Molecular Simulation of NaF Chemisorption on Lepidocrocite for Water Purification

The large volume of water produced alongside crude oil during extraction is a significant challenge in the oil and gas industry. Since crude oil emulsions can have a pH ranging from 2 to 13, inhibitors effective in acidic, basic, and neutral media are essential for reducing the corrosion rate of transportation lines. This paper aims to elucidate the capability of corrosion inhibition of 1, 3-diphenyl prop-2-en-1-one, a chalcone on mild steel corrosion in near acidic, neutral, and near basic media using 1 N HCl, 3.5 % NaCl and 3.5 % NaCl with 0.1 N NaOH made up to pH 8.5. The inhibitive competence of the inhibitor was determined by gravimetric, potentiodynamic polarization, and electrochemical impedance spectroscopic studies. Nuclear magnetic resonance spectroscopy was used to characterize the carbon and hydrogen environment of the synthesized chalcones. The proposed inhibitor was found to give a good inhibition efficiency of 98.7 % in acidic media and a reasonable decrease in corrosion rate in basic and neutral media. As the concentration of the inhibitor increases, the efficiency of inhibition increases, whereas, the efficiency decreases as the temperature increases. Potentiodynamic polarization confirmed that the inhibitor is of the mixed type. Impedance analysis explains the presence of an inhibitor as an adsorbed layer by a change in resistance and inhibition efficiency. The adsorption mechanism of the inhibitor on the mild steel surface is found to obey Langmuir’s adsorption isotherm.

A detailed kinetic investigation of As(III) oxidation was performed on gold surface within pH between ∼3.0 and ∼9.0. It was found that the As(III) oxidation on the gold surface follows a purely adsorption-controlled process irrespective of pH. The evaluated adsorption equilibrium constant decreased from 3.21 × 105 to 1.61 × 105 mol L−1 for acidic to basic medium, which implies the strong affinity of the arsenic species in the acidic medium. Besides, the estimation of Gibbs free energy revealed that an acidic medium promotes arsenic oxidation on gold surface. In mechanistic aspect, the oxidation reaction adopts a stepwise pathway for acidic medium and a concerted pathway for neutral and basic medium. From the substantial kinetic evaluation, it is established that a conducive and compatible environment for the oxidation of arsenic was found in an acidic medium rather than a basic or neutral medium on gold surface. Besides, in sensitivity concern, neutral and highly acidic medium is quite favourable for the arsenite oxidation on gold surface.

Experimental kinetics of sulfide oxidation by hydrogen peroxide presents a pH-dependent profile. In this article, it was carried out a detailed study of the mechanism and kinetics of dimethyl sulfide (DMS) oxidation by H2O2 in neutral, acid, and basic aqueous medium using ab initio calculations. The results point out that DMS oxidation in neutral aqueous medium occurs through its direct reaction with H2O2. In acid medium, cluster-continuum model calculations shows that cluster is the best representation of the very reactive species. In basic medium, there is formation of the species. However, the pathway involving this species has high free energy barrier, making this pathway unfeasible. The theoretical pH-rate profile is in good agreement with the experimental observations. © 2013 Wiley Periodicals, Inc.

Introduction Fuel cells still command great attention even as the need for an alternative source of energy increases annually. Following the unison conclusion by various researchers on the use of platinum and precious group metals (PGM) catalysts, as an expensive venture, many researchers are still committed to alternative and effective catalysts for fuel cell application. First-row transitional metals (TM) have been proposed as effective alternatives for oxygen reduction reaction (ORR) applications due to their promising high ORR activity and durability besides their abundance and cheaper unit cost compared to PGM [1]. In the last decade in particular, many researchers have reported catalysts derived from carbon doped with iron (Fe) and nitrogen (N). Such catalysts have shown high ORR activity, especially under alkaline conditions. The formation of Fe-N4 moieties in the carbon lattice through the interaction between the doped heteroatoms has been reported as an effective active site during the ORR process [2,3]. However, most of these catalysts have only shown high activity under alkaline conditions while their activity in neutral and acidic mediums still lags hindering wide applications in fuel cells [4]. This negative trend has been attributed to the limited number of active sites and the ease of catalyst degradation in acidic or neutral medium hence low durability in these media. To address these challenges, an increase in the number and concentration of dopants has been considered and shown positive results towards increasing the activity. However, such ventures have a net effect on the sustainability of this technology. Herein an eco-friendly and sustainable approach is presented. In this approach, we harness waste rice husk biomass rich in silica as a source of carbon (C) and self-doped silica (Si) together with pyrite, an abundant byproduct of chalcopyrite smelting following the increased demand of copper for the electric vehicle (EV) industry, as a source of Fe. These materials are then synthesized through a chain of hydrothermal and template carbonization synthesis processes to yield a tri-doped catalyst which is referred to as Fe-N-Si-C in this work. The synthesized catalyst was characterized for both physiochemical properties using TEM, SEM, XAFS and DFT while the electrochemical properties were analyzed using linear sweep voltammetry, cyclic voltammetry, and durability in both alkaline, neutral, and acidic mediums. Results and discussion The template carbonization method adopted in the synthesis in this work, resulted in a porous carbon material with successfully doped heteroatoms as confirmed by the TEM and SEM analysis. A homogenous and evenly distributed of C, Fe, N and Si elements was further confirmed by the EDS mapping. The significance of this heteroatom doping that was achieved from wastes was exhibited in the Fourier transform EXAFS where the coexistence of Fe-N4 moieties and Fe clusters with surface oxides was confirmed by the presence of Fe-N and Fe-O peaks were detected on the catalyst spectrum. Similarly, a Si-K-edge X-ray absorption revealed the formation of an amorphous SiO2 peak on the Fe-N-Si-C catalyst. These Fe-N4 porphyrin-like moieties and the SiO2 provided effective active sites for ORR as demonstrated in the LSV analysis in all universal pH media, where Fe-N-Si-C showed an onset potential of 1.02V, 0.96V and 0.86V which was close to those of commercial PtC ;1.04V, 0.99V and 0.91V in ORR analysis conducted in 0.1M KOH (alkaline),0.1M PB (neutral) and 0.1M HClO4 (acidic) respectively as highlighted in Table 1. Motivated by superior ORR performance in alkaline conditions, a Zn-air battery (ZAB) was assembled with 6M KOH and 0.2M Zinc acetate. Fe-N-Si-C cell achieved a 103mW/cm2 power density, nearly 50% higher than what the PtC (78mW/cm2) cell achieved. This work not only showcases value addition to unused environmental waste resources considered pollutants but also introduces a cost-effective and environmentally friendly catalyst, serving as a substitute for precious and expensive metal-derived electrocatalysts.

Study on electrochemical oxidation of 4-Chlorophenol on a vitreous carbon electrode using cyclic voltammetry

The inhibitive effect of 2-aminothiazole on corrosion of Zn-Al-Cu alloy was investigated in acid, neutral and alkaline media by impedance and polarization techniques. The impedance data were fitted to a simple equivalent circuit model and indicated control by a diffusion process. Satisfactory inhibition was obtained in alkaline and neutral media. On the other hand, quite low inhibition efficiency was observed in acid medium. Increase in concentration of the inhibitor increased the inhibitive effect, whereas the reverse was observed when the temperature was increased. Different adsorption isotherms were tested in neutral and alkaline media, the best fit was obtained for the Freundlich isotherm. The highest value for the free energy change was recorded in alkaline medium in accordance with the high inhibition efficiency observed in this medium.

Microwave-assisted degradation of dyes is being extensively investigated for wastewater remediation. The present work reports, for the first time, degradation of methyl orange (MeO) in neutral, acidic, and basic media under microwave irradiation using semiconducting polymer [poly(1-naphthylamine) (PNA)] nanotubes as catalyst in the absence of any light source. Degradation of the dye was followed spectrophotometrically and by total organic content (TOC) analysis. In the presence of poly(1-naphthylamine), microwave irradiation in 15 min caused 70, 84, and 64 % degradation at 280 nm, while at 460 nm the degradation values were 80, 96 and 78 % of MO in neutral, acidic, and basic media, respectively. The degradation of the dye in the absence and presence of PNA catalyst and in acidic, basic, and neutral media followed first-order rate kinetics. Rate constant, k, values were found to be slightly dependent on the pH of the dye solutions. Liquid chromatography–mass spectroscopy analysis confirmed that dye degrades into smaller fragments of low molar masses.

Investigation of the chemical stability of Zn2SnO4 in aqueous media by using ICP-OES and TEM analyses

The keto–enol interconversion of 2-acetyl-1-tetralone (ATLO) and of 2-acetyl-cyclohexanone (ACHE) occurs at measurable rates in aqueous acid or neutral medium. This finding allowed us to determine the keto–enol equilibrium constants, KE, by following two distinct methods. Both methodologies afford results in complete agreement. The first one is a test of the Beer-Lambert law under two different experimental conditions that contain the substrate only in the enol form or in a mixture of both tautomers in equilibrium. The second method analyses the UV-absorption spectrum of each substrate under keto–enol equilibrium in aqueous β-cyclodextrin (β-CD) solutions of variable concentration: the presence of β-CD increases the percentage of the enol due to the formation of 1:1 inclusion complexes between this tautomer and β-CD. Rates of keto–enol tautomerization, in neutral and acid medium, and of nitrosation in acid medium under non equilibrium conditions have also been measured. Throughout the study, the presentation of the results is done by comparing the different behaviour observed between ATLO and ACHE. While the enol of ACHE included into the β-CD cavity shows to be unreactive either in tautomerization or in nitrosation, in the case of ATLO it is observed tautomerization through the complexed enol. In addition, with ACHE only the enol tautomer forms inclusion complexes with β-CD, whereas with ATLO the keto tautomer entries also to the β-CD cavity, however the stability constant with the enol is near 3-fold that of the keto isomer. These main differences can be rationalized on the basis of the molecular structure of these diketones.

Electrochemical Corrosion Behavior of Titanium-Zirconium-Molybdenum Alloy

The oxygen reduction reaction (ORR) is a crucial chemical process, which contributes significantly to the energy efficiency of fuel cells and metal-air batteries. Although Pt and its alloys are known as the most efficient catalysts for ORR, its high costs and scarcity limits their applications. Alternatively, Fe–N–C catalysts have attracted the most interest owing to their low-cost and facile preparation methods. Metal macrocycles, such as iron phthalocyanines (FePc), are the precursors most widely used to prepare the ORR catalysts in fuel cells and metal-air batteries. However, limited progress has been made in improving their electrocatalytic activity and durability. In particular, FePc has demonstrated to suffer from severe degradation in the acidic fuel cell environments, commonly attributed to demetalation and/or degradation by ORR intermediates. The typical life cycle for FePc catalysts is less than 100 cycles in either acidic or basic media.In this work we use ab initio molecular dynamic (MD) simulations to investigate the degradation of graphene-supported FePc in acidic and basic media at operational conditions. The acidic media includes hydrochloric, acetic, sulfuric and phosphoric acids, whereas the basic media includes sodium hydroxide. The solution concentration in all cases is 1M. The MD simulations were performed by dispersion-corrected DFT calculations, using periodic boundary conditions, at 80°C through a simulation time of 2 ps. Our results show that demetalation is not the cause of the FePc degradation, instead we find that for acetic and hydrochloric acids, the acid anion binds to the Fe center, with binding energies of -1.1 and -6.5 eV, respectively. The formation of this ligand would hinder the O2 adsorption on the metal center, limiting the FePc catalytic activity. On the contrary, for phosphoric and sulfuric acids, as well as for basic media, the ion-Fe binding is energetically unlikely, enabling the O2-metal interaction. Therefore, our results suggest that the catalytic activity and degradation of FePc are connected to the acidic media involved.This work was supported by ANID/FONDECYT under Grant No. 1170480

Using cyclic voltammetry, for both p- and n-type GaAs, an anodic peak is observed just before an unlimited anodic current, in acidic, neutral, or basic media in liquid ammonia. This anodic peak is due to an adsorption/precipitation phenomenon. It led to the passivation of the SC surface regardless of pH. After passivation of the electrode, a reduction wave appeared in acidic medium as well in unbuffered neutral medium, but in this last case only for high scan rates. In basic medium, no reduction wave appeared. The initial electrochemical state of GaAs, corresponding to the presence of the anodic peak, is recovered by reduction of the adsorbed film in acidic media, in unbuffered neutral medium, or in buffered neutral medium. No recovery is observed in basic media. These last results show that this recovery occurred only when protons came from the medium (acidic medium) or protons came from the anodic reaction associated to the anodic peak, or if a proton donor was present. Quantitative analysis of gallium and arsenic elements released in solution showed clearly that a dissolution happened without limitation as soon as the unlimited anodic current was reached. Coulometric measurements point out that the efficiency of dissolution increased with current density in the anodic unlimited current, showing that ammonia oxidation and material oxidation were concomitant reactions. The anodic wave may be considered as the result of the precipitation of a nitrogenous-based complex (i.e., amide complex) which implies the superficial lattice contribution. The passivation film may be initiated by the formation of strongly adsorbed atoms of nitrogen, intermediaries arising from ammonia oxidation, which can behave as site poison at the interface during nitrogen evolution. © 2003 The Electrochemical Society. All rights reserved.

Pt-W/C catalyst was synthesized by slow reduction of platinum and tungsten solutions in the desired ratio with subsequent deposition on the Vulcan carbon already added to the solution. Crystallite size of catalyst was about 9 nm and its density, cell volume, d-spacing and lattice parameter were also calculated. EDX analysis of the catalyst was also done. Electrochemical surface area of the catalyst was determined by cyclic voltammetry (CV). CV of the catalyst was done both in acidic and basic media to find out the peak potential, peak current, specific activity and mass activity of the catalyst. Peak potential versus scan rate plots showed that the electro oxidation of methanol is an irreversible process. Tafel equation was used to plot polarization curves to find out the exchange current density. Higher values of exchange current indicate better catalysts. Specific activities of the catalyst were determined in acidic and basic media and it was found that the specific activity in basic media increased substantially as compared to acidic media. The specific activity in acidic media was 83 mA/mg pt whereas in basic media it was 137mA/mg pt which is a substantial increase. Heterogeneous rate constant in acidic media was 6.15 x 10−6 cm/ s and in basic media it was 4.92 x 10−5 cm/s which is much higher in basic media. In this binary catalyst addition of tungsten has increased the catalytic activity but it is non-noble metal thus will decrease the cost. Stability studies of the catalyst were done upto fifty cycles both in acidic and basic media and was found quite stable in both the media.

Different ZnS nanostructures synthesized by a chemical solution process were deposited on the glass substrate by dip coating technique. The effect of the sol-pH variation on the surface morphology, crystalline quality, optical band gap, and photoluminescence of the ZnS nanosphere, nanorod and mixed hexagonal cubic and rectangular-like nano-grains were investigated. It was found that the triethanolamine/ethanolamine played a key role to get desired nanostructure quality with different surface morphology. All nanostructures, derived in acidic and basic medium showed mixed cubic and hexagonal crystalline structure with preferred orientation along (111) plane of predominant cubic phase without oxidation. Nanospherical grains, sharply increased in size observed in acidic medium, whereas nanorods and mixed hexagonal, cubic and rectangular-like grains, slightly increased and then decreased in size, observed in basic medium with increased pH. The increased/decreased in average crystallite size was confirmed by reduced/enhanced dislocation density and micro strain. Comparatively, the band gap (Eg) of the nano-grains, reduced by pH derived in acidic medium was higher than those derived in basic medium. Comparatively, the higher surface defects related emission was observed in nanostructures derived in acidic medium than those derived in basic medium, whereas a higher green emission was observed in nanostructures fabricated at basic medium compared with the nanostructures synthesized in acidic medium. The results showed better crystalline quality in nano-grains derived at acidic medium than those derived at basic medium, whereas better optical quality in nanostructures derived at basic medium compared with the nanostructures derived at acidic medium. The observed characteristics are highly attractive for solar cells and optoelectronic applications.

The decomposition of N′‐benzoyl‐N‐methyl‐N‐nitrosourea (BMNU) in aqueous media over the 0−14 pH range has been studied. In basic and neutral media (6 < pH < 14) the reaction proceeds through abstraction of the acidic proton of BMNU (pKa = 7.8) and subsequent decomposition of the conjugate base of the thus formed nitrosourea, via an intermediate benzoyl isocyanate. Support for this mechanism is provided by the presence of N,N′‐dibenzoylurea in the final reaction mixtures, as the result of the trapping of benzoyl isocyanate with benzamide generated from hydrolysis of the former. The hydrolysis of BMNU takes place through three competitive pathways: spontaneous decomposition of the conjugate base of BMNU, and buffer‐catalyzed and hydroxide ion catalyzed water addition to the carbonyl group of the deprotonated nitrosourea. N′‐Benzoyl‐N,N′‐dimethyl‐N‐nitrosourea (BDMNU), a benzoyl nitrosourea lacking the acidic proton of BMNU, is hydrolyzed in basic media by attack of hydroxide ion on the carbonyl group of the urea. In acid media (0 < pH < 6), BMNU gives only deamination products, differing from the reported behavior of other N‐nitroso compounds and of the isoster nitrosoguanidine, in which denitrosation is almost quantitative. The reaction is acid‐catalyzed in the 0−2.5 pH range and pH‐independent in the 3−5 pH range. The presence of general acid catalysis (α = 0.60), the absence of nucleophilic catalysis, and the thermodynamic activation parameters for the reaction support the mechanism proposed in the literature for the deamination of N‐nitrosoureas in acidic media. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)