1,720,994 research outputs found
Cobalt(II) adsorption on haematite at room temperature
No full textThe interactions of Co2+ with haematite are investigated as a function of the solution pH by means of acid-base titrations and direct adsorption measurements. Adsorption determinations show that Co2+ is adsorbed on haematite to increasing extents with increasing solution pH. The desorption of Co2+ from haematite is also found to be a function of pH as Co2+ can be desorbed on acidification of the solution; however, the adsorption is never fully reversible. From acid-base titrations the adsorption of Co2+ appears to modify the primary H+-OH- adsorption equilibria of the oxide surface. © 1983
Influence of alkali metal cations on the rate of oxygen evolution from a mixed Mg-LiCoO2 oxide
No full textO2 evolution from a mixed Mg-LiCoO2 oxide is investigated in base solns. of alkali metal cations (0.1-2 M LiOH, KOH, or CsOH) without added supporting electrolytes. Tafel slopes range 30-50 mV/decade, scarcely related to the used alkali. However, Tafel lines are displaced to more pos. potentials in passing from CsOH to KOH and LiOH at the same concn., and moreover, reaction orders with respect to OH- decrease from .apprx.3 to .apprx.1 along the same alkali sequence with fractional rather than integer values. The reaction mechanism is examd. on the assumption that Temkin-type adsorption conditions apply to the many reacting intermediates possibly involved. Anal. equations representing Tafel slope and reaction order in Temkin conditions are derived and reported for many frequently adopted O2 evolution pathways, most of them for the first time. From these equations, the exptl. behavior is reconciled with a const. mechanism (Kobussen's path) in which the rate-detg. step varies depending on the alkali cation, shifting forward in the sequence of elementary steps from early positions in CsOH and KOH to the last one in which mol. O2 is released in LiOH. Cation interactions with reacting surface sites apparently intervene to modify the reaction activation free energy profile, stronger for Li+ than K+ and Cs+. Structural relations with Li+ (and Mg2+) sites in the oxide lattice may be involved. However, the interaction sequence, Li+ > K+ > Cs+, is similar to that obsd. at the interface of many structurally and chem. unrelated oxides suspended in water and is attributed to entropic contributions from interactions of water mols. bonded in the hydration ion cosphere and at the oxide-soln. interface
Hydrothermal preparation of goethite crystals
No full textThe precipitation of goethite particles by a hydrothermal procedure is described. The reaction involves the oxidation of ferrous ion solutions with hydroxylamine salts at 85 °C. The phase purity of the oxide is discussed as a function of the solution composition, and in particular of the pH and of the different anions. Scanning electron microscope examinations show that goethite particles are acicular crystals with a rather narrow size distribution. The crystals show prismatic shape and appear to be elongated in the [001] crystallographic direction. The attribution of 110 form to the side faces of the prisms is proposed. © 1985
Anodic deposition of γ-FeOOH layers on inert metal electrodes
No full textThe anodic oxidation of Fe2+ ions in neutral and alkaline solutions was studied by using a gold or platinum rotating disk electrode in the region of limiting currents. The reaction occurs with the quantitative deposition on the base metal of a ferric oxyhydroxide (γ-FeOOH). The effects introduced by the different experimental variables on the limiting currents are reported. The kinetic contributions of the mass transport and of the subsequent reaction steps are individually examined. It is proposed that the reaction scheme involves the coupling of two diffusion steps occurring in solution within the Levich layer and across the thickness of the deposited oxide. These two reaction steps are kinetically connected by the ion transport of Fe2+ ions across the interfacial potential difference; this latter reaction step is considered to introduce the pH dependence of the overall reaction. © 1983
Temperature induced phase transformation of metastable Fe(OH)3 in the presence of ferrous ions
No full textReactivity of ferrous and ferric ions has been investigated by means of re dox potential measurements at room temperature and at 85°C. Results show that in the pH range 6.5-8.0 the prevailing redox system at room temperature is ra presented by Fe(OH)3/Fe2+; upon heating, the system transforms to Fe304/Fe2+ by a reaction which does not seem to involve the Schikorr reaction. The change of composition of the solid phase is interpreted as a phase transformation of Fe(OH)3 due to an interaction with Fe2+
Water-dependent structural and chemical relaxation of bulk Co3O4 from cobalt nitrate decomposition
No full textThe cell size of Co3O4 powders from exhaustively long cobalt nitrate decompn. (T = 260-850 °C) is shown to depend on heating temp. and to expand upon prolonged room temp. aging (1 yr). By means of TGA and gas mass analyses, water from the starting salt is bound to the product oxide to various extents and is spontaneously released over time. Excess non-stoichiometric oxygen is left unaffected. A relation is obsd. between expanding cell size and decreasing bound water, suggesting that water is dissociatively accommodated in the lattice. The result is not within current defectivity models for Co3O
Oxide electrodes. A new technique to bind oxide powders onto Au substrates
No full textA technique is described to obtain supported oxide powder electrodes. Preformed powders are first embedded into a gold substrate, as reported by Totir et al. and then made to adhere in place by heating at T .apprx. 260-300°C in flowing oxygen. This latter step is of the greatest importance to obtain reliable electrodes for use in aq. electrolytes. Electrode stability and reproducibility are examd. on voltammetric and O2 evolution results using Co3O4 as a sensitive test oxide material. Many independent Au-Co3O4 electrodes compare favorably with one another and with graphite-supported electrodes from an unrelated, room temp. binding technique. Substrate and oxide-substrate junction effects are minimized
Precipitation of nickel ferrites: Nature of intermediate reaction steps
No full textOne of the reactions, by which substituted ferrites are formed, (controlled oxidation by nitrates of Fe2+ in the presence of the substituent cations) is examined using the data referring to the precipitation of a family of nickel ferrous ferrites. The precipitation of all the oxides involves two distinctly different pH regimes connected by a very sharp pH transition. The transition occurs for only fractional additions of the oxidizing ions and is a continuons function of the Ni Fetot ratio
Mass transport electrophoresis: An improved apparatus
No full textA mass-transport electrophoresis apparatus is proposed by which high stability with time of the impressed current is achieved. This is obtained by introducing a salt bridge between the inner electrode compartment and the chamber where enrichment of the colloid occurs. This disposition allows the use of concentrated solutions of electrolytes in which reversible behavior of the employed electrodes is easily obtained; in the mean time, it is possible to avoid both the growth of films at the electrode surface and the electrical transport of the ions produced by the electrode reaction into the colloidal suspension, thus preventing possible coagulation phenomena of the colloid. © 1978
Effect of thermal treatment on the EPR spectrum and on catalytic properties of pure Co3O4
No full textThermal treatment of Co3O4 decreases the number of cationic vacancies by reducing the oxygen excess in the ''as prepared'' sample. Correspondingly, a narrower Lorentzian-shaped EPR line is detectable after higher temperature treatment, because of a more efficient Co-O-Co super-exchange effect, favored by the Co2+ enrichment of the sample. Furthermore, the linewidth increases linearly with electron paramagnetic resonance (EPR) recording temperature. We attribute this effect to the onset of antisymmetric exchange in layer-like structures. Catalytic activity tests have been carried out at 323, 373, and 423 K on oxidation of CO by air oxygen as a probe reaction. Samples calcined at 536 and 773 K were tested. Both the catalysts proved to be practically inactive at 323 K. However, at 423 K the specific activity of the catalyst calcined at higher temperature was approximately twice as high as that shown by the sample calcined at lower temperature. Also, this effect has been attributed to the enrichment in Co2+ ions, which would act as catalytic centers when located at the solid surface
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