•The Ni/Y-doped ZrO2 catalysts show highly catalytic activity for CO2 methanation.•Bidentate carbonate is a major adsorption spice on the Ni/Y-doped ZrO2 catalysts.•The oxide support of t-ZrO2 and/or ...c-ZrO2 with oxygen vacancies plays a key role.
The catalytic methanation of CO2 was carried out on Ni catalysts supported on Y-doped ZrO2 with various Y3+ concentrations and Ni/(Zr+Y) molar ratio=1. The catalysts were characterized by X-ray diffraction, scanning transmission electron microscopy, specific surface area, temperature-programmed desorption of CO2, and temperature-programmed reaction. In addition, operando diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFT) was used to identify the adsorbed reaction intermediate. Catalysts supported on Y-doped ZrO2 show higher catalytic activity than the catalyst on Y-free ZrO2 with a monoclinic ZrO2 phase. The catalytic activity is also dependent upon the Y3+ concentration, and the highest activity was obtained for the catalyst with a Y/(Zr+Y) molar ratio of 0.333, which consists mainly of fcc Ni and cubic ZrO2 phase. Y3+ doping into ZrO2 introduces oxygen vacancies, which play an important role in enhancing the catalytic activity. The operando DRIFT study reveals that a CO adsorption intermediate is absent, and bidentate carbonate is an important intermediate for CH4 formation.
► Wear-resistant coatings are formed on β-titanium alloy by plasma electrolytic oxidation. ► Better coatings are formed at lower electrolyte temperatures. ► High speed video imaging was used to ...monitor the discharge behavior during the process.
Plasma electrolytic oxidation of single β-phase Ti–15 mass% V–3 mass% Al–3 mass% Cr–3 mass% Sn (hereafter denoted as Ti–15–3) alloy has been conducted in alkaline aluminate electrolyte at different electrolyte temperatures between 278 and 313K. The results obtained disclose the highly improved wear resistance of the coatings formed at the lowest temperature of 278K. The coating formed at this temperature has lower porosity and contains higher concentration of α-Al2O3 phase in addition to the Al2TiO5 major phase. In contrast, non-uniform coatings are formed at higher temperatures and their porosity is relatively high. Thus, the highest wear resistance of the Ti–15–3 is obtained when the coating is formed at 278K. Such influence of the electrolyte temperature on the coating morphology and composition is discussed by direct imaging based on direct video imaging data during the coating process.
This paper reports highly active CO2 methanation catalysts of Ni supported on tetragonal ZrO2 for supply of renewable energy in the form of CH4 via electrolytic H2 generation. The Ni/tetragonal ZrO2 ...catalysts with the nominal cationic compositions of 50 at.% Ni-(Zr + Ca) (Ca/(Zr + Ca) molar ratios of 0.125–0.333), were prepared by calcination of a ZrO2 sol mixed with nickel and calcium nitrates at 650 and 800 °C and subsequent H2 reduction of NiO. All the catalysts prepared exhibited almost 100% CH4 selectivity, and the activity increased with the calcium content up to a Ca/(Zr + Ca) molar ratio between 0.167 and 0.200, above which the activity tended to decrease. At the optimized calcium content, a tetragonal ZrO2 phase with cation vacancies introduced by Ca2+ and Ni2+ doping was predominant, in addition to fcc Ni. A monoclinic ZrO2 phase was also present below the optimized calcium content, while a Ca-rich amorphous or CaZrO3 phase was formed with tetragonal ZrO2 above the optimized calcium content. The high catalytic activity of the catalyst with the optimized composition could be correlated with the presence of tetragonal ZrO2 with oxygen vacancies.
•Ni/tetragonal-ZrO2 catalysts doped with Ni2+ and Ca2+ show high activity for CO2 methanation.•There is an optimized Ca2+ content to enhance the activity.•The oxygen vacancies in ZrO2 may play an important role in enhancing the catalytic activity.
The development of pores in a classical porous anodic film formed on aluminium in phosphoric acid solution is investigated. The study employs a tungsten tracer layer that is incorporated into the ...anodic film from the aluminium substrate, followed by detection of the tracer by transmission electron microscopy and Rutherford backscattering spectroscopy. Distortions of the tungsten layer on entry into the film and retention of tungsten species in the film are compatible with porosity arising mainly from flow of anodic oxide beneath the pore bases towards the cell walls. The behaviour is contrary to expectations of a dissolution model of pore formation.
Redox-based resistive switching memories (ReRAMs) are the strongest candidates for next generation nonvolatile memories. These devices are commonly composed of metal/solid electrolyte/metal ...junctions, where the solid electrolyte is usually an oxide layer. A key aspect in the ReRAMs development is the solid electrolyte engineering, since it is crucial to tailor the material properties for obtaining excellent switching properties (
e.g.
retention, endurance,
etc.
). Here we present an anodizing process as a non vacuum and low temperature electrochemical technique for growing oxides with tailored structural and electronic properties. The effect of the anodizing conditions on the solid state properties of the anodic oxides is studied in relation to the final ReRAM device performances demonstrating the great potentiality of this technique to produce high quality oxide thin films for resistive switching memories.
Electrochemically grown anodic oxides of different compositions and properties were tested as solid electrolytes for resistive switching memories.
The rapid inward migration of fluoride ions in growing anodic titanium oxide under a high electric field has been elucidated by anodizing a Ti–12
at% silicon alloy, where film growth proceeds at ...nearly 100% efficiency in selected electrolytes. Further, incorporated silicon species in the anodic film are immobile, acting as marker species. The migration rate of fluoride ions is determined precisely by three-stage anodizing, consisting of initial anodic film formation at a constant current density to 50
V in ammonium pentaborate electrolyte, subsequent incorporation of fluoride ions by reanodizing to 55
V in ammonium fluoride electrolyte and, finally, anodizing again in ammonium pentaborate electrolyte at high current efficiency. The resultant films were analyzed by glow discharge optical emission spectroscopy to reveal the depth distribution of fluoride ions and the location of the silicon marker species. The fluoride ions migrate inward at twice the rate of O
2− ions. Consequently, anodizing of titanium in fluoride-containing electrolytes develops a fluoride-rich layer that separates the alloy substrate from the anodic oxide, with eventual detachment of the film from the substrate.
Ti–15V–3Al–3Cr–3Sn (Ti–15–3) is one of the important practical titanium alloys with high cold deformability and high mechanical strength, but its wear resistance is poor. This paper reports the ...formation of wear-resistant and adhesive ceramic coatings on Ti–15–3 by two-step plasma electrolytic oxidation (PEO). The PEO of Ti–15–3 has been carried out first in alkaline aluminate electrolyte to form a wear-resistant oxide layer and then in acid electrolyte containing both phosphoric acid and sulfuric acid to improve adhesion of the coating. The coating formed in the alkaline aluminate electrolyte is more than 10μm thick, and highly crystalline. The main phase is Al2TiO5. This coating shows high wear resistance, but is not adherent to substrate due to the development of a number of voids and pores in the oxide layer close to the substrate. A new oxide layer with amorphous structure is formed next to the substrate in the subsequent PEO in the acid electrolyte, during which the voids are filled with a new oxide formed in the acid electrolyte, reducing the porosity. As a consequence, the adhesion of the coating is markedly improved without deteriorating the high wear resistance.
► Two-step PEO was carried out to form a wear-resistant coating on Ti–15V–3Al–3Cr–3Sn. ► The first step PEO in alkaline aluminate electrolyte formed a wear-resistant coating. ► The second step PEO in acid electrolyte improved the coating adhesion markedly. ► During the second step PEO, voids near the alloy/coating interface were healed.
Anodic films were grown to 20 V on sputtering-deposited Al-Ta alloys in ammonium biborate and borate buffer solutions. According to glow discharge optical emission spectroscopy, anodizing in ammonium ...containing solution leads to the formation of N containing anodic layers. Impedance measurements did not evidence significant differences between the dielectric properties of the anodic films as a function of the anodizing electrolyte. Photoelectrochemical investigation allowed evidencing that N incorporation induces a red-shift in the light absorption threshold of the films due to the formation of allowed localized states inside their mobility gap. The estimated Fowler threshold for the internal photoemission processes of electrons resulted to be independent of the anodizing electrolyte confirming that N incorporation does not appreciably affect the density of states distribution close to the conduction band mobility edge. The transport of photogenerated carriers has been rationalized according to the Pai-Enck model of geminate recombination.
The study examined the influence of fluorozirconic acid additions to the electrolyte on the growth rate and composition of porous anodic films formed on AA 2024-T3 alloy in sulphuric acid and ...tartaric-sulphuric acid electrolytes. A range of anodizing voltages, electrolyte temperatures, fluorozirconic acid concentrations and anodizing times was employed. High purity aluminium was employed as a reference material. Fluorozirconic acid increased the film growth rate, with a reducing influence as the temperature increased from 0 to 37 °C. The growth rate was enhanced by increased additions of fluorozirconic acid to the electrolyte, although chemical dissolution of the film was also accelerated. The films contained fluoride and sulphate ions from the electrolyte, with the fluoride concentrations decreasing with increasing temperature whereas the sulphate concentration was relatively unaffected.
•Fluorozirconic acid (FZ) enhances film growth on aluminium/AA 2024 alloy in SA/TSA.•Effect increases with FZ concentration and decreases with acid temperature.•Fluoride and sulphate incorporated into film, but negligible zirconium species.•FZ enhances film dissolution, increasing with FZ concentration and temperature.•Negligible influence on corrosion protection due to absence of inhibiting species.
The formation of self-organized nanoporous anodic films on Type 304 stainless steel in fluoride-containing ethylene glycol electrolyte is reported. A key factor in the formation of the nanoporous ...anodic films is the water concentration in the electrolyte, which must be 0.3moldm−3 or less for the present conditions of film formation, although nanoporous anodic films have been formed on iron at higher water concentrations. The films contain relatively high concentrations of iron, chromium, nickel, oxygen and fluorine species distributed throughout the film thicknesses. The fluoride species can be removed by post-annealing in air, which improves the chemical stability of the films without damaging the nanoporous morphology of the remaining oxide.
► Self-organized nanoporous anodic films were formed on stainless steel. ► The films grew by the reduction of H2O content in the ethylene glycol electrolytes. ► Post-annealing converted water-soluble oxyfluoride to insoluble oxide.
PDF
