Showing papers on "Reaction rate constant published in 2017"

Selective Catalytic Reduction over Cu/SSZ-13: Linking Homo- and Heterogeneous Catalysis.

Abstract: Active centers in Cu/SSZ-13 selective catalytic reduction (SCR) catalysts have been recently identified as isolated Cu2+ and [CuII(OH)]+ ions. A redox reaction mechanism has also been established, where Cu ions cycle between CuI and CuII oxidation states during SCR reaction. While the mechanism for the reduction half-cycle (CuII → CuI) is reasonably well-understood, that for the oxidation half-cycle (CuI → CuII) remains an unsettled debate. Herein we report detailed reaction kinetics on low-temperature standard NH3-SCR, supplemented by DFT calculations, as strong evidence that the low-temperature oxidation half-cycle occurs with the participation of two isolated CuI ions via formation of a transient [CuI(NH3)2]+–O2–[CuI(NH3)2]+ intermediate. The feasibility of this reaction mechanism is confirmed from DFT calculations, and the simulated energy barrier and rate constants are consistent with experimental findings. Significantly, the low-temperature standard SCR mechanism proposed here provides full consiste...

Oxygen Vacancy Promoted Heterogeneous Fenton-like Degradation of Ofloxacin at pH 3.2-9.0 by Cu Substituted Magnetic Fe3O4@FeOOH Nanocomposite

Abstract: To develop an ultraefficient and reusable heterogeneous Fenton-like catalyst at a wide working pH range is a great challenge for its application in practical water treatment We report an oxygen vacancy promoted heterogeneous Fenton-like reaction mechanism and an unprecedented ofloxacin (OFX) degradation efficiency of Cu doped Fe3O4@FeOOH magnetic nanocomposite Without the aid of external energy, OFX was always completely removed within 30 min at pH 32–90 Compared with Fe3O4@FeOOH, the pseudo-first-order reaction constant was enhanced by 10 times due to Cu substitution (904/h vs 094/h) Based on the X-ray photoelectron spectroscopy (XPS), Raman analysis, and the investigation of H2O2 decomposition, •OH generation, pH effect on OFX removal and H2O2 utilization efficiency, the new formed oxygen vacancy from in situ Fe substitution by Cu rather than promoted Fe3+/Fe2+ cycle was responsible for the ultraefficiency of Cu doped Fe3O4@FeOOH at neutral and even alkaline pHs Moreover, the catalyst had an ex

Aqueous singlet oxygen reaction kinetics of furfuryl alcohol: Effect of temperature, pH, and salt content

Abstract: The rate constant for the reaction between furfuryl alcohol (FFA) and singlet oxygen (1O2) in aqueous solution was measured as a function of temperature, pH and salt content employing both steady-state photolysis (β value determination) and time-resolved singlet oxygen phosphorescence methods. The latter provided more precise and reproducible data. The reaction rate constant, krxn,FFA, had a relatively small temperature dependence, no pH dependence and showed a small increase in the presence of high salt concentrations (+19% with 1 M NaCl). A critical review of the available literature suggested that the widely used value of 1.2 × 108 M−1 s−1 is likely overestimated. Therefore, we recommend the use of 1.00 × 108 M−1 s−1 for reactions performed in low ionic strength aqueous solutions (freshwater) at 22 °C. Furthermore, corrections are provided that should be applied when working at higher or lower temperatures, and/or at high salt concentrations (seawater).

A New Michael-Reaction-Resistant Benzoquinone for Aqueous Organic Redox Flow Batteries

Abstract: We report here the synthesis, characterization and properties of 3,6-dihydroxy-2,4-dimethylbenzenesulfonic acid (DHDMBS) as a new positive side electrolyte material for aqueous organic redox flow batteries (ORBAT). We have synthesized this material in pure form in high yield and confirmed its structure. We have determined that the standard reduction potential, the rate constant of the redox reaction, and the diffusion coefficient are ideally suited for use in ORBAT. Specifically, DHDMBS overcomes the major issue of Michael reaction with water faced with 4,5-dihydroxybenzene-1,3-disulfonic acid (BQDS) and similar unsubstituted benzoquinones in the selection of positive electrolyte materials. DHDMBS can be synthesized relatively inexpensively. We have demonstrated the chemical stability of DHDMBS to repeated electrochemical cycling through NMR and electrochemical studies proving the absence of products of the Michael reaction. A flow cell with DHDMBS and anthraquinone-2,7-disulfonic acid has now been shown to operate close to 100% coulombic efficiency for over 25 cycles when continuously cycled at 100 mA/cm2, and can sustain current densities as high as 500 mA/cm2 without noticeable chemical degradation. However, there was a slow decrease in the capacity of the flow cell attributable to the crossover of DHDMBS from the positive side of the cell. Thus, the present study has shown DHDMBS as a promising candidate for the positive side material for an all-organic aqueous redox flow battery in acidic media, and our future efforts will focus on understanding the crossover of DHDMBS and the effects of long-term cycling.

Insights into the Kinetics and Reaction Network of Aluminum Chloride-Catalyzed Conversion of Glucose in NaCl–H2O/THF Biphasic System

Abstract: We performed a systematic experimental kinetics study on AlCl3-catalyzed conversion of glucose to 5-hydroxymethylfurfural (HMF) in NaCl–H2O/tetrahydrofuran (THF) biphasic solvent. The kinetics model covers an extensive reaction network including the parallel and tandem reactions of isomerization, dehydration, decomposition, and polymerization from glucose. The accuracy of the model was verified by a parity plot and statistical significance analysis of the kinetic parameters. A deliberate insight into the intrinsic kinetic properties (reaction rate constant and apparent activation energy) of each subreaction elaborates the regulatory role of THF and NaCl on reaction pathways within the network. That is, THF suppresses the rehydration, degradation, and polymerization of HMF to unwanted byproducts, inhibits fructose-to-HMF dehydration and fructose-to-humins polymerization, but promotes the generation of formic acid (FA) from the direct degradation of both glucose and fructose by facilitating the generation o...

Kinetic Analysis of the Reduction of 4-Nitrophenol Catalyzed by CeO2 Nanorods-Supported CuNi Nanoparticles

Abstract: CuxNi100–x (x = 0, 20, 40, 60, 80, and 100) nanoparticles were uniformly grown on the surface of CeO2 by the liquid impregnation method. The as-prepared nanocomposite abbreviated CuxNi100–x–CeO2 was characterized by various techniques including, X-ray powder diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, Brunauer–Emmett–Teller surface area analyzer, and transmission electron microscopy. The catalytic activity of CuxNi100–x–CeO2 nanocomposites was investigated in 4-nitrophenol (4-NP) reduction reaction. Among the synthesized nanocomposites, Cu60Ni40–CeO2 exhibited the best catalytic activity (rate constant as 0.1654 s–1) with high recyclability for five consecutive runs. The mechanism of the reduction was studied, and the adsorption equilibrium constant of 4-NP (K4-NP) and borohydride (KBH4–) was calculated by using the Longmuir–Hinshelwood model. The energy of activation (Ea) and thermodynamic parameters such as activation enthalpy (ΔH⧧), entropy (ΔS⧧), and...

Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes

Abstract: The kinetics of photoelectrochemical (PEC) oxidation of methanol, as a model organic substrate, on α-Fe2O3 photoanodes are studied using photoinduced absorption spectroscopy and transient photocurrent measurements. Methanol is oxidized on α-Fe2O3 to formaldehyde with near unity Faradaic efficiency. A rate law analysis under quasi-steady-state conditions of PEC methanol oxidation indicates that rate of reaction is second order in the density of surface holes on hematite and independent of the applied potential. Analogous data on anatase TiO2 photoanodes indicate similar second-order kinetics for methanol oxidation with a second-order rate constant 2 orders of magnitude higher than that on α-Fe2O3. Kinetic isotope effect studies determine that the rate constant for methanol oxidation on α-Fe2O3 is retarded ∼20-fold by H/D substitution. Employing these data, we propose a mechanism for methanol oxidation under 1 sun irradiation on these metal oxide surfaces and discuss the implications for the efficient PEC m...

Catalytic reduction of 4‐nitrophenol using silver nanoparticles‐engineered poly(N‐isopropylacrylamide‐co‐acrylamide) hybrid microgels

Abstract: Nearly monodisperse poly(N-isopropylacrylamide-co-acrylamide) [P(NIPAM-co-AAm)] microgels were synthesized using precipitation polymerization in aqueous medium. These microgels were used as microreactors to fabricate silver nanoparticles by chemical reduction of silver ions inside the polymer network. The pure and hybrid microgels were characterized using Fourier transform infrared and UV–visible spectroscopies, dynamic light scattering, X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry and transmission electron microscopy. Results revealed that spherical silver nanoparticles having diameter of 10–20 nm were successfully fabricated in the poly(N-isopropylacrylamide-co-acrylamide) microgels with hydrodynamic diameter of 250 ± 50 nm. The uniformly loaded silver nanoparticles were found to be stable for long time due to donor–acceptor interaction between amide groups of polymer network and silver nanoparticles. Catalytic activity of the hybrid system was tested by choosing the catalytic reduction of 4-nitrophenol as a model reaction under various conditions of catalyst dose and concentration of NaBH4 at room temperature in aqueous medium to explore the catalytic process. The progress of the reaction was monitored using UV–visible spectrophotometry. The pseudo first-order kinetic model was employed to evaluate the apparent rate constant of the reaction. It was found that the apparent rate constant increased with increasing catalyst dose due to an increase of surface area as a result of an increase in the number of nanoparticles.

Synthesis of magnetic core–shell Fe3O4@TiO2 nanoparticles from electric arc furnace dust for photocatalytic degradation of steel mill wastewater

Abstract: The study was undertaken to design magnetic core–shell Fe3O4@TiO2 nanoparticles from electric arc furnace dust and evaluate its photocatalytic activity on organic pollutant degradation from steel industry wastewater. Different molar ratios of Fe3O4 to TiO2 were tested on Fe3O4@TiO2 nanoparticles. The materials were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform-infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy, and a zeta potential analyzer and vibrating sample magnetometer. The behavior of Fe3O4@TiO2 nanoparticles under different molar ratios of Fe3O4 to TiO2, pH, photocatalyst dose and temperature was investigated. The apparent rate constant of organic pollutant degradation using Fe3O4@TiO2 was found to be pH dependent as it influenced the surface properties and therefore the photocatalytic activity of Fe3O4@TiO2, which was higher under acidic condition. The degradation of organic pollutants was as high as 96% at pH 3, 1 g L−1 photocatalyst dose, 30 °C temperature, after 90 min reaction time, and the apparent rate constant was 0.043 min−1. The thermodynamic parameters of activation, estimated by the Eyring equation and based on transition state theory (TST), indicated a nonspontaneous process in nature with positive Δ‡Go values, an endothermic reaction with positive Δ‡Ho and negative Δ‡So values. High degradation rate and catalyst recovery were maintained after five consecutive recycling cycles.

Synergistic effects of plasma–catalyst interactions for CH4 activation

Abstract: The elucidation of catalyst surface–plasma interactions is a challenging endeavor and therefore requires thorough and rigorous assessment of the reaction dynamics on the catalyst in the plasma environment. The first step in quantifying and defining catalyst–plasma interactions is a detailed kinetic study that can be used to verify appropriate reaction conditions for comparison and to discover any unexpected behavior of plasma-assisted reactions that might prevent direct comparison. In this paper, we provide a kinetic evaluation of CH4 activation in a dielectric barrier discharge plasma in order to quantify plasma–catalyst interactions via kinetic parameters. The dry reforming of CH4 with CO2 was studied as a model reaction using Ni supported on γ-Al2O3 at temperatures of 790–890 K under atmospheric pressure, where the partial pressures of CH4 (or CO2) were varied over a range of ≤25.3 kPa. Reaction performance was monitored by varying gas hourly space velocity, plasma power, bulk gas temperature, and reactant concentration. After correcting for gas-phase plasma reactions, a linear relationship was observed in the log of the measured rate constant with respect to reciprocal power (1/power). Although thermal catalysis displays typical Arrhenius behavior for this reaction, plasma-assisted catalysis occurs from a complex mixture of sources and shows non-Arrhenius behavior. However, an energy barrier was obtained from the relationship between the reaction rate constant and input power to exhibit ≤∼20 kJ mol−1 (compared to ∼70 kJ mol−1 for thermal catalysis). Of additional importance, the energy barriers measured during plasma-assisted catalysis were relatively consistent with respect to variations in total flow rates, types of diluent, or bulk reaction temperature. These experimental results suggest that plasma-generated vibrationally-excited CH4 favorably interacts with Ni sites at elevated temperatures, which helps reduce the energy barrier required to activate CH4 and enhance CH4 reforming rates.

Enhancing the catalytic activity of hydronium ions through constrained environments.

Abstract: The dehydration of alcohols is involved in many organic conversions but has to overcome high free-energy barriers in water. Here we demonstrate that hydronium ions confined in the nanopores of zeolite HBEA catalyse aqueous phase dehydration of cyclohexanol at a rate significantly higher than hydronium ions in water. This rate enhancement is not related to a shift in mechanism; for both cases, the dehydration of cyclohexanol occurs via an E1 mechanism with the cleavage of Cβ-H bond being rate determining. The higher activity of hydronium ions in zeolites is caused by the enhanced association between the hydronium ion and the alcohol, as well as a higher intrinsic rate constant in the constrained environments compared with water. The higher rate constant is caused by a greater entropy of activation rather than a lower enthalpy of activation. These insights should allow us to understand and predict similar processes in confined spaces.

Reaction Parameters Influencing Cobalt Hydride Formation Kinetics: Implications for Benchmarking H2-Evolution Catalysts

Abstract: The need for benchmarking hydrogen evolution catalysts has increasingly been recognized. The influence of acid choice on activity is often reduced to the overpotential for catalysis. Through the study of a stable cobalt hydride complex, we demonstrate the influence of acid choice, beyond pKa, on the kinetics of hydride formation. A linear free energy relationship between acid pKa and second-order rate constants is observed for weaker acids. For stronger acids, however, further increases in pKa do not correlate to increases in rate constants. Further, steric bulk around the acidic proton is shown to influence rate constants dramatically. Together, these observations reveal the complex factors dictating catalyst performance.

A MoO3–Metal–Organic Framework Composite as a Simultaneous Photocatalyst and Catalyst in the PODS Process of Light Oil

Abstract: Photo-oxidative desulfurization (PODS) properties of MoO3–metal–organic framework composite photocatalysts were investigated by introducing the proper weight percent of MoO3 into a Zn(II)-based MOF, [Zn(oba)(4-bpdh)0.5]n·1.5DMF (TMU-5), for the mineralization of dibenzothiophene from model oil. The addition of 3 wt % of MoO3 into a TMU-5 host acting as a crystal growth inhibitor was confirmed by PXRD and BET results. For the first time, under mild and green reaction conditions, 5 wt % MoO3–TMU-5 composite (MT-5) exhibited good photocatalytic activity in the model oil PODS reaction, which has no limitations in the current oxidative desulfurization catalytic systems. Only 3% of the total amount of MoO3 content in the MT catalyst is leached during the reaction. In addition, the rate of PODS of MT-5 obeys a pseudo-first-order equation with an apparent rate constant of 0.0305 min–1 and half-life t1/2 of 22.7 min. Radical scavenger experiments and terephthalic acid fluorescence techniques confirmed that OH• and...

A Novel Magnetically Recoverable Ni-CeO2-x/Pd Nanocatalyst with Superior Catalytic Performance for Hydrogenation of Styrene and 4-Nitrophenol.

Abstract: Metal/support nanocatalysts consisting of various metals and metal oxides not only retain the basic properties of each component but also exhibit higher catalytic activity due to their synergistic effects. Herein, we report the creation of a highly efficient, long-lasting, and magnetic recyclable catalyst, composed of magnetic nickel (Ni) nanoparticles (NPs), active Pd NPs, and oxygen-deficient CeO2–x support. These hybrid nanostructures composed of oxygen deficient CeO2–x and active metal nanoparticles could effectively facilitate diffusion of reactant molecules and active site exposure that can dramatically accelerate the reaction rate. Impressively, the rate constant k and k/m of 4-nitrophenol reduction over 61 wt % Ni-CeO2–x/0.1 wt % Pd catalyst are 0.0479 s–1 and 2.1 × 104 min–1 g–1, respectively, and the reaction conversion shows negligible decline even after 20 cycles. Meanwhile, the optimal 61 wt % Ni-CeO2–x/3 wt % Pd catalyst manifests remarkable catalytic activity toward styrene hydrogenation wi...

Chlorination of Amino Acids: Reaction Pathways and Reaction Rates

Abstract: Chlorination of amino acids can result in the formation of organic monochloramines or organic dichloramines, depending on the chlorine to amino acid ratio (Cl:AA). After formation, organic chloramines degrade into aldehydes, nitriles and N-chloraldimines. In this paper, the formation of organic chloramines from chlorination of lysine, tyrosine and valine were investigated. Chlorination of tyrosine and lysine demonstrated that the presence of a reactive secondary group can increase the Cl:AA ratio required for the formation of N,N-dichloramines, and potentially alter the reaction pathways between chlorine and amino acids, resulting in the formation of unexpected byproducts. In a detailed investigation, we report rate constants for all reactions in the chlorination of valine, for the first time, using experimental results and modeling. At Cl:AA = 2.8, the chlorine was found to first react quickly with valine (5.4 × 104 M-1 s-1) to form N-monochlorovaline, with a slower subsequent reaction with N-monochlorovaline to form N,N-dichlorovaline (4.9 × 102 M-1 s-1), although some N-monochlorovaline degraded into isobutyraldehyde (1.0 × 10-4 s-1). The N,N-dichlorovaline then competitively degraded into isobutyronitrile (1.3 × 10-4 s-1) and N-chloroisobutyraldimine (1.2 × 10-4 s-1). In conventional drinking water disinfection, N-chloroisobutyraldimine can potentially be formed in concentrations higher than its odor threshold concentration, resulting in aesthetic challenges and an unknown health risk.

Synthesis and characterization of Bi4Si3O12, Bi2SiO5, and Bi12SiO20 by controlled hydrothermal method and their photocatalytic activity

Abstract: This is the first report that three compounds, Bi4Si3O12, Bi2SiO5, and Bi12SiO20, can be selectively synthesized by a controlled and template-free hydrothermal method. Crystalline Bi4Si3O12, Bi2SiO5, and Bi12SiO20 are prepared from the reaction of Na2SiO3 and Bi(NO3)3 • 5H2O in an alkaline aqueous solution at 150 °C–250 °C for 24 h, whereas conventional syntheses necessitate operational temperatures above 650 °C. The composition and morphology of bismuth silicates could be controlled by adjusting some growth parameters, including reaction pH and temperature. The products are characterized by X-ray diffraction scanning electron microscope–energy–dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, Diffuse Reflectance UV–visible spectroscopy, Brunauer–Emmett–Teller, and high-resolution X-ray photoelectron spectroscopy. The UV–Vis spectra demonstrate that the three materials are indirect semiconductors with optical bandgaps of 2.65, 3.30, and 3.44 eV for Bi12SiO20, Bi2SiO5, and Bi4Si3O12, respectively. The photocatalytic efficiency of the powder suspension is evaluated by measuring the crystal violet concentration under visible-light irradiation. The order of rate constants shows Bi2SiO5 > Bi4Si3O12 > Bi12SiO20.

Improving Ni Catalysts Using Electric Fields: A DFT and Experimental Study of the Methane Steam Reforming Reaction

Abstract: This work demonstrates the benefits of applying an external electric field to the methane steam reforming reaction (MSR) in order to tune the catalytic activity of Ni. Through combined DFT calculations and experimental work, we present evidence for the usefulness of an electric field in improving the efficiency of current MSR processes—namely by reducing coke formation and lowering the overall temperature requirements. We focus on the influence of an electric field on (i) the MSR mechanisms, (ii) the rate-limiting step of the most favorable MSR mechanism, (iii) the methanol synthesis reaction during the MSR reaction, and (iv) the formation of coke. Our computational results show that an electric field can change the most favorable MSR mechanism as well as alter the values of the rate constants and equilibrium constants at certain temperatures and, hence, significantly affect the kinetic properties of the overall MSR reaction. Both computational and experimental results also suggest that a positive electri...

Perfluoroaryl Azide Staudinger Reaction: A Fast and Bioorthogonal Reaction

Abstract: We report a fast Staudinger reaction between perfluoroaryl azides (PFAAs) and aryl phosphines, which occurs readily under ambient conditions. A rate constant as high as 18 m−1 s−1 was obtained between methyl 4-azido-2,3,5,6-tetrafluorobenzoate and methyl 2-(diphenylphosphanyl)benzoate in CD3CN/D2O. Furthermore, the iminophosphorane product was stable toward hydrolysis and aza-phosphonium ylide reactions. This PFAA Staudinger reaction proved to be an excellent bioothorgonal reaction. PFAA-derivatized mannosamine and galactosamine were successfully transformed into cell-surface glycans and efficiently labeled with phosphine-derivatized fluorophore-conjugated bovine serum albumin.

Isostructural metal-carboxylates MIL-100(M) and MIL-53(M) (M: V, Al, Fe and Cr) as catalysts for condensation of glycerol with acetone

Abstract: The synthesis of solketal from acetone and glycerol (I) was investigated in the presence of the isostructural MOFs of the families MIL-100(M) and MIL-53(M) (M = V, Al, Fe and Cr) and mixed MIL-53(Al,V) (Al/V – 100/0, 75/25, 50/50, 25/75 and 0/100 atom/atom). The main products were a five-membered solketal (2,2-dimethyl-1,3-dioxane-4-methanol, (II)) and a six-membered acetal (2,2-dimethyl-dioxane-5-ol, (III)). It was demonstrated that the reaction rate and isomer selectivity depend on different parameters such as the type of metal ion, the length of the M-O bond, the rate constant for the exchange of the water molecules from the first coordination sphere of a metal ion and the value of the zero point of the surface charge (pH PZC ). Investigation of mixed MIL-53(Al,V) shows that the reaction rate and selectivity towards (II) increase with increasing V 3+ content in MIL-53(Al,V). V-containing MOFs possess a high activity and selectivity at 25 °C. The efficiencies of MIL-100(V) and MIL-47(V) were higher than those of H 2 SO 4 , SnCl 2 and p -toluenesulfonic acid at 25 °C. The MIL-100(V) catalyst showed good reusability for 4 catalyst recycles.