ABO3-type Perovskite Research Articles

A Novel Series of the New Visible-Light-Driven Photocatalysts MCo1/3Nb2/3O3 (M = Ca, Sr, and Ba) with Special Electronic Structures

A new series of ABO3-type perovskite photocatalysts MCo1/3Nb2/3O3 (M = Ca, Sr, and Ba) were synthesized by a solid-state reaction method, in which the B site is occupied by Co2+ and Nb5+ randomly in a charge-balanced manner. They were characterized by powder X-ray diffraction, UV−vis diffuse reflectance spectroscopy and photocatalytic activity measurements with Pt and NiOx cocatalyst under visible-light irradiation (λ > 420 nm), respectively. It is determined from the reflectance spectra that the optical transitions for these compounds, corresponding to visible-light absorption, are directly forbidden, and the band structures are suggested to be composed of the Co2+ 3d states and the Nb5+ 4d states. The feature of the electronic structure for BaCo1/3Nb2/3O3 calculated by the full potential-linearized augmented plane wave (F-LAPW) method shows that the O 2p states are strongly hybridized with the Co2+ 3d states, and little hybridized with the Nb5+ 4d states. The difference in their photocatalytic activity ...

Photophysical and Photocatalytic Properties of MIn0.5Nb0.5O3 (M = Ca, Sr, and Ba)

New photocatalysts MIn0.5Nb0.5O3 (M = Ca, Sr, and Ba) with the ABO3 type perovskite structure in which B sites are occupied by In3+ or Nb5+ ions randomly in a charge-balanced manner were synthesized. They were characterized by structural analysis, diffuse reflectance spectroscopy, and photocatalytic activity measurement in evolving H2 and O2 from pure water with cocatalysts Pt and NiOx under UV and visible light irradiation (λ > 420 nm), respectively. It is suggested that the conduction bands of these photocatalysts are composed of In 5s and Nb 4d levels, and the valence bands are composed of O 2p levels. With the decrease of the ionic radius in the A site, the CaIn0.5Nb0.5O3, SrIn0.5Nb0.5O3, and BaIn0.5Nb0.5O3 photocatalysts show different photocatalytic activities. Under UV light irradiation, the BaIn0.5Nb0.5O3 photocatalyst powder with cocatalyst NiOx split pure water into H2 and O2 in a nearly stoichiometric ratio. Under visible irradiation, all of these photocatalysts show obvious photocatalytic activity in evolving H2 from pure water with cocatalyst NiOx.

Visible photoluminescence in amorphous ABO3 perovskites

The photoluminescence observed in ABO3 type perovskite in their highly structural disordered state can be explained by a model in which is assumed a distribution of electronic states localized within the energy band gap coupled to lattice local vibrational states. The model fits very well the experimental results and indicates that photoluminescence in the visible region can be considered as a general behavior of disordered solids.

Catalytic oxidation of 1,2-dichlorobenzene over ABO 3-type perovskites

Several ABO 3-type perovskite oxides (A = La, Y, Nd or Gd; B = Fe, Mn, Cr or Co) have been investigated as catalysts for the oxidation of 1,2-dichlorobenzene ( o-DCB), a model compound for the highly toxic polychlorinated dibenzodioxins. Initial transient and steady state activity measurements were conducted with all catalysts in the absence and presence of water. Perovskites containing Cr in the B-site were more active than perovskites containing other transition metals, with YCrO 3 being the most active catalyst among the different systems studied. Furthermore, YCrO 3 did not show any loss of its initial activity after several hours on stream. Other perovskites lost 10–20% of their initial activity within the first 5–10 h on stream. This loss was associated with a corresponding loss in BET surface area. With the exception of LaCoO 3, all perovskites retained their crystalline structure upon exposure to o-DCB under reaction conditions. LaCoO 3 was converted to LaOCl and Co 3O 4. The presence of water appeared to enhance the catalytic activity of some perovskites. This effect can be attributed to a faster removal of Cl − ions from the catalyst surface via their reaction with water.

A method for increasing the surface area of perovskite-type oxides

A method based on hydrothermal treatments is described for increasing the surface area of sintered ABO3-type perovskite oxides. Influence of hydrothermal treatments, such as water treatment at 125–300°C under autogeneous pressure and steam treatment at 350–800°C, to low surface area (or sintered) LaCoO3 and LaMnO3 perovskite oxides on their surface properties (viz. surface area, crystal size and morphology and surface La/(Co or Mn) ratio) and also catalytic activity in complete combustion of methane at different temperatures (450–600°C) has been thoroughly investigated. The hydrothermal treatments result in the activation of the perovskite oxides by increasing their surface area very markedly.

Structural Investigations on (BaxSr1-x)(ZryTi1-(y+δ)Taδ)O3 Dielectric Resonator Compounds Used for Microwave Applications

Structural investigations on ABO3 type perovskite dielectricresonator (DR) compounds, namely (Ba0.29Sr0.71)(Zr_0.973Ti_0.027)O_3 [BSZT] and (Ba_0.29Sr_0.71)(Zr_0.951Ti_0.039Ta_0.01)O_3 [BSZTTa] were carried out using X-ray Rietveld method. The structures were found to be isostructural with SrZrO_3 and belong to the space group Pbnm. In both the structures, the tilt angles of the BO_6 octahedra were lower than that in SrZrO_3. The volume of the octahedron was comparable for all the three. When Ta was substituted in BSZT, dielectric resonator properties of the resulting product BSZTTa improved substantially.

Protonic conduction in Sr 1 − y(Zr 1 − xDy x)O 3 − δ ceramics

Ceramic materials derived from strontium zirconate were prepared by high-temperature solid-state reaction starting from oxides and carbonates. Distorted ABO 3-type perovskite structures, indexed to the orthorhombic system, were obtained for A-site sub-stoichiometric and/or B-site Dy-doped materials. The conductivity of Sr(Zr 1 − x Dy x )O 3 − δ is slightly lower than found for Y-doped strontium zirconate with identical trivalent dopant content, and increases with water vapour pressure, as expected for protonconducting materials. For Dy-free perovskites with slight A-site substoichiometry (Sr 1 − y ZrO 3 − δ, with y ≤ 0·02), the conductivity drops a few orders of magnitude and is nearly independent of water vapour pressure. The corresponding B-site doped materials [Sr 1 − y (Zr 1 − x Dy x )O 3 − δ] have the highest conductivities, again dependent on water vapour pressure. This indicates that B-site doping is essential to obtain significant proton conductivity. The behaviour of these materials can be understood based on a classical defect chemistry type of approach, if one assumes that electron hole mobilities at low temperature (≈300 °C) are smaller than for protons. This trend is reversed at higher temperatures (>500 °C). For highly substoichiometric perovskites ( y ≥ 0·05), even when B-site doped, the conductivity is minimal and independent of water vapour pressure. A blocking intergrain phase is believed to control the electrical transport properties of these materials.