ray diffraction patem at 500°C which sharpens with heating to 1000°C. The magnesium alumina spinel precipitates as a distinct fine crystal upon finther heating. 

The various support materials have different effects on potential carbon formation. This seems to go in parallel with the Lewis/Bronsted acidity. The main commercially used catalyst supports can be ranked as follows in decreasing order of carbon forming tendency (and thus in decreasing order of the important minimum practical steam/ carbon ratio) a-alumina > magnesium aluminate (spinel) > calcium aluminate > alkalized calcium aluminate [419],... 

Most steam-reforming catalysts are based on nickel as the active material. Also, cobalt and noble metals catalyze the steam-reforming reaction, but they are generally too expensive to find widespread use. A number of different carriers including alumina, magnesium-aluminum spinel, zirconia, and calcium aluminate are employed. 

Sutorik AC, Gilde G, Swab JJ, Cooper C, Gamble R, Shanholtz E (2012) Transparent solid solution magnesium aluminate spinel polycrystalline ceramic with the alumina-rich composition MgO-1.2 AI2O3. J Am Ceram Soc 95 636-643... 

They reported similar findings to those of Bodrov et al (1967) including the attainment of a steady state after 40 hours and they represented the kinetic data by similar expressions. Xu and Froment (1989) studied the steam reforming of methane over a crushed nickel-magnesium-alumina catalyst containing 15% nickel in a reactor of 10.7mm diameter at temperatures between 500 and 575°C and pressures between 5 and 15 bar. Soliman et al (1992) studied steam reforming over a nickel-calcium aluminate-spinel catalyst in a commercial microreactor 6mm in diameter at temperatures from 475 to 550 C and pressures from 2 to 4 atmospheres. 

Kazakov AI, Rubtsov Yul, Lempert DB, Manelis GB (2003) Kinetics of oxidation of organic acids by ammonium nitrate. Russ J Appl Chem 76 1214-1220 Behera SK, Barpanda R Prathar SK, Bhattacharyya S (2004) Synthesis of magnesium-aluminium spinel from autoignition of citrate-nitrate gel. Mater Lett 58 1451-1455 Chandradass J, Kim K, Ki H (2004) Effect of activity on the citrate-nitrate combustion synthesis of alumina-zirconia composite powder. Metals Mater Int 15 2039-2043 Purohit RD, Saha S, Tyagi AK (2006) Nanocrystalline ceria powders through citrate-nitrate combustion. J Nanosci Nanotech 6 209-214... 

Both structure and composition affect phonon conductivity in singlephase crystalline ceramics. Complex structures scatter lattice waves to a greater extent. Hence, thermal conductivity is lower in those structures. For example, magnesium aluminate spinel, consisting of two oxides, has a lower thermal conductivity than the single oxides alumina or magnesia. 

At high temperatures the spinel MgAl204 can take in excess alumina to a composition of approximately 70 mol% A1203 (Fig. 4.5). (a) What are the possible formulas that fit the composition of this spinel Write the defect formation equation for the reaction if the excess A1 is (b) distributed over both magnesium and aluminum sites and (c) only over aluminum sites. Assume that there is no electronic compensation in the insulating oxide. 

There is no clear evidence to identify the active material for SO2 removal in a MgAl20 stoichiometric system. Figure 13 shows results for a 50-50 mole% magnesia-alumina material prepared from magnesium hydroxide and alumina sol and calcined at various temperatures. An attempt was made to correlate SO2 removal with compound formation, as measured by X-ray diffraction, and surface area. As indicated in the figure, SO2 removal ability decreased with Increasing calcination temperature as did surface area. X-ray diffraction analysis showed spinel formation increases as... 

The chapters in Characterization and Catalyst Development An Interactive Approach, assembled from both academic and industrial contributors, give a unique perspective on catalyst development Some chapters thoroughly characterize the catalyst prior to plant evaluation, whereas others utilize characterization to explain performance variances. Some new types of catalysts incorporated into this volume include the preparation of novel catalyst supports based on alumina and hydrous titanates. Attrition-resistant catalysts and ultrafine ceramics were prepared by modified spray-drying methods. New catalyst compositions based on vanadium-containing anionic clays were proposed for oxidation. A recently commercialized catalyst based on magnesium spinel was proposed for use in the abatement of sulfur oxide pollutants in fluid... 

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). 

There are two other variants of the structure we should mention, the P "- and )8 -aluminas. These have spinel sheets 6 oxygen layers thick, as shown in Figure 47 (i). They are related one to another as are P- and jS"-alumina, that is, /S "-alumina is hexagonal while j5""-alumina is rhombohedral. The structure of P " has been determined and it has an idealized formula of NaMg2Ali5025. The structure is analogous to j8-alumina in that all of the alkali-metal atoms are to be found in partly occupied planes lying between the spinel sheets. These latter contain the aluminium and magnesium in octahedral and tetrahedral positions, as in spinel itself. These forms of jS-alumina are also not found in the ternary Na-Al-0 system but are formed in a number of quaternary systems, notably Mg-Na-Al-0. 

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. 

Figure 8.25 Growth of a layer of spinel (MgAl204) between crystals of magnesium oxide (MgO) and alumina (AI2O3) (a) MgO and AI2O3 crystals in contact (b) separation of the reacting oxides by a layer of spinel...

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