Showing papers on "Reaction rate published in 2013"
TL;DR: In condensed matter, strong interactions alter chemical activities and create variations that can dramatically affect the reaction rate as mentioned in this paper, and the extreme case is that of a reaction coupled to a phase transformation whose kinetics must depend not only on the order parameter but also on its gradients at phase boundaries.
Abstract: Advances in the fields of catalysis and electrochemical energy conversion often involve nanoparticles, which can have kinetics surprisingly different from the bulk material. Classical theories of chemical kinetics assume independent reactions in dilute solutions, whose rates are determined by mean concentrations. In condensed matter, strong interactions alter chemical activities and create variations that can dramatically affect the reaction rate. The extreme case is that of a reaction coupled to a phase transformation, whose kinetics must depend not only on the order parameter but also on its gradients at phase boundaries. Reaction-driven phase transformations are common in electrochemistry, when charge transfer is accompanied by ion intercalation or deposition in a solid phase. Examples abound in Li-ion, metal–air, and lead–acid batteries, as well as metal electrodeposition–dissolution. Despite complex thermodynamics, however, the standard kinetic model is the Butler–Volmer equation, based on a dilute s...
497 citations
TL;DR: New understanding is unearthed of the one-pot catalytic conversion of cellulose to EG, a typical petroleum-dependent bulk chemical widely applied in the polyester industry whose annual consumption reaches about 20 million metric tons, which will greatly decrease the dependence on petroleum resources and the associated CO₂ emission.
Abstract: With diminishing fossil resources and increasing concerns about environmental issues, searching for alternative fuels has gained interest in recent years. Cellulose, as the most abundant nonfood biomass on earth, is a promising renewable feedstock for production of fuels and chemicals. In principle, the ample hydroxyl groups in the structure of cellulose make it an ideal feedstock for the production of industrially important polyols such as ethylene glycol (EG), according to the atom economy rule. However, effectively depolymerizing cellulose under mild conditions presents a challenge, due to the intra- and intermolecular hydrogen bonding network. In addition, control of product selectivity is complicated by the thermal instabilities of cellulose-derived sugars. A one-pot catalytic process that combines hydrolysis of cellulose and hydrogenation/hydrogenolysis of cellulose-derived sugars proves to be an efficient way toward the selective production of polyols from cellulose. In this Account, we describe our efforts toward the one-pot catalytic conversion of cellulose to EG, a typical petroleum-dependent bulk chemical widely applied in the polyester industry whose annual consumption reaches about 20 million metric tons. This reaction opens a novel route for the sustainable production of bulk chemicals from biomass and will greatly decrease the dependence on petroleum resources and the associated CO₂ emission. It has attracted much attention from both industrial and academic societies since we first described the reaction in 2008. The mechanism involves a cascade reaction. First, acid catalyzes the hydrolysis of cellulose to water-soluble oligosaccharides and glucose (R1). Then, oligosaccharides and glucose undergo C-C bond cleavage to form glycolaldehyde with catalysis of tungsten species (R2). Finally, hydrogenation of glycolaldehyde by a transition metal catalyst produces the end product EG (R3). Due to the instabilities of glycolaldehyde and cellulose-derived sugars, the reaction rates should be r₁ << r₂ << r₃ in order to achieve a high yield of EG. Tuning the molar ratio of tungsten to transition metal and changing the reaction temperature successfully optimizes this reaction. No matter what tungsten compounds are used in the beginning reaction, tungsten bronze (HxWO₃) is always formed. It is then partially dissolved in hot water and acts as the active species to homogeneously catalyze C-C bond cleavage of cellulose-derived sugars. Upon cooling and exposure to air, the dissolved HxWO₃ is transformed to insoluble tungsten acid and precipitated from the solution to facilitate the separation and recovery of the catalyst. On the basis of this temperature-dependent phase-transfer behavior, we have developed a highly active, selective, and reusable catalyst composed of tungsten acid and Ru/C. Our work has unearthed new understanding of this reaction, including how different catalysts perform and the underlying mechanism. It has also guided researchers to the rational design of catalysts for other reactions involved in cellulose conversion.
402 citations
TL;DR: In this article, the microstructural development and hardening rate of fly ash-slag geopolymers were investigated using SEM-EDS, and the reaction rate was monitored using semi-adiabatic calorimetry.
337 citations
TL;DR: In this article, the influence of surface area and mineral type (e.g., quartz or limestone) on cement reaction rates was analyzed through extensive experimentation and simulation, and it was shown that the extent of the acceleration is linked to the magnitude of the surface area increase and the capacity of the filler's surface to offer favorable nucleation sites for hydration products.
Abstract: Finely ground mineral powders are known to accelerate cement hydration rates. This “filler effect” has been attributed to the effects of dilution (w/c increase) when the cement content is reduced or to the provision of additional surface area by fine powders. The latter contribution (i.e., surface area increase) is speculated to provide additional sites for the nucleation of the hydration products, which accelerates reactions. Through extensive experimentation and simulation this study describes the influence of surface area and mineral type (e.g., quartz or limestone) on cement reaction rates. Simulations using a boundary nucleation and growth (BNG) model and a multiphase reaction ensemble (MRE) indicate that the extent of the acceleration is linked to the: (1) magnitude of surface area increase and (2a) capacity of the filler's surface to offer favorable nucleation sites for hydration products. Other simulations using a kinetic cellular automaton model (HydratiCA) suggest that accelerations are linked to: (2b) the interfacial properties of the filler that alters (increases or decreases) its tendency to serve as a nucleant, and (3) the chemical composition of the filler and the tendency for its dissociated ions to participate in exchange reactions with the calcium silicate hydrate product. The simulations are correlated with accelerations observed using isothermal calorimetry when fillers partially replace cement. The research correlates and unifies the fundamental parameters that drive the filler effect and provides a mechanistic understanding of the influence of filler agents on cementitious reaction rates.
285 citations
TL;DR: In this article, the influence of red mud addition on the reaction, structure and properties of fly ash geopolymer was studied using isothermal conduction calorimetry (ICC), Fourier transform infrared spectroscopy (FTIR), electron probe microanalysis (EPMA), and mechanical testing.
261 citations
TL;DR: At higher temperatures, only about 50% of dissociation is found to take place under quasi-stationary state conditions, suggesting the necessity of explicitly including some rovibrational levels, when solving a global kinetic rate equation.
Abstract: A rovibrational collisional model is developed to study energy transfer and dissociation of N2(1Σg+) molecules interacting with N(4Su) atoms in an ideal isochoric and isothermal chemical reactor. The system examined is a mixture of molecular nitrogen and a small amount of atomic nitrogen. This mixture, initially at room temperature, is heated by several thousands of degrees Kelvin, driving the system toward a strong non-equilibrium condition. The evolution of the population densities of each individual rovibrational level is explicitly determined via the numerical solution of the master equation for temperatures ranging from 5000 to 50 000 K. The reaction rate coefficients are taken from an ab initio database developed at NASA Ames Research Center. The macroscopic relaxation times, energy transfer rates, and dissociation rate coefficients are extracted from the solution of the master equation. The computed rotational-translational (RT) and vibrational-translational (VT) relaxation times are different at low heat bath temperatures (e.g., RT is about two orders of magnitude faster than VT at T = 5000 K), but they converge to a common limiting value at high temperature. This is contrary to the conventional interpretation of thermal relaxation in which translational and rotational relaxation timescales are assumed comparable with vibrational relaxation being considerable slower. Thus, this assumption is questionable under high temperature non-equilibrium conditions. The exchange reaction plays a very significant role in determining the dynamics of the population densities. The macroscopic energy transfer and dissociation rates are found to be slower when exchange processes are neglected. A macroscopic dissociation rate coefficient based on the quasi-stationary distribution, exhibits excellent agreement with experimental data of Appleton et al. [J. Chem. Phys. 48, 599–608 (1968)]10.1063/1.1668690. However, at higher temperatures, only about 50% of dissociation is found to take place under quasi-stationary state conditions. This suggest the necessity of explicitly including some rovibrational levels, when solving a global kinetic rate equation.
207 citations
TL;DR: In this paper, the selectivity of Pd-based alloys for the selective hydrogenation of acetylene to ethylene was investigated using density functional theory calculations to understand both the acetylene hydrogenation activity and selectivity for ethylene formation.
202 citations
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07 Aug 2013TL;DR: In this article, the steady state oxide thickness, the time to achieve steady state, and the steady-state recession rate were derived from the paralinear kinetics model for SiC and Si3N4 in water vapor-containing environments.
Abstract: At high temperatures SiC and Si3N4 react with water vapor to form a silica scale. Silica scales also react with water vapor to form a volatile Si(OH)4 species. These simultaneous reactions, one forming silica and the other removing silica, are described by paralinear kinetics. A steady state, in which these reactions occur at the same rate, is eventually achieved, After steady state is achieved, the oxide found on the surface is a constant thickness and recession of the underlying material occurs at a linear rate. The steady state oxide thickness, the time to achieve steady state, and the steady state recession rate can all be described in terms of the rate constants for the oxidation and volatilization reactions. In addition, the oxide thickness, the time to achieve steady state, and the recession rate can also be determined from parameters that describe a water vapor-containing environment. Accordingly, maps have been developed to show these steady state conditions as a function of reaction rate constants, pressure, and gas velocity. These maps can be used to predict the behavior of silica formers in water-vapor containing environments such as combustion environments. Finally, these maps are used to explore the limits of the paralinear oxidation model for SiC and Si3N4
202 citations
TL;DR: A detailed study of p-NP reduction by NaBH4 in the presence of the raspberry-like poly(allylamine hydrochloride)-modified polymer poly(glycidyl methacrylate) composite sub-microspheres with tunable gold nanoparticles (PGMA@PAH@AuNPs) was presented.
Abstract: The early reported pseudo-first-order reaction kinetics of the polymer-supported metallic nanocatalysts for the model reaction of p-nitrophenol (p-NP)/NaBH4 were probably oversimplified. Here a detailed study of p-NP reduction by NaBH4 in the presence of the raspberry-like poly(allylamine hydrochloride)-modified polymer poly(glycidyl methacrylate) composite sub-microspheres with tunable gold nanoparticles (PGMA@PAH@AuNPs) was presented. Effects of polyelectrolyte concentration, the ratio of polymer spheres to gold nanoparticles, and the solution pH value for composite synthesis on the induction period, reaction time, average reaction rate and average turnover frequency were systematically investigated. Experimental results in all cases of our study revealed an nth order (n > 1) of the p-NP/NaBH4 catalytic reaction by the prepared polymer composite particles. The apparent order of reaction, n, is dependent on the total surface area of the coated gold nanoparticles on the polymer spheres, which can be closely correlated with the tunable gold nanoparticle surface coverage. The mechanism of the observed catalytic activity enhancement was proposed based on active epoxy groups of the polymer spheres and a large adsorption of p-nitrophenolate anions onto the positively-charged spheres.
158 citations
TL;DR: By analyzing the obtained profiles from the cyclic voltammetry, PXRD and XPS, it is demonstrated that the Cr (VI) is reduced to insoluble Cr (III) compound in the reaction.
157 citations
TL;DR: A series of Anderson-type Q 4 NiMo 6− x W x O 24 H 6 (x ǫ = 0, 2, 4, 6) catalysts were synthesized and employed in ionic liquids (ILs) extraction coupled with catalytic oxidation desulfurization systems (ECODS) for removal of benzothiophene (BT), dibenzothiophen (DBT) and their derivates in a model diesel and an actual commercial diesel as mentioned in this paper.
Abstract: A series of Anderson-type Q 4 NiMo 6− x W x O 24 H 6 ( x = 0, 2, 4, 6) catalysts were synthesized and employed in ionic liquids (ILs) extraction coupled with catalytic oxidation desulfurization systems (ECODS) for removal of benzothiophene (BT), dibenzothiophene (DBT) and their derivates in a model diesel and an actual commercial diesel. The catalyst [(C 4 H 9 ) 4 N] 4 NiMo 6 O 24 H 6 in ECODS system, containing H 2 O 2 and [Bmim]PF 6 , exhibited so high catalytic activity that DBT removal can reach 98% at 30 °C in 3 h. The catalytic activity of the catalysts depends on the polyoxometalate anion and cation. The DBT removal gradually increases with the increasing of the molybdenum ions in the Anderson-type polyoxometalates. Quaternary ammonium cations can effectively promote the oxidative desulfurization reaction, which was confirmed by the presence of the mixed-valence molybdenum ions in the Anderson-type polyoxometalate catalyst. ILs was used as an extractant, however it also promoted the oxidative desulfurization reaction greatly. The reaction rate was so sensitive to the oxidant dosage that the removal DBT first increased and then decreased with the increasing of O/S molar ratio. The change is probably due to the introduction of excess water in the system. Sulfur removal selectivity for S-compounds followed the order of DBT > 4-MDBT > 4,6-DMDBT > BT > 5-MBT. The reactivity of these S-compounds is sensitive to the electron density on sulfur atoms and the steric hindrance of the substituted groups of S-compounds. The sulfur level of an actual commercial diesel can be decreased from 700 to about 30 ppm after desulfurization reaction. The desulfurization system for the actual commercial diesel can be recycled ten times with an unnoticeable decrease in activity.
TL;DR: The present paper investigates the various features of NaOH aqueous solution when applied as an absorbent to capture carbon dioxide emitted with relatively high concentration in the flue gas.
TL;DR: In this paper, the authors report on the thermal behavior of a Reactor with direct heat transfer between the gaseous reactant and the solid material, and the impact of the most significant parameters such as the maximum possible enthalpy difference of the heat transfer fluid between inlet and outlet, heat transfer, the particle reaction rate and the mass transport is derived.
Abstract: Heat storage technologies are used to improve energy efficiency of power plants and recovery of process heat. Storing thermal energy by reversible thermo-chemical reactions offers a promising option for high storage capacities especially at high temperatures. Due to its low material cost the use of the reversible reaction Ca(OH)2 ⇌ CaO + H2O has been suggested. This paper reports on the thermal behavior of a reactor with direct heat transfer between the gaseous reactant and the solid material. Cycling stability is confirmed and the impact of the most significant parameters such as the maximum possible enthalpy difference of the heat transfer fluid between inlet and outlet, the heat transfer, the particle reaction rate and the mass transport is derived. In the test system the particle reaction rate could be identified as the main limiting parameter.
01 Feb 2013-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this paper, the authors determined comprehensive kinetics of the flash reduction reaction of magnetite concentrate particles by hydrogen and found that the reduction kinetics had a 1/2-order dependence on hydrogen partial pressure and activation energy of 463 kJ/mol.
Abstract: A novel ironmaking technology is under development at the University of Utah. The purpose of this research was to determine comprehensive kinetics of the flash reduction reaction of magnetite concentrate particles by hydrogen. Experiments were carried out in the temperature range of 1423 K to 1673 K (1150 °C to 1400 °C) with the other experimental variables being hydrogen partial pressure and particle size. The nucleation and growth kinetics expression was found to describe the reduction rate of fine concentrate particles and the reduction kinetics had a 1/2-order dependence on hydrogen partial pressure and an activation energy of 463 kJ/mol. Unexpectedly, large concentrate particles reacted faster at 1423 K and 1473 K (1150 °C and 1200 °C), but the effect of particle size was negligible when the reduction temperature was above 1573 K (1300 °C). A complete reaction rate expression incorporating all these factors was formulated.
TL;DR: In this paper, the authors investigated the production of biodiesel from non-edible Ceiba Pentandra oil by two-step acid base transesterification and found that the conversion of triglycerides into methyl esters obeys the first-order mechanism.
TL;DR: In this article, a thermogravimetric analysis of the reduction of hematite (Fe2O3) in a continuous stream of CH4 (15, 20, and 35%) was conducted at temperatures ranging from 700 to 825°C over ten reduction cycles.
TL;DR: In this paper, well-crystalline anatase TiO 2 nanoparticles were prepared by facile solution process and characterized in terms of their morphological, structural, optical and photocatalytic properties.
TL;DR: Pine bark biochar generated by slow pyrolysis (950°C) was used as a low cost catalyst to decompose toluene (model tar compound) over a temperature range of 600-900°C.
TL;DR: The kinetics and mechanism of glycerol photo-oxidation and photo-reforming reactions have been investigated over irradiated TiO2 and Pt/TiO2 suspensions as mentioned in this paper.
TL;DR: Experiments suggest that TBBPA can be effectively transformed by the naturally occurring laccase enzyme from Trametes versicolor, and data from acute immobilization tests with Daphnia confirm that T BBPA toxicity is effectively eliminated by lAccase-catalyzed TBB PA removal.
Abstract: The widespread occurrence of the brominated flame retardant tetrabromobisphenol A (TBBPA) makes it a possible source of concern. Our experiments suggest that TBBPA can be effectively transformed by the naturally occurring laccase enzyme from Trametes versicolor. These reactions follow second-order kinetics, whereby apparent removal rate is a function of both substrate and enzyme concentrations. For reactions at different initial concentrations and with or without natural organic matter (NOM), reaction products are identified using liquid or gas chromatography with mass spectrometry. Detailed reaction pathways are proposed. It is postulated that two TBBPA radicals resulting from a laccase-mediated reaction are coupled together via interaction of an oxygen atom on one radical and a propyl-substituted aromatic carbon atom on the other. A 2,6-dibromo-4-isopropylphenol carbocation is then eliminated from the radical dimer. All but one of the detected products arise from either substitution or proton elimination of the 2,6-dibromo-4-isopropylphenol carbocation. Three additional products are identified for reactions in the presence of NOM, which suggests that reaction occurs between NOM and TBBPA radical. Data from acute immobilization tests with Daphnia confirm that TBBPA toxicity is effectively eliminated by laccase-catalyzed TBBPA removal. These findings are useful for understanding laccase-mediated TBBPA reactions and could eventually lead to development of novel methods to control TBBPA contamination.
TL;DR: The high reaction selectivity of vinyl sulfones over acrylates via thiol–Michael addition reaction in ternary systems is used to control gelation behavior in crosslinked polymer networks formed by thiol-Michael addition reactions.
TL;DR: In this paper, the cycloaddition of propylene oxide (PO) and CO2 to give propylene carbonate (PC) is studied in a microreactor using a HETBAB ionic liquid catalyst.
TL;DR: In this article, the isothermal differential characteristics of the gas-solid reaction occurring in a micro-fluidized bed reactor were studied using the indigenously developed Micro-Fluidized Bed Reaction Analyzer (MFBRA).
TL;DR: In this paper, the activation energy of hydrogen absorption for the catalyzed Mg was evaluated to be 38kJ/mol, which was significantly smaller than that of MgH2 without the catalyst.
TL;DR: In this paper, the second-order rate constant k1 was obtained using two-film model using zwitterionic mechanism and the overall reaction rate for CO2 absorption into promoted K2CO3 were described as the following equation: - r CO 2 = 2.58 × 10 16 exp - 8645 T C Arg + 4.32 × 10 13 exp - 6666 T C OH - C CO 2.
TL;DR: In this paper, a concave Cu2O microcrystals mainly enclosed by {hhl} high-index facets have been successfully prepared by reducing Cu(CH3COO)2 with glucose in the presence of sodium dodecyl sulphate (SDS), and the concave degree of truncated octahedra can be controlled by adjusting the concentration of SDS.
Abstract: Due to the fact that crystal facets with high surface energy usually exhibit superior performance in many fields, such as catalysis, the importance of the synthesis of micro/nano-crystals with exposed high surface energy facets is becoming a hot research field. In this article, concave Cu2O microcrystals mainly enclosed by {hhl} high-index facets have been successfully prepared by reducing Cu(CH3COO)2 with glucose in the presence of sodium dodecyl sulphate (SDS). SDS was proved to be important in the formation of the concave Cu2O microcrystals. The concave degree of truncated octahedra can be controlled by adjusting the concentration of SDS. In addition, we found the reaction rate also affected the morphology of Cu2O microcrystals. Octahedron-based branched particles, truncated concave octahedra and truncated octahedra can be obtained by adjusting the concentration of glucose. In the catalytic oxidation of CO, truncated concave octahedral Cu2O enclosed by {332} high-index facets exhibited the highest catalytic activity among the high-index {332} facets, low index {111} and {100} facets, due to the existence of high density steps on {332} facets and the CO catalytic activities of the crystal facets are in the sequence: {332} > {111} > {100}.
TL;DR: High reaction rates and tunable capacity make ILs, and AHA ILs in particular, attractive solvents for CO2 separations.
Abstract: The reaction kinetics between CO2 and trihexyl(tetradecyl)phosphonium ([P66614])-based ionic liquids (ILs) with prolinate ([Pro]), 2-cyanopyrrolide ([2-CNpyr]), and 3-(trifluoromethyl)pyrazolide ([3-CF3pyra]) anions are studied at temperatures from 22-60 °C. The absorption of CO2 is carried out in a stirred reactor under pseudo first order conditions. ILs are diluted to concentrations of 0.05, 0.1 and 0.15 M with tetraglyme--a nonreactive, low volatility solvent with much lower viscosity than the ILs. Physical solubility of CO2 in the mixtures is calculated using correlations developed from CO2 solubility measurements in tetraglyme and the N2O-analogy for ILs and dilute IL solutions. The diffusivity of CO2 is estimated from viscosity-dependent correlations chosen after a thorough literature review. The results indicate partial first order reaction kinetics with respect to IL with values ranging from 19,500 L mol(-1) s(-1) ([P66614][Pro]) to 3200 L mol(-1) s(-1) ([P66614][3-CF3pyra]) at 22 °C. The second order reaction rate constants follow Arrhenius behavior with the highest activation energy of 43 kJ mol(-1) measured for [P66614][Pro]. ILs with aprotic heterocylic anions (AHA), on the other hand, show small activation energies of 18 and 11 kJ mol(-1) for [P66614][3-CF3pyra] and [P66614][2-CNpyr], respectively. The ILs studied in this work exhibit reactivity comparable to or higher than common aqueous amines. High reaction rates and tunable capacity make ILs, and AHA ILs in particular, attractive solvents for CO2 separations.
TL;DR: In this article, iodo-BODIPYs were used as visible-light-driven photocatalysts for the photocatalysis of thioanisole under visible light.
Abstract: Boron-dipyrromethene (BODIPY) compounds have been used extensively. However, their application in photocatalysis has not been well-studied. In this report, iodo-BODIPYs were utilized as visible-light-driven photocatalysts. They demonstrated very high catalytic efficiencies and reaction rates for the photocatalyzed oxidation of thioanisole under visible light.
TL;DR: In this article, a simplified reaction pathway for gases production from supercritical water gasification of glycerol was proposed, and a quantitative kinetics model for describing the gaseous products (H 2, CO, CH 4 and CO 2 ) was developed.
Abstract: In this paper, the liquid products from supercritical water gasification (SCWG) of glycerol were analyzed and some intermediates were identified. A simplified reaction pathway for gases production from SCWG of glycerol was proposed. The first quantitative kinetics model for describing the gaseous products (H 2 , CO, CH 4 and CO 2 ) of SCWG of glycerol was developed. The model comprises seven reactions to describe the typical reactions in SCWG, and the reaction rate constant of each reaction was obtained by using the nonlinear least-square fitting method. The reaction rate analysis showed that the main sources of hydrogen yield were glycerol pyrolysis and steam reforming of intermediates, while the hydrogen yield from water–gas shift reaction (WGSR) was very small. The temperature estimated by the kinetics model for completely SCWG of glycerol solution was given. In addition, the sensitivity analysis of rate constant of WGSR was done based on the model.
TL;DR: In this paper, the effect of milling time on structure of pure Al powder and consequently on efficiency of Al-water reaction and hydrogen production is studied considering changes in morphology, size, lattice imperfections and grains orientation of Al particles.