No abstract

Ultrafine powder samples of Cu, Fe, Ni, Mo and W were examined voltammetrically in aqueous suspensions with hanging mercury drop electrode. According to chemical interaction of the metallic powder particles with the dispersing medium the voltammetric curves either consist of a set of individual current peaks given by particulate reduction of surface oxides on impingements of particles from their suspension upon the electrode, or correspond to electroactivity of dissolved products of chemical reaction between particles and the medium. In connection with increasing importance of metallic powders in contemporary industrial technology we attempted to find a way of their characterization by an electrochemical method. Although the present literature on electrochemistry of metallic nanoparticles is fairly rich (e.g. [1 -3]), we did not find so far any paper, which would compare voltammograms of powder suspensions of different metals.When the dispersed electroactive particles are of the size of several nanometers, their diffusion coefficients are near to those of molecules, and their motion towards the electrode is controlled by the rate of diffusion, as in the case of true solutions [4]. In our present case the dimensions of the particles are of the order of hundred nanometers and their movements in the solution follow more or less the laws of Brownian motion. The voltammetric curves of some of the powder samples dispersed in neutral and/or alkaline solutions displayed characteristic irregular peaks of irreversible cathodic current, due obviously to electroreduction of surface oxides, notoriously present on the surfaces of metallic powder particles kept open to air (cf. [5]). Suspensions of different insoluble compounds had been studied several decades ago with polarography [6,7] and with voltammetry [8]; in contradistinction to those results we obtained current signals without simultaneously stirring the suspensions, both when using dropping mercury electrode or HMDE. That was due to the smaller size of our particles, which allowed their relatively faster thermal motion in the suspension.In cases when the particles do not undergo a chemical reaction with the supporting solution, their electroactivity is limited to reduction of the surface oxides by electron transfer when they, in course of their thermal motion, approach the electrode surface sufficiently closely. This approach depends on thermal energy of the particle, and on electric charge and surface tension of the mercury electrode; it varies from a direct physical contact at positive potentials, to a distance, which allows electron tunneling, at negative potentials. Starting from the positive side, with increasing negative potential the surface tension of the mercury electrode increases to its maximum at the point of zero charge. There the particles have the highest affinity to the mercury surface and hence the highest tendency to adsorb on it. Further in the negative direction the surface tension gradually decreases, and the orientation of polarized water...

The regularities of the oxidation of electroexplosion iron nanopowder, produced by the wire elec tric explosion, heated in air under conditions of linearly increasing temperature and in the isothermal mode are examined. The oxidation process under conditions of linear heating is demonstrated to occur stepwise due to the combined influence of the fractional composition of the powder, its phase composition, and the struc ture of the oxide layer formed on the surface of the particles. It is shown that, under isothermal conditions (250-600°C), the oxidation of the nanopowder, as opposed to micron sized powders, obeys a linear law and proceeds in the kinetic regime with E a = 100 ± 7 kJ/mol. The conditions of thermogravimetry analysis at which the thermal self ignition of the nanopowder occurs are determined. Based on the numerical evaluation of the sample surface heating parameter, the experimentally measured critical temperature is verified.

While following voltammetric behavior of ultrafine metallic powders, we realized that the results obtained with six metals (Al, Fe, Ni, Cu, Mo, and W) were providing us with material for treating the connection between electroactivity and the state of dispersion of matter. The electroactive species were metallic oxides formed spontaneously on surfaces of the metallic powder particles, and we could follow their electrochemical reactivity in the states of coarse and fine suspensions, colloids, and true solutions. Each state of dispersion can be characterized by a distinctive form of electroactivity, which we illustrated by experimental results with all six metals.

No abstract

No abstract

No abstract

The interaction of urea and nitrogen oxides produces N 2. This reaction is used in neutralization of nitrogencontaining technological gases and wastes of power, metallurgic, and metal working plants. 1 It is generally accepted 2-4 that urea reacts with nitrous acid according to the equation CO(NH2) 2 + 2 HNO 2 9 2 N 2 + CO 2 +3 H20.However, in acidic solutions urea may also undergo hydrolysis producing CO: and NH4 + ions. 4 Available data on the product composition and kinetics of urea reactions with nitrogen oxides and nitrous acid are insufficient for complete description of occurring processes.We investigated in detail the interaction of HNO 2 with urea in aqueous solutions of HNO3 containing 0.05--0.2 M CO(NH2) 2 and NaNO2 and 0.02--1.0 M HNO 3 with magnetic stirring. Time variation of the solution contents was monitored by titrimetry (determination of H +, HNO 2 and CO(NH2)2) and ionometry . .with use .of.ion-selective electrodes _(NH4~" .andNO.if).Gaseous products were identified by IR spectroscopy and with the use of a ~I'esto-3Y' automatic gas analyzer. We found that the reaction between CO(NH2)2 and HNO 2 within a temperature range of 10--50 ~ follows the stoichiometric equation CO(NH2) 2 + HNO 2 + H + ,, 9 N 2 + NH4 + + CO 2 + H20.The reaction orders with respect to CO(NH2)2 and HNO 2 are equal to unity. The effective reaction rate constant keff grows proportionally to the H * ion concentration within the range from 0.01 L tool -t s -I when [H +] = 0.02 tool L -I to 0.30 L mo1-1 s -1 when [H +] = 1.0 mol L -1 (25.0+_0.1 ~ In our opinion, the essential determining factor in the H + concentration range under study is the fraction of the protonated form of CO(NH2) 2. Assuming that the reaction involves H2NCONH3 +, we calculated the rate constant /c o independent of the H + ion concentration with the use of the dissociation constant of the urea protonated formS:The concentration of the urea protonated form is determined by the expression K+[H +1