Specific performance of silica-coated Ni catalysts for the partial oxidation of methane to synthesis gas

Silica-supported Ni catalysts were prepared by using water-in-oil typed microemulsion. By preparation using the microemulsion, Ni metal particles with diameters < ca. 5 nm were covered uniformly with silica layers with thickness of ca. 10 nm, whereas Ni metal particles were supported on the outer surface of silica by a conventional impregnation method. The Ni metal covered with silica (coat-Ni) showed excellent catalytic performance for the partial oxidation of methane into synthesis gas. Coat-Ni showed a high activity and a long life for the partial oxidation of methane at temperatures above 973 K to form CO and hydrogen, whereas Ni metal catalyst supported on silica was rapidly deactivated for the reaction. Ni K-edge XANES/EXAFS and temperature-programmed reduction for these catalysts showed that Ni metal particles in coat-Ni strongly interacted with silica. The strong interaction of Ni metal particles with silica would improve their catalytic performance for the partial oxidation of methane into synthesis gas.     

Production of Carbon Nanotube by Ethylene Decomposition over Silica-Coated Metal Catalysts

Formation of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) through the decomposition of ethylene at 973 K was achieved using various metal catalysts covered with silica layers. CNFs of various diameters were formed by ethylene decomposition over a Co metal catalyst supported on the outer surface of the silica. In contrast, silica-coated Co catalysts formed CNTs with uniform diameters by ethylene decomposition. Silica-coated Ni/SiO₂ and Pt/carbon black also formed CNTs with uniform diameters, while CNFs and CNTs with various diameters were formed over Ni/SiO₂ and Pt/carbon black without a silica coating. These results indicate that silica layers that envelop metal particles prevent sintering of the metal particles during ethylene decomposition. This results in the preferential formation of CNTs with a uniform diameter.     

Hydrocarbon steam reforming on Ni alloys at solid oxide fuel cell operating conditions

We demonstrate that supported Sn/Ni alloy catalyst is more resistant to deactivation via carbon deposition than supported monometallic Ni catalyst in steam reforming of isooctane at moderate steam to carbon ratios, irrespective of the average size of metal particles and the metal loading. The experiments were performed for average diameters of catalytic particles ranging from 30 to 500 nm and for the loading of active material ranging from 15 to 44 wt% with respect to the total mass of catalyst. The steam reforming reactions were performed at conditions that are consistent with typical solid oxide fuel cell (SOFC) operating conditions. DFT calculations show that the reasons for the enhanced carbon-tolerance of Sn/Ni compared to monometallic Ni are high propensity of Sn/Ni to oxidize carbon and lower driving force to form carbon deposits on low-coordinated metal sites.     

Structure of Co and Co oxide clusters in MCM-41

The structural properties of Co/MCM-41 with pore diameters between 2.9 and 3.6 nm prepared by direct synthesis and impregnation were investigated. For both preparation methods, the size of the metal particles decreased with the pore diameter. For Co/MCM-41 with the same pore diameter we observed that the direct synthesis method led to significantly smaller metal clusters compared to the impregnation method. For all Co/MCM-41 samples constraints of the metal cluster sizes were observed, which are speculated to result from influences of the micro structure during the formation of the catalyst precursor.     

Increased CNT growth density with an additional thin Ni layer on the Fe/Al catalyst film

Density of 3 × 10¹¹/cm² and diameter of CNTs of 9–12 nm were successfully controlled by using the multi-layered catalyst film consisting of an additional Ni layer on Fe/Al catalyst film. EDS analysis for the annealed catalyst films revealed that the increase of the density of Fe catalyst particles corresponded with the decrease of Ni in the films, which strongly suggested that the additional thin Ni layer on the Fe/Al multi-layered catalyst films prevented the fine Fe catalyst particles from agglomeration, resulting in the growth of high-density, and uniform diameter of CNTs.     

Catalytic growth of semiconductor micro- and nano-crystals using transition metal catalysts

The catalytic reaction concept was introduced in the growth of semiconductor micro- and nano-crystals. It was found that gallium nitride (GaN) micro- and nano-crystal structures, carbon nanaotubes, and silicon carbide (SiC) nanostructures could be efficiently grown using transition metal catalysts. The use of Ni catalyst enhanced the growth rate and crystallinity of GaN micro-crystals. At 1,100 ‡C, the growth rate of GaN micro-crystals grown in the presence of Ni catalyst was over nine times higher than that in the absence of the catalyst. The crystal quality of the GaN microcrystals was almost comparable to that of bulk GaN. Good quality GaN nanowires was also grown over Ni catalyst loaded on Si wafer. The nanowires had 6H hexagonal structure and their diameter was in the range of 30–50 nm. Multiwall nanotubes (MWNTs) were grown over 20Fe : 20Ni : 60Al₂O₃ catalyst. However, single wall nanotubes (SWNTs) were grown over 15Co : 15Mo : 70MgO catalyst. This result showed that the structure of CNTs could be controlled by the selection of catalysts. The average diameters of MWNTs and SWNTs were 20 and 10 nm, respectively. SiC nanorod crystals were prepared by the reaction of catalytically grown CNTs with tetrametysilane. Structural and optical properties of the catalytically grown semiconductor micro- and nano-crystals were characterized using various analytic techniques. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Layered double hydroxides as catalysts for the efficient growth of high quality single-walled carbon nanotubes in a fluidized bed reactor

A family of layered double hydroxides (LDHs), such as Fe/Mg/Al, Co/Mg/Al, and Ni/Mg/Al LDHs, were used as catalysts for the efficient growth of single-walled carbon nanotubes (SWCNTs) in a fluidized bed reactor. The LDH flakes were agglomerated into clusters with sizes ranging from 50 to 200 μm, and they can be easily fluidized with a gas velocity ranging from 2.3 to 24 cm/s. After calcination and reduction, small metal catalyst particles formed and distributed uniformly on the flakes. At the reaction temperature, the introduction of methane realized the growth of SWCNTs with the diameter of 1-4 nm. The loose structure of LDH agglomerates afforded a yield as high as 0.95 g_CNT/(g_cat h) of SWCNTs with a surface area of 930 m²/g. Compared with Fe/Mg/Al LDH, Ni/Mg/Al and Co/Mg/Al LDHs showed a better selectivity to SWCNTs. The highest selectivity for metallic SWCNTs was obtained using Co/Mg/AI LDHs as the catalyst.     

Preparation of partly hydrophobized,crosslinked polyacrylamide particles by terpolymerization of acrylamide/ N,N‐methylenebisacrylamide/styrene in inverse microemulsion

Free‐radical polymerization of a termonomer system comprising acrylamide (AAm) N,N ′‐methylenebisacrylamide (MBAAm) and styrene (S) initiated by water‐soluble ammonium peroxodisulphate (APS) or by toluene‐soluble dibenzoyl peroxide (DBP) in inverse microemulsion (toluene/S/AOT//water//AAm/MBAAm), leads to the formation of partly hydrophobized crosslinked polymer particles of tailored chemical composition, degree of crosslinking and polymer particle size. Styrene strongly decreases the rate of terpolymerization, while the presence of MBAAm has almost no effect on the polymerization rates observed. This conclusion is valid for both APS and DBP initiators. Increase of the S/T mass ratio (T is toluene) in inverse microemulsion leads to an increase of polymer particle diameter from about 20 nm to about 50 nm attributed to toluene swelling of the styrene‐rich structural moieties of AAm‐co‐S copolymer located on the surface of polymer particles. Polymerization kinetics measurements pointed to the important role of exiting water soluble AAm and MBAAm monomer radicals generated by thermal decomposition of APS in water pools of inverse micelles for initiation of polymerization reactions of sparingly water‐soluble S monomer in the oil‐phase of the inverse microemulsion. It was shown that the polymerization and copolymerization reactions of S in the presence of AAm and/or MBAAm are effectively initiated by water‐soluble APS and also by oil‐soluble DBP initiators. During dialysis the polymerized single‐phase water/oil Winsor IV inverse microemulsion gradually converts itself into a two‐phase oil/water Winsor I dispersion system with volume fraction of aqueous phase Φ_aw ≈ 0.950. The water phase contains water swelled, crosslinked polymer particles of diameters 80–300 nm. During dialysis, toluene and the sodium salt of bis(2‐ethyl hexyl)sulphosuccinic acid (AOT) partition between the oil phase of the dialysed dispersion system and the water dialysate. After evaporation of water from the dialysed inverse microemulsion, solid, dried, crosslinked polymer particles in the form of a transparent film, almost uncontaminated by AOT surfactant, were obtained. © 2000 Society of Chemical Industry     

Growth of large-diameter (∼4 nm) single-wall carbon nanotubes in the nanospace of mesoporous material SBA-15

Single-wall carbon nanotubes (SWCNTs) have been synthesized by supported-catalyst chemical vapor deposition (CCVD) using one-dimensional (1D) channels of mesoporous silica (SBA-15; mean channel diameter, 6.0 nm) functionalized with carboxyl groups where Co and Fe complexes are encapsulated. The synthesized SWCNTs have much larger diameters than the SWCNTs synthesized by conventional CCVD. Transmission electron microscope observations reveal that large-diameter SWCNTs (<4.2 nm) are grown in 1D channels of SBA-15. Large metal particles formed in the channels should play an important role in the growth of the SWCNTs with larger diameters.     

CATALYTIC PRODUCTION OF GRAPHITIC NANORODS VIA THE GAS PHASE DECOMPOSITION OF ETHYLENE OVER SUPPORTED NICKEL

The catalytic growth of high aspect ratio carbon nanorods by ethylene decomposition over nickel/silica is presented as a viable low-cost selective route to a high-value product. This study focuses on the role of catalyst preparation in determining carbon yield and structural characteristics and considers the application of a 10% w/w Ni loading prepared by impregnation and precipitation/deposition. The latter is characterized by a narrower dispersion of smaller Ni particles (average diameter?=?2.4?nm) but lower carbon yields. Doping this catalyst with KBr resulted in a 40-fold increase in carbon production with >95% having rod diameters <10?nm; doping the impregnated catalyst had little effect on catalyst performance. High-resolution transmission electron microscopy and temperature programmed oxidation were employed to characterize the catalysts and the carbon product; the effect of temperature (673–873?K) on carbon yield is also reported.     

Fe对Ni/SBA-16催化CO低温甲烷化促进作用的研究

为了提高Ni/SBA-16催化剂在低温下CO甲烷化中的活性,通过引入Fe助剂制备了Ni-Fe/SBA-16双金属催化剂。对催化剂进行XPS、XRD、HRTEM及EDS-mapping表征的结果表明,Fe的加入与Ni形成了Ni₃Fe合金,减小了金属颗粒尺寸,使得还原后金属颗粒平均粒径从60 nm降低到30 nm左右。同时H₂-TPR的结果表明,Ni₃Fe合金的形成增强了金属Ni与载体之间的相互作用,从而能够减弱Ni颗粒在还原过程及反应过程中的团聚。最后,由CO-TPD和H₂-TPD的测试结果可知,Ni₃Fe合金的形成促进了催化剂对反应气体CO和H₂的解离,从而提高了催化剂在低温下的CO甲烷化活性。当空速为150000 h^-1、压力为0.1 MPa、V(H₂)∶V(CO)∶V(N₂)=3∶1∶1时,CO最低完全转化温度可以从300°C降低到250°C,同时CH₄的选择性保持在90%。     

Polymerization of vinyl acetate in microemulsions stabilized with a mixture of dodecyltrimethylammonium bromide and didodecyldimethylammonium bromide

Summary Polymerization of vinyl acetate (VA) in three component oil/water (o/w) microemulsions stabilized with a mixture of two cationic surfactants (DTAB/DDAB in a weight ratio of 3), was carried out at 40, 50 and 60°C using a water soluble initiator, V-50. In all cases studied, stable latexes containing particles with diameters between 70 and 110 nm were obtained. Particle size increased with conversion yielding uncommonly large particles for microemulsion polymerization, probably because particle coagulation. Multimodal molar mass distributions with average molar masses between 1.2 to 2.0×10⁶ g/mol were obtained. Chain transfer to polymer and bimolecular termination reactions play important roles in the microemulsion polymerization of this monomer. Received: 5 October 1998/Revised version: 12 February 1999/Accepted: 12 February 1999     

Catalytic etching of {100}-oriented diamond coating with Fe, Co, Ni, and Pt nanoparticles under hydrogen

Etching of a highly {100}-oriented diamond coating, {100}HODC, with hydrogen gas using Fe, Co, Ni, and Pt nanoparticles as a catalyst was examined at high temperatures over 700 °C by high-resolution scanning electron microscopy and Raman spectroscopy. The metal atoms vacuum-evaporated onto the {100}HODC formed nanoparticles themselves when heated at high temperatures; e.g. 700 °C, in a flowing gas mixture of H₂ (10%) + N₂ (90%). At 800 °C, short nano-channels and etch pits holding metal nanoparticles were formed by Fe, Co, and Ni. The shapes of the Co and Ni nanoparticles in the etch pits were affected by the shape of the etch pits; reversed pyramidal shape. On the other hand, the top view of the Fe nanoparticles embedded in the etch pits showed a distorted round shape, probably due to the formation of something such as iron carbide, while the carbon content was unknown. Apparently, etching of the {100}HODC by Pt nanoparticles was observed after the treatment at 1000 °C. The difference in the catalytic etching behavior among these metal particles, the potential etching mechanism of diamonds with hydrogen by metal nanoparticles, probably as melted metal nanoparticles, and the formation mechanism of vacant etch pits were discussed.     

Synthesis of narrow-diameter carbon nanotubes from acetylene decomposition over an iron-nickel catalyst supported on alumina

Carbon nanotubes (CNTs) were synthesized by the catalytic decomposition of acetylene over 40Fe:60Al₂O₃, 40Ni:60Al₂O₃ and 20Fe:20Ni:60Al₂O₃ catalysts. High density CNTs of 20 nm diameter were grown over the 20Fe:20Ni:60Al₂O₃ catalyst, whereas low growth density CNTs of 40 and 50 nm diameter were found over 40Fe:60Al₂O₃ and 40Ni:60Al₂O₃ catalysts. Smaller catalyst particles enabled the synthesis of highly dense, long and narrow-diameter CNTs. It was found that a homogeneous dispersion of the catalyst was an essential factor in achieving high growth density. The carbon yield and the quality of CNTs increased with increasing temperature. For the 20Fe:20Ni:60Al₂O₃ catalyst, the carbon yield reached 121% after 90 min at 700 °C. The CNTs were grown according to the tip growth mode. Based on reports regarding hydrocarbon adsorption and decomposition over different faces of Ni and Fe, the growth mechanism of CNTs over the 20Fe:20Ni:60Al₂O₃ catalyst are discussed.     

CVD法可控制备碳包覆金属纳米复合材料

以Fe质量分数分别为0.3%、1.6%、3.3%和5.2%的氯化钠担载铁为催化剂,化学气相沉积法催化裂解乙炔400℃下进行反应,系统探讨了碳包覆金属纳米颗粒的可控制备。通过扫描电子显微镜和高分辨透射电子显微镜对产物进行了表征,结果表明,w(Fe)=0.3%的催化剂制备的样品粒径在20～50 nm,平均直径约为30 nm;w(Fe)=1.6%的催化剂制备的样品粒径在35～60 nm,平均直径约为49 nm;而w(Fe)=3.3%和5.2%的催化剂制备的样品粒径差别不大,在40～100 nm,平均粒径约为65 nm。所制备的碳包覆金属纳米颗粒具有清晰的同心石墨壳层结构,并存在一定的结构缺陷。     

Trial for diameter-selective synthesis of single-walled carbon nanotubes

The diameter-controlled synthesis of single-walled carbon nanotubes (SWNTs) has been examined experimentally. The catalysis of the Rh and Pd atoms has been confirmed by blending with Co atoms at 950 °C in a furnace, although the Rh/Pd catalyst has not been recorded to act efficiently at this temperature before. Raman spectra indicate that the Rh/Co and Pd/Co catalysts can synthesize narrow-diameter SWNTs more selectively than the Fe/Co and Ni/Co catalysts, which can only synthesize SWNTs with slightly larger diameters. These results suggest that by changing the combination of catalysts, the persistent problem of controlling the diameter of SWNTs can be solved without any expensive setup or complicated techniques.     

过渡金属氧化物催化剂对丙酮氧化的催化性能

以氧氯化锆和过渡金属硝酸盐为原料,溶胶-凝胶法制备氧化锆载体,采用溶液浸渍法制备出负载型纳米复合氧化物催化剂。对过渡金属氧化物(Mn、Ni、Cu、Fe和Co)催化剂进行初步筛选,其中Mn₂O₃/ZrO₂催化活性最佳。考察了活性组分含量、焙烧温度和焙烧时间等对催化剂催化活性的影响。催化剂的最佳制备条件：锰负载量为6%,450 ℃焙烧6 h。丙酮的转化率达62%。通过TEM、SEM、XRD和TG-DTA对催化剂结构进行了表征。结果显示,催化剂颗粒均匀,大小约为20 nm;无定形的Mn2O3是催化剂的活性中心。     

胰岛素W/O型微乳液的制备及体外释药性能

制备了包封胰岛素(INS)的Tween 80-Span 80/乙醇/丁酸乙酯/水体系的W/O型微乳液。以最大增溶水量为指标,选择了合适的微乳液组分包封INS。考察了温度、盐度和pH对微乳液区域的影响。电导率法区分了微乳液的O/W、W/O和B.C.区域。动态光散射测定了微乳液的粒径和多分散度。125I同位素示踪法测定了INS微乳液体外释放效果。结果表明,微乳体系在水/乙醇(质量比为1.8∶1)的质量分数小于41%时形成W/O型微乳液,温度、盐度升高和pH降低使微乳区稍有减小。微乳液粒径和多分散度分别为35~45 nm和0.29~0.37。pH的降低对微乳液粒径影响不大,而药物的加入使微乳液粒径略有减小。载药微乳液粒径在制备3 d后突降,以后的27 d内保持在37 nm左右。该载药微乳液在7.5 h后进入缓释阶段,40 h时INS的释放率为66.20%。     

Low-Temperature Growth of Carbon Nanotubes on Bi- and Tri-metallic Catalyst Templates

Low temperature growth process of carbon nanotubes (CNTs) over bi-metallic (Co–Fe) and tri-metallic (Ni–Co–Fe) catalysts on Si/Al/Al₂O₃ substrates is carried out from acetylene precursor using hydrogen, ammonia or nitrogen as a carrier in a low pressure chemical vapor deposition system. Using the tri-metallic Ni–Co–Fe catalyst template, vertically aligned CNTs of ~700 nm length could be grown already at 450 °C within 10 min using ammonia as a carrier. Within the same period of time, on bi-metallic Co–Fe catalyst templates, ~250 nm long aligned nanotubes emerged already at 400 °C in nitrogen carrier. At low temperatures most of the catalyst materials were elevated from the support by the grown nanotubes indicating tip growth mechanism. The structure of catalyst layers and nanotube films was studied using scanning and transmission electron microscopy and atomic force microscopy.     

Synthesis of carbon nanofibers with a polygonal cross section by the catalytic decomposition of C2H2 over bi-metal catalysts

Carbon nanofibers with a polygonal cross section were synthesized using catalytic chemical vapor deposition over Fe–Sn, Ni–Sn or Co–Sn catalysts. Their morphologies were characterized by scanning and transmission electron microscopy. Edges connecting of two walls can be clearly observed and some adjacent walls have a V-shape. The lengths of the sides of the polygonal cross sections range from 150 to 500 nm over Fe–Sn or Co–Sn nanoparticles, increasing to 700–900 nm over Ni–Sn nanoparticles. Polygonal catalyst particles can be seen at the ends of the fibers.