X-ray amorphous

Zeolites and Catalytic Cracking. The best-understood metal oxide catalysts are zeoHtes, ie, crystalline aluminosihcates (77—79). The zeoHtes are well understood because they have much more nearly uniform compositions and stmctures than amorphous metal oxides such as siUca and alumina. Here the usage of amorphous refers to results of x-ray diffraction experiments the crystaUites of a metal oxide such as y-Al202 that constitute the microparticles are usually so small that sharp x-ray diffraction patterns are not measured consequendy the soHds are said to be x-ray amorphous or simply amorphous. 

Considerably complicated realizes ablation of water from Zn Co j P O -H O. Heating of it to 603 K is accompanied with practically full destaiction of diphosphate stmcture. In composition of X-ray amorphous solid phase take place the processes of anion condensation. On their realization indicates formation of triphosphate with lineai anion stmcture (5,6 mas.% in count on P,0 ) in composition of burning products. 

X-ray amorphous clays + H2Si04 + cations + HCOf -> cation-rich aluminosilicates + CO2 + H2O... 

Attempts to electrodeposit M0S2 in a way similar to that used for MoSe2 - in this case from solutions of molybdate and thiosulfate under various conditions of pH and temperature - have been unsuccessful. Instead, thin films of M0S2 were convenienfly deposited from tetrathiomolybdate solutions Ponomarev et al. [149] observed that reduction of MoO in slightly alkaline solutions results in the formation of an X-ray amorphous film which by annealing at 550 in Ar for 1 h... 

Liu S, Huang W, Chen S et al (2001) Synthesis of X-ray amorphous silver nanoparticles by the pulse sonoelectrochemical method. J Non-Cryst solids 283 231-236... 

Zeolites possess the remarkable property of exhibiting shape-selective catalysis even when they are X-ray amorphous. Clearly, even though there is no long range order, there is still a degree of structural organization in the aluminosilicate adequate to exert shape-selectivity in the "noncrystalline" regions of the samples. Thanks to HREM we can now understand how this state of affairs arises (17). 

Figure 1. High-resolution electron micrograph and corresponding optical transform (inset) of an x-ray amorphous zeolite-Y specimen that has undergone ion-exchange with a solution containing U022+ ions. The microcrystalline regions are rendered visible by the locally ordered U022+ ions. ( See text.)...

Figure 2. These high-resolution micrographs show how a so-called x-ray amorphous, nonstoichiometric molybdenum sulfide catalyst exhibits structural (as well as compositional) heterogeneity. Amorphous, quasi-crystalline, and crystalline regions coexist at the ultramicro level (18,).

Although a chemical substance with the PON formulation was reported as early as 1846 (186), this phosphorus oxynitride was not well known for a long time. That is essentially due to some difficulty in its preparation and to the X-ray amorphous character of the commonly obtained powders. 

The resulting product has a low apparent density. Other synthesis techniques have been reported (192), mainly the reaction of a stoichiometric mixture of P3N5 and P4O10, heated at 780°C for 48 h, which directly results in a crystalline powder (193). Crystallization of X-ray amorphous PON produced using the other preparation methods is obtained after heating the powder in an evacuated quartz ampoule at 700-800°C for several days. 

High crystallization rates and the possibility to stabilize X-ray amorphous phases, which exhibit ZSM-5 like properties, were among the reasons why we decided to investigate the procedure B in more detail. In order to optimize the particle size, homogeneity, morphology and composition, we have questioned more systematically the influence of secondary synthesis variables such as the pH, solvent viscosity or the nature of the alkali cation, added as chloride. 

Preparation of X-ray amorphous ZSM-5 crystallites according to procedure BT It is important that the gel formation takes place as homogeneously as possible. Because of the particular sensitivity of various silica and alumina species to the pH (63,64), the pH range between 4.5 and 8.5 was avoided. Nucleation was performed at pH 3.5-4, where a low viscous gel containing essentially monomeric silica species is rapidly formed (65).The, pH is theii raised to about 9, in order to form tetrahedral A1(0H) entities and to favour the further A1 incorporation within the zeolitic framework. NaCl was used to increase the (super)saturation of the gel, which will flocculate into a macromolecular colloid (V) and initiate the nucleation. This procedure yields 100 % crystalline zeolite after... 

The 500 nm size is a limit value crystallites below this size tend to broaden the diffraction peaks in a spectrum, while size distributions above this value produce particularly sharp signals whose half width is a function only of the wavelength of the X-ray beam and the equipment. Signal broadening is at its maximum in materials known as X-ray amorphous substances, featuring particle size distributions below 8 nm. These afford flattened, washed-out spectra of little analytical value. 

Especially methods of electron microscopy are important at study of X-ray amorphous substances and polyphase nanomixtures which are distributed very widely in the nature such as agate, bauxite, bitumen, coal, natural glasses etc., as X-ray diffraction is almost useless at analyzing such mostly disordered materials. 

The AI2O3 films obtained by CVD of [(tBuO)2AlH]2 at 300-400 °C on metal targets are transparent, X-ray amorphous and show no major contamination by other elements (measured Al O = 2.0 2.98(0.05)). When tempered at 600 °C or 1000 °C under aerobic conditions, the layers become crystalline and consist of a mixture of aluminum oxide phases, y-Al203 being the major one at low temperature and o -Al203 at high temperature. 

The solid phase resulting from this reaction typically forms a white precipitate, which on account of its low density often forms impressive masses of white foam where the water, from which it is precipitating, is subjected to turbulent eddies. Generally, this white precipitate is X-ray amorphous, although if it is allowed to settle for periods in excess of six months, crystalline forms of Al(OH)3, most notably gibbsite, are... 

Although a number of secondary minerals have been predicted to form in weathered CCB materials, few have been positively identified by physical characterization methods. Secondary phases in CCB materials may be difficult or impossible to characterize due to their low abundance and small particle size. Conventional mineral identification methods such as X-ray diffraction (XRD) analysis fail to identify secondary phases that are less than 1-5% by weight of the CCB or are X-ray amorphous. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), coupled with energy dispersive spectroscopy (EDS), can often identify phases not seen by XRD. Additional analytical methods used to characterize trace secondary phases include infrared (IR) spectroscopy, electron microprobe (EMP) analysis, differential thermal analysis (DTA), and various synchrotron radiation techniques (e.g., micro-XRD, X-ray absorption near-eidge spectroscopy [XANES], X-ray absorption fine-structure [XAFSJ). 

The most serious limitation of XRD as a catalyst characterization method is often related to the fact that many of the phases present in a catalyst may not give rise to any well-defined diffraction line at all. Absence of a diffraction pattern is a consequence of the requirement that a structure must contain a periodicity extending more than about 2-3 nm to yield a diffraction pattern measurable in a sense of the Bragg equation [Eq. (1)]. Thus, particles or domains with sizes smaller than 2-3 nm will appear to be X-ray amorphous in XRD experiments i.e., they do not exhibit sharp diffraction lines. 

Fig. 1. Schematic representation of different size distributions of structures present in typical catalysts. The X-ray amorphous region is indicated by the shaded area.

At the present time much effort is being devoted to tailor-making of new nanomaterials with specific catalytic properties. In this quest for constantly decreasing the dimensions of the catalytically active components, one will unavoidably encounter materials that will be partly or completely X-ray amorphous. The present review has shown that the combined EXAFS/ XRD techniques are uniquely well suited for providing the necessary structural understanding. Thus, in view of the trend in catalyst technologies and advances in technique developments, the application of the combined techniques will no doubt play an increasing role in future catalyst characterization efforts. We now briefly discuss some likely applications and technique developments which involve the X-ray techniques discussed presently. 

Transparent red iron oxides containing iron oxide hydrate can also be produced directly by precipitation. A hematite content of > 85 % can be obtained when iron(II) hydroxide or iron(II) carbonate is precipitated from iron(II) salt solutions at ca. 30 °C and when oxidation is carried out to completion with aeration and seeding additives (e.g., chlorides of magnesium, calcium, or aluminum) [5.271], Transparent iron oxides can also be synthesized by heating finely atomized liquid pentacarbonyl iron in the presence of excess air at 580-800 °C [5.272], [5.273]. The products have a primary particle size of ca. 10 nm, are X-ray amorphous, and have an isometric particle form. Hues ranging from red to orange can be obtained with this procedure, however, it is not suitable for yellow hues. 

Therefore, the mesostructured tungsten sulfides MTS-W, MTS-M and MTS-C must consist of organic templates intercalated between condensed inorganic walls made up of layered WS2 and chain-like WS3. On the other hand. The product prepared at room temperature in aqueous solution (i.e. MTS-RT) mainly contains organic templates and discrete WS42 clusters. It should be noted that WS3is X-ray amorphous. Its presence in the mesostructured materials was further confirmed by the elemental analysis results of the products (Table 1). 

Under the optimized conditions given in the experimental section the reaction products from TEOS consist exclusively of nanotubes as deduced from TEM-micrographs (Fig. 1). The length of the tubes varies between 50 nm and about 4 pm and the inner diameter range from 10 nm up to 300 nm with a maximum of frequency around 50 nm. The silica walls are X-ray amorphous, and their thickness is about 30 nm. 

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