X-ray amorphous material

Starting from an aqueous acidic Al3+ solution (for example an aluminium sulphate solution) precipitation occurs if the pH of the solution is increased above about pH = 3 by addition of a base. The first precipitate is a gel-like substance in which minute crystals of boehmite (A10(0H)) are present. If this is filtered without aging and then calcined at temperatures up to 600°C an X-ray amorphous material is obtained. The material remains amorphous until after firing to temperatures greater than 1100°C. (X-AI2O3 is formed at higher temperatures. 

However, as mentioned in the introduction, the main advantage of XAS compared to other structural techniques is its element specificity, its potential operation under more or less any experimental conditions, its appUcabiUty for amorphous and X-ray amorphous materials and its straightforward sample preparation. In the following, some applications of XAS in catalytic systems will be discussed. 

Ferric oxyhy dr oxide minerals or phases which occur naturally are listed in Table I, and except for akaganeite, their occurrences have been described by Palache et al. (3). The oxyhydroxides found as precipitates in natural waters are usually goethite and x-ray amorphous material. Amorphous material, which comprises a relatively large proportion of most fresh precipitates, is formed under conditions of substantial supersaturation with respect to the crystalline oxyhydroxides. An amorphous phase develops by rapid hydrolysis of dissolved ferric species, particularly at pH s below 4-5 where the total concentration of such species can exceed 0.01 ppm. Amorphous material is also produced during the rapid oxidation and hydrolysis of ferrous iron-rich solutions. 

Figure 4. Mid infrared spectra of several pentasils from Table I (A) Sample No. 8 (B) Sample No. 9 (C) Sample No. 10 and (D) Sample No. 10, seeded. (E) is the spectrum of an x-ray amorphous material obtained from a gel with x = 60.

Monolithic zirconia networks can also be formed using a similar procedure giving porous 2xQ>2 structures [9]. As the titania and zirconia precursors are miscible, binary inorganic networks of various Ti Zr ratios could be produced [9]. The crystallinity and photocatalytic properties of the mixed material were studied X-ray amorphous materials were produced for Ti Zr ratios of 2 8 to 7 3, and the binary material containing 10% zirconia (the presence of which inhibited crystal transformation to the rutile phase) showed the highest photocatalytic activity for the photodecomposition of sahcylic acid and 2-chlorophenol [9]. 

X-Ray amorphous materials (see Jacobs ideas), especially for very large molecules or where reactant or product shape selectivities are not required. 

Glasses have been defined as X-ray amorphous materials which exhibit the glass transition (7 ), this being a more or less abrupt change in the derivative thermodynamic properties with increasing temperature, such as change in heat capacity at constant pressure and thermal expansion coefficient, from crystal-Hke values to hquid-like values (see Figure 10.1) [ 1 ]. 

An important advantage of TGA is its sensitivity towards X-ray amorphous materials, such as C-S-H, M-S-H or AH3, such that TGA is often... 

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. 

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). 

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. 

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). 

DADB is usually obtained as an X-ray amorphous white powder, but reports of its presence as a microcrystalline material with a characteristic XRPD diffractogram also exist [8, 13]. It appears in two modifications. Modification I is stable at ambient temperature [13]. Modification II is only formed at higher temperatures [8] and changes at ambient temperature to modification I within a few days. DADB is stable up to 80 °C and decomposes at 90 °C with melting [4]. It is soluble in NH3 and is. 

Since the material obtained by ball-milling is X-ray amorphous, the identification as BH3NH2Li is not straightforward. It was eventually carried out by by Xiong et al. by applying high resolution powder X-ray diffraction analysis [125]. 

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