Oxide crystallite

Andreeva and coworkers518 improved on the preparation of Au/a-Fe203 by using a deposition-precipitation method. The preparation involved deposition of Au hydroxide with a solution of sodium carbonate at 60 °C and pH = 8.0 on a fresh precipitate of iron hydroxide. The precipitate was aged for 1 hour at 60 °C, filtered, washed, dried under vacuum at 80 °C, and calcined in air at 400 °C for 2 hours. A comparison of properties and activities is provided in Table 122. The iron oxide crystallites were about 10 nm in both samples investigated. 

The XRD powder patterns of V-containing silicalite samples indicate in all cases the presence of only a pentasyl-type framework structure with monoclinic lattice symmetry, characteristic of silicalite-1 no evidence was found for the presence of vanadium oxide crystallites. The analysis of cell parameters of VSU545 does not indicate significant modifications with respect to those found for pure silicalite-1. This is in agreement with that expected on the basis of the small amount of V atoms present in V-containing silicalite. 

Metal-oxide catalysts and support suffer a decrease in the surface area and porosity upon exposure to high temperatures due to the coalescence and growth of the bulk oxide crystallites. 

It may be noted that an oxide crystallite wets a metal substrate better than the metal crystallite wets the oxide substrate. This happens because acg is... 

Fig. 11. A view of copper and zinc oxide crystallites with dispersed copper ions in the binary Cu/ZnO catalyst = 30/70 derived from diffraction and characteristic X-ray emission analysis in TEM and STEM.

In Ni/Al203 catalysts prepared by impregnation there is a weaker interaction between the impregnating salt and the support than there is when the catalyst is prepared by deposition-precipitation. Calcination of the former catalysts gave large nickel oxide crystallites and, on reduction, the supported metal was rather easily sintered. Catalysts prepared by precipitation-deposition, on the other hand, were resistant to sintering because of the strong interaction... 

Pt/Al203, Pt/Si02 O2,573-873°C H2,02 redox, TPR Pt oxidation limited to surface layers for all crystallite sizes redn. of highly-dispersed oxidized crystallites difficult due to lack of metal sites for H, dissociation. i 7... 

From the infrared spectroscopy of adsorbed CO it appears that aging treatments, as low as 923 K, lead to a migration of the active isolated copper ions to inaccessible sites. In these conditions an agglomeration is not detected but, after aging at 1173 K, an agglomeration is evidenced both by XRD and by the infrared bands of CO adsorbed on partially reduced bulk CuO oxide. These accessible copper oxide crystallites are probably located at the external surface of the zeolite and are inactive. In fact, the activity remains correlated to the number of Cu VCu isolated ions deduced from the infrared spectra of adsorbed CO and located in the zeolite structure. This correlation holds whatever the treatment and whatever the Si/Al ratio (Table 4). 

When a modification either during the synthesis or by post synthesis procedure has been performed the first question which everyone is asking himself is to determine if the modifier element has been incorporated in the lattice framework at the lattice position or at cationic sites location (exchangeable sites for instance) or at last entrapped in the cavities or pores as tiny clusters or metallic oxide particles or even metallic oxide crystallites as impregnated or deposited near the zeolitic crystallites. 

Pure CaO and Ce02 have rather low oxygen-ion conductivities. However, this conductivity notably increased first and then decreased as the substitution of Ce by Ca increased, having a maximum around the solubility limit. As the Ca content exceeded this limit, the conductivity decreased gradually due to the formation of additional calcium oxide crystallites. The dependence of selectivity to C2 for OCM reaction on the Ca content... 

Rebours, B., D-espinose Dc La Caillcric. J-B. and Clause, O. (1994). Decoration of nickel and magnesium oxide crystallites with spinel-type phases. J. Amer. Chem. Soc.. 116, 1707-1717. 

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