CO oxidation with oxygen of the catalyst and gas-phase oxygen over (0.5−15)%СоО/СеО<Subscript>2</Subscript>

The CO adsorption species on Co₃O₄ and (0.5-15%)CoO/CeO₂ catalysts have been investigated by temperature-programmed desorption and IR spectroscopy. At 20°C, the largest amount of CO is adsorbed on the 5%CoO/CeO₂ sample to form, on Co_m²⁺O_n²⁺ clusters, hydrogen-containing, bidentate, and monodentate carbonate complexes, whose decomposition is accompanied by CO₂ desorption at 300 and 450°C (1.1 × 10²⁰ g^–1). The formation of the carbonates is accompanied by the formation of Co⁺ cations and Co⁰, on which carbonyls form. The latter decompose at 20, 90, and 170°C to release CO (2.7 × 10¹⁹ g^–1). Part of the carbonyls oxidizes to CO₂ upon oxygen adsorption, and the CO₂ undergoes desorption at 20°C. Adsorbed oxygen decreases the decomposition temperature of the H-containing and bidentate carbonates from 300 to 100-170°C and maintains the sample in the oxidized state, which is active in subsequent CO adsorption and oxidation. CO oxidation by oxygen of the catalyst diminishes the activity of the sample in these processes and increases the decomposition temperature of the carbonate complexes. Taking into account the properties of the adsorption complexes, we concluded that the H-containing and bidentate carbonates are involved in CO oxidation by oxygen of the catalyst at ~170°C under isothermal conditions. The rate limiting step is the decomposition of the carbonates, a process whose activation energy is 65-74 kJ/mol.     

Syntheses, structure and rare earth metal photoluminescence of new and known isostructural A2Mo4Sb2O18 (A=Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) compounds

Nine new A₂Mo₄Sb₂O₁₈ (A=Ce, Pr, Eu, Tb, Ho, Er, Tm, Yb, Lu) compounds have been synthesized by solid-state reactions. They are isostructural with six reported analogues of yttrium and other lanthanides and the monoclinic unit cell parameters of all fifteen of them vary linearly with the size of A³⁺ ion. Single crystal X-ray structures of eight A₂Mo₄Sb₂O₁₈ (A=Ce, Pr, Eu, Gd, Tb, Ho, Er, Tm) compounds have been determined. Neat A₂Mo₄Sb₂O₁₈ (A=Pr, Sm, Eu, Tb, Dy, Ho, Er, Tm) compounds exhibit characteristic rare earth metal photoluminescence.     

Electronic structure and photocatalytic properties of ABi2Ta2O9 (A=Ca, Sr, Ba)

A new series of layered perovskite photocatalysts, ABi₂Ta₂O₉ (A=Ca, Sr, Ba), were synthesized by the conventional solid-state reaction method and the crystal structures were characterized by powder X-ray diffraction. The results showed that the structure of ABi₂Ta₂O₉ (A=Ca, Sr) is orthorhombic, while that of BaBi₂Ta₂O₉ is tetragonal. First-principles calculations of the electronic band structures and density of states (DOS) revealed that the conduction bands of these photocatalysts are mainly attributable to the Ta 5d+Bi 6p+O 2p orbitals, while their valence bands are composed of hybridization with O 2p+Ta 5d+Bi 6s orbitals. Photocatalytic activities for water splitting were investigated under UV light irradiation and indicated that these photocatalysts are highly active even without co-catalysts. The formation rate of H₂ evolution from an aqueous methanol solution is about 2.26 mmol h^-1 for the photocatalyst SrBi₂Ta₂O₉, which is much higher than that of CaBi₂Ta₂O₉ and BaBi₂Ta₂O₉. The photocatalytic properties are discussed in close connection with the crystal structure and the electronic structure in details.     

Synthesis, structure and magnetic properties of A2MnB′O6 (A=Ca, Sr; B′=Sb, Ta) double perovskites

A₂MnB′O₆ (A=Ca, Sr; B=Sb, Ta) double perovskites have been synthesized and their structural and magnetic properties have been investigated. Rietveld refinement of the powder X-ray diffraction data for Sr₂MnSbO₆ indicated significant ordering of Mn and Sb at the B-site while all other phases showed mostly a random distribution of the B-site cations. X-ray absorption spectroscopic data established the presence of Mn in the 3+ and Sb/Ta in the 5+ oxidation states in all the phases. Magnetic susceptibility data indicated ferromagnetic correlations for all the A₂MnB′O₆ phases with Weiss temperatures varying from 64 to 107 K.     

Comparative Studies on the Deactivation and Regeneration of TiO2 Nanoparticles in Three Photocatalytic Oxidation Systems: C7H16, SO2, and C7H16-SO2

Three photocatalytic oxidation (PCO) systems: C₇H₁₆-O₂, SO₂-O₂ and C₇H₁₆-SO₂-O₂ were carried out with the aid of UV-illuminated TiO₂ nanoparticles at room temperature in a batch reactor. In C₇H₁₆-O₂-TiO₂ system, no catalyst deactivation was observed, while for SO₂-O₂-TiO₂ and C₇H₁₆-SO₂-O₂-TiO₂ systems, the photocatalytic activity of used TiO₂ powder showed decreasing and eventually no activity after used consecutively. The reaction products such as sulfur trioxide or sulfuric acid adsorbed onto the surface of TiO₂ catalyst were poisoning species. Photocatalytic activity of the deactivated TiO₂ powder could be regenerated by sonicating treatment with water and methanol for the two systems, respectively.     

Mossbauer Investigation of Fe-Doped Ni(III) Perovskites ANi0.98Fe0.02O3 (A=Pr, Nd, Sm, Y, Lu, Tl) versus Temperature

Nickelates ANiO₃ (A=Pr, Nd, Sm, Lu, Y, Tl) containing Mössbauer probe ⁵⁷Fe atoms were synthesized. In the case of nickelates with larger rare earth (A=Pr, Nd, Sm) the Mössbauer spectra confirm that ferric ions are located in single type of crystallographic positions. On the contrary, the spectra of ANi_0.98Fe_0.02O₃ with small cations (A=Lu, Y, Tl) can be described as a superposition of two sub-spectra which indicate that ⁵⁷Fe probe atoms are simultaneously stabilized in two non-equivalent crystallographic positions. These results have been interpreted in terms of partial charge disproportionation of Ni³⁺ cations associated with the electronic localization in monoclinic distorted Lu, Y, Tl nickelates. The modification of ⁵⁷Fe spectra for TlNi_0.98Fe_0.02O₃ as a function of temperature has shown that this charge disproportionation occurs in varying degrees, corresponding to the charge states Fe^(3+σ)+ and Fe^(3−σ′)+. On the contrary, the spectra for Lu and Y nickelates show that charge variation (σ,σ′) for dopant Fe(1) and Fe(2) cations does not depend on temperature.     

Les oxydes A2BaCuO5 (A = Y,Sm, Eu,Gd, Dy,Ho, Er,Yb)

A series of new phases, A₂BaCuO₅ (A = Y, Sm, Eu, Gd, Dy, Ho, Er, Yb), has been isolated. These compounds are orthorhombic, with $\text{a ? 7.1, b ? 12.2}$, and $\text{c ? 5.6}$Å. The probable space groups deduced from the electron diffraction patterns are Pbnm and Pbn2₁. The structure has been resolved from X-ray powder patterns. The framework can be considered as built up from distorted monocapped trigonal prisms AO₇ which share one triangular face forming A₂O₁₁ blocks. The edge-sharing A₂O₁₁ blocks form a three-dimensional network which delimits cavities where Ba²⁺ and Cu²⁺ are located. Barium is coordinated to 11 oxygen atoms, while the coordination polyhedron of copper is a distorted tetragonal pyramid CuO₅.     

Cation disorder and phase transitions in the four-layer ferroelectric Aurivillius phases ABi4Ti4O15 (A=Ca, Sr, Ba, Pb)

Crystal structures of a series of bi-layered compounds ABi₄Ti₄O₁₅ (A=Ca, Sr, Ba, Pb) have been investigated using a combination of synchrotron X-ray and neutron powder diffraction data. All four oxides adopt an orthorhombic structure at room temperature and the structures have been refined in space group A2₁am. This orthorhombic structure is a consequence of a combination of rotation of the TiO₆, resulting from the less than optimal size of the A-type cation, and displacement of the Ti atoms towards the Bi₂O₂ layers. There is partial disorder of the Bi and A-type cations over two of the three available sites, which increases in the order Ca<Sr and Pb<Ba.     

Syntheses, structures, and second-harmonic generating properties in new quaternary tellurites: A2TeW3O12 (A=K, Rb, or Cs)

The syntheses, structures, and characterization of a new family of quaternary alkali tungsten tellurites, A₂TeW₃O₁₂ (A=K, Rb, or Cs), are reported. Crystals of the materials were synthesized by supercritical hydrothermal methods using 1 M AOH (A=K, Rb, or Cs), TeO₂, and WO₃ as reagents. Bulk, polycrystalline phases were synthesized by standard solid-state methods combining stoichiometric amounts of A₂CO₃, TeO₂, and WO₃. Although the three materials are not iso-structural, each exhibits a hexagonal tungsten oxide layer comprised of corner-sharing W⁶⁺O₆ octahedra. Te⁴⁺O₃ groups connect the WO₆ layers in K₂TeW₃O₁₂, whereas the same groups cap the WO₆ layers in Rb₂TeW₃O₁₂ and Cs₂TeW₃O₁₂. This capping results in non-centrosymmetric structures for Rb₂TeW₃O₁₂ and Cs₂TeW₃O₁₂. Powder second-harmonic generation measurements on Rb₂TeW₃O₁₂ and Cs₂TeW₃O₁₂ revealed strong SHG efficiencies of 200 and 400×SiO₂, respectively. These values indicate an average non-linear optical susceptibility, 〈d_eff〉_exp of 16 and 23 pm/V for Rb₂TeW₃O₁₂ and Cs₂TeW₃O₁₂, respectively. Crystallographic information: K₂TeW₃O₁₂, monoclinic, space group P2₁/n (No. 14), a=7.3224(13) Å, b=11.669(2) Å, c=12.708(2) Å, β=90.421(3)°, Z=4; Rb₂TeW₃O₁₂, trigonal, space group P31c (No. 159), a=b=7.2980(2) Å, c=12.0640(2) Å, Z=2.     

Synthesis and characterization of the pseudo-hexagonal hollandites ALi2Ru6O12 (A=Na, K)

The crystal structures, synthesis and physical properties of ruthenium hollandites ALi₂Ru₆O₁₂ (A=Na, K) with a new pseudo-hexagonal structure type are described. Analogous to tetragonal hollandites, the framework is made of MO₆ octahedra in double chains that share corner oxygens with each other to create interstitial tunnels. The tunnels are either hexagonal or triangular in cross-section. Magnetic susceptibilities, low temperature specific heat, and electrical resistivities are reported. The data indicate that these materials are normal, low density of states metals. This new structure type can be extended from A=Group I to A=Group II ions with the synthesis of CaLi₂Ru₆O₁₂ and SrLi₂Ru₆O₁₂.     

Syntheses and structures of six compounds in the A2LiMS4 (A=K, Rb, Cs; M=V, Nb, Ta) family

Six new compounds in the A₂LiMS₄ (A=K, Rb, Cs; M=V, Nb, Ta) family, namely K₂LiVS₄, Rb₂LiVS₄, Cs₂LiVS₄, Rb₂LiNbS₄, Cs₂LiNbS₄, and Rb₂LiTaS₄, have been synthesized by the reactions of the elements in Li₂S/S/A₂S₃ (A=K, Rb, Cs) fluxes at 773 K. The A and M atoms play a role in the coordination environment of the Li atoms, leading to different crystal structures. Coordination numbers of Li atoms are five in K₂LiVS₄, four in A₂LiVS₄ (A=Rb, Cs) and Cs₂LiNbS₄, and both four and five in Rb₂LiMS₄ (M=Nb, Ta). The A₂LiVS₄ (A=Rb, Cs) structure comprises one-dimensional chains of tetrahedra. The Rb₂LiMS₄ (M=Nb, Ta) structure is composed of two-dimensional layers. The Cs₂LiNbS₄ structure contains one-dimensional chains that are related to the Rb₂LiMS₄ layers. The K₂LiVS₄ structure contains a different kind of layer.     

Electronic and structural properties of A Al2Se4 (A=Ag, Cu, Cd, Zn) chalcopyrite semiconductors

We have studied the structural and electronic properties of defect chalcopyrite semiconductors A Al₂Se₄ (A=Ag, Cu, Cd, Zn) using density functional theory (DFT) based first principle technique within tight binding linear muffin-tin orbital (TB-LMTO) method. Our calculated structural parameters such as lattice constants a and c, tetragonal distortion (η=c/2a) are in good agreement with experimental work. Anion displacement parameters, bond lengths and bulk modulus are also calculated. Our band structure calculation suggests that these compounds are direct band gap semiconductors having band gaps 2.40, 2.50, 2.46 and 2.82 eV for A Al₂Se₄ (A=Ag, Cu, Cd, Zn) respectively. Calculated band gaps are in good agreement with other experimental and theoretical works within LDA limitation. We have made a quantitative estimation of the effect of p-d hybridization and structural distortion on the electronic properties. The reduction in band gap due to p-d hybridization is 19.47%, 21.29%, 0% and 0.7% for A Al₂Se₄ (A=Ag, Cu, Cd, Zn) respectively. Increment of the band gap due to structural distortion is 11.62%, 2.45%, 2.92% and 9.30% in case of AgAl₂Se₄, CuAl₂Se₄, CdAl₂Se₄ and ZnAl₂Se₄ respectively. We have also discussed the bond nature of all four compounds.     

High-temperature thermoelectric studies of A11Sb10 (A=Yb, Ca)

Large samples (6-8 g) of Yb₁₁Sb₁₀ and Ca₁₁Sb₁₀ have been synthesized using a high-temperature (1275-1375 K) flux method. These compounds are isostructural to Ho₁₁Ge₁₀, crystallizing in the body-centered, tetragonal unit cell, space group I4/mmm, with Z=4. The structure consists of antimony dumbbells and squares, reminiscent of Zn₄Sb₃ and filled Skutterudite (e.g., LaFe₄Sb₁₂) structures. In addition, these structures can be considered Zintl compounds; valence precise semiconductors with ionic contributions to the bonding. Differential scanning calorimetry (DSC), thermogravimetry (TG), resistivity (ρ), Seebeck coefficient (α), thermal conductivity (κ), and thermoelectric figure of merit (zT) from room temperature to at minimum 975 K are presented for A₁₁Sb₁₀ (A=Yb, Ca). DSC/TG were measured to 1400 K and reveal the stability of these compounds to ∼1200 K. Both A₁₁Sb₁₀ (A=Yb, Ca) materials exhibit remarkably low lattice thermal conductivity (∼10 mW/cm K for both Yb₁₁Sb₁₀ and Ca₁₁Sb₁₀) that can be attributed to the complex crystal structure. Yb₁₁Sb₁₀ is a poor metal with relatively low resistivity (1.4 mΩ cm at 300 K), while Ca₁₁Sb₁₀ is a semiconductor suggesting that a gradual metal-insulator transition may be possible from a Ca_11−xYb_xSb₁₀ solid solution. The low values and the temperature dependence of the Seebeck coefficients for both compounds suggest that bipolar conduction produces a compensated Seebeck coefficient and consequently a low zT.     

Cation and anion ordering in the layered oxyfluorides Sr3−xAxAlO4F (A=Ba, Ca)

The synthesis and structural characterization of mixed oxyfluorides of the type Sr_3−xA_xAlO₄F is reported, where A is either calcium or barium. In these compounds the fluoride and oxide ions are ordered onto two distinct crystallographic sites. There is also an ordering of the alkaline earth cations over two crystallographic sites upon substitution of Ba²⁺ or Ca²⁺ for Sr²⁺. The solid solubility limits extend to x∼1 for substitution of both barium and calcium, but the larger Ba²⁺ cations show a strong site preference for the ten-coordinate strontium sites, while the smaller Ca²⁺ cations prefer the eight-coordinate strontium sites.     

Synthesis, crystal structure, and photocatalytic activity of the new three-layer aurivillius phases, Bi2ASrTi2TaO12 (A=Bi, La)

Two new three-layer Aurivillius phases Bi₂ASrTi₂TaO₁₂ (A=Bi, La) have been synthesized. The detailed structure determination of Bi₂ASrTi₂TaO₁₂ (A=Bi, La) performed by powder X-ray diffraction (XRD) and selected area electron microscopy (SAED) shows that they all crystallize in the space group I/4mmm. UV-visible diffuse reflection spectrum of the prepared Bi₂ASrTi₂TaO₁₂ (A=Bi, La) indicates that it had absorption in the ultraviolet (UV) region. The photocatalytic activity of the Bi₂ASrTi₂TaO₁₂ (A=Bi, La) powders was evaluated by degradation of rhodamine B (RB) molecules in water under UV light irradiation. The results showed that Bi₂ASrTi₂TaO₁₂ (A=Bi, La) has high photocatalytic activity at room temperature. Therefore, the preparation and properties studies of Bi₂ASrTi₂TaO₁₂ (A=Bi, La) with a three-layer Aurivillius structure suggest potential future applications in photocatalysis.     

A large series of isotypic Mo(V) diphosphates with a tunnel structure: From A(MoO)10(P2O7)8 with A=Ba, Sr, Ca, Cd, Pb to A(MoO)5(P2O7)4 with A=Ag, Li, Na, K

The single crystal structure of a series of nine isotypic Mo(V) diphosphates was determined from crystals with composition A²⁺(MoO)₁₀(P₂O₇)₈ (A=Ba, Sr, Ca, Cd, Pb) and A⁺(MoO)₅(P₂O₇)₄ (A=Ag, Li, Na, K). The structure of those phosphates, built up of corner sharing MoO₆ octahedra, MoO₅ tetragonal pyramids and P₂O₇ diphosphates groups, forms eight-sided tunnels as described by Lii et al. for A=Ag. New features are evidenced: (1) existence of two orientations, up and down along b for the MoO₅ pyramids; (2) maximum insertion rate of the divalent cations which is twice less than that of the univalent cations; (3) different behavior of the series “Pb, Sr, Ba, Li, Na, K” which exhibits only one kind of site for the inserted cation, compared to the “Cd, Ca, Ag” series for which two kinds of sites are observed; (4) off-centering of the A-site cations with respect to the tunnel axis; and (5) unusually high thermal factors along the tunnel axis, but absence of ionic conductivity.     

Structural studies of five layer Aurivillius oxides: A2Bi4Ti5O18 (A=Ca, Sr, Ba and Pb)

The room temperature structures of the five layer Aurivillius phases A₂Bi₄Ti₅O₁₈ (A=Ca, Sr, Ba and Pb) have been refined from powder neutron diffraction data using the Rietveld method. The structures consist of [Bi₂O₂]²⁺ layers interleaved with perovskite-like [A₂Bi₂Ti₅O₁₆]²⁻ blocks. The structures were refined in the orthorhombic space group B2eb (SG. No. 41), Z=4, and the unit cell parameters of the oxides are a=5.4251(2), b=5.4034(1), c=48.486(1); a=5.4650(2), b=5.4625(3), c=48.852(1); a=5.4988(3), b=5.4980(4), c=50.352(1); a=5.4701(2), b=5.4577(2), c=49.643(1) for A=Ca, Sr, Ba and Pb, respectively. The structural features of the compounds were found similar to n=2-4 layers bismuth oxides. The strain caused by mismatch of cell parameter requirements for the [Bi₂O₂]²⁺ layers and perovskite-like [A₂Bi₂Ti₅O₁₆]²⁻ blocks were relieved by tilting of the TiO₆ octahedra. Variable temperature synchrotron X-ray studies for Ca and Pb compounds showed that the orthorhombic structure persisted up to 675 and 475 K, respectively. Raman spectra of the compounds are also presented.     

A structure, conductivity and dielectric properties investigation of A3CoNb2O9 (A=Ca, Sr, Ba) triple perovskites

The room temperature structures as well as the temperature-dependent conductivity and dielectric properties of the A₃CoNb₂O₉ (A=Ca²⁺, Sr²⁺ and Ba²⁺) triple perovskites have been carefully investigated. A constrained modulation wave approach to Rietveld structure refinement is used to determine their room temperature crystal structures. Correlations between these crystal structures and their physical properties are found. All three compounds undergo insulator to semiconductor phase transitions as a function of increasing temperature. The hexagonal Ba₃CoNb₂O₉ compound acts as an insulator at room temperature, while the monoclinic Ca₃CoNb₂O₉ compound is already a semiconductor at room temperature. The measured dielectric frequency characteristics of the A=Ba compound are excellent.     

Synthesis of cyclic carbonates and dimethyl carbonate using CO<Subscript>2</Subscript> as a building block catalyzed by MOF-5/KI and MOF-5/KI/K<Subscript>2</Subscript>CO<Subscript>3</Subscript>

The synthesis of cyclic carbonates or dimethyl carbonate (DMC) using CO₂ as a building block is a very interesting topic. In this work, we found that the metalorganic framework-5 (MOF-5)/KI was an active and a selective catalytic system for the synthesis of cyclic carbonates from CO₂ and epoxides, and MOF-5/KI/K₂CO₃ was efficient for the preparation of DMC from CO₂, propylene, and methanol by a sequential route. The impacts of temperature, pressure, and reaction time length on the reactions were investigated, and the mechanism of the reactions is proposed on the basis of the experimental results.     

Structure property relationships in the ATi2O4 (A=Na, Ca) family of reduced titanates

Reduced titanates in the ATi₂O₄ (A=Li, Mg) spinel family exhibit a variety of interesting electronic and magnetic properties, most notably superconductivity in the mixed-valence spinel, Li_1+xTi_2−xO₄. The sodium and calcium analogs, NaTi₂O₄ and CaTi₂O₄, each differ in structure, the main features of which are double rutile-type chains composed of edge-sharing TiO₆ octahedra. We report for the first time, the properties and band structures of these two materials. XANES spectroscopy at the Ti K-edge was used to probe the titanium valence. The absorption edge position and the pre-edge spectral features observed in the XANES data confirm the assignment of Ti³⁺ in CaTi₂O₄ and mixed-valence Ti³⁺/Ti⁴⁺ in NaTi₂O₄. Temperature-dependent resistivity and magnetic susceptibility studies are consistent with the classification of both NaTi₂O₄ and CaTi₂O₄ as small band-gap semiconductors, although changes in the high-temperature magnetic susceptibility of CaTi₂O₄ suggest a possible insulator-metal transition near 700 K. Band structure calculations agree with the observed electronic properties of these materials and indicate that while Ti-Ti bonding is of minimal importance in NaTi₂O₄, the titanium atoms in CaTi₂O₄ are weakly dimerized at room temperature.