RESUMO
The in situ growth of N-doped multi-walled carbon nanotubes (N-MWCNTs) from the products of graphitic carbon nitride (g-C3N4) etching by Ni nanoparticles in a hydrogen atmosphere has been confirmed for the first time. During the etching process of g-C3N4, the building blocks, notably methane, ammonia, and hydrogen cyanide, are formed. The formation of N-MWCNTs was confirmed by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning (SEM) and transmission electron microscopy (TEM). A sponge-like carbonaceous structure was obtained with a specific surface area of 384 m2 g-1 from initial g-C3N4 (32 m2 g-1).
RESUMO
Pt, Ru, and Ir were introduced onto the surface of graphitic carbon nitride (g-C3N4) using the wet impregnation method. A reduction of these photocatalysts with hydrogen causes several changes, such as a significant increase in the specific surface area, a C/N atomic ratio, a number of defects in the crystalline structure of g-C3N4, and the contribution of nitrogen bound to the amino and imino groups. According to the X-ray photoelectron spectroscopy results, a transition layer is formed at the g-C3N4/metal nanoparticle interphase, which contains metal at a positive degree of oxidation bonded to nitrogen. These structural changes significantly enhanced the photocatalytic activity in the production of hydrogen through the water-splitting reaction. The activity of the platinum photocatalyst was 24 times greater than that of pristine g-C3N4. Moreover, the enhanced activity was attributed to significantly better separation of photogenerated electron-hole pairs on metal nanoparticles and structural distortions of g-C3N4.
RESUMO
Research was carried out on the incorporation of divalent cobalt cations into the crystalline structure of MgF2 to form Mg x Co1-x F2 binary fluorides, which had not been investigated before. The above fluorides were obtained by the precipitation from aqueous solution of magnesium and cobalt nitrates with ammonium fluoride. Binary fluorides containing 0.6, 7.5 and 37.7 mol% CoF2 were prepared. The effects of treatment temperature (300, 400 °C) and atmosphere (oxidizing or reducing) on the structure (XRD, TPR-H2, UV-Vis), texture (low-temperature N2 adsorption), surface composition (XPS) and surface acidity (NH3-TPD) of the binary fluorides were determined. It has been found that in Mg x Co1-x F2 an isomorphic substitution occurs of Mg2+ cations by Co2+ cations which results in the formation of a rutile-type solid solution. The obtained binary fluorides are characterized by a mesoporous structure and relatively large surface area. It has been found that thermal treatment of the binary fluorides in oxidizing conditions results in the oxidation of CoF2 to Co3O4 even at 300 °C; therefore it is not possible to obtain pure Mg x Co1-x F2 binary fluorides in the presence of air. The preparation of the latter requires reducing conditions, namely thermal treatment of dry precipitate at 300 °C in an atmosphere of hydrogen. If the treatment is conducted at a higher temperature (400 °C), CoF2 undergoes a partial reduction to metallic cobalt. An XPS study has shown the presence of hydroxyl groups in the investigated samples. However, these are solely surface groups because their presence was not detected by XRD measurements. The binary fluorides obtained by our method are characterized by a very narrow optical energy gap (5.31-3.50 eV), considerably narrower than that recorded for bulk fluorides. Measurements of temperature-programmed desorption of ammonia have shown that the incorporation of cobalt cations into the crystal structure of MgF2 results in a decrease in the surface acidity of the binary fluorides.
