Kinetics and mechanism of deposition of zinc from zincate in concentrated alkali hydroxide solutions

Galvanostatic investigation has been carried out of the kinetics of the reaction on zinc amalgam hanging mercury drop immersed in alkaline zincate solutions at KOH concentrations ranging from 1 to 14 normal. Three kinds of data have been extracted: (i) the pseudo-capacitance as a function of overpotential from the portion of the charging curves prior to plateaux, (ii) the Tafel plots from quasi-steady state values at the plateaux and (iii) the Sand's products as function of current density from transition times. In (i), the pseudo-capacitance was found to increase with overpotential, in (ii), two slopes (60 mV dec^?1 and 120 mV dec^?1) have been found in the cathodic direction and a limiting current in the anodic one; in (iii), the Sand's product was found to decrease with increasing (i). On the basis of the above findings the reaction mechanism has been suggested to be of the cece type, with the chemical dissociation of the intermediate univalent zinc complex in between the two elctrochemical steps as rate-determining. A modification of the method of evaluating the reaction orders was introduced to account for a simultaneous change of more than one activity of reactants in concentrated solutions. Using this, reaction order was found to be 2 with respect to both OH^? ions and water, suggesting that the electroactive species is not only reduced in the number of ligands but also dehydrated with respect to the prevailing species. The anodic limiting current appears to be due to the accumulation of Zn(OH) species to cover all the surface free of adsorbed Zn(OH)₂. Exchange cd and rate constants of all the four steps of the reaction have been estimated.     

Analysis of catalyst effectiveness in trickle-bed reactors processing volatile or nonvolatile reactants

The problem of diffusion and first-order reaction in catalyst pellets whose exterior surface is partially wetted, as encountered in trickle-bed reactors; is formulated to account for all cases of interest such as gas or liquid limiting reactants, finite or negligible external surface resistance, and volatile or nonvolatile liquids. It is shown that the problem of evaluating catalyst effectiveness for a slab, cylindrical, or spherical shape results in a general dual-series equation which for a given geometry may be reduced to a variety of cases of interest by proper assignment of Biot numbers for dry and wetted surfaces. The catalyst effectiveness factor is given by a simple expression that depends solely on the Thiele modulus and first series coefficient which is found by numerical solution of the dual-series using the method of weighted residuals. A comparison between effectiveness factors predicted by approximate formulas to those given by the more accurate numerical solutions suggests that these former expressions underestimate the catalyst effectiveness and consequently are safe to employ for initial design estimates. The opposite effect of contacting efficiency on catalyst effectiveness for nonvolatile liquid limiting and gas or volatile liquid limiting reactants is illustrated and quantified. Possible pitfalls of employing reaction studies to determine contacting are also discussed.     

On the accurate calibration of an electrochemical oxygen meter in the 10−7 bar range

A method for the accurate standardization of an electrochemical oxygen meter, in the 10^–6-10^–7 bar range is described. The method is based on the use of an electrochemical oxygen leak and a chemical getter pump. In this manner problems connected with traces of hydrogen, originating from the pyrolysis of water, and traces of oxygen of unknown origin are overcome.     

Calculation of local current densities and terminal voltage for a monopolar sandwich electrolyser: application to chlorate cells

Chemical engineering calculations are performed for a new type of monopolar electrolyser with power leads located on its sides, used for chlorate production. The calculation gives the value of the total cell voltage as well as of local current densities for given current load and given electrode dimensions, interelectrode gap etc. On this basis the optimization of the system is possible.List of symbols a _A constanta for the calculation of anode potential, see Equation 2a (V) - a _K constanta for the calculation of cathode potential, see Equation 2b (V) - b _A constantb for the calculation of anode potential, see Equation 2a (V) - b _K constantb for the calculation of cathode potential, see Equation 2b (V) - a _A,a _K constants in linearized Equations 3a and 3b (V) - b _A,b _K constants in linearized Equations 3a and 3b ( cm²) - d electrode distance (cm) - D _G average diameter of bubbles at pressureP ₀ (cm) - F Faraday's constant; 964 96 C - F _G,F _E effective cross-section of inter-electrode channel for the flow of gas and the electrolyte, respectively (cm²) - F _p cross-section occupied by current leads placed in inter-electrode channel for one cell (cm²) - F _T total cross-section of inter-electrode channel for one cell (cm²) - F _R cross-section for one copper rod outside the cell (cm²) - g acceleration due to gravity; 981 cm s^–2 - I _o total current (A) - I _T current flowing through one cell=I _o/n _c (A) - I _x current flowing through an electrode strip at a heightx and a distancey from the origin (A) - I _T,x current flowing through an electrode strip at a heightx andy=0 (A) - I _p,x current consumed by electrochemical reaction at a heightx betweeny=0 andy=y (A) - i _x,y local current density (A cm^–2) - _x average current density at a heightx (A cm^–2) - ¯i average current density=I _T/wL (A cm^–2) - K ₁ criterion cf. Equation 32 - K _1,x criterion cf. Equation 25 - K ₂ criterion cf. Equation 26 - K ₃ criterion cf. Equation 22 - K _3,x criterion cf. Equation 21 - K ₄ criterion cf. Equation 33 - K _4,A criterion cf. Equation 27 - K _4,K criterion cf. Equation 28 - K _4,L criterion cf. Equation 29 - K ₅ criterion cf. Equation 38 - L height of electrode (cm) - L _A,L _K length of copper rods outside the electrolyser (cm) - n _A number of equivalents per mole for anodic process yielding a gaseous phase - n _C number of cells in electrolyser - n _K number of equivalents per mole for cathodic process yielding a gaseous phase - n _r number of copper rods outside the electrolyser - P local pressure (atm) - P _o pressure on top of electrolyser (atm) - P _w pressure of water vapour in equilibrium with electrolyte (atm) - R gas constant (cm³ atm [mol° K]^–1) - (Re)_G Reynolds number for bubbles - S _A anode thickness (cm) - S _K cathode thickness (cm) - s _E specific gravity of electrolyte (g cm^–3) - s _G specific gravity of gas (g cm^–3) - s _M specific gravity of gas-electrolyte mixture (g cm^–3) - T absolute temperature (° K) - U _A,U _K ohmic voltage drops in anode and cathode, respectively (V) - U _LA,U _LK ohmic voltage drop in anode and cathode leads, respectively (V) - U _M ohmic voltage drop in the electrolyte in thez direction (V) - U _T cell voltage (V) - U _AB,U _CD ohmic voltage drop between copper rods and electrodes, see Fig. 5 (V) - _E rate of electrolyte flow in inter-electrode channel (cm s^–1) - _ET rate of electrolyte flow in inter-electrode channel at the top (cm s^–1) - _G rate of gas flow in inter-electrode channel (cm s^–1) - _GT rate of gas flow in inter-electrode channel at the top (cm s^–1) - _R velocity of bubbles corresponding to buoyance (cm s^–1) - V _E volume flow rate of electrolyte in inter-electrode channel (for one cell) (cm³ s^–1) - V _G volume rate of gas flow in inter-electrode channel (for one cell) (cm³ s^–1) - V _GT volume rate of gas flow in inter-electrode channel at the top (cm³ s^–1) - w width of the active surface of an electrode (cm) - w _A width of the inactive part of an anode (cm) - w _K width of the inactive part of cathode (cm) - w _AE width of the inactive part of an anode embedded in electrolyte (cm) - w _KE width of the inactive part of a cathode embedded in electrolyte (cm) - x, y, z length in the direction of co-ordinates - , _T volume fraction of bubbles at a heightx and at the top - _A, _K anodic and cathodic potentials - _A anodic current efficiency for gas evolution - _K cathodic current efficiency for gas evolution - v kinematic viscosity of electrolyte (cm² s^–1) - _A specific resistance of an anode ( cm) 5.76×10^–5 - _K specific resistance of a cathode ( cm) 1.21×10^–5 - _E specific resistance of electrolyte ( cm) - _M specific resistance of a gas-electrolyte mixture between electrodes ( cm) - _A ratio of active anode surface to the productwL - _K ratio of active cathode surface to the productwL     

The effect of foreign atoms on the properties of electrolytic zinc powders

Kinetic and morphological studies have been made of the deposition of zinc powder on steel substrate, from pure alkaline zincate electrolytes and with small amounts of lead and tin ions in the electrolyte. The rate of co-deposition of hydrogen has been followed volumetrically and the current efficiency of zinc deposition calculated as a function of potential.The addition of lead and tin was found to have a profound effect on the properties of the powder. The dendritic powder, obtained from pure zincate solutions, changes with the addition of lead into a mossy deposit consisting of very small crystallites. Both additives increase the specific surface area of the powder and the average lattice parameters. Hydrogen evolution per unit real surface area and hence the corrosion of the powder are somewhat inhibited by both additives.Reported at the Symposium on Electrocrystallization, 140th Meeting of the Electrochemical Society, Cleveland, Ohio, October 1971.     

Study of the mechanical properties of TiCN–WC–CO hardmetals by the interpretation of internal friction spectra

TiCN–WC–Mo–Co mixed carbide hardmetals have an interesting application potential for cutting tool fabrication combining the high toughness of WC–Co with the resistance to plastic deformation of TiCN–Co cermets. Mechanical spectroscopy (MS) is used in order to separate the effects of the constituents on the mechanical properties. Internal friction (IF) spectra are measured in a torsion pendulum on WC–TiCN–Mo–Co samples where TiCN/WC ratio is varied as well as the Co content. Six components of the characteristic IF spectrum of WC–TiCN–Mo–Co have been identified and interpreted. Two peaks are located in the cobalt, two peaks in the TiCN phase and two peaks in the ceramic grain boundaries. Four temperature domains are defined depending on the mechanical behaviour: brittle (I), anelastic (IIa), limited plasticity (IIb) and extended plasticity (III). The anelastic domain is characterized by the bulk deformation of cobalt. In the limited plasticity domain, both cobalt and TiCN are deformed by dislocation movement. The high temperature extended plasticity should be attributed to grain boundary sliding in the ceramic phase (mainly WC) enhanced by cobalt diffusion in the grain boundaries.     

Fracture toughness of coated TiCN–WC–Co cermets with graded composition

Mixed TiCN–WC–Co cermets are developed to improve at the same time toughness and resistance to deformation of materials for cutting tool applications. Moreover, graded materials joining optimum properties according to the functional part of the tool are elaborated. To this end, TiCN–WC–Co cermets are interesting because they develop a WC–Co layer at the surface during the sintering. This tough layer at the surface limits the crack propagation that can lead to the rupture of the tool. Such materials show a good resistance to the deformation in the bulk and a good toughness at the surface, where the cracks are initiated upon machining. Cutting tools are often coated by CVD to improve the wear resistance. This paper proposes a method to measure the toughness K_IC at high temperature by using this CVD coating for initial crack formation. The coating thickness is the precrack length of traditional K_IC measurements. Samples are fractured by three point bend tests. The rupture stress is measured by Weibull statistics. This method is particularly interesting for graded structure materials where the influence of surface layers on toughness must be estimated. The comparison between cermets with and without WC–Co layer shows an improvement of 28% of the toughness when the layer is present. The possible bias of internal stresses on the results is discussed.     

Diffusion and filtration properties of self-assembled gold nanocrystal membranes

Close-packed nanoparticle monolayers have recently been shown to form mechanically robust, free-standing membranes. We report the first measurements of molecular transport through such ultrathin sheets, self-assembled from dodecanethiol-ligated gold nanocrystals. For aqueous solutions we find filtration coefficients 2 orders of magnitude larger than those observed in polymer-based filters, sieving of large solutes, and for smaller solutes a pronounced dependence of rejection on being charged. These results open up new possibilities for controlled delivery and separation of nano-objects.