I. V. Chernykh

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

CuO+MxOy/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.

Silicate coatings on titanium modified by cobalt and/or copper oxides and their activity in CO oxidation

SiO2 + TiO2/Ti composites have been formed by plasma electrolytic oxidation (PEO) of titanium in alkaline aqueous solution of sodium silicate. By modifying them using impregnation in nitrate solutions followed by annealing, the oxide composites Co3O4/SiO2 + TiO2/Ti, CuO/SiO2 + TiO2/Ti, and Co3O4 + CuO/SiO2 + TiO2/Ti have been obtained. The latter composites were tested in the model reaction of CO oxidation into CO2. The activation energy has been calculated: for the initial composites, it is equal to 42.6 kJ/mol, whereas, for the composites modified by cobalt oxide, copper oxide, and cobalt/copper oxides, it is equal to 141.8, 86.9, and 97.8 kJ/mol, respectively. The surface morphology and the composition of coatings have been investigated by the methods of scanning electron microscopy, X-ray spectral microprobe analysis, and X-ray photoelectron spectroscopy. Surface morphological structures with the highest concentrations of transition metals, which could serve as catalytically active sites, have been determined: “flakes” (Co3O4), “grains’” (CuO), and “hedgehogs” (Co3O4 +CuO).

Catalytically active cobalt-copper-oxide layers on aluminum and titanium

Oxide coatings modified with cobalt and copper oxides are obtained on titanium and aluminum by means of combining plasma electrolytic oxidation (PEO) in silicate and zirconate electrolytes and impregnation in nitrate solutions followed by annealing. The effect of PEO coatings that were preliminarily formed on aluminum and titanium in different electrolytes on the composition and surface morphology of cobaltcopper oxide composites and their activity with respect to CO oxidation is studied. The maximum total concentration of cobalt and copper is found to be typical of composite layers based on SiO2 + Al2O3/Al, while the minimum content is observed in the case of layers based on ZrO2 + TiO2/Ti, the PEO bases being characterized by the highest and lowest water-absorbing capacity, respectively. The effect of PEO coatings on the catalytic activity of cobalt-copper oxide catalysts decreases in the series SiO2 + TiO2/Ti > SiO2 + Al2O3/Al > Ce2O3 + ZrO2 + TiO2/Ti > ZrO2 + TiO2/Ti.

Bifunctional Fe-containing coatings formed on aluminum by plasma-electrolytic oxidation

Oxide coatings with a thickness of about 30 μm were deposited. The coatings contain approximately 21 at % Fe and 2.3 at % Cu and exhibit, together with ferromagnetic properties, a catalytic activity in CO oxidation at temperatures higher than 260°C.

Oxide coatings modified with transition and rare-earth metals on aluminum and their activity in CO oxidation

Oxide catalysts, which, in addition to Al2O3 + SiO2, contain from one (Ni) to four oxides or compounds of transition metals (Ni, Cu, Mn, and Co) and oxides or compounds of rare-earth elements (Ce, La), are produced on D16 aluminum alloy by plasma electrolytic oxidation combined with impregnation and subsequent annealing. The composites formed begin to catalyze the CO oxidation in a temperature range of from 100 to 300°C. The catalysts used can be arranged in the following series of decreasing catalytic activity: Ni-Cu-Mn-Co-Ce > Ni-Cu-Mn-Co-Ce-La ≈ Ni-Cu-Mn-Co > Ni-Cu-Mn > Ni-Cu > Ni. Oxygen compounds of Cu+, Cu2+, Mn4+, Co3+, Ce3+, and Ce4+, which seem to determine the catalytic properties of the oxide systems studied, are found on the surface and in the subsurface layer with a total thickness of ∼6 nm of the most active Ni-Cu-Mn-Co-Ce catalyst.

An X-ray photoelectron spectroscopy study of Ni, Cu-containing coatings formed by plasma electrolytic oxidation on aluminum and titanium

A series of Ni- and/or Cu-containing coatings formed by plasma electrolytic oxidation on aluminum and titanium are examined by X-ray photoelectron spectroscopy. Binding energies of core electrons, elemental composition, chemical state of elements, and features of the structural organization of the surface and nearsurface layers of the coatings are determined. A combination of the data collected indicates similar regularities of the composition and significant distinctions in the structure of the coatings formed. It is shown that the coatings formed on titanium are characterized by a considerably higher phosphorus concentration, and correspondingly, phosphates, unlike the coatings formed on aluminum, in which base metal and 3d element (Ni or Cu) oxides are dominant. In both cases, Cu is mainly concentrated in the surface layers of the coatings whereas Ni is mainly concentrated in the near-surface layers.

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.

Growth of nanowires on the surfaces of multicomponent oxide coatings on titanium

The thermal behavior of Ni- and Cu-containing coatings on titanium formed by plasma electrolytic oxidation and additionally modified with nickel and copper oxides is studied. Annealing of the produced multiphase coatings in air at a temperature of 750°C or higher is shown to result in the growth of surface nanowires, the main components of which are nickel, oxygen, and titanium. The sizes of nanowires depend on the temperature of annealing, and the diameters can be as large as tens or hundreds of nanometers at a length of several to tens of microns. Experimental and literature data show that the combination of plasma electrolytic oxidation with impregnation and annealing is promising for the production of both nanowires bound to metal-oxide substrates and individual nanostructures of certain compositions.

Formation, composition, and catalytic activity of multicomponent oxide structures on aluminum

Possibility is considered of obtaining multicomponent oxide systems as afterburning catalysts for automobiles by flame-electrolytic oxidation and impregnation of oxide structures on aluminum.