Sintering and recrystallization of small metal particles. Loss of surface area by platinum-black fuel-cell electrocatalysts

Operation of high-surface-area platinum-black electrodes in hot concentrated phosphoric acid produces a loss of surface area with time, giving consequent loss of performance.

The loss of surface area is the result of sintering produced by surface migration of platinum atoms on the individual crystallites to sites of lower surface energy. As the sintering process is a surface phenomenon, potential changes cause changes in the surface tensions of the metal crystallites giving rise to variations in sintering rate. The rate of sintering is greatest at low potentials and smallest at high.

Carbon monoxide and phosphoric acid impurities adsorbed on the surfaces of the platinum crystallites retard the sintering at low potentials. Similar adsorption-induced changes in physical properties of materials (Rebinder effects) have been observed with ionic solids, but in only a few instances is this effect known to operate on metals. The presence of a strongly adsorbed molecule on the metal surface alters the inter-atomic bonding between the surface metal atoms and illustrates the dependence of the sintering mechanism on the mobility of the surface atoms. Changes in crystallite morphology accompany the sintering process and are considered to be the result of a surface rearrangement of ad-atoms or clumps with elimination of the [100] faces.

The activation energy for the sintering process is 25 ± 2 kcal/mole at 0·5 V and is not significantly changed by adsorbed carbon monoxide on the platinum.     

The investigation of passivating oxide layers on zirconium metal using small single shot anodic current pulses

Passivated metals such as Al, Nb, Ta and particularly Zr were anodically polarized in aqueous sulphate solution by single shot intensiostatic charge pulses, and the overtensions recorded on CRO photograms.

The initial slope of the overtension/time curves is given by the capacitance of the passivating oxide film.

The slope decreases when, with rising electrical field strength across the oxide film, the ionic current flow increases. In agreement with the known theories (Wagner; Mott; Verwey et al.) a straight line results, from plots of the overtension measured at charge densities of 10⁻⁴ C/cm² vs. the logarithm of the current density. From the slope of this straight line the thickness of the passivating layer can be obtained without interference of the surfaces roughness.

For the measurement of the thickness of the passivating oxide film of an individual specimen surface, two successive pulses were applied, the first of 3·3 × 10⁻⁵ A/cm² and the second of 3·3 × 10⁻³ A/cm² current density; the pulse duration was so chosen that each pulse represented the same charge density, 10⁻⁴ C/cm². Thus the modifications of the original surface state of the electrode metal caused by each pulse were negligible. From the difference of the maximum overtensions and the difference of the logarithms of the current densities of the two pulses the thickness of the passivating oxide film of the investigated sample can be calculated.

The method was checked measuring zirconium samples after gravimetrically controlled oxidation. The results obtained applying the method to differently prepared metal surfaces are discussed.     

A study on mechanism of charging/discharging at amorphous manganese oxide electrode in 0.1 M Na2SO4 solution

In the present work, the mechanism of charging/discharging at the amorphous manganese oxide electrode was investigated in 0.1 M Na₂SO₄ solution with respect to amount of hydrates and valence (oxidation) states of manganese using a.c.-impedance spectroscopy, anodic current transient technique and cyclic voltammetry. For this purpose, first the amorphous manganese oxide film was potentiostatically electrodeposited, followed by heat-treatment at 25–400 °C to prepare the electrode specimen with different amounts of hydrates and oxidation states of manganese. For as-electrodeposited electrode with the most hydrates, the anodic current transient clearly exhibited a linear relationship between the logarithm of current density and the logarithm of time, with a slope of −0.5, indicating that the charging/discharging is purely limited by Na⁺/H⁺ ion diffusion. From the analyses of the impedance spectra combined with anodic current transients measured on the hydrated electrode heat-treated at 25–150 °C, it was found that as the amount of hydrates decreases, the depth of cation diffusion in the electrode becomes shallower and the ratio of charge-transfer resistance to diffusion resistance also increases. This suggests that a transition occurs of pure diffusion control to a mixed diffusion and charge-transfer reaction control. For the dehydrated electrode heat-treated at 200–400 °C, the charging/discharging purely proceeds by the charge-transfer reaction. The reversibility of the redox reaction increases with increasing amount of hydrates and oxidation states of manganese, which provides us the higher power density. On the other hand, the pseudocapacitance decreases in value with increasing heat-treatment temperature, thus causing the lower energy density.     

The kinetics of copper dissolution in acetonitrile-water copper(II) solutions

The rate of dissolution of rotating copper discs in acidified copper(II) solutions prepared in an acetonitrile-water solvent (mole fraction ACETONITRILE = 0.·25) was measured in the temperature range 270–360 K. The rate of dissolution was found to be proportional to the square root of the rotation speed and to have an activation energy of 18 ± 2kJ mol⁻¹. The reaction is therefore considered to be diffusion controlled.

The reduction of copper(II) to copper(I) on a rotating platinum electrode, and the oxidation of copper(O) to copper(I) on a rotating copper electrode, were investigated. These two reactions together make up the dissolution reaction. From the intersection of the polarization curves, it was confirmed that the dissolution reaction was diffusion controlled. Dissolution rates calculated from the electrochemical measurements agreed with the kinetic measurements. Estimates of some electrochemical parameters in the acetonitrile mixed solvent were made.

The surface characteristics of annealed and unannealed copper discs which had been etched in the copper(II) solution are discussed briefly.     

Kinetics of aqueous oxidation of tantalum from chronopotentiometric measurements

Theoretical mechanisms and growth laws for the initial oxidation of metals are briefly reviewed. The oxidation of tantalum was studied chronopotentiometrically at temperatures between 0°C and 40°C in an oxygen-free electrolyte.

The probable rate-determining steps for the two different growth laws observed are deduced by comparison of the time dependence of the experimentally determined local oxidation current to theoretical local-current/time functions derived from assumed growth laws.

Tantalum oxidizes initially according to a direct logarithmic law, the slow step being tunnelling of electrons through the oxide film. At 40°C a transition from a direct logarithmic to a parabolic growth law is observed. The interpretation is that the diffusion of electrons which have been injected thermionically into the oxide film becomes rate-determining at film thicknesses for which current due to electron tunnelling becomes negligible.     

The anodic oxidation of thiocyanate ion dissolved as KSCN in dimethylsulphoxide

The anodic oxidation of SCN⁻ ion dissolved as KSCN in dimethylsulphoxide, on platinum electrodes, was investigated at ca 25, 60 and 160°C by means of various non-stationary electrochemical techniques. At low temperatures one anodic and two main cathodic processes were found. The anodic oxidation of SCN⁻ ion yields as primary product the SCN radical, which readily produces (SCN)₂. The latter can be in part reduced back to SCN⁻ ion, because it reacts in part yielding solvated hydrogen ions which cause the second cathodic reaction. In the region of 60°C, no (SCN)₂ cathodic current is observed. In the region of 160°C the only reaction is SCN⁻ ion oxidation and the primary product polymerizes to (SCN)_x, which forms a film on the electrode, causing passivation.

On the basis of kinetic data obtained for the different reactions, a mechanism for the anodic oxidation of the SCN⁻ ion and for the passivating film formation is suggested. The second process only partly fits the Müller model for the electrochemical growth of insoluble layers.     

The effect of thermal cycling on oxide scale structure and morphology on a hot-pressed silicon nitride

Dense Si₃N₄ hot-pressed with the aid of 9 wt% Y₂O₃ was oxidised in ‘dry’ synthetic air at both 1000 and 1200°C for up to 500 h under thermal cycling conditions. The experiments revealed that thermal cycling has little effect on oxidation kinetics although the morphology of surface oxidation products is affected by the cycling frequency. Cracks formed in the oxide layers on cooling healed immediately on re-exposure to high temperature, and there was no apparent change in the oxidation rate controlling mechanism over the time period investigated. The exposure of the material at 1000°C did not result in catastrophic oxidation as observed for some other Y₂O₃-doped hot-pressed Si₃N₄ compositions. Additionally, it was observed that crystallisation of the oxide layer with time (assisted in part by the outward diffusion of intergranular phase cations from the bulk ceramic to the surface scale) leads to non-parabolic kinetics owing to reduced rates of diffusion through crystalline phases in the surface scale. ©     

Structural properties of CoPt films patterned using ion irradiation

Patterned CoPt films were fabricated using a combination of e-beam lithography and He⁺ ion irradiation to produce a planar array of ordered CoPt squares separated by disordered CoPt areas: (i) by molecular beam epitaxy was deposited a CoPt ordered film which corresponds to a “natural” multilayer: alternating pure cobalt and pure platinum (0 0 1) planes. (ii) The film was covered by a 300 nm thick Pt layer mask. (iii) Irradiation with appropriate ion beam and fluence disorders the CoPt film where not protected by the mask. X-ray diffraction, as well as atomic and magnetic force microscopy, is used to characterise the structural and magnetic changes in the film. The He⁺ ion irradiation does not significantly modify the surface of the CoPt film: the roughness almost remains identical (2 nm). This is promising for applications in magnetic recording technologies.     

基于原位红外光谱的水相苯酚电氧化机理研究

采用电化学原位红外光谱技术,研究了苯酚在Pt电极表面的电化学氧化机理。在0.1 mol/L Na₂SO₄溶液中,Pt电极上电化学氧化苯酚的反应电位为+0.9~1.0 V（vs SCE）、析氧电位为+1.3 V;电化学原位红外光谱结果表明,当电位<0.9 V时,苯酚氧化产物主要为苯二酚、醌及少量醇类物质;电位0.9~1.1 V时,苯环结构被破坏,氧化产物主要为酮、酸、醇和CO₂;根据官能团吸收峰的变化,苯酚在Pt电极表面氧化经历如下途径：苯酚→苯二酚→苯醌→酮、醇、酸→CO₂。同时研究了NH₄⁺对苯酚在Pt电极表面的电氧化的影响,结果表明在低电位区（<0.9 V）对苯酚氧化构成竞争。     

Growth of oxide layers on platinum electrodes in molten alkali nitrates

The formation of oxide layers on Pt anodes and on Pt cathodes in each of the molten alkali nitrates, LiNO₃, NaNO₃ and KNO₃, has been investigated under galvanostatic and potentiostatic conditions at 340°C.The coverage by oxygen on the anode increased with the polarization potential and time and reached a limiting value of about 2·6 oxygen atoms per apparent surface platinum atom. No cation effect on the anodic formation of the oxide film was observed.Thick multilayer oxide films grew very rapidly on the “cathode” in the KNO₃ melt and slowly on that in the NaNO₃ melt. In the KNO₃ melt and also in the NaNO₃ melt, the growth rate of the multilayer oxide reached a maximum at ?1.65 V (vs a Ag 0.1 M Ag⁺ in 1 kg KNO₃ reference electrode) and rapidly fell at more negative potentials because of the dissolution of the oxide into the melt. Above ?2.0 V, neither the formation of the multilayer oxide nor corrosion of the electrode was observed. In the LiNO₃ melt, no multilayer oxide film was formed at any potentials.     

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.     

Kinetics of anodic reactions of molten potassium thiocyanate on platinum: Formation and growth of anodic film

The film-growth process occurring on platinum anodes when molten potassium thiocyanate is electrolysed at 190°C has been investigated. The kinetics of the process has been studied by the application of different relaxation techniques. The film growth controls the rate of the process, and its mechanism comprises a surface diffusion process occurring after SCN⁻-ion oxidation. The film-thickening stage approaches a parabolic growth law. The faradaic rectification of the film is discussed in terms of its physicochemical properties and the actual site where anodic and cathodic reactions take place.     

载铂聚苯胺/聚砜复合膜修饰电极对甲醇的电催化氧化行为研究

采用循环伏安法制备聚苯胺(PAN)/聚砜(PSF)复合膜修饰电极,在其上电沉积铂粒子,制得载铂聚苯胺/聚砜复合膜修饰电极,用循环伏安法和交流阻抗法研究它对甲醇的电催化氧化行为。复合膜的化学组分用FTIR进行表征,复合膜内层载铂后的表面形态用SEM进行表征。结果表明,复合膜的内层(与工作电极接触的一面)是聚苯胺,外层(与溶液接触的一面)是聚砜,铂粒子在复合膜内层的多孔聚苯胺上均匀沉积,从而使载铂聚苯胺/聚砜复合膜修饰电极对甲醇有好的电催化氧化性能。     

Wood液态合金在NaOH溶液中的电化学反应及表面张力变化行为

通过改变液体金属在NaOH溶液中所处的环境介质,利用石墨电极对其施加电场,研究了液态金属的表面张力、界面电化学反应、氧化膜的产生、溶液中电润湿等动态过程。实验发现Wood合金液滴在阳极发生电化学反应产生的氧化膜会迅速减小表面张力并发生铺展,在阴极发生还原反应会使带氧化膜的金属在3 s内恢复到原状,电毛细作用力使氧化膜破裂,各氧化物与NaOH反应产生Sn（OH）₆^2-、SnO₃^2-、SnO₂^2-、PbO₃^2-、Cd（OH）₄^2-使溶液的颜色发生改变,溶液中生成Bi₂O₃-Bi（OH）₃白色共聚物。薄膜电介质层润湿和溶液中电润湿机理在本质上相同,表面张力随着电压的增大而减小并发生明显的电润湿铺展过程,但由于金属液滴通过电化学反应所能吸附的OH^-数量有限,润湿角存在饱和性。     

The electrochemical reduction of nitrosonium ion in concentrated sulphuric acid: The NO/NO redox couple

The electrochemical reduction of NO⁺ ion in concentrated sulphuric acid has been studied on gold and platinum electrodes in the temperature range from 4·5 to 55·5°C with the rotating disk electrode, linear potential-sweep voltammetry and chronopotentiometry. On gold, NO⁺ is reduced to NO, the reaction being a reversible one-electron transfer reaction. The experimental diffusion coefficients of the NO⁺ ion and NO molecule are evaluated. On platinum, in the potential region where reduction of NO⁺ ion and oxidation of NO occur, oxides formed on the metal interfere, thus making the interpretation of the process difficult.     

Propene oxidation over MoO3 film deposited on an Au|YSZ|Ag system

Propene oxidation was carried out with an electrochemical reactor, MoO₃/Au|YSZ|Ag (YSZ: 8 mol.-% yttria-doped ZrO₂), at 475°C under oxygen pumping, and the catalytic activity of the thin MoO₃ film related to its crystal morphologies was discussed. The thin MoO₃ film was deposited on an Au anode by means of vacuum deposition or sputtering method at room temperature or 300°C. Each method showed a characteristic texture as well as crystal morphology of the MoO₃ film, resulting in variations in the catalytic activity. Sputtering at 300°C gave porous films composed of leaf-like crystals with preferential orientation of (010) plane parallel to the pore channel and perpendicular to the Au surface, resulting in the highest activity for acrylaldehyde production. A relatively high step density was observed on the oriented (010) plane of the leaf-like crystal. The high activity of this MoO₃ film is probably due to the high density of active sites for the partial oxidation of propene and also to their highly porous structure, which is favorable to the surface migration of oxygen to the reduced active sites.     

Platinum oxidation and its effect on concanavalin A adsorption

Oxidation of platinum electrode by chemical and electrochemical means was investigated through electrochemical impedance spectroscopy and cyclic voltammetry. It was possible to observe that concanavalin A readily adsorbs on platinum surface. The degree of adsorption is found to be related to oxidation of the electrode surface. It is argued that the protein may have a strong affinity for hydrated oxide layer on the metal surface, similarly as it has affinity for carbohydrates. Using an equivalent circuit as a model for the interface, it was possible to obtain the thickness of the oxide layer. These results were compared with an electron tunneling model across the oxide layer and showed reasonable agreement. There are differences between the thickness of the oxides prepared chemically and electrochemically, which reflect over the adsorption of the protein.     

Oxidation of H2S on activated carbon KAU and influence of the surface state

Properties of the oxidized activated carbon KAU treated at different temperatures in inert atmosphere were studied by means of DTA, Boehm titration, XPS and AFM methods and their catalytic activity in H₂S oxidation by air was determined. XPS analysis has shown the existence of three types of oxygen species on carbon catalysts surface. The content of oxygen containing groups determined by Boehm titration is correlated with their amount obtained by XPS. Catalytic activity of the KAU catalysts in selective oxidation of hydrogen sulfide is connected with chemisorbed charged oxygen species (O_3.1 oxygen type with BE 536.8–537.7 eV) present on the carbons surface.

Formation of dense sulfur layer (islands of sulfur) on the carbons surface and removal of active oxygen species are the reason of the catalysts deactivation in H₂S selective oxidation. The treatment of deactivated catalyst in inert atmosphere at 300 °C gives full regeneration of the catalyst activity at low temperature reaction but only its partial reducing at high reaction temperature. The last case is connected with transformation of chemisorbed charged oxygen species into CO groups.

The KAU samples treated in flow of inert gas at 900–1000 °C were very active in H₂S oxidation to elemental sulfur transforming up to 51–57 mmol H₂S/g catalyst at 180 °C with formation of 1.7–1.9 g S_x/g catalyst.     

Selective oxidation with air on metal catalysts

Oxidation of organic molecules with air on metal catalysts has been known for a long time but there has been a renewed interest in recent years because these catalytic reactions are environmentally safe and could replace stoichiometric oxidations. This paper describes several oxidation reactions conducted either at high temperatures in the gas phase or at moderate temperatures in the liquid phase; in both cases they proceed via a mechanism of oxidative dehydrogenation on the metal surface. Ethylene glycol was converted to glyoxal at 550°C on Ag/SiC catalyst with a 70% yield provided promoters were added to the reaction feed (diethylphosphite or iodine) or deposited on the catalyst (LiPO₄ or H₃PO₄). The promoters improve the conversion and selectivity by modifying the structure and the oxygen concentration on the surface of silver. Oxidation of glyoxal to glyoxylic acid, glucose to gluconic acid and glycerol to various oxygenated derivatives were conducted in water at 60°C in the presence of carbon-supported palladium or platinum catalysts. Bismuth promoter, deposited on the platinum metals by redox reaction, improves the catalyst activity by preventing over-oxidation of the metal surface and favors the oxidation of secondary alcohol functions into keto-derivatives. At higher reaction temperatures, platinum catalysts produce C-C bond rupture with the formation of carboxylic acids with smaller chains. Thus, cyclohexanol was converted into C₆, C₅, and C₄ diacids with a 45% selectivity to adipic acid on Pt/C catalysts at 150°C.