Determination of conduction and valence band electronic structure of anatase and rutile TiO 2

Electronic structures of rutile and anatase polymorph of TiO2 were determined by resonant inelastic X-ray scattering measurements and FEFF9.0 calculations. Difference between crystalline structures led to shifts in the rutile Ti d-band to lower energy with respect to anatase, i.e., decrease in band gap. Anatase possesses localized states located in the band gap where electrons can be trapped, which are almost absent in the rutile structure. This could well explain the reported longer lifetimes in anatase. It was revealed that HR-XAS is insufficient to study in-depth unoccupied states of investigated materials because it overlooks the shallow traps. Graphical Abstract The resonant X-ray emission spectroscopy around Ti k-edge was applied to probe local electronic structure of TiO2 rutile and anatase. By measuring 1s→3d excitation and 3p→1s decay channel, differences between localized and delocalized orbitals were determined. The 3d pre-edge structures were compared with ab initio multiple scattering simulations

Nanocrystalline TiO2/SnO2 heterostructures for gas sensing

The aim of this research is to study the role of nanocrystalline TiO2/SnO2 n–n heterojunctions for hydrogen sensing. Nanopowders of pure SnO2, 90 mol % SnO2/10 mol % TiO2, 10 mol % SnO2/90 mol % TiO2 and pure TiO2 have been obtained using flame spray synthesis (FSS). The samples have been characterized by BET, XRD, SEM, HR-TEM, Mössbauer effect and impedance spectroscopy. Gas-sensing experiments have been performed for H2 concentrations of 1–3000 ppm at 200–400 °C. The nanomaterials are well-crystallized, anatase TiO2, rutile TiO2 and cassiterite SnO2 polymorphic forms are present depending on the chemical composition of the powders. The crystallite sizes from XRD peak analysis are within the range of 3–27 nm. Tin exhibits only the oxidation state 4+. The H2 detection threshold for the studied TiO2/SnO2 heterostructures is lower than 1 ppm especially in the case of SnO2-rich samples. The recovery time of SnO2-based heterostructures, despite their large responses over the whole measuring range, is much longer than that of TiO2-rich samples at higher H2 flows. TiO2/SnO2 heterostructures can be intentionally modified for the improved H2 detection within both the small (1–50 ppm) and the large (50–3000 ppm) concentration range. The temperature Tmax at which the semiconducting behavior begins to prevail upon water desorption/oxygen adsorption depends on the TiO2/SnO2 composition. The electrical resistance of sensing materials exhibits a power-law dependence on the H2 partial pressure. This allows us to draw a conclusion about the first step in the gas sensing mechanism related to the adsorption of oxygen ions at the surface of nanomaterials

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 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