Distribution Of Reactive Sites Research Articles

Controlled construction of Co3S4@CoMoS yolk-shell sphere for efficient hydrodesulfurization promoted by hydrogen spillover effect

A yolk-shell structured Co3S4@CoMoS catalyst was prepared through Oswald ripening method and subsequently used for hydrodesulfurization (HDS) reaction. The shells, constructed from Co-promoted MoS2 nanosheets, possess abundant active sites and facilitate the adsorption of reactants through the development of pore channels. The MoS2 sheets are arranged in a staggered and convoluted manner, creating numerous defect sites. The shorter MoS2 slabs provide a wide distribution of reactive sites, ensuring efficient reactions. The Co3S4 species within the inner core acts as an auxiliary active phase, inducing the hydrogen spillover effect in the HDS system. This facilitates the transfer of active hydrogen species to the CoMoS shell, where both CoMoS and Co3S4 phases synergistically enhance the HDS reaction. The Co3S4@CoMoS catalyst achieved up to 99.2% dibenzothiophene (DBT) conversion and 94.9% 4,6-dimethyldibenzothiophene conversion at the lower dosage (30 mg). The structure-activity relationships between active phase and HDS activity were investigated via in-situ infrared spectroscopy and other characterizations as well as density functional theory calculations.

Light-driven Orderly Assembly of Ir-atomic Chains to Integrate a Dynamic Reaction Pathway for Acidic Oxygen Evolution.

This work suggests an intriguing light-driven atomic assembly proposal to orderly configure the distribution of reactive sites to optimize the spin-entropy-related orbital interaction and charge transfer from electrocatalysts to intermediates. Herein, the introduced fluorine (F) atoms acting as photo-corrosion centres in MnO1.9 F0.1 effectively soften the bonding interaction of Mn-O bonds in the IrCl3 solution. Therefore, partial Mn atoms can be successively replaced to form orderly atomic-hybridized catalysts with a spin-related low entropy due to the coexistence of Ir-atomic chains and clusters. The time-related elemental analysis demonstrates that the dynamic dissolution/redeposition of Ir clusters in acidic oxygen evolution leads to a reintegration of the reaction pathway to seek the switchable rate-limiting step with a lower activation energy.

Light‐driven Orderly Assembly of Ir‐atomic Chains to Integrate a Dynamic Reaction Pathway for Acidic Oxygen Evolution

AbstractThis work suggests an intriguing light‐driven atomic assembly proposal to orderly configure the distribution of reactive sites to optimize the spin‐entropy‐related orbital interaction and charge transfer from electrocatalysts to intermediates. Herein, the introduced fluorine (F) atoms acting as photo‐corrosion centres in MnO1.9F0.1 effectively soften the bonding interaction of Mn−O bonds in the IrCl3 solution. Therefore, partial Mn atoms can be successively replaced to form orderly atomic‐hybridized catalysts with a spin‐related low entropy due to the coexistence of Ir‐atomic chains and clusters. The time‐related elemental analysis demonstrates that the dynamic dissolution/redeposition of Ir clusters in acidic oxygen evolution leads to a reintegration of the reaction pathway to seek the switchable rate‐limiting step with a lower activation energy.

Super-resolution imaging of photogenerated charges on CdS/g-C3N4 heterojunctions and its correlation with photoactivity.

Construction of heterostructures is an effective way to improve photo-induced charge separation and photocatalytic performance. Among various structures, type II and direct Z-scheme heterojunctions with distinct charge separation mechanisms are the two typical representatives attracting much research attention. Here we prepared type II and Z-scheme CdS/g-C3N4 nanocomposites by thermal treatment and self-assembly chemisorption methods, respectively. High-resolution microscopy techniques including (scanning) transmission electron microscopy (TEM/STEM) and super-resolution fluorescence microscopy (SRM) were used to investigate the charge distribution and flow mechanism. The charge tracking results reveal that the nanocomposite prepared by thermal treatment has a type II heterostructure with charges flowing in the opposite direction, while the self-assembly sample possesses a Z-scheme structure. It was found that the type II system exhibited the lowest charge migration resistance and the best charge separation ability and stability of photoactivity, leading to the highest H2 generation rate of 2410 μmol h-1 g-1. The SRM technique was applied for the first time to map the reactive sites of type II and Z-scheme structures at nanometer resolution. The photoactive species (i.e., e- and h+) were found to be preferentially distributed at the two end segments of CdS nanorods and the edge boundaries of g-C3N4. Therefore, our findings shed more light on the charge distribution and photocatalytic heterogeneity of composite materials at the nanoscale. Such results would provide guidance on optimizing nanocomposite properties and help to design better photocatalysts for efficient solar-to-chemical energy conversion.

Distribution of Reaction Sites in SOFC Cathode through Oxygen Isotope Labeling with Three-Dimensional Microstructural Analysis

Distribution of electrochemical reaction sites in a composite cathode of solid oxide fuel cell is investigated through active sites imaging by oxygen isotope labeling combined with three-dimensional microstructure observation by focused ion beam–scanning electron microscopy (FIB-SEM). Power generation with oxygen isotope and subsequent cell quenching is conducted at 973 K for LSM/YSZ cathode using a button cell. Through FIB-SEM observation and secondary ion mass spectroscopy (SIMS) analysis, the 3D microstructure of the quenched cathode with 2D 18O concentration mapping in the middle plane of the characterized volume is obtained. Some LSM particles which show quite weak 18O concentration are identified as the isolated phase, while several percolated LSMs near the electrolyte also show quite weak 18O concentration, which indicates the existence of inactive reaction sites or ion/electron paths even inside the percolated phases. In addition, the oxide ion flux incorporated from the cathode/electrolyte interface is estimated from the 18O concentration profiles at two locations in the observed area. A clear difference between the calculated oxide ion fluxes at the two locations is observed, which is discussed with the distributions of percolated TPB density and YSZ volume fraction in terms of the effective reaction thickness.

Use of Amperometric and Potentiometric Probes in Scanning Electrochemical Microscopy for the Spatially-Resolved Monitoring of Severe Localized Corrosion Sites on Aluminum Alloy 2098-T351.

Amperometric and potentiometric probes were employed for the detection and characterization of reactive sites on the 2098-T351 Al-alloy (AA2098-T351) using scanning electrochemical microscopy (SECM). Firstly, the probe of concept was performed on a model Mg-Al galvanic pair system using SECM in the amperometric and potentiometric operation modes, in order to address the responsiveness of the probes for the characterization of this galvanic pair system. Next, these sensing probes were employed to characterize the 2098-T351 alloy surface immersed in a saline aqueous solution at ambient temperature. The distribution of reactive sites and the local pH changes associated with severe localized corrosion (SLC) on the alloy surface were imaged and subsequently studied. Higher hydrogen evolution, lower oxygen depletion and acidification occurred at the SLC sites developed on the 2098-T351 Al-alloy.

Tracking the rotation of single CdS nanorods during photocatalysis with surface plasmon resonance microscopy

While rotational dynamics of anisotropic nanoobjects has often been limited in plasmonic and fluorescent nanomaterials, here we demonstrate the capability of a surface plasmon resonance microscopy (SPRM) to determine the orientation of all kinds of anisotropic nanomaterials. By taking CdS nanorods as an example, it was found that two-dimensional Fourier transform of the asymmetrical wave-like SPRM image resulted in a peak in its angular spectrum in k space. Consistency between the peak angle and the geometrical orientation of the nanorod was validated by both in situ scanning electron microscope characterizations and theoretical calculations. Real-time monitoring of the rotational dynamics of single CdS nanorods further revealed the accelerated rotation under appropriate reaction conditions for photocatalyzed hydrogen generation. The driving force was attributed to the asymmetric production of hydrogen molecules as a result of inhomogeneous distribution of reactive sites within the nanorod. The present work not only builds the experimental and theoretical connections between the orientation of anisotropic nanomaterials and its SPRM images; the general suitability of SPRM also sheds light on broad types of nonfluorescent and nonplasmonic anisotropic nanoobjects from semiconductors to bacteria and viruses.

Reactivity of heterogeneous surfaces: Modeling precursor–surface interaction using absorbing Markov chains

Heterogeneous reactive surfaces involving a spatial distribution of reactive sites are ubiquitous in thin film growth, from defect nucleation to step flow growth. In this work, the interaction of gas phase species with heterogeneous reactive surfaces was explored using a model combining absorbing Markov chains and local balance equations. Through this model, single particle statistics can be extracted of the adsorption–diffusion–desorption/reaction process even beyond the transport-limited regime, where the surface coverage of adsorbates, and hence adsorbate–adsorbate interaction, is not negligible, and local rates can be connected with macroscopic quantities, such as the sticking probability and desorption rates. In the transport-limited regime, a similarity law was identified for the sticking probability for both step flow growth and nucleation on randomly distributed reactive sites in which the sticking probability is a function of the product of the local diffusion and reaction probabilities. Finally, the model can be applied to extract local reactivity maps providing the probability that a particle adsorbing on a given site reacts with the surface. This allows one to explore the development of capture zones and the impact of site-blocking and desorption on the local sticking probabilities.

The nano necklace made of single-walled carbon nanotubes and an azo-containing polymer: Island-like carboxyl distribution, bead-like morphology formation, and photophysical properties manipulation

The formation of a bead-shaped microstructure of poly{[3-octylthiophene-2,5-diyl]-[p-amino-azobenzylidenequinomethane]}bonded SWNT (PDTAABQ-SWNT) reveals the inhomogeneous distribution of reactive sites along a SWNT in a covalent functionalization. The bead-shaped microstructure arises from the twisting and entangling of polymer chains due to spatial confinement and geometrical restriction, as a result of dense functionalization into island-like small areas containing a high density of carboxyl groups on the SWNT sidewall. Accordingly a portion of (Ca. 24.14%) trans-azobenzene side groups of PDTAABQ are forced to isomerize to the cis form due to a steric effect arising from the overcrowded environment in the ‘bead’. The experimental findings are well-correlated with the molecular simulation, particularly for the physical realization of a bead-shaped microstructure and a trans-cis isomerization. The distortion in morphology due to the inhomogeneous distribution of reactive sites leads to a method for controlling the electronic structure and photophysical properties of SWNT. The transient optical properties of PDTAABQ-SWNT show a faster decaying component on a picosecond timescale but a very unusual slower decaying tail on a nanosecond timescale as compared to the timescale of the parent polymer (PDTADBQ). The severely distorted polymer chains with cis azobenezene side groups result in the reduction of the exciton mobility, leading to the longer fluorescence lifetime. This is confirmed by a similar ratio of the undepleted photoexcitons after 2.2 ns (Ca. 23.28%) and the ratio of the severely distorted polymers (Ca. 24.14%).

Characterization and reactivity of sodium aluminoborosilicate glass fiber surfaces

Multicomponent complex oxides, such as sodium aluminoborosilicate glass fibers, are important materials used for thermal insulation in buildings and homes. Although the surface properties of single oxides, such as silica, have been extensively studied, less is known about the distribution of reactive sites at the surface of multicomponent oxides. Here, we investigated the reactivity of sodium aluminoborosilicate glass fiber surfaces for better understanding of their interface chemistry and bonding with acrylic polymers. Acetic acid (with and without a 13C enrichment) was used as a probe representative of the carboxylic functional groups in many acrylic polymers and adhesives. Inverse gas chromatography coupled to a mass spectrometer (IGC–MS), and solid state nuclear magnetic resonance (NMR), were used to characterize the fiber surface reactions and surface chemical structure. In this way, we discovered that both sodium ions in the glass surface, as well as sodium carbonate salts that formed on the surface due to the intrinsic reactivity of this glass in humid air, are primary sites of interaction with the carboxylic acid. Surface analysis by X-ray photoelectron spectroscopy (XPS) confirmed the presence of sodium carbonates on these surfaces. Computer simulations of the interactions between the reactive sites on the glass fiber surface with acetic acid were performed to evaluate energetically favorable reactions. The adsorption reactions with sodium in the glass structure provide adhesive bonding sites, whereas the reaction with the sodium carbonate consumes the acid to form sodium-carboxylate, H2O and CO2 without any contribution to chemical bonding at the interface.

Anodic Functionalization of Titania Nanotube Arrays for the Electrochemical Detection of Tuberculosis Biomarker Vapors

Anodic titania nanotube arrays have been extensively explored for gas sensing applications due to their high chemical and thermal stability and robust synthesis method. In this study, we explore the use of functionalized titania nanotube arrays for the electrochemical detection of gaseous volatile organic biomarkers associated with tuberculosis. The synthesis method developed allows for in situ functionalization of titania nanotubes with oxygen containing metal species (i.e., Co(OH)2) during anodization. Amperometric detection of volatile organic biomarkers with concentrations of 275 ∼ 360 ppm using next generation titania nanotubes functionalized with Co(OH)2 where compared with Co(OH)2 functionalized titania nanotubes via a wet incipient method. Improved sensor response and sensitivity was observed with next generation titania nanotubes, which can be attributed to homogeneous distribution of reactive sites directly in contact with the parent titania. A mechanism for sensor response and in situ functionalization is presented.

Calcite Dissolution Kinetics

We compare the canonical treatment of calcite’s dissolution rate from the literature in a closed system, particle batch reactor, with the alternative approach suggested by Truesdale (Aquat Geochem, 2015). We show that the decay of rate over time can be understood in terms of the evolution and distribution of reactive sites on the surface of these particles. We also emphasize that interpretation of observed rates must not exclude the fundamental role of crystal defects, whose importance is already implicitly reflected in the common form of rate laws in geochemistry. The empirical behavior of overall rate in closed systems, such as those described by Truesdale, may thus reflect relationships between defect centers and the generation of steps over the calcite surface (previously documented for silicates), such that below a critical free energy limit, there is insufficient driving force to open hollow cores and thus a loss of reaction mechanism. Dissolution in this very-near-equilibrium regime will be dependent on the distribution of extant steps and the energetics of new kink site nucleation. However, these sensitivities are complicated in the case of particle systems by grain boundaries, edges, corners, and other terminations. Such discontinuities constitute a defect class whose overall kinetic importance will be strongly tied to particle diameter and which can act independently of the internal strain field imposed by screw and edge dislocations.

Phase-Segregated Dendrigraft Copolymer Architectures

Dendrigraft polymers have a multi-level branched architecture resulting from the covalent assembly of macromolecular building blocks. Most of these materials are obtained in divergent (core-first) synthetic procedures whereby the molecule grows outwards in successive grafting reactions or generations. Two main types of dendrigraft polymers can be identified depending on the distribution of reactive sites over the grafting substrate: Arborescent polymers have a large and variable number of more or less uniformly distributed sites, while dendrimer-like star polymers have a lower but well-defined number of grafting sites strictly located at the ends of the substrate chains. An overview of the synthesis and the characterization of dendrigraft copolymers with phase-segregated morphologies is provided in this review for both dendrigraft polymer families. The tethering of side-chains with a different composition onto branched substrates confers unusual physical properties to these copolymers, which are highlighted through selected examples.

Atomistic thermodynamics study of the adsorption and the effects of water–gas shift reactants on Cu catalysts under reaction conditions

Density-functional theory (DFT) calculations were performed to determine the structure and stability of oxygen, carbon monoxide and sulfur adsorption on Cu(111), (100) and (110) surfaces that are in equilibrium with a water–gas shift (WGS) reactive environment of H 2, H 2S, H 2O and CO. An atomistic thermodynamic framework based on DFT was used for describing the phase behaviors of the adsorbates on different Cu facets. Phase diagrams of each possible adsorbate on each surface were constructed as a function of the corresponding chemical potential which showed sulfur poisoning occurs even at ppm levels of H 2S in the environment at low temperatures. Under reaction conditions relevant to WGS at low temperature, CO and S adsorbed surface structures were found to be more stable then the clean catalyst surfaces. At high temperatures and high hydrogen pressures, a poisoned surface can be regenerated back to a clean surface. The shapes of a Cu nanoparticle in the WGS reaction conditions under various sulfur chemical potentials were determined using the Wulff construction. We found that the crystal shape changes significantly from one dominated by (111) and (100) facets at very low sulfur chemical potentials to a shape dominated by (110) facets at higher sulfur chemical potentials, suggesting that reactive site distributions may change under reaction conditions.

Determination of reactive site distributions of molybdenum sulfide catalyst during hydrodesulfurization of dibenzothiophene

The nonparametric determination of the distributions of the rate constant and active sites was studied on the basis of the Fredholm integral of the first kind during the hydrodesulfurization (HDS) of dibenzothiophene. The reactivities of the intermediates accounted for better fitting of the kernel of exp ( - kt ) rather than 1 / ( 1 + kt ) . The two distinct distributions support the fact that two active sites with different activities are present on the Mo sulfide catalysts for the C–S hydrogenolysis and hydrogenation of dibenzothiophene during the HDS reaction. The former active sites are 15–16 times stronger than the latter ones. The distributions of the active sites ( k 2 ) decreased with the increasing sulfidation temperature. The 573 K-sulfided Mo catalyst contained one strong active site distributed at 52.4% while the other contained weak active sites at 47.6%. The strength and distributions of the active sites of the Mo sulfide catalysts during the HDS of dibenzothiophene are discussed.

On-line study of lignin behaviour in dilute alkaline solution by the SEC-UV method

Abstract The dynamic method for monitoring ionisation of the lignin functional groups in a 0.1 N NaOH solution at ambient temperature by means of the SEC technique with multiwave UV detection was performed. An increase in absorbance at 210, 278, 304, 320 and 350 nm was examined to reveal the ionisation dynamics in respect to functional groups’ acidity; that is, carboxyl groups, carbonyl- and double-bond conjugated phenol hydroxyls. A distinctive response of absorbance at 210 and 278 nm on the ionisation of carbonyl- and double-bond-conjugated phenols was found. Simultaneous characterisation of lignin by molecular weight (MW) and chemical heterogeneity was performed. A non-uniform distribution of reactive sites over the MW distribution for commercial kraft lignin (Curan) was found employing the family of chemical composition distribution curves. The redistribution of extended chromophore structures over time along the lignin’s MW distribution causes a misrepresentation of the UV chromatogram pattern and influences the accuracy of MW values.

Which sites react first? Functional site distribution and kinetics on solid supports investigated using confocal Raman and fluorescence microscopy.

Fluorescence microscopy is a powerful technique for analyzing beads with very low loadings of fluorophores; however, the method is flawed when looking at more highly loaded beads as a result of severe problems with absorption. To probe distributions at higher loading levels, Raman spectroscopy avoids many of these issues. These studies show that there is a uniform distribution of reactive sites throughout the beads but that the spatial distribution of reacted sites depends on the polymer type, with a fine balance between reaction and diffusion rate.

Evidence for Localization of Reaction upon Reduction of Carbon Tetrachloride by Granular Iron

The distribution of reaction sites on iron particles exposed to water containing carbon tetrachloride has been examined by measuring the locations of reaction products. The uniformity or localization of reaction sites has implications for understanding and modeling the reduction of environmental contaminants by iron in ground water systems. Granular iron surfaces similar to those being used for environmental remediation applications were studied using surfaces analysis techniques to develop an understanding of the physical and chemical structure of the surface and oxide films. Scanning Auger microscopy and imaging time-of-flight secondary ion mass spectrometry revealed that granular iron exposed to carbon tetrachloride-saturated water exhibits chloride-enriched regions occurred at pits rather than on the passive oxide film on the metal. Understanding the nature of the local solute reduction sites will play an important role in modeling the kinetics of reaction at passive iron oxide films in environmental systems.

Site Distribution in Resin Beads as Determined by Confocal Raman Spectroscopy

Scanning confocal Raman spectroscopy was used to study the distribution of reactive sites within a resin bead used for solid-phase synthesis. The distribution of NH2 groups in aminomethylated polystyrene resin (APS) was determined by doping with varying amounts of 4-cyanobenzoic acid. The extent of loading was determined by both elemental analysis and ninhydrin assays. The spatial distribution of the coupled 4-cyanobenzamide within the bead was determined to an in-plane resolution of 1 microm and depth resolution of about 4 microm, using the strong Raman CN stretching vibrational transition at 2230 cm(-1). Dry and swollen beads were studied and the distribution was found to be essentially uniform throughout the bead in all cases.

Site distribution in resin beads as determined by confocal Raman spectroscopy.

Scanning confocal Raman spectroscopy was used to study the distribution of reactive sites within a resin bead used for solid-phase synthesis. The distribution of NH2 groups in aminomethylated polystyrene resin (APS) was determined by doping with varying amounts of 4-cyanobenzoic acid. The extent of loading was determined by both elemental analysis and ninhydrin assays. The spatial distribution of the coupled 4-cyanobenzamide within the bead was determined to an in-plane resolution of 1 microm and depth resolution of about 4 microm, using the strong Raman CN stretching vibrational transition at 2230 cm(-1). Dry and swollen beads were studied and the distribution was found to be essentially uniform throughout the bead in all cases.