A thermodesorption method for the investigation of hydrogen adsorption on electrodes; its application to tungsten carbide,along with electrochemical methods

A method for direct determination of hydrogen adsorption on large surface area electrodes or on electro-conductive powders in electrolyte solutions, at any given potential, is proposed. It consists of hydrogen desorption from the electrode by heating (thermodesorption) after complete adsorption and a quantitative chromatographic analysis of the gas collected during thermodesorption. The proposed method was used in this work to investigate hydrogen adsorption on tungsten carbide powder in a H₂SO₄ solution. The potential-dependence of adsorption was found, confirming the authors previous suggestion of an extensive coverage of tungsten carbide by chemisorbed hydrogen and of the potential region of adsorption (up to 0·35–0·4 V).The tungsten carbide surface was also investigated by the method of charging and potentiodynamic curves. Reversibility of these curves up to 0·35–0·4 V is shown. Hydrogen adsorption is correlated with the quantity of electricity calculated from the charging curve and it is shown that a considerable portion of the charge is associated with hydrogen chemisorption but some portion of the total charge is consumed even in the hydrogen region for other non-defined redox processes occurring on the tungsten carbide surface.     

Extraction of some mineral acids by tri-n-octyl phosphine oxide

The distribution equilibria of hydrochloric, nitric, perchloric, sulphuric and phosphoric acids between aqueous solutions and a solution of tri-n-octyl phosphine oxide (TOPO) in organic solvents have been investigated. The organic phases were studied by infrared and nuclear magnetic resonance spectroscopy. As a result, it is found that the extraction of these acids is governed by the formation of HCl·TOPO, HNO₃·TOPO and HClO₄·TOPO, of H₂SO₄·2 TOPO and H₂SO₄·TOPO, and of H₃PO₄·3 TOPO, H₃PO₄·2 TOPO and H₃PO₄·TOPO, respectively.     

Combined photoelectrochemical conditioning and surface analysis of InP photocathodes: II. Photoelectron spectroscopy

The photoelectrochemical modification of p-InP(111) surfaces in HCl and H₂SO₄ is investigated. In 0.5 M HCl a photoreduction/oxidation procedure via cyclic voltammetry leads to a solar energy conversion efficiency of the p-InP/vanadium^2+,3+—4 M HCl/C photoelectrochemical solar cell (PECS) of η=11.6% in natural sunlight. The optimisation procedures are followed by surface analysis methods performed ex-situ at the modified InP electrodes after cathodic and after anodic polarisation in the supporting acidic electrolytes and in the redox solution. X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED) are used. At cathodic polarisation in H₂SO₄, XPS shows formation of indium metal and In_x(SO₄)_y compounds, whereas in HCl a stable interfacial film consisting of indium monochloride to a smaller extent of indium oxides is formed. This film is optimised by cycling in HCl having a thickness of 10±3 Å. Cyclic polarisation in the vanadium redox electrolyte changes the composition and thickness. Whereas in HCl, the optimised interphase consists of 0.7 parts InCl and 0.3 parts In(PO₃)₃, phosphates are absent after optimisation in V^2+/3+ and more indium oxide is found. The film thickness is reduced to 6±2 Å for the InCl component and about 0.6 monolayers of indium oxide are found. UPS measurements are mainly used to derive an energy band diagram of the semiconductor/film/electrolyte junction. The results show in that the stabilisation of the film in chloride containing electrolytes can be explained by the relative conduction band positions of p-InP with respect to cathodic decomposition energies. Since the electron affinity increases with surface phase formation in HCl, whereas it stays low in sulphuric acid, the ongoing corrosion in the latter solution can be understood, the LEED data show that the surface phase is ordered with lattice constants close to InP, indicating the formation of a hitherto unknown InOCl phase. According to the results we propose reaction mechanisms for dissolution and stabilisation and present schematic energy band diagrams which explain the corrosion processes in H₂SO₄ and the passivation phenomenon in HCl. The influence of the properties of the interfacial film on the functioning of the solar cell is outlined.     

On adsorption and electro-oxidation of some compounds on tungsten carbide; their effect on hydrogen electro-oxidation

Adsorption and electro-oxidation of carbon monoxide, ethylene, acetylene, and hydrogen sulphide on tungsten carbide, in solutions of these compounds in 1 N H₂SO₄, have been investigated. It was found that CO, C₂H₄, and C₂H₂ do not undergo adsorption and oxidation and do not affect adsorption and electro-oxidation of hydrogen. H₂S does not oxidise as well, and it does not displace adsorbed hydrogen in any measurable amounts, though it does inhibit electro-oxidation of molecular hydrogen. Methanol is inert on tungsten carbide like carbon monoxide and hydrocarbons. Electro-oxidation of formaldehyde and formic acid proceeds without apparent WC-surface coverage by the adsorbed compound.     

Combined photoelectrochemical conditioning and photoelectron spectroscopy analysis of InP photocathodes. I. The modification procedure

Photoelectrochemical modification procedures at p-InP (111)A-surfaces for improved photocurrent-voltage behaviour in acidic vanadium^2+/3+ redox electrolyte are presented. Preconditioning in H₂SO₄ and HCl results in marked differences of the I-V characteristics in the redox—as well as in the supporting electrolytes: in H₂SO₄, the photocathodic reaction of hydrogen evolution is accompanied by photocorrosion leading to metallic indium at the surface. Cyclic polarisation in HCl, however, results in an improved photovoltage and fill factor. A new two-step conditioning procedure is presented yielding a solar-to-electrical conversion efficiency of 11.6% in a two-electrode non-optimised configuration. Electrochemical analyses of the dark current-voltage curves and Mott-Schottky plots show shifts of the flatband potential due to the surface conditioning. In HCl-cycled electrodes, an increased barrier height Φ_bh, is observed whereas upon cyclic polarisation in H₂SO₄, Φ_bh decreases. In HCl, the formation of a passivating interfacial film of thickness of 5-7 Å is assumed, the thickness estimate depending on the assumed corrosion reaction. The film allows efficient charge transfer and also improves the anodic protection of the p-InP electrodes in forward direction. The chloride ion appears to strongly influence the surface chemistry with respect to film formation.     

Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives

In this work, cemented tungsten carbide nano-particles were prepared by a chemical method called acrylamide gel. In this process, first, a xerogel containing tungsten and cobalt oxide particles was synthesized. Then, it was carburized by a hydrogen reduction heating process.Acrylamide and N, N-methylene-bis-acrylamide monomers were used as an in-situ carbon. Ammonium meta tungstate (NH₄)₆H₂W₁₂O₄₀·xH₂O, and cobalt nitrate Co(NO₃)₂·6H₂O salts were used as the precursor.Both reduction and carburization reactions were carried out simultaneously and the formation of the intermediate phases of W₂C, Co₃W₃C, and Co₆W₆C led to decrease in the activation barrier.Transactions of reduction and carburization processes were studied by X-ray diffraction analysis at various temperatures. Accordingly, tungsten carbide phase formation was completed at 1100 °C. The formation of W–C and V–C bonds was verified by Raman spectroscopy. SEM images showed the average nano particle size of 50 nm.     

A green process for recovery of H2SO4 and Fe2O3 from FeSO4·7H2O by modeling phase equilibrium of the Fe(П)– –H+–Cl– system

Ferrous sulfate heptahydrate FeSO₄·7H₂O is a major waste produced in titanium dioxide industry by the sulfate process and has caused heavy environmental problem. A new green process for the treatment of FeSO₄·7H₂O was proposed to make use of iron source and recycle sulfate source as H₂SO₄. It was found that by adding concentrated HCl to the FeSO₄ solution, FeCl₂·4H₂O was crystallized out, which was subsequently calcined to produce Fe₂O₃ and HCl. Concentrated H₂SO₄ solution (about 65 wt %) was obtained by evaporating the FeCl₂·4H₂O‐saturated filtrate. To facilitate the process development and design, the solubilities of FeCl₂·4H₂O in HCl, H₂SO₄, and HCl + H₂SO₄ solutions were measured and the experimental data were regressed with both the mixed‐solvent electrolyte model and the electrolyte NRTL model. On the basis of the prediction of the optimum conditions for the crystallization of FeCl₂·4H₂O, material balance of the new process was calculated. FeCl₂·4H₂O and Fe₂O₃ were obtained from a laboratory‐scale test with about 70% recovery of ferrous source for a single cycle, indicating the feasibility of the process. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4549–4563, 2017     

Isomer Directed Synthesis of Two 3D Cadmium(II) Compounds: Structure Variation and Energetic Performance

Two inorganic mixtures of copper and sodium compounds have been synthesized and characterized with different measurement techniques. The thermal property of these mixtures has been studied to low temperature up to 223 from 573 K with DSC. The specific heat capacity of this mixture has been measured in atmospheric O₂ at a rate of 10 K min^?1 from 573 to 223 K and vice versa in two thermal cycles. The net specific heat capacity of these mixtures is found endothermic in first and second thermal cycles. The net specific heat capacity of 0.5Cu₂(PO₄)(OH); 3.5CuH(PO₄)₂·H₂O; 2NaHSO₄·H₂O (CuPHS) during first thermal cycle is ?71203.05 J kg^?1 K^?1 and in second thermal cycle is ?73881.67 J kg^?1 K^?1 between temperature range of 303–223 K. The net specific heat capacity of mixture 0.5Cu₂(PO₄)(OH); 3.5CuH(PO₄)₂; 2Na₂SO₄(CuPS) in first thermal cycle is ?21158.37 J kg^?1 K^?1 and in second thermal cycle is?45739.92 J kg^?1 K^?1 between temperature range of 298–573 K. As both mixtures are endothermic in nature in all cycles, it can be used as heat storage material.The average crystallite size of mixture 0.5Cu₂(PO₄)(OH); 3.5CuH(PO₄)₂·H₂O; 2NaHSO₄·H₂O and 0.5Cu₂(PO₄)(OH); 3.5CuH(PO₄)₂; 2Na₂SO₄ is ~47 and ~17.3 nm, respectively.     

Characterization and performance of melamine enhanced urea formaldehyde resin for bonding southern pine particleboard

Urea‐formaldehyde resins modified by melamine were synthesized by four catalysts (H₂SO₄, HCl, H₃PO₄, and NaOH/NH₄OH) with a F/U/M molar ratio of 1.38/1/0.074. Resin structure and thermal behavior were studied by ¹³C‐NMR and DSC techniques. For H₂SO₄, HCl, and H₃PO₄ catalysts, resins were prepared by two stage pH adjustment: the first pH stage was set at 1.25 (H₃PO₄ pH 1.60) and second pH stage was set at 5.0. For the NaOH/NH₄OH catalyst, the resin was set at pH 5.0 from the start. Of the four catalysts, HCl catalyzed resins, with the highest free urea and lowest free formaldehyde, consistently yielded the lowest formaldehyde emission; NaOH/NH₄OH catalyst resulted in the best IB strength tested at dry conditions and also after 24 h cold water soak and the lowest water absorption and thickness swell. The resins catalyzed with H₃PO₄ had the highest free formaldehyde and no free urea yielding the highest formaldehyde emission. Each DSC thermogram was proceeded by a weak exothermic peak and followed by an obvious endothermic peak. The exothermic peak temperatures were 125.0, 131.1, 111.4, and 125.2°C, and endothermic peak temperatures were 135.8, 147.6, 118.9, and 138.4°C, respectively, for H₂SO₄, HCl, H₃PO₄, and NaOH/NH₄OH catalysts. The close proximity of the peak temperatures of the exothermic and endothermic reactions strongly suggests that there is potential interference of heat flow between the exothermic and endothermic reactions which may impact resin curing. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Kinetics and mechanism of the hydrogen discharge on Ti in HCl-DMSO solutions

The kinetics of hydrogen discharge in solutions of HCl-DMSO is investigated within 10^?3 1 N HCl concentrations, in the presence of KClO₄ as supporting electrolyte. The cathodic slope is 0·120 V/decade for all solutions. The electrochemical reaction order with respect to the hydrogen ion concentration at E = ?0·6 V is found to be 0·52. The stoichiometric number is practically equal to 2. The experimental activation energy is 11·2 kcal/mole.The cathodic process is interpreted with the discharge reaction as rate determining step, followed by the adatom recombination reaction.     

Catalytic activity and capacitance of tungsten carbide as a function of its dispersity and surface state

The catalytic activity and total capacitance¹ of tungsten carbide (WC) have been studied on various samples without free carbon content. It was found that the true cd in hydrongen oxidation is about the same for all carbides, independently of the total volume content of carbon, starting materials for carbide preparation, temperature, and other technological parameters of carburization, as well as of the particle size (specific surface) of tungsten carbide. It was also shown that in the anodic polarization of tungsten carbide in H₂SO₄ in the presence of H₂, no true activation, ie no increase in the specific activity, takes place, whereas the observed current rise is due to an increase in the catalyst surface.A close dependence of the total çapacitance on the particle sizes of carbide, in the range of the largest ones (up to smooth WC) up to 0.1 μ dia (S ≈ 5m²/g) has been observed. A further decrease of the particle size involved almost no change in capacitance.     

Studies on the mechanism of the hydrogen electrode reaction by means of deuterium tracer—II. The reaction mechanism

The exchange reaction between deuterium gas and light water was conducted with Ni, Rh, and Pt catalyst. Through analysis of isotopic composition of the gaseous hydrogen during the exchange reaction, the exchange rates of the individual steps, H₂ ? 2H(a) and H(a) + B ? H⁺B + e, of the hydrogen electrode reaction were determined at various hydrogen pressures and solution pH's, where H(a) is a hydrogen adatom and B = H₂O or OH^?. The rates of the steps were widely different among the metals studied but, throughout these metals, the hydrogen pressure dependence of the rate of the first step was with the power of 1·0–0·9, and that of the second step, 0·2–0·5. Both the rates were independent of solution pH. Generally, the former step is virtually rate-determining under low hydrogen pressures, whereas the latter takes over the role with increasing hydrogen pressure. Under ordinary pressures, the first step is virtually rate-determining on Pt but neither step is singly rate-determining on Rh and Ni.     

Electrochemical reduction of acetone on mercury electrode in aqueous sulphuric acid solution

Electrochemical reduction of acetone in aqueous H₂SO₄ solution was studied on Hg electrode by galvanostatic, potentiodynamic and polarographic methods. Electrocapillary curves show that acetone molecule adsorbs to a monolayer at concentrations larger than 0·5 M in a potential range of 0 > V > ? 1·0 V (nhe). It orients with the positive end of the dipole toward electrode surface and extends an attractive interaction to its neighbours.Kinetics oberved by galvanostatic pulse method and polarography leads the following conclusions; (1) adsorbed acetone molecule undergoes elecrochemical reduction at potentials more negative than ?1·0 V (nhe), (2) reaction rate is the first and second order with respect to the amount of adsorbed acetone and proton activity, and (3) one electron transfer step determines the reaction rate (Tafel slope is 0·12 V). Reaction intermediate is concluded from the potentiodynamic study to be isopropanol radical, (CH₃)₂??OH, whose amount is proportional to that of adsorbed acetone and to a_H+. From the above results, the rate of the electrochemical reduction of acetone is H₂SO₄ solution is concluded as determined by the step, (CH₃)₂?OH(a) + H⁺ + e^? → (CH₃)₂CHOH.     

Studies of anodic dissolution and film growth on iron in acid chloride solutions

The rate of dissolution of Armco Fe has been measured as a function of time for deaerated solutions of constant ionic strength (HCl + NaCl).For pH < 1·5, no appreciable change of the measured polarisation resistance was observed. For 1·5 < pH < 3 the polarisation resistance gave straight lines when plotted against time. The slope of the lines is represented by a = 2·0√(C_OH^?.10¹⁴ ? 45) Ωcm²/min, where C_OH^? is the concentration of the bulk.The Tafel slopes were 64–70 mV when 0 < pH < 1·2.The experimental data are explained by two very similar reaction mechanisms. The theory presented involves the formation of two surface complexes denoted by FeOH Fe·H₂O and FeO Fe·H₂O. It is suggested that the complex FeOH Fe·H₂O serves as nuclei for film formation when pH > 1·5. This would explain the change of the reaction mechanism at pH = 1·5 from a 60 mV mechanism when pH < 1·5 to a 30 mV mechanism when pH > 1·5.The theory is supported by data from the literature. It gives a reasonable explanation of the nonstationary 60 mV Tafel slopes. The mechanisms are in harmony with a hyperbolic relation between anodic current density and time. The logarithmic law of film growth may be derived from the same principles.     

Effect of different acids during the synthesis of urea-formaldehyde adhesives and the mechanical properties of medium-density fiberboards bonded with them

The characteristics of urea-formaldehyde (UF) adhesives condensed by catalysis with four different acids, namely formic (HCOOH), hydrochloric (HCl), phosphoric (H₃PO₄), and sulfuric (H₂SO₄) acids, under alkaline–acidic–alkaline conditions at a molar ratio F/U = 1.12 were studied. The thermal curing properties of UF adhesives catalyzed with acid were characterized by differential scanning calorimetry at 10 °C/min heating rate. The resin structure examined by ¹³C-NMR spectroscopy showed that the resin catalyzed with HCl had a lower proportion of methylol groups, resulting in a lower level of formaldehyde emission. It was interesting to note that HCOOH resulted in the best overall mechanical properties of the medium-density fiberboard (MDF) panels. The HCl catalyst resulted in the poorest performance, providing the lowest internal bond strength, modulus of elasticity, and thickness swelling, with the exception of the free formaldehyde content. The resin catalyzed with H₂SO₄ had the highest free formaldehyde and the highest formaldehyde emission. H₂SO₄ and H₃PO₄ resulted in MDF mechanical properties relatively lower than for HCOOH. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47256.     

Hydrogen evolution reaction on sodium-tungsten bronzes

The hydrogen evolution reaction has been investigated on sodium-tungsten bronzes (Na_xWO₃) of various stoichiometries (0·34 < × < 0·89) in sulfuric acid solutions. Steady-state current-potential relationships and capacity-potential profiles were examined. The first cathodic steady-state current-potential curve in the ascending direction indicates the presence of two distinct Tafel lines with slopes of about ?2RT/F and ?RT/2F. This behavior is interpreted in terms of alternative reactions. At low overvoltages the mechanism is fast discharge-slow electrochemical desorption, while at higher overvoltages a fast discharge-slow recombination mechanism is indicated. The change of the rate-determining step from one potential range to the other is attributed to a change in the crystallographic structure of the electrode surface, eg, a lattice dimensional change of the hydrogen tungsten bronze. The Tafel slope for an electrode pre-polarized cathodically is close to ?RT/F. In this case the rate-determining step is proposed to be the surface diffusion of adsorbed hydrogen atoms to recombination sites. The x-value in Na_xWO₃ and the platinum content of the electrode were found to have no significant effect on the rate of the hydrogen evolution reaction.     

Effect of different catalysts on urea–formaldehyde resin synthesis

Four catalysts (H₂SO₄, HCl, H₃PO₄, and NaOH/NH₄OH) were studied in the preparation of melamine modified urea–formaldehyde (UFM) resins. ¹³C‐nuclear magnetic resonance spectroscopic analysis of the UFM resins at different synthesis stages revealed the polymer structure and detailed reaction mechanism. Three acidic catalysts (H₂SO₄, HCl, and H₃PO₄) enhanced the resin polymerization through the formation of various contents of methylene, ether linkages, and urons. H₃PO₄ yielded the most terminal ether linkages at the first stage and enhanced polycondensation by depleting all free urea and glycols to form the most linear methylene linkages NHCH₂NH in the end. However, at the initial synthesis stage, NaOH/NH₄OH catalyzed the formation of UFM prepolymer to a limited extent with a large amount of free urea left, and therefore produced the final polymer with relatively more substituted methylolureas and linear ether linkages. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40644.     

New open-framework beryllium phosphates with hcb,sql, and bnn topologies

Three new beryllium phosphates, namely, H₃aep·Be₂(PO₄)₂(H₂PO₄)·H₂O (1), H₂tmdp·Be₃(HPO₄)₃(PO₄CH₃)·2.25H₂O (2), and H₂dmpda·Be₃(PO₄)(HPO₄)₂(H₂PO₄) (3), were prepared under solvothermal conditions. These compounds display different open-framework structures with hcb (for 1), sql (for 2), and bnn (for 3) topologies, respectively. The formation of phosphate ester from in situ esterification reaction of phosphoric acid and methanol is noteworthy.     

High temperature reaction of kaolin with inorganic acids

Abstract

A new method is described in which raw materials prepared by mixing kaolin and concentrated acid are heated in a furnace at temperatures between 100 and 700°C. Experiments with HCl, HNO₃, H₃PO₄, and H₂SO₄ showed that only H₂SO₄ gave high reaction yields, which grew steadily as reaction temperature was raised. Reaction products were mostly Al₂(SO₄)₃. Leached kaolin samples showed BET specific surface areas of over 100 m² g^-1. Furthermore, reaction between kaolin and H₂SO₄ required no previous calcination of kaolin.     

Alternating current impedance characteristics of reduced and oxidised sodium-tungsten bronze electrodes

The differential capacity of sodium—tungsten bronze electrodes (Na_xWO₃ with x between 0·58 and 0·89) has been examined in helium-saturated 1 N H₂SO₄ using an ac impedance bridge. Low capacitance values obtained at potentials more anodic than 0·8 V vs nhe give a linear¹/C²vs E (Mott—Schottky) plot. Donor concentrations determined from the slopes of the Mott—Schottky plots show that the surface layer is depleted to sodium. A typical equivalent x-value of the semiconducting surface layer is less than 0·01. Strongly oxidised electrodes give Mott—Schottky plots with a break at a potential of about 1·5 V vs NHE, indicating that an increase in donor concentration occurs as the potentials is made more anodic. A similar effect has been reported for WO₃ and interpreted as an activation of certain donor levels at high anodic potentials. The flatband potential is about 0·6 V vs nhe and independent of the platinum content of the bronze electrode. At about 0·2 V vs nhe a high pseudo-capacitance peak is observed in the capacity vs potential curve. The pseudo-faradaic process associated with this maximum is believed to be hydrogen bronze formation.