Reactant Composition Research Articles

Combustion of Na 2B 4O 7 + Mg + C to synthesis B 4C powders

Boron carbide powder was fabricated by combustion synthesis (CS) method directly from mixed powders of borax (Na 2B 4O 7), magnesium (Mg) and carbon. The adiabatic temperature of the combustion reaction of Na 2B 4O 7 + 6 Mg + C was calculated. The control of the reactions was achieved by selecting reactant composition, relative density of powder compact and gas pressure in CS reactor. The effects of these different influential factors on the composition and morphologies of combustion products were investigated. The results show that, it is advantageous for more Mg/Na 2B 4O 7 than stoichiometric ratio in Na 2B 4O 7 + Mg + C system and high atmosphere pressure in the CS reactor to increase the conversion degree of reactants to end product. The final product with the minimal impurities’ content could be fabricated at appropriate relative density of powder compact. At last, boron carbide without impurities could be obtained after the acid enrichment and distilled water washing.

Hydrothermal synthesis and growth mechanism of Zn xCd 1− xS nanoflake dendrites

Novel Zn x Cd 1− x S nanoflake dendrites were synthesized by an original hydrothermal process at 95 °C for 1 h. Highly ordered embranchments were found on the trunks of the dendrites and each of the embranchments consisted of numbers of uniform regular hexagon nanoflakes. The nanoflake dendrites were oriented along the [0 0 0 1] direction. With a fixed composition of reactants, Cd/Zn ratio of the product Zn x Cd 1− x S was close to the composition in the recipe and gradually decreased from trunks to embranchments and then to nanoflakes. By adjusting the initial Zn/Cd ratio of the reactants, gradual changes in morphology of the final products were observed. The variation of Cd/Zn ratio in different parts of the nanoflake dendrite and morphology transformation of the final product Zn x Cd 1− x S were governed by a competition between the trunk-like and the flake-like growth mode.

Space and time resolving molecular acoustics as a tool to visualize epoxy formation at a planar hardener–resin interface

Scanning Brillouin microscopy visualizes the temporal evolution of a heterogeneous hypersonic profile during chemical reactions and transport processes around a planar phase boundary between an epoxy resin and hardener. The hypersonic investigations are interpreted in terms of structural formation and annihilation in the layered sample and are compared to results obtained for a well mixed bulk sample of the same composition of reactants. Chaotic transport channels are made responsible for the creation of local glassy heterogeneities not observable in the bulk material on a macroscopic level.

A Revisit to Carbon Monoxide Oxidation on Pd(111) Surfaces

Carbon monoxide (CO) oxidation on Pd(111) surfaces has been studied by molecular beam methods with mixed molecular beams (CO + O2) between 400 and 900 K and a CO:O2 ratio of 7:1 to 1:10. A new aspect of the above reaction observed in the transient kinetics regime is the evidence for oxygen diffusion into Pd(111) subsurface layers and its significant influence toward CO oxidation at high temperatures (≥600 K). An overall influence of subsurface oxygen on the kinetics of the CO oxidation reaction is addressed. Interesting information derived from the above studies is the necessity to fill up the subsurface layers with oxygen atoms to a threshold coverage (θOsub), above which the reactive CO adsorption occurs on the surface with subsequent CO2 production. The above observation was demonstrated with CO-rich reactant compositions (CO + O2) above 600 K via instant oxygen adsorption on Pd surfaces; however, onset of CO adsorption as well as CO2 production occurs after a time delay. θOsub and the time delay in CO...

Semisynthesis of a Controlled Stimuli-Responsive Alginate Hydrogel

Benefits of the use of natural polymers include biodegradability, biocompatibility, natural abundance, and unique physicochemical/biological properties. Native alginate was used to semisynthesize a new class of biomaterial in which the physical properties such as swelling and pore size can be chemically tailored for desired end use. Semisynthetic network alginate polymer (SNAP) was prepared by reaction with glutaraldehyde, forming an acetal-linked network polymer gel with carboxylate moieties preserved as stimuli-responsive sensors. The molecular structure of the hydrogel was confirmed by cross-polarization magic-angle spinning (13)C solid state NMR, and reaction parameters affecting the polymer synthesis, including reactant, catalyst concentrations, and solvent composition, were characterized by gel equilibrium swelling. The acetalization reaction can be thermodynamically controlled, offering fine-tuned control of gel swelling and pore properties. In addition, SNAP demonstrated pronounced swelling at alkaline pH and contraction in acidic environment with oscillatory response to repeated pH-stimuli, yielding a potential pulsatile, oral drug delivery vehicle. Through selection of reaction conditions, gel swelling, pore size, and stimuli-responsive characteristics can be specifically tailored for applications such as a tissue scaffold in regenerative medicine, as a targeted delivery vehicle, and as a superabsorbent in environmental cleanup.

Evaluation of the net water transport through electrolytes in Proton Exchange Membrane Fuel Cell

Electro-osmotic drag and back diffusion are the primary water transport mechanisms in PEMFC (Proton Exchange Membrane Fuel Cell) electrolytes. These two phenomena occur competitively in the membrane, and ultimately determine the net water movement. The chemical compositions of reactants and product (i.e., H 2, O 2, and H 2O) in a porous catalyst layer vary with respect to the electro-chemical reactions and water transport through the membrane. The tendency of the chemical compositions was estimated by analyzing the net water transport coefficient ( α), defined as the ratio of the reaction rate to the water transport rate. New criteria were suggested for predicting species mole fractions from the flow direction, and these were validated by CFD analysis. The hydrogen mole fraction had different tendencies to rise or fall based on the flow direction at α = 0.5, while the oxygen mole fraction extreme was located at α = −0.75. Finally, α was shown to influence the membrane conductivity and activation losses, which are the main factors that contribute to fuel cell performance.

Plasma Synthesis of Tungsten Carbide Nanopowder from Ammonium Paratungstate

A thermal plasma process has been applied to the synthesis of nanosized tungsten carbide powder with ammonium paratungstate (APT) as the precursor. The reduction and carburization of vaporized APT produced nanosized tungsten carbide (WC1−x) powder, which sometimes contained a small amount of W2C phase. The effects of reactant gas composition, plasma torch power, the flow rate of plasma gas, and the addition of secondary plasma gas (H2) on the product composition and particle size were investigated. The produced tungsten carbide (WC1−x) powder was <20 nm in particle size. The synthesized powders were also subjected to a hydrogen heat treatment to fully carburize the WC1−x and W2C phases to the WC phase as well as to remove excess carbon. Finally, WC powder of particle size <100 nm was obtained.

Hierarchically macro/mesoporous silica monoliths constructed with interconnecting micrometer-sized unit rods

Under typical dilute reactant compositions (3 ~ 5 wt% of surfactant template concentration) and conventional hydrothermal conditions for mesoporous materials synthesis, successful preparation of hierarchically macro/mesoporous silica monoliths was reported in this paper. The resultant materials were characterized by a series of techniques including powder X-ray diffraction, N2 adsorption–desorption, SEM, TEM/EDS, and Hg porosimetry. A new kind of stable and hierarchically porous pure silica monoliths was confirmed, which are featured with highly ordered mesoporous structures, rod-shaped unit particles, large specific surface area of 492 m2/g, continuous macropores of about 4.0 μm in size and high macropore volume of about 13.1 cm3/g. Moreover, using the resultant silica monoliths as hard templates, carbon monoliths have been successfully replicated, which inherit the structural characters of parent silica materials.

Formation of needle-like hydroxyapatite by hydrothermal treatment of CaHPO4 2H2O combined with .BETA.-Ca3(PO4)2

Conditions were investigated for preparing needle-like hydroxyapatite (HAp; Ca10(PO4)6(OH)2) by hydrothermal processing of mixed calcium phosphate powders consisting of dicalcium phosphate dihydrate (DCPD; CaHPO4·2H2O) and beta-tricalcium phosphate (β-TCP; β-Ca3(PO4)2). The powder mixture was subjected to two-step hydrothermal processing in which the compacted powder was treated first by exposure to vapor of the liquid and subsequently by immersion in the liquid. High aspect ratios were obtained in samples with crystal sizes around 60 μm in length from the reactant mass ratio composition of β-TCP/DCPD = 1/9 by treatment at 160°C for 0.5 h under vapor conditions, followed by liquid conditions for 24 h with renewal of the liquid at 12 h. Initial compositions and pH conditions were the main parameters determining the formation of needle-like HAp by hydrothermal processing.

Stable .DELTA.15N and .DELTA.13C isotope ratios in aquatic ecosystems

In the past 20 years, rapid progress in stable isotope (SI) studies has allowed scientists to observe natural ecosystems from entirely new perspectives. This report addresses the fundamental concepts underlying the use of the SI ratio. The unique characteristics of the SI ratio make it an interdisciplinary parameter that acts as a chemical fingerprint of biogenic substances and provides a key to the world of isotopomers. Variations in SI ratios of biogenic substances depend on the isotopic compositions of reactants, the pathways and kinetic modes of reaction dynamics, and the physicochemical conditions. In fact, every biogenic material has its own isotopic composition, its “dynamic SI fingerprint”, which is governed by its function and position in the material flow. For example, the relative SI ratio in biota is determined by dietary lifestyle, e.g., the modes of drinking, eating, and excreting, and appears highly regular due to the physicochemical differences of isotopomers. Our primary goal here is to elucidate the general principals of isotope partitioning in major biophilic elements in molecules, biogenic materials, and ecosystems (Wada, E. et al., 1995). To this end, the nitrogen and carbon SI distribution ratios (δ15N and δ13C, respectively) are used to examine materials cycling, food web structures, and their variability in various kinds of watershed-including aquatic ecosystems to elucidate an “isotopically ordered world”.

Simultaneous Determination of Orthophosphate and Silicate Ions in River Water by Ion-Exclusion Chromatography with Postcolumn Spectrophotometric Detection Using Molybdate and Malachite Green

The simultaneous determination of orthophosphate and silicate ions in river water was examined using ion-exclusion chromatography with spectrophotometric detection at 680 nm after derivatization with molybdate and malachite green. In this study, optimization of the color-forming reactant composition to form the ion association complex of heteropolyacids with malachite green was examined. The optimum concentration of ethanol (30%) in the reactant accelerated the formation of the ion-association complex, resulting in high sensitivity for the detection of ions. Using the optimized reactant containing 100 mM H2SO4/10 mM Na2MoO4/0.05 mM malachite green/30% ethanol, the calibration curve of orthophosphate and silicate ions was linear in the range of 0∼200 μg L−1 as P and 0∼2000 μg L−1 as Si with a good correlation coefficient of 0.998. The relative standard deviations of the peak areas and the retention time of orthophosphate and silicate ions were between 0.5 and 3.2% for five repeated measurements. The detection limits of orthophosphate and silicate ions were 0.3 and 2.5 μg L−1, respectively. The detection limit for the orthophosphate ion was 5 to 10-times improved when compared with those of previous studies using molybdenum-yellow or molybdenum-blue as a derivatization with molybdate. The recovery tests using river water for orthophosphate and silicate ions were 97.2% and 102.5%, respectively. The developed method was successfully applied to the simultaneous determination of orthophosphate and silicate ions in practical river-water samples.

Measuring the Current Distribution with Sub-Millimeter Resolution in PEFCs: II. Impact of Operating Parameters

In the first paper of this series, an experimental technique for measuring the current-density distribution with a resolution better than the sub-millimeter scale of the channel and rib structures in the flow-field plates of polymer electrolyte fuel cells (PEFCs) was introduced. This method is extended to the determination of local membrane resistance with the same spatial resolution in the present paper. The combined measurement of current and resistance allows for investigating the interaction of mass- and charge-transport processes, which determine the local rate distribution across the domain of channels and ribs. Therewith, the influence of relevant operating parameters such as reactant composition, dew points, and cell compression on local current generation is investigated. The results show that the distribution of water and oxidant across the channel and rib are the main reasons for significant current gradients on a scale smaller than a millimeter. Humidity variation mainly affects the membrane resistance under the channel, while reactant concentration predominantly influences current generation under the rib-covered cell area.

CO Oxidation on Pt-Group Metals from Ultrahigh Vacuum to Near Atmospheric Pressures. 2. Palladium and Platinum

CO oxidation on Pd(100), -(111), -(110), and Pt(110) single crystals was studied at steady-state conditions at low (≤2 × 10−3 Torr) and high (2−88 Torr) pressures at various reactant compositions. At low pressures the reaction fell into two regimes, one with a CO-dominant surface where the CO2 formation rate is low, and a second with an O-dominant surface where the reaction rate is high. Within this second regime, the reaction is collision-limited with no oxygen inhibition. Under high-pressure reaction conditions, three reaction regimes are evident: (i) a CO-inhibited metallic regime displaying a low CO2 formation rate; (ii) an oxygen-rich metallic regime with a high CO2 formation rate; and (iii) a high-temperature regime where the CO2 formation rate is either mass transfer limited on a metallic surface or limited by the reduced reactivity of the oxidized surface. The superior activity of Pt group metal oxides compared to the reduced metal, as proposed recently, was not observed in this study.

CO Oxidation on Pt-Group Metals from Ultrahigh Vacuum to Near Atmospheric Pressures. 1. Rhodium

The CO oxidation reaction on Rh(111) was studied both at low pressures (≤2 × 10−4 Torr) under steady-state conditions and at high pressures (0.01−88 Torr) in a batch reactor at various gaseous reactant compositions. Surface CO and O coverages were determined using polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and X-ray photoelectron spectroscopy (XPS). CO titration experiments were also carried out on surfaces with known oxygen coverages. Both CO and O inhibition were evident at low pressures so that only within a relatively narrow temperature range were the reaction conditions optimized such that the CO conversion reached ∼20% of the CO flux to the surface. For high pressures and with stoichiometric or slightly oxidizing reactant ratios (O2/CO ≤ 2), the reaction fell into three regimes: (i) a CO-inhibited low temperature regime where the reaction rate was determined by CO desorption; (ii) a mass transfer limited regime at high temperatures; and (iii) a transient, high-rate regime lying between regimes (i) and (ii) where the reaction was not completely controlled by mass transfer limitation. For all reaction conditions investigated (when O2/CO ≤ 2), the surface oxygen coverage did not exceed ∼0.5 monolayer. With very oxidizing reactants (O2/CO ≥ 5), the reactivity of the Rh surface decreased dramatically at high temperatures due to oxidation. Furthermore, the so-called “superior oxide reactivity” for CO oxidation that has been proposed in several recent studies is not evident in this investigation.

CO oxidation trends on Pt-group metals from ultrahigh vacuum to near atmospheric pressures: A combined in situ PM-IRAS and reaction kinetics study

The CO oxidation reaction on Pt-group metals (Pt, Rh, and Pd) has been investigated at low (⩽10−3Torr) and near atmospheric (1–102Torr) pressures in a batch reactor under steady-state conditions and at various gaseous reactant compositions using PM-IRAS and kinetic measurements. The results indicate that Langmuir–Hinshelwood kinetics adequately provides a general description of the kinetic trends over a wide range of pressures provided that mass transfer effects are considered. At high pressures, the reaction kinetics fall into three regimes: a CO-inhibited low temperature regime where the reaction rate is determined by CO desorption; a mass transfer limited regime at high temperatures; and a transient, high-rate regime which lies in between the other two regimes. The data show that the most reactive surface phase, at both low and high pressures, is a CO-uninhibited phase. This surface phase is not an oxide phase, but a surface phase that contains primarily chemisorbed atomic oxygen and a low coverage of CO.

CH∗ chemiluminescence modeling for combustion diagnostics

CH∗ chemiluminescence is often employed in combustion diagnostics, for example as a measure of heat release rate and for equivalence ratio sensing. However, most interpretations of CH∗ chemiluminescence rely either on heuristic arguments or empirical data gathered under limited conditions. More rigorous analysis is required to understand the effects of combustion conditions, e.g., pressure, reactant composition and preheat, strain and exhaust gas recirculation, on CH∗ chemiluminescence. Chemiluminescence modeling holds promise in this regard. The predictive accuracy of four proposed CH∗ chemiluminescence formation models were experimentally tested in premixed, methane–air and prevaporized Jet-A–air flames. Two of the models, based on CH∗ formation via reactions between C2H and O or O2, are able to predict the experimental data within the experimental uncertainty, in both room temperature methane and preheated Jet-A flames. The utility of CH∗ chemiluminescence for sensing heat release rate and equivalence ratio (ϕ), when combined with OH∗ chemiluminescence, is then analyzed in methane flames for varying pressure, preheat temperature and strain. The CH∗/OH∗ chemiluminescence ratio is found to be useful for sensing equivalence ratio in lean methane systems, but only at certain pressure and reactant temperature conditions. The relationship between CH∗ chemiluminescence and heat release varies with ϕ, pressure, temperature and strain. At high pressures, however, the dependence on ϕ and strain are small, making CH∗ attractive for heat release sensing applications in gas turbine combustors.

Model of hyperbranched polymers formed by monomers A 2 and B g with end-capping molecules

Hyperbranched polymers, HBPs, formed via a stepwise polymerization of A 2, B g type monomers with the addition of end-capping molecules, AR, were investigated by means of recursive and kinetic models. First, gelation curves were established based on the initial compositions of reactants at various functionalities, g, of monomers B g . According to this guide, the hyperbranched polymers without gel fraction can be obtained. The molecular structures of HBPs, such as molecular weight and the degree of branching were calculated as related to conversion. It is shown that they can be controlled by the composition of reactants. With the addition of molecules AR, the gelation can be avoided at high conversion, and the distribution of molecular weights of polymers becomes narrower.

High-throughput approach to the catalytic combustion of diesel soot

A methodology for the evaluation of diesel soot oxidation catalysts by high-throughput (HT) screening was developed. The optimal experimental conditions (soot amount, catalyst/soot ratio, type of contact, composition and flow rate of gas reactants) ensuring a reliable and reproducible detection of light-off temperatures in a 16 parallel channels reactor were set up. The temperature profile measured in the catalyst/soot bed under TPO conditions when the exothermic combustion of soot takes place was shown to provide an accurate measurement of the ignition. Its reproducibility and relevance were checked. The results obtained with a reference noble metal free catalyst (La 0.8Cr 0.8Li 0.2O 3 perovskite) agree very well with literature data. Qualitative mechanistic features could be derived from these experiments, stressing the likely limiting step of oxygen transfer from catalyst surface to soot particulates to ignite the soot combustion. Ceria material was shown to be more appropriate than perovskite one. From an HT screening of a large diverse library (over 100 mixed oxides catalysts) under optimized conditions, about 10 new formulations were found to perform better than selected noble metal free reference materials.

Innovative membrane reformer for hydrogen production applied to PEM micro-cogeneration: Simulation model and thermodynamic analysis

This paper illustrates an innovative concept of hydrogen production, applied to micro-cogeneration systems based on polymer electrolyte membrane (PEM) fuel cells fed with natural gas. Three options to generate the hydrogen rich mixture required by the fuel cell are compared: (i) a conventional steam reformer, (ii) an auto-thermal reformer and (iii) an innovative membrane reformer. In the first two cases, the syngas generated by the reformer requires a water gas shift reactor (WGSR), where the majority of carbon monoxide is converted into hydrogen, as well as a preferential oxidation (PROX) reactor to further reduce the CO content before entering the fuel cell. In the third case, hydrogen is simultaneously produced by the reformer and separated at high temperature by a Pd-based membrane, selective to pure hydrogen, and, directly fed to the fuel cell after cooling. The model simulates the PEM performance with a lumped-volume approach. It calculates energy balance and flow composition at the cell outlet based on available information about utilization factors and reactant compositions; cell voltage is predicted with theoretical correlations calibrated over available experimental data. After a description of the adopted calculation model, the paper carries out a thermodynamic analysis of the considered system configurations and discusses the issue of thermal integration among the various heat exchange processes, in order to achieve maximum heat recovery efficiency and avoid the use of an external water supply. The obtained results show that the innovative membrane reforming solution yields several advantages in terms of electric efficiency and plant layout simplicity, suggesting interesting potential applications and the possibility to achieve relevant energy savings when applied to typical residential loads.

Rapid and environmentally friendly preparation of starch esters

The effects of microwave heating and reactant composition on the iodine-catalyzed acetylation of starch by acetic anhydride were investigated. After only 2 min at 100 °C, significant amounts of acetylation occurred using small amounts of iodine (0.16–2.5 mol%). Values of degree of substitution (DS) increased as iodine/starch and acetic anhydride/starch ratios increased. Starch acetates having DS of up to 3 were prepared using 2.5 mol% iodine. Molecular weights decreased as iodine levels increased. This method of starch acetylation is attractive since no solvent is needed, reaction is rapid, energy inputs are relatively small, and the catalyst has low toxicity.