Theoretical and Experimental Studies of the Nature of the Catalytic Activity of VO x /TiO2 Systems

The states of supported vanadium and the nature of activation of ammonia adsorbed on vanadium sites of V _x /Ti₂ catalysts are studied by ⁵¹V NMR spectroscopy and diffuse-reflectance IR Fourier-transform (DRIFT) spectroscopy using cluster quantum chemical calculations of N₃ adsorption. We employ the V _x /Ti₂ catalyst of two types: the monolayer catalyst in which vanadium is located on the surface of well-crystallized anatase and the catalyst in which vanadium embedded in the anatase lattice at a rather great depth. It is shown that ammonia is predominantly adsorbed on Lewis acid sites of the monolayer catalyst, whereas most of N₃ adsorbed on the catalyst containing bulk vanadium is in the form of ammonium ions. Analysis of experimental and calculated data suggests that, in the monolayer catalyst, N₃ molecules in the selective reduction of nitrogen oxides are activated on Lewis acid sites. Ammonia activation involves the dissociation of the N–H bond in a coordinated molecule, which results in the formation of the amide V–N₂ group and a water molecule coordinated by a V⁵⁺ ion. It is likely that, in the case of the catalyst containing bulk vanadium, this reaction occurs with the predominant participation of ammonium ions.     

Correlations between the adsorption and structural properties of SV-1 phytoadsorbent and its main components

The structural and morphological characteristics, as well as the adsorption properties with respect to benzene, methanol, and water vapors, were studied for the main components of SV-1 phytoadsorbent (lignin, cellulose, and melanin). Lignin and cellulose determine the capillary-porous structure of the material as a whole. The adsorption properties of the adsorbent are determined by the porous structure of its main constituents and by differences in the sensitivity of the adsorptives to the chemistry of the surface of the materials under study.     

Investigation of n-hexane dehydrocyclization over supported Pt catalyst modified by adsorbed rhenium

A -Al₂O₃ supported Re–Pt bimetallic catalyst prepared by electrochemical adsorption has been studied in n-hexane dehydrocyclization, using the slug-pulse method. The activity of the Re–Pt/Al₂O₃ catalyst treated only in hydrogen was low, but markedly increased by a calcination step before hydrogen treatment. The modified catalyst showed higher activity and increased selectivities for benzene and iso-hexanes than the base Pt/Al₂O₃ catalyst.

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Pd/Pt, -Al₂O₃, -. Pd–Pt/Al₂O₃, , , . Pd–Pt/Al₂O₃ , Pt/Al₂O₃.

     

1, 2-Dithiolium salts

4-(p-Tolyl)-1,2-dithiolium hydrogen sulfate (I) is not a specific analytical reagent for the majority of anions, and it is not very selective, only in a few cases attaining pD=5 (MoO₄ ^2–, [Hg(CNS)₄]^2–, and [PtCl₆]^2–). All strongly colored anions give colored salts with I. Among the weakly colored anions, the [Fe(CN)₆]^4– anion is worthy of special note, since it forms a deeply colored salt with I. This anion can be detected with I in the presence of Cl^–, Br^–, I^–, CNS^–, ClO₄ ^–, IO₄ ^–, and ReO₄ ^–, which form fairly readily soluble salts with I. Compound I is also a fairly selective reagent for Pd²⁺ in acid solution (pD=5).For part II, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, Vol. 6, No. 5, pp. 595–597, May, 1970.     

Ammoxidation of methylpyrazine over phosphorus-modified vanadium-titanium catalysts

Vanadium-titanium catalysts modified with phosphorus additives (1-15 wt.% of P₂O₅) have been studied in methylpyrazine ammoxidation. Introduction of high amounts of the additive (≥ 10 wt.% of P₂O₅) results in a decrease in activity and selectivity of the catalysts due to formation of low active phase - a ternary compound with the component ratio V : P : Ti ≈ 1 : 1 : 1 with only one type of slightly distorted tetrahedral vanadium (5+) bound via oxygen to phosphorus (5+). The nature of the active sites of the samples modified with 1-5 wt.% of P₂O₅, similarly to that in the V-Ti-O catalyst, was found to include the V⁵⁺ cations strongly bound with TiO₂ and located in the significantly distorted octahedral oxygen environment. This revised version was published online in June 2006 with corrections to the Cover Date.     

Methylpyrazine Ammoxidation over Binary Oxide Systems: V. Effect of Phosphorus Additives on the Physicochemical and Catalytic Properties of a Vanadium–Titanium Catalyst in Methylpyrazine Ammoxidation

Oxide vanadium–titanium catalysts modified by phosphorus additives (20V₂O₅–(80 –n)TiO₂–nP₂O₅, n = 1, 3, 5, 10, and 15 wt %) are studied in methylpyrazine ammoxidation. Two regions of compositions are found corresponding to radically different catalytic properties, namely, catalysts with a low (5 wt % P₂O₅) and high (10 wt % P₂O₅) concentration of the additive. In the first case, the introduction of phosphorus is accompanied by a gradual increase in the activity. In the second case, an increase in the additive concentration results in a decrease in the activity and selectivity to the target product, pyrazineamide, and a simultaneous increase in the selectivities to by-products, pyrazine and carbon oxides. The catalysts are characterized by X-ray diffraction analysis, differential dissolution, IR, and NMR spectroscopic data. As in the binary system, the active sites of the samples with a low concentration of phosphorus contain V⁵⁺ cations in a strongly distorted octahedral oxygen environment, which are strongly bound to a support due to the formation of V–O–Ti bonds. The catalytic properties of the samples containing 10 wt % P₂O₅ are due to the presence of the phase of a triple V–P–Ti compound with an atomic ratio V : P : Ti approximately equal to 1 : 1 : 1. The V⁵⁺ cations in this compound occur in a weakly distorted tetrahedral oxygen environment and are bound to the tetrahedral P⁵⁺ cations.     

51V-NMR spectra of vanadates and oxosulfato-vanadates of alkali metals

A relation between the type of vanadium environment and the chemical shift anisotropy in the⁵¹V-NMR spectra has been established. The results obtained are compared with spectra of real catalysts.

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⁵¹V , .

     

Active component of vanadium catalysts in oxidation of gases with low concentrations of SO2

EPR and NMR studies show that in oxidation of gases with low concentrations of SO₂ the molten active component of K/V and Cs/V catalysts becomes inhomogeneous and contains V⁵⁺ compounds having different activities.

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, SO₂ K/V Cs/V , V⁵⁺.

     

Direct (without degassing) determination of sulfur-containing compounds in unstable gas condensate by gas chromatography

A procedure has been developed for direct determination of sulfur-containing components in the concentration range 0.0002?C0.7 wt % in samples of unstable gas condensate under a pressure of 8 MPa using a gas chromatograph equipped with a flame photometric detector. The determination of the overall composition of samples (including hydrocarbons and inorganic gases) takes no more than 1 h, i.e., is 5 times more rapid than the standard procedure with preliminary sample degassing. In addition, direct (without degassing) determination results is lower errors (the repeatability for the main components is 0.01?C0.06; n = 10).