Substituted Ti Atoms Research Articles

Bottom-Up Construction of Mesoporous Cerium-Doped Titania with Stably Dispersed Pt Nanocluster for Efficient Hydrogen Evolution.

Hydrogen generation is one of the crucial technologies to realize sustainable energy development, and the design of advanced catalysts with efficient interfacial sites and fast mass transfer is significant for hydrogen evolution. Herein, an in situ coassembly strategy was proposed to engineer a cerium-doped ordered mesoporous titanium oxide (mpCe/TiO2), of which the abundant oxygen vacancies (Ov) and highly exposed active pore walls contribute to good stability of ultrasmall Pt nanoclusters (NCs, ∼ 1.0 nm in diameter) anchored in the uniform mesopores (ca. 20 nm). Consequently, the tailored mpCe/TiO2 with 0.5 mol % Ce-doping-supported Pt NCs (Pt-mpCe/TiO2-0.5) exhibits superior H2 evolution performance toward the water-gas shift reaction with a 0.73 molH2·s-1·molPt-1 H2 evolution rate at 200 °C, which is almost 6-fold higher than the Pt-mpTiO2 (0.13 molH2·s-1·molPt-1 H2). Density functional theory calculations confirm that the structure of Ce-doped TiO2 with Ce coordinated to six O atoms by substituting Ti atoms is thermodynamically favorable without the deformation of Ti-O bonds. The Ov generated by the six O atom-coordinated Ce doping is highly active for H2O dissociation with an energy barrier of 2.18 eV, which is obviously lower than the 2.37 eV for the control TiO2. In comparison with TiO2, the resultant Ce/TiO2 support acts as a superior electron acceptor for Pt NCs and causes electron deficiency at the Pt/support interface with a 0.17 eV downshift of the Pt d-band center, showing extremely obvious electronic metal-support interaction (EMSI). As a result, abundant and hyperactive Ti3+-Ov(-Ce3+)-Ptδ+ interfacial sites are formed to significantly promote the generation of CO2 and H2 evolution. In addition, the stronger EMSI between Pt NCs and mpCe/TiO2-0.5 than that between Pt and mpTiO2 contributes to the superior self-enhanced catalytic performance during the cyclic test, where the CO conversion at 200 °C increases from 72% for the fresh catalyst to 99% for the used one. These findings reveal the subtle relationship between the mesoporous metal oxide-metal composite catalysts with unique chemical microenvironments and their catalytic performance, which is expected to inspire the design of efficient heterogeneous catalysts.

Green synthesis of Mn-doped TiO2 nanoparticles and investigating the influence of dopant concentration on the photocatalytic activity

• An environmentally friendly method for preparing photocatalysts to treat organic dyes. • A green synthesis approach is adopted to prepare Mn-doped TiO 2 nanoparticles. • Methylene blue is degraded most efficiently using a 0.05% Mn-TiO 2 catalyst. • 0.1 g L -1 of catalyst is found to remove 86% of Methylene blue after 35 minutes. • The regeneration of the Mn 4+ /Mn 2+ cycle is crucial for photocatalytic activity. A tea plant extract is used in the presence of titanium and manganese sources to synthesize three different concentrations of Mn x Ti 1-x O 2 (x=0.03, 0.05, and 0.1). By substituting Ti atoms for Mn, the lattice of TiO 2 is successfully incorporated with Mn. The degradation of Methylene Blue (MB) in the presence of visible light is used to test the photocatalytic activity of the synthesized samples. Among the Mn-doped TiO 2 samples, the highest degradation ratio is observed for samples with a dopant concentration of 0.05 at%. A dose of 0.1 g L -1 and 5 mM of Mn-TiO 2 is found to remove 86% of MB after 35 minutes of reaction. Mn x Ti 1-x O 2 is critically important for speeding up the MB degradation because of electron transfer between Mn and TiO 2 , and for the regeneration of the Mn 4+ /Mn 2+ cycle on its surface. Optical irradiation of the photocatalytic system under visible light led to higher visible photocatalytic activity than pure TiO 2 . We propose a novel and environmentally friendly method for preparing photocatalysts to treat organic dyes.

Comparing the role of Zr and Hf atoms on microstructure and mechanical properties optimization of Ti2AlN reinforced Ti48Al 0.5W composites

In order to further optimize the mechanical properties of TiAl composites, Ti2AlN/Ti48Al0.5W composites with 1 at% Zr/Hf addition were prepared by arc melting, which were termed as Base alloy, Zr-containing alloy and Hf-containing alloy. The grain size, element distribution, phase content and compressive properties were investigated and compared in this work. With 1 at% Zr/Hf addition, the lamellae colony size decreased from 33 µm to 25/23 µm, and Zr atoms almost dissolved into the single γ phase in the form of solid solution atoms, while Hf atoms distributed in both α2/γ lamellae and single γ phase, and part Hf atoms also existed in the form of B2 phase, which illustrated the higher solid solution of Zr atoms in TiAl composites comparing with Hf atoms. The γ phase content increased and the α2 phase content decreased, especially with 1 at% Zr addition, which acted as γ-phase stabilizing element. With 1 at% Hf addition, the compressive strength and strain increased from 2010 to 2221 MPa and from 23.9 % to 26.2 %, respectively, which was attribute to the solid solution strengthening of Hf atoms by substituting Ti atoms, refinement of α2/γ lamellae and increased γ phase content. Compared with 1 at% Hf addition, 1 at% Zr addition led to higher compressive strength and strain, which were 2344 MPa and 26.8 %, respectively, and this result was due to the higher concentration of Zr atoms in γ phase and higher content of γ phase. On account of density, cost and performance, Zr atoms are more suitable as an alloying element to further optimize the microstructure and mechanical properties of TiAl composites.

First-principles study of phase stability and elastic properties in metastable Ti-Mo alloys with cluster structure

ABSTRACTThis paper provided a novel approach for evaluating phase stability and elastic properties in metastable Ti–Mo alloys with low Mo content by first-principles combined with cluster structure. In 54-atom body-centered-cubic supercell by substituting Ti atoms with 2–7 Mo atoms (7.1–23.0 wt% Mo), individual cluster structure of β-phase was constructed by ‘-Mo-Ti-Mo-’ cluster unit having the lowest cohesive energy. The distorted supercell was more stable than undistorted one at a low Mo content. With increasing Mo content, the density of state at Fermi level decreased, and bonding electron number increased, indicating β-phase stability was gradually promoted. Tetragonal shear elastic constant (C′ = (C11 – C12)/2), shear modulus (G111) and anisotropy factor (A = C44/C′) exhibited a fluctuation with Mo addition, while the change trend of A was opposite to C′ and G111. Calculated Young’s modulus exhibited similar changing trend to the C′, implying that the softening of C′ resulted in low Young’s modulus of β-phase. Measured Young’s modulus exhibited significant difference from calculated one, which was mainly caused by formation of α″-martensite and ω-phase. The values of C′, G111 and A were considered to associate with not only elastic properties of β-phase itself but also transition from β-phase to α″-martensite and/or ω-phase.

First principles study of oxidation of Si-segregated α-Ti(0001) surfaces

The oxidation of the α-Ti(0001) surface was studied using the density functional theory. To enhance the oxidation resistance, we substituted Ti atoms with Si atoms on the Ti(0001) surface. We observed that Si prefers to segregate at the surface layer of Ti(0001) compared with other subsurface layers. The Si solubility on the Ti(0001) surface, based on the segregation energy, was found to be much larger than the corresponding bulk solubility. Si segregation was found to reduce the binding between the oxygen atom and the Ti(0001) surface and hinder the diffusion of oxygen atoms into the slab. The dissociation of the oxygen molecule on the clean and Si-segregated surfaces of Ti was found to be barrierless. Overall, the Si segregation on the Ti(0001) surface was found to hinder the dissolution of oxygen in Ti.

TDPAC study of Fe-implanted titanium dioxide thin films

Fe-doping in TiO2 has been proven to improve several of its properties, including the photocatalytic activity. Time-differential perturbed angular correlation (TDPAC) as the applied spectroscopy method is particularly interesting because it can probe the electric and magnetic interactions on a local atomic scale. In this work the hyperfine interactions on 111Cd atoms substituting Ti atoms in TiO2 due to nearby Fe atoms also diluted within the TiO2 lattice were measured as a function of temperature. The results review two fractions with distinct quadrupole interaction parameters. One site, occupied by the 111Cd probes, presents the smaller quadrupole interaction frequency, namely υq1 = 45 MHz, and can be ascribed to sites that are more distant from the Fe substitutional site whereas the second site characterized with υq2 = 62 MHz is related to Cd probe atoms that are closer to the Fe defect. Additionally, the system has been characterized using electron dispersive spectroscopy.

First-principles studies of the influences mechanism of Y, Hf elements on the high temperature oxidation of γ-TiAl alloys

The influences mechanisms of Y, Hf elements on the high temperature oxidation of γ-TiAl alloys were studied by using a first-principle plane wave pseudopotential method within the density functional theory. It is shown that Y, Hf atoms segregate to the γ-TiAl surface by substituting Ti atoms, and Y atom is easier to segregate than Hf. Y, Hf atoms segregated at γ-TiAl surface decrease the adhesion of O and γ-TiAl surface, which decreases the oxidation rate of γ-TiAl alloys and thereby hinders the growth of oxide films. The electron structure calculations suggest that the interaction between O and Ti, Al, Y, Hf atom exists both ionic and covalent binding characteristics, the decrease of the adhesion of O and γ-TiAl surface with the doped Y, Hf is mainly attributed to the weaken covalent interactions. As a result, the results obtained by first principles can make us get a deeper understanding of the mechanism of the segregation of Y, Hf to the γ-TiAl surface and the influence mechanism of surface active elements Y, Hf on the initial oxidation of γ-TiAl alloys.

Photocatalytic activity of TiO2-WO3 mixed oxides in relation to electron transfer efficiency

Aiming at producing photocatalysts with minimized photoproduced electron – hole pairs recombination, a series of titanium – tungsten mixed oxides has been prepared, by coupling TiO2 with different amounts of WO3, according to an alkaline-catalyzed sol-gel method followed by an incipient wetting procedure. The photocatalysts were characterized by surface and bulk techniques and tested in both an oxidation and a reduction photocatalytic reaction, i.e. in formic acid (FA) mineralization and in Cr(VI) reduction. Tungsten was mainly present as hexagonal WO3 on the TiO2 surface, though a fraction of W migrates into the TiO2 lattice substituting Ti atoms, as evidenced by XRPD analysis. Different photoactivity scales were found in the two test reactions, both occurring in the same pH range under similar substrate-photocatalyst electrostatic interactions. In fact, photoexcited electrons transferred from the conduction band (CB) of TiO2 to the CB of coupled WO3, being energetically unable to reduce O2 molecules, easily recombine with photoproduced holes, with a consequent photoactivity decrease in FA photo-mineralization with increasing W/Ti ratio. On the contrary, coupling TiO2 with small WO3 amounts (0.2–1.0mol%) is beneficial in the removal of Cr2O72− anions. These species, being characterized by a redox potential more positive than the CB edge of WO3, may efficiently accept the electrons trapped in WO3 domains, converting into less toxic Cr(III) species. Thus, WO3 surface domains effectively promote photoproduced charge separation by efficiently trapping CB electrons; increased photocatalytic efficiency depends on the redox potential of the electron acceptor species directly (or indirectly) involved in the photocatalytic process.

Crystal chemistry of synthetic Ti-Mg-bearing hibonites: A single-crystal X-ray study

Hibonite single crystals were synthesized using two different techniques: hot-pressing of polycrystalline hibonite by means of piston-cylinder apparatus and solid state reaction using citrate-based sol-gel precursors. Chemical analyses, UV/Vis spectroscopy, and single-crystal X-ray characterization were performed on four sets of Ti-Mg-bearing hibonites to investigate the substitution mechanism of Ti3+, Ti4+, and Mg2+, relevant for hibonites found in calcium-aluminum-rich inclusions in meteorites. The unit-cell volume of hibonite depends linearly on the concentration of Ti and Mg: V = 8.21(3)·(Titot + Mg)apfu + 586.06(1). Structural refinements, carried out in the space group P63/mmc, demonstrate that Ti occupies two sites: M2, a trigonal bipyramid, and M4, a distorted octahedron occurring in facesharing pairs. The Ti occupancy factor was refined at both sites. Due to the repulsion of neighboring Ti cations the bond distance M4-O3 increases with increasing Ti content and the cations are displaced from the central position of the polyhedron. The displacement factors (U33)M2 and the site occupancy factor for Ti in the M2 site positively correlate for the samples, which have more than 0.3-0.4 Tiapfu, while (U33)M2 remains that of pure hibonite for small Ti occupancies at the M2 site. This plateau of displacement factor reflects the local strain fields around the substituted Ti atoms and its magnitude indicates that these strain fields begin to overlap at a Ti-Ti separation of about 1-2 unit cells. For a sample synthesized at low oxygen fugacity we detected Ti3+ by means of UV/Vis absorption spectroscopy. The presence of Ti3+ has a clear effect on the M4-M4 distance, which deviates from the linear trend described by samples containing mainly Ti4+. The average bond length M3-O depends linearly on the Mg content of these synthetic hibonites clearly indicating that Mg occupies this site.

Neutron Diffraction Analysis of NASICON‐type Li1+xAlxTi2–xP3O12

AbstractPowder neutron diffraction analysis was carried out on a sample of Li1+xAlxTi2–xP3O12 at two temperatures, 10 K and 300 K. After heat treatment the material contained small amounts of LiTiOPO4 as impurity. Refinement of the neutron diffraction data confirmed that the main phase crystallized with a NASICON structure. No structural change was observed at low temperature. Al atoms randomly substituted Ti atoms, and additional Li atoms occupied a second site in the crystal structure. A detailed analysis of the diffraction data, including the inspection of Fourier difference maps, indicated that these excess Li+ ions occupied a 6a site within the ion conduction channels of the NASICON‐type structure instead of the formerly reported 18e or 36f sites.

Microstructure and mechanical properties of high niobium containing TiAl alloys elaborated by spark plasma sintering

Spark Plasma Sintering (SPS) is used to process TNB alloys. Pre-alloyed powders (Ti–46Al–9Nb) were sintered between 975 °C and 1300 °C under 100 MPa. The microstructures of the products are determined by X-ray diffraction and by scanning and transmission electron microscopy. Their mechanical properties are evaluated by tensile tests performed at room temperature and creep experiments at 700 °C under 300 MPa stress. Depending on the sintering temperature, two-phased, duplex and lamellar microstructures have been obtained. A quasi-α 2 phase has been observed, which is in fact an α 2 phase in which Nb atoms have substituted Ti atoms. The TNB alloys consolidated by SPS are stronger than the alloys previously elaborated by SPS using Ti–47Al–2Cr–2Nb and Ti–44Al–2Cr–2Nb–1B powders but they exhibit a lower ductility. Moreover, some specimens broke prematurely due to Fe inclusions.

Reactive magnetron cosputtering of hard and conductive ternary nitride thin films: Ti–Zr–N and Ti–Ta–N

Ternary transition metal nitride thin films, with thickness up to 300 nm, were deposited by dc reactive magnetron cosputtering in Ar–N2 plasma discharges at 300 °C on Si substrates. Two systems were comparatively studied, Ti–Zr–N and Ti–Ta–N, as representative of isostructural and nonisostructural prototypes, with the aim of characterizing their structural, mechanical, and electrical properties. While phase-separated TiN–ZrN and TiN–TaN are the bulk equilibrium states, Ti1−xZrxN and Ti1−yTayN solid solutions with the Na–Cl (B1-type) structure could be stabilized in a large compositional range (up to x=1 and y=0.75, respectively). Substituting Ti atoms by either Zr or Ta atoms led to significant changes in film texture, microstructure, grain size, and surface morphology, as evidenced by x-ray diffraction, x-ray reflectivity, and scanning electron and atomic force microscopies. The ternary Ti1−yTayN films exhibited superior mechanical properties to Ti1−xZrxN films as well as binary compounds, with hardness as high as 42 GPa for y=0.69. All films were metallic, the lowest electrical resistivity ρ∼65 μΩ cm being obtained for pure ZrN, while for Ti1−yTayN films a minimum was observed at y∼0.3. The evolution of the different film properties is discussed based on microstructrural investigations.

Synthesis of a Sinter-Resistant, Mixed-Oxide Support for Au Nanoclusters

Following the discovery that small gold clusters highly dispersed on metal oxide supports are active catalysts at low temperature for a variety of reactions, a number of studies have been carried out to determine the structure of the clusters and the mechanism leading to their activity. A major deterrent to the use of these catalysts, however, is that under reaction temperatures and pressures, the clusters tend to sinter, or agglomerate, leading to a dramatic decrease in activity. In an attempt to make these highly active Au catalysts more stable, mixed-oxide supports have been developed by substituting Ti atoms for Si in a silica thin film network. Depending on the amount of Ti deposited, the TiO2−SiO2 surface consists of substituted Ti atoms and/or TiOx islands. With deposition of Au onto these TiO2−SiO2 surfaces (at low and high Ti coverages), the substituted Ti and/or TiOx islands act as Au cluster nucleation sites, leading to a marked increase in the cluster number density compared to the Ti-free SiO...

Electrochemical properties of peroxotitanium-containing phosphotungstate with Dawson–Wells structure

Abstract The electrochemical behavior of titanium-containing and peroxotitanium-containing phosphotungstates with Dawson–Wells structure, i.e. MaHbP2W18−nTinO62, MaHbP2W18−n(TiO2)nO62−n (n=1, a+b=8; n=3, a+b=12, M=K+, Et4N+ or Bu4N+), were studied systematically. A reduction wave of O22− in the peroxo polyanion at −0.1 V and a reduction wave of Ti4+ at −0.2 V were observed. The oxidation potential of the peroxotitanium-containing phosphotungstate is higher than that of the titanium-containing phosphotungstate. The peroxide group and substituted Ti atoms have a large effect on the redox behavior of the heteropoly anions.

Neutron diffraction study of the boride Y(Fe11.04Ti0.52B0.43)Ti0.39: a new phase of a ThMn12-type intermetallic interstitial compound

A sample with nominal composition YFe10.5TiB0.5 has been prepared through careful processing. X-ray diffraction analysis revealed that the main phase in the sample is in the tetragonal ThMn12-type structure (space group, 14/mmm), and a small amount (less than 3%) of alpha -Fe exists as a second phase. Neutron diffraction measurements indicated that, in the tetragonal structure, the boron atoms occupy the 8i sites, substituting for a few Ti atoms, while the substituted Ti atoms enter the interstitial 2b sites. The real composition of the main phase was determined to be Y(Fe11.04Ti0.52B0.43)Ti0.39.