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We report here a simple one-pot directed synthesis of an oxomolybdate urea composite in which elementary molybdenum oxide building blocks are linked together with the aid of urea. This type of directed material design resulted in large rod-like crystals of an inorganic-organic hybrid extended structure of {MoO3(NH2-CO-NH2)}∞ consisting of right- and left-handed helical units. In the crystal structure urea acts both as a glue that links the inorganic molybdenum units into a helix and as a supramolecular linker for the stabilization of the crystal structure as a whole. This type of molecular topology resulted in an unexpectedly high thermal stability.

The valence and local symmetry of iron in framework-substituted FeZSM-5 with a high Fe dilution (Si/Fe = 360) was studied by means of Kbeta-detected X-ray absorption spectroscopy. This technique combines high-resolution (DeltaE similar to1 eV) fluorescence detection of the 3p to 1s (Kbeta) transition with the X-ray absorption near-edge structure (XANES) at the Fe K-edge. An absorption-like spectrum is recorded by detecting the Kbeta fluorescence intensity as a function of the incident energy that is scanned through the K absorption edge. Kbeta-detected XANES spectra allow for a more precise separation of the weak K pre-edge structure from the main edge as compared to conventional absorption spectroscopy. Subsequent analysis and interpretation of the pre-edge spectral features therefore is more accurate. The pre-edge is sensitive to changes in the local coordination and oxidation state of Fe. Using this technique we were able to quantitatively determine the degree of iron extraction out of a zeolite framework upon steaming. With the use of appropriate reference compounds, the pre-edge analysis was used to monitor the activation of low-loaded, framework-substituted FeZSM-5 (0.3 wt % Fe). Template removal and calcination distort the zeolite framework and induce a deviation from T-d symmetry for incorporated iron. The (deliberate) presence of water at high temperature (T > 500 degreesC) facilitates the hydrolysis of the Si-O-Fe bonds and increases the formation of extraframework iron species. The amount of Fe-III occupying tetrahedral sites in the MFI-type zeolite decreases to 32% and 19%, respectively, for mild- and hard-steamed samples.

The homogeneity of typical electrolessly plated Pd/Ag membranes for H2 separation was investigated with XAFS and XPS depth profiling. Next the crucial mixing of the separate Pd and Ag layers, obtained after the two sequential plating steps, was followed in situ with XRD. The results show that some low temperature alloying procedures found in the field are insufficient for obtaining a well-mixed Pd/Ag alloy membrane. Finally the Pd/Ag-H XANES was determined for the first time. In contrast to the Pd-H and Pd/Ag systems, shifts in electron density from the Ag or H atoms to the Pd could not satisfactorily explain the observed XANES. Not only is the intensity of the whiteline higher than expected, the shape and the intensity of the anti-bonding Pd 4d H 1s feature have also significantly changed. Local geometry is expected to account for these deviations.

Synchrotron small-angle x-ray diffraction images of random-stacking-induced Bragg scattering rods are obtained in a wide range of wave vectors from a single colloidal crystal. The results reveal a strong multiple scattering effect, which leads to new features in the diffraction pattern-secondary Bragg rods. We argue that dynamic x-ray diffraction is rather common for high-quality colloidal photonic crystals and should be taken into account.

Postsynthesis ammonium treatment induces a substantial increase in the catalytic activity of Au/Ti- SBA-15 catalysts for the direct vapor-phase epoxidation of propylene using hydrogen and oxygen. The PO formation rate of a calcined Au/Ti-SBA-15 catalyst prepared by this method increased from 4.3 mgPO h−1 g−1 cat to 37.2 mgPO h−1 g−1 cat at 200 ◦C compared with a catalyst prepared in an identical manner without this treatment. The catalysts were characterized by XRD, N2-sorption, UV–vis–NIR DRS, 29Si MAS NMR, FT-IR spectroscopy, and TEM to gain insight into the relationship between ammonia treatment and catalyst activity. 29Si MAS NMR measurements proved that the ammonium nitrate solution caused hydrolysis of ≡Si–O–Si≡ or ≡Ti–O–Si≡ bonds, resulting in a Ti-SBA-15 surface with a greater amount of surface hydroxyl groups. FT-IR measurements indicated the presence of amine species that favor the homogeneous deposition of Au nanoparticles. This was confirmed by TEM measurements showing greater metal dispersion for NH4NO3-treated Au/Ti-SBA-15 materials.

The acidity of catalytic systems based on tungsten oxide or niobium oxide supported on titania was compared. Two series with metal contents up to 3.6 atom nm � 2 were prepared by incipient wetness impregnation of the titania support with ammonium metatungstate or niobium oxalate solutions. Characterization of both systems by X-ray diffraction and Raman spectroscopy studies did not show evidence of bulk metal oxide formation. The acidity was monitored by adsorption of 2,6-dimethylpyridine (2,6-lutidine) followed by infrared spectroscopy. The catalytic activity was tested for the reaction of isopropanol dehydration. At a reaction temperature of 403 K, WOx/TiO2 catalysts were inactive for a surface density of W � 1.2 W atom nm � 2 . Above this loading, the activity increased progressively with increasing W content. Similar evolution was observed for the abundance of relatively strong Bronsted acid sites (i.e. able to retain lutidine at 573 K). In contrast, NbOx/TiO2 catalysts were essentially inactive at this reaction temperature and a higher reaction temperature (473 K) was required to reach a comparable catalytic activity. No threshold of Nb loading for the development of catalytic activity was observed. Similar behavior was evidenced for the abundance of medium strength Bronsted acid sites (able to retain lutidine at 523 K). For both systems, a direct correlation between the catalytic activity and the abundance of Bronsted acid sites was observed. # 2005 Elsevier B.V. All rights reserved.

The homogeneity of typical electrolessly plated Pd/Ag membranes for H2 separation was investigated with XAFS and XPS depth profiling. Next the crucial mixing of the separate Pd and Ag layers, obtained after the two sequential plating steps, was followed in situ with XRD. The results show that some low temperature alloying procedures found in the field are insufficient for obtaining a well-mixed Pd/Ag alloy membrane. Finally the Pd/Ag-H XANES was determined for the first time. In contrast to the Pd-H and Pd/Ag systems, shifts in electron density from the Ag or H atoms to the Pd could not satisfactorily explain the observed XANES. Not only is the intensity of the whiteline higher than expected, the shape and the intensity of the anti-bonding Pd 4d H 1s feature have also significantly changed. Local geometry is expected to account for these deviations.

The valence and local symmetry of iron in framework-substituted FeZSM-5 with a high Fe dilution (Si/Fe = 360) was studied by means of Kbeta-detected X-ray absorption spectroscopy. This technique combines high-resolution (DeltaE similar to1 eV) fluorescence detection of the 3p to 1s (Kbeta) transition with the X-ray absorption near-edge structure (XANES) at the Fe K-edge. An absorption-like spectrum is recorded by detecting the Kbeta fluorescence intensity as a function of the incident energy that is scanned through the K absorption edge. Kbeta-detected XANES spectra allow for a more precise separation of the weak K pre-edge structure from the main edge as compared to conventional absorption spectroscopy. Subsequent analysis and interpretation of the pre-edge spectral features therefore is more accurate. The pre-edge is sensitive to changes in the local coordination and oxidation state of Fe. Using this technique we were able to quantitatively determine the degree of iron extraction out of a zeolite framework upon steaming. With the use of appropriate reference compounds, the pre-edge analysis was used to monitor the activation of low-loaded, framework-substituted FeZSM-5 (0.3 wt % Fe). Template removal and calcination distort the zeolite framework and induce a deviation from T-d symmetry for incorporated iron. The (deliberate) presence of water at high temperature (T > 500 degreesC) facilitates the hydrolysis of the Si-O-Fe bonds and increases the formation of extraframework iron species. The amount of Fe-III occupying tetrahedral sites in the MFI-type zeolite decreases to 32% and 19%, respectively, for mild- and hard-steamed samples.