Active Catalyst Particles Research Articles

Forecasting carbon nanotube diameter in floating catalyst chemical vapor deposition

Carbon nanotube (CNT) diameter control is essential for many conductive and structural applications; to date, a general diameter methodology applicable to all floating catalyst chemical vapor deposition (FC-CVD) reactors is not yet established. Single-walled CNT (SWCNT) diameter was robustly controlled by systematically exploring an extensive variety of fundamental FC-CVD reactor parameters (631 experiments) including: CO2, H2, and Ar flowrates; fuel flowrate; added H2O (up to an unprecedented 30% of fuel by mass); ferrocene concentration; reactor temperature (covering temperatures with solid and liquid floating catalyst); and quartz reactor tube age. As a function of these experimental input factors, across three Raman wavelengths, validated statistical models robustly predicted: 1) diameter of SWCNTs, via the weighted average of the radial breathing modes (RBMs); 2) the conditions which led to SWCNT growth, via the categorical appearance of viable RBMs. This is noteworthy because it illustrates a new paradigm of FC-CVD reactor control, accounting for interaction between reactor input factors, system noise, and other non-linear responses. Oxidative influences (including laboratory humidity) yield smaller CNT diameters, although H2O becomes important only at higher temperatures and CO2 is countered by H2 addition. Increasing reactor temperature increased diameters. Pyrolysis calculations showed species inside the reactor differ substantially from species injected. A kinetic-thermodynamic model of catalyst particle size and chemical state revealed that adding oxidative species kept the active catalyst particles small, explaining the resulting small CNT diameters. The powerful combination of our multivariate statistical approach with kinetic-thermodynamic modeling enables unprecedented insight and control over CNT synthesis.

Enhanced CO2-assisted growth of single-wall carbon nanotube arrays using Fe/AlOx catalyst annealed without CO2

Controlling catalyst-particle formation is essential for the growth of single-wall carbon nanotube (SWCNT) arrays with improved alignment, areal mass, and height. We have previously reported the positive effect of CO2 on SWCNT growth via chemical vapor deposition, and in this study, we found its negative effect on catalyst-particle formation during annealing. A Fe (1 nm)/AlOx (15 nm) catalyst that was sputter-deposited on SiO2/Si substrates demonstrated a prolonged lifetime and enabled the growth of SWCNT arrays with better alignment, twice the height, and three times higher areal mass when the catalyst was annealed under 10 vol% H2/Ar without CO2 than with 1 vol% CO2. Detailed analysis indicated that the Fe particles could remain partially oxidized during annealing in H2 with mildly oxidative CO2, resulting in the bulk diffusion of Fe into the AlOx layer. In contrast, Fe is reduced sufficiently in H2 in the absence of CO2, thereby remaining on the AlOx surface and active for SWCNT growth. The findings of this study emphasize the importance of maintaining a highly reductive atmosphere during annealing to achieve active catalyst particles with a higher number density and longer lifetime.

Multi-Scale Studies of 3D Printed Mn–Na–W/SiO2 Catalyst for Oxidative Coupling of Methane

This work presents multi-scale approaches to investigate 3D printed structured Mn–Na–W/SiO2 catalysts used for the oxidative coupling of methane (OCM) reaction. The performance of the 3D printed catalysts has been compared to their conventional analogues, packed beds of pellets and powder. The physicochemical properties of the 3D printed catalysts were investigated using scanning electron microscopy, nitrogen adsorption and X-ray diffraction (XRD). Performance and durability tests of the 3D printed catalysts were conducted in the laboratory and in a miniplant under real reaction conditions. In addition, synchrotron-based X-ray diffraction computed tomography technique (XRD-CT) was employed to obtain cross sectional maps at three different positions selected within the 3D printed catalyst body during the OCM reaction. The maps revealed the evolution of catalyst active phases and silica support on spatial and temporal scales within the interiors of the 3D printed catalyst under operating conditions. These results were accompanied with SEM-EDS analysis that indicated a homogeneous distribution of the active catalyst particles across the silica support.

Polymer Nanocomposites for Photocatalytic Applications

In the present comprehensive review we have specifically focused on polymer nanocomposites used as photocatalytic materials in fine organic reactions or in organic pollutants degradation. The selection of the polymer substrates for the immobilization of the active catalyst particles is motivated by several advantages displayed by them, such as: Environmental stability, chemical inertness and resistance to ultraviolet radiations, mechanical stability, low prices and ease availability. Additionally, the use of polymer nanocomposites as photocatalysts offers the possibility of a facile separation and reuse of the materials, eliminating thus the post-treatment separation processes and implicitly reducing the costs of the procedure. This review covers the polymer-based photocatalytic materials containing the most popular inorganic nanoparticles with good catalytic performance under UV or visible light, namely TiO2, ZnO, CeO2, or plasmonic (Ag, Au, Pt, Pd) NPs. The study is mainly targeted on the preparation, photocatalytic activity, strategies directed toward the increase of photocatalytic efficiency under visible light and reuse of the hybrid polymer catalysts.

Получение синтез-газа сухим реформингом метана над стеклоткаными катализаторами

The application field of materials based on lanthanum orthophosphate (LaPO4) including nanomaterials, has been permanently extending recently. The high level of mechanical properties and the compatibility with numerous oxides make it possible to consider the possibility of using lanthanum orthophosphate as a composite material for construction purposes. This application is particularly promising when nanoparticles with quasi-1D morphology (nanorods) are used. The high isomorphic capacity of the LaPO4-based phase for alkaline-earth ions and ions of lanthanides and actinides, high chemical stability, and high radiation hardness make promising the application of this compound as a matrix for immobilization of radioactive wastes. The possibility of obtaining lanthanum phosphate (LaPO4) by the hydrothermal method is considered in the work. Effects of pH, temperature and time of processing of hydrothermal synthesis on the morphology and structure of monostructured lanthanum phosphate are studied. It has been established that, with the increase of pH, the morphology of phosphate changed, the size of the crystallites increased, while the crystal structure changed from hexagonal to monoclinic. The catalytic activity of nanostructured low-percentage Mg-Ni-Co-catalysts based on high-temperature KT-11-TO grade fiberglass obtained by “solution combustion” (SC) method was studied at carbon dioxide conversion of methane (CDCM). The physico-chemical characteristics of samples were studied using X-ray diffraction phase analysis, temperature-programmed reduction (TPR) and transmission electron microscopy (TEM). The X-ray phase method showed the formation of several phases during the synthesis: NiCo2O4, 3CoO·5NiO, MgO, and Co3O4. According to TEM, active catalyst particles have a size of 5-10 nm proving the nanoscale size of the active component. TPR method showed the shift of maximum hydrogen absorption to higher temperatures. Apparently, it occurs due to the interaction of the active components with the carrier till the new phase formation. On the basis of the gas chromatographic analysis the high activity of fiberglass catalysts at the carbon dioxide conversion of methane into synthesis gas with a conversion of the initial components close to ~ 100% was disclosed. The optimal technological conditions for the CDCM process were established – a temperature in the range of 850-900°С, the volumetric rate of initial reactants 4000-10000 h-1 with a ratio of methane to carbon dioxide equal to 1.

Gas phase pyrolysis synthesis of carbon nanotubes at high temperature

In this study, carbon nanotubes were synthesized at high temperature (1150–1500°C) using the substrate-free gas phase pyrolysis method. The CNT sock morphology, CNT structure, impurity, process yield and growth efficiency at high temperatures were investigated. It was found that the CNTs transform from single-walled to multi-walled nanotubes at elevated temperature. The amount of amorphous impurities increases with higher temperature, possibly due to an increased non-catalytic decomposition of hydrocarbons. However the CNT quality increases, as indicated by a higher Raman spectroscopy IG/ID ratio. The process yield increase by two folds at higher temperature (1500°C) compared to a lower temperature (1200°C), but the catalyst efficiency is highest at 1400°C with 1g Fe producing 5.1g CNT. The calculated carbon conversion rate is lower than 4%. The CNT growth is not limited by the availability of carbon around the catalyst, instead of by the availability of active catalyst particles. Based on a pristine CNT sheet, the measured electrical conductivity is highest at 1400°C, due to a balance between impurities and CNT quality. The tensile strength of the CNT sheet increases with the temperature, possibly because of the “gluing” effect of the carbonaceous impurities.

Unveiling the Evolutions of Nanotube Diameter Distribution during the Growth of Single-Walled Carbon Nanotubes.

In situ and ex situ Raman measurements were used to study the dynamics of the populations of single-walled carbon nanotubes (SWCNTs) during their catalytic growth by chemical vapor deposition. Our study reveals that the nanotube diameter distribution strongly evolves during SWCNT growth but in dissimilar ways depending on the growth conditions. We notably show that high selectivity can be obtained using short or moderate growth times. High-resolution transmission electron microscopy observations support that Ostwald ripening is the key process driving these seemingly contradictory results by regulating the size distribution and lifetime of the active catalyst particles. Ostwald ripening appears as the main termination mechanism for the smallest diameter tubes, whereas carbon poisoning dominates for the largest ones. By unveiling the key concept of dynamic competition between nanotube growth and catalyst ripening, we show that time can be used as an active parameter to control the growth selectivity of carbon nanotubes and other 1D systems.

Flexible Paper Based N-, P- Doped 3-D Graphene Framework Hosting Metal Oxides for Efficient Oxygen Evolution Reactions

The electrocatalytic oxygen evolution reaction (OER) is a critical anode reaction that is often coupled with an electronic/photoelectronic water splitting reaction or used in rechargeable metal-air batteries for renewable energy conversion and storage. [1] However, the sluggish OER reaction kinetics require the use of efficient electro-catalysts, such as metal oxides (e.g. IrO2, RuO2, MnO2, Co3O4, NiFeOx, etc.) or metal-free materials (e.g. N- and P- doped graphene, MWCNT, graphitic carbon nitrite, etc.). To enhance the electro-catalytic activity of OER, great efforts have been devoted to the design of hybrid materials with functionalities tuned by tailoring their micro/nano structure. In addition, pore structure and surface chemistry of supporting materials also play an important role in determining the efficiency of OER catalysis. Traditionally OER catalysts are deposited on a plenary glassy carbon electrode or indium titanium oxide (ITO) glasses, of which the low surface area greatly limits the catalyst loading. Additionally, owning to the lack of surface functionality, these substrates interact weakly with the OER catalyst, adversely affecting OER electro-catalytic activities. More importantly, the bulky and rigid electrodes make them incompatible for broad applications in energy conversion and storage systems. Herein, we present a flexible, free standing oxygen electrode featuring 3-D architectures of high activity and durability for OERs. Conductive microfibers and paper sheets were fabricated using layer-by-layer (LbL) nano-assembly techniques, in which the cationic poly(ethyleneimine) (PEI) has been used in alternate deposition with anionic conductive PEDOT–PSS and solubilized CNT–PSS on lignocellulose wood microfibers [2]. By creating alternating layers of oppositely charged components on the surface of wood microfibers, we have produced a nanocoating of 20–150 nm thickness that enables the microfibers to exhibit electrical conductivity. Moreover, the surface charge of resultant paper can be well tuned by the LbL polyelectrolyte coating. P-, N- dual doped 3-D graphenes were then prepared with an imposing opposite charge to the composite paper. When casting functional porous graphenes onto the composite paper, it is observed that they interconnected, driven by the strong electro-static interaction. The resulting graphene coated paper, which shows high porosity and functionality, was then employed as the host framework, in which a various types of metal oxide particles of high OER activity were decorated in-situ. The obtained oxygen electrode of this hierarchy structure has exhibited improved mass and ion transport and OER efficiency was significantly enhanced. In summary, the developed smart paper consisting of orderly assembled P-, N- doped graphene layers incorporated with active OER catalyst particles performs as a flexible oxygen electrode that shows great promise in being directly applied in flexible electronic devices for efficient energy storage and conversion.

Sugar hydrogenation in continuous reactors: From catalyst particles towards structured catalysts

The density, viscosity and hydrogen solubility of selected sugars (l-arabinose, d-galactose, d-maltose and l-rhamnose) were determined at different temperatures (generally 60, 90 and 130°C). The role of internal diffusion resistance in porous catalyst layers for sugar hydrogenation was confirmed by numerical simulations based on kinetic data and physical properties. The simulations suggested the use of small catalyst particles or structured catalysts in continuous hydrogenation of the sugars to sugar alcohols. Continuous hydrogenation of l-arabinose was carried out in a laboratory-scale fixed bed reactor with ruthenium catalysts on three different supports (active carbon clothes, carbon nanotubes on sponge-like metallic structures, conventional active carbon catalyst particles). It was proved that continuous hydrogenation is a feasible alternative to batch technology for sugar hydrogenation over conventional catalyst particles and structured catalysts: l-arabinose was converted to arabitol with a very high selectivity.

Synthesis and characterization of Ni-Mo catalyst using Jatropha curcas leaves as carbon support and its catalytic activity for hydrotreating of gas oil, Jatropha oil, and their blends

ABSTRACTA novel carbon-based Ni-Mo catalyst has been synthesized successfully from Jatropha curcas leaves using boric acid as a surface modifying agent. The Ni-Mo catalyst prepared on Jatropha curcas leaves had shown BET surface area of 316 m2/g whereas the Ni-Mo catalyst prepared without boric acid activation had shown BET surface area of only 14 m2/g. XRD and SEM data have shown that the active catalyst particles such as Ni and Mo have been found to be uniformly distributed. The inventive catalyst was studied for hydrotreating of gas oil, Jatropha curcas oil and 20% Jatropha oil in gas oil at 370°C, 90 bar H2 pressure, liquid hour space velocity of 1 h−1, and gas-to-oil ratio of 500 Nm3/m3 and the results obtained were found to be comparable with that of the commercial Ni-Mo catalyst supported on alumina.

Efforts for long-term protection of palladium hydrodechlorination catalysts

Nano-sized palladium (Pd) catalysts are known for their high intrinsic hydrodechlorination (HDC) activity towards many chlorinated water pollutants, converting them into non-harmful compounds. Optimization of catalyst protection by embedding various types of Pd catalysts (Pd on oxidic supports and in-situ generated Pd clusters) into poly(dimethylsiloxane) (PDMS) layers was studied together with their use in HDC reactions. The catalyst performance in standard batch and long-term experimental set-ups was used to better understand and optimize the Pd-PDMS system. The phenomenon of polymer deterioration by entrapped HDC-generated HCl is studied for various systems and found to be the main reason for limited life times of Pd-PDMS systems.Pd-PDMS composites were found to provide long-term protection from ionic catalyst poisons. This gain in prolonged catalyst protection goes along with an inevitable decrease in specific catalyst activity which is unhesitantly accepted for the benefit of a longer-lasting catalytic activity. However, when comparing the relative loss of the specific Pd activity due to embedding, it is found to be much lower for initially less active catalyst particles (e.g. a factor of 2 for Pd/Al2O3) compared to about two orders of magnitude for the highly active nanoscale Pd particles. It could be shown that embedded catalysts in continuously operated long-term set-ups have enhanced resistance and longevity compared to those tested in repeated batch-type reaction cycles. PDMS-embedded Pd/Al2O3 maintains its full initial activity up to turnover numbers of >750 (for trichloroethene as probe substance), whereas the catalyst system loses more than 50% of its initial activity for a comparable catalyst utilization (TON≈750) operated in batch cycles.

Gas Transport Resistance in Polymer Electrolyte Thin Films on Oxygen Reduction Reaction Catalysts.

Significant reductions in expensive platinum catalyst loading for the oxygen reduction reaction are needed for commercially viable fuel cell electric vehicles as well as other important applications. In reducing loading, a resistance at the Pt surface in the presence of thin perfluorosulfonic acid (PFSA) electrolyte film, on the order of 10 nm thick, becomes a significant barrier to adequate performance. However, the resistance mechanism is unresolved and could be due to gas dissolution kinetics, increased diffusion resistance in thin films, or electrolyte anion interactions. A common hypothesis for the origin of the resistance is a highly reduced oxygen permeability in the thin polymer electrolyte films that coat the catalyst relative to bulk permeability that is caused by nanoscale confinement effects. Unfortunately, the prior work has not separated the thin-film gas transport resistance from that associated with PFSA interactions with a polarized catalyst surface. Here, we present the first characterization of the thin-film O2 transport resistance in the absence of a polarized catalyst, using a nanoporous substrate that geometrically mimics the active catalyst particles. Through a parametric study of varying PFSA film thickness, as thin as 50 nm, we observe no enhanced gas transport resistance in thin films as a result of either interfacial effects or structural changes in the PFSA. Our results suggest that other effects, such as anion poisoning at the Pt catalyst, could be the source of the additional resistance observed at low Pt loading.

Biofuel-Promoted Polychlorinated Dibenzodioxin/furan Formation in an Iron-Catalyzed Diesel Particle Filter.

Iron-catalyzed diesel particle filters (DPFs) are widely used for particle abatement. Active catalyst particles, so-called fuel-borne catalysts (FBCs), are formed in situ, in the engine, when combusting precursors, which were premixed with the fuel. The obtained iron oxide particles catalyze soot oxidation in filters. Iron-catalyzed DPFs are considered as safe with respect to their potential to form polychlorinated dibenzodioxins/furans (PCDD/Fs). We reported that a bimetallic potassium/iron FBC supported an intense PCDD/F formation in a DPF. Here, we discuss the impact of fatty acid methyl ester (FAME) biofuel on PCDD/F emissions. The iron-catalyzed DPF indeed supported a PCDD/F formation with biofuel but remained inactive with petroleum-derived diesel fuel. PCDD/F emissions (I-TEQ) increased 23-fold when comparing biofuel and diesel data. Emissions of 2,3,7,8-TCDD, the most toxic congener [toxicity equivalence factor (TEF) = 1.0], increased 90-fold, and those of 2,3,7,8-TCDF (TEF = 0.1) increased 170-fold. Congener patterns also changed, indicating a preferential formation of tetra- and penta-chlorodibenzofurans. Thus, an inactive iron-catalyzed DPF becomes active, supporting a PCDD/F formation, when operated with biofuel containing impurities of potassium. Alkali metals are inherent constituents of biofuels. According to the current European Union (EU) legislation, levels of 5 μg/g are accepted. We conclude that risks for a secondary PCDD/F formation in iron-catalyzed DPFs increase when combusting potassium-containing biofuels.

Accelerated Stress Tests on Fuel Cell Cathode Catalysts: A Material Balance Approach Combining Modeling and Experiment

Proton Exchange Fuel Cells (PEFCs) represent an environmentally benign power sources for automotive transportation. The current major hurdles for the mass commercialization of automotive PEFC technology are cost and durability of the cathode catalyst layer. Catalyst degradation occurs due to voltage transients experienced by the cathode catalyst during normal drive cycles and start-up/shut-downs (SUDS). Urban drive cycles involve rapid stop-go transients (idle-to-peak power) that take the cathode catalyst potential from >0.9 V (idle) to around 0.6 V (peak power). Frequent potential changes with high peak potentials (> 1.2 V) drastically accelerate the loss of active catalyst particles and the decay of the of electrochemically active surface area (ECSA). The main degradation mechanisms affecting the carbon-supported Pt-based catalyst are dissolution/redeposition, coagulation and detachment. Accelerated stress tests (ASTs), performed to decouple and quantify contributions of different catalyst degradation mechanisms are typically performed ex situ to eliminate degradation effects caused by the other materials within the cell, such as the proton conducting membrane and the gas diffusion media. The AST results presented herein evaluated Pt/C degradation under operationally relevant conditions temperature (22°C and 70°C), upper potential limit (UPL, 0.9 V and 1.2 V) and potential wave form (triangle vs. square waves). A comprehensive material balance analysis was performed to track changes in the state of Pt during the AST. This analysis included the determination of ECSA, particle size distribution and amount of dissolved Pt in the electrolyte solution. These results were analyzed with a dynamic model that couples the three catalyst degradation mechanisms to quantify their relative contributions. The model relates the kinetic rates of the degradation processes to the evolution of the particle size distribution. The loss of ECSA during 25,000 AST cycles at 70°C by periodically performing cyclic voltammetry (0.02 V « 0.6 V at 10 mV sec-1) and integrating the hydrogen desorption charge (210 µC cm2). Generally, ECSA loss was found to be accelerated by the SW profile and by the high UPL of 1.2V. The Pt particle size distribution was measured by Transmission Electron Microscopy (TEM). The beginning of life (BOL) particle median size was 2 nm. Figure 1 shows the change in the particle size distribution after 25,000 AST cycles at 70°C. The tests with higher UPL had the most significant increase in mean particle size and width of the distribution. The combined modeling and material balance preliminary results demonstrate a significant role of the effective surface tension of the catalyst dominating the kinetics of the dissolution/Redeposition mechanism at 1.2 V, suggesting that Pt dissolution is strongly coupled to Pt oxide formation and reduction. The analysis fit the enhanced degradation due to the square wave acceleration of dissolution/Redeposition compared to the triangular wave. Figure 1. Histograms from TEM particle size analysis for 25k AST cycles at 70°C. Upper potential limit of (A) 0.9V and (B) 1.2V. Lines are simulated particle size distributions from dynamic model analysis. Figure 1

Controlling Carbon Nanotube Type in Macroscopic Fibers Synthesized by the Direct Spinning Process

We report on the synthesis of kilometers of continuous macroscopic fibers made up of carbon nanotubes (CNT) of controlled number of layers, ranging from single-walled to multiwalled, tailored by the addition of sulfur as a catalyst promoter during chemical vapor deposition in the direct fiber spinning process. The progressive transition from single-walled through collapsed double-walled to multiwalled is clearly seen by an upshift in the 2D (G′) band and by other Raman spectra features. The increase in number of CNT layers and inner diameter results in a higher fiber macroscopic linear density and greater reaction yield (up to 9%). Through a combination of multiscale characterization techniques (X-ray photoelectron spectroscopy, organic elemental analysis, high-resolution transmission electron microscopy, thermogravimetric analysis, and synchrotron XRD) we establish the composition of the catalyst particles and position in the isothermal section of the C–Fe–S ternary diagram at 1400 °C. This helps explain the unusually low proportion of active catalyst particles in the direct spinning process (<0.1%) and the role of S in limiting C diffusion and resulting in catalyst particles not being in thermodynamic equilibrium with solid carbon, therefore producing graphitic edge growth instead of encapsulation. The increase in CNT layers is a consequence of particle coarsening and the ability of larger catalyst particles to accommodate more layers for the same composition.

Nanostructuring Ultra-thin Co Films to Active Catalyst Particles for Vertically Aligned Single-walled CNT Growth

In the field of material synthesis using chemically-derived technique, nanostructuring metal catalyst particles towards high quality production of carbon nanotubes (CNTs) has been very attractive. In this work, cobalt (Co) which used as catalyst for vertical growth of CNTs were studied by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Aluminum (Al) films (20 nm) were thermally-oxidized to form aluminum oxide (Al-O) to support 0.5 nm Co catalyst during CNT growth. In growing CNTs by using chemical vapor deposition (CVD) technique, the role and characters of all involving materials are crucial to the growth result. From the Co/Al-O substrate and at 650°C of CVD temperature, 33-μm thick of vertically aligned single-walled CNT (VA-SWCNT) forest has been grown. TEM particle analysis revealed that the Co particles have an average of 3.50 nm experimentally and in principle favored the growth of highly demanded VA-SWCNTs. The as-prepared Co particles are suggested chemically active for the CNT growth. In addition, XPS analysis confirmed the surface chemical state of Co particles prior to the VA-SWCNT growth using ethanol based CVD system.

In situ and ex situ time resolved study of multi‐component FeCo oxide catalyst activation during MWNT synthesis

AbstractComparative in situ X‐ray diffraction (XRD) study of activation Fe, Co, and FeCo catalysts on different supports (Al2O3 and CaCO3) for multi‐walled carbon nanotube (MWNT) growth combined with ex situ high‐resolution transmission electron microscopy (HRTEM) characterization has been performed. The catalyst contained Fe as an active component (mono‐component Fe catalyst) demonstrates the simultaneous formation of FeC alloys (austenite and tetragonal ferrite like structure) with their subsequent transformation into stable iron carbide Fe3C (cohenite). The active component of bimetal FeCo catalysts (Fe:Co = 2:1) is FeCo alloyed nanoparticles, which does not demonstrate carbide formation. The absence of carbides promotes carbon diffusion through metal particle providing much higher activity of bimetal FeCo catalyst in comparison with the mono‐component Fe and Co catalysts. Tubes produced using the bimetal catalysts have a high degree of structural perfection and narrow diameter distribution. The most active metal catalyst particles formed under reaction conditions are solid or at least have crystalline core. Pechini‐type method used for FeCo catalyst production provides much higher MWNT yields than it was described elsewhere. magnified imageStructure of active metal particles was proposed according to XRD data during MWNT growth on different type of catalysts with mono‐ and bimetal active component.

Comparison of CFD simulations to experiment under methane steam reforming reacting conditions

The highly endothermic commercial methane steam reforming (MSR) reaction was studied experimentally in a single-pellet-string fixed bed reactor. The temperatures inside the active catalyst particles, the temperatures on the outer surfaces of selected pellets and the exit gas composition were measured. The MSR reaction showed strong effects on the temperature profile along the reactor, causing it to decrease initially. A computational fluid dynamics (CFD) model was used to predict temperature and species profiles under the experimental MSR reaction conditions. Comparison of CFD and experimental data showed very good qualitative as well as quantitative agreement for temperature inside catalyst particles at different inlet gas temperatures, and for the temperature drop from outside to inside the pellets due to the reaction heat sink. Trends in methane conversion were also well-represented by the CFD simulation.

Inorganic Materials with Double‐Helix Structures

Two different methods recently yielded inorganic materials with double-helix structures: Silicon microtubes (see picture) formed when high inner pressure forced NaSi melt through an opening in the surface of a disc, and carbon nanotubes were prepared when plates of layered double hydroxide coated with active catalyst particles were used as substrate. These reports open the door for the application of double-helical inorganic materials in chemistry and biology.

Synthesis and field emission properties of carbon nanotubes grown in ethanol flame based on a photoresist-assisted catalyst annealing process

Carbon nanotubes (CNTs) have been grown directly on a Si substrate without a diffusion barrier in ethanol diffusion flame using Ni as the catalyst after a photoresist-assisted catalyst annealing process. The growth mechanism of as-synthesized CNTs is confirmed by scanning electron microscopy, high resolution transmission-electron microscopy and energy-dispersive spectroscopy. The photoresist is the key for the formation of active catalyst particles during annealing process, which then result in the growth of CNTs. The catalyst annealing temperature has been found to affect the morphologies and field electron emission properties of CNTs significantly. The field emission properties of as-grown CNTs are investigated with a diode structure and the obtained CNTs exhibit enhanced characteristics. This technique will be applicable to a low-cost fabrication process of electron-emitter arrays.