Shell oxide

Tubercles consisted of hard, hlack oxide shells overlaid with friable carbonate-containing deposits. In places, several laminate black magnetite shells existed. The outer crust could be crushed by gentle pressure with a finger. Tubercles were riddled with white crystalline fibers. Other detritus was incorporated into the tubercle core and crust. Metal loss was less than 0.030 in. (0.076 cm) below each tubercle. Wall thickness was almost 0.25 in. (0.64 cm). 

Propanediol is produced either from the reductive hydration of acrolein (Degussa-DuPont process), or through reductive carbonylation of ethylene oxide (Shell process), or through fermentation of glucose via glycerol (DuPont-Genencor process). 

Besides the complex cases of mixed oxides, there exist more simple problems of oxide and scale formation in alloy production. The detrimental effect of oxide shells around metal particles preventing intermixing is well known. The compositional changes resulting from preferential oxidation of one component have also to be taken into account. Instability of the product and/or drastic changes in the thermochemical properties of the material after shell formation (such as massive increases in the required fusion temperature in noble metal eutectic mixtures) are common, in particular in small-scale preparations. These effects still set limits to the availability of catalytically desired alloys for practical purposes (e.g. for compounds with Zr, Si, alkali, Mg). 

FIGURE 13 Graded oxide nanoparticle. Mo02 was oxidized with air at 723 K to give a core-shell structure of molybdenum dioxide and possibly molybdenum trioxide that was identified by its different electron energy loss spectrum. No structural description of the highly disordered and catalytically relevant outer oxide shell could be determined, either with XRD or with TEM, (or even with EXAFS spectroscopy) as the signals are dominated by the core structure. 

Fig. 8.8 Processes for 1,3PD (a) from acrolein (Degussa) (b) from ethylene oxide (Shell) (c) from glycerol, via anaerobic fermentation (Henkel) (d) from glucose,...

Fif. 1.7. Hydrogen munuracturu hy partial oxidation. Shell process. 

Hiis method, initially intended for the more selective production of propylene oxide, is commercialized by ARCO Chemical (formerly Oxirane an Atlantic Richfield Co subsidiary, and by Sheli The first industrial plant was built in 1973 by Montoro, a Joint venture of Oxirane and Empecrol, at Alcudia, Spain. This plant can now manufacture 100.0CX) t/year of styrene and 40,000 t/year of propylene oxide. Two other facilities based on this technology are also in operadoo, one at Channelview, Texas, and the second in Japan, owned jointly by Sumitomo and Showa Denka (Nippon Oxirane), capable of producing 455,000 and 225,000 t year of styrene respectively, as well as about 180,000 and 90.0001/ year of propylene oxide. Shell has also built production capacities of 330,000 and 12SJXX) tf year of these two products at its Moerdijk complex in the Netherlands. 

Fig. 11.5 Rationally fabricated SERS substrates. From (a) to (I), triangular nanoparticle array [30], silver nanowire bundles [32], Ag nanoparticle-assembled silica nanoparticle (SERS dots) [33], metal nanoparticle aggregates (COINs) [34], gold nanocrescent [49], gold nanoparticles with thin oxide shells [50], hollow-type gold nanoparticles [35-37], gold nanorods [38 10], nanocubes [41,42], flower-like gold nanoparticles [43], nanodisks [46], and gold nanorods immobilized on silica nanoparticles [48]...

Fig. 4) or just metal oxides (ZnO, CtO, Fig. 5). In the first case, the nanoparticles are more likely to be oxidized within their outer layer therefore they must have a core-shell structure (metallic core and metal oxide shell). The composition of some of the samples was confirmed by EPR and solid state NMR studies.

Ethylene glycol via ethylene oxide Shell MMlb/y 500 1,000 5.792 0.6... 

Fig. 15.5 Oxidation and reduction of epitaxially grown polyhedral Rh nanoparticles (mean size 5 nm) on alumina, monitored ex situ by HRTEM. In the as-prepared state, most of the Rh particles were half-octahedra with 111 and 100 surface facets, as revealed by combining results from HRTEM and WBDF (a, d), and SAED (b). Upon oxidation in 1 bar at 723 K, an epitaxial Rh-oxide shell developed on top of a Rh core (c, e). Reduction in 1 bar H at 523 and 723 K led to polycrystalline (f) and rounded crystalline (g) nanoparticles, respectively. The microstructural changes were correlated with changes in catalytic hydrogenolysis activity (see text for details) adapted in part from [20] with permission. Copyright (1998) Elsevier...

An ideal study of support effects requires model catalysts with metal particles that are identical in size and shape (so that only the support oxide varies). This is difficult to achieve for impregnated catalysts, but identical metal particles can be prepared via epitaxial model catalysts [36]. Well-faceted Rh nanocrystals were grown on a 100-cm area NaCl(OOl) thin film at 598 K. One half of a Rh/NaCl sample was covered with Al Oj, and the other half with TiO. The preparation of Rh particles for both Al Oj- and TiO -supported model catalysts in a single step prevents any differences in particle size, shape, and surface structure which could occur if the samples were prepared in separate experiments. Three model catalysts were prepared, with a mean Rh particle size of 7.8, 13.3, and 16.7 mn (the films were finally removed from the NaCl substrate by flotation in water). Activation was performed by O /H treatments, with the structural changes followed by TEM (Fig. 15.6). Oxidation was carried out in 1 bar O at 723 K prodncing an epitaxially grown rhodium oxide shell on a Rh core (cf Fig. 15.5e), whereas the hydrogen reduction temperature was varied. 

HRTEM shows the typical SiNW Si core encapsulated by a Si02 sheath and the 111 planes of crystalline silicon. The diameters of the crystalline silicon core varied from 13 to 30 nm, and the mean value was about 20 nm. The thickness of the amorphous silicon oxide shell varied from 2 to 10 nm, and the mean value was about 5 nm. 

It is well known that nanotubes and nanowires with sharp tips are promising materials for application as cold cathode field emission devices. We have investigated the field emission of different nanowire structures. The first is from SiNWS. SiNWs exhibit well-behaved and robust field emission fitting a Fowler-Nordheim (FN) plot. The turn-on field for SiNWs, which is needed to achieve a current density of 0.01 mA cm , was 15 V p.m [26]. The field emission characteristics may be improved by further optimization, such as oriented growth or reducing the oxide shell, and may be promising for applications. 

Zhou ZY, Brus L, Friesner R (2003) Electronic structure and luminescence of 1.1- and 1.4-nm silicon nanocrystals oxide shell versus hydrogen passivation. Nano Lett 3 163-167... 

Tavs, P, Esters of aryl- and vinylphosphonic acids, aryl- and vinylphosphinic acids and aryl- and vinylphosphine oxides. Shell, U.S. PatentAppl. DE 1810431, 1970 Chem. Abstr., 13, 77387, 1970. 

A. Partial oxidation (Shell Gasification or catalytic partial oxidation)... 

The Fe-Au nanoparticles were reported to consist of metallic cores, having an average diameter of 6.1 nm, surrounded by an oxide shell, averaging 2.7 nm in thickness, for a total average particle diameter of 11.5 nm [101]. A surfactant solution is prepared with nonylphenol poly(ethoxylate) ethers. Au-coated Fe nanoparticles were also prepared in a reverse micelle formed by cetyltrimethylammonium bromide (CTAB), 1-butanol and octane as the surfactant, the co-surfactant and the oil phase, respectively [100]. The nanoparticles were prepared in aqueous solutions of micelles by reduction of Fe(II) and Au precursors with NaBH4. The typical size of the nanoparticles is about 20 nm. The existence of Fe and Au is again confirmed by energy dispersive X-ray microanalysis. 

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