Atomic dispersion

Pumping of liquids. Compression of gases Mixing (solids, liquids, gases possibly multiphase) Atomization, dispersion... 

Powerforming is basically a conversion process in which catalytically promoted chemical reactions convert low octane feed components into high octane products. The key to a good reforming process is a highly selective dual-function catalyst. The dual nature of this catalyst relates to the two separate catalyst functions atomically dispersed platinum to provide... 

Silver vapor cocondensed with matrices of HjO or paraffin wax (C22H4 ) at 12 K gives mainly an atomic dispersion of Ag. However when these Ag atom matrices are warmed briefly (to 77 K for HjO and up to 80 K for C22H46), thermal diffusion takes place with aggregation of the Ag atoms into small clusters of up to Ag4. These thermal aggregation methods can be used to prepare small clusters, but a mixture of metal polymers is usually obtained. 

The processes that govern the formation of ash particles are complex and only partially understood (Figure 7.12). The mineral matter in pulverized coal is distributed in various forms some is essentially carbon-free and is designated as extraneous some occurs as mineral inclusions, typically 2-5 pm in size, dispersed in the coal matrix and some is atomically dispersed in the coal either as cations on carboxylic acid side chains or in porphyrin-type stmctures. The behavior of the mineral matter during combustion depends strongly on the chemical and physical state of the mineral inclusions. 

In both cases, the Au nanoparticles behave as molecular crystals in respect that they can be dissolved, precipitated, and redispersed in solvents without change in properties. The first method is based on a reduction process carried out in an inverse micelle system. The second synthetic route involves vaporization of a metal under vacuum and co-deposition of the atoms with the vapors of a solvent on the walls of a reactor cooled to liquid nitrogen temperature (77 K). Nucleation and growth of the nanoparticles take place during the warm-up stage. This procedure is known as the solvated metal atom dispersion (SMAD) method. 

The increasing volume of chemical production, insufficient capacity and high price of olefins stimulate the rising trend in the innovation of current processes. High attention has been devoted to the direct ammoxidation of propane to acrylonitrile. A number of mixed oxide catalysts were investigated in propane ammoxidation [1]. However, up to now no catalytic system achieved reaction parameters suitable for commercial application. Nowadays the attention in the field of activation and conversion of paraffins is turned to catalytic systems where atomically dispersed metal ions are responsible for the activity of the catalysts. Ones of appropriate candidates are Fe-zeolites. Very recently, an activity of Fe-silicalite in the ammoxidation of propane was reported [2, 3]. This catalytic system exhibited relatively low yield (maximally 10% for propane to acrylonitrile). Despite the low performance, Fe-silicalites are one of the few zeolitic systems, which reveal some catalytic activity in propane ammoxidation, and therefore, we believe that it has a potential to be improved. Up to this day, investigation of Fe-silicalite and Fe-MFI catalysts in the propane ammoxidation were only reported in the literature. In this study, we compare the catalytic activity of Fe-silicalite and Fe-MTW zeolites in direct ammoxidation of propane to acrylonitrile. 

Color can also be induced into colorless crystals by the incorporation of impurity atoms. The mineral corundum, 01-AI2O3, is a colorless solid. Rubies are crystals of A1203 containing atomically dispersed traces of Cr203 impurity. The formula of the crystal can be written (CrvAli r)203. In the solid the Al3+ and Cr3+ cations randomly occupy sites between the oxygen ions, so that the Cr3+ cations are impurity substitutional, CrA1, defects. When x takes very small values close to 0.005, the crystal is colored a rich ruby red. 

The fact that evaporated potassium arrives at the surface as a neutral atom, whereas in real life it is applied as KOH, is not a real drawback, because atomically dispersed potassium is almost a K+ ion. The reason is that alkali metals have a low ionization potential (see Table A.3). Consequently, they tend to charge positively on many metal surfaces, as explained in the Appendix. A density-of-state calculation of a potassium atom adsorbed on the model metal jellium (see Appendix) reveals that the 4s orbital of adsorbed K, occupied with one electron in the free atom, falls largely above the Fermi level of the metal, such that it is about 80% empty. Thus adsorbed potassium is present as K, with 8close to one [35]. Calculations with a more realistic substrate such as nickel show a similar result. The K 4s orbital shifts largely above the Fermi level of the substrate and potassium becomes positive [36], Table 9.2 shows the charge of K on several metals. 

Atom atoms dispersion low very low very soft formed between atoms with no elecbronegativity difference all Group 18 (VIIIA) atoms... 

Characterization techniques continue to develop and will impact their application to zeolitic systems. Aberration corrected electron microscopes are now being used to improve our understanding of catalysts and other nano-materials and will do the same for zeolites. For example, individual Pt atoms dispersed on a catalyst support are now able to be imaged in the STEM mode [252]. The application of this technique for imaging the location of rare-earth or other high atomic number cations in a zeolite would be expected to follow. Combining this with tomography... 

Figure 4.3 Different configurations of surface hydroxyl groups produce different, atomically dispersed surface bound metal species.

Atomically Dispersed Titanium and Vanadium, Single Site Catalysts... 

Figure 4.12 V SS NMR (MAS) for three atomically dispersed vanadyl containing Si8O20-building block matrices, (a) Embedded (3-connected) vanadyl (b) mainly 2-connected vanadyl (c) surface (1-connected) vanadyl moieties.

More pragmatically, one can attempt to address the question of the benefits of nanostructured catalysts by comparison against the properties of similar catalysts prepared by traditional methods. There are a couple of well-known reactions that are thought to be catalyzed by atomically dispersed metals on supports [95]. Epoxi-dation of olefins by titanium on silica is one [96, 97]. 

We have also investigated the properties of several of our nanostructured catalysts as solid acids in reactions such as the dehydration of alcohols and transesterification reactions [99]. One of the best examples of atomically dispersed solid acid catalysts is aluminosilicates [100]. When aluminium is substituted into silicate frameworks and remains isolated from other A1 centers it can behave as a strong acid site [101]. 

Figure 4.15 Light-off curve for the dehydration of 2-propanol by a Si8O20-building block catalyst initially containing 3-connected, atomically dispersed aluminium atoms. Conditions 55 mg catalyst, 95ccmin total flow across catalyst, WHSV 0.4 h. ...

Takahashi et al. [25] reported that the dispersed tetravalent vanadium (l 7/2) showed a hyperfine structure but broad band could be observed in the agglomerated vanadium. Miyamoto et al. [8] and Jhung et al. [7] reported that EPR spectra of VAPO -S showed hyperfine structure. Miyamoto et al. [8] suggested that the hyperfine structure indicated atomically dispersion of vanadium in VAPO -S molecular sieve, in other words, vanadium was substituted in the framework of AIPO -S. 

Copper ions are supported with atomic dispersion due to the ion-exchange properties of zeolites and are difficult to collect owing to the framework structure of zeolite. 

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