Vanadium, atomically dispersed

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

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. 

A further Raman investigation of the effect of the antimony-to-vanadium atomic ratio, gave evidence that the formation of VSbCh resulted in higher yields of acrylonitrile when surface vanadium oxide species were also present (Banares et al., 2002 Guerrero-Perez and Banares, 2004). The presence of surface alkoxy species was not observed in the absence of dispersed surface vanadium oxide species. 

Recently the large pore vanadium containing molecular sieve, V-NCL-1 with a pore size of 7 A, has been shown to be an active catalyst for the oxidation of larger molecules, such as napthalenes, 1,4-napthoquinones and phthalic anhydride (Scheme 22)[187]. The as synthesised form of V-NCL-1 contains atomically dispersed V4+ ions located in fiamework postions although not neccessarily in tetrahedral coordination. The vandium ions can be oxidised to the pentavalent state by calcination, as evidenced by ESR [157], with some... 

The similar structural and catalytic properties of the SiOj-supported and unsupported samples prepared from the same precursor suggest that the same active surface is formed on both types of samples. The higher conversions obtained with the supported samples could be attributed to higher dispersions of the VPO compounds. The slightly lower maleic anhydride selectivity observed for catalyst A than B or the bulk catalyst could be due to some phosphorus atoms interacting with the silica surface rather than with vanadium atoms, such that the P/V ratio is less than two in the VPO compounds. Addition of phosphorus to catalyst B replenished this lost phosphorus. Previous studies of supported vanadium-phosphorus oxides have shown that some phosphorus atoms can be associated with the silica [2,8]. The catalytic properties of the supported samples as well as the LRS are similar to the SiOj-supported PA =2 VPO samples prepared previously [2,3]. These earlier samples were prepared by adding H3PO4 to PA =1 samples synthesized by various synthesis routes. Thus, for the supported samples, the method of preparation is much less important than the composition. 

The sample NbA =l/l (precipitate) reveals the stoichiometric crystalline phase NbVOs instead, the sample NbA =l/l cit, synthesized by adding citric acid as vanadium ion complexing agent, confirms a second crystalline phase (Nbi8V405s) in addition to NbVOs. It is reasonable to assume that the formation of a homogeneous gel has some influence on the structure of the material because it can favour a better atomic dispersion. 

When all-silica MCM-41 is subjected to chemical vapor deposition of VOCI3 at 100°C, the reaction time must be restricted After a treatment for more than 15 min about 20% of amorphous non-porous by-products were found [88]. After a reaction time of 5 min, a vanadium content of 1.5 wt.% was achieved which could be increased to 2.1 wt.% without significant damage to the well-ordered MCM-41 structure by triple repetition of the CVD cycle. XP and UV-Vis spectra showed the presence of vanadium atoms mononuclearly dispersed in the pores of MCM-41 and largely tetrahedrally coordinated with oxygen [87]. In... 

For a 1 wt % V loading, 37 V atoms are deposited per 5 nm M0S2 slab or 8 per 2 nm M0S2 slab. In this case the number of vanadium per M0S2 slab approaches the number of edge sites and complete contamination or promoter ion substitution appears possible for a 1 wt % V loading with 100% dispersion. 

V—0-Support (930cm ) and V—O—V (625 cm ) bonds. Similar distributions of monomeric and polymeric surface VO4 species are found on other oxide supports with the exception of Si02 [30]. For the supported V20s/Si02 catalyst system, only isolated surface VO4 species are present below the maximum dispersion limit (<3 V atoms/nm ). For all supported vanadium oxide catalysts, crystalline V2O5 NPs are also present above the monolayer surface coverage or maximum dispersion limit [31]. 

Sn-silicalites of MFI, MEL and MTW stmctures with Si/Sn > 30 have been synthesized hydrothemally under basic conditions. The unit cell volume eraansion in each case, though linear with respect to Sn content (upto 3 Sn per unit cell in MFI and MEL silicalites), does not correspond to theoretical T-atom substitution by Sn " ions. The well-dispersed SnOx units can be described as structural defects with octahedral coordination and are active in the oxidation of a number of organic substrates (phenol, toluene, m-cresol and m-xylene) with aqueous H2O2. These are similar to vanadium silicalites (VS-1 and VS-2), as both hydroxylation or the aromatic nucleus and the oxidation of the alkyl substituent are catalysed. Due to the presence of Sn + in large pores, Sn-ZSM-12 sample is able to oxidize bulkier naphthalene and 2-methylnaphthalene more effectively than the medium pore Sn-MFI and Sn-MEL silicalites. 

Nickel and vanadium along with iron and sodium (from the brine) are the major metallic constituents of crude oil. These metals can be determined by atomic absorption spectrophotometric methods (ASTM D-5863, IP 285, IP 288, IP 465), wavelength-dispersive X-ray fluorescence spectrometry (IP 433), and inductively coupled plasma emission spectrometry (ICPES). Several other analytical methods are available for the routine determination of trace elements in crude oU, some of which allow direct aspiration of the samples (diluted in a solvent) instead of time-consuming sample preparation procedures such as wet ashing (acid decomposition) or flame or dry ashing (removal of volatile/combustible constituents) (ASTM D-5863). Among the techniques used for trace element determinations are conductivity (IP 265), flameless and flame atomic absorption (AA) spectropho-... 

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