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排序方式: 共有558条查询结果,搜索用时 15 毫秒
91.
结合工程实例,对超限框支结构设计原则进行简要论述。通过对设计探索过程的剖析,我们试图阐明:对于较为复杂的结构体系,在缺乏较为明确或常规的规范依据情况下,需要回归到结构设计最基本的原理上。先期针对结构不同部位、不同受力状态,分别设定出在小震、中震、大震作用下不同的抗震设防目标,并根据目标期望,进行有目的的结构分析,并取得相对应的、可靠的内力值。这样,才更能够最真实的反应出结构状态,从而确保结构安全。 相似文献
92.
Ru-active carbon (Ru/C) catalysts are prepared by impregnation reduction method for hydrogen generation via hydrolysis of alkaline sodium borohydride (NaBH4) solution. The corresponding activity and durability of the prepared catalysts are tested in an immobile bed reactor. The variation of hydrogen generation rate with the increasing of flux and concentration of NaBH4 solution is measured. The durability of the catalysts prepared under various reductive pH values and reductants is tested by using different concentrations of NaBH4 solution (10 & 15 wt%). It is found that the durability of catalyst in 15 wt% NaBH4 solution is longer than that in 10 wt% NaBH4 solution. The deactivation of Ru/C catalysts is considered as the comprehensive effect of three factors: the loss of Ru, the deposition of byproducts on the catalyst surface and the aggregation of Ru particles. 相似文献
93.
Aine Desikusumastuti Mathias Laurin Markus Happel Zhihui Qin Shamil Shaikhutdinov Jörg Libuda 《Catalysis Letters》2008,121(3-4):311-318
Based on a well-defined model-catalyst approach, we study the particle size dependent properties of NO
x
storage materials. The single-crystal based model systems are prepared on an ordered Al2O3 film, on which BaO nanoparticles are grown under ultrahigh-vacuum (UVH) conditions. Particle size and density are characterized
by scanning tunneling microscopy (STM). The interaction with NO2 is probed by molecular beam (MB) methods in combination with time-resolved IR reflection absorption spectroscopy (TR-IRAS).
It is found that both, the stability and the formation kinetics of alumina supported barium nitrate nanoparticles show a strong
dependence on particle size. Very small BaO particles are rapidly converted into nitrates, however, the resulting aggregates
exhibit a strongly reduced thermal stability. Surface and bulk nitrate and nitrate features are identified by means of vibrational
spectroscopy. It is concluded that the size dependencies are related to the formation and decomposition of surface-related
BaNO
x
species the decomposition temperature of which can be tuned over an exceptionally large temperature interval. It is suggested
that the stability of these surface NO
x
species is strongly modified by the underlying support. 相似文献
94.
Xiao-dan SunXiao-dong Yi Wei-qi HuaHao Jin Wei-zheng Weng Hui-lin Wan 《Fuel Processing Technology》2011,92(8):1662-1669
SBA-3 and SiO2-supported MoVTe mixed oxide catalysts have been prepared by impregnation and/or direct synthesis methods and tested for selective oxidation of isobutane to methacrolein (MAL). It was found that the supported catalysts showed much higher activity than the bulk MoVTe mixed oxide for the reaction. Among the supported catalysts, better isobutane conversion and MAL yield were achieved on the 3% MoV0.8Te0.23Ox/SBA-3 catalyst prepared by the impregnation method. The catalysts were characterized with BET, XRD, Raman, H2-TPR, XPS and FT-IR of pyridine adsorption. The good performance of the SiO2 and SBA-3 supported MoV0.8Te0.23Ox catalysts was attributed to a combination of different properties: (i) formation of well dispersed active phases on large surface areas of SiO2 and SBA-3 supports, which is beneficial for the isolation of active site and preventing the further oxidation of unstable reaction intermediate as well as product; (ii) improved activity for hydrogen abstraction of C-H bond of isobutane due to the formation of isolated pseudotetrahedral VO4 species. 相似文献
95.
In order to improve proton exchange membrane water electrolyzer (PEMWE) performance, some factors related to the processes of preparing the Membrane Electrode Assemblies (MEAs), such as iridium (Ir) electrocatalyst loading and Nafion® content at the anode, thicknesses of proton exchange membrane and gas diffusion layers (GDLs), were examined. In addition, a home-made supported Ir/titanium carbide (Ir/TiC, 20% Ir by weight) was developed for the anode. With best commercial Ir catalyst loading of 1.5 mg cm−2 Ir at the anode, the cell's current densities of 1346 mA cm−2, 1820 mA cm−2 and 2250 mA cm−2 were achieved at the cell potentials of 1.80 V, 1.90 V and 2.00 V, respectively. A PEMWE with 0.3 mg cm−2 Ir loading of Ir/TiC anode catalyst was comparatively stable and gave current densities of 840 mA cm−2, 1130 mA cm−2 and 1463 mA cm−2 at the cell potentials of 1.80 V, 1.90 V and 2.00 V, respectively. Based on catalysis efficiency of Amperes per milligram of Ir, the Ir/TiC catalyst is found to be more active than unsupported Ir catalyst. 相似文献
96.
97.
研究设计了一种变压器手动拆卸装置,主要包括承重滑道、支撑横梁、脚梯、支撑架、固定杆和带式捆绑器等结构。各连接部位采用插接式微调结构和粘贴式带式捆绑器,不仅可以方便地调整元件安装位置,而且省去了螺栓连接等工作。同时对主要承重部件的支撑横梁进行优化设计,运用Ansys Workbench软件建立多种桁架模型,并分析比较得到最优的桁架模型。该设备具有安装拆卸简单快捷、使用方便、占地小的特点,解决了在有限空间内或因其他条件限制而无法使用大型起重机械安装变压器的问题,具有较好的推广应用价值。 相似文献
98.
Masayuki Shirai Chandrashekhar V. Rode Eiichi Mine Akiyoshi Sasaki Osamu Sato Norihito Hiyoshi 《Catalysis Today》2006,115(1-4):248-253
Catalytic ring hydrogenations of naphthalene and 1-naphthol were studied over several supported metal catalysts in supercritical carbon dioxide solvent at low temperature. Higher concentration of hydrogen in supercritical carbon dioxide and lower reaction temperature were responsible for higher catalyst activity and selectivities to the desired partial ring hydrogenated products as compared with those observed in organic solvent for the same catalyst. 相似文献
99.
M.V. Martínez-Huerta X. Gao H. Tian I.E. Wachs J.L.G. Fierro M.A. Baares 《Catalysis Today》2006,118(3-4):279-287
The influence of vanadium oxide loading in the supported VOx/Al2O3 catalyst system upon the dehydrated surface vanadia molecular structure, surface acidic properties, reduction characteristics and the catalytic oxidative dehydrogenation (ODH) of ethane to ethylene was investigated. Characterization of the supported VOx/Al2O3 catalysts by XPS surface analysis and Raman spectroscopy revealed that vanadia was highly dispersed on the Al2O3 support as a two-dimensional surface VOx overlayer with monolayer surface coverage corresponding to 9 V/nm2. Furthermore, Raman revealed that the extent of polymerization of surface VOx species increases with surface vanadia coverage in the sub-monolayer region. Pyridine chemisorption-IR studies revealed that the number of surface Brønsted acid sites increases with increasing surface VOx coverage and parallels the extent of polymerization in the sub-monolayer region. The reducibility of the surface VOx species was monitored by both H2-TPR and in situ Raman spectroscopy and also revealed that the reducibility of the surface VOx species increases with surface VOx coverage and parallels the extent of polymerization in the sub-monolayer region. The fraction of monomeric and polymeric surface VOx species has been quantitatively calculated by a novel UV–Vis DRS method. The overall ethane ODH TOF value, however, is constant with surface vanadia coverage in the sub-monolayer region. The constant ethane TOF reveals that both isolated and polymeric surface VOx species possess essentially the same TOF value for ethane activation. The reducibility and Brønsted acidity of the surface VOx species, however, do affect the ethylene selectivity. The highest selectivity to ethylene was obtained at a surface vanadia density of 2.2 V/nm2, which corresponds to a little more than 0.25 monolayer coverage. Below 2.2 V/nm2, exposed Al support cations are responsible for converting ethylene to CO. Above 2.2 V/nm2, the enhanced reducibility and surface Brønsted acidity appear to decrease the ethylene selectivity, which may also be due to higher conversion levels. Above monolayer coverage, crystalline V2O5 nanoparticles are also present and do not contribute to ethane activation, but are responsible for unselective conversion of ethylene to CO. The crystalline V2O5 nanoparticles also react with the Al2O3 support at elevated temperatures via a solid-state reaction to form crystalline AlVO4, which suppresses ethylene combustion of the crystalline V2O5 nanoparticles. The molecular structure–chemical characteristics of the surface VOx species demonstrate that neither the terminal VO nor bridging VOV bonds influence the chemical properties of the supported VOx/Al2O3 catalysts, and that the bridging VOAl bond represents the catalytic active site for ethane activation. 相似文献
100.
Jian Liu Zhen Zhao Chunming Xu Aijun Duan Ling Zhu Xuezhong Wang 《Catalysis Today》2006,118(3-4):315-322
Vanadium oxides supported on γ-Al2O3, SiO2, TiO2, and ZrO2 were studied on their molecular structures and reactive performances for soot combustion. To investigate the effect of different alkali metals on the structures and reactivities of supported-vanadium oxide catalysts, they were doped into the V4/TiO2 catalyst which had the best intrinsic activity for soot combustion in the selected supported vanadium oxide catalysts. The experimental results demonstrated that the catalytic properties of these catalysts depended on the vanadium loading amount, support nature, and the presence or the absence of alkali metals. The spectroscopic analysis (FT-IR and UV–vis) and H2-TPR results revealed that the higher activity of alkali-promoted vanadium oxide catalysts could be related to the ability of alkali metal promoting the redox cycle of the active vanadyl species. TG results showed that adding alkali to Vm/TiO2 catalyst was beneficial to lowering their melting points. Low melting points could ensure the good surface atom migration ability, which would improve the contact between the catalyst and soot. Due to the alkali metal components promoting the redox ability and the mobility of the catalysts, alkali-modified vanadium oxide catalysts could remarkably improve their catalytic activities for soot combustion. The catalytic activity order for soot combustion followed Li > Na > K > Rb > Cs in the catalyst system of alkali-V4/TiO2, and the reason why it followed this sequence was discussed. 相似文献