Combined in situ EXAFS and electrochemical investigation of the oxygen reduction reaction on unmodified and Se-modified Ru/C

In this work we study the kinetics of the oxygen reduction reaction on carbon-supported Ru nanoparticles modified with various amounts of Se. Rotating disk electrode is used to determine kinetic currents for the ORR in 0.1 M H₂SO₄ at 298 K and O₂ partial pressures from 1 to 0.01 atm. The dependence of the ORR activity on Se/Ru ratio shows volcano-type behavior with ca. 10 fold increase of the mass activity at 0.1 < Se/Ru < 0.3. The reaction order in O₂ is close to 1 in the interval of overpotentials from 0.4 to 0.7 V, and is independent of the presence of Se. Regardless the amount of Se, the Tafel slope demonstrates continuous increase from ca. 70 mV/dec at 0.4 V to ca. 140 mV/dec at 0.6 V overpotential. In situ EXAFS spectra are measured at Ru K-edge (in the transmission mode) and Se K-edge (in the fluorescence mode) in argon and oxygen saturated 0.1 M H₂SO₄ solutions in the interval of electrode potentials from 0.050 to 0.750 V RHE. The data are used to explore the surface state changes of Ru and Ru_xSe_y particles and clarify the promoting role of Se during the ORR.     

Parallel oxygen and chlorine evolution on Ru1−xNixO2−y nanostructured electrodes

Nanocrystalline materials with chemical composition corresponding to formula Ru_1−xNi_xO_2−y (0.02 < x < 0.30) were prepared by sol-gel approach. Substitution of Ru by Ni has a minor effect on the structural characteristics extractable from X-ray diffraction patterns. The electrocatalytic behavior of Ru_1−xNi_xO_2−y with respect to parallel oxygen (oxygen evolution reaction, OER) and chlorine (chlorine evolution reaction, CER) evolution in acidic media was studied by voltammetry combined with differential electrochemical mass spectrometry (DEMS). The DEMS data indicate a significant decrease of the over-voltage for chlorine evolution with respect to that of pure RuO₂. The oxygen evolution is slightly hindered. The increasing Ni content affects the electrode material activity and selectivity. The overall material's activity increases with increasing Ni content. The activity of the Ru-Ni-O oxides towards Cl₂ evolution shows a distinguished maximum for material containing 10% of Ni. Further increase of Ni content results in suppression of Cl₂ evolution in favor of O₂ evolution. A model reflecting the cation-cation interactions resulting from Ni-doping is proposed to explain the observed trends in electrocatalytic behavior.     

Evaluation of RuxWySez Catalyst as a Cathode Electrode in a Polymer Electrolyte Membrane Fuel Cell

The oxygen reduction reaction (ORR) on Ru_xW_ySe_z is of great importance in the development of a novel cathode electrode in a polymer electrolyte membrane fuel cell (PEMFC) technology. The Ru_xW_ySe_z electrocatalyst was synthesised in an organic solvent for 3 h. The powder was characterised by transmission electron microscopy (TEM), and powder X‐ray diffraction (XRD). The electrocatalyst consisted of agglomerates of nanometric size (∼50–150 nm) particles. In the electrochemical studies, rotating disc electrode (RDE) and rotating ring‐disc electrode (RRDE) techniques were used to determine the oxygen reduction kinetics in 0.5 M H₂SO₄. The kinetic studies include the determination of Tafel slope (112 mV dec^–1), exchange current density at 25 °C (1.48 × 10^–4 mA cm^–2) and the apparent activation energy of the oxygen reaction (52.1 � 0.4 kJ mol^–1). Analysis of the data shows a multi‐electron charge transfer process to water formation, with 2% H₂O₂ production. A single PEMFC with the Ru_xW_ySe_z cathode catalysts generated a power density of 180 mW cm^–2. Performance achieved with a loading of 1.4 mg cm^–2 of a 40 wt% Ru_xW_ySe_z and 60 wt% carbon Vulcan (i.e. 0.56 mg cm^–2 of pure Ru_xW_ySe_z). Single PEMFC working was obtained with hydrogen and oxygen at 80 °C with 30 psi.     

The electrocatalytic behavior of Ru0.8Co0.2O2−x—the effect of particle shape and surface composition

The electrocatalytic activity of Ru_0.8Co_0.2O_2−x nanocrystals was studied using diffraction, microscopic and spectroscopic techniques to elucidate the role of particle shape and surface chemical composition in the electrocatalytic evolution of oxygen and chlorine. The prepared Ru_0.8Co_0.2O_2−x samples are of the rutile structure, their chemical composition, however, differs. The samples with the smallest particle size compensate for the Co doping by oxygen deficiency. The materials featuring bigger particle size show tendency to compensate for the presence of cobalt by higher valency of Ru. Regardless of the particle size or actual surface composition of the Ru_0.8Co_0.2O_2−x electrodes, the chlorine evolution precedes that of oxygen by ca. 100 mV. The Ru_0.8Co_0.2O_2−x electrodes retain high affinity to oxygen evolution once the reaction becomes possible. This behavior can be ascribed to a stabilization of the six-valent ruthenium, which represents the major intermediate for the oxygen evolution process, at the electrode surface due to the presence of di-valent and tri-valent cobalt. This effect precludes the possible effects of the particle shape and crystal edge distribution.     

Study of IrxRu1−xO2 oxides as anodic electrocatalysts for solid polymer electrolyte water electrolysis

Electrocatalysts of the general formula Ir_xRu_1−xO₂ were prepared using Adams’ fusion method. The crystallite characterization was examined via XRD, and the electrochemical properties were examined via cyclic voltammetry (CV) in, linear sweep voltammetry (LSV) and chronopotentiometry measurements in 0.5 M H₂SO₄. The electrocatalysts were applied to a membrane electrode assembly (MEA) and studied in situ in an electrolysis cell through electrochemical impedance spectroscopy (EIS) and stationary current density–potential relations were investigated. The Ir_xRu_1−xO₂ (x = 0.2, 0.4, 0.6) compounds were found to be more active than pure IrO₂ and more stable than pure RuO₂. The most active electrocatalyst obtained had a composition of Ir_0.2Ru_0.8O₂. With an Ir_0.2Ru_0.8O₂ anode, a 28.4% Pt/C cathode and the total noble metal loading of 1.7 mg cm⁻², the potential of water electrolysis was 1.622 V at 1 A cm⁻² and 80 °C.     

Mechanism of the Formation of Ba2Ti9O20-Based Phases in the Course of Solid-Phase Interaction in the BaO–TiO2(ZrO2) and Cs2O–BaO–TiO2(ZrO2) Systems

The mechanism of solid-phase interaction in the BaO–TiO₂(ZrO₂) and Cs₂O–BaO–TiO₂(ZrO₂) systems is investigated. It is established that the formation of the Ba₂Ti₉O₂₀ compound and Ba₂Ti₉O₂₀-based solid solutions is a multistage process proceeding through the formation of intermediate phases. The solid-phase interaction in the BaO–TiO₂(ZrO₂) system occurs through the formation of the BaTi₄O₉ intermediate compound. The Ba₂Ti₉O₂₀ single-phase product is formed only in the presence of ZrO₂ (0.82 mol %) upon heat treatment at a temperature of 1250°C for 5 h. In the Cs₂O–BaO–TiO₂(ZrO₂) system, the BaTi₅O₁₁ metastable intermediate phase is formed at the first stage of the solid-phase interaction. The Cs _x Ba_{2 – x/2}Ti_{9 – y} Zr _y O₂₀ single-phase solid solutions are prepared upon heat treatment at 1100°C for 1 h. It is demonstrated that, in the Ba₂Ti₉O₂₀ structure, cesium can isomorphously substitute for barium with the formation of Cs _x Ba_{2 – x/2}Ti_{9 – y} Zr _y O₂₀ solid solutions (0 x 0.8, y = 0 and 0.09).     

Self-assembly of BixTi1−xO2 visible photocatalyst with core–shell structure and enhanced activity

Mesoporous Bi_xTi_1−xO₂ spheres with core–shell chamber were prepared by alcoholysis under solvothermal conditions. The cross-condensation between Ti–OH and Bi–OH ensured complete incorporation of Bi-dopants into TiO₂ lattice, though Bi atom is much bigger than Ti. Meanwhile, the aggregation of titania building clusters into spheres and their subsequent reactions including dissolution and re-deposition processes lead to the hollow spheres with tunable interior structure. The Bi-doping induced strong spectral response in visible region owing to the formation of narrow intermediate energy band gaps. Meanwhile, multiple reflections within the sphere interior voids promoted the light absorbance. As a result, the as-prepared Bi_xTi_1−xO₂ spheres exhibited much higher activity than the undoped TiO₂, the Bi₂O₃/TiO₂ obtained by impregnating the TiO₂ with Bi(NO₃)₃ solution, and the Bi_xTi_1−xO₂ after being ground during photodegradation of p-chlorophenol under visible light irradiation. Meanwhile, the Bi_xTi_1−xO₂ could be used repetitively for 10 times owing to the high hydrothermal stability and the absence of Bi-leaching.     

Preparation and electrochemical characteristics of a new Li-Mn-V-O system formed from heat-treatment of a MnO2, NH4VO3 and LiNO3 mixture

New quarternary oxides (Li₂O) _x · MnO₂ · yV₂O₅ (x = 0.125 0.25, y = 0.125 0.25), formed by heating mixtures of MnO₂, NH₄VO₃ and LiNO₃ at various Li/Mn and V/Mn atomic ratios and at different temperatures (300 400 °C in air, have been characterized by X-ray diffraction, X-ray photoelectron spectroscopy (ESCA) and infrared spectroscopy. The quarternary oxide with x = 0.25 and y = 0.25 showed a discharge capacity of 220 A h (kg oxide)^–1 and an energy density of ca. 600 W h (kg oxide)^–1 at a current density of 0.20 mA cm^–1 in 1 m LiClO₄-propylene carbonate at 25 °C. When charge-discharge cycling with the (Li₂O) · MnO₂ · 0.25V₂O₅ electrode was performed at a constant capacity of 30 A h (kg oxide)^–1 and at a constant current density of 0.10 0.20 mA cm^–2, the electrode sustained over 100 cycles at a high mean discharge potential of ca. 3 V vs Li/Li⁺.This paper was originally presented at the 183rd meeting of the Electrochemical Society (Honolulu, Hawaii, 1993).     

Ceramics and Thick Films Based on Bi2 – x(Pb, Sb)xSr2Ca2Cu3Oy Compounds as High-Tc Superconductors and Electrode Materials

Complex cuprates of the compositions Bi_1.6Pb_0.4Sr_1.8Ca₂Cu₃O _y and Bi_1.7Pb_0.2Sb_0.1Sr₂Ca₂Cu₃O _y are synthesized. The electrical properties of the superconductors prepared are investigated. Thin films based on the bismuth cuprates studied are produced. It is demonstrated that the thin films prepared can be used as electrodes being in contact with a solid electrolyte for designing oxygen partial pressure gauges.     

Hypochlorite generation on Ru–Pt binary oxide for treatment of dye wastewater

Ruthenium–platinum binary oxides [(Ru + Pt)O _x ] were coated on titanium substrates by thermal decomposition. The surface morphologies and elemental analyses of these electrodes were examined by means of scanning electron microscopy. The electrochemical behaviour was characterized by cyclic voltammetry (CV) and linear scanning voltammetry (LSV). The effects of electrolyte conditions on the current efficiency (CE) of hypochlorite production on binary (Ru + Pt)O _x electrodes and the treatment of a high salt-containing dye wastewater using this hypochlorite were also investigated. The highest CE for hypochlorite production occurred on the RP1 (20 mol% Pt in precursor) electrode. The major factors influencing CE for hypochlorite production were the electrolyte flow rate, current density and chloride ion (C1^–) concentration. The RP1 electrode exhibited the best removal of organics and chromophoric groups in the dye wastewater. On this electrode, better removal of organics and chromophoric groups was obtained at 300 mA cm^–2. The colour of black–red dye wastewater became light yellow when a charge of 13.2 A h was passed while the COD of the wastewater decreased from 10 500 to 1250 mg L^–1.     

Development of ceramic electrochemical sensor based on Bi2Ru2O7+x − RuO2 sub-micron oxide sensing electrode for water quality monitoring

Sub-micron Bi₂Ru₂O_7+x + RuO₂ oxide sensing electrodes (SE) for water quality sensors were prepared on platinised ceramic substrate of the sensor. Their morphology was analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). Sensing properties of the Bi₂Ru₂O_7+x + RuO₂-SE were investigated for potentiometric detection of pH and dissolved oxygen (DO) in water in the temperature range of 4–30 °C. Sensor was capable to measure DO from 0.5 to 8.0 ppm and pH from 2.0 to 13.0, respectively. The obtained results show acceptable linearity of the measuring characteristics. Long-term stability trial for Bi₂Ru₂O_7+x + RuO₂-SE revealed that bio-fouling can be one of the main destructive factors affecting the performance of the sensors in the long run. The screen-printing technology used in the multi-sensory implementation provides fundamental properties of miniaturization, reasonable accuracy and low cost.     

Structural,magnetic, electrical,and electrochemical properties of Sr–Co–Ru–O: A hybrid‐capacitor application

In this study, we report the synthesis of SrCo_1?xRu_xO_3?δ nominal compositions, where x = 0.0‐1.0, using solid‐state reaction technique. XRD analysis confirms the structure of x = 0 sample as hexagonal Sr₆Co₅O₁₅. As the Co ions are substituted by Ru, a two‐phase structure (hexagonal R32 and orthorhombic Pbnm) emerges up to x ≤ 0.5. As the Ru content is increased further, the hexagonal R32 phase disappears completely and an orthorhombic Pbnm phase becomes the main phase. SEM images show that grain size of the samples decreases with increasing Ru content. Temperature‐dependent electrical conductivity studies indicate upon Ru substitution in the nominal SrCo_1?xRu_xO_3?δ compounds, resistivity decreases due to appearance of metallic SrRuO₃ phase. The cyclic voltammogram (CV) of the samples show capacitive properties upon Ru substitution. The cycle measurements of the capacitors yield promising results for potential supercapacitor applications.     

EXAFS and FTIR characterization of tetrarhodium carbonyl clusters attached on tris-(hydroxymethyl)phosphine grafted silica catalytically active for olefin hydroformylation

A series of perovskites of the formula Ca_1–x Sr _x Ti_1–y M _y O_3–, M=Fe, Co, Cr or Ni,x = 0–1,y = 0–0.6, has been synthesized by a modified sol-gel method using citrate. Several of these materials were proved to be stable under operating conditions in reducing atmospheres of air and hydrocarbons. An outline of the synthesis procedure is given, together with the results of XRD, SEM, BET, TG, DTA and IR characterization before and after catalytic testing. The solubility of Ni and Cr in this perovskite was very limited, and the solubility of Co decreased abruptly above 1173 K. The solubility range of Ca and Sr on alkaline earth sites is 100%.     

Kinetics and Mechanism of the Interaction of Alkali Calcium Silicate Glasses with Salt Melts in the KNO3–Pb(NO3)2 System

The interaction of alkali calcium silicate glasses with salt melts in the KNO₃–Pb(NO₃)₂ system is investigated at temperatures of 420–520°C. The chemical composition of crystalline coatings formed upon treatment contains both components of the initial glass (SiO₂, 9–12 wt %; CaO, 0.8–1.2 wt %) and components of the salt melt (PbO, 82–89 wt %). The treatment temperature is the main factor affecting the structure of the modified surface layer. The mechanism of the interaction of alkali calcium silicate glasses with salt melts is analyzed. According to this mechanism, the interaction involves the ion exchange (with the participation of Na⁺, K⁺, Ca²⁺, and Pb²⁺ ions), crystallization of modified surface layers, and incorporation of Pb _x O _y nanoparticles (formed in the salt melt) into the coating structure.     

The Abnormal Optical Property and Room‐Temperature Exchange Bias Behavior in Na‐ and Ru‐Codoped BiFeO3 Nanoparticles

Bi_0.98Na_0.02Fe_1?xRu_xO₃ (x = 0, 0.01, 0.02) samples were prepared via a sol–gel method. The formation of the desired materials was confirmed using X‐ray diffraction. The tetravalent Ru doping could effectively reduce the number of oxygen vacancy, and subsequently reduce the leakage current. Interestingly, the optical band gap was narrowed gradually in our samples, which seems contradictory with the variation in oxygen vacancy. To better understand this abnormal optical property, the effect of bandwidth was analyzed based on the change in Fe–O bond length and Fe–O–Fe bond angle, and the corresponding phenomenological qualitative model was proposed. The magnetization was increased with Ru substitution. In addition, an exchange bias (EB) phenomenon without field cooled process was observed at room temperature for all the samples, which was explained by introducing a core–shell structure model. Moreover, the EB behavior becomes more pronounced at 5 K for 1% Ru‐doped sample.     

Carbon supported Ru–Se as methanol tolerant catalysts for DMFC cathodes. Part I: preparation and characterization of catalysts

Carbon supported Ru _x Se _y O _z catalysts were prepared from Ru₃(CO)₁₂ and RuCl₃ · xH₂O as ruthenium precursors and H₂SeO₃ and SeCl₄ as the selenium sources. Highly active catalysts for the oxygen reduction reaction (ORR) in direct methanol fuel cells (DMFC) were obtained via a multi-step preparation procedure consisting of a CO₂-activation of the carbon support prior to the preparation of a highly disperse Ru particles catalyst powder that is subsequently modified by Se. Ultimately, an excess of Se was removed during a final thermal annealing step at 800 °C under forming gas atmosphere. The morphology of the catalysts was analyzed by transmission electron microscopy (TEM) and X-ray diffraction (XRD), which shows that the catalysts consist of crystalline Ru-particles with sizes ranging from 2 to 4 nm exhibiting a good dispersion over the carbonaceous support. The corresponding catalytic activity in the process of oxygen reduction was analyzed by cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements. The nature of the carbon support used for the preparation of RuSe cathode catalysts is of significant importance for the activity of the final materials. Catalysts supported on CO₂-activated Black Pearls 2000 gave the highest ORR-activity. Se stabilizes the Ru-particles against bulk oxidation and actively contributes to the catalytic activity. An exceptional property of the carbon supported Ru-particles modified with Se is their resistance to coalescence up to temperatures of 800 °C under inert or reducing conditions. Additional effects of Se-modification are the enhanced stability towards electrochemical oxidation of Ru and a lowering of the H₂O₂ formation in the ORR.     

Oxidative coupling of methane over some titanates based perovskite oxides

A series of perovskites of the formula Ca_1–xSr_xTi_1–yM_yO_3– (M = Fe or Co,x = 0–1,y = 0–0.6 for Fe,y = 0–0.5 for Co) were prepared and tested as the catalyst for the oxidative coupling of methane. The catalysts were stable under the reaction conditions. The catalysts of high p-type and oxide ionic conductivity afforded the high selectivity. Some catalysts containing Co on B-sites are thermally unstable and decomposed to metal oxide components at high temperature, giving rise to synthesis gas production.     

Selective oxidation of hydrogen sulfide containing excess water and ammonia over Bi-V-Sb-O catalysts

We investigated the selective oxidation of hydrogen sulfide to elemental sulfur and ammonium thiosulfate by using Bi₄V_2-xSb_xO_11-y catalysts. The catalysts were prepared by the calcination of a homogeneous mixture of Bi₂O₃, V₂O₅, and Sb₂O₃ obtained by ball-milling adequate amounts of the three oxides. The main phases detected by XRD analysis were Bi₄V₂O₁₁, Bi_1.33V₂O₆, BiSbO₄ and BiVO₄. They showed good H₂S conversion with less than 2% of SO₂ selectivity with a feed composition of H₂S/O₂/NH₃/H₂O/He=5/2.5/5/60/27.5 and GHSV=12,000 h^-1 in the temperature ranges of 220–260 ‡C. The highest H₂S conversion was obtained for x=0.2 in Bi₄V_2-xSb_xO_11-y catalyst. TPR/TPO results showed that this catalyst had the highest amount of oxygen consumption. XPS analysis before and after reaction confirmed the least reduction of vanadium oxide phase for this catalyst during the reaction. It means that the catalyst with x=0.2 had the highest reoxidation capacity among the Bi₄V_2-xSb_xO_11-y catalysts.     

Oxygen octahedra distortion-induced multiferroic properties of Er and Zr co-doped BiFeO3 nanoparticles

The present work unveiled the distortion of oxygen octahedra influencing magnetic and magnetoelectric properties of novel Bi_1−xEr_xFe_1−yZr_yO₃ (x = 0, .05, .1, y = .02, .05) polycrystalline nanoparticles by sol–gel route. X-ray diffraction patterns analysis reveals that pristine BiFeO₃ and doped BiFeO₃ are crystalized in the rhombohedral structure (R3c). The Fe–O–Fe bond angle of Bi_1−xEr_xFe_1−yZr_yO₃ (x = 0, .05, .1, y = .02, .05) varies between 141° and 159.62° as the concentration of Er (via Bi site) and Zr (via Fe site) ions increases in BiFeO₃. As a result, the tilt angle of oxygen octahedra and the canting angle of spiral spin arrangement increase. Hence, the maximum magnetization varies between .03144 and .37558 emu/g in Er and Zr co-doped BiFeO₃ system. The number of electrons per unit cell of Bi_1−xEr_xFe_1−yZr_yO₃ (x = 0, .05, .1, y = .02, .05) lies between 733.38 and 831, respectively. Further, the number of coherently diffracting domains increases from 3.07 to 5.21, and then it decreases when Er and Zr are increased in BiFeO₃. Consequently, the magnetoelectric coupling coefficient varies between .0265 and .2511 mV/cm Oe, respectively. Particularly, Bi_0.95Er_0.05Fe_0.98Zr_0.02O₃ shows enhanced magnetic and magnetoelectric behaviors compared to other samples.     

High energy ball-milled Pt and Pt–Ru catalysts for polymer electrolyte fuel cells and their tolerance to CO

High energy ball milling, an industrially amenable technique, has been used to produce CO tolerant unsupported Pt–Ru based catalysts for the oxidation of hydrogen in polymer electrolyte fuel cells. Nanocrystalline Pt_0.5–Ru_0.5 alloys are easily obtained by ball-milling but their performances as anode catalysts are poor because nanocrystals composing the material aggregate during milling into larger particles. The result is a low specific area material. Improved specific areas were obtained by milling together Pt, Ru and a metal leacheable after the milling step. The best results were obtained by milling Pt, Ru, and Al in a 1:1:8 atomic ratio. After leaching Al, this catalyst (Pt_0.5–Ru_0.5 (Al₄)) displays a specific area of 38 m²g^–1. Pt_0.5–Ru_0.5 (Al₄) is a composite catalyst. It consists of two components: (i) small crystallites (4 nm) of a Pt–Al solid solution (1–3 Al wt%) of low Ru content, and (ii) larger Ru crystallites. It shows hydrogen oxidation performance and CO tolerance equivalent to those of Pt_0.5–Ru_0.5 Black from Johnson Matthey, the commercial catalyst which was found to be the most CO tolerant one in this study.