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991.
CO氧化可能是多相催化领域最常见的反应,它不仅能作为探针反应研究催化剂结构、反应活性位等,而且在诸多实际过程如空气净化、汽车尾气污染物控制、燃料电池所用氢源净化等扮演重要角色.最早的 CO氧化催化剂为霍加拉特剂,其组分主要为 CuO与 MnO2混合氧化物,然而在实际应用过程中存在低温活性低、吸湿易失活等缺点.1987年, Haruta等发现湿化学法制备的氧化物负载 Au催化剂表现出非常高的低温 CO氧化活性及耐水稳定性,其 Au粒子以纳米尺度分散,进而引发了催化研究领域的“淘金热”及纳米催化研究热潮.而 CO氧化通常作为考察 Au催化剂结构性质的探针反应,也成为考核其它金属催化剂是否具有高活性的判据之一. Pt族金属上 CO氧化反应从 Langmuir等研究开始至今已有100多年,然而低温下该金属催化剂活性与 Au催化剂相比要低一个数量级.本质原因为 Pt族金属上 CO吸附较强, O2吸附与活化受到抑制,而该步骤被认为是 CO氧化的速控步,因而表现出较低的催化活性.通常 Pt族金属催化剂需要100oC以上 CO才能脱附, O2进而得以吸附.目前研究人员采取多种策略,其基本原则为削弱 Pt族金属上 CO吸附强度或者提供其它活性位供 O2吸附与活化.本综述将概括近十年来Pt族金属催化剂 CO氧化研究进展,主要总结室温甚至超低温条件下的研究成果.高活性 CO氧化催化剂主要是通过采用可还原氧化物为载体或助剂,或者改变催化剂表面性质如使表面富 OH基物种来形成. Au催化剂的研究发现,改变金属粒子尺寸极有可能获得不同寻常的催化性能,而常规的 Pt族金属催化剂研究主要是在纳米尺度.近期人们发现逐渐减小 Pt族金属粒子尺寸,从纳米到亚纳米甚至单原子时,其电荷状态逐渐呈正价形式,这有利于削弱其 CO吸附强度.此外,可通过增强金属载体间的相互作用,改变金属载体接触方式,如从核壳到交叉结联结构,构筑出更多的金属载体界面,使得 O2更容易吸附与活化或稳定更多的 OH基物种进而在此界面与吸附的 CO反应.伴随着表征技术的发展, CO氧化机理的认识也更加深入,这给催化剂的设计带来更多新的思路.(1)改变 CO吸附活化位,将 CO吸附活化位从金属转移到载体上,从而大大降低 CO吸附强度,活化的 CO物种在反应过程中容易溢流到金属载体界面处,这甚至有利于超低温度下(–100oC左右) CO氧化.(2)改变 O2活化形式. O2通常在 Pt族金属上容易以解离氧原子形式存在,通过改变载体、金属载体界面性质使得 O2以分子氧形式活化,如形成超氧或过氧物种,这有利于降低 CO氧化的活化能垒,进而提高其低温甚至超低温下 CO氧化活性.今后,设计并合成出在超低温度下能够氧化 CO的 Pt族金属催化剂将成为 CO氧化催化剂研究的重要方向之一. 相似文献
992.
Realignment of Nanocrystal Aggregates into Single Crystals as a Result of Inherent Surface Stress 下载免费PDF全文
Zhaoming Liu Dr. Haihua Pan Genxing Zhu Yaling Li Dr. Jinhui Tao Biao Jin Prof. Ruikang Tang 《Angewandte Chemie (International ed. in English)》2016,55(41):12836-12840
Crystallization by particle attachment is widely observed in both natural and synthetic environments. Although this form of nonclassical crystallization is generally described by oriented attachment, random aggregation of building blocks to give single‐crystal products is also observed, but the mechanism of crystallographic realignment is unknown. We herein reveal that random attachment during aggregation‐based growth initially produces a nonoriented growth front. Subsequent evolution of the orientation is driven by the inherent surface stress applied by the disordered surface layer and results in single‐crystal formation by grain‐boundary migration. This mechanism is corroborated by measurements of orientation rate versus external stress, which demonstrated a predictive relationship between the two. These findings advance our understandings about aggregation‐based growth via nanocrystal blocks and suggest an approach to material synthesis that takes advantage of stress‐induced coalignment. 相似文献
993.
Identification of HcgC as a SAM‐Dependent Pyridinol Methyltransferase in [Fe]‐Hydrogenase Cofactor Biosynthesis 下载免费PDF全文
Dr. Takashi Fujishiro Liping Bai Dr. Tao Xu Dr. Xiulan Xie Dr. Michael Schick Jörg Kahnt Prof. Dr. Michael Rother Prof. Dr. Xile Hu Dr. Ulrich Ermler Dr. Seigo Shima 《Angewandte Chemie (International ed. in English)》2016,55(33):9648-9651
Previous retrosynthetic and isotope‐labeling studies have indicated that biosynthesis of the iron guanylylpyridinol (FeGP) cofactor of [Fe]‐hydrogenase requires a methyltransferase. This hypothetical enzyme covalently attaches the methyl group at the 3‐position of the pyridinol ring. We describe the identification of HcgC, a gene product of the hcgA‐G cluster responsible for FeGP cofactor biosynthesis. It acts as an S‐adenosylmethionine (SAM)‐dependent methyltransferase, based on the crystal structures of HcgC and the HcgC/SAM and HcgC/S‐adenosylhomocysteine (SAH) complexes. The pyridinol substrate, 6‐carboxymethyl‐5‐methyl‐4‐hydroxy‐2‐pyridinol, was predicted based on properties of the conserved binding pocket and substrate docking simulations. For verification, the assumed substrate was synthesized and used in a kinetic assay. Mass spectrometry and NMR analysis revealed 6‐carboxymethyl‐3,5‐dimethyl‐4‐hydroxy‐2‐pyridinol as the reaction product, which confirmed the function of HcgC. 相似文献
994.
Enhanced Electrochemical Kinetics on Conductive Polar Mediators for Lithium–Sulfur Batteries 下载免费PDF全文
Hong‐Jie Peng Ge Zhang Xiang Chen Ze‐Wen Zhang Wen‐Tao Xu Prof. Jia‐Qi Huang Prof. Qiang Zhang 《Angewandte Chemie (International ed. in English)》2016,55(42):12990-12995
Lithium–sulfur (Li–S) batteries have been recognized as promising substitutes for current energy‐storage technologies owing to their exceptional advantage in energy density. The main challenge in developing highly efficient and long‐life Li–S batteries is simultaneously suppressing the shuttle effect and improving the redox kinetics. Polar host materials have desirable chemisorptive properties to localize the mobile polysulfide intermediates; however, the role of their electrical conductivity in the redox kinetics of subsequent electrochemical reactions is not fully understood. Conductive polar titanium carbides (TiC) are shown to increase the intrinsic activity towards liquid–liquid polysulfide interconversion and liquid–solid precipitation of lithium sulfides more than non‐polar carbon and semiconducting titanium dioxides. The enhanced electrochemical kinetics on a polar conductor guided the design of novel hybrid host materials of TiC nanoparticles grown within a porous graphene framework (TiC@G). With a high sulfur loading of 3.5 mg cm?2, the TiC@G/sulfur composite cathode exhibited a substantially enhanced electrochemical performance. 相似文献
995.
保留指数是气相色谱中用于化合物结构鉴定的重要工具。本研究对基于正构烷烃和直链脂肪酸甲酯的线性保留指数进行线性拟合,发现在所设定的4种不同的升温条件下两种保留指数之间呈现较为固定的线性关系:在弱极性柱(固定液为5%苯基和95%的甲基聚硅氧烷)和极性柱(固定液为聚乙二醇)条件下,正构烷烃保留指数(y)与脂肪酸甲酯保留指数(x)之间的关系分别为y=1.005 1x+318.51(r2=1)和y=1.036 2x+562.519(r2=1)。采用文献保留指数对所建立的线性关系进行验证,发现通过公式换算得到的保留指数均在其对应文献值的均值加减方差范围内,说明换算公式真实有效。两种保留指数之间关系的建立对于化合物结构鉴定时扩大保留指数搜索范围、减少保留指数测定实验具有重要意义。 相似文献
996.
Jiajing Wu Dr. Jianyu Tong Dr. Yuan Gao Dr. Aifei Wang Prof. Tao Zhang Prof. Hairen Tan Prof. Shuming Nie Prof. Zhengtao Deng 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(20):7812-7816
A novel triphenylphosphine (TPP) treatment strategy was developed to prepare the near-infrared emission CsPbI3 nanocrystal (NC)-polymer composite thin-film luminescent solar concentrators (LSCs) featuring high absolute photoluminescence quantum yield (PLQY), low reabsorption, and high stability. The PL emission of the LSCs is centered at about 700 nm with 99.4±0.4 % PLQY and narrow full width at half maximum (FWHM) of 75 meV (30 nm). Compared with LSCs prepared with classic CsPbI3 NCs, the stability of the LSCs after TPP treatments has been greatly improved, even after long-term (30 days) immersion in water and strong mercury-lamp irradiation (50 mW cm−2). Owing to the presence of lone-pair electrons on the phosphorus atom, TPP is also used as a photoinitiator, with higher efficiency than other common photoinitiators. Large-area (ca. 75 cm2) infrared LSCs were achieved with a high optical conversion efficiency of 3.1 % at a geometric factor of 10. 相似文献
997.
998.
Novel bio-based and biodegradable block copolymers were synthesized by "click" reaction between poly(L-lactide)(PLLA) and polyamide 4(PA4). Upon tuning the molar mass of PLLA block, the properties of copolymers and electrospun ultrafine fibers were investigated and compared with those of PLLA and PA4 blends. PLLA and PA4 were found incompatible and formed individual crystalline regions, along with reciprocal inhibition in crystallization. Electrospun fibers were highly hydrophobic, even if hydrophilic PA4 was the rich component. The crystallinity of either PLLA or PA4 decreased after electrospinning and PLLA-rich as-spun fibers were almost amorphous. Immersion tests proved that fibers of block copolymers were relatively homogeneous with micro-phase separation between PLLA and PA4. The fibrous structures of copolymers were different from those of the fibers electrospun from blends, for which sheath-core structure induced by macro-phase separation between homopolymers of PLLA and PA4 was confirmed by TEM, EDS, and XPS. 相似文献
999.
1000.
利用四氰基构筑单元和不同位阻的吡啶类配体,合成了3例氰基桥联的Fe2ⅢNiⅡ链状化合物。化合物{[Fe(bpy)(CN)4]2[Ni(bp)2]·2H2O}n(1)(bpy=2,2''-联吡啶;bp=4-苯基吡啶),{[Fe(bpy)(CN)4]2[Ni(papy)2]·H2O}n(2)(papy=4-(苯基氮烯)吡啶)和{[Fe(bpy)(CN)4]2[Ni(azp)]·4H2O}n(3)(azp=1,2-二(吡啶-4-基)二氮烯)均为双之字型的链状结构。磁性测试表明化合物1~3均表现为链内的铁磁相互作用。化合物1表现出单链磁体行为,有效能垒为10.9 K。 相似文献