Reaction Rate Constants For Reduction Research Articles

Plant Extract Induced Biogenic Preparation of Silver Nanoparticles and Their Potential as Catalyst for Degradation of Toxic Dyes

This study focusses on the synthesis of silver nanoparticles (Ag-nPs) by citrus fruit (Citrus paradisi) peel extract as reductant while using AgNO3 salt as source of silver ions. Successful preparation of biogenic CAg-nPs catalyst was confirmed by turning the colorless reaction mixture to light brown. The appearance of surface Plasmon resonance (SPR) band in UV-Vis spectra further assured the successful fabrication of nPs. Different techniques such as FTIR, TGA and DLS were adopted to characterize the CAg-nPs. CAg-nPs particles were found to excellent catalysts for reduction of Congo red (CR), methylene blue (MB), malachite green (MG), Rhodamine B (RhB) and 4-nitrophenol (4-NP). Reduction of CR was also performed by varying the contents of NaBH4, CR and catalyst to optimize the catalyst activity. The pseudo first order kinetic model was used to explore the value of rate constants for reduction reactions. Results also interpret that the catalytic reduction of dyes followed the Langmuir–Hinshelwood (LH) mechanism. According to the LH mechanism, the CAg-nPs role in catalysis was explained by way of electrons transfer from donor (NaBH4) to acceptor (dyes). Due to reusability and green synthesis of the CAg-nPs catalyst, it can be a promising candidate for the treatment of water sources contaminated with toxic dyes.

A rapid microwave-assisted synthesis of silver nanoparticles using Ziziphus jujuba Mill fruit extract and their catalytic and antimicrobial properties

An eco-friendly and sustainable approach was developed for the synthesis of silver nanoparticles (AgNPs) of Ziziphus jujuba Mill (ZJM) fruit extract by a microwave-assisted technique. The developed method was simple, economically viable, sustainable, and eco-friendly. Concentrations of AgNO3 and ZJM fruit extract and reaction time were optimized for the final properties of synthesized AgNPs. The prepared AgNPs were characterized by UV-visible and FTIR spectrophotometers, powder XRD, TEM, and DLS. XRD results showed a face-centered cubic structure for the prepared AgNPs. The obtained size of AgNPs was 8 ± 1 nm with spherical shape. The FTIR spectrum indicates that ZJM is a good capping agent to stabilize AgNPs. A negative zeta potentials value of − 21.2 mV indicates the stability of AgNPs. The AgNPs has good stability over a wide range of pH and ionic strength. The role and efficacy of AgNPs in the catalytic degradation of methylene blue (MB) and Congo red (CR) dye were studied, and the reduction reaction rate constants were found to be 0.033 s−1 (180 s) and 0.025 s−1 (200 s) respectively. The synthesized AgNPs showed rapid and excellent catalytic reduction of MB and CR dyes. The rapid catalytic reduction reactions followed pseudo-order kinetics. The ZJM fruit extract-capped AgNPs showed good antimicrobial activity against tested bacterial and fungal cultures.

Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e-/4H+ O2 Reduction: Activation of O-O Bond by 2nd Sphere Hydrogen Bonding.

Facile and selective 4e-/4H+ electrochemical reduction of O2 to H2O in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e-/4H+ electrochemical O2 reduction to H2O is possible with rate constants as fast as 105 M-1 s-1 under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of O-O bond cleavage determines the overall rate of O2 reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient O-O bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O2 to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O2 with rate constants >107 M-1 s-1 while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst.

Dynamic surface composition in a Mars-van Krevelen type reaction: CO oxidation on Au/TiO2

Aiming at a better mechanistic understanding of Mars-van Krevelen type catalytic reactions, we have systematically investigated possible correlations between the composition of the reaction gas atmosphere, the concentration of surface oxygen vacancies (surface oxidation state), and the catalytic activity of a Au/TiO2 catalyst in the CO oxidation reaction. In the meantime, there is considerable experimental and theoretical evidence that this reaction follows a Au-assisted Mars-van Krevelen mechanism at reaction temperatures ≥80 °C. Employing quantitative pulse experiments in a temporal analysis of products (TAP) reactor we found that both the surface oxidation state and the activity of the catalyst under steady-state conditions depend sensitively on the composition of the reaction gas atmosphere, specifically on the CO:O2 ratio, and that there is a strict correlation between these quantities. A simple kinetic model is introduced, which allows to quantitatively determine the ratio of the effective reaction rate constants for reduction and oxidation of the TiO2 support surface. These results and ideas are considered to be of general relevance for the understanding of Mars-van Krevelen type reactions and specifically of oxidation reactions on supported Au catalysts, and they lend further support to previous proposals of a Au-assisted Mars-van Krevelen mechanism for this reaction.

Green Synthesis and Catalytic Activity of Gold Nanoparticles Synthesized by Artemisia capillaris Water Extract.

Gold nanoparticles were synthesized using a water extract of Artemisia capillaris (AC-AuNPs) under different extract concentrations, and their catalytic activity was evaluated in a 4-nitrophenol reduction reaction in the presence of sodium borohydride. The AC-AuNPs showed violet or wine colors with characteristic surface plasmon resonance bands at 534~543 nm that were dependent on the extract concentration. Spherical nanoparticles with an average size of 16.88 ± 5.47~29.93 ± 9.80 nm were observed by transmission electron microscopy. A blue shift in the maximum surface plasmon resonance was observed with increasing extract concentration. The face-centered cubic structure of AC-AuNPs was confirmed by high-resolution X-ray diffraction analysis. Based on phytochemical screening and Fourier transform infrared spectra, flavonoids, phenolic compounds, and amino acids present in the extract contributed to the reduction of Au ions to AC-AuNPs. The average size of the AC-AuNPs decreased as the extract concentration during the synthesis was increased. Higher 4-nitrophenol reduction reaction rate constants were observed for smaller sizes. The extract in the AC-AuNPs was removed by centrifugation to investigate the effect of the extract in the reduction reaction. Interestingly, the removal of extracts greatly enhanced their catalytic activity by up to 50.4 %. The proposed experimental method, which uses simple centrifugation, can be applied to other metallic nanoparticles that are green synthesized with plant extracts to enhance their catalytic activity.

Ca–alginate-entrapped nanoscale iron: arsenic treatability and mechanism studies

The use of nanoscale zero-valent iron (NZVI, diameter 10–90 nm with an average value of 35 nm) entrapped in calcium (Ca)–alginate beads shows great promise for aqueous arsenic treatment. This research evaluated Ca–alginate-entrapped NZVI as an advanced treatment technique for aqueous arsenic removal. Arsenic is a serious threat to human health and millions of people are affected by arsenic contamination in various parts of the world including the USA. In bench scale batch studies with initial As(V) concentrations of 1–10 mg L−1, ~85–100 % arsenic removal was achieved within 2 h. While the reaction kinetics differ between bare and entrapped NZVI, the overall reductions of arsenic are comparable. Surface area-normalized arsenic reduction reaction rate constants (k sa) for bare and entrapped NZVI were 3.40–5.96 × 10−3 and 3.92–4.43 × 10−3 L m−2 min−1, respectively. The entrapped NZVI removed ~100 μg L−1 As(V) to below detection limit within 2 h and groundwater with 53 μg L−1 As(V) was remediated to below instrument detection limit (10 μg L−1) within 1 h. The presence of Na+, Ca2+, Cl−, and $${\rm HCO}_{3}^{ - }$$ did not affect arsenic removal by entrapped NZVI and there was no leaching of iron from the beads. X-ray diffraction and Fourier transform infrared spectroscopic techniques have been used to understand the mechanism of arsenic removal by the entrapped NZVI. Ca–alginate polymer is an excellent choice as an entrapment medium as it is non-toxic and has little solubility in water.

Reduction Kinetics of Iron Oxides in Molten Lunar Soil Simulant by Graphite

In space exploration including the utilization of materials and energy outside the earth, construction of a moon base is one of considerable measures to develop the front line. Due to the huge cost of transportation from the earth, structural materials such as iron should be in-situ produced on the moon. Fortunately, the Apollo 14 mission has reported that the lunar soil contains about 10 mass% of iron oxides. When considering ironmaking from lunar soils by graphite on the moon, understanding the reduction behaviour should come first but has not yet been attempted for iron resources whose concentration of iron oxides is as low as 10 mass%. Thus, the present work has focused on reduction kinetics of iron oxides in molten lunar soil simulant by graphite, and apparent rate constants for reduction reactions have been derived from experiments with systematic variation in reduction conditions for comprehensive discussion about the reaction mechanism. The apparent rate constant derived is 2.0×10−4 cm/s at 1803 K and the corresponding activation energy estimated is about 360 kJ/mol. The use of a graphite rod rotation mechanism enabled the stirring condition to be changed, resulting in a finding that stronger stirring shifts the rate-determining step from reduction of FeO by CO gas to direct reduction of FeO by graphite. On the basis of all these kinetic findings, it is likely that ironmaking from lunar soils by graphite on the moon is possible in that high vacuum and low gravity conditions would probably promote the removal of CO gas film from the graphite surface.

Photoreactions of Mercury in Surface Ocean Water: Gross Reaction Kinetics and Possible Pathways

We present pseudofirst order rate constants for gross photoreduction and gross photooxidation of mercury in surface water from the open Atlantic Ocean, determined under controlled laboratory conditions. Experiments using both unfiltered and filtered ocean water were carried out to characterize the importance of microbes and colloids on reaction kinetics. Results indicate that reduction and oxidation of mercury in ocean water does not follow a simple two-species reversible reaction pathway. We suggest two possible redox pathways that reproduce the pattern of dissolved gaseous mercury (DGM) concentrations observed in our laboratory experiments, and evaluate them using a controlled outdoor experiment. In both proposed pathways Hg(0), the major constituent of DGM, is converted to an unidentified oxidized species that is different from the reducible form present initially. This reaction step plays a major role in the net formation of DGM in our experiments. Our results represent new quantitative information about the gross reaction kinetics for both reduction and oxidation of mercury in open ocean surface water. Pseudofirst order rate constants for reduction reactions that form DGM were determined to be in the range of 0.15-0.93 h(-1) and pseudofirst order rate constants for oxidation of Hg(0) to be in the range of 0.4-1.9 h(-1). Microbes and colloids did not appreciably influence the reduction and oxidation kinetics.

Numerical Prediction of NOx Reduction Amount by NOx Storage-Reduction Catalysts under Sulphur Poisoning Condition (Application of Catalyst Model Considering Diffusion into Wash-Coat)

The catalyst model which considers diffusion phenomena in the wash-coat layer has been developed and applied to NOx storage reduction (NSR) catalysts under sulphur poisoning condition. In this model, the effective reaction rate, which can be measured by experiments, is expressed as the harmonic mean of the diffusion rate from bulk gas phase to wash-coat surface in the channel, the diffusion rate in the wash-coat layer which has a large number of pores and reaction rate on the noble metal surface. Analysis of numerical results of NOx reduction phesomena of NSR catalysts by means of this model leads to following conclusions. Sulphur poisoning level doesn't affect NOx reduction reaction rate constants on the noble metal surface. The effective NOx reduction rate is dominated by the diffusion rate in the wash-coat layer at temperatures approximately over 600 K.

Flow and Diffusion Analysis on the Kinetics of Reduction of Fused Silica in Hydrogen

The kinetics of the reduction of fused silica in hydrogen is studied from the viewpoint of the water vapor generation. The water vapor content in exhaust hydrogen gas is monitored at temperatures ranging from 700 to 1200°C. A model of the flow and diffusion of water vapor generated at the inner surface of a fused-silica tube in a hydrogen atmosphere is assumed for estimating the water vapor content. Then the reduction reaction rate constants are determined by the best fit to the measured water vapor concentrations at various temperatures in the range. Arrhenius plots of the rate constants give the activation energy to be approximately 85 kcal/mol, independent of gas flow rates and tube diameters, indicating that the reduction rate is reaction-limited. It is worth noting that this study demonstrates the validity of this numerical flow and diffusion analysis method on water vapor generation.

Electrolytic Reduction of Low Molecular Weight Chlorinated Aliphatic Compounds: Structural and Thermodynamic Effects on Process Kinetics

A series of chlorinated low molecular weight alkanes and alkenes was transformed electrolytically using a porous nickel cathode at potentials from −0.3 to −1.4 V (versus standard hydrogen electrode). Kinetics were first-order with respect to the concentration of the halogenated targets. The dependence of the first-order rate constants on cathode potential followed the Butler−Volmer equation, modified to account for mass transfer resistance to reaction. The mass-transfer-limited rate constant for reaction of all species was about 1.55 L m-2 min-1. Log-transformed reaction rate constants for reduction of chlorinated alkanes, derived via experiments at the same cathode potential (Ec = −1.0 or −1.2 V vs SHE), were linearly related to carbon−halogen bond enthalpies, as expected based on a physical model that was developed from transition state theory. The chlorinated ethenes reacted much faster than predicted from bond enthalpy calculations and the alkane-based correlation, suggesting that alkenes are not tran...

Pulse Radiolytic Reduction Study of Bovine Serum Albumin and Lysozyme in Quaternary Microemulsion

Reduction reactions of bovine serum albumin (BSA), lysozyme (Lz) and dimethyldisulfide (DMDS) with hydrated electron in quaternary micro emulsion were studied using pulse radiolysis. The reduction of proteins in this organized assembly is found to be dependent on w o values of the micro emulsions. At larger radii [r (A) ~ 1.5 w o ] of the water pools, reaction tends to become similar to that which occurs in bulk water, but is slower at smaller radii. This is quite evident from the reduction reaction rate constants, which vary from 5.5 X 10 7 dm 3 mol -1 s -1 to 3.94 X 10 9 dm 3 mol -1 s -1 for BSA, from 2.0 X 10 8 dm 3 mol -1 s -1 to 6.66 X 10 9 dm 3 mol -1 s -1 for Lz, and from 7.7 X 10 7 dm 3 mol -1 S -1 to 3.04 X 10 8 dm 3 mol -1 s -1 for DMDS with w o values from 16 to 60.

Significant contribution of arginine-112 and its positive charge of Pseudomonas putida cytochrome P-450 cam in the electron transport from putidaredoxin

Cytochrome P-450 cam of Pseudomonas putida is a prototype of various eukaryotic cytochrome P-450 molecules. Arg-112 located on the surface of this protein is highly conserved among various other cytochromes P-450. In this study, we constructed mutant genes for P-450 cam in which Arg-112 was replaced by Gln or Glu, expressed them in Escherichia coli and purified the mutant proteins. Their enzymic activities were analyzed in the reconstituted system to determine the function of Arg-ll2. K a values for d-camphor of Arg 112-Gln and Arg 112-Glu were much the same as those of the wild-type enzyme, whereas K d values for the oxidized form of putidaredoxin, which is an acidic protein and is the redox partner of P-450 cam, were 240 and 530 μM, respectively. These values are 8 and 19 times larger than that of the wild-type enzyme (28 μM), thereby indicating lower affinities of the mutant enzymes for the oxidized putidaredoxin. Reaction rate constants for reduction by the reduced form of putidaredoxin, measured using the stopped flow method, were 45.5, 9.0·10 −3 and 9.0·10 −4 s −1 for the wild type, Arg 112-Gln and Arg 112-Glu, respectively. Thus, Arg-ll2 of P-450 cam plays an important role in the interaction with putidaredoxin and in the high efficiency of the electron transfer; the positive charge of the residue seeming to contribute to the process. The yields in Escherichia coli, the heme contents in the purified fractions and heat stability of the mutant proteins were lower than those of the wild type enzyme, suggesting that Arg-ll2 of P-450 cam also important for stability of P-450 cam.