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The structure of amorphous precursor species formed under hydrothermal conditions, prior to the onset of crystallization of microporous aluminosilicate zeolites, is determined employing high-energy X-ray diffraction (HEXRD). The investigation, combined with the use of reverse Monte Carlo modelling suggests that even numbered rings, especially 4R (R: ring) and 6R, which are the dominant aluminosilicate rings in zeolite A, have already been produced in the precursor. The model implies that the formation of double 4Rs occurs at the final step of the crystallization of zeolite A.

The evolution of iron in over-exchanged Fe/ZSM5 prepared via chemical vapor deposition of FeCl 3 was studied at each stage of the synthesis. Different characterization techniques (EXAFS, HR-XANES, 57 Fe Mössbauer spectroscopy, 27 Al NMR, EELS, HR-TEM, XRD, N 2 physisorption, and FTIR spectroscopy) were applied in order to correlate the changes occurring in the local environment of the Fe atoms with migration and aggregation phenomena of iron at micro-and macroscopic scale. Mononuclear isolated Fe-species are formed upon FeCl 3 sublimation, which are transformed into binuclear Fe-complexes during washing. During calcination, iron detached from the Brønsted sites migrates to the external surface of the zeolite, finally leading to significant agglomeration. Nevertheless, agglomeration of Fe can be strongly suppressed by adequately tuning the conditions of the calcination.  2002 Elsevier Science (USA). All rights reserved.

Au/Fe 2 O 3 catalysts prepared using co-precipitation are described and discussed for the preferential oxidation of CO in the presence of H 2 , H 2 O and CO 2 . A catalyst prepared using a two-stage calcination procedure (400 uC followed by 550 uC) achieves target conversion and selectivity (.99.5% CO conversion and .50% selectivity, based on O 2 , for the competing conversion of H 2 with O 2 at 80-100 uC) for the competitive oxidation of dilute CO in the presence of moist excess H 2 and CO 2 . The effect of the preparation method of the uncalcined precursor is described and the effects of calcination on the catalyst activity in the absence of H 2 , CO 2 and H 2 O is initially explored. The catalysts are characterised in detail using electron microscopy (TEM), X-ray photoelectron spectroscopy and Mo ¨ssbauer spectroscopy. For the target conversion to be achieved, it is necessary that the activity for the reverse water gas shift activity (CO 2 + H 2 A CO + H 2 O) of the catalyst is suppressed, since under the fuel cell conditions this reaction reforms CO at high CO conversions due to the presence of excess CO 2 and H 2 . It is proposed that the two stage calcination procedure removes active sites for the water gas shift reaction whilst retaining active sites for preferential CO oxidation.

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Calcination of mixtures of (c-C 5 H 9 ) 7 Si 7 O 9 (OH) 3 , 1, and (c-C 5 H 9 ) 7 Si 7 O 12 Fe(tmeda), 2 (tmeda ) N,N,N′,N′tetramethylethylenediamine), led to microporous amorphous Fe-Si-O materials with adjustable iron content in the range 1-11 wt %. A set of different complementary techniques including N 2 physisorption, XRD, XPS, DRUV-vis, RS, IR, HRTEM, and Mo ¨ssbauer spectroscopy was used to follow the variation of the textural properties, metal dispersion, and speciation with the iron content along the whole mixing series. The calcination of these mixtures produced Fe-Si-O materials having basically the same properties as those observed for the individually calcined iron silsesquioxane. The N 2 physisorption indicates high surface areas, rather large pore volumes, and a very narrow pore size distribution with an average pore size diameter around 6-7 Å. The TEM and the spectroscopic analysis of the Fe-Si-O materials indicate that the iron is present mainly as small iron oxide particles highly dispersed throughout silica and to a minor extent as clustered and isolated species. The particle size distribution was estimated to be about 2-8 nm for 11% Fe-Si-O and 2-4 nm for samples with lower iron content. These materials showed catalytic activity in NH 3 oxidation and N 2 O decomposition.