Oxidative coupling of methane in a mixed-conducting perovskite membrane reactor

Ionic-electronic mixed-conducting perovskite-type oxide La0.6Sr0.4Co0.8Fe0.2O3 was applied as a dense membrane for oxygen supply in a reactor for methane coupling. The oxygen permeation properties were studied in the pO2-range of 10¿3¿1 bar at 1073¿1273 K, using helium as a sweeping gas at the permeate side of the membrane. The oxygen semi-permeability has a value close to 1 mmol m¿2 s¿1 at 1173 K with a corresponding activation energy of 130¿140 kJ/mol. The oxygen flux is limited by a surface process at the permeate side of the membrane. It was found that the oxygen flux is only slightly enhanced if methane is admixed with helium. Methane is converted to ethane and ethene with selectivities up to 70%, albeit that conversions are low, typically 1¿3% at 1073¿1173 K. When oxygen was admixed with methane rather than supplied through the membrane, selectivities obtained were found to be in the range 30¿35%. Segregation of strontium was found at both sides of the membrane, being seriously affected by the presence of an oxygen pressure gradient across it. The importance of a surface limited oxygen flux for application of perovskite membranes for methane coupling is emphasized

Mass-fractal growth in niobia/silsesquioxane mixtures: a small-angle X-ray scattering study

The nucleation and growth of niobium pentaethoxide (NPE)-derived clusters in ethanol, through acid-catalyzed hydrolysis/condensation in the presence and absence of the silsesquioxane 1,2-bis(triethoxysilyl)ethane (BTESE), was monitored at 298-333 K by small-angle X-ray scattering. The data were analyzed with a newly derived model for polydisperse mass-fractal-like structures. At 298-313 K in the absence of BTESE the data indicated the development of relatively monodisperse NPE-derived structures with self-preserving polydispersity during growth. The growth exponent was consistent with irreversible diffusion-limited cluster agglomeration. At 333 K the growth exponent was characteristic for fast-gelling reaction-limited cluster agglomeration. The reaction yielded substantially higher degrees of polydispersity. In the presence of BTESE the growth exponents were substantially smaller. The smaller growth exponent in this case is not consistent with irreversible Smoluchowski-type agglomeration. Instead, reversible Lifshitz-Slyozov-type agglomeration seems to be more consistent with the experimental dat

Structure–property tuning in hydrothermally stable sol–gel-processed hybrid organosilica molecular sieving membranes

Supported microporous organosilica membranes made from bridged silsesquioxane precursors by an acid-catalyzed sol–gel process have demonstrated a remarkable hydrothermal stability in pervaporation and gas separation processes, making them the first generation of ceramic molecular sieving membranes with sufficient performance under industrially relevant conditions. The commercial availability of various α,ω-bis(trialkoxysilyl)alkane and 1,4-bis(trialkoxysilyl)benzene precursors facilitates the tailoring of membrane properties like pore size and surface chemistry via the choice of precursor(s) and process variables. Here, we describe the engineering of sols for making supported microporous thin films, discuss the thermal and hydrothermal stability of microporous organosilicas and give a short overview of the developments and applications of these membranes in liquid and gas separation processes since their first report in 2008

Synthesis and characterization of microporous titania membranes

A procedure for the preparation of microporous titania membranes by the polymeric sol-gel technique is reported. The influence of acid/titanium ratio, water/titanium ratio, method of mixing components and refluxing time on particle size and sol stability was investigated. The thermal evolution, structural characteristics and liquid permeation properties of calcined materials were studied. Highly reproducible amorphous microporous titania layers with pore sizes le0.8 nm were obtained on both mesoporous gamma-alumina and titania/zirconia coated substrates. The upper limit of thermal stability of the amorphous phase is sim300°C. Higher calcination temperatures led to phase transformation into anatase, which was accompanied by a collapse of the microstructure. The material was found to be chemically stable in a wide pH interval

Crystal and magnetic structure of substituted lanthanum cobaltites

The crystal and magnetic structures of the lanthanum cobaltites La0.6Sr0.4CoO3, La0.6Sr0.4Co0.9Fe0.1O3 and La0.6Ba0.4Co0.9Fe0.1O3 have been studied by neutron powder diffraction at temperatures of 2, 300 and 900 K. All compounds undergo a phase transition from cubic to rhombohedral structure. Below the room temperature La0.6Sr0.4CoO3 becomes ferromagnetic while for the components with 10% Fe substituted for Co, we found an antiferromagnetic order

Complexation of lithium and sodium cations with B-phosphorylate ethers, modelling terminal groups of organophosphorus podands. An experimental and theoretical study

The organophosphorus compounds o-Ph2P(O)C6H4OCH3 and Ph2P(O)C2H4OCH3, which are analogs of podands' terminal groups, have been synthesized. The thermodynamic characteristics of their complexation with LiNCS and NaNCS in acetonitrile were obtained by calorimetry. Molecular mechanics calculations on M+ L complexes with different stoichiometries M+:L = 1:1, 1:2 and 1:4 (M+ = Li+, Na+) were performed, as well as on their solvates with a limited number of MeCN molecules. It has been shown that the experimental data could be explained by taking into account both the specific features of the complex structure and the solvent effects. The possibility of the application of additive schemes in the investigation of the complexation of polydentate molecules is discussed

Manipulation of Crystal Orientation and Phase Distribution of Quasi-2D Perovskite through Synergistic Effect of Additive Doping and Spacer Engineering

The diammonium precursor 1,4-phenylenedimethanammonium (PDMA) was used as a large organic spacer for the preparation of Dion-Jacobson-type quasi-2D perovskites (PDMA)(MA)n−1PbnI3n+1 (MA = methylammonium). Films with composition ⟨n⟩ = 5 comprised randomly orientated grains and multiple microstructural domains with locally differing n values. However, by mixing the Dion-Jacobson-type spacer PDMA and the Ruddlesden-Popper-type spacer propylammonium (PA), the crystal orientation in both the vertical and the horizonal directions became regulated. High crystallinity owing to well-matched interlayer distances was observed. Combining this spacer-engineering approach with the addition of methylammonium chloride (MACl) led to full vertical alignment of the crystal orientation. Moreover, the microstructural domains at the substrate interface changed from low-n (n = 1, 2, 3) to high-n (n = 4, 5), which may be beneficial for hole extraction at the interface between perovskite and hole transport layer due to a more finely tuned band alignment. Our work sheds light on manipulating the crystallization behavior of quasi-2D perovskite and further paves the way for highly stable and efficient perovskite devices.</p

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and reduction half reactions, and fast electron (hole) diffusion and charge separation. Nanowires present suitable architectures to meet these requirements. Axially segmented Ag|ZnO and radially segmented (coaxial) TiO2-Ag nanowires with a diameter of 200 nm and a length of 6-20 µm were made by templated electrodeposition within the pores of polycarbonate track-etched (PCTE) or anodized aluminum oxide (AAO) membranes, respectively. In the photocatalytic experiments, the ZnO and TiO2 phases acted as photoanodes, and Ag as cathode. No external circuit is needed to connect both electrodes, which is a key advantage over conventional photo-electrochemical cells. For making segmented Ag|ZnO nanowires, the Ag salt electrolyte was replaced after formation of the Ag segment to form a ZnO segment attached to the Ag segment. For making coaxial TiO2-Ag nanowires, a TiO2 gel was first formed by the electrochemically induced sol-gel method. Drying and thermal annealing of the as-formed TiO2 gel resulted in the formation of crystalline TiO2 nanotubes. A subsequent Ag electrodeposition step inside the TiO2 nanotubes resulted in formation of coaxial TiO2-Ag nanowires. Due to the combination of an n-type semiconductor (ZnO or TiO2) and a metal (Ag) within the same nanowire, a Schottky barrier was created at the interface between the phases. To demonstrate the photocatalytic activity of these nanowires, the Ag|ZnO nanowires were used in a photocatalytic experiment in which H2 gas was detected upon UV illumination of the nanowires dispersed in a methanol/water mixture. After 17 min of illumination, approximately 0.2 vol% H2 gas was detected from a suspension of ~0.1 g of Ag|ZnO nanowires in a 50 ml 80 vol% aqueous methanol solution