Adsorption des cations a des potentiels anodiques

As is known from the literature, the nature of the cation has a marked influence upon the kinetics of the anodic process of the formation of persulphuric acid at a platinum electrode, occurring at such potentials where the adsorption of cations would not seem to be of essential importance. The study of the dependence of the rate of evolution of oxygen in HClO₄ solutions on the concentration and radius of the cation has shown that the adsorption of cations is connected with the covering of the electrode surface with chemisorbed oxygen resulting in a discontinuous rise of oxygen overvoltage. This adsorption effect can be explained by the dipole nature of the platinum/oxygen bond.

The phenomena observed during a change in the properties of surface oxides in the region of considerable anodic polarization are somewhat similar to those found in the initial stage of the oxidation of platinum, which was shown to impart to the electrode the ability to adsorb cations.

The absorption of organic cations at positively charged metal/solution interfaces depends to a large extent on the presence of primarily adsorbed surface active anions and on the interaction of the π-electrons of the aromatic nuclei with the metal surface. The latter effect is demonstrated by the difference of the adsorbabilities of anilinium and cyclohexylammonium cations on positively charged mercury.

Some inorganic cations, for instance thallium on mercury, or thallium and cadmium on platinum, display a specific adsorbability similar to the adsorbability of bromide or iodide anions.     

Base and acid sites in alkaline earth cation-exchanged X zeolites

Alkaline earth exchanged X zeolites are characterized by pyrrole chemisorption using FTIR spectroscopy. Lewis basic, Lewis acidic (cations) and also Brønsted acidic sites were detected in these zeolites. Based on a local environmental model, the charges on oxygens (basic sites) and cations are further calculated using the electronegativity equivalence method. The NH stretching frequencies of pyrrole chemisorbed on both basic and acidic sites are well correlated with these charges. This relationship supports the early idea that the alkaline earth cations in faujasite exist in the form of (MOH)¹⁺.     

Influence of anion adsorption on the action of inhibitors on the acid corrosion of iron and cobalt

It has been shown by the method of current/potential (i/) curves and by differential capacitance measurements that the chemisorption of halogen anions from H₂SO₄ solutions on cobalt leads, as in the case of iron, to an increase in the overvoltage of the cathodic reaction of hydrogen-ion discharge and of the anodic reaction of metal ionization. Due to a positive charge of the cobalt surface in H₂SO₄ solutions, organic cations are poorly adsorbed and ineffective as inhibitors, but in the presence of halogen anions their adsorption increases and they exert an additional inhibitive effect upon the above reactions, although less pronounced than in the case of iron.

Hydrogen sulphide accelerates the electrochemical reactions occurring on iron and cobalt electrodes. In small concentration in acid solutions, it increases the adsorption of organic bases, which become effective inhibitors. This leads to the disappearance of the accelerating effect of hydrogen sulphide. The distortion of the anodic i/ curves (decrease in the slope in the case of inhibitors and increase in the case of stimulators) is connected with the finite rate of the establishment of adsorption equilibrium and with the removal of the adsorbed organic substance together with the ionized metal atoms. A hypothesis is advanced that the acceleration of the anodic reaction of iron and cobalt ionization in the presence of hydrogen sulphide is due to the formation of a surface catalyst from the chemisorbed HS⁻ ions and the metal atoms.     

In Vitro Efficacy of Bacterial Cellulose Dressings Chemisorbed with Antiseptics against Biofilm Formed by Pathogens Isolated from Chronic Wounds

Local administration of antiseptics is required to prevent and fight against biofilm-based infections of chronic wounds. One of the methods used for delivering antiseptics to infected wounds is the application of dressings chemisorbed with antimicrobials. Dressings made of bacterial cellulose (BC) display several features, making them suitable for such a purpose. This work aimed to compare the activity of commonly used antiseptic molecules: octenidine, polyhexanide, povidone-iodine, chlorhexidine, ethacridine lactate, and hypochlorous solutions and to evaluate their usefulness as active substances of BC dressings against 48 bacterial strains (8 species) and 6 yeast strains (1 species). A silver dressing was applied as a control material of proven antimicrobial activity. The methodology applied included the assessment of minimal inhibitory concentrations (MIC) and minimal biofilm eradication concentration (MBEC), the modified disc-diffusion method, and the modified antibiofilm dressing activity measurement (A.D.A.M.) method. While in 96-well plate-based methods (MIC and MBEC assessment), the highest antimicrobial activity was recorded for chlorhexidine, in the modified disc-diffusion method and in the modified A.D.A.M test, povidone-iodine performed the best. In an in vitro setting simulating chronic wound conditions, BC dressings chemisorbed with polyhexanide, octenidine, or povidone-iodine displayed a similar or even higher antibiofilm activity than the control dressing containing silver molecules. If translated into clinical conditions, the obtained results suggest high applicability of BC dressings chemisorbed with antiseptics to eradicate biofilm from chronic wounds.     

NMR studies of supported metal catalysts

Oxide-supported metals constitute one of the most important classes of heterogeneous catalysts, and for this reason they have been investigated by many techniques: adsorption isotherms, IR of chemisorbed molecules, electron microscopy, EXAFS, etc. However, the fact that they have been studied by so many methods proves that no one technique is totally satisfactory. The physical and chemical properties of sufficiently small particles, in particular their magnetism, must depend on their size, whence the idea of using NMR to study them. Because of the theoretical and technical difficulties associated with the NMR detection of metals, and especially since only a few can be detected, NMR work has been mainly devoted to the indirect study of metallic properties, by the intermediary of easily detected chemisorbed phases (¹H, ¹³C) or of physically adsorbed probes (¹²⁹Xe). However, when the metal can be detected directly (mainly Pt), it is possible to relate the form of the NMR spectrum to the dispersion of the metal and to calculate the electron density of states at the Fermi level.     

Extraction of Lithium Using TBP and the Noncoordinating Cation Exchanger Tetraphenylborate: Principles of Selectivity from Sodium and Higher-Valent Cations

Extraction of lithium, calcium, and some other metal cations using tetraphenylborate/tributyl phosphate mixtures has been studied. The possibility of a qualitative change in the cation exchange extraction series towards the preferential recovery of lowly-charged (singly-charged) metal cations was shown for these systems. A change in the separation factors for singly- over multiply-charged cations in this system, compared to the known commercially available extractants such as alkylphosphoric, carboxylic, and other organic acids, is several orders of magnitude and increases when passing from doubly- to triply-charged metal cations, reaching, for example, values of 10³ and more for the lithium/yttrium couple. It was assumed that the change in the extraction series was caused by the instability of the salts of the multiply-charged metal cations compared to that for the singly-charged cations due to the ionic character of the bond with the extracted metals and structural problems caused by the large volume of the tetraphenylborate anion. The investigated system can serve as a model for the development of a new class of extractants having a higher selectivity for singly-charged metal cations, in particular, for lithium.     

SYNTHESIS AND CHARACTERIZATION OF THE ION EXCHANGE PROPERTIES OF A SPHERICALLY GRANULATED SODIUM ALUMINOPHOSPHATESILICATE

ABSTRACT

Spherically granulated sodium aluminosilicophosphate (APS) of the empirical formula Na₄Al₄PS₁₈O_46.5 18H₂O was synthesized by a gel method. The APS was characterized by elemental analysis, X-ray diffraction, TGA and ²⁷AI, ²⁸Si and ³¹P MAS NMR spectroscopy methods. Ion exchange of alkali, alkaline earth and some other divalent metal cations by APS was studied in batch conditions. It was found that the APS has a cation exchange capacity of 2.5 meq/g and exhibits rapid kinetics of ion exchange. The ion exchange isotherms of alkali, alkaline earth and some other divalent cations were determined and the corrected selectivity coefficients as a function of metal loading were analyzed. It was found that APS exhibits a high affinity for cesium ion, a moderate affinity for some heavy metal cations (Pb²⁺, Zn⁺) and a low affinity for alkali and alkaline earth metal ions. A testing of the APS in complex solutions suggests that it could be a promising exchanger for treatment of some specific nuclear waste and contaminated environmental and biological liquors from radioactive cesium and toxic heavy metal ions.     

Grain Boundary Oxidation in PTCR Barium Titanate Thermistors

The phenomenon of grain boundary oxidation in PTCR BaTiO₃ thermistors is discussed. In particular, the energy spectra of the surface states were calculated for different samples, and these were related to the nominal composition, the impurity content of the base BaTiO₃ powder used, and the prevalent atmospheric conditions during cooling and/or annealing. It is proposed that the interaction of manganese with oxygen creates deep-lying traps, and, in general, some proof is offered that the majority of the surface states are due to different oxidizing chemisorbed gases. It is believed that the ability of a particular sample to adsorb such gases in adequate amounts, and thus exhibit an appreciable PTCR effect, is related to the presence of acceptor-type dopants perferentially segregated onto the grain surfaces. Notably, the role of 3 d transition metal cations in this process is discussed in some detail.     

Solid Solutions of Silicon Oxide in Mullite

An explanation is proposed to account for the possibility of formation of solid solutions of silicon oxide in mullite. The difference in the diffusion coefficients of cations in complex oxides leads to the formation of a solid solution of the oxide containing the cation with the lower diffusion coefficient in the complex oxide and a phase containing the cation with the higher diffusion coefficient or rich in this oxide. Using yttrium or scandium additives, which are chemisorbed on the surface of aerosil (SiO₂ source), it is possible to make the rate of diffusion mass transfer of aluminum cations higher than that of silicon cations and thus obtain a solid solution of silicon oxide in mullite.     

Struktur-Aktivitätsbeziehungen von Hydrierkatalysatoren

Structure / Activity Correlations of Hydrogenation Catalysts The reason for different activities of group VIII metal catalysts in several hydrogenation reactions is unknown. The activity of group VIII metal catalysts decreases in the hydrogenation of crotonaldehyde to butyraldehyde in the following sequence Pd > Pt > Ni > Rh > Co; Ir. However in the hydrogenation of butyraldehyde to butanol the sequence of metals is inversely. The hydrogenations of crotonaldehyde and butyraldehyde proceed as ligand addition reactions. The reaction rate depends on the nucleophilicity of atomically chemisorbed hydrogen. Whereas the reaction rate in the hydrogenation of crotonaldehyde increases, the hydrogenation rate of butyraldehyde decreases with the nucleophilicity of atomically chemisorbed hydrogen.     

Kinetic study of gas-surface molecular exchange between CO/CO on metals

Isotopic switching technique has been used to study the exchange of gaseous CO* with chemisorbed CO molecules on metal surface. The kinetic curves observed on Re(0001) exhibit a nonlinear dependence of desorption rate on the surface coverage of CO. However, the desorption rate for CO on Rh(111) in the same temperature range is strictly linear with the coverage of CO. A concerted adsorption-desorption mechanism is postulated which emphasizes that the desorption of CO in enhanced by the pressure of gas phase CO* on the one hand, and may be retarded by the screening effect of the CO* molecule chemisorbed on the surface on the other hand. It is pointed out that the adsorption of CO* (or CO) and the desorption of CO (or CO*) on a surface at high coverage, i.e. the molecular exchange processes, are primarily taking place simultaneously and concertedly.     

Removal of Divalent Metal Cations and Their Mixtures from Aqueous Streams Using Micellar-Enhanced Ultrafiltration

ABSTRACT

Micellar-enhanced ultrafiltration (MEUF) is a novel membrane-based separation technique that can be used to remove multivalent metal cations from aqueous streams. In this technique an anionic surfactant is added to the aqueous stream containing the metal cations to be removed. The surfactant forms highly charged aggregates called micelles onto which the metal cations adsorb or bind. The aqueous stream is then passed through an ultrafiltration membrane with pores small enough to block the passage of the micelles and adsorbed metal cations. In this study, MEUF has been shown to remove divalent cadmium, zinc, copper, and calcium ions and their mixtures with rejections of at least 96%. A previously developed equilibrium binding model describes the results successfully. Under reasonable conditions the flux rates are not substantially below that of pure water, indicating the feasibility of MEUF for industrial application.     

A simple method to generate spontaneous chemisorption of metallic particles mediated by carboxylate groups from silylated oligomeric poly(amide‐imide)s

The present work describes oligomeric poly(amide‐imide)s (PAIs) containing several l ‐amino acidic residues and two silicon atoms in their repeat unit, whose carboxylate terminal group was chemisorbed onto metallic particles (Cu, Ag or Au) previously deposited in controlled conditions via physical vapor deposition. Thus, for each prepared polymer–metallic hybrid, the surface morphology, particle size distribution, and percentage of organic material, silicon and metal were studied using scanning electron microscopy and energy‐dispersive X‐ray spectroscopy. The results show that the hybrids are formed probably via electrostatic interaction between the carboxylate anions of the PAIs and nanoparticle cations. This bridging ligand was visualized using Raman spectroscopy and corroborated with X‐ray diffraction. Optical studies and resistivity measurements (conductivity) of each hybrid were developed using UV‐visible and the four‐point probe method, respectively. X‐ray photoelectron spectroscopy was used to study the oxidation states of the metallic particles at surface level. Thus, a simple and spontaneous protocol is proposed for the preparation of metallic particles stabilized in situ by an oligomer, a procedure that takes place from seconds to a few minutes. Finally, particle diameters were measured using atomic force microscopy in order to study possible agglomeration of the metallic particles with time. © 2017 Society of Chemical Industry     

Preparation of polysulfonebenzylthiourea-reactive ultrafiltration plate membranes and their rejection properties for heavy toxic metal cations

The asymmetric membranes were prepared via phase inversion method, by using chloromethyl polysulfone as membrane materials, polyethylene glycol (PEG) as pore forming agent to improve the morphology and function of resultant membranes, N,N-dimethylacetamide as solvent, and water as the extraction solvent. Then the highly qualified polysulfonebenzylthiourea-reactive ultrafiltration plate membrane was prepared successively through the reactions between the chloromethyl polysulfone matrix membrane and thiourea. The thiourea-functionalized polysulfone plate reactive ultrafiltration membrane was used for the rejection of heavy toxic metal cations such as Cd²⁺ and Zn²⁺ through the coordination of the thiourea group and heavy toxic metal cations, in which the effects of the morphological and the structure of the membrane on the rejection properties were investigated. The rejection conditions, including the concentration of heavy toxic metal cations, temperature and pH of the solution had significant effects on the rejection capacity of polysulfonebenzylthiourea-reactive ultrafiltration membrane. The reactive ultrafiltration membrane containing thiourea group can be conveniently recovered by dilute hydrochloric acid for coordination of heavy toxic metal cations, which would have wide application for the treatment of waste-water-containing heavy toxic metal cations. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Extraction of Zinc,Copper, and Lead Ions with a Zeolite Loaded by a Multidentate Schiff Base Ligand Followed by Flame Atomic Absorption Spectrometric Analysis

A multi-element preconcentration–separation technique for metal ions is established. The procedure is based on complex formation of metal ions with Bis(2-hydroxy-1-naphtaldimine)-N-diethylentriamine (BHNDT) on nanoporous Analcime as sorbent followed by their determination using flame atomic absorption spectrometry. The effects of analytical parameters like pH, flow rates, eluent types, and sorption capacity on the recoveries of metal ions are investigated. In order to predict the extraction ability of BHNDT for different metallic ions, DFT method is used to compute the binding energies of the cations to the title Schiff base compound. The calculated binding energies are in good agreement with the observed extraction power of the Shiff-base ligand.     

Preparation and surface characterization of nonsupported ruthenium catalysts

A study of the preparation of ruthenium powder catalysts, starting from ruthenium compounds from various sources, has been made. In many cases it is found that the extent of the chemisorbed hydrogen monolayer is lower than expected on the grounds of the BET surface area of the powder. From TGA, XPS, and EM observations it is concluded that this is caused by the presence of tightly bound chlorine and oxygen on the surface. These poisoning substances can be removed by reduction in hydrogen at room temperature, but above 700 K the metal surface is repoisoned by chlorine and oxygen originating from the bulk of the metal and diffusing to the surface. Methods are indicated to circumvent this difficulty and to prepare very pure samples.     

Potentials of zero charge,interaction of metals with water and adsorption of organic substances—III. The role of the water dipoles in the structure of the dense part of the electric double layer

Two types of solvent particles resident on the electrode surface have been considered: (1) associates of water molecules freely oriented along the electric field and (2) chemisorbed water dipoles oriented with the oxygen atom towards the metal surface. On the basis of this model, a semi-quantitative interpretation has been given to the dependence of the differential capacity of the dense layer on the electrode charge. It has been shown that a change in this dependence, when passing from mercury to gallium, can be described assuming the energy of interaction of chemisorbed water dipoles with the metal surface to increase.     

Pyrolysis of a lignite in an entrained flow reactor: 1. Effect of cations on total weight loss

The role of exchangeable metal cations (Ca, Mg, Na, K, Sr and Ba) in the rapid pyrolysis of a Montana lignite has been studied. The effects of variation of the cations present, and cation concentration, on weight loss and pyrolysis kinetics are presented. The reactor utilized was a dilute-phase entrained flow reactor. The reactor conditions decrease the degree of secondary reactions occurring to the primary volatile species and allow a clearer view of the fundamental pyrolysis processes. A model of the heat and momentum transfer in the reactor has been employed to estimate the particle time-temperature history. The results of pyrolysing the lignite samples display a significant role of metal cations. At 1173 K, the presence of metal cations resulted in a decrease in total weight loss at maximum residence time (0.25 s) from 50% (dry inorganic constituent free basis, dicf) for 0 wt% metal cations (acid washed) to 30% (dicf) (raw lignite, 2.8 wt% cations). Data are also presented concerning the maximum weight loss when only calcium, sodium or magnesium exchangeable cations were present. The apparent activation energy obtained from a first-order Arrhenius kinetic treatment is shown to be dependent on the presence of cations.     

Molecular Metal Clusters as Catalysts

Abstract

In earlier analyses [1–8] we have established a correlation between metal clusters and metal surfaces with chemisorbed molecules in the specific contexts of (1) the metal frameworks wherein the metal cluster core structures are fragments of cubic and hexagonal close packed or body centered cubic metal bulk structures; (2) ligand stereochemistry where the geometric features of ligands bound to clusters and to metal surfaces are similar; (3) thermodynamic features where the average bond energies for ligand-metal and metal-metal bonds are comparable, for a specific metal, in the metal cluster and the metal surface regime; and (4) mobility of ligands bonded to metal cluster frameworks and to metal surfaces. Nevertheless, there are sharp distinctions between surfaces and clusters. The average coordination numbers for metal-metal interactions and for metal-ligand interactions are distinctly different for clusters and for surfaces: generally, the former are larger for surfaces and the latter are larger for clusters. Additionally, the surface state is typically differentiated from the cluster state in the degree of coordination saturation—the metal atoms in the surface state are typically less coordinately saturated even for the states in which molecules or molecular fragments are chemisorbed at the surface than those metal atoms at the periphery of a molecular metal cluster. In the crucial chemical issue, metal surfaces are far more reactive than metal clusters. Metal surfaces exhibit a wide range and high level of catalytic activity whereas most metal clusters are catalytically inert, at least under modest reaction conditions, Most reported clusters are relatively stable and nonreactive; they are not the products of sophisticated synthesis procedures designed to generate highly reactive metal clusters. They commonly have been the products of reaction mixtures run at forcing conditions and are thermodynamically controlled, not kinetically controlled, products.     

Molecular Metal Clusters as Catalysts

In earlier analyses [1-8] we have established a correlation between metal clusters and metal surfaces with chemisorbed molecules in the specific contexts of (1) the metal frameworks wherein the metal cluster core structures are fragments of cubic and hexagonal close packed or body centered cubic metal bulk structures; (2) ligand stereochemistry where the geometric features of ligands bound to clusters and to metal surfaces are similar; (3) thermodynamic features where the average bond energies for ligand-metal and metal-metal bonds are comparable, for a specific metal, in the metal cluster and the metal surface regime; and (4) mobility of ligands bonded to metal cluster frameworks and to metal surfaces. Nevertheless, there are sharp distinctions between surfaces and clusters. The average coordination numbers for metal-metal interactions and for metal-ligand interactions are distinctly different for clusters and for surfaces: generally, the former are larger for surfaces and the latter are larger for clusters. Additionally, the surface state is typically differentiated from the cluster state in the degree of coordination saturation—the metal atoms in the surface state are typically less coordinately saturated even for the states in which molecules or molecular fragments are chemisorbed at the surface than those metal atoms at the periphery of a molecular metal cluster. In the crucial chemical issue, metal surfaces are far more reactive than metal clusters. Metal surfaces exhibit a wide range and high level of catalytic activity whereas most metal clusters are catalytically inert, at least under modest reaction conditions, Most reported clusters are relatively stable and nonreactive; they are not the products of sophisticated synthesis procedures designed to generate highly reactive metal clusters. They commonly have been the products of reaction mixtures run at forcing conditions and are thermodynamically controlled, not kinetically controlled, products.