Selective synthesis of single-walled carbon nanotubes on Fe–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of single-walled carbon nanotubes (SWCNTs) with narrow chirality and diameter distribution by methane decomposition over Fe–MgO catalyst is reported. The catalyst was examined by nitrogen physisorption, X-ray diffraction, temperature programmed reduction, X-ray photoelectron spectroscopy, and UV–Vis diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. High resolution electron microscopy, Raman and optical absorption spectroscopy, temperature programmed oxidation, energy dispersive X-ray spectroscopy and nitrogen physisorption were used to probe reaction selectivity, SWCNT chirality and diameter distribution, carbon yield and effectiveness of purification protocols. The yield of carbon increased with an increase in temperature, although SWCNTs selectivity decreased above the optimum synthesis temperature. Results established a clear link between the degree of dispersion of iron oxide species inside the MgO lattice and the catalyst selectivity for SWCNT growth.     

Impact of nanometal catalysts on the laser vaporization synthesis of single wall carbon nanotubes

The effects of catalyst particle size on the purity, yield, and purification efficiency of single wall carbon nanotubes (SWCNTs) synthesized via pulsed laser vaporization were investigated. The purity of as-produced SWCNT material synthesized using Ni and Co nanometal (∼13 nm diameter) catalyst particles was compared to material synthesized using conventional micronmetal (2-3 μm diameter) particles. The SWCNT material from nanometal catalysts demonstrated a 50% increase in SWCNT purity as assessed by optical absorption spectroscopy and thermogravimetric analysis (TGA). A change in the thermal oxidation properties was also observed with the nanometal-SWCNTs exhibiting a suppression of the exothermic oxidation of post-synthesis catalyst. Statistical analysis of the TGA residue yielded mean post-synthesis catalyst particle diameters of 18 ± 6 nm and 3 ± 1 nm for the micronmetal and nanometal produced material, respectively. When a thermal oxidation profile was performed, the micronmetal-produced material showed the typical decrease in SWCNT purity with increasing oxidation temperature while the nanometal-produced material showed increasing SWCNT purity with increasing temperature. Overall, the use of nanometal catalysts significantly increases synthesis yield and offers novel thermal oxidation procedures to thermally remove carbonaceous impurities without the aid of acid treatments for the development of potential large-scale purification processing.     

Room-temperature synthesis of single-wall carbon nanotubes by an electrochemical process

Single-wall carbon nanotubes (SWCNTs) were produced by an electrochemical route by applying a small negative potential to a solution of acetic acid over a Au surface supporting Ni nanocatalysts. Ni nanocatalysts were grown electrochemically on Au surface and their particle sizes were controlled by deposition time. Raman spectroscopy and scanning probe microscopy observations of the catalyst and as-deposited samples and revealed that the catalyst structure strongly affects the SWCNT diameter distribution. The deposited carbon structure depended on the catalyst particle size and structure. Raman spectra confirmed the existence of selectively grown semiconducting SWCNTs with very narrow diameter distribution.     

The synthesis of single-walled carbon nanotubes with narrow diameter distribution using polymerized hemoglobin

Single-walled carbon nanotubes (SWCNTs) with a narrow diameter distribution were synthesized by using polymerized hemoglobin (PolyHb). PolyHb containing 11 Hb molecules (11-PolyHb) was separated from a mixture of PolyHb with various degrees of polymerization by using size-exclusion chromatography (SEC). After the deposition and oxidation of 11-PolyHb, we obtained iron oxide nanoparticles with an average diameter of 1.30 ± 0.36 nm. SWCNT were grown from these nanoparticles with a narrow diameter distribution, 1.08 ± 0.26 nm. This technique has the potential to synthesize SWCNT with an identical diameter, and to control the diameter of SWCNT at atomic resolution through improvements to the SEC processing.     

Formation of fine Fe-Ni particles for the non-supported catalytic synthesis of uniform carbon nanofibers

The morphological changes of Fe-Ni catalyst for the preparation of carbon nanofiber (CNF) were examined at 5 steps; (1) the precipitation of Fe-Ni carbonate from Fe-Ni nitrate solution, (2) the calcination of Fe-Ni carbonate into Fe-Ni oxide, (3) the reduction of Fe-Ni oxide, (4) the second reduction of Fe-Ni metal before the growth of CNF, and (5) the reaction with CO/H₂ for the growth of CNF. The Fe-Ni fine particle was formed from the Fe-Ni aggregate through the second reduction and successive CNF growth from CO/H₂. The temperature of these two steps is the most important factor which determines the size and shape of the Fe-Ni fine particle as a catalyst for CNF growth. The lower temperature of 580 °C provided hexagonal particles with very smooth surface sized around 100-200 nm which allowed the growth of platelet CNFs of the same diameter and cross-sectional shape of the formed catalyst particle. At the higher temperature of 630 °C, the Fe-Ni aggregate was found to give the very fine Fe-Ni particles by the two steps; the first step did the Fe-Ni particle sized around 100-500 nm which was successively degraded into smaller particles sized around 20-40 nm, thinner tubular CNFs growing with the contact of CO/H₂. Such smaller particles definitely originated from as-precipitated Fe-Ni carbonate through the steps. The metal particle on the top of CNF was almost exclusively composed of Fe although the catalyst particle before the growth of CNFs carried around 65% of iron and 35% of nickel. The preferential activity of Fe to CO gas may cause such the selectivity. The major role of Ni in the present reaction should be limited to provide the uniform particle of Fe. Controlling the size of the Fe-Ni particle through the reduction and reaction steps was proved to be a key factor to determine the dimension and structure of resultant CNF.     

Production of single-walled carbon nanotubes with narrow diameter distribution using iron nanoparticles derived from DNA-binding proteins from starved cells

Iron nanoparticles derived from DNA-binding proteins from starved cells (Dps) were used to grow single-walled carbon nanotubes (SWCNTs) with narrow diameter distribution. An atomic force microscopy, Raman spectroscopy, and photoluminescence were used for evaluation of diameter or chirality distribution of the SWCNTs. We found that thin SWCNTs (1.1 nm diameter) were grown from the large Dps-derived nanoparticles (2.4 nm diameter) on and above the substrates. From the size comparison with ferritins and Co-filled apoferritins, we also found that SWCNTs become thinner as the catalyst becomes smaller. The synthesis of smaller catalysts (ca. 1 nm diameter) and their use for growth becomes crucial for the control of SWCNT diameter.     

Mechanistic investigations of single-walled carbon nanotube synthesis by ferrocene vapor decomposition in carbon monoxide

The single-walled carbon nanotubes (SWCNTs) were synthesized by the carbon monoxide disproportionation reaction on Fe catalyst particles formed by ferrocene vapor decomposition in a laminar flow aerosol (floating catalyst) reactor. On the basis of in situ sampling of the product collected at different locations in the reactor, kinetics of the SWCNT growth and catalyst particle crystallinity were studied. Catalyst particles captured before SWCNT nucleation as well as inactive particles were determined to have cementite (Fe₃C) phase, while particles with γ- and α-Fe phases were found to be embedded in the SCWNTs. The growth rate in the temperature range from 804 to 915 °C was respectively varied from 0.67 to 2.7 μm/s. The growth rate constant can be described by an Arrhenius dependence with an activation energy of E_a = 1.39 eV, which was attributed to the carbon diffusion in solid iron particles. CNT growth termination was explained by solid-liquid phase transition in the catalyst particles. A high temperature gradient in the reactor was found to not have any effect on the diameter during the SWCNT growth and as a result on the chirality of the growing SWCNTs.     

Selective growth of single-walled carbon nanotubes over Co–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of SWCNTs with narrow chirality and diameter distribution by methane decomposition over a Co–MgO catalyst is reported. Raman spectroscopy, temperature programmed oxidation (TPO), UV–Vis–NIR absorption spectroscopy, and nitrogen physisorption were used to probe SWCNTs morphology, reaction selectivity, SWCNTs chirality and diameter distribution, and carbon yield. The catalyst was examined by nitrogen physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), and UV–Vis-diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. The results established a clear link between the degree of dispersion of Co species inside the MgO lattice and the catalyst activity and selectivity for SWCNT growth. High dispersion and stabilization of Co species influenced catalytic activity for methane decomposition and the high SWCNT selectivity. The yield of carbon and SWCNT selectivity increased with an increase in temperature, however, SWCNTs diameter distribution shifts to larger diameter tubes as synthesis temperature was increased.     

On-line detection of single-walled carbon nanotube formation during aerosol synthesis methods

Differential electrical mobility (DMA) method for the on-line detection of single-walled carbon nanotubes (SWCNTs) formation was used for the first time. Three different gas-phase synthesis processes were used to produce SWCNTs via CO disproportionation in the presence of catalyst nanoparticles formed either by a hot wire generator method or via thermal decomposition of ferrocene or iron pentacarbonyl. The typical product measured with the DMA method was bundles of SWCNTs, which further agglomerated prior to the measurement. Despite the different product morphology and concentration, the on-line measurement was able to distinguish SWCNT formation in each experimental set-up as an increase in the geometric mean particle diameter and as a decrease in the total particle number concentration. Furthermore, information regarding the relative SWCNT concentration can also be obtained from the DMA measurement. A theoretical approach to the mobility of nonspherical particles in the electric field was successfully developed in order to convert the electrical mobility size of the high aspect ratio SWCNTs measured with DMA to the physical size of the product. Size-selected SWCNTs were studied with transmission electron microscopy in order to find the correlation between the on-line DMA measurement data and the SWCNT morphology.     

Size-Resolved Particle Fluxes and Vertical Gradients over and in a Sparse Pine Forest

Although a number of flux data sets indicate apparent upward fluxes of particles over forests, the causes of such fluxes remain only partially understood. Using data collected during the 2011 BEACHON experiment, it is shown that over one third of fluxes of both sub- and super-30-nm diameter particles over this sparse pine forest are upward. Quadrant analysis, timescale analysis using a particle dynamics model, and frequency power spectra of particle concentrations demonstrate that, in this environment, the morning upward fluxes tend to be associated with downward “sweeps” of particle depleted air during break down of the nocturnal inversion, and it is the most common mechanism resulting in upward fluxes of particle size distributions with diameters above 30 nm. Upward fluxes of particles later in the day are more strongly linked to “ejections” of particle enriched air from the canopy that are attributable to growth of fairly recently nucleated particles by both addition of oxidation products of biogenic volatile organic compounds and coagulation. This mechanism appears to dominate upward fluxes of sub-30-nm particles, although the resulting destabilization of the particle size distribution can result in upward fluxes of larger particles. Vertical gradients of particle size distribution above, through, and below the canopy are also analyzed to investigate the size dependence of canopy uptake of particles and indicate that, in accord with wind tunnel analyses, penetration efficiencies are lower for smaller geometric mean diameters (～15–20 nm) and increase with diameter up to approx. 80 nm (the largest diameter considered here).

Copyright 2013 American Association for Aerosol Research     

Single-walled carbon nanotube synthesis using ferrocene and iron pentacarbonyl in a laminar flow reactor

A gas-phase process of single-walled carbon nanotube (SWCNT) formation, based on thermal decomposition of iron pentacarbonyl or ferrocene in the presence of carbon monoxide (CO), was investigated in ambient pressure laminar flow reactors in the temperature range of 600-1300 °C. Ferrocene was found to be a better catalyst precursor in the studied conditions since iron pentacarbonyl decomposes at lower temperatures resulting in the excessive growth of catalyst particles. In situ sampling carried out at 1000 °C showed that the SWCNT growth occurred from individual metal particles in the heating section of the furnace in the temperature range of 891-928 °C, in which the growth rate was estimated to exceed . FT-IR measurements of gaseous products revealed that the majority of the CO disproportionation took place on the reactor walls. Raman measurements confirmed the results of TEM observations, namely, the formation of very high purity SWCNT product. On-line aerosol number size distributions were measured to detect the conditions of SWCNT formation and the product morphology changes. Mechanism of SWCNT formation in the gas phase from ferrocene and CO is discussed.     

A study of the surface-catalyzed polymerization of ethylene on chromic oxide–silica–alumina catalysts

This investigation was an exploratory study of the phenomena associated with the polymerization of ethylene on a chromic oxide–silica–alumina catalyst. The polymerization was carried out in a batch reactor fed on demand at 500 psi gage and at 135°C. The catalyst was 3% chromic oxide on an 87% silica–13% alumina support, activated at 450°C for 5.25 hr in dry air. The catalyst particles were found to reduce in size during the polymerization. The particles decreased in size from 18/20 mesh to an average diameter of 9.6 μ. This reduction in particle size was found to cease at a yield of 40.0 g per gram of catalyst. By determining the monomer reaction rate versus time, the catalyst activity was found to change greatly during the polymerization. The activity increased to a maximum, then decreased. The period of increasing catalyst activity corresponded to the period of catalyst particle size reduction. The period of decreasing activity was considered to be due to the build-up of a polymer film diffusion layer and true catalyst deactivation. The molecular weight distributions for a series of samples from each polymerization test were determined on gel permeation chromatograph. These distributions for each test were characterized by an increase in the high molecular weight tail as the yield increased, while the peak position and low molecular weight end remained essentially constant.     

Spray coating as a simple method to prepare catalyst for growth of diameter-tunable single-walled carbon nanotubes

Spray coating is proposed as an optional wet method for preparing nano-sized particles suitable for the growth of single-walled carbon nanotubes (SWCNTs). The obtained SWCNT films are characterized by Raman spectroscopy and electron microscopy, and are confirmed to be comparable to SWCNTs produced by the conventional dip-coating process in terms of crystallinity, tube diameter and carbon yield. The mean diameter of SWCNTs can be effectively reduced from 1.85 to 1.35 nm by prolonging the deposition of Mo. In addition, spray coating allows catalyst preparation on supports other than flat wafers, as demonstrated by the synthesis of high-quality SWCNTs on Al₂O₃ fiber and quartz wool supports.     

Cefditorene-Mediated Synthesis of Silver Nanoparticles and Its Catalytic Activity

Ag(0) NPs were prepared by chemical reduction method in which silver nitrate was taken as the metal precursor and cefditorene as a reducing/capping agent and NaOH as the catalyst for reaction enhancement. The formation of the Ag(0) NPs was monitored using UV–Vis absorption spectroscopy confirmed the formation of Ag(0) NPs by exciting the typical surface plasmon absorption maxima at 405 nm. Transmission electron microscopy (TEM) confirmed the spherical morphology of the (Ag(0) NPs). The crystallite (11 ± 3 nm) and particle size (14.1 ± 2.2 nm) obtained from TEM and XRD analysis were coinciding with each other. Prepared Ag(0) NPs were then used as catalyst against 2-nitroaniline, 3-nitroaniline and 4-nitroaniline, which all showed best catalytic activity.     

Studies on product distribution of nanostructured iron catalyst in Fischer–Tropsch synthesis: Effect of catalyst particle size

The dependencies of hydrocarbon product distributions of Fischer–Tropsch synthesis by iron catalysts on catalysts particle size are studied. The concept of two superimposed Anderson–Schulz–Flory distributions applied for represent size dependency of product distributions. A series of catalysts with different particle size are prepared by microemulsion method. It is found that the carbon number of produced hydrocarbon decreased with decreasing the catalyst particle size. These results indicate the H₂ concentration on catalyst surface decreased by increasing the catalyst particle size. Thus the concentration of monomers that exhibited higher degree of hydrogenation (like CH₂ species) on the surface of catalyst increased with decreasing the catalyst particle size.     

络合还原法制备的Pd/C催化剂对甲酸氧化的电催化性能

分别以硼氢化钠和乙二醇为还原剂,经络合还原法制备了炭载钯（Pd/C）催化剂。透射电镜（TEM）和X射线粉末衍射谱（XRD）结果表明,以乙二醇为还原剂制备的Pd/C催化剂中Pd粒子具有较小的粒径、均匀的粒径分布和较大的相对结晶度,Pd粒子的平均粒径和相对结晶度分别为4.2±2 nm和1.88。电化学测试结果显示,以乙二醇为还原剂制备的Pd/C催化剂具有较大的电化学活性面积,对甲酸氧化表现出较高的电催化活性和稳定性。     

Modification of Laminar Flow Ultrafine Condensation Particle Counters for the Enhanced Detection of 1 nm Condensation Nuclei

This paper describes simple modifications to thermally diffusive laminar flow ultrafine condensation particle counters (UCPCs) that allow detection of ～1 nm condensation nuclei with much higher efficiencies than have been previously reported. These non-destructive modifications were applied to a commercial butanol-based UCPC (TSI 3025A) and to a diethylene glycol-based UCPC (UMN DEG-UCPC). Size and charge dependent detection efficiencies using the modified UCPCs (BNL 3025A and BNL DEG-UCPC) were measured with high resolution mobility classified aerosols composed of NaCl, W, molecular ion standards of tetra-alkyl ammonium bromide, and neutralizer-generated ions. With negatively charged NaCl aerosol, the BNL 3025A and BNL DEG-UCPC achieved detection efficiencies of 37% (90× increase over TSI 3025A) at 1.68 nm mobility diameter (1.39 nm geometric diameter) and 23% (8× increase over UMN DEG-UCPC) at 1.19 nm mobility diameter (0.89 nm geometric diameter), respectively. Operating conditions for both UCPCs were identified that allowed negatively charged NaCl and W particles, but not negative ions of exactly the same mobility size, to be efficiently detected. This serendipitous material dependence, which is not fundamentally understood, suggests that vapor condensation might sometimes allow for the discrimination between air “ions” and charged “particles.” As a detector in a scanning mobility particle spectrometer (SMPS), a UCPC with this strong material dependence would allow for more accurate measurements of sub-2 nm aerosol size distributions due to the reduced interference from neutralizer-generated ions and atmospheric ions, and provide increased sensitivity for the determination of nucleation rates and initial particle growth rates.

Copyright 2012 American Association for Aerosol Research     

The Formation of Aerosol Particles from Solution-Based Pressurized Metered Dose Inhalers and Implications of Incomplete Droplet Drying: Theoretical and Experimental Comparison

The aerosol particle size distributions of solution-based pressurized metered dose inhalers containing 15%w/w ethanol and different quantities of nonvolatile component (NVC) (drug and glycerol) were evaluated at 25°C and 55°C, using a custom-built heating rig that preheated air prior to aerosolization. Particle size distributions were assessed using an Andersen cascade impactor and mass‐weighted cumulative aerodynamic diameter distributions were compared to a theoretical model that predicts the final size distribution, based on initial droplet size, vapor pressure of the formulation containing HFA 134a and percent NVC. In general, the mass median aerodynamic diameter was proportional to NVC^1/3, with experimental particle size distributions following theoretical values. However, when comparing theoretical vs. experimental data over the range of mass-weighted cumulative aerodynamic diameter distributions between 10 and 90%, the 55°C experimental measurements more closely fitted the theoretical equation when compared to 25°C. This was attributed to incomplete drying of some of the larger initial droplets prior to impaction. Additionally, postinduction port measurements of volumetric size distribution using laser diffraction, showed a reduction in median particle diameter at 55°C, compared to 25°C and a change from bimodal to monomodal distribution, indicating complex drying kinetics under ambient conditions.

Copyright 2015 American Association for Aerosol Research     

铁矿石尾矿粉催化还原NO的实验研究

以马鞍山钢铁集团南山矿铁矿石尾矿为主要原料制备无载体催化剂,对其进行了XRF, BET和XRD表征,并于电加热石英玻璃管固定床中实验研究了催化剂对NH3还原NO的催化活性. 结果表明,用铁尾矿所制催化剂在反应温度500?600℃下具有良好的催化活性,600℃时催化还原率达97.5%,且粒径越小催化还原率越高;氧含量对催化还原率有较大影响,350℃时增加氧含量可提高催化还原率,500℃时增加氧含量会降低催化还原率;氧化铁作为主要活性组分,在催化剂中含量越高催化还原率越高,但氧化铁高温时易烧结,会降低催化还原率.     

Evaluation of a Method to Measure Black Carbon Particles Suspended in Rainwater and Snow Samples

We conducted a detailed evaluation of a method for measuring the mass concentrations and size distributions of black carbon (BC) particles in rainwater and snow. The method uses an ultrasonic nebulizer (USN) and a single particle soot photometer (SP2). The USN disperses sample water into micron-size droplets at a constant rate and then dries them to release BC particles into the air. The masses of individual BC particles are measured by the SP2, using the laser-induced incandescence technique. The loss of BC particles during the extraction from liquid water to air depends on their sizes. We determined the size-dependent extraction efficiency using polystyrene latex (PSL) spheres with 12 different diameters between 107 and 1025 nm. The PSL concentrations in water were measured by the light extinction at 532 nm. The extraction efficiency of the USN showed a broad maximum of about 10% in the diameter range 200–500 nm and decreased substantially at larger sizes. The accuracy and reproducibility of the measured mass concentration of BC in sample water after long-term storage were about ±25% and ±35%, respectively. We tested the method by analyzing rainwater and surface snow samples collected in Okinawa and Sapporo, respectively. The measured number size distributions of BC in these samples showed negligible contributions of BC particles larger than 300 nm to the total number of BC particles. A dominant fraction of BC mass in these samples was observed in the diameter range 100–500 nm.

Copyright 2013 American Association for Aerosol Research