Copper aluminate

The replacement of vanadia-based catalysts in the reduction of NOx with ammonia is of interest due to the toxicity of vanadium. Tentative investigations on the use of noble metals in the NO + NH3 reaction have been nicely reviewed by Bosch and Janssen [85], More recently, Seker et al. [86] did not completely succeed on Pt/Al203 with a significant formation of N20 according to the temperature and the water composition. Moreover, 25 ppm S02 has a detrimental effect on the selectivity with selectivity towards the oxidation of NH3 into NO enhanced above 300°C. Supported copper-based catalysts have shown to exhibit excellent activity for NOx abatement. Recently Suarez et al and Blanco et al. [87,88] reported high performances of Cu0/Ni0-Al203 monolithic catalysts with NO/NOz = 1 at low temperature. Different oxidic copper species have been previously identified in those catalytic systems with Cu2+, copper aluminate and CuO species [89], Subsequent additions of Ni2+ in octahedral sites of subsurface layers induce a redistribution of Cu2+ with a surface copper enrichment. Such redistribution... 

Lv W, Luo Z, Yang H, Liu B, Weng W, Liu J (2010) Effect of processing conditions on sonochemical synthesis of nanosized copper aluminate powders. Ultrason Sonochem 17(2) 344—351... 

The interaction of cupric ions with alumina supports has subsequently been studied more extensively as a function of the support surface area, metal loading, and calcination temperature (93,279) by means of EXAFS and X-ray absorption-edge shifts, in conjunction with XRD, EPR, XPS, and optical reflectance spectroscopy. These techniques, each sensitive to certain structural and electronic aspects, allow a unified picture of the phases present and the cation site location. Four Cu2 + ion sites are distinguished in the catalysts. In low concentrations (typically below about 4 wt. % Cu/100 m2/g support surface area) Cu2 + ions enter the defect spinel lattice of the A1203 support. The well-dispersed surface copper aluminate has Cu2+ ions predominantly occupying tetragonally (Jahn-Teller) distorted octahedral sites, although... 

E24.24 The Cu site in Egyptian blue is square planar and thus has a centre of symmetry (see Fig. 24.63). The d-d transitions that give rise to the colour are, consequently, symmetry-forbidden (Laporte rule) and less intense. In copper aluminate spinel blue, the site is tetrahedral with no centre of symmetry and the transitions are not symmetry-forbidden. This leads to the increased intensity of blue colour in the spinel. 

The stage of thermal treatment involves the formation of C12A7, y-alumina, and solid solution of aluminum and copper oxides, which is followed by the precipitation of the excess of highly dispersed copper oxide and by the formation of copper aluminate spinels with various degrees of disorder. 

With copper oxide, a mixed phase was also obtained a poorly crystallized copper aluminate containing a large excess of alumina. This structure was stable at 900°C, but between 900° and 1000°C stoichiometric copper aluminate was formed and excess alumina was rejected from the structure as -Al203. A2 and A3 carriers could not withstand this transformation types Cl and D were less degraded. 

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. 

The dependence of the activity from the nature of support indicates a strong interaction between copper and oxide support this latter playing a role during the molecular H2 activation. This is in agreement with the literature concerning the interaction of H2 with copper chromite and copper aluminate (ref. 13,14). 

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