Structure and Electrochemical Properties of Carbon Nanotubes Synthesized with Catalysts Obtained by Decomposition of Co,Ni, and Fe Polyoxomolybdates Supported by MgO

Carbon nanotubes (CNTs) were synthesized by thermal decomposition of methane at 900 °C using Co–Mo/MgO, Fe–Mo/MgO, and Ni–Mo/MgO catalysts. To obtain metallic nanoparticles, polyoxomolybdate clusters of Co, Ni, and Fe deposited on MgO were thermally decomposed at 700 °С, and the obtained oxides were heated in a carbon-containing atmosphere. The method of transmission electron microscopy (TEM) testified formation of one to ten walled CNTs with the average outer diameter depending on the catalyst used. Raman spectroscopy data confirmed the presence of single-walled CNTs in the samples obtained with Co–Mo/MgO and Fe–Mo/MgO catalysts. The electrochemical properties demonstrated by the obtained materials in supercapacitors are shown to be functions of their structural and compositional features.     

MgO负载Fe基催化剂选择性合成单/双/多壁碳纳米管

通过浸渍及水热处理获得MgO负载的Fe基催化剂,并将其用于化学气相沉积过程裂解甲烷获得碳纳米管.结果表明,单/双/多壁碳纳米管可选择性地生长在Fe负载量不同的Fe/MgO催化剂上.当Fe负载量仅为0.5%时,铁原子在载体表面烧结为0.8～1.2nm的铁颗粒,碳在这种小颗粒上以表面扩散为主,导致单壁碳纳米管形成,并且单壁碳纳米管的选择性高达90%.当Fe负载量提高到3%时,铁原子聚集成约2.0nm的颗粒,在化学气相沉积中生长碳纳米管时,碳在Fe催化剂颗粒中的体相扩散的贡献增大,在表相扩散和体相扩散的共同作用下,双壁碳纳米管的选择性显著增高.当进一步增加Fe负载量时,铁原子烧结形成1～8nm的颗粒,经过化学气相沉积,在催化剂上生长了单、双、多壁碳纳米管.随着Fe在MgO载体上负载量的增加,管径、管壁数以及半导体管的含量都增加.本研究提供了一种适合大批量选择性生长单/双/多壁碳纳米管的方法.     

助剂铬对Ni/MgO催化剂CVD法制备碳纳米管的促进作用

采用溶胶-凝胶法制备了助剂Cr改性的Ni/MgO催化剂, 用化学气相沉积(CVD)法在600 ℃下裂解甲烷生长碳纳米管, 研究了助剂Cr的引入对催化剂微结构和制备碳纳米管性能的影响. 催化剂样品用XRD, TPR和CO-TPD进行了分析, 制备的碳纳米管用TEM和XRD进行了表征. 实验结果表明, NiO和MgO之间存在着强相互作用而形成固溶体, Ni/MgO催化剂经氢气处理后其中的镍氧化物只有极少部分被还原成为镍. 助剂铬的引入明显促进了镍的还原, 使得催化剂表面的Ni活性中心数增多, 从而使催化剂的活性和性能得到了明显的改进. 在加入助剂后碳纳米管的产率明显增加, 当Cr质量分数为8%时, 碳纳米管的产量为未加助剂时产量的5倍, 碳纳米管和催化剂的质量比达到1928. 当Cr含量进一步增加时, Ni在催剂表面聚集形成大颗粒, 制备出的产品中含有大量的碳纳米纤维和无定形碳. 以8%Cr-Ni/MgO催化剂合成的碳纳米管具有比较高的产率且质量较好.     

Nonoxidative conversion of methane into aromatic hydrocarbons on Ni-Mo/ZSM-5 catalysts

The nonoxidative conversion of methane into aromatic hydrocarbons on high-silica zeolites ZSM-5 containing nanosized powders of molybdenum (4.0 wt %) and nickel (0.1–2.0 wt %) was studied. Data on the acid characteristics of the catalysts and the nature and amount of coke deposits formed on the surface of the catalysts were obtained using the thermal desorption of ammonia and thermal analysis. The microstructure and composition of Ni-Mo/ZSM-5 catalysts were studied by high-resolution transmission electron microscopy and energy-dispersive X-ray analysis. The formation of various chemical species in the samples was detected: oxide-like clusters of Mo within zeolite channels (∼1 nm), molybdenum carbide particles (5–30 nm) on the outer surface of the zeolite, and Ni-Mo alloy particles with different compositions (under reaction conditions, carbon filaments grew on these particles). It was found that, as the Ni content was increased from 0.1 to 2.0 wt %, the rate of deactivation of the catalytic system increased because of blocking pores in the zeolite structure by filamentous carbon up to the formation of condensed coke deposits.     

Promoting effect of group VI metals on Ni/MgO for catalytic growth of carbon nanotubes by ethylene chemical vapour deposition

The incorporation of 1 mass % of group VI metals (chromium, molybdenum, and tungsten) into 4 mass % of Ni/MgO catalysts was evaluated for the synthesis of carbon nanotubes (CNTs) by the catalytic chemical vapour deposition of ethylene. All materials were characterised by XRD, surface area, TEM, SEM, Raman spectroscopy, and TGA-DTA. The resulting data demonstrated that the addition of group VI metals improved the surface area and metal dispersion, thereby achieving a remarkable enhancement in catalytic growth activity. Among the metals of group VI, Mo was found to be the most effective promoter for catalysing the CNTs’ growth. From TEM observation, long CNTs with a higher degree of graphitization were obtained on the Ni-Mo/MgO catalyst. TGA and DTA analysis showed that the as-grown CNTs over both Ni-Mo and Ni-W/MgO catalysts exhibited higher thermal stability.     

催化剂中Cu含量对CVD法制备碳纳米管的影响

本文以溶胶-凝胶法制备了以铜为助剂的Ni/MgO催化剂，X-射线衍射(XRD)表明，经400 ℃氢气处理，催化剂中只有部分镍被还原，原因是NiO和MgO间存在强相互作用，形成固溶体。XRD和程序升温还原(TPR)表明，加入铜促进了镍的还原。CO化学吸附得出，随着催化剂中铜含量的增加，还原后催化剂表面镍原子数目增多，因此，催化剂的活性和反应寿命增加，C₂H₄裂解生成碳纳米管(CNT)的产率随之增加；但是，铜含量过高会引起催化剂表面镍颗粒增大，导致产物中纳米碳纤维（CNF）量增多，CNT量减少。对于约50% Ni/MgO催化剂，铜的最佳含量为4%～6%，此时得到的CNT产率最高，达36 g·g^-1，质量较好(纯度高、管径均匀、石墨化程度高)。     

活性金属不同浸渍顺序对Mo-Ni/Al2O3催化剂硫醚化反应性能的影响

针对硫醚化反应过程使用的Mo-Ni/Al₂O₃催化剂,考察了不同浸渍方法对其催化性能和表面结构性质的影响。结果表明,通过同步浸渍和分步浸渍方法制得Mo-Ni/Al₂O₃催化剂的活性顺序为：先浸渍Mo后浸渍Ni的催化剂（SI-mn）≈Ni和Mo共同浸渍的催化剂（MN）>先浸渍Ni后浸渍Mo的催化剂（SI-nm）。对于SI-mn催化剂,先负载Mo后减弱了二次浸渍的Ni金属与载体间的相互作用,有利于负载金属的活化,并在二次浸渍后焙烧过程显现出显著的电子效应,形成新的Mo-Ni前体物种,有利于在预硫化过程形成适宜硫醚化和二烯烃选择性加氢的活性中心相,促进硫醚化反应和二烯烃选择性加氢。对于共同浸渍的MN催化剂也有类似的性质,因而也具有较好的催化性能。     

Growth, new growth, and amplification of carbon nanotubes as a function of catalyst composition

Carbon nanotubes (CNTs) have been grown using Fe, Co, Ni, and Co/Fe spin-on-catalyst (SOC) systems, involving the metal salt dispersed with a spin-on-glass precursor. During initial growth runs (CH4/H2/900 degrees C), the CNT yield followed the order Co-SOC > Fe-SOC > Ni-SOC. The Fe catalysts produced the longest nanotubes at the expense of a larger average CNT diameter and broader diameter distribution than the Co-SOC system. A series of Co/Fe-SOCs were prepared where as the atomic percentage of Co is increased nucleation of CNT increases but the CNT length decreases. The linear relationship between the diameter and length of CNTs grown from the Co/Fe-SOC suggests that slow growth is beneficial with respect to control over CNT diameter. After initial CNT growth, the original samples were subjected to additional growth runs. Four individual reactions were observed in the Fe-SOC and binary Co/Fe-SOC: regrowth (amplification), double growth (a second CNT growing from a previously active catalyst), CNT etching, and nucleation from initially inactive catalysts (new growth). CNT etching was observed for the mixed catalyst systems (Co/Fe-SOC) but not for either Fe-SOC or Co-SOC. During the regrowth experiments, CNTs were observed that were not present after the initial growth run (and were not as a result of amplification or double growth). Thus, catalysts, which were initially inactive toward nucleation of CNTs in the original growth run, are capable of becoming activated when placed back into the furnace and submitted to regrowth under identical conditions.     

碱性助剂的添加对Ni/CaO-Al2O3催化剂性能的影响

在CH4、 CO2和O2制合成气的反应中, 通过在Ni/CaO-Al2O3催化剂中添加碱性助剂K2O、 MgO和La2O3, 使催化剂的性能得到了改善. 实验结果表明, MgO和La2O3助剂的添加, 有利于提高催化剂的活性;添加K2O, 却相反. 测得催化剂上积炭量的顺序为: Ni-La2O3/CaO-Al2O3相似文献   

Design of superhydrophobic surfaces by synthesis of carbon nanotubes over Co-Mo nanocatalysts deposited under microwave irradiation on Ti-containing mesoporous silica thin films

The superhydrophobic surface has been designed by the synthesis of carbon nanotubes (CNTs) on Ti-containing mesoporous silica thin films (Ti-MSTFs) with Co-Mo binary nanocatalysts. The active Co-Mo catalysts have been successfully deposited on Ti-MSTFs under microwave irradiation. SEM and TEM observations after CNT synthesis revealed that surfaces of Ti-MSTFs were densely covered with CNTs having a diameter of 15 nm. Raman spectra indicated that the undesired structural defects in the carbon network of the synthesized CNTs, which would lead to the formation of hydroxyl groups, were scarce. Interestingly, hydrophobic properties of samples after CNT synthesis were enhanced with increasing titanium concentration of Ti-MSTF, and the water contact angle reached up to 165° on Ti-MSTF with a titanium concentration of 10 at%. The combination of dispersed titanium oxide moieties within the silica frameworks and the microwave irradiation made a great contribution to deposit active Co-Mo catalysts responsible for the formation of well-dense CNTs.     

Effect of elemental molar ratio on the synthesis of higher alcohols over Co-promoted alkali-modified Mo_2C catalysts supported on CNTs

A series of molybdenum carbide catalysts promoted by potassium and cobalt,supported on carbon nanotubes(CNTs) were prepared by carbothermal hydrogen reduction method using CNTs as a carbon precursor.Firstly,molybdenum and cobalt were loaded by co-precipitation method,and then potassium and additional molybdenum were impregnated to previous resultant.Different Mo/Co and K/Co molar ratio were used in catalyst synthesis.All the catalysts were characterized by ICP,BET,TEM,TPR,XRD and XPS,and the catalysts performances for higher alcohols synthesis(HAS) were investigated in a fixed-bed micro-reactor.The maximum selectivity to higher alcohols(C2+OH) was obtained at Mo/Co and K/Mo molar ratios of 1.66 and 0.6,respectively.XRD results confirmed the formation of K-Mo-C site and Co3Mo3 C phase that might play important role in producing C2+OH.     

Synthesis of multiwalled carbon nanotubes through a modified Wolff-Kishner reduction process

We report the synthesis of highly crystallized multiwalled carbon nanotubes through a modified Wolff-Kishner reduction process at a low temperature of 180 degrees C without adding the conventional catalysts of Fe/Co/Ni into the reaction vessel. The as-synthesized carbon nanotubes are about 10-40 nm in diameter and several tens of micrometers in length. The experimental results indicate that a high concentration of NaOH is essential to the formation of carbon nanotubes. This technique opens a new route for the synthesis of CNTs and other carbon nanostructured materials.     

Production of hydrogen-rich gas and multi-walled carbon nanotubes from ethanol decomposition over molybdenum modified Ni/MgO catalysts

A series of molybdenum modified Ni/MgO catalysts(Ni-Mo/MgO) with different loading ratios of Ni : Mo were prepared by impregnation method. Ethanol decomposition to co-produce multi-walled carbon nanotubes and hydrogen-rich gas at temperatures of 600–800 ℃ was investigated over the synthesized Ni-Mo/MgO catalysts. The results showed that the catalytic activity depended strongly on the reaction temperature and loading ratio of Ni : Mo. According to the gaseous and solid products obtained, the reaction pathways for ethanol decomposition were suggested.     

Effect of pH and Temperature on Adsorption of Dimethyl Phthalate on Carbon Nanotubes in Aqueous Phase

Adsorptions of dimethyl phthalate (DMP) on carbon nanotubes (CNTs) in aqueous phase at various pH and temperatures were studied. The increase in pH results in the increase in adsorption coefficient. The adsorption is governed by the π-π electron interaction which is affected by the changes in pH of the medium. The outer diameter of the CNTs greatly influences the adsorption behavior of CNT for DMP. Under the same working temperature, the adsorption capacity of CNTs for DMP is inversely related to the average outer diameter of the CNT: single-walled SWCNT (1.4 nm)>multi-walled MWCNT10 (9.4 nm)>MWCNT30 (27.8 nm)>MWCNT40 (42.7 nm). The larger surface area of CNTs provides many active sites for adsorption of DMP molecules. The Freundlich model can describe well the adsorption isotherms of DMP on CNTs. The thermodynamic parameters of standard free energy, standard enthalpy (ΔH), and standard entropy changes are determined, showing that the adsorption of DMP on CNTs is an endothermic and spontaneous reaction. The ΔH value of 27.8 nm-sized MWCNT (22.69 kJ/mol) is higher than 1.4 nm-sized SWCNT (6.05 kJ/mol), inferring that the adsorption process becomes more endothermic with the increase in the outer diameter of CNTs.     

水促进Co/Mo/Al2O3催化剂上碳纳米管的生长

对水促进Co/Mo/Al2O3催化剂裂解乙烯生长碳纳米管(CNTs)的研究发现,通入体积分数(φ)为0.6％的水蒸汽在1h内可将CNTs的生长倍率从3.7 g·g-1提高至70 g·g-1.水的作用在于恢复被无定形碳包覆的催化剂颗粒的活性,水的加入量由于其积碳(促进同体碳生成)和消碳(去除固体碳)的竞争作用而存在最佳值.不同反应时间下乙烯的转化率与有效催化剂含量的分析表明,在CNTs生长后期,水的催化促进作用减弱.将催化剂的相对活性与CNT聚团的相对密度关联发现,反应后期的CNTs主要在聚团内部缠绕生长,催化剂被包覆失活.拉曼测试与差热热重分析表明,生长阻力导致所得CNTs缺陷增多,CNT聚团密度变化与CNT缺陷间存在对应关系.聚团内外CNTs的生长阻力不同,生长倍率不同,导致产品纯度不均匀.     

载体对担载Ni催化剂甲烷与二氧化碳重整反应活性的影响

制备了 Zr O2 、Mg O改性的 Al2 O3、Ti O2 复合载体 ,并应用 X-射线粉末衍射 (XRD)、比表面积测定、扫描电镜 (SEM)等手段进行了表征 .结果表明 ,这些氧化物在 Al2 O3上的晶粒尺寸小、比表面积大 ,分散较好 ,而在Ti O2 上的分散性较差 .对经 10 73K焙烧的 Mg O/ Ti O2 ,还发现部分 Ti O2 载体由锐钛矿变为金红石 ,同时生成Mg Ti O3 新相 .考察了载体对 Ni催化剂的 CH4与 CO2 重整反应活性的影响 ,其次序为 :Mg O/ Al2 O3>Zr O2 /Al2 O3>Al2 O3>Mg O >Zr O2 >Ti O2 >Mg O/ Ti O2 . Ti O2 及 Mg O/ Ti O2 担载 Ni催化剂的低活性可能与 Ti O2 本身的还原性有关     

镍基板上低温合成定向纳米碳管

纳米碳管具有非常优异的场发射效应 ,亮度高、均匀且稳定的纳米碳管场效应发射器 ,例如平板显示器、阴极射线管以及信号灯等有着非常广阔的应用前景 [1] .由于纳米碳管的场发射效应与纳米碳管的方向性有关[2 ] ,因此定向纳米碳管的制备及其场发射性能研究是当前的一个研究热点     

碳纳米管结构对碳纳米管载Pt催化剂电催化性能的影响

在制备单、双壁及不同管径的多壁碳纳米管(CNTs)的基础上, 用液相还原法把Pt沉积到单、双壁和管径不同的多壁CNTs上. 发现制得的CNTs载Pt(Pt/CNTs)催化剂对甲醇氧化的电催化活性随CNTs管径减小而增加. 这归结于管径小的CNTs的比表面积较大, 含氧基团多, 有利于提高Pt粒子分散度, 加上管径小的单壁CNTs具有更高的导电性, 这些因素都有利于提高Pt/CNTs催化剂对甲醇氧化的电催化活性.     

石墨烯负载镍催化CO_2加氢甲烷化

采用改进的Hummers法制备了氧化石墨烯(GO),经水合肼还原得到石墨烯(RGO),通过浸渍法制备了石墨烯负载的镍基催化剂(Ni/RGO);对其催化二氧化碳甲烷化反应的性能进行了研究,并与以碳纳米管(CNTs)和活性炭(AC)为载体负载的Ni基催化剂进行了比较.由于催化剂的载体分别为RGO,CNTs和AC,所以Ni将会表现出不同的形态.利用红外光谱(FTIR)、比表面积(BET)测试、程序升温还原(H2-TPR)、X射线衍射(XRD)分析和透射电子显微镜(TEM)等表征手段对其结构及物理性质进行了表征.结果表明,Ni/RGO具有相对较大的比表面积(316 m~2/g),Ni在Ni/RGO上的颗粒尺寸(5.3 nm)小于其在Ni/CNTs(8.9 nm)和Ni/AC(11.6 nm)上的颗粒尺寸;该催化剂在二氧化碳甲烷化反应中具有更高的催化活性和选择性,而且具有良好的使用寿命.     

Comparing the deactivation behaviour of Co/CNT and Co/γ-Al2O3 nano catalysts in Fischer-Tropsch synthesis

An extensive study of Fischer-Tropsch (FT) synthesis on cobalt nano particles supported on γ-alumina and carbon nanotubes (CNTs) catalysts is reported.20 wt% of cobalt is loaded on the supports by impregnation method.The deactivation of the two catalysts was studied at 220 C,2 MPa and 2.7 L/h feed flow rate using a fixed bed micro-reactor.The calcined fresh and used catalysts were characterized extensively and different sources of catalyst deactivation were identified.Formation of cobalt-support mixed oxides in the form of xCoO yAl2O3 and cobalt aluminates formation were the main sources of the Co/γ-Al2O3 catalyst deactivation.However sintering and cluster growth of cobalt nano particles are the main sources of the Co/CNTs catalyst deactivation.In the case of the Co/γ-Al2O3 catalyst,after 720 h on stream of continuous FT synthesis the average cobalt nano particles diameter increased from 15.9 to 18.4 nm,whereas,under the same reaction conditions the average cobalt nano particles diameter of the Co/CNTs increased from 11.2 to 17.8 nm.Although,the initial FT activity of the Co/CNTs was 26% higher than that of the Co/γ-Al2O3,the FT activity over the Co/CNTs after 720 h on stream decreased by 49% and that over the Co/γ-Al2O3 by 32%.For the Co/γ-Al2O3 catalyst 6.7% of total activity loss and for the Co/CNTs catalyst 11.6% of total activity loss cannot be recovered after regeneration of the catalyst at the same conditions of the first regeneration step.It is concluded that using CNTs as cobalt catalyst support is beneficial in carbon utilization as compared to γ-Al2O3 support,but the Co/CNTs catalyst is more susceptible for deactivation.