Number Of Reactive Sites Research Articles

Binding of proton and iron to lignite humic acid size-fractions in aqueous matrix

The bioavailability of trivalent iron (Fe3+) to plants can be enhanced using fertilizer solutions containing humic acids (HA) as manifested from the increased crop yield at an iron stress conditions. The lignite-derived HA (HAlignite) facilitates higher diffusion of Fe3+ between the soil layers as attributable to more number of reactive sites in the assemblage compared to those from other origins. In the current work, the proton-binding of HAlignite size-fractions (5–10, 10–30, 30–100, and >100 kDa), as segmented based on the molecular weight distribution, and their complexation with Fe3+ have been studied at varying pH ranging from low to high. The protonation or formation of Fe3+-complexes exhibited a comparable pattern despite the differences in the conformational distribution of HAlignite size-fractions. The protonation behavior specified that the behavior of HAlignite size-fractions has similarity with that of a dibasic acid. The results are interpreted using reactive structural units (RSU) concept to show that the carboxyl and phenolic-hydroxyl groups in the HAlignite size-fractions simultaneously available as the Fe3+-binding sites. The stability constants for larger MW fractions of HAlignite (>100 kDa) was the lowest, as attributed to the increased aggregation rate in an aqueous matrix. The trend in conditional stability constants of HAlignite-size fractions and other Fe-chelators point to a better Fe-binding capability of HAlignite (30–100 kDa) size-fraction than the biodegradable alternatives (GLDA, HIDS, EDDS, IDSA, or NTA), while the Fe-interaction was stronger with classical synthetic chelators (EDTA, DTPA, or EDDHA).

The Effect of Additionally Introduced Impurities of Fe and Pb Ions on Silver Azide Decomposition

The experiments have emphasized that additionally introduced impurities of Fe and Pb ions are relevant to the initiation of slow and explosive decomposition of silver azide crystals by the external electric field and UV irradiation. External gas release was used for research purpose. Test-sensitivity is 10−12 mole. Time-to-explosion was measured by a stop watch. It’s been found out that the reaction of slow decomposition in silver azide under the action of electric field turns into explosion faster in samples with the additionally introduced impurities. The samples with additionally introduced Fe are the most explosive ones (time-to-explosion 3 minutes). The authors have assumed that external influence can generate non-equilibrium charge carriers (holes), which become localized on cationic vacancies and support formation of reactive sites. As soon as cut off concentration of these sites is reached, the solid-phase chain reaction turns into explosion. The growing concentration of impurity influences on the number of reactive sites, making their concentration critical. In view of the results obtained in experiments a procedure for monitoring the decomposition of crystals is suggested, which necessitates additional introduction of Fe and Pb ions.

Chemodynamics of metal ion complexation by charged nanoparticles: a dimensionless rationale for soft, core-shell and hard particle types.

Soft nanoparticulate complexants are defined by a spatial confinement of reactive sites and electric charges inside their 3D body. In turn, their reactivity with metal ions differs significantly from that of simple molecular ligands. A revisited form of the Eigen mechanism recently elucidated the processes leading to metal/soft particle pair formation. Depending on e.g. particle size and metal ion nature, chemodynamics of nanoparticulate metal complexes is controlled by metal conductive diffusion to/from the particles, by intraparticulate complex formation/dissociation kinetics, or by both. In this study, a formalism is elaborated to achieve a comprehensive and systematic identification of the rate-limiting step governing the overall formation and dissociation of nanoparticulate metal complexes. The theory covers the different types of spherical particulate complexants, i.e. 3D soft/permeable and core-shell particles, and hard particles with reactive sites at the surface. The nature of the rate-limiting step is formulated by a dynamical criterion involving a power law function of the ratio between particle radius and an intraparticulate reaction layer thickness defined by the key electrostatic, diffusional and kinetic components of metal complex formation/dissociation. The analysis clarifies the intertwined contributions of particle properties (size, soft or hard type, charge, density or number of reactive sites) and aqueous metal ion dehydration kinetics in defining the chemodynamic behavior of nanoparticulate metal complexes. For that purpose, fully parameterized chemodynamic portraits involving the defining features of particulate ligand and metal ion as well as the physicochemical conditions in the local intraparticulate environment, are constructed and thoroughly discussed under conditions of practical interest.

Geometrical structures of catalyst layer and their impact on oxygen reduction in proton exchange membrane fuel cell

The oxygen diffusion and reduction in the catalyst layer (CL) of proton exchange membrane fuel cells is often calculated by approximating the CL structures using different geometries in fuel cell modelling, yet they all produced polarization curves that agree well with measurements. This raises a question about the importance of the CL structure and how to use the agglomerate model to help CL design. In this paper, we address this by comparing six widely used CLs in the literature to calculate the oxygen reduction: i) packing of isolated cylinders, ii) packing of isolated spheres, iii) packing of touched spheres of different diameters, iv) and v) two real CLs acquired using tomography, vi) a stochastically generated CL. To facilitate comparison, we assume that all CLs have the same catalyst loading and agglomerate volume, so that the number of reactive sites in them is the same. We also coat ionomer films approximately10nm thick over the agglomerate surfaces. The average oxygen diffusion and reduction in each CL is directly calculated using pore-scale simulations, and the results are then used in an electrode model to elucidate how the CL structure affects cell performance. We first prove that the oxygen reduction in all six CLs with contrasting geometrical structures can be described by a proposed agglomerate model with its parameters directly calculated from the CL structure. We then demonstrate that the spherical agglomerate model widely used in the literature is just to bridge simulated results and measurements, and explain why its diameter needs to vary so unrealistically widely.

Direct preparation of hierarchical macroporous β-SiC using SiO2 opal as both template and precursor and its application in water splitting

β-SiC is a suitable photocatalyst for hydrogen evolution reaction because of its proper band structure (2.36 eV) and high chemical stability. Porous β-SiC has a large surface area that can increase the number of reactive sites and shorten the charge transport pathways between the material interior and surface, thereby significantly improving the hydrogen evolution reaction performance. In the present study, hierarchical macroporous β-SiC monolith composed of nanocrystals was directly prepared using SiO2 opal as both template and precursor via the magnesiothermic reduction method at 750 °C. An interfacial magnesiothermic reaction model was presented as well. The β-SiC sample exhibited high photoelectric conversion performance and stability as a catalyst for hydrogen evolution.

Origin of Large Onset Overpotential and Sluggish Kinetics of the Oxygen Reduction Reaction on Pt(111) Surface

The origin of the large onset overpotential and sluggish kinetics of the oxygen reduction reaction (ORR) on Pt has been a challenging problem in electrochemistry, which hinders the optimization of the cathode catalyst in fuel cells. This is largely due to the lack of a clear mechanistic view of the ORR on Pt, despite extensive studies during the past years. It remains uncertain whether the 4-electron oxygen reduction starts through the direct adsorption and dissociation of O2 (dissociation pathway) or proton-coupled adsorption of O2 (association pathway); and the exact nature of the rate-determining step (rds) is still elusive. In this study, the ORR mechanism is investigated theoretically by emphasizing the importance of various oxygenated adsorbates in the reaction kinetics. The adsorption of oxygenated species may affect ORR kinetics by modulating both the reaction energetics and the number of reactive sites. We first investigate the oxygenation processes of Pt(111) surface using combined density functional theory (DFT) calculations and Monte Carlo simulation, which allows the surface phase structures of oxygenated adsorbates at different potentials to be determined. The results show that the conversion between the adsorbed hydroxyl groups (OH*) and water molecules (H2O*) in the (Ö3´Ö3)R30°-structured OH*-H2O* network, and the phase transition between the OH*-H2O* network and the (Ö3´Ö3)-structured O* adsorption phase, can reasonably describe the cyclic voltammetric responses of Pt(111) electrode. Based on this, it is concluded that there are mainly two types of oxygenated surface phases at potentials of ORR relevance (below ca. 1.1 V), namely, the (Ö3´Ö3)R30°-patterned OH*-H2O* networks and the Ö3´Ö3-structured O* phase. The reaction energy calculations indicated that the adsorption of O2, which is the initial step of the ORR, is thermodynamically much more difficult to take place on the surface covered with the ordered O* adsorption phase than that on the surface bears the OH*-H2O* co-adsorption network. This suggests that the latter provides the main active sites for the ORR below ca. 1.07 V. The large onset overpotential of the ORR thus can be attributed to the full occupation of the strong spectator phase of O* at potentials above ca. 1.07 V. Besides, the potential-dependent Tafel slopes can be explained in terms of the coverage variation of this spectator phase with potential. By investigating the energetics of various possible reaction channels for O2 adsorption, we identify a new type of proton/electron-transfer (PET) coupled O2 adsorption reaction as the initial and also the activity-determining step in the ORR, in which the O2 adsorption is accompanied by a PET to an OH* in the OH*-H2O* network, rather than to the O2 itself to form an end-on OOH as commonly believed. That is, (1) The O2 molecule adsorbs on surface in a t-b-t configuration with one O atom occupying the central Pt atom inside the OH*-H2O* hexagonal structure and the other one displaces an OH* in the hexagonal structure. The present study thus reveals the distinct roles of various oxygenated adsorbates in the ORR. The adsorbed oxygen atoms, O*s, act mainly as surface spectators for the ORR; while OH*s and H2O*s on the one hand act as spectators to block the surface sites, but on the other hand provide reaction sites by forming hexagonal networks, with some OH*s and H2O*s participating into the reaction as intermediates and/or PET mediators. The sluggish ORR kinetics even after the onset potential should be due to the limited reaction sites on the surface, on which OH*, H2O* and O* occupy the most of surface sites (>2/3 ML).

Graphite-Conjugated Rhenium Catalysts for Carbon Dioxide Reduction.

Condensation of fac-Re(5,6-diamino-1,10-phenanthroline)(CO)3Cl to o-quinone edge defects on graphitic carbon surfaces generates graphite-conjugated rhenium (GCC-Re) catalysts that are highly active for CO2 reduction to CO in acetonitrile electrolyte. X-ray photoelectron and X-ray absorption spectroscopies establish the formation of surface-bound Re centers with well-defined coordination environments. GCC-Re species on glassy carbon surfaces display catalytic currents greater than 50 mA cm(-2) with 96 ± 3% Faradaic efficiency for CO production. Normalized for the number of Re active sites, GCC-Re catalysts exhibit higher turnover frequencies than that of a soluble molecular analogue, fac-Re(1,10-phenanthroline)(CO)3Cl, and turnover numbers greater than 12,000. In contrast to the molecular analogue, GCC-Re surfaces display a Tafel slope of 150 mV/decade, indicative of a catalytic mechanism involving rate-limiting one-electron transfer. This work establishes graphite-conjugation as a powerful strategy for generating well-defined, tunable, heterogeneous electrocatalysts on ubiquitous graphitic carbon surfaces.

Modeling of mass and charge transport in a solid oxide fuel cell anode structure by a 3D lattice Boltzmann approach

A 3D model at microscale by the lattice Boltzmann method (LBM) is proposed for part of an anode of a solid oxide fuel cell (SOFC) to analyze the interaction between the transport and reaction processes and structural parameters. The equations of charge, momentum, heat and mass transport are simulated in the model. The modeling geometry is created with randomly placed spheres to resemble the part of the anode structure close to the electrolyte. The electrochemical reaction processes are captured at specific sites where spheres representing Ni and YSZ materials are present with void space. This work focuses on analyzing the effect of structural parameters such as porosity, and percentage of active reaction sites on the ionic current density and concentration of H2 using LBM. It is shown that LBM can be used to simulate an SOFC anode at microscale and evaluate the effect of structural parameters on the transport processes to improve the performance of the SOFC anode. It was found that increasing the porosity from 30 to 50 % decreased the ionic current density due to a reduction in the number of reaction sites. Also the consumption of H2 decreased with increasing porosity. When the percentage of active reaction sites was increased while the porosity was kept constant, the ionic current density increased. However, the H2 concentration was slightly reduced when the percentage of active reaction sites was increased. The gas flow tortuosity decreased with increasing porosity.

Synthesis and characterisation of a pH-sensitive magnetic nanocomposite for controlled delivery of doxorubicin

We prepared a magnetic chitosan-cis-aconitic anhydride-doxorubicin nanocomposite, denoted by MCS-CAA-DOX. Chitosan (CS) was linked to magnetic nanoparticles (Fe3O4) to decrease cytotoxicity of the composite and provided a large number of reactive sites for coupling of drug molecules. DOX was attached to the magnetic chitosan (MCS) via a pH-sensitive linker, cis-aconitic anhydride, which hydrolyses in the acidic lysosomal environment to allow pH-responsive release of DOX. The prepared nanocomposites were within 15 nm and had good superparamagnetic properties. The loading rate of DOX was up to 83%. It was found that nearly 88% DOX was released within 60 h at pH 5.0, compared with only 29% at pH 7.4.

A Highly Efficient Sunlight Driven ZnO Nanosheet Photocatalyst: Synergetic Effect of P‐Doping and MoS2 Atomic Layer Loading

AbstractA novel kind of highly efficient photocatalyst composed of ultrathin P‐doped ZnO nanosheets decorated with atomic MoS2 layers is reported. Under natural sunlight, 95 % of the organic dyes can be degraded within six minutes. The photodegradation rate constant reaches up to 1.413 min−1, which is 3.4 times larger than that of commercial P25 under the same reaction conditions. The superior photocatalytic activity of the hybrid photocatalyst can be attributed to the synergetic effects of many advantages, including enhanced light adsorption efficiency, suppression of charge recombination, improvement of interfacial charge transfer, and an increase in the number of reaction sites.

Development of Porous Polypyrrole Electrode for Fuel Cell Applications

One method of increasing number of reaction sites was by the introduction of a porous structure into the electrodes to provide a high surface area for catalyst deposition. This work focused on the development of a method for the fabrication of a porous polypyrrole electrode; a mixture of two monomers (i.e. pyrrole and methylene blue) was simultaneously electropolymerised and one of which was selectively removed from the composite film by solvent extraction. The porous polypyrrole had a suitably porous structure whilst maintaining excellent electrical properties. The application of this novel material to miniaturized fuel cells was shown to have improved power density of 2-fold and 3-fold higher than bulk polypyrrole and bare gold electrodes respectively.

Immobilization of quaternary ammonium salts on silica gel for perchlorate ions removal

Abstract Silica surface was modified with quaternary ammonium salts to give three matrices of different chain length. The synthesized supports were characterized by elemental analysis and infrared spectroscopy, as well as scanning electron microscopy and semi-empirical parametric method 5. Their exchange capacities for ClO4−, F−, NO3− and CH3COO− ions were investigated by conductometric titration. The amount of exchanged chloride ions from the matrix was studied for different initial anions concentration. The functionalized silica gel showed very good ability of perchlorate ions exchange from the water solution in room temperature. The anion-exchange properties were also studied in comparison to different number of reactive sites in the matrices. The increased number of chloride at the silica surface took effect on increased ability of ClO4− anion exchange

Probing the Mechanisms of the Yellow Vine Syndrome Development in the American Cranberry: Shade Effect

Yellow vine symptoms are often observed in cranberry bogs. To explore the mechanisms of the formation of yellow vine syndrome in cranberry leaves, the shade effect on the chlorophyll (Chl) content and photosynthetic activities in cranberry bogs were investigated by spectrometric, high-performance liquid chromatography (HPLC), and in vivo Chl fluorescence kinetics. Spectrometric and HPLC analyses revealed that the yellow vine leaves were associated with a 11% ± 5% and 14% ± 5% increase in Chl a/Chl b ratio after shading, respectively. The Chl a/Chl b ratio was the same in both types of leaves, suggesting the photosystem (PS) II organization remains invariant. The rise in chlorophyll content suggested that the number of reaction sites on PS II is increased in the shaded yellow vine leaves. The results of in vivo chlorophyll fluorescence analysis also indicated that the electron transport chain in the PS II is enhanced and that the size of the quinone pool is increased. In addition, the overall photosynthesis index is drastically improved by shading. These three lines of evidence imply that the shading of cranberry plants appeared to reduce the syndrome by improving the photosynthetic activity and increasing the chlorophyll content. The techniques presented here may be valuable for characterizing variations of plants by stress or disease.

Microstructural study of the LiBH4–MgH2 reactive hydride composite with and without Ti-isopropoxide additive

An exhaustive microstructural characterization is reported for the LiBH4–MgH2 reactive hydride composite (RHC) system with and without titanium isopropoxide additive. X-ray diffraction with Rietveld analysis, transmission electron microscopy coupled to energy dispersive X-ray analysis, selected-area electron diffraction and electron energy loss spectroscopy are presented in this paper for the first time for this system for all sorption steps. New data are reported regarding average crystallite and grain size, microstrain, phase formation and morphology; these results contribute to the understanding of the reaction mechanism and the influence of the additives on the kinetics. Microstructural effects, related to the high dispersion of titanium-based additives, result in a distinct grain refinement of MgB2 and an increase in the number of reaction sites, causing acceleration of desorption and absorption reactions. Considerations on the stability of phases under electron beam irradiation have also been reported.

Graft Polymerization of Vinyl Acetate onto Potato Starch Dissolved in an Alkaline Solution

AbstractGraft polymerization of vinyl acetate (VAc) onto gelatinized and dissolved granular potato starch has been carried out using an initiation system of ceric ammonium nitrate (CAN)/nitric acid (HNO3) at 50°C. The granular starch was dissolved in an aqueous alkaline solution (0.25 M NaOH) for 24 h before graft reaction. The effects of reaction variables, such as the concentrations of acid, monomer, and initiator, on the grafting parameters of prepared starch/VAc copolymers have been investigated in both grafting methods for comparison. Although the scanning electron microscopy (SEM) images showed a rather similar homogeneous structure for dissolved and gelatinized starch, a higher grafting level, especially at lower monomer concentrations, was obtained for the copolymers prepared using the dissolved starch. A larger number of reactive sites uniformly created on the starch molecules led to greater accessibility of the monomer and improved considerably the grafting efficiency in the grafting conditions using the dissolved starch. The effects of the grafting methods on the microstructure, morphology, and thermal stability of the resulting copolymers have been studied by Fourier‐transform infrared (FTIR) spectroscopy, scanning electron microscopy, and thermogravimetric analysis (TGA), respectively. The TGA results showed that the grafting of VAc onto dissolved starch led to the formation of more thermally stable graft copolymers.

Mechanism of cellulase reaction on pure cellulosic substrates

Cellulase reaction mechanism was investigated with the use of following pure cellulosic substrates: Microcrystalline cellulose (Avicel), alpha-cellulose (Sigma), filter paper, cotton, and non-crystalline cellulose (NCC). NCC is amorphous cellulose prepared in our laboratory by treatment with concentrated sulfuric acid. When hydrolyzed with cellulase, NCC produces significant amount of cello-oligosaccharides (COS) as reaction intermediates along with glucose and cellobiose. The COS produced by cellulase were categorized into two different moieties based upon their degree of polymerization (DP): low DP (less than 7) COS (LD-COS) and high DP COS (HD-COS). Endo-glucanase (Endo-G) reacts rapidly on the NCC reducing its DP to 30-60, after which the Endo-G reaction with NCC ceases. HD-COS is produced from NCC by the action of Endo-G, whereas LD-COS is produced by exo-glucanase (Exo-G). beta-Glucosidase (beta-G) hydrolyzes LD-COS to produce cellobiose, but it does not hydrolyze HD-COS. DP of NCC affects the action of Exo-G in such a way that the overall yield is high for high DP NCC. This is in line with previous findings that substrate-recognition by Exo-G requires binding on beta-glucan chain with DP of 10 for the hydrolysis to take place. The individual cellulose chain residues within solid having DP less than 10 therefore remain unreacted. The percentage of the unreacted portion would be lower for high DP NCC, which results high overall conversion. The surface area and the number of reactive sites on the substrate facilitate adsorption of enzyme therefore the initial rate of the hydrolysis. The overall extent of conversion of cellulose, however, is controlled primarily by its inherent characteristics such as DP and crystallinity.

Number of arm selection in two-dimensional diffusion processes

We introduce an algorithm to generate two-dimensional diffusion-limited star-branched aggregates (DLSA) attaching bi-functional monomers successively to a central colloidal particle with any desired number of reactive sites. The proposed algorithm produces star-shaped aggregates that grow forever and show a power law polydispersity in the chemical length of the arms near the central colloid. More interestingly, it gives rise to a number of arm selection consisting in that only a small number of arms (around five) define the final structure at relatively large distances from the central colloid, independently of the initial number of reactive sites and the size of the central colloid. We characterize the structure of the aggregates by means of the particle-particle correlation function, analyze its scaling properties and obtain the fractal dimension.

The role of crosslinkers in epoxy–amine crosslinked silicon sol–gel barrier protection coatings

The search for chromate replacements in corrosion prevention materials has identified the use of hybrid sol–gel coatings as one, very promising approach. Appropriately functionalized hybrid sol–gel materials can be crosslinked to enhance their chemical durability and mechanical strength. In this work, we evaluate three crosslinkers used in a tetramethoxysilane–glycidoxypropyltrimethoxysilane binary sol–gel system in order to identify the role of the crosslinkers in corrosion protection. The crosslinkers examined were ethylenediamine, N-aminethylepiperazine, and diethylenetriamine. The sol–gel coatings were examined by contact angle, atomic force microscopy, and electrochemical impedance spectroscopy (EIS). Circuit modeling of the EIS results yielded valuable insights into the significant differences between the durabilities of the variously crosslinked coatings. Crosslinker hydrophobicity was identified as not playing a significant role whereas the number of reactive sites per crosslinker and the resulting morphology of the material may be an important parameter.

In Situ Cross-Linking of Elastin-like Polypeptide Block Copolymers for Tissue Repair

Rapid cross-linking of elastin-like polypeptides (ELPs) with hydroxymethylphosphines (HMPs) in aqueous solution is attractive for minimally invasive in vivo implantation of biomaterials and tissue engineering scaffolds. In order to examine the independent effect of the location and number of reactive sites on the chemical cross-linking kinetics of ELPs and the mechanical properties of the resulting hydrogels, we have designed ELP block copolymers comprised of cross-linkable, hydrophobic ELP blocks with periodic Lys residues (A block) and aliphatic, hydrophilic ELP blocks with no cross-linking sites (B block); three different block architectures, A, ABA, and BABA were synthesized in this study. All ELP block copolymers were rapidly cross-linked with HMPs within several minutes under physiological conditions. The inclusion of the un-cross-linked hydrophilic block, its length relative to the cross-linkable hydrophobic block, and the block copolymer architecture all had a significant effect on swelling ratios of the cross-linked hydrogels, their microstructure, and mechanical properties. Fibroblasts embedded in the ELP hydrogels survived the cross-linking process and remained viable for at least 3 days in vitro when the gels were formed from an equimolar ratio of HMPs and Lys residues of ELPs. DNA quantification of the embedded cells indicated that the cell viability within triblock ELP hydrogels was statistically greater than that in the monoblock gels at day 3. These results suggest that the mechanical properties of ELP hydrogels and the microenvironment that they present to cells can be tuned by the design of the block copolymer architecture.

Contribution of lignin to the reactivity of wood in chemical modifications II: influence of delignification on reaction with vaporous formaldehyde

Participation of lignin in the reaction between vapor-phase formaldehyde and wood was examined by using gradually delignified wood meal. A fi rst-order rate equation was successfully applied to the weight gain data. From the estimated reaction parameters such as rate constant, k, and ultimate weight gain, a, the reactivity toward formaldehyde was discussed among wood components, and compared with that for acetylation. k decreased monotonously with progress of the elimination of lignin, suggesting that the reaction rate of lignin is dominant over that of whole wood, and the decrease in the ratio of lignin retarded the reaction of wood as a whole. On the other hand, a increased with decreasing lignin content. This may be attributable to the enhanced reactivity of the remaining lignin due to some structural changes and to the increase in the number of reactive sites in polysaccharides as a result of their exposure accompanying the elimination of lignin. The dependencies of k and a on the lignin content were not similar to the case for acetylation, probably because of the difference in the reaction phase. In vapor-phase formaldehyde treatment, the remaining lignin reacts as it is, whereas in liquid-phase acetylation it would undergo rearrangement or swelling of the structure in the reaction solution.