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排序方式: 共有43条查询结果,搜索用时 109 毫秒
1.
M. Kaltchev D. Stacchiola H. Molero G. Wu A. Blumenfeld W.T. Tysoe 《Catalysis Letters》1999,60(1-2):11-14
The reaction between gas‐phase acetylene and alumina‐supported palladium saturated with 13C‐labelled vinylidene is studied using both one‐pulse, 13C magic‐angle spinning, nuclear magnetic resonance (NMR) spectroscopy and by mass spectroscopic analysis of the reaction products
to probe the reaction pathway. The presence of vinylidene on alumina‐supported palladium is confirmed by comparing the infrared
spectra of the species formed on the supported sample with those found on a Pd(111) single crystal. It is shown using NMR
that a high pressure (∼350 Torr) of gas‐phase acetylene reacts with adsorbed vinylidene at the same rate at which benzene
is formed catalytically on the same sample. The resulting benzene incorporates two 13C atoms. This indicates that benzene is formed by a slow reaction between gas‐phase (12C‐labelled) acetylene and adsorbed vinylidene (13CH2=13C=) to form a C4 intermediate which reacts rapidly with further acetylene to yield benzene. There are precedents for such reactions in homogeneous
phase. The proposed reaction pathway differs from that elucidated previously from ultrahigh vacuum studies on clean Pd(111),
where it was found that benzene synthesis also proceeds via a C4 intermediate, in this case formed from two adsorbed acetylenes.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
Alvarez Romero JT Tovar Muñoz VM de León BS Oviedo JO Barcenas LS Milo CM Monterrubio JM 《Radiation protection dosimetry》2006,120(1-4):95-99
A thermoluminescence dosimetry (TLD) system was characterised at SSDL-ININ to verify the air-kerma strength (S(K)) and dose-to-water (D(W)) values for (137)Cs sources used in low dose rate (LDR) brachytherapy treatments at the Hospital General de Mexico (HGM). It consists of a Harshaw 3500 reader and a set of TLD-100 powder capsules. The samples of TLD-100 powder were calibrated in terms of D(W) vs. nC or nC mg(-1), and their dose response curves were corrected for supralinearity. The D(W) was calculated using the AAPM TG-43 formalism using S(K) for a CDCSM4 (137)Cs reference source. The S(K) value was obtained by using a NE 2611 chamber, and with two well chambers. The angular anisotropy factor was measured with the NE 2611 chamber for this source. The HGM irradiated TLD-100 powder capsules to a reference dose D(W) of 2 Gy with their (137)Cs sources. The percent deviations between the imparted and reference doses were 1.2% < or = Delta < or = 6.5%, which are consistent with the combined uncertainties: 5.6% < or = u(c) < or = 9.8% for D(W). 相似文献
3.
4.
M Prieto-Carrasquero A Molero N Carrasquero A Del Villar S González-Ferrer A Rojas J Brito R Mena L González F Pérez F Alvarez M Quintero W Fulcado 《Canadian Metallurgical Quarterly》1998,39(4):257-272
The Medical Genetic Unit of the University of Zulia (MGUUZ) has developed a Prenatal Diagnosis Program (PDP) since January-1993, in which Genetic Risk Factors are determined in couples who request prenatal genetic counseling. In this program, different prenatal diagnostic procedures are performed to detect congenital defects during intrauterine life. One of these procedures is the Fetal Sonogram (FS). FS is a non invasive technique which permits the prenatal diagnosis of many genetic dysmorphic syndromes. Through the search of abnormal specific characteristics in the fetus, chromosomopathies may be suspected. These findings are named "Echosonographic Markers of Chromosomal Abnormalities" (EMCA). During three years (January-1993 to December-1996), patients attended in the PDP included 321 pregnant women in which 312 FS were performed. Abnormal outcomes were 22 (17 with isolated congenital malformations and 5 with EMCA). Only one fetus with chromosome abnormality (46,XX21q-) could not be detected by FS. The goals of this paper are: 1) to report 5 patients with sonographic markers suggestive of chromosomal abnormalities and 2) to show the FS usefulness in prenatal diagnosis of chromosompathies. We conclude that, in the search of the EMCA the FS should be offered systematically to all pregnant women without recognizable genetic risk. They are the main group with optimal reproductive age and in consequence, with the possibility of having a relatively major number of conception outcomes with congenital defects, with or without chromosomic etiology. The majority of those defects can be detected by FS and could allow us to select the patients in which the use of an invasive prenatal diagnostic procedure could be justified. 相似文献
5.
Enam Alsrayheen Eric McLeod Richard Rateick Jr. Hebert Molero Viola Birss 《Electrochimica acta》2011,(17):6041
An ac/dc spark anodization method was used to deposit an oxide film (6 ± 3 μm in thickness) on the Al–Cu alloy AA2219. The oxide films were formed at 10 mA/cm2 for 30 min in an alkaline silicate solution, showing three main stages of growth. Scanning electron microscopy and electron microprobe analysis revealed that the oxide films are not uniform and consist of three main layers, an inner Al-rich barrier layer (∼1 μm), an intermediate Al–Si mixed oxide layer (∼2 ± 1 μm), and an outer porous Si-rich layer (∼3 ± 3 μm). In addition, microscopic analysis showed that the Al2Cu intermetallics present in the alloy have not been excessively oxidized during the anodization process and thus are retained beneath the oxide film, as desired. The coating passivity and corrosion resistance, evaluated using linear sweep voltammetry (LSV) in pH 7 borate buffer solution and electrochemical impedance spectroscopy (EIS) in 0.86 M NaCl solution, respectively, were both significantly improved after spark-anodization. 相似文献
6.
A. Molero G mez E. Mart nez de la Ossa 《Journal of the American Oil Chemists' Society》2000,77(9):969-974
The extraction of wheat germ oil by liquid and supercritical CO2 is described from the point of view of both operative method and pretreatment of raw material. The best conditions for wheat
germ oil extraction are: pressure, 150 bar; temperature, 40°C; and solvent flow rate, 1.5 L/min at standard temperature and
pressure. The yields and fatty acid compositions obtained are very similar to those resulting from the conventional extraction
process using hexane as solvent (8.0 wt%), although a higher-quality oil is obtained by using CO2 as solvent (free fatty acids, 12.4%; tocopherol content, 416.7 mg tocopherol/g wheat germ oil). These factors lead to the
conclusion that the extraction process using CO2 could be economically competitive with the conventional process, since it considerably simplifies the oil refinement stages
and completely eliminates the solvent distillation stage, which are the most costly processing steps in terms of energy consumption. 相似文献
7.
W. Huber A. Molero C. Pereyra & E. Martínez de la Ossa 《International Journal of Food Science & Technology》1996,31(2):143-151
The feasibility of different extraction techniques for the selective removal of cholesterol from anhydrous milk fat (AMF) by supercritical carbon dioxide has been studied. A dynamic extraction system was used to determine both the experimental solubility of anhydrous milk fat in the supercritical solvent and the selectivity of cholesterol over anhydrous milk fat at 40–70°C and 8–40MPa at various stages during extraction. In addition, adsorbents were used for the selective removal of cholesterol from anhydrous milk fat. The results indicate that a direct extraction alone or with several separators in series are not practical, but a selective removal of 97% of the cholesterol from the extracted anhydrous milk fat is possible by using an adsorbent with recovery of solvent and cholesterol. A schematic industrial countercurrent process for the removal of cholesterol and the fractionation of milk fat is proposed. 相似文献
8.
The influence of a series of metals—iron, copper, tin, and lead—on the thermal stability of olive oils of different origins
and refined grades has been studied by applying thermal analysis techniques: thermogravimetry (TG) and derivative thermogravimetry
(DTG). From thermogravimetric data it is deduced that iron and tin have a greater influence on oil oxidation, since the degradation
rate increases. In the presence of copper and lead, important changes in the degradation rate are not observed, compared with
the degradation in the absence of metals. The kinetic parameters, activation energy, and frequency factor of the oil degradation
reaction were also calculated. The results obtained for both parameters confirm the negative influence of iron and tin on
the oil oxidation process, regardless of the kind of oil tested. 相似文献
9.
M. L. Ruiz del Castillo M. Herraiz M. D. Molero A. Herrera 《Journal of the American Oil Chemists' Society》2001,78(12):1261-1265
A new procedure is presented for off-line coupling of high-performance liquid chromatography and proton nuclear magnetic resonance
spectroscopy (1H NMR) in hazelnut oil analysis. The optimization of some parameters affecting both the liquid chromatography preseparation
step and the effective multiple-solvent suppression required for the NMR study enabled us to determine the presence in a hazelnut
oil of (E)-5-methyl-hept-2-en-4-one (filbertone), a marker previously proposed to detect the adulteration of olive oil with hazelnut
oil. The described procedure requires the filtration of the oil prior to its introduction into the chromatographic system
and combines the advantages of providing sufficient sensitivity and selectivity with simple methodology and reduced sample
handling. 相似文献
10.
JA Chicharro Molero I Santos G-Vaquero JM del Rosal Samaniego C Marchal Escalona R Burgos Rodríguez 《Canadian Metallurgical Quarterly》1993,17(6):357-360
Lymphadenectomy is mandatory in vesical and prostatic tumours for a precise staging of candidates to radical surgery. Presentation of our series of 18 ilioobturative lymphadenectomies between May 1992 and February 1993, performed in 14 prostate carcinoma and 4 vesical tumours. As most interesting results, the average number of nodes removed, which was 8, and the discovery of nodular affectation in 5 cases, understaged by the non-invasive procedures, are highlighted; likewise, the reduction in surgery duration up to the current frame of 80 to 120 minutes must be emphasized. Mortality due to the procedure was null. We believe that laparoscopic lymphadenectomy should be the choice technique in the staging of tumoral dissemination in the ilioobturative domain. 相似文献