Versatile application of BiVO4/TiO2 S-scheme photocatalyst: Photocatalytic CO2 and Cr(VI) reduction.

Herein, the synthesis, characterization, and reduction properties of 2D TiO2 aerogel powder decorated with BiVO4 (TiO2/BiVO4) were investigated for versatile applications. First, 2D TiO2 was prepared via lyophilization and subsequently modified with BiVO4 using a wet impregnation method. The morphology, structure, composition, and optical properties were evaluated using transmission electron microscopy (TEM), X-ray diffractometry (XRD), laser-induced breakdown spectroscopy (LIBS), and diffuse reflectance spectroscopy (DRS), respectively. Significantly enhanced photocurrent densities (by 3-15 times) were obtained for TiO2/BiVO4 compared to those of pure TiO2 and BiVO4. The reduction of toxic Cr(VI) to Cr(III) was assessed, including the effect of pH on overall photocatalytic efficiency. Under acidic conditions (pH âˆ¼ 2), Cr(VI) reduction efficiency reached 100% within 2 h. For photocatalytic CO2 reduction, the highest yields of CH4 and CO were obtained using TiO2/BiVO4. A higher efficiency for both applications was achieved because of the better separation of the electron-hole pairs in TiO2/BiVO4. The excellent stability of TiO2/BiVO4 over repeated runs highlights its potential for use in versatile environmental applications. The efficiency of TiO2/BiVO4 is due to the interplay of the structure, morphology, composition, and photoelectrochemical properties that favour the material for the presented herein photocatalytic applications.

Synthesis of vacant graphitic carbon nitride in argon atmosphere and its utilization for photocatalytic hydrogen generation.

Graphitic carbon nitride (C3N4) was synthesised from melamine at 550 °C for 4 h in the argon atmosphere and then was reheated for 1-3 h at 500 °C in argon. Two band gaps of 2.04 eV and 2.47 eV were observed in all the synthetized materials. Based on the results of elemental and photoluminescence analyses, the lower band gap was found to be caused by the formation of vacancies. Specific surface areas of the synthetized materials were 15-18 m2g-1 indicating that no thermal exfoliation occurred. The photocatalytic activity of these materials was tested for hydrogen generation. The best photocatalyst showed 3 times higher performance (1547 µmol/g) than bulk C3N4 synthetized in the air (547 µmol/g). This higher activity was explained by the presence of carbon (VC) and nitrogen (VN) vacancies grouped in their big complexes 2VC + 2VN (observed by positron annihilation spectroscopy). The effect of an inert gas on the synthesis of C3N4 was demonstrated using Graham´s law of ammonia diffusion. This study showed that the synthesis of C3N4 from nitrogen-rich precursors in the argon atmosphere led to the formation of vacancy complexes beneficial for hydrogen generation, which was not referred so far.

Reductive Modification of Carbon Nitride Structure by Metals-The Influence on Structure and Photocatalytic Hydrogen Evolution.

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.

Influence of High Temperature Synthesis on the Structure of Graphitic Carbon Nitride and Its Hydrogen Generation Ability.

Graphitic carbon nitride (g-C3N4) was obtained by thermal polymerization of dicyandiamide, thiourea or melamine at high temperatures (550 and 600 °C), using different heating rates (2 or 10 °C min-1) and synthesis times (0 or 4 h). The effects of the synthesis conditions and type of the precursor on the efficiency of g-C3N4 were studied. The most efficient was the synthesis from dicyandiamide, 53%, while the efficiency in the process of synthesis from melamine and thiourea were much smaller, 26% and 11%, respectively. On the basis of the results provided by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-vis), thermogravimetric analysis (TGA), elemental analysis (EA), the best precursor and the optimum conditions of synthesis of g-C3N4 were identified to get the product of the most stable structure, the highest degree of ordering and condensation of structure and finally the highest photocatalytic activity. It was found that as the proton concentration decreased and the degree of condensation increased, the hydrogen yields during the photocatalytic decomposition of water-methanol solution were significantly enhanced. The generation of hydrogen was 1200 µmol g-1 and the selectivity towards hydrogen of more than 98%.

Successful Immobilization of Lanthanides Doped TiO2 on Inert Foam for Repeatable Hydrogen Generation from Aqueous Ammonia.

We describe the successful possibility of the immobilization of a photocatalyst on foam, which is beneficial from a practical point of view. An immobilized photocatalyst is possible for use in a continuous experiment and can be easily separated from the reactor after the reaction concludes. Parent TiO2, La/TiO2, and Nd/TiO2 photocatalysts (containing 0.1 wt.% of lanthanide) were prepared by the sol-gel method and immobilized on Al2O3/SiO2 foam (VUKOPOR A) by the dip-coating method. The photocatalysts were investigated for the photocatalytic hydrogen generation from an aqueous ammonia solution under UVA light (365 nm). The evolution of hydrogen was compared with photolysis, which was limited to zero. The higher hydrogen generation was observed in the presence of 0.1 wt.% La/TiO2 than in 0.1 wt.% Nd/TiO2. This is, besides other things, related to the higher level of the conduction band, which was observed for 0.1 wt.% La/TiO2. The higher conduction band's position is more effective for hydrogen production from ammonia decomposition.

The Role of Fluorine in F-La/TiO2 Photocatalysts on Photocatalytic Decomposition of Methanol-Water Solution.

F-La/TiO2 photocatalysts were studied in photocatalytic decomposition water-methanol solution. The structural, textural, optical, and electronic properties of F-La/TiO2 photocatalysts were studied by combination of X-ray powder diffraction (XRD), nitrogen physisorption, Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), Electrochemical impedance spectroscopy (EIS), and X-ray fluorescence (XPS). The production of hydrogen in the presence of 2.8F-La/TiO2 was nearly up to 3 times higher than in the presence of pure TiO2. The photocatalytic performance of F-La/TiO2 increased with increasing photocurrent response and conductivity originating from the higher amount of fluorine presented in the lattice of TiO2.

Photocatalytic Decomposition of N2O Over Ceramics Cordierite/CeO2 Nanoparticles.

The study is focused on the testing of the photocatalytic ability to decompose nitrous oxide (N2O) over cordierite/CeO² nanoparticles ceramic photocatalysts. The activity of ceramic materials was compared with the activity of industrially produced TiO² (Evonik photocatalyst). Photocatalytic decomposition of N2O over the ceramic samples and the TiO² Evonik was performed in annular batch reactor illuminated with 8 W Hg lamp (λ ═ 254 nm wavelength). Reaction kinetics was well described by pseudo 1st rate law. Photocatalytic activity of cordierite/CeO² was better in comparison with TiO² Evonik P25. The highest N2O conversion (56%) after 20 h of irradiation in inert gas was achieved over the sample with higher amount of CeO². This photocatalyst sample was examined for photocatalytic activity in the decomposition of N2O in the three various gaseous feed mixtures. The gaseous feed mixtures were: N2O enriched with O² (6.5 mol.%); N2O enriched with H2O(25 mol.%) and N2O enriched with mixture of O² and H2O(6.5 mol.% and 25 mol.%, respectively). It is assumed that the reduced conversion of N2O (47%) observed in the flow of the mixture of N2O and H2Ocould be affected by the sorption of water vapor on/onto the photocatalyst "active sites" causing less penetration of light and thus reducing the efficiency of photocatalytic decomposition of N2O. The presence of oxygen in the N2O mixture had only little effect to photocatalytic decomposition of N2O.

TiO2 and Nitrogen Doped TiO2 Prepared by Different Methods; on the (Micro)structure and Photocatalytic Activity in CO2 Reduction and N2O Decomposition.

TiO2 as nanostructured powders were prepared by (1) sol-gel process and (2) hydrothermal method in combination with (A) the processing by pressurized hot water and methanol or (B) calcination. The subsequent synthesis step was the modification of prepared nanostructured TiO2 with nitrogen using commercial urea. Textural, structural, surface and optical properties of prepared TiO2 and N/TiO2 were characterized by nitrogen physisorption, powder X-ray diffraction, X-ray photoelectron spectroscopy and DR UV-vis spectroscopy. It was revealed that TiO2 and N/TiO2 processed by pressurized fluids showed the highest surface areas. Furthermore, all prepared materials were the mixtures of major anatase phase and minor brookite phase, which was in nanocrystalline or amorphous (as nuclei) form depending on the applied preparation method. All the N/TiO2 materials exhibited enhanced crystallinity with a larger anatase crystallite-size than undoped parent TiO2. The photocatalytic activity of the prepared TiO2 and N/TiO2 was tested in the photocatalytic reduction of CO2 and the photocatalytic decomposition of N2O. The key parameters influencing the photocatalytic activity was the ratio of anatase-to-brookite and character of brookite. The optimum ratio of anatase-to-brookite for the CO2 photocatalytic reduction was determined to be about 83 wt.% of anatase and 17 wt.% of brookite (amorphous-like) (TiO2-SG-C). The presence of nitrogen decreased a bit the photocatalytic activity of tested materials. On the other hand, TiO2-SG-C was the least active in the N2O photocatalytic decomposition. In the case of N2O photocatalytic decomposition, the modification of TiO2 crystallites surface by nitrogen increased the photocatalytic activity of all investigated materials. The maximum N2O conversion (about 63 % after 18 h of illumination) in inert gas was reached over all N/TiO2.

Nd/TiO2 Anatase-Brookite Photocatalysts for Photocatalytic Decomposition of Methanol.

Neodymium enriched TiO2 anatase-brookite powders were prepared by unconventional method via using pressurized hot fluids for TiO2 crystallization and purification. The photocatalysts were tested in the CH3OH photocatalytic decomposition and they were characterized with respect to the textural (nitrogen adsorption), structural (XRD, XPS, and Raman spectroscopies), chemical (XRF), and optical (DR UV-Vis spectroscopy) and photoelectrochemical measurement. All prepared materials were nanocrystalline, had biphasic (anatase- brookite) structure and relatively large specific surface area (125 m2.g-1). The research work indicates that the doping of neodymium on TiO2 photocatalysts significantly enhances the efficiency of photocatalytic reaction. The photocatalytic activity increased with increasing portion of hydroxyl oxygen to the total amount of oxygen species. It was ascertained that the optimal amount of 1 wt% Nd in TiO2 accomplished the increasing of hydrogen production by 70% in comparison with pure TiO2. The neodymium doped on the titanium dioxide act as sites with accumulation of electrons. The higher efficiency of photocatalytic process was achieved due to improved electron-hole separation on the modified TiO2 photocatalysts. This result was confirmed by electrochemical measurements, the most active photocatalysts proved the highest photocurrent responses.

Photocatalytic reduction of carbon dioxide over Cu/TiO2 photocatalysts.

The photocatalytic reduction of CO2 with H2O was investigated using Cu/TiO2 photocatalysts in aqueous solution. For this purpose, Cu/TiO2 photocatalysts (with 0.2, 0.9, 2, 4, and 6 wt.% of Cu) have been synthesized via sol-gel method. The photocatalysts were extensively characterized by means of inductively coupled plasma optical emission spectrometry (ICP-OES), N2 physisorption (BET), XRD, UV-vis DRS, FT-IR, Raman spectroscopy, TEM-EDX, and photoelectrochemical measurements. The as-prepared photocatalysts contain anatase as a major crystalline phase with a crystallite size around 13 nm. By increasing the amount of Cu, specific surface area and band gap energy decreased in addition to the formation of large agglomeration of CuO. Results revealed that the photocatalytic reduction of CO2 decreased in the presence of Cu/TiO2 in comparison to pure TiO2, which might be associated to the formation of CuO phase acting as a recombination center of generated electron-hole pair. Decreasing of photoactivity can also be connected with a very low position of conduction band of photocatalysts with high Cu content, which makes H2 production necessary for CO2 reduction more difficult.

Photocatalytic decomposition of methanol over La/TiO2 materials.

Lanthanum-modified TiO2 photocatalysts (0.2-1.5 wt% La) were investigated in the methanol decomposition in an aqueous solution. The photocatalysts were prepared by the common sol-gel method followed by calcination. The structural (X-ray diffraction, Raman, X-ray photoelectron spectroscopy), textural (N2 physisorption), and optical properties (diffuse reflectance spectroscopy, photoelectrochemical measurements) of all synthetized nanomaterials were correlated with photocatalytic activity. Both pure TiO2 and La-doped TiO2 photocatalysts proved higher yields of hydrogen in comparison to photolysis. The photocatalyst with optimal amount of lanthanum (0.2 wt% La) showed almost two times higher amount of hydrogen produced at the same time as in the presence of pure TiO2. The photocatalytic activity increased with both increasing photocurrent response and decreasing amount of lattice and surface O species. It has been shown that both direct and indirect mechanisms of methanol photocatalytic oxidation participate in the production of hydrogen. Both direct and indirect mechanisms take part in the formation of hydrogen.