Enhanced electrocatalytic activity and stability of Pd nanoparticles supported on TiO2-modified nitrogen-doped carbon for ethanol oxidation in alkaline media

TiO₂-modified nitrogen-doped carbon (TiO₂-NC), prepared by a polymerization-pyrolysis process, is used to support the Pd catalyst for ethanol oxidation reaction (EOR) in alkaline media. X-ray photoelectron spectroscopy characterization indicates that the incorporation of TiO₂ and nitrogen into the carbon matrix could improve the percentage of Pd⁰ in Pd/TiO₂-NC catalyst. Electrochemical characterization shows that the Pd/TiO₂-NC catalyst presents higher electrocatalytic activity and stability for EOR than the nitrogen-doped carbon-supported Pd (Pd/NC) catalyst and the carbon black-supported Pd (Pd/CB) catalyst, which can be mainly attributed to the high percentage of Pd⁰ in Pd/TiO₂-NC catalyst (65%) than those in Pd/NC (48%) and Pd/CB (31%) catalysts. The results indicate that the Pd/TiO₂-NC catalyst holds great potential as high-performance anode catalyst for direct ethanol fuel cells.     

基于近场测试的相控阵天线自动化校准 与阵面监测方法

相控阵天线装配好之后,由于各组成部件机械加工误差、装配误差、部件老化更换和环境温度改变等因素,各 单元通道的初始幅相产生差异,因此必须对天线的所有系统进行校准。本文针对小型化相控阵平台,通过硬连接将相控 阵天线的波控系统与测试设备相结合,提出一种简便的自动化近场逐点校准方法。同时,本文还提出一种简单的外监测 方法。当相控阵天线工作期间,可对阵面的幅相分布进行监测。可在相控阵天线工作期间,对近场幅相校准数据进行修 正,达到阵面自身校准的目的。经对一个16阵元的相控阵天线进行实验测量可知,该自动化校准与阵面自身校准方法可 以准确、快捷测试出天线阵面的幅相分布。非常适合一维、二维相控阵天线,尤其是小型化相控阵天线的幅相校准与监 测。     

Seasonal pollution characteristics of organic compounds in atmospheric fine particles in Beijing

Beijing is a rapidly developing city with severe and unique air pollution problems. Organic matter is the most abundant fraction in fine particles in Beijing, occupying 30-50% of the total mass, indicating its key role in air pollution control. However, detailed chemical characterization of particulate organic matter in Beijing has never been reported. In this study, fine particles in the urban atmosphere in Beijing were investigated for its organic components by GC/MS technique. Over 100 individual organic compounds were identified and quantified in 25 PM2.5 samples from the summer, autumn and winter of 2002-2003. Alkanes, fatty acids, dicarboxylic acids, polycyclic aromatic hydrocarbons and some important tracer compounds (hopanes, levoglucosan and steroids) were the major constituents with the sum of their concentrations of 502, 1471 and 1403 ng m(-3) in summer, autumn and winter, respectively. Different organic compounds presented apparently different seasonal characteristics, reflecting their different dominant emission sources, such as coal combustion, biomass burning and cooking emission. The abundance and origin of these organic compounds are discussed to reveal seasonal air pollution characteristics of Beijing.     

Effect of carbon nanofibers microstructure on electrocatalytic activities of Pd electrocatalysts for ethanol oxidation in alkaline medium

Pd electrocatalysts supported on three types of carbon nanofibers (CNFs), viz. platelet CNFs (p-CNFs), fish-bone CNFs (f-CNFs) and tubular CNFs (t-CNFs) are prepared and the effect of CNFs microstructure on the activities of the electrocatalysts for ethanol oxidation reaction (EOR) is investigated. The information on structural characteristics is obtained by high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. Electrochemical techniques are employed to characterize the microstructure effect of CNFs on the catalytic activities of catalysts. HRTEM images indicate the microstructure of CNFs has a powerful influence on the distribution of Pd particles. The results of the electrochemical characterization also indicate that the structure of CNFs significantly influences the catalytic activities of the electrocatalysts and p-CNFs supported Pd electrocatalyst has the best performance for ethanol oxidation in an alkaline medium because p-CNFs has the highest ratio of edge atoms to basal atoms and correspondingly the fastest electrode kinetics and strongest Pd–CNFs interaction.     

Fine particle emissions from on-road vehicles in the Zhujiang Tunnel, China

Little is known about the characteristics of particulate matter emissions from vehicles in China, although such information is critical in source apportionment modeling, emission inventories, and health effect studies. In this paper, we report a comprehensive characterization of PM2.5 emissions in the Zhujiang Tunnel in the Pearl River Delta region of China. The chemical speciation included elemental carbon, organic carbon, inorganic ions, trace elements, and organic compounds. The emission factors of individual species and their relative distributions were obtained for a mixed fleet of heavy-duty vehicles (19.8%) and light-duty vehicles (80.2%). In addition, separate emission factors of PM2.5 mass, elemental carbon, and organic matter for heavy-duty vehicles and light-duty vehicles also were derived. As compared to the results of other tunnel studies previously conducted, we found that the abundances and distributions of the trace elements in PM2.5 emissions were more varied. In contrast, the characteristics of the trace organic compounds in the PM2.5 emissions in our study were consistent with characteristics found in other tunnel studies and dynamometer tests. Our results suggested that vehicular PM2.5 emissions of organic compounds are less influenced by the geographic area and fleet composition and thereby are more suitable for use in aerosol source apportionment modeling implemented across extensive regions.     

Controllable synthesis of carbon nanofiber supported Pd catalyst for formic acid electrooxidation

Carbon nanofiber (CNF) supported Pd nanoparticles are synthesized with sodium citrate and sodium borohydride served as stabilizing agent and reducing agent, respectively. The size and distribution of the supported Pd nanoparticles are controlled by adjusting the pH value of the synthesis solution. Analyses of the obtained Pd/CNF catalysts indicate that the supported Pd nanoparticles become more uniform in size and the average particle size is decreased from 5.85 to 3.62 nm with pH value of the synthesis solution increasing from 3.2 to 6.0. However, the further increasing of the pH value to 6.5 leads to an increased particle size and the formation of PdO phase in the synthesized Pd/CNF catalyst. The Pd/CNF catalyst synthesized at the pH value of 6.0 exhibits superior catalytic activity and stability for formic acid electrooxidation due to its small particle size and uniform size distribution.     

Development of a kinetics-integrated CFD model for the industrial scale-up of DHA fermentation using Schizochytrium sp.

In this work, a kinetics-integrated CFD model based on coupling computational fluid dynamics (CFD) and biokinetic was developed to guide the industrial scale-up for fermentation of docosahexaenoic acid (DHA). Two-substrate biokinetic equations for a 5 L bioreactor with oxygen and nitrogen source as limiting substrates were established, which were further coupled with CFD to simulate the biomass growth and lipid accumulation as well as flow field environment in the fermentation process. The coupled CFD-biokinetic model can accurately reflect the changes of each relevant variable in the 50 L bioreactor under different operating conditions. The optimal working conditions of a 35 m³ bioreactor were predicted by this model and have secured the one-step scale-up of DHA fermentation, resulting in a biomass, lipid concentration, and DHA content in lipids of 99.2 g/L, 55.7 g/L, and 52.5%, respectively. This study thus provides an effective strategy for rapid industrial scale-up of DHA production.     

Electrophoretic deposition of network-like carbon nanofibers as a palladium catalyst support for ethanol oxidation in alkaline media

A network-like carbon nanofiber (CNF) film with an open porous structure formed by the open space between entangled CNFs is fabricated by electrophoretic deposition. The performance of the CNF film as an electrocatalyst in the presence of electrodeposited Pd nanoparticles for ethanol oxidation in alkaline media is investigated. Cyclic voltammetric analyses show the electrocatalyst has a good electrocatalytic activity toward ethanol oxidation in KOH solution. This is believed to be due to the high dispersion of Pd on the CNF film with a three-dimensional network structure which can provide a large number of available Pd active sites for ethanol oxidation, and to the structural and electrical properties of the film.