Design guidelines for titania-silica-alumina ceramics with tuned anatase to rutile transformation

Titania-based ceramics with adjustable anatase-rutile fractions were obtained by milling of anatase, quartz and corundum precursors, uniaxial pressing and firing at 1100?°C. The influence of silica and alumina, combined with milling time and compaction pressure, was studied by design of experiments. The L9 orthogonal array with a three-level noise factor was employed. Firing of pure titania at 1100?°C yielded complete anatase to rutile transformation (ART), whereas stabilized samples show that an optimum amount of 9% silica and 33% alumina reduces phase transformation to only about 5?wt% rutile. An extended correlation matrix combined with analysis of variance (ANOVA) was applied to assess the combined effects of quartz, alumina, milling time and uniaxial compressing pressure on relative density, and anatase to rutile transformation. Results show absence of ART after milling, and controlled partial conversion of anatase to rutile after firing. Very good fitting was obtained by multivariate analysis on considering first and second order terms for dependence on silica contents and interactions between silica and each of the remaining factors, including milling time. This empirical dependence could be interpreted on a sound physicochemical basis, allowing the prediction of suitable compositions and processing conditions to obtain rutile-free samples by conventional ceramic processing, and to design ceramic samples with controlled fractions of anatase and rutile.     

Vanadia doped tungsten-titania SCR catalysts as functional materials for exhaust gas sensor applications

Urea-SCR systems (selective catalytic reduction) are required to meet future NO_x emission standards of heavy-duty and light-duty vehicles. It is a key factor to control the SCR systems and to monitor the catalysts’ functionalities to achieve low emissions. The novel idea of this study is to apply commercially available SCR catalyst materials based on vanadia-doped tungsten-titania as gas sensing films for impedimetric thick-film exhaust gas sensor devices. The dependence of the impedance on the surrounding gas atmosphere, especially on the concentrations of NH₃ and NO₂, is investigated, as well as cross interferences from other components of the exhaust. The sensors provide a good NH₃ sensitivity at 500 °C. The sensor behavior is explained in light of the literature combining the fields of catalysts and semiconducting gas sensors.     

TiO2对烧成镁钙砖烧结性能的影响

研究了两种晶型的TiO2(锐钛矿型和金红石型)作为添加剂对烧成镁钙砖烧结性的影响.结果表明,两种晶型的TiO2都能显著促进镁钙砖的烧结,其中以锐钛矿晶型为添加剂的效果好于金红石型的.XRD分析结果表明,TiO2与CaO反应生成了CaTiO3,这是加入TiO2使镁钙砖烧结性变好的主要原因.     

Porous Titania Thin Films Grown by Anodic Oxidation for Hydrogen Sensors

Porous titanium dioxide (Titania) thin films were grown by anodic oxidation using high purity (99.7%) titanium foil in a dilute sulphuric acid (1 M) medium. The anodization process was carried out for 30 minutes with 20 mA/cm² and 50 mA/cm² current densities. The samples were characterized by XRD, SEM, and AFM techniques. It was found that the grown porous titania films were less sensitive to 500 ppm hydrogen in air ambient below 300°C; however, the sensitivity and response behavior of the film at 300°C are very much dependent on the growth conditions. Particularly, the films grown at current density 50 mA/cm² and 1 M acid concentration exhibited the lowest response time of 151 sec at 300°C.     

Fabrication of TiO2 nanoflowers with bronze (TiO2(B))/anatase heterophase junctions for efficient photocatalytic hydrogen production

Light harvesting and charge separation are both significant in the photocatalysis, but it is challenging to synchronously realize both in a single-component material. The novel porous TiO₂ nanoflowers (NFs) photocatalysts with stable bronze (TiO₂(B))/anatase heterophase junctions and large pore sizes are prepared via a hydrothermal/annealing method. The presence of porous nanoflower structure enhances the light absorption through reflection/refraction of light. The stable TiO₂(B)/anatase heterophase junctions can efficiently promote the separation of photoinduced electrons and holes pairs and therefore suppress the charge recombination. The large pore sizes provide multi-level channels for the absorption and diffusion of reactants. With the increase of annealing temperatures from 350 to 550 °C, the H₂ evolution activity is promoted. However, overhigh annealing temperature (650 °C) cause the broken of nanoflower structure and TiO₂(B)/anatase heterophase junctions, thus inducing even decrease of H₂ evolution activity. As a consequence, the obtained TiO₂ NFs exhibit an enhanced photocatalytic H₂ evolution activity at the optimal annealing temperature (550 °C) with Pt as co-catalysts (5.013 mmol h⁻¹g⁻¹), exceeding that of TiO₂ NFs without annealing (0 mmol h⁻¹g⁻¹) and pure anatase TiO₂ NFs (TiO₂ NFs-650 °C, 4.722 mmol h⁻¹g⁻¹), respectively. Interestingly, TiO₂ NFs-550 °C still show a high hydrogen evolution rate of 4.317 mmol h⁻¹g⁻¹ in the absence of co-catalysts.     

Nano-to-macroporous TiO2 (anatase) by cold sintering process

Cold Sintering Process (CSP) was applied on commercial nanopowders to produce nanostructured TiO₂ anatase with nano-to-macro porosity. Nanoporous TiO₂ based materials were obtained by applying CSP at 150 °C and pressures up to 500 MPa on three TiO₂ nanopowders with different specific surface area (s.s.a. = 50, 90 and 370 m²/g), using water as transient aqueous environment. Although TiO₂ is insoluble in water, a density of 68% and s.s.a. = 117 m²/g were achieved from the powder with the highest specific surface area. A post annealing process at 500 °C increased the density up to 73% with a s.s.a. = 59 m²/g, and the crystallites dimensions passed from 110 Å in the powder to 130 Å in CSP material and 172 Å after post annealing. Finally, macroporosity was produced by using thermoplastic polymer beads as sacrificial templates within TiO₂ nanopowder during CSP, followed by a debonding at 500 °C.     

Effect of titania synthesis conditions on the catalytic performance of mesoporous Ni/TiO2 catalysts for carbon dioxide reforming of methane

The catalytic performance of 5 wt% Ni/TiO₂ catalysts with different physicochemical properties was studied for the CO₂ reforming of methane reaction. The TiO₂ supports were prepared by the evaporation-induced self-assembly method using three different titania metal precursors. The catalysts were characterized by XRD, BET, TGA, and TEM techniques. The results showed that the phase composition of TiO₂ support plays a crucial role in catalyst performance. Furthermore, the variation of synthesis conditions significantly affects the physicochemical properties of TiO₂ support. NH₃-treatment helped maintain the higher surface area by retaining a significant fraction of the amorphous content of titania support. Catalysts deactivation was caused by the phase transformation of TiO₂ from anatase to rutile and the sintering of Ni metal. Phase transformation into rutile was more significant, with the catalysts possessing a higher content of amorphous TiO₂. Ni/TiO₂ catalyst prepared using the titanium ethoxide precursor performed better in the dry reforming reaction. Anatase titania offers strong metal-support interaction, whereas weak metal-support interaction was observed in the amorphous and rutile phase.     

Suspension high velocity oxy-fuel spraying of TiO2: A quantitative approach to phase composition

A range of coatings from a water based suspension of anatase has been prepared by suspension high velocity oxy-fuel spraying with the aim to study effects of heat power of the flame on phase composition, microstructure and surface topography. Three most commonly used approaches of quantitative phase analysis have been scrutinized with respect to their applicability and as some of the coatings showed presence of preferred orientation and it was argued that quantitative Rietveld refinement is the most accurate method for phase composition determination. Coatings had a layered duplex anatase/rutile microstructure with fraction of rutile increasing exponentially with heat power. Spraying at the lower heat power led to a lower surface roughness and higher power resulted in surfaces with pronounced humps, which were distributed homogeneously on the surface. The emergence of humps is related to an increase in macroscopic surface area of up to 30% with respect to the flat coating.     

Supercritical hydrothermal synthesis of titanium dioxide nanostructures with controlled phase and morphology

A novel template- and organic-free synthesis of TiO₂ nanostructures with controlled phase and morphology was realized through batch supercritical hydrothermal treatment (400 °C) of titanate nanotubes (TNTs) with H₂O₂ in NaOH aqueous solution. Well-defined 3D titanate hierarchical spheres (THSs), 2D multilayered titanate nanosheets (TNSs), and 1D monodisperse anatase nanorods (ANRs) exposing (0 1 0) facets were prepared in 15 min by slightly varying the NaOH solution pH. Specifically, the obtained Na/H-THSs (without/with HCl neutralization) exhibited highly porous structures with large specific surface area (109 m² g⁻¹ and 196 m² g⁻¹, respectively). Temperature-dependent phase and morphology evolutions of products under subcritical condition (200 and 300 °C) were investigated. The formation of the TiO₂ nanostructures from TNTs was proposed mainly following a dissolution–nucleation-growth mechanism, suggesting that both supercritical temperature and NaOH solution pH were determinant factors governing the nucleation and growth process and thus the phase and morphology.     

Porous TiO2/rGO nanocomposites prepared by cold sintering as efficient electrocatalyst for nitrogen reduction reaction under ambient conditions

Haber–Bosch process as the current dominant artificial NH₃ production process in industry, requires relatively high temperature (350–550 °C) and pressure (150–350 atm). Electrocatalytic nitrogen reduction reaction (NRR) as a green and sustainable strategy for ammonia production has raised intensive research interest in recent years but still remains a significant challenge because of the lack of high performance electrocatalysts. In this work, porous TiO₂-reduced graphene oxide (TiO₂/rGO) nanocomposite as self-supporting efficient electrocatalyst for NRR under ambient conditions were prepared by cold sintering associated with sacrificial template method. The porous TiO₂/rGO nanocomposite with grain size of ～40 nm were prepared by cold sintering process at 220 °C and 147 MPa. Given the 220 °C as cold sintering temperature, anatase TiO₂ were preserved as the final phase which exhibit much better NRR electrocatalytic performance than the rutile phase. The oxygen vacancy densities in the nanocomposites were also tuned by heat treatment at 450 °C under different atmosphere, while samples heat treated under H₂/Ar atmosphere gave the best electrocatalytic NRR performance with a FE of 8.88 % and an NH₃ yield of 7.75 μg h^?1 cm^?2 at ambient conditions. Experiments also shows that the addition of rGO significantly improved the electrocatalytic NRR performance especially the conductivity. This work not only designed a framework of ceramic nanocomposites based self-supporting and durable electrocatalysts system but also paves a feasible way towards preparing electrocatalysts that are sensitive to high temperature fabrication process.