Effect of Nb doping on structural, optical and photocatalytic properties of flame-made TiO2 nanopowder

TiO2:Nb nanopowders within a dopant concentration in the range of 0.1-15 at.% were prepared by one-step flame spray synthesis. Effect of niobium doping on structural, optical and photocatalytic properties of titanium dioxide nanopowders was studied. Morphology and structure were investigated by means of Brunauer–Emmett–Teller isotherm, X-ray diffraction and transmission electron microscopy. Diffuse reflectance and the resulting band gap energy were determined by diffuse reflectance spectroscopy. Photocatalytic activity of the investigated nanopowders was revised for the photodecomposition of methylene blue (MB), methyl orange (MO) and 4-chlorophenol under UVA and VIS light irradiation. Commercial TiO2-P25 nanopowder was used as a reference. The specific surface area of the powders was ranging from 42.9 m2/g for TiO2:0.1 at.% Nb to 90.0 m2/g for TiO2:15 at.% Nb. TiO2:Nb particles were nanosized, spherically shaped and polycrystalline. Anatase was the predominant phase in all samples. The anatase-related transition was at 3.31 eV and rutile-related one at 3.14 eV. TiO2:Nb nanopowders exhibited additional absorption in the visible range. In comparison to TiO2-P25, improved photocatalytic activity of TiO2:Nb was observed for the degradation of MB and MO under both UVA and VIS irradiation, where low doping level (Nb < 1 at.%) was the most effective. Niobium doping affected structural, optical and photocatalytic properties of TiO2. Low dopant level enhanced photocatalytic performance under UVA and VIS irradiation. Therefore, TiO2:Nb (Nb < 1 at.%) can be proposed as an efficient selective solar light photocatalyst

WO3/CeO2/TiO2 Catalysts for Selective Catalytic Reduction of NOx by NH3: Effect of the Synthesis Method

WO3/CeO2/TiO2, CeO2/TiO2 and WO3/TiO2 catalysts were prepared by wet impregnation. CeO2/TiO2 and WO3/TiO2 showed activity towards the selective catalytic reduction (SCR) of NOx by NH3, which was significantly improved by subsequent impregnation of CeO2/TiO2 with WO3. Catalytic performance, NH3 oxidation and NH3 temperature programmed desorption of wet-impregnated WO3/CeO2/TiO2 were compared to those of a flame-made counterpart. The flame-made catalyst exhibits a peculiar arrangement of W-Ce-Ti-oxides that makes it very active for NH3-SCR. Catalysts prepared by wet impregnation with the aim to mimic the structure of the flame-made catalyst were not able to fully reproduce its activity. The differences in the catalytic performance between the investigated catalysts were related to their structural properties and the different interaction of the catalyst components

Flame-made visible light active TiO2:Cr photocatalysts: correlation between structural, optical and photocatalytic properties

TiO2:Cr nanoparticles with dopant concentration from 0.1 to 10 at.% were synthesized by a liquid-fed one-step flame spray synthesis. All investigated nanopowders were synthesized with a specific surface area of about 60 m2 g-1. Undoped flame-made TiO2 consisted mainly of anatase. The increase of Cr content in TiO2:Cr nanopowder was accompanied by a decrease of the anatase phase and a consequent increase of the rutile polymorph. The comparison of the XANES spectra of samples with different doping level of Cr showed that Cr is incorporated in the structure. The presence and the concentration of Cr significantly affected the optical properties of TiO2 and caused a red-shift of the fundamental absorption edge. Photocatalytic performance of TiO2 for the gas phase formaldehyde decomposition under visible light irradiation was enhanced by Cr doping and reached its maximum at 3 at.% Cr

Methane abatement under stoichiometric conditions on perovskite-supported palladium catalysts prepared by flame spray synthesis

Three-way catalysts (TWC) are the key technology to reduce emissions of pollutants from stoichiometric engines. Perovskite-type catalysts of general formula ABO3±δ (A = La, Y; B = Mn, Fe) containing 2 wt% Pd were produced by flame spray synthesis (FSS) using metal nitrate precursors. The structural properties of the catalysts were characterized by X-ray diffraction (XRD), surface area determination (BET) and transmission electron microscopy (TEM). Crystalline metal oxide nano-particles of 20 nm average size were accompanied by minority La2O3 and Y2O3 phases. The state of Pd in the catalysts was characterized using X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge spectroscopy (XANES) and CO adsorption by infrared spectroscopy. Metallic Pd coexisted with Pd in oxidation state +2 and higher on all fresh samples. TEM confirmed the presence of dispersed Pd particles 2-5 nm in diameter. Therefore, under the chosen synthesis conditions, FSS provides supported palladium nano-particles rather than a solid solution. PdO was the dominant Pd species after calcination at 700 °C. The TWC activity was tested in a simulated stoichiometric gas mixture comprising CH4, CO, NOx and O2. PdO in combination with YFeO3±δ exhibited the lowest temperature for CH4 oxidation (T50 = 450°C), which was ca. 100°C lower than that of the sample obtained by the conventional wet-chemical method. After cycling under reaction conditions up to 850°C, a large improvement of catalytic activity for CH4 oxidation was observed which associated with the formation of metallic Pd particles (ca. 20 nm) and the hexagonal → orthorhombic phase transition of YFeO3±δ

Flame-Made WO₃/CeO_<i>x</i>‑TiO₂ Catalysts for Selective Catalytic Reduction of NO_<i>x</i> by NH₃

Materials based on a combination of cerium–tungsten−titanium are potentially durable catalysts for selective catalytic NO_<i>x</i> reduction using NH₃ (NH₃–SCR). Flame-spray synthesis is used here to produce WO₃/CeO_<i>x</i>-TiO₂ nanoparticles, which are characterized with respect to their phase composition, morphology, and acidic properties and are evaluated by NH₃–SCR. HR-TEM and XRD revealed that flame-made WO₃/CeO_<i>x</i>-TiO₂ consists of mainly rutile TiO₂, brannerite CeTi₂O₆, cubic CeO₂, and a minor fraction of anatase TiO₂. These phases coexist with a large portion of amorphous mixed Ce–Ti phase. The lack of crystallinity and the presence of brannerite together with the evident high fraction of Ce³⁺ are taken as evidence that cerium is also present as a dopant in TiO₂ and is well dispersed on the surface of the nanoparticles. Clusters of amorphous WO₃ homogeneously cover all particles as observed by STEM. Such morphology and phase composition guarantee short-range Ce–O–Ti and Ce–O–W interactions and thus the high surface concentration of Ce³⁺. The presence of the WO₃ layer and the close Ce–O–W interaction further increased the Ce³⁺ content compared to binary Ce–Ti materials, as shown by XPS and XANES. The acidity of the materials and the nature of the acid sites were determined by NH₃ temperature-programmed desorption (NH₃-TPD) and DRIFT spectroscopy, respectively. TiO₂ possesses mainly strong Lewis acidity; addition of cerium, especially the presence of surface Ce³⁺ in close contact with titanium and tungsten, induces Brønsted acid sites that are considerably increased by the amorphous WO₃ clusters. As a result of this peculiar element arrangement and phase composition, 10 wt % WO₃/10 mol % CeO_<i>x</i>-90 mol % TiO₂ exhibits the highest NO_<i>x</i> reduction efficiency, which matches that of a V₂O₅–WO₃/TiO₂ catalyst. Preliminary activity data indicate that the flame-made catalyst demonstrates much higher performance after thermal and hydrothermal aging at 700 °C than the V-based analogue despite the presence of the rutile phase. Ce³⁺ remains the dominating surface cerium species after both aging treatments, thus confirming its crucial role in NH₃–SCR by Ce–W–Ti-based catalysts