難溶于水的非金屬氧化物是（ ?。? A．MnO 2 B．CO 2 C．NO 2 D．SiO
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難溶于水的非金屬氧化物是（ ） A．MnO 2 B．CO 2 C．NO 2 D．SiO 2
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- 難溶于水的非金屬氧化物是（ ） A．MnO 2 B．CO 2 C．NO 2 D．SiO
- 難溶于水的非金屬氧化物是（ ?。? A．MnO 2 B．CO 2 C．NO 2 D．SiO 2
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- Keywords:Epoxides;Amines;2-Aminoalcohols;Yttriumnitratehexahydrate;RegioselectivityAminoalcoholsconstituteanimportantclassofcompoundshavingbothchemicalandbiologicalapplic... Keywords: Epoxides; Amines; 2-Amino alcohols; Yttrium nitrate hexahydrate; Regioselectivity Amino alcohols constitute an important class of compoundshaving both chemical and biological applications. They are important pharmacophores present in various inhibitors and are used as building blocks for the synthesis of biologically active natural products， insecticidal agents and chiral auxillaries for asymmetric reactions.1a–f They act as starting materials for the synthesis of oxazolines which are useful in the polymer industry.1g Owing to their widespread applications， the synthesis of amino alcohols has received much attention in recent years. Traditionally， 2-amino alcohols are prepared by heating an epoxide with an excess of amine at elevated temperature. The use of a high temperature leads to undesired side reactions and also limits the applicability towards temperature sensitive substrates. To overcome these drawbacks various promoters such as ZnCl2，2 ScOTf，3 MgBr2OEt2，4 67 9 bismuth salts，5 CoCl2， CuBF4， DIPAT，8 Ti(OiPr)4， 1011 12TaCl5， ZrCl4， Sm(OTf)3， potassium dodecatungstocobaltate， 13 and aluminosilicate14 have been employed for the above transformation. The use of non-conventional techniques such as microwave irradiation at elevated temperaturehas also been reported.15 Recently， the use of Al(OTf)3 (1 mol %) as catalyst in toluene at reflux has also been reported.16 Although significant advances have been made in this area， low regioselectivity， longer reaction time， use of elevated temperature， high catalyst loading， toxic solvents and lower substrate compatibilitylimit their applications.Thus there is a need to develop an efficient catalytic protocol for ring opening of various epoxides with aliphatic， aromatic andheteroaromatic amines under ambient conditions 展開
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- 請逐步分析以下題中所有步驟目的及加星號的試劑的簡單性質(zhì)：準確稱取0.1500g Mg-Al合金（含有鎂鋁硅等）試樣于150毫升燒杯中，加入約10mL*混合酸，并立即加蓋表面皿，待試樣溶解后，用水沖洗表面皿和杯壁，將溶液轉(zhuǎn)移至100mL容量瓶中，稀釋至刻度，搖勻。用移... 請逐步分析以下題中所有步驟目的及加星號的試劑的簡單性質(zhì)：準確稱取0.1500g Mg-Al合金（含有鎂鋁硅等）試樣于150毫升燒杯中，加入約10mL*混合酸，并立即加蓋表面皿，待試樣溶解后，用水沖洗表面皿和杯壁，將溶液轉(zhuǎn)移至100mL容量瓶中，稀釋至刻度，搖勻。用移液管吸取25mL試液于250mL錐形瓶中，加入0.02mol\L的EDTA溶液20mL，*二甲酚橙指示劑，用氨水調(diào)至溶液恰呈紫紅色，然后滴加稀鹽酸三滴，將溶液煮沸三分鐘左右，冷卻，加入百分之20的六次甲基四胺溶液20mL，此時溶液應呈黃色，如不呈黃色，可再用鹽酸調(diào)節(jié)。再補加二甲酚橙2滴，用cmol\L的*鋅標準溶液滴定至溶液由黃色變?yōu)樽霞t色，用去鋅標準溶液體積V1毫升，加入20%NH4F溶液10mL，將溶液加熱到微沸，流水冷卻，再補加稀鹽酸使溶液顯黃色。再用cmol\L的鋅標準溶液滴定至溶液由黃色變?yōu)榧t紫色時，即為終點，用去鋅標準溶液體積V2. 附問題 <1>列出計算合金中鋁的質(zhì)量分數(shù)的表達式 <2>為什么不用標準的EDTA溶液直接滴定來測鋁離子 <3>加入20%NH4F溶液10mL的目的是什么 <4>實驗完畢后要注意什么 展開
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- 3.6. Advantages of chromia as promoter in copper-based Catalysts It should also be noted from Figs. 5 and 6 that in the steam reforming reaction to produce hydrogen for fuel cell applications， trace quantities (<0.4 wt.%) of Cr2O3 on Cu... 3.6. Advantages of chromia as promoter in copper-based Catalysts It should also be noted from Figs. 5 and 6 that in the steam reforming reaction to produce hydrogen for fuel cell applications， trace quantities (<0.4 wt.%) of Cr2O3 on Cu not only doubled the activity for the MSR but also halved the amount of CO formed， thereby enhancing H2 production. This improved selectivity reduces the problem of H2 separation from the reaction products in fuel cell applications. An important finding in this research is the role that very small amounts of Cr2O3 play in all the reactions investigated， namely， methanol synthesis， water gas shift and methanol steam reforming. In commercial co-precipitatedcopper-based methanol synthesis catalysts， copper is known to be the active component [3]. More recently， it has becomeestablished that ZnO promotes methanol synthesis and that surface species formed by Cu-ZnO interaction are responsible for methanol synthesis [42]. The role of Cr2O3 in commercial catalysts is thought to be similar to that of Al2O3， which is to act as stabilizer of the structure of the copper catalyst， thereby reducing sintering. In this study， as in a previous one [18] using a different technique todeposit chromia on the surface of skeletal copper， we have shown that Cr2O3 has a significant role in copper-based methanol synthesis from CO2. That role is to improve the methanol yield by reducing the RWGS reaction (Fig. 3) aswas observed in the earlier study [18]. A major finding of this study has been the very strong evidence the Cr2O3 has a synergistic effect on the activity of copper for methanol synthesis， methanol steam reforming and the water gas shift reactions. From Figs. 4 and 5 it can be seen that 0.85 wt.% Cr2O3 enhances the specific activity (mol/hm2 Cu) of skeletal copper 270% for the WGS and 150% for methanol steam reforming. In the case of methanol synthesis (Fig. 2) 0.61 wt.% Cr2O3 increases the specific activity of copper by 67%. The results for the WGS and MS reactions are similar to those obtained under the same reaction conditions using skeletal copper promoted by Cr2O3 which was deposited from sodium chromate in the caustic leach liquor [28]. In that study， the effect of chromia was more pronounced， with an increase in activity of 950% for theWGSand 168% for the MSR reaction， respectively， using skeletal copper containing 0.75 wt.% Cr2O3. For methanol synthesis over Cr2O3 promoted skeletal copperprepared using sodium chromate in the leach liquor， Ma et al. [18] observed no increase in the specific activity of copper. 展開
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- Summarizing the experimental data available today， thiol-capped CdTe nanocrystals synthesized in aqueous solution represent a kind of stable highly luminescent core-shell QD system with a naturally sulfur-capped surface (CdS shell) created ... Summarizing the experimental data available today， thiol-capped CdTe nanocrystals synthesized in aqueous solution represent a kind of stable highly luminescent core-shell QD system with a naturally sulfur-capped surface (CdS shell) created by mercapto- groups covalently attached to the surface cadmium atoms. All chemicals used were of analytical grade or of the highestpurity available. Al2Te3 (lumps) used as a source of H2Te waspurchased from CERAC Inc.， WI. Milli-Q water (Millipore)was used as a solvent. UV-vis absorption spectra were recorded with a Cary 50 spectrophotometer (Varian). Photoluminescence (PL) measurements were performed at room temperature using a FluoroMax-2 spectrofluorimeter (Instruments SA). The room-temperature PL quantum efficiency (QE) of CdTe nanocrystals was estimated following the procedure of ref 41 by comparison with Rhodamine 6G (laser grade， Lambda Physik) in ethanol (Uvasol) assuming its PL QE as 95%.42 High-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray analysis (EDX) were performed on a Philips CM-300 microscope operating at 300 kV. TEM samples were prepared by dropping diluted solutions of transferred in toluene (by surface exchange of capping agent with 1-dodecanethiol) CdTe nanocrystals onto 400-mesh carbon-coated copper grids with the excessive solvent immediately evaporated. Powder X-ray diffraction (XRD) measurements were carried out with a Philips X￠Pert diffractometer (Cu KR-radiation， variable entrance slit， Bragg-Brentano geometry， secondary monochromator). Samples for this study were prepared by placing finely dispersed powders of CdTe nanocrystals on standard Si supports. To investigate the photostability of CdTe QDs， dilute colloidal solutions were irradiated for different time intervals with light of a 450 W xenon lamp cut around 400 nm by a band-pass filter having a bandwidth of _40 nm and a peak transmission of _40%. a “Stable” means here that colloidal solutions of CdTe QDs are stable for months and even years being stored under air in the dark at room temperature. “Moderate” means that colloidal solutions coagulate occasionally during the storage; however， they are generally stable for months as well. Independent of the stabilizer nature， CdTe QDs are generally stable (no oxidation， no or only very minor changes of the optical properties) for years in powder form and in closely packed or nanocrystal/polymer films being kept in the dark under air. b The charge of nanocrystals was evaluated from ú-potential measurements with employment of oppositely charged latex beads. The experimental conditions were chosen so that the ú-potential of the beads changed its sign after adsorption of a monolayer of nanoparticles. 展開
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