摘要:

AbstractThe rate constants of the reactions of1CH2with C2H4, CH2= CF2, CHF = CHF, CF2= CHF, C2F4, and CF2= CHCl were investigated with a laser photolysis/laser induced fluorescence apparatus at room temperature under quasistationary conditions.1CH2was produced via photolysis of ketene. The obtained rate constants show a regular dependence on the amount of fluorine substitution: an increasing fluorine part leads to a decrease of the overall rate constant. This also holds for CF2= CHCl, which shows nearly the same reactivity towards1CH2as CF2= CHF does. Some of the investigated reactions were measured for both1CH2(v= 0) and1CH2(v= 1). The obtained rate constants are very similar.In the reactions of1CH2with CH2= CF2, CF2= CHF and C2F4the radical1CF2could be detected as a reaction product. In the1CH2reactions with CHF = CHF and CF2= CHF the radical1CHF could be observed.These results underline the importance of the addition channel for this kind of reactions.

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000502

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractThe atmospheric chemical behaviour of 1,1,1,2‐tetrafluoroethane (CF3CFH2, HFC‐134a) with respect to its rate and mechanism of degradation in the troposphere has been investigated. The rate coefficient for the reaction of (1 a) CF3CFH2+ OH→CF3CFH + H2O has been determined in direct time‐resolved experiments using laser‐pulse initiation and laser long‐path absorption. A value ofk1a= (4.6±0.5)×10−15cm3/s atT= 295K has been found. The ratio of the rate coefficients for the reactions of the CF3CFHO‐radical with O2, (4) CF3CFHO + O2→CF3CFO + HO2, and C‐C bond fission, (5) CF3CFHO + M→CFHO + CF3+ M, forT= 295 K andptotal= 50 mbar (O2) has been obtained to bek4/(k5[M]) = 1.5×10−19cm3, with the individual values beingk4= 2.7×10−15cm3/s andk5[M] = 1.8×104s−1. The mechanism of the atmospheric degradation of HFC‐134a has been investigated in the temperature range 244‐295 K andptotal= 1000 mbar using UV photolysis/FTIR product studies. From these experiments the Arrhenius expressionk4/(k5[M]) = 8.7±10−25exp ((3240+990−610) K/T) cm3was determined. Moreover, it is concluded that the major carbonyl products are CF3CFO, CFHO and CF2O which result from the two possible reactions of the oxy radical CF3CFHO. As a consequence the yield of these carbonyl products is dependent on temperature, O2partial pressure and total pressure. For the reaction of CFHO with Cl atoms the Arrhenius expressionk18= 5.7×10−14exp (−(1130 ± 160) K/T) cm3/s wa

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000503

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractThe oxidative degradation of HCFC 141 b (CFCl2CH3) and HCFC 142 b (CF2ClCH3) in the presence of NO atT= 293 K andp= 50 mbar has been studied using laser pulse initiation combined with cw laser long path absorption/LiF for the detection of OH and NO2. The absolute yield and temporal behaviour of these products have been found to be sensitive indicators for the reaction behaviour of the alkoxy radicals CFCl2CH2O and CF2ClCH2O, respectively. The rate coefficients for the reactions of the alkoxy radicals with O2were determined to bek(CFCl2CH2O+O2→CFCl2CHO+HO2) = (2.0±1.0)×10−15cm3/s andk(CF2ClCH2O+O2→CF2ClCHO+HO2) = (2.5±1.5)×10−15cm3/s. Upper limits for the rate coefficients for unimolecular decomposition of the alkoxy radicals by C‐C‐bond fission ofk(CFCl2CH2O+M→CFCl2+CH2O+M)×1.0±103s−1andk(CF2ClCH2O+M→CF2Cl+CH2O+M)≤1.5 T≤ 103s−1were derived. We conclude from these data that CFCl2CHO and CF2ClCHO will be formed as the dominant primary carbonyl products in the oxidation of HCFC 141 b and 142 b throughout the troposphere. Moreover, the rate coefficients for the reactions of the HCFCs with OH radicals atT= 293 K have been measured. The valuesk(CFCl2CH3+OH→CFCl2CH2+H2O) = (4.6±0.8)×10−15cm3/s andk(CF2ClCH3+OH→CF2ClCH2+H2O)

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000504

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractHomogeneous gas phase reactions in F/F2/CHFO mixtures with helium as a bath gas were studied pure and with addition of oxygen at ambient temperature. A computer‐controlled discharge flow system with mass spectrometric detection was used. The accessible pressure range was 1.6–3.5 mbar. A more precise value for the reaction CFO+CFO→CF2O+CO was determined:k(2a)=(1.7+0.4−0.3)·10−11cm3molecules−1s−1. No discernible pressure dependence was found for CFO+F→CF2O:k(3a)= (5.0+3.3−1.4)·10−12cm3molecules−1s−1. Fluoroformyl peroxy radicals were formed by CFO+O2+M→CF(O)O2+M. The pressure dependence of this reaction could be described byk0(4)= (4.5+2.2−1.1)·10−31cm−6molecules−2s−1andk·(4)= (1.7±0.3)×10−12cm3molecules−1s

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000505

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractComplexes of 5‐octadecyloxyisophthalic acid with piperazine, 1,4,7‐triazacyclononane and 1,4,7,10‐hexaazacyclooctadecane were selectively deuterated in various positions, and investigated with variable‐temperature solidstate2H n. m. r. spectroscopy. All systems form stable, enantiotropic liquid‐crystalline phases at high temperature, and the dynamics in these phases can be described by a single motional model. This model involves fast rotation around the director coupled with a rapid phenyl flip. Macroscopically oriented liquid‐crystalline phases with the director parallel to the magnetic field are found on cooling in a magnetic field of 7 T for the piperazine complexes, but only partial macroscopic order was achieved for the complexes with 1,4,7,10‐hexaazacyclooctadecane and 1,4,7‐triazacyclononane under these conditions, and also for the former in a magnetic field of 11 T. In contrast to the pure acid C18ISA, the stability of the phases is determined by the hydrogen/ionic bonds between the acid and the bases, and not by the alkyl‐a

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000506

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractIn the present work we report a study of the molecular relaxation processes in tetrachloro‐m‐xylene in the solid state. The techniques of dielectric relaxation spectroscopy and thermally stimulated discharge currents were used in order to identify and characterise the different relaxation processes and it was found that a solid rotator phase appears on heating at a temperature of ∼−125°C. It was also observed by thermally stimulated currents that a higher temperature relaxation occurs above ∼−15°C. The attribution, at the molecular level, of this relaxation is not straightforward and it was suggested that it probably corresponds to the onset of the self‐diffusional motio

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000507

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractThe growth of nanocrystalline platinum particles in zeolite [Pt(NH3)4]NAX was investigated by in situ X‐ray diffraction in combination with Rietveld refinement atT= 509, 519, 530 and 562 K. The reaction was directly followed by the gradual decrease of the Pt2+‐occupation and the simultaneous increase of the metallic platinum particles, which grow up to an average size of 4 nm. An increase of the zeolite lattice constant and a gradual decomposition of the framework up to 20% were simultaneously observed.The process of crystal growth can be influenced by the preparation conditions of the zeolite. After dehydration up to 383 K at 10−7bar more than 90% of the platinum cations are located near the centre of the 12‐ring‐window. Dehydration in the temperature range from 403 to 443 K under vacuo results in a partial shift of the platinum cations to a second position and to an increase of the r

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000508

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractThe catalytic and structural properties of two different preparations of manganese oxide supported on high surface area zirconia are compared. The catalyst were prepared by grafting from an alkoxide precursor and by conventional aqueous wet impregnation technique. Reaction of manganese ethoxide with the hydroxyl groups at the surface of the zirconia support is highly specific and allows manganese oxide to be deposited layer by layer. The zirconia used in this study has a mesoporous structure; pores with diameters ≦ 5.8 nm were found to be inaccessible to the manganese ethoxide precursor. Hence, only about half of the total surface area is available for the reaction with ethoxide. A quantitative evaluation of the XPS intensities for manganese and zirconia shows that about 2.5 layers of manganate are deposited after five grafting steps. Activity tests made on these catalysts confirm the monolayer buildup. The specific activity of the isolated manganese ions which result from the first grafting step is very low; the specific activity increases by an order of magnitude once the first monolayer is completed, and shows a further increase in the second and third layers. In contrast, impregnation with manganese acetate results in small manganate crystallites, which most likely form inside the pores. The specific activity of these catalysts and their selectivity towards acetone in isopropanol decomposition are very similar to that of bulk manganese dioxide whereas the grafted samples show a high selectivity towards propen

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000509

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractPotassium‐iron(III)‐hexacyanoferrate(II) (Prussian blue) is deposited electrochemically as a thin film (thickness: 300 nm) on a Pt‐disk substrate electrode. The relative change in optical reflectivity of this electrode is detected time‐resolved upon galvanostatic reduction of Prussian blue to Everitt's salt and subsequent reoxidation. Considering different transport properties of electrons and cations within the film, the experimental result is fitted by numerical modelling. The specific diffusion coefficient for electrons is deduced as 5·10

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000510

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY

摘要:

AbstractThe growth of an oxide layer on Pt, Ru and codeposited Pt‐Ru was monitored ellipsometrically during the electrooxidation of CH3OH, HCOOH and adsorbed CO. In the potential region where these molecules or their intermediates are oxidized at the Pt‐Ru surface, the Ru‐oxide is no longer observed. This supports the bifunctional mechanism that has been proposed for the promoting role of Ru on Pt. PtO is not involved in the HCOOH and COadsoxidation. The adsorbed CO layer on Pt and on Pt‐Ru could also be observed with ellip

ISSN:0005-9021    

DOI:10.1002/bbpc.19961000511

出版商:Wiley‐VCH Verlag GmbH&Co. KGaA    

年代:1996

数据来源: WILEY