Pore formation during the pyrolysis of C-Nb2O5 and C-Ta2O5 xerogels

Binary organic-inorganic gels have been prepared by mixing a carbonaceous hydrosol and a Nb₂O₅ or Ta₂O₅ sol derived by hydrolysis of the alkoxides. The gels are pyrolyzed under an inert atmosphere into precursors in which carbon and the metal oxides are mixed very intimately. High temperature treatment converts the precursors into the cubic face centered carbides. The precursors as well as the carbides have been shown to be micro- and mesoporous materials. Measurements of nitrogen adsorption reveal a characteristic change of the shapes of the isotherms (Type I Type IV) and of the hysteresis loops (H4H2H1) during the thermal processes. Pore widening has been observed with rising temperature. The phenomena of crystallization, carbothermal reduction and sintering were found to control the pore shape and size. The results of the adsorption measurement correlate well with those of the thermoanalytical and X-ray diffraction studies.Dedicated to Professor Dr. rer.nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Structure of the UV irradiation product of 5-methyl-2-phenyl-1,2,3-diazaphosphole

Conclusion UV irradiation of 5-methyl-2-phenyl-1,2,3-diazaphosphole leads to 4,4-didehydro-bis(5-methyl-2-phenyl-1,2,3-diazaphosphole).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 633–636, March, 1987.The authors thank A. V. Il'yasov and K. M. Enikeev for discussion of the¹³C NMR spectra.     

Effects of solar UV irradiation on the tensile properties and structure of PPTA fiber

This paper comprehensively studied the effects of simulated solar ultraviolet irradiation on the mechanical and structural properties of the Twaron2000 para-aramid fiber by use of mechanical test, SEM, XRD, DSC, DMA and ATR-IR measurements. The results showed that after UV irradiation, the mechanical properties of the fiber were decreased obviously, and UV irradiation deteriorated the surface and defect areas of the fiber severely by photo-induced chain scission and end group oxidation in air with the crystalline structure remained almost unchanged although some local rearrangement of the crystalline area might occur. It was found that surface etching and shortening of the crystalline correlation length along the fiber axis were the main causes of the mechanical loss induced by UV irradiation.     

Crystal structure of some niobium and tantalum oxides. IV. The structure of KTa5O13 and its relationship to the α-PbO2 structure

KTa₅O₁₃ crystallizes in the orthorhombic system with unit-cell dimensions a = 5.653(3), b = 10.708(5), and c = 16.799(7) Å with space group Pbcm, z = 4. The structure was solved by conventional methods; of 1445 unique reflections measured by counter methods, 982 that obeyed the condition I ? 3σ(I) were used in the refinement of the model to a conventional R of 0.053 (R_w = 0.052). The structure consists of edge-and corner-shared tantalum-oxygen octahedra joined such that tunnels are formed in which the potassium ions are located. Slabs of octahedra are of the α-PbO₂ type and are related to one another by a mirror plane (in which the potassium ions are located) producing a structure that may be considered as a “chemical twin” of the α-PbO₂ structure.     

Nanocomposites with the layered structure of V2O5 · nH2O xerogel

New layered nanocomposites of V₂O₅ · nH₂O xerogels with poly(vinyl alcohol) (PVA), pyrocatechol (PC), and hydroquinone (HQ) were synthesized with the compositions (C₂H₃)_0.32V₂O_4.90 · nH₂O, (C₆H₄)_xV₂O_4.60 · nH₂O, and (C₆H₄)_0.17V₂O_4.94 · nH₂O and the interlayer distances d ₀₀₁ = 11.73, 12.85, and 15.28 ± 0.05 Å, respectively. IR and Raman spectroscopy was used to analyze which structural changes occur in the V-O layers of the xerogel upon composite formation. X-ray photoelectron spectroscopy showed V⁴⁺ and V⁵⁺ ions in the layers with binding energies lower than in V₂O₅ · nH₂O. The electrical conductivity of the nanofilms and the thermal properties of the nanopowders were studied.     

Comparative Effect of UV,UV/H2O2 and UV/H2O2/Fe on Terbuthylazine Degradation in Natural and Ultrapure Water

Different advanced oxidation processes (AOPs) (ultraviolet radiation, hydrogen peroxide photolysis and photo-Fenton) were applied to test the degradation of terbuthylazine in three types of water: (a) ultrapure water, (b) surface water from the Gaditana area (Los Hurones reservoir, Cádiz, Spain) and (c) groundwater from the Tempul spring in Jerez de la Frontera (Cádiz, Spain). The experiments were carried out on a laboratory scale, using two different types of reactors, batch and semi-continuous. In batch reactors, the most efficient process for the experiments carried out with both ultrapure water and underground groundwater was ultraviolet radiation, whereas for surface water from the Gaditana area, the process that obtained the best results was the photolysis of hydrogen peroxide with 2.5 mg L⁻¹ of H₂O₂. In semi-continuous reactors, the most efficient process was the photolysis of hydrogen peroxide with 2.5 mg L⁻¹ of H₂O₂ for all the matrices studied. In both types of reactors, terbuthylazine degradation percentages higher than 90% were achieved; the main difference was in the reaction time, which varied from minutes in the batch reactor to seconds in the semi-continuous reactor. In all the applied AOPs, N-terbutyl-6-hydroxy-N′ethyl-1,3,5-triazine-2,4-diamine (TBA-212) was generated as a reaction intermediate.     

Facile formation of an intrastrand cross-link lesion between cytosine and guanine upon pyrex-filtered UV light irradiation of d((Br)CG) and duplex DNA containing 5-bromocytosine

Pyrex-filtered UV light irradiation of d(BrCG) and 5-bromocytosine-containing duplex DNA leads to facile formation of a cross-link lesion between the C5 carbon atom of cytosine and the C8 carbon atom of its adjacent guanine. A similar cross-link lesion has been previously found in the X-ray irradiation mixture of d(CGTA).     

Mechanisms of direct and TiO2-photocatalysed UV degradation of phenylurea herbicides.

Phenylurea herbicides undergo low-yield (phi(PI) <15 %) monophotonic photoionisation upon 193-nm laser flash excitation. The so-formed radical cations (phenylurea.+) are highly acidic (-1.5 < pKa <0.5) and deprotonate readily to yield the corresponding neutral radical (phenylurea.). Pulse radiolysis experiments allowed limitation of the reduction potential of phenylurea.+ within 2.22 V versus the normal hydrogen electrode (NHE) < E degrees (phenylurea.+/phenylurea) < 2.43 V versus NHE. The main photoproducts of UVC (lambda=193 nm) photodegradation of phenylureas correspond to a photo-Fries rearrangement. One-electron reduction with e-(aq) yields the corresponding radical anions (phenylurea.-), for which 4.3< pKa < 5.33. The rate constants for reaction with e-(aq) show that in photocatalysis the generation of phenylurea.- and O2.- on the surface of the photocatalyst may be competitive. High reactivity toward e-(aq) is predicted from linear free-energy relationships (LFER) for phenylureas bearing electron-withdrawing groups. Reaction with HO. takes place mainly via addition to the aromatic ring and/or H. abstraction from a saturated carbon atom (98 %), rather than one-electron oxidation (2 %). High reactivity toward oxidation by HO. is predicted from LFER for phenylureas bearing electron-donating groups. Adsorption studies for TiO2 in its polymorphic forms of rutile and anatase, as well as with the commercial mixture Degussa P-25, show photocatalysis is independent of the specific area of the catalyst. A variety of compounds are generated during the photocatalytic degradation of Diuron, while only two hydroxychloro derivatives are observed upon prolonged direct 365 nm irradiation. The photocatalytic degradation proceeds mainly by oxidation of the Me group of the side chain, hydroxylation of the aromatic ring, and dechlorination. The photoproducts of photocatalytic degradation differ from one polymorphic form of TiO2 to another.