An X-ray photoelectron spectroscopic study of electrocatalytic activity for chlorine evolution on amorphous palladium-phosphorus alloys containing rhodium,iridium or platinum

The relationship between the electrocatalytic activity for chlorine evolution and the nature of the surface film formed on amorphous palladium-base alloy anodes during electrolysis of a hot concentrated sodium chloride solution was investigated. Chlorine evolution took place on the surface film in which cations were mainly those of additive group metals such as rhdoium, iridium or platinum ions. Amorphous PdIrP alloys showed a high activity for chlorine evolution, while a high rate of chlorine evolution on amorphous PdPtP alloys to produce chlorine gas at a further high rate even at very high overpotentials. This was interpreted in term of formation of a surface film of a low electronic conductivity due to an increase in concentration of a higher valency platinum ion Pt⁴⁺ at high overpotentials. A large amount of neutral chlorine was found on the surface of the amorphous PdRhP andPdPtP alloys polarized at high overpotentials. This was assigned mainly to adsorbed molecular chlorine; that is, the final products of the chlorine evolution reaction, and the difficulty of their release seemed responsible for the difficulty in increasing the rate of chlorine evolution on the amorphous alloys containing rhodium and platinum. The amorphous PdIrP alloys were significantly active for chlorine evolution because they were able to release chlorine molecules without difficulty in addition to the fact that the relative amounts of the higher valency cations in the surface film were not appreciably changed with potential.     

Anodic characteristics of amorphorous palladium-base alloys in sodium chloride solutions

The anodic characteristics of a variety of amorphous palladium-base alloys were examined with a view to their use for the production of sodium hypochlorite by electrolysis of dilute sodium chloride solutions at ambient temperature. The corrosion resistance of palladium-metalloid alloys was obtained by alloying with platinum group metals and/or valve metals. Among these alloys, rhodium-containing alloys showed high electrocatalytic activities for chlorine evolution. Surface activation treatment was, however, necessary to obtain sufficiently high activities for chlorine evolution at low overpotentials. Surface-activated amorphous alloys possessed considerably higher current efficiency for chlorine evolution in comparison with currently used anodes.     

Anodic characteristics of amorphous nickel-value metal alloys containing small amounts of platinum group elements in 0.5 M NaCl

By utilizing the characteristics of amorphous alloys capable of possessing specific electrocatalytic activity and corrosion resistance by alloying and surface activation treatment, an attempt was made to find amorphous alloys which are active as the anode material for production of sodium hypochlorite by electrolysis of seawater. Amorphous Ni-Nb and Ni-Ta alloys containing only 0.5–2.0 at % palladium and other platinum group metals showed a very high activity for chlorine production by electrolysis of 0.5M NaCl at 30°C when they were previously immersed in 46% HF for surface activation. The current efficiency of these surface-activated alloys for chlorine evolution considerably exceeded that of the currently used, most active Pt-Ir/Ti electrode for electrolysis of seawater. The surface activation treatment resulted in preferential dissolution of alloy constituents unnecessary for the electrocatalytic activity, i.e. nickel and valve metals, with a consequent enrichment of electrocatalytically active platinum group elements in the surface-activated layer. The corrosion weight loss of the surface-activated amorphous alloys under the steady state conditions for chlorine production was undetectable by a microbalance.     

Effect of cathodic reduction on catalytic activity of amorphous alloy electrodes for electrooxidation of sulfite

Active electrode materials for a new zinc electrowinning process, in which the thermodynamic cell voltage is about half that of the conventional process by replacing oxygen evolution by anodic oxidation of SO₂ produced in the zinc smelting process have been studied. Immersion in HF solution and subsequent cyclic voltammetry (CV) in sulfuric acid are known to be effective surface activation treatments of the amorphous alloy electrodes. The galvanostatic cathodic reduction (CR) treatment was applied to obtain further activation for sulfite oxidation for HF- and CV-treated electrodes prepared from amorphous nickel-valve metal-platinum group metal alloys. This treatment has been found to be effective in enhancing the activity. Among the amorphous Ni-40Nb alloys containing platinum group elements, the platinum-containing electrode showed the highest catalytic activity, which was higher than that of platinized platinum. Furthermore, the electrocatalytic activities of CR-treated electrodes prepared from amorphous alloys containing platinum and rhodium, and platinum and ruthenium were higher than that of the electrode containing only platinum. According to XPS analysis of the amorphous Ni-40Nb-1Pt-1Ru alloy specimen the enrichment of platinum and ruthenium occurred by CV treatment, and a small amount of oxidized platinum and ruthenium species remained on the electrode surfaces, but most of them were cathodically reduced to the metallic state by CR treatment. High catalytic activities for sulfite oxidation can be attributed to the metallic state of platinum and ruthenium contained in the alloy electrodes, even though the activity of these electrocatalysts is higher than that of pure Pt or Ru.     

An X-ray photoelectron spectroscopic study of electrocatalytic activity of platinum group metals for chlorine evolution

A correlation of the catalytic activity for anodic chlorine evolution of platinum group metals to the nature of the surface film formed during chlorine evolution in a sodium chloride solution was studied by X-ray photoelectron spectroscopy. The change in the surface film with increasing potential was found on platinum, including an increase in the cationic valence. This seemed responsible for the decrease in the activity for chlorine evolution on platinum in the high potential region. Increasing potential did not result in the appreciable increase in the cationic valence in the surface film on the other platinum group metals. Replacement of hydroxyl ions in the surface film by chloride ions became easier in the order of rhodium, iridium and palladium, and the activity for anodic chlorine evolution increased in this order due possibly to an increase in the amount of chloride ions in the film which seemed to be one of the reactants in the rate determining electrochemical desorption of adsorbed chlorine atom. Chlorine molecules adsorbed on the surface film were also found. It was assumed that the activity for anodic chlorine evolution might be low when the metal surface was covered by a large amount of molecular chlorine which was the reaction product.     

The oxygen evolution reaction on platinum,iridium, ruthenium and their alloys at 80°C in acid solutions

Kinetic parametes were determined for the oxygen evolution reaction on 50–50 atom percent alloys of RuIr, RuPt, and IrPt and compared with results obtained using ruthenium, iridium, platinum, and RuO₂/TiO₂ electrodes. The potentiostatic studies were made on oxide covered electrodes at 80°C in both 1.0 M H₂SO₄ and 1.0 M CF₃SO₃H. Cyclic voltammetric studies showed that while these noble metals and alloys are about equally effective as electrocatalysts for the hydrogen evolution reaction, striking differences in activity are found for the oxygen evolution reaction. The order of electrocatalytic activity towards oxygen evolution in H₂SO₄ is Ru > RuIr alloy ～ RuO₂/TiO₂ ～ Ir > IrPt alloy > RuPt alloy > Pt. The type of acid used had very little effect on the kinetic parameters. The lower electrocatalytic activities when platinum is present is probably due to the formation of a platinum oxide film. The dual barrier model is used to interpret the results for the electrodes containing platinum. The best electrocatalysts for oxygen evolution in acid solutions consist of noble metals which form oxide films (RuO₂, IrO₂) possessing metallic characteristics.     

Electrocatalytic activities of metal electrodes in anodic oxidation of hydrazine in alkaline solution

The electrocatalytic activities of various metals and alloys in the anodic oxidation of hydrazine in alkaline solution have been studied by means of palladium membrane method in which the contact side of the membrane was electrodeposited with a thin layer of the electrocatalytic metals. The electrode materials studied can be divided into two groups. In the first group, platinum, rhodium, cobalt, cobalt—phosphor and cobalt—boron, anodic current of hydrogen oxidation on the diffusion side decreased remarkably with an increase of the electro-oxidation of hydrazine on the contact side. The anodic oxidation of hydrazine occurs through the preliminary stepwise dehydrogenation on this group metals.On the other hand, the amount of sorbed hydrogen in the palladium, gold, nickel and nickel—phosphor electrodes increased with an increase of the electro-oxidation of hydrazine on the contact side. Thus, the anodic oxidation of hydrazine on the latter group metals may proceed through the anodic formation of the intermediate radicals which readily decompose into hydrogen and the related compounds.     

Formation of anodic films on sputtering-deposited Al-Hf alloys

The growth of barrier-type anodic films at high efficiency on a range of sputtering-deposited Al-Hf alloys, containing from 1 to 95 at.% Hf, has been investigated in ammonium pentaborate electrolyte. The alloys encompassed nanocrystalline and amorphous structures, the latter being produced for alloys containing from 26 to 61 at.% Hf. Except at the highest hafnium content, the films were amorphous and contained units of HfO₂ and Al₂O₃ distributed relatively uniformly through the film thickness. Boron species were confined to outer regions of the films. The boron distributions suggest that the cation transport number decreases progressively with increasing hafnium concentration in the films, from ∼0.4 in anodic alumina to ∼0.2 for a film on an Al-61 at.% Hf alloy. The distributions of Al³⁺ and Hf⁴⁺ ions in the films indicate their similar migration rates, which correlates with the similarity of the energies of Al³⁺-O²⁻ and Hf⁴⁺-O²⁻ bonds. For an alloy containing ∼95 at.% Hf, the film was largely nanocrystalline, with a thin layer of amorphous oxide, of non-uniform thickness, at the film surface. The formation ratios for the films on the alloys changed approximately in proportion to the hafnium content of the films between the values for anodic alumina and anodic hafnia, ∼1.2 and 1.8 nm V⁻¹ respectively.     

The anodic characteristics of manganese dioxide electrodes prepared by thermal decomposition of manganese nitrate

Manganese dioxide electrodes were prepared by a thermal decomposition of manganese nitrate solution on a titanium or a platinum substrate, and their anodic characteristics were investigated mainly in 1N H₂SO₄ and 1N KOH. The platinum-supported manganese dioxide electrode shows the good anodic characteristics with a relatively low overvoltage for oxygen evolution while the use of the titanium-supported one as an anode needs further modification to reduce its high resistivity resulting from the thick film of titanium dioxide in spite of good adhesion of the oxide film with the titanium substrate.The primary water or hydroxide ion discharge step is rate-controlling in the anodic evolution of oxygen on the platinum-supported manganese dioxide electrode in both acidic and alkaline solutions. The oxygen overvoltage is raised and the mechanical strength of the catalytic oxide on the platinum substrate is weakened by the anodic polarization in acidic solution. These phenomena are explicable on the basis of an increase in the oxygen content in the oxide. In conclusion, the presented results suggest that the manganese dioxide film is a practical usable material for the anodic evolution of oxygen as well as chlorine, especially in alkaline solutions.     

Microstructure of new composite electrocatalyst and its anodic behavior for chlorine and oxygen evolution

The first layer of active coating made from a rutile-structured solid solution of ruthenium and titanium dioxides having an average crystal grain size of 30 nm was thermally deposited on an adequately prepared titanium metal substrate. Then, at a temperature of 500 °C, the second layer was formed on the first layer from a mixture of amorphous particles of metallic platinum and rutile-structured iridium dioxide nanocrystals having an average crystal grain size of 26 nm.Rutile phase nanocrystals are characterized by a high density of chaotically distributed dislocations and high internal microstrain values. The coatings exhibit a compact granular morphology without cracks on the surface. Their catalytic activity is similar to that of conventional DSAs for the anodic oxidation of chloride ions from both concentrated and dilute sodium chloride solutions. The anodic current efficiency both during chlorate formation and active chlorine production was several percentage points higher in electrolyzers containing these anodes than in those containing DSAs. The catalytic activity of anodes having these coatings is about 50 mV lower than that of DSAs and about 350 mV higher than that of lead/antimony alloy electrodes, for oxygen evolution from acid sulfate solutions (0.5 mol dm^?3 H₂SO₄) characteristic of processes for the production of some metals. An accelerated corrosion test showed that the stability of the double-layer anodes is about twelve-fold higher than that of conventional DSAs.     

Electrodeposition of Ni-transition alloys for the oxygen evolution reaction

The oxygen evolution reaction (OER) is the anodic reaction in several industrial electrolytic processes. The objective of this work was to develop a new electrocatalytic material for long-lasting and economical high performance electrodes. New electrodes were prepared by electrodeposition of nickel, nickel-ruthenium and nickel-iridium alloys. They were then activated by anodic polarization at 100 mA cm^–2 to form an oxide layer. The electrocatalytic activity was characterized for the OER in 5M KOH solution. The results show that nickel-iridium alloys provide greater electrocatalytic activity for the OER and better corrosion resistance than nickel-ruthenium in alkaline solution. The effects of transition elements on improving the performance of the nickel electrode are then discussed.Notation b Tafel slope - i _ex exchange current density - i _dp electrodeposition current density - i _OER oxygen evolution reaction current density - C _dl double-layer capacity - o₂ oxygen overpotential     

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.     

Amorphous PdZr alloys for water electrolysis cathode materials

Amorphous Pd_0.35Zr_0.65 alloys prepared by a melt—quench technique were investigated for their possible use as water electrolysis cathodes. The Tafel—Volmer reaction route of the hydrogen electrode reaction was concluded. Kinetic parameter values of the constituent steps were evaluated by a transient technique. The electrocatalytic activity for cathodic hydrogen evolution on the as-obtained electrode was 10² times lower than Pd foil electrodes, but it was improved after the electrodes were treated in acids, typically aqueous HF. The activity in the highest state exceeds that of Pd by one order of magnitude. SEM and XPS observations indicated that the improvement in activity is due to increased surface Pd concentration after removal of a Zr-enriched surface layer which was produced during fabrication of the amorphous alloy specimens. Crystalline alloy electrodes prepared by heat treatment of the amorphous alloy were very brittle but showed electrocatalytic activity close to that of the amorphous electrodes.     

The anodic evolution of oxygen and chlorine on foreign metal-doped SnO2 film electrodes

The anodic evolution of oxygen and chlorine on foreign metal-doped SnO₂ film electrodes were studied, in connection with their physical and chemical properties. In general, the anodic characteristics of noble metal-doped electrodes were much better than those of base metal-doped electrodes. The oxygen evolution reaction on noble metal-doped electrodes reflected each characteristic of the doped noble metals and the active center for this reaction was considered to be on the noble metal sites. On the other hand, the chlorine evolution reaction on such electrodes was found to be independent of the kind of doped noble metals and the probable mechanism for this reaction was proposed in which the spillover of Cl radical from noble metal to Sn sites was involved.     

Hydrogen evolution reaction (h.e.r.) in an acidic or basic medium on nickel electrodeposited with PW12O 40 3- and Cu2+

The hydrogen evolution reaction (h.e.r.) on electrodeposited nickel from a chemical bath containing Cu^2-, with and without PW₁₂O ₄₀ ^3- , was investigated. Hydrogen cathodes based on nickel-copper alloys and PW₁₂O ₄₀ ^su3- reduced species are better electrocatalysts in 1 M H₂SO₄ or 3m KOH at 298 K than nickel-copper alloys deposited without PW₁₂O ₄₀ ^3- . The electrocatalytic performance of the former cathodes was attributed to their chemical composition. This electrocatalytic activity for the h.e.r. was examined as a function of the [Cu²⁺] or [PW₁₂O ₄₀ ^3- ] in the electrodeposition bath. The influence of nickel salt anions on the h.e.r. electrocatalytic activity of the electrodes was also investigated. The chemical surface composition of the electrodes was analysed by X-ray photoelectron spectroscopy (XPS). It was shown that the electrocatalytic parameters were correlated to the quantity of tungsten in the electrode surface. The various factors causing the improvement in the electrocatalytic activity are discussed.     

Performance and stability of Pt-based ternary alloy catalysts for PEMFC

Carbon-supported Pt-based ternary alloy electrocatalysts were prepared by incipient wetness method in order to elucidate the origin of the enhanced activity of oxygen reduction reaction in PEMFC. To measure the catalytic activity and stability of the cathode alloy catalysts (electrodes containing Pt loading of 0.3 mg/cm², 20 wt.% Pt/C, E-TEK), the I-V polarization curves were obtained. All alloy catalysts showed higher performances than Pt/C. It can be concluded that as platinum formed alloys with transition metals, the electronic state of Pt and the nearest neighbor Pt-Pt distance changes, which significantly influence the electrocatalytic activity for oxygen reduction.Long-term stability test was performed with the Pt₆Co₁Cr₁/C alloy catalyst for 500 h. According to XPS analysis, the lower oxide component with Pt₆Co₁Cr₁/C electrocatalyst provides a large portion of platinum in metallic species in the electrocatalyst and it seems to be mainly responsible for its enhanced activity towards oxygen reduction.     

Electrochemical performance of Ni/TiO2 hollow sphere in proton exchange membrane water electrolyzers system

This work presents the electrocatalytic evaluation of Ni/TiO₂ hollow sphere materials in PEM water electrolysis cell. All the electrocatalysts have shown remarkably enhanced electrocatalytic properties in comparison with their performance in aqueous electrolysis cell. According to cyclic voltammetric results, 0.36 A cm^?2 peak current density has been exhibited in hydrogen evolution reaction (HER) from 30 wt% Ni/TiO₂ electrocatalyst. 15 wt% Ni-doped titania sample has shown the best result in oxygen evolution reaction (OER) with the anodic peak current density of 0.3 A cm^?2. In the anodic polarization curves, the performance of 15 wt% Ni/TiO₂ hollow sphere electrocatalyst was evaluated up to 140 mA cm^?2 at comparatively lower over-potential value. 20 wt% Ni/TiO₂ hollow sphere electrocatalyst has also shown electrochemical stability in PEM water electrolyzer for 48 h long analysis. The comparative electrocatalytic behavior of hollow spherical materials with non-sphericals is also presented, which clearly shows the influence of hollow spherical structure in greater electrocatalytic activity of the materials. The physical characterization of all the hollow spherical materials is presented in this work, which has confirmed their better electrochemical behavior in PEM water electrolyzer.     

On the development of metallic inert anode for molten CaCl2–CaO System

Some fundamental aspects related to inert anode development in molten CaCl₂–CaO were investigated based on thermodynamic analysis, electrochemistry of metals and solubility of oxide measurements. The Gibbs free energy change of several key anodic reactions including electro-stripping of metals, electro-formation of metallic oxides, electro-dissolution of metallic oxides as well as oxygen and chlorine evolution was calculated and documented, for the first time, as a reference to develop metallic inert anode in chloride based melts. The anodic behaviors of typical metals (Ni, Fe, Co, Mo, Cu, Ag, and Pt) in the melt were investigated. The results confirmed the thermodynamic stability order of metals in the melts and revealed that surface oxide formation can increase the stability of the electrodes in CaO containing melt. Furthermore, solubility of several oxides (NiO, Fe₂O₃, Cr₂O₃, Co₃O₄, NiFe₂O₄) in pure CaCl₂ or CaCl₂–CaO melts was measured to evaluate the stability of oxide coating or a cermet inert anode in the melt. It was found that the solubility of NiO decreased with increasing CaO concentration, while that of Fe₂O₃ increased. Ni coated with NiO film had much higher stability during anodic polarization.     

The Development of Palladium(II)-Specific Amine-Functionalized Silica-Based Microparticles: Adsorption and Column Separation Studies

The adsorption and separation of platinum(IV) and palladium(II) chlorido species ([PtCl₆]^2? and [PdCl₄]^2?) on silica-based microparticles functionalized with ammonium centers based on ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetriamine (TETA) and tris-(2-aminoethyl)amine (TAEA) were investigated. The functionalized sorbent materials were characterized using SEM, XPS, BET, and FTIR. The sorbents were used in the batch and column study for adsorption and selective separation of [PtCl₆^2? and PdCl₄]^2?. The adsorption model for both [PtCl₆]^2? and [PdCl₄]^2? on the different sorbent materials fitted the Freundlich isotherm with R² values > 0.99. The S-TETA sorbent material was palladium(II) specific. Pd(II) loaded on the silica column was recovered using 3% m/v thiourea solution as the eluting agent. Separation of platinum and palladium was achieved by selective stripping of [PtCl₆]^2? with 0.5 M of NaClO₄ in 1.0 M HCl while Pd(II) was eluted with 0.5 M thiourea in 1.0 M HCl. The separation of palladium (Pd) from a mixture containing platinum (Pt), iridium (Ir), and rhodium (Rh) was successful on silica functionalized with triethylenetriamine (TETA) showing specificity for palladium(II) and a loading capacity of 0.27 mg/g. S-TETA showed potential for use in the recovery of palladium from platinum group metals such as from solutions of worn out automobile emission control catalytic convertors and other secondary sources.     

The anodic characteristics of the massive β-MnO2 doped with noble metals in sodium chloride solution

The anodic characteristics of the massive β-MnO₂ doped with a slight amount of noble metals were investigated in NaCl aqueous solution. The doping of noble metal, especially Pd, decreases the chlorine overvoltage enormously. By the kinetic considerations, it was clarified that noble metal sites dispersed on the oxide surface serve effectively as an active site for the chlorine evolution reaction. Furthermore, it was suggested that new effective catalysts for the chlorine evolution might be developed by using such noble metal-doped β-MnO₂ systems.