Silicate coatings on titanium, modified with transition metal oxides and their activity in CO oxidation

CuO+M _x O _y /TiO₂+SiO₂/Ti composites (M = Mn, Fe, Co, Ni) were produced by plasma-electrolytic oxidation and impregnation, followed by annealing. The elemental and phase composition of these composites were examined and their activity series in CO oxidation was determined.     

Catalytic oxidation of CO on platinum

Using concepts recently developed in thermal decompositions of solids and reduction of bulk oxides by gases and (re)analysis of experimental literature data, a novel mechanism for the catalytic oxidation of CO by PtO₂ is proposed. Instead of the conventional Mars–van Krevelen scheme, the reactions proposed are: PtO₂(s) + 2CO ? Pt(g) + 2CO₂ and Pt(g) + O₂ ? PtO₂(g) → PtO₂(s). The first reaction determines the kinetics of CO oxidation and the second determines the kinetics of restoration of the PtO₂ layer. Thermochemical consideration of the kinetic features of this model, based on Langmuir’s quasi-equilibrium equations for evaporation of simple substances, allowed calculation of the reaction enthalpy and the absolute rate of CO oxidation. These results are in good agreement with experimental data. The proposed mechanism explains the origin of the surface-retexturing effect, the limited loss of Pt metal from the catalyst, the mechanism of PtO₂ regeneration by oxygen, the strong effect of CO₂ in reducing the CO oxidation rate and the three-fold variation of the Arrhenius E parameter with temperature.     

Formation of oxide layers modified with polytetrafluoroethylene or graphite on aluminum and titanium alloys by plasma electrolytic oxidation

For one-stage formation of coatings containing polytetrafluoroethylene or graphite, it was proposed to use Na₂SiO₃ + NaOH aqueous electrolyte containing siloxane-acrylate emulsion and dispersed powders of polytetrafluoroethylene or graphite. The phase and elemental composition and surface morphology of the formed coatings were determined. It was shown that the introduction of polytetrafluoroethylene markedly increases the hydrophobic and wear-resistant properties of coatings.     

Anodic-spark layers on aluminum and titanium alloys in electrolytes with sodium phosphotungstate

The influence exerted by the pH of an aqueous 0.0083 M solution of Na₂H[PW₁₂O₄₀] on the composition and morphology of anodic coatings galvanostatically formed on aluminum and titanium alloys at sparking and breakdown voltages was studied.Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 9, 2004, pp. 1472–1480.Original Russian Text Copyright © 2004 by Lukiyanchuk, Rudnev, Kuryavyi, Gordienko.     

Iron- and nickel-containing oxide-phosphate layers on aluminum and titanium

Oxide-phosphate surface structures with thicknesses of 5 to 50 μm containing iron and nickel compounds have been synthesized by plasma electrolysis (PE) in aqueous electrolytes with polyphosphate nickel(II) and iron(III) complexes. The elemental and phase compositions have been studied as affected by polarization parameters, electrolytes, and annealing parameters. Simple and complex oxides and phosphates crystallize in layers: on aluminum, AlPO₄, NiAl₂O₄, and Fe₂O₃ crystallize; on titanium, Ni₂P₂O₇, Ni_0.5TiOPO₄, NaTi₂(PO₄)₃, M^(II)M^(III)Ti(PO₄)₃, FePO₄, and Fe₂Fe(P₂O₇)₂ crystallize.     

Chemisorption of CO and mechanism of CO oxidation on supported platinum nanoclusters

Kinetic, isotopic, and infrared studies on well-defined dispersed Pt clusters are combined here with first-principle theoretical methods on model cluster surfaces to probe the mechanism and structural requirements for CO oxidation catalysis at conditions typical of its industrial practice. CO oxidation turnover rates and the dynamics and thermodynamics of adsorption-desorption processes on cluster surfaces saturated with chemisorbed CO were measured on 1-20 nm Pt clusters under conditions of strict kinetic control. Turnover rates are proportional to O(2) pressure and inversely proportional to CO pressure, consistent with kinetically relevant irreversible O(2) activation steps on vacant sites present within saturated CO monolayers. These conclusions are consistent with the lack of isotopic scrambling in C(16)O-(18)O(2)-(16)O(2) reactions, and with infrared bands for chemisorbed CO that did not change within a CO pressure range that strongly influenced CO oxidation turnover rates. Density functional theory estimates of rate and equilibrium constants show that the kinetically relevant O(2) activation steps involve direct O(2)* (or O(2)) reactions with CO* to form reactive O*-O-C*=O intermediates that decompose to form CO(2) and chemisorbed O*, instead of unassisted activation steps involving molecular adsorption and subsequent dissociation of O(2). These CO-assisted O(2) dissociation pathways avoid the higher barriers imposed by the spin-forbidden transitions required for unassisted O(2) dissociation on surfaces saturated with chemisorbed CO. Measured rate parameters for CO oxidation were independent of Pt cluster size; these parameters depend on the ratio of rate constants for O(2) reactions with CO* and CO adsorption equilibrium constants, which reflect the respective activation barriers and reaction enthalpies for these two steps. Infrared spectra during isotopic displacement and thermal desorption with (12)CO-(13)CO mixtures showed that the binding, dynamics, and thermodynamics of CO chemisorbed at saturation coverages do not depend on Pt cluster size in a range that strongly affects the coordination of Pt atoms exposed at cluster surfaces. These data and their theoretical and mechanistic interpretations indicate that the remarkable structure insensitivity observed for CO oxidation reactions reflects average CO binding properties that are essentially independent of cluster size. Theoretical estimates of rate and equilibrium constants for surface reactions and CO adsorption show that both parameters increase as the coordination of exposed Pt atoms decreases in Pt(201) cluster surfaces; such compensation dampens but does not eliminate coordination and cluster size effects on measured rate constants. The structural features and intrinsic non-uniformity of cluster surfaces weaken when CO forms saturated monolayers on such surfaces, apparently because surfaces and adsorbates restructure to balance CO surface binding and CO-CO interaction energies.     

Effect of cerium oxide on the activity of platinum catalysts of hydrogen and CO oxidation

Effective catalytic systems based on Pt/CeO₂-C were prepared. The use of a cerium oxide addition in the substrate stabilized the platinum catalysts against their poisoning with carbon monoxide at room temperature.     

Effects of anodic spark oxidation by pulse power on titanium substrates

The characteristics of the oxide layer of titanium generated by anodic spark oxidation are affected significantly by the process variables. In this study, electrochemical treatments were performed while applying a direct current, a pulse current, and a reverse pulse current during anodic spark oxidation. A mixed solution of 0.015 M DL‐α‐GP (DL‐α‐glycerophosphate disodium salt) and 0.2 M CA (calcium acetate) was used as the electrolyte. The pore size generated after anodic spark oxidation was smallest in the group exposed to the reverse pulse current followed in order by the pulse current and direct current. Anatase was the major crystal phase of the TiO₂ produced on the surfaces subjected to 280 V, and the rutile phase was additionally detected in the group subjected to 320 V. The crystals precipitated on the surface after the hydrothermal treatment were hydroxyapatite (HA) crystals that had a polygonal bar‐shaped needle structure. Good activity was observed in the 320‐V pulse treated group, in which very thin needle‐shaped crystals were observed after immersing the samples in Hanks' solution for 4 weeks. The cell viability was increased greatly by anodic spark oxidation, and the surface roughness was also increased. It is believed that the surface treated using a pulse current has excellent characteristics, making it suitable for applications in biomaterials. Copyright © 2010 John Wiley & Sons, Ltd.     

Deposition of cobalt-containing films on titanium by plasma electrolytic oxidation

The possibility of forming Co-containing coatings on titanium by plasma electrolytic oxidation and by a combination of oxidation and impregnation methods using an aqueous solution of sodium silicate as the base electrolyte was examined. Comparative analysis of the composition and structure of the systems obtained and of their activity in oxidation of CO to CO₂ was performed.     

Reaction of CO oxidation on platinum,rhodium, a platinum-rhodium alloy,and a heterophase bimetallic platinum/rhodium surface

The reaction of CO oxidation on thin metal films of platinum, rhodium, and their alloy and on a heterophase bimetallic Pt/Rh surface that consisted of platinum particles of size 10–20 nm on the surface of rhodium was studied in the region of low reactant pressures (lower than 2 × 10^?5 mbar). At low temperatures (T < 200°C), the activity of samples increased in the order Rh > Pt/Rh > Pt-Rh alloy > Pt. Above 200°C, the rate of reaction on the heterophase Pt/Rh surface was almost twice as high as the sum of the rates of reaction on the individual metals; this fact is indicative of a synergistic effect. The nature of this effect is considered.     

Effective platinum catalysts for low-temperature oxidation of CO

A number of platinum-based catalysts bonded on carbon fiber karbopon were prepared using alcohol solutions of H₂PtCl₆. The catalytic activity of the samples was determined in the low-temperature oxidation of carbon monoxide by oxygen. The effect of the solvent nature of the precursor on the activity of the catalysts was determined. The best results were obtained in the presence of 1.5% Pt/karbopon catalyst prepared using isobutyl alcohol; 100% CO conversion was achieved in respiratory treatment (12000 h^?1, room temperature).     

Multi-method characterization of anodic oxidation of a titanium alloy in fluoride-containing electrolytes

Journal of Solid State Electrochemistry - Anodic film growth on a Ti alloy in water-containing ethylene glycol electrolyte with addition of fluoride...     

Oxide layers with ferro- and ferrimagnetic characteristics formed on aluminum via plasma electrolytic oxidation

(Fe+Co)-containing oxide coatings are formed on aluminum alloy. It is established that at a temperature of ～2 K, the coatings manifest ferromagnetic properties. The coatings, formed within 5 min, are characterized by an anomalously high coercive force H _c = 1300 Oe. At room temperature (300 K), they manifest ferromagnetic properties, whereas in external fields of ～30 kOe they tend to transition into the antiferromagnetic state. It is concluded that magnetism is in this case due to the presence in the coating’s pores of crystallites ～50–100 nm in size that consist predominantly of reduced iron, cobalt, and aluminum.     

Intercalation of lithium into porous titanium oxide produced by anodic oxidation

Process of lithium intercalation in a 1 M anhydrous propylene carbonate solution of LiClO₄ into porous titanium oxide produced by anodic oxidation of titanium in aqueous and nonaqueous electrolytes containing fluoride ions was studied. It is shown that the mass of intercalated lithium, which determines the efficiency of the cathodic reaction of lithium titanate formation, strongly depends on the specific surface area of the oxide. The results obtained make it possible to formulate approaches to solving the problem of raising the reversible electrical capacity of a lithium power source.     

Fabrication of polytetrafluoroethylene- and graphite-containing oxide layers on aluminum and titanium and their structure

Stable aqueous electrolyte emulsions with negatively charged micelles containing dispersed particles of polytetrafluoroethylene (PTFE) or graphite are obtained using siloxane-acrylate emulsion as an emulsifier. The oxide coatings formed in such electrolytes contain carbon, polytetrafluoroethylene, or graphite. The coatings with PTFE particles are similar to monolithic polytetrafluoroethylene with respect to its hydrophobic characteristics. According to X-ray photoelectron spectroscopy data, the surface of the formed coatings predominantly contains aliphatic carbon (C-C and C-H bonds) and some fraction of oxidized (or, in the case of PTFE-containing electrolytes, fluorinated) carbon.     

阳极氧化条件对氧化钛纳米管阵列形貌及光催化性能的影响

以阳极氧化法制备了高度有序的TiO2纳米管阵列,采用扫描电镜观察了不同制备条件下得到的TiO2纳米管阵列的微观形貌,分别考察了电解时间、电压、电解液组成对TiO2纳米管阵列形貌的影响;此外,以酸性红3R染料作为标志物,测定了制得的TiO2纳米管阵列片的光催化性能.结果表明,以0.5%NH4F(质量分数)和乙二醇与水按体积比5∶1混合得到的溶液作为电解液,在15V电压下电解2h,得到的TiO2纳米管阵列的纳米管管径适中(60nm)、大小均一、排列整齐.所制备的TiO2纳米管阵列对染料的降解率显著优于纳米TiO2水溶胶的,且其光催化性能与形貌、电解时间及煅烧温度等密切相关.