甲醇在聚苯胺修饰铂钼共沉积电极上的催化氧化

用恒电位法和循环伏安法在铂电极上分别制备了聚苯胺修饰的分散氢钼青铜电极和分散铂电极,以及聚苯胺修饰的不同铂钼比例的铂与氢钼青铜共沉积电极。用循环伏安法研究了制备电极在c(H2SO4)=0.5mol/L水溶液中的电化学行为,以及对c(CH3OH)=0.1 mol/L的催化氧化行为。其中,分散氢钼青铜电极对甲醇无催化氧化的作用,铂与氢钼青铜共沉积电极对甲醇的催化氧化效果优于分散铂电极。铂-氢钼青铜共沉积电极对甲醇氧化的催化能力与共沉积铂钼的比例有关,当制备电极所用的溶液中n(氯铂酸)∶n(钼酸钠)=2∶1时,共沉积电极对甲醇的催化氧化活性最高,此时甲醇在共沉积电极上的氧化峰电流是单纯铂电极的2.632倍。     

The role of platinum as the high voltage electrode in the enhancement of Fenton''s reaction in liquid phase electrical discharge

Pulsed electrical discharges in water produce a variety of oxidative and reductive species including hydroxyl radicals, hydrogen peroxide, and hydrogen. The reaction of ferrous ions with hydrogen peroxide (Fenton's reaction) provides additional hydroxyl radicals. Previous experiments with pulsed electrical discharges in water have shown that when ferrous sulfate is used as an electrolyte with a platinum high voltage electrode significantly higher organic compound degradation can be obtained in comparison to the case with an electrode made of nickel-chromium. In the work presented here, it is shown that particles emitted into solution from the platinum high voltage electrode enhance the production of hydroxyl radicals by forming a catalytic cycle between ferric and ferrous ions. The ferrous ions are converted to ferric ions by the Fenton's reaction utilizing hydrogen peroxide from the electrical discharge and the ferric ions are in turn converted to ferrous ions by reactions on the platinum particles emitted into solution from the high voltage electrode with molecular hydrogen formed by the electrical discharge. Based upon experiments with various scavengers it is concluded that the catalytic effect of the platinum particles is due to the presence of adsorbed hydrogen, while in contrast the nickel-chromium, which does not adsorb hydrogen, high voltage electrode and particles emitted by this electrode have no effect on the ferric ion regeneration.     

Platinum catalysed decomposition of hydrogen peroxide in aqueous-phase pulsed corona electrical discharge

Electrical discharges in water produced by a pulsed high voltage power supply generate chemically active species (OH, H₂, O₂, H₂O₂, HO₂ and O) that are capable of degrading various hazardous chemicals. Previous experimental studies showed that platinum high voltage electrodes in a pulsed corona electrical discharge lead to significantly higher pollutant removal in comparison to that with other electrode materials. In the present work it was observed that when nickel–chromium was used as a high voltage electrode, the pulsed corona electrical discharge in water produces hydrogen peroxide at a constant rate regardless of the initial pH of the solution. Replacement of the nickel–chromium electrode with a platinum high voltage electrode leads to the decomposition of hydrogen peroxide where the rate of decomposition increases with increasing pH. An Eley-Rideal mechanism describing heterogeneous catalytic hydrogen peroxide decomposition is proposed. It is assumed that the decomposition occurs on the surface of the platinum particles ejected from the platinum high voltage electrode. Combination of the experimental measurements and a mathematical model describing the platinum catalysed hydrogen decomposition suggests that the pH dependent hydrogen peroxide decomposition is caused by the adsorption of molecular hydrogen produced by the discharge and hydroxyl ions on the platinum surface. The influence of gases bubbled into the reactor (argon, oxygen and hydrogen) on the hydrogen peroxide decomposition was also tested by both experiments and the model. Finally, the model was utilized to predict molecular hydrogen and oxygen concentrations at three pH values when either nickel–chromium or platinum high voltage electrodes are used.     

Simulation studies on the fuel electrode of a H2---O2 polymer electrolyte fuel cell

The macro-homogeneous porous electrode theory is used to develop a model which describes the catalyst layer of the hydrogen electrode formed by catalyst particles that are bonded to the membrane. The water transport in the catalyst layer and polymer electrolyte membrane is considered. The effects of catalyst layer structure parameters such as polymer volume fraction, catalyst layer thickness, platinum loading and reactant gas humidity as well as CO poison on the hydrogen electrode behavior are examined. The results show that the catalyst layer thickness has a significant effect on the electrode performance. A thicker catalyst layer will result in a larger ohmic voltage loss and higher catalyst cost. The optimal polymer volume fraction and catalyst layer thickness are 0.5 and 1.5–4 μm, respectively, for this electrode. An optimal platinum surface coverage on carbon need not exceed 20% (20 wt% Pt/C). Larger platinum coverage will increase the cost, but only slightly enhance the electrode performance.     

Adsorption de l'acide maleique et de l'hydrogene sur le platine-platine en presence de cuivre adsorbe

According to a previous study, the catalytic hydrogenation of maleic acid on platinum catalyst is inhibited by copper adsorption. A copper adatom deactivates five accesible atoms of platinum. A cyclic voltammetry study of hydrogen and maleic acid adsorptions on platinum and on platinum modified by copper adatoms accounts for this result.Indeed a copper adatom inhibits the adsorption of one hydrogen atom on one platinum atom and of one maleic acid molecule on 5 ± 1 platinum atoms.     

Influence of mercury on hydrogen overvoltage on solid metal electrodes—III. Adsorption and evolution of hydrogen on poisoned platinum

The adsorption capacity for hydrogen as well as the exchange cd of hydrogen evolution and the double-layer capacitance of a platinum electrode depend on the degree of poisoning by mercury. Mercury electrodeposited with simultaneous hydrogen evolution on smooth platinum forms droplets, the curvature radius of which is of the order 10^?6 cm, and for complete inhibition of hydrogen adsorption, an amount equivalent to several monolayers of mercury is necessary. By submerging a pure platinum electrode in a dilute solution of mercuric compounds without external polarization, a monolayer is formed and one atom of mercury replaces just one atom of hydrogen. On platinum-black electrodes, electrodeposited drops of mercury cover the orifice of the pores and one mercury atom shields on an average 2–3 active sites of the electrode surface.     

Electrocatalytic hydrogenation processes controlled potential—1. The electrocatalytic reduction of nitric acid

Methods are described to control the catalyst potential during the electrocatalytic hydrogenation of nitric acid in a slurry electrode cell. The influence of traces of GeO₂ on the hydrogenation reaction is studied with a platinum electrode. Deposition of Ge on the platinum surface limits the amount of adsorbed hydrogen and enhances the nitric acid reduction. This changes the mixed potential of the hydrogen-nitric acid system on the platinum (catalyst) surface. A similar potential change occurs, when the partial hydrogen pressure in the system is altered.     

氢钼青铜修饰铂电极对甲醇氧化的电催化作用

利用循环伏安法研究了氢钼青铜在铂电极上的修饰作用和修饰铂电极在c(H2 SO4 ) =0 5mol/L溶液中对甲醇的催化作用。研究结果表明 :铂电极因钼酸盐的还原和钼青铜的氧化而得到修饰 ,低电位范围内修饰铂电极对甲醇的氧化有催化作用 ,催化氧化电流是未修饰电极上的1 6倍。酸性条件下 ,含高价态钼的钼青铜不稳定 ,会不断溶解对铂失去修饰作用 ,对甲醇的氧化效果与未修饰铂电极上的效果相同 ;而低电位时 ,钼青铜修饰铂电极则相对稳定     

Metal electrodes bonded on solid polymer electrolyte membranes (SPE)—The behaviour of platinum bonded on SPE for hydrogen and oxygen electrode processes

The electrochemical characteristics of a platinum electrode directly bonded to one side of a solid polymer electrolyte membrane (Pt-SPE) were studied. Current-potential relations of hydrogen and oxygen adsorbed on Pt-SPE were observed to be similar to those on a platinum metal electrode independently of the presence/absence of electrolytic solution at the platinum side of Pt-SPE. Hydrogen evolution, hydrogen ionisation, and oxygen reduction were examined at the condition that the side at the platinum of Pt-SPE was free from liquid but filled with the gases concerned     

Electrochemical activity of electrodeposited and heat-treated platinum electrodes

The electrochemical activity of the platinum electrode for hydrogen reaction in 0.5 M H₂SO₄ has been studied with special reference to the state of the electrode surface. The electrode, prepared by electrodepositing a small amount of platinum on a platinum substrate from H₂PtCl₄ solution, is characterized by extremely high and stable activity, but this is decreased remarkably by annealing in vacuum. The activity decay on annealing is associated with the settling down of superficial metastable platinum atoms to stable lattice positions.     

Reaction pathways and poisons—II: The rate controlling step for electrochemical oxidation of hydrogen on Pt in acid and poisoning of the reaction by CO

The reaction mechanism for hydrogen molecule oxidation on platinum electrocatalysts in acid solutions is deduced by comparing kinetic rate parameters obtained electrochemically, to those rate parameters obtained in the gas phase from H₂—D₂ exchange. The electrochemical rate parameters were obtained from potentiodynamic scanning of smooth platinum and from polarisation curves of porous platinum black flooded electrode structures. The rate controlling step for hydrogen molecule oxidation on Pt is the dual site dissociative chemisorption of the hydrogen molecule H₂ → 2H (known as the Tafel reaction). Specific poisons for this reaction are chemisorbed carbon monoxide and adsorbed hydrogen, producing a simple site elimination for dissociative chemisorption of the hydrogen molecule and confirm the dual site mechanism. The electrochemical reaction rate parameters are the same on smooth platinum, unsupported platinum black and platinum crystallites supported on graphitised carbon, correlating with the H₂-H₂ exchange in the gas-phase, both with and without chemisorbed carbon monoxide and are dependent only upon the surface areas of platinum catalysts.     

Electrolytic hydrogenation of phenols in aqueous acid solutions

On platinum electrodes, may substituted phenols are electrolytically hydrogenated to form corresponding cyclohexanols with a 100% current efficiency on a platinum electrode in aqueous acid solutions. The reaction mechanism was concluded to be the surface reaction between adsorbed phenols and hydrogen. Studies on the substituent effect showed that the hydrogenation is most favoured for unsubstituted phenol, suggesting that the polarity of adsorbed hydrogen atoms are essentially neutral.     

The study of adsorption by measurement of electrode resistance

The feasibility of studying adsorption processes by following changes in the lateral resistance of a thin-film platinum electrode has been investigated. The principal difficulty associated with this technique is to correct for the parallel conduction through the electrolyte. A theoretical treatment of this problem indicates that, providing the slope of the current-potential relation for the electrode reaction is small, a satisfactory correction for these co-conduction effects may be applied.The method has been used to study the adsorption of hydrogen, oxygen, thallium, chloride and bromide of platinum. The results are in general accord with those obtained by other methods.     

Amorphous nickel-valve metal-platinum group metal alloy electrodes for hydrogen-oxygen sulphuric acid fuel cells

Powder catalysts were prepared by immersion of amorphous Ni-40Zr and Ni-40Ti alloys containing a few at % of platinum group elements in HF solution. This treatment led to preferential dissolution of the valve metal and nickel with a consequent formation of microcrystalline alloy powders consisting of concentrated platinum group elements and some nickel and valve metal. Porous gas-diffusion electrodes prepared by using these alloy catalyst powders were employed for electrochemical reduction of oxygen and oxidation of hydrogen in 1 M H₂SO₄ at 25°C. The activity of the electrodes prepared from the amorphous alloys containing Pt–Ru, Pt–Rh, Pt and Pd for oxygen reduction was considerably higher than that of the platinum black electrode. Oxidation of hydrogen occurred readily close to the equilibrium potential. Amorphous alloy electrodes containing Pt–Ru, Pt–Rh and Pt were more active than the platinum black electrode for the hydrogen oxidation.     

Electrochemical study of benzene on Pt of various surface structures in alkaline and acidic solutions

The electrochemical behaviour of benzene on platinum electrodes (polycrystalline and single-crystal electrodes) has been studied in acidic and alkaline solutions. In acid solutions the reduction of benzene to cyclohexane takes place in all the platinum surface structure employed, however it does not occur in alkaline media (0.1 M NaOH). In this case, the hydrogen adsorption-desorption processes displace the adsorbed benzene from the electrode surface. The oxidation of benzene is also affected by the pH of the electrolyte and also by the surface structure of the platinum electrode used. In acid solutions, this oxidation at higher potentials (1.4 V vs. RHE) yields CO₂, benzoquinone and α,β-unsaturated esters or lactones, however in alkaline media carbonate anions coming from CO₂ and salts of carboxylic acids have been detected by in situ FTIR spectroscopy using a platinum polycrystalline electrode. Bulk electrolysis of benzene solutions using a platinum electrode in acid and alkaline media was performed in order to confirm the results obtained by spectroscopic measurements.     

General scheme of adsorption,electroreduction and catalytic hydrogenation of nitro-compounds of platinum—I. Kinetics and mechanism of adsorption

The kinetics and mechanism of adsorption of nitro-compounds (nitro-methane, nitro-ethane and nitro-benzene) on platinum and platinum group metals have been investigated by complex potentiodynamic pulses' and charging curves' techniques. The adsorption of nitro-compounds on platinum in the potential range from 0 to 1 V has been shown to proceed mainly with the formation of two types of chemisorbed particles. For potentials more positive than 0.35 V nitro-compounds are adsorbed due to a nitro-group with the formation of NPt and OPt bonds. In this region of potentials the adsorption of nitro-compounds is not accompanied with the flow of considerable amounts of electricity through the electrode—solution interface. These adsorbed particles react very rapidly with adsorbed hydrogen with the formation of semi-reduced chemisorbed particles, whose formation can be observed at 0 < E_r < 0.35 V, that is in the range of hydrogen desorption on platinum. Two hydrogen atoms are spent on the formation of a semi-reduced chemisorbed particle from a nitro-compound molecule, and the particle formed occupies 4 adsorption centres on the surface.The dependence of steady-state coverage of the platinum electrode surface with chemisorbed particles in nitro-compounds solutions is described by the Temkin isotherm and the adsorption kinetics by the Roginsky—Zel'dovich equation.     

The behaviour of platinized platinum electrodes in acid solutions: correlation between voltammetric data and electrode structure

The electrochemical behaviour of platinized platinum electrodes prepared by different methods was studied in the H-adatoms and O-adatoms electroadsorption-electrodesorption potential range in 1m HClO₄ at 30°C. Porous platinized platinum electrodes of different degrees of platinization prepared at a constant potential show an anomalous voltammetric behaviour. The magnitude of the anomalous effect depends on the electroplating conditions and it becomes more remarkable at the lowest voltammetric sweep rate. The anomalous behaviour is explained on the basis of absorption of hydrogen into the porous structure and its influence on the different stages of the hydrogen electrode reaction. The absorption of hydrogen occurs either during electroplating and/or during the potentiodynamic stabilization of the porous electrode.     

Electrochemical reduction of perchlorate ions on platinum-activated nickel

The electrochemical reduction of perchlorate anion has been studied in a cell with a nickel working electrode and a platinum counter electrode in concentrated solutions of HClO₄. Significant reduction of perchlorate to chloride ions occurs on the nickel cathode in the potential region where hydrogen evolution occurs. It is shown that the mechanism of this process involves platinum deposited in small amounts on the cathode as a result of oxidation of the platinum anode in the perchloric acid solution. The reduction of perchlorate is accompanied by oxidation of the nickel cathode, which is attributed to chlorate ions formed in the initial step of perchlorate reduction. Changes in the surface structure of the nickel electrode have been followed using atomic force microscopy.     

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. II. Identification of potential fluctuation sources for unstressed electrodes

In this second part of the study of electrochemical noise (EN) generated by hydrogen evolution on a vertical cylindrical AISI 4140 steel electrode under galvanostatic control in 0.5 M sulphuric acid, the potential fluctuations induced by the growth and detachment of hydrogen bubbles at the electrode surface were analysed. They could be related to fluctuations of various quantities: electrode active surface due to bubble screening effects, concentration of dissolved hydrogen in the electrolyte close to the electrode surface, and metal-hydrogen interactions (MHI) on or beneath the electrode surface. The existence of MHI and their influence on faradaic potential fluctuations could be revealed by comparing the noise features on steel and platinum. The influence on EN of the charging cathodic current density, the presence of dissolved oxygen in the solution, and the electrode rotation speed was investigated in the absence of stress applied to the electrode. In the third paper of this series, the effect of hydrogen embrittlement on potential fluctuations of stressed electrodes will be examined.     

Untersuchungen zur anodischen oxidation des hydrazins im alkalischen elektrolyten—II. Der einfluβ der wasserstoffüberspannung

Electrodes with low hydrogen overpotential such as platinum or skeleton nickel are very suitable catalysts for the anodic oxidation of hydrazine. On the other hand, there are electrodes with remarkably high hydrogen overpotential, such as amalgamated sintered nickel, on which hydrazine can be oxidised in the vicinity of the thermodynamic hydrogen potential. In contrast to the reaction on platinum, the hydrazine molecule undergoes no dehydrogenation at the amalgamated sinter nickel electrode, but the hydrogen is directly discharged without being previously split off the hydrazine molecule. The reason for this different behaviour is evidently the small heat of adsorption of hydrogen on mercury: On electrodes with high hydrogen overpotential such as amalgamated sinter nickel the heat of adsorption of hydrogen atom is not sufficiently large to split the hydrazine molecule. On electrocatalysts with low hydrogen overpotential, such as platinum or nickel, the splitting of the (N₂H₃---H)-bond and, therefore, the dehydrogenation of the hydrazine molecule is energetically preferred.