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51.
Arenas-Alatorre J Gómez-Cortés A Avalos-Borja M Díaz G 《The journal of physical chemistry. B》2005,109(6):2371-2376
The surface properties of bimetallic Ni-Pt/SiO2 catalysts with variable Ni/Ni + Pt atomic ratio (0.75, 0.50, and 0.25) were studied using N2O decomposition and N2O reduction by hydrogen reactions as probes. Catalysts were prepared by incipient wetness impregnation of the silica support with aqueous solutions of the metal precursors to a total metal loading of 2 wt %. For both model reactions, Pt/SiO2 catalyst was substantially more active than Ni/SiO2 catalyst. Mean particle size by TEM was about the same (in the range 6-8 nm) for all catalysts and truly bimetallic particles (more than 95%) were evidenced by EDS in the Ni-Pt/SiO2 catalysts. CO adsorption on the bimetallic catalysts showed differences in the linear CO absorption band as a function of the Ni/Pt atomic ratio. Bimetallic Ni-Pt/SiO2 catalysts showed, for the N2O decomposition, a catalytic behavior that points out an ensemble-size sensitive behavior for Ni-rich compositions. For the N2O + H2 reaction, the bimetallic catalysts were very active at low temperature. The following activity order at 300 K was observed: Ni75Pt25 > Ni25Pt75 approximately Ni50Pt50 > Pt. TOF values for these catalysts increased 2-5 times compared to the most active reference catalyst (Pt/SiO2). The enhancement of the activity in the Ni75Pt25 bimetallic catalysts is explained in terms of the presence of mixed Ni-Pt ensembles. 相似文献
52.
V. Manríquez C. Díaz G. González I. Brito 《Journal of Thermal Analysis and Calorimetry》1996,46(6):1875-1880
Benzenesulfenamides with the formula R-S-N-(R)2 (R=C6H5 andR=NC4H8O, C7H7 and C6H11) and their chromium carbonyl complexes were studied by means of TG and mass spectrometric methods. The thermal behaviour of the compounds the stabilities of free sulfenamides are lower than those observed for the corresponding chromium carbonyl complexes. Combined thermogravimetry — mass spectrometry results suggest that the fragmentation mechanism of the carbonyl complexes involves cleavage of the Cr-S and Cr-CO bonds while that of sulfenamide depends mainly on the dissociation rates of the NR2 groups.This work was partially supported by the Departamento técnico de Investigation of the Universidad de Chile, Grant Q3280/9324. 相似文献
53.
[reaction: see text] A new Prins-type cyclization between homopropargylic alcohol and aldehydes in the presence of FeX(3) to obtain 2-alkyl-4-halo-5,6-dihydro-2H-pyrans in good yield is described. Osmium-catalyzed cis dihydroxylation provided direct access to trans-2-alkyl-3-hydroxy-tetrahydro-pyran-4-ones. Anhydrous ferric halides are also shown to be excellent catalysts for the standard Prins cyclization using homoallylic alcohol. Isolation of an intermediate acetal provides substantiation of a proposed mechanism. 相似文献
54.
55.
Chromatographic retention data and thermodynamic properties of six pine terpenes (α-pinene, β-pinene, camphene, limonene, α-phellandrene and p-cymene) on six different liquid phases, with a wide range of polarity, are reported at temperatures from 80 to 120 °C. The retention behaviour and selectivity of liquid phases for the separation of binary solute systems are discussed in terms of activity coefficients and specific interactions. Solubility parameters for the liquid phases are reported using the regular solution theory and the Takamiya equation and a good correlation between polarity and solubility parameters is observed. 相似文献
56.
1,3-Cyclopentanedione bis(4-methylthiosemicarbazone) monohydrochloride produces colored solutions with chlorate ions in strongly acid medium. The yellow color obtained has been used to propose a spectrophotometric method of C103? determination in the concentration range 0.5–6.0 ppm (molar absorptivity 1.26 × 104 liters mol?1 cm?1 at a wavelength of 397 nm). 相似文献
57.
C. Díaz M. L. Valenzuela E. Spodine Y. Moreno O. Peña 《Journal of Cluster Science》2007,18(4):831-844
Pyrolysis of the organometallic polymer: {{[N=P(R1)]0.8[N=P(OC6H4CH2CN[Ru])2]0.15[N=P((OC6H5)(OC6H4CH2CN[Ru]]0.05}{Cl}0.31}
n
, [Ru]=CpRu(PPh3)2, R1 = O2C12H8 (1) as well of the cyclic specie {N3P3 (OC6H5)5(OC6H4CH2CN[Ru])}{PF6} (2) under a flow of air at 800°C affords nanostructured RuO2. Nanoparticles near to 10 nm were observed. The differences in the use of cyclic or polymeric phosphazenes, as solid state
template, influence strongly the morphology and slightly the composition of the pyrolytic product. Temperature variable (SQUID)
measurements in the range of 5–300 K of the material obtained from the polymer, indicate an antiferromagnetic interaction
between the Ru atoms, although lower than that found for the crystalline ruthenium oxide, probably due to some amorphous product
present in the pyrolytic material. The possible formation mechanism is discussed and the differences in using the cyclic or
the polymeric compound as precursor is analyzed in terms of the relative content of Ru to P, N. A general formation method
of nanostructured metal oxides is proposed. 相似文献
58.
O. Díaz Rizo M. V. Manso Guevara E. Herrera Peraza I. Alvarez Pellón M. C. López Reyes 《Journal of Radioanalytical and Nuclear Chemistry》1997,221(1-2):241-244
Using the facilities of the Triga Mark III reactor at the NNRI, Mexico and the HAV-1 multipurpose monitor, the reactor power dependency for thek
0-standardization essential neutron flux parameters as: epithermal shape factor (), thermal to epithermal ratio (f) and neutron temperature (T
n
) were experimentally obtained. Evaluation of the obtained dependencies shows that it is unnecessary to analyze the possible introduction of correction factors in thek
0-INAA experimental results. A single experimental procedure to determine throughf is suggested. 相似文献
59.
60.
M. T. Fernández-Díaz J. Rodríguez-Carvajal J. L. Martínez G. Fillion F. Fernández R. Saez-Puche 《Zeitschrift für Physik B Condensed Matter》1991,82(2):275-282
Neutron powder diffraction and magnetization measurements have been performed in stoichiometric Pr2NiO4+(0), which at room temperature (RT) is orthorhombic (Bmab). Ni2+ becomes three dimensionally (3D) antiferromagnetically ordered atT
N
325K, with a propagation vectork=[100], and spins oriented along thea axis (parallel to the propagation vector). The magnetic structure belongs to the
7g
(––+)-representation (g
x
mode, Shubnikov groupB
pmab') ofBmab, and a magnetic moment of 1.5
B
is measured at 155K. This compound undergoes two different structural phase transitions. First, going down in temperature, Pr2NiO4 transforms by a first order phase transition from orthorhombic (Bmab) to tetragonal (P42/ncm) atT
c1115K. At high temperature, we can predict a transformation from orthorhombic (Bmab) to tetragonal (I4/mmm) in a continuous way. The extrapolated temperature for this second structural transition isT
c21500±100K. The low temperature structural transformation allows a change in the magnetic structure which forT<T
c1 is better described in the orthorhombic symmetry. Just belowT
c1 the magnetic structure is described by the:
3g
(–+)-representation ofPccn(gxcyfz mode, Shubnikov groupPccn), this magnetic phase begins to disappear going down in temperature and at the same time a new magnetic phase grows. This new magnetic structure is described by the
1g
ofPccn (c
xgyaz mode, Shubnikov groupPccn). Both magnetic structures coexist in a certain temperature range. At 1.5 K thec
xgyaz mode represents the total of the magnetic ordering. To reproduce the observed magnetic intensities we are forced to consider that Pr3+ is polarized below 40 K, with a magnetic structure which is coupled to the Ni sublattices (i.e.
1g
and
3g
). The magnetic moment at low temperature for Pr is about 1.28
B
. 相似文献