Catalysts based on cordierite modified with transition metal oxides

Catalysts active in ammonia oxidation have been obtained by the substitution of transition metal (Mn, Fe, Co, Ni, and Cu) ions for Mg ions in the cordierite structure 2MgO · 2Al₂O₃ · 5SiO₂ at 1100°C. Their phase composition, texture, and activity depend on the type and amount of introduced transition metal oxide. The Mn- and Cu-containing catalysts, which consist of substituted cordierites 2(Mg_{1 ? x} M _x )O · 2Al₂O₃ · 5SiO₂ and Mn₂O₃ or CuO crystallites located on their surface, are most active in ammonia oxidation. The catalysts are characterized by a small specific surface area and have large pores, whose total volume is small. The Fe-containing catalysts consist of the Fe-substituted cordierite phase and particles of an iron oxide phase. These particles are mostly located in internal pores of the catalysts and are, therefore, hardly accessible to ammonia molecules. The introduction of Co or Ni oxide leads to the formation of a low-active spinel phase rather than the cordierite phase.     

Influence of surface properties of the titanium dioxide porous films on the characteristics of solar cells

Porous films formed by cylindrical geometrically anisotropic fragments of TiO₂ have been produced by electrochemical anodization of titanium. The specific surface area and pore volume of the samples were determined by the BET method. It is shown that the samples have a bimodal pore-size distribution with maxima depending on the anodization voltage: by increase in voltage the inner diameter of the cylindrical pores grows, which leads to a decrease in the specific surface area. Dye sensitized solar cells were assembled on the basis of the obtained materials to study the effect of certain characteristics on the efficiency of solar energy conversion. The electrical transport properties of the films were studied by impedance spectroscopy.     

Immobilization of phosphomolybdic acid on the surface of mesoporous silica functionalized with alkylammonium groups

Silica containing protonated 3-aminopropyl groups has been synthesized by a template method using Pluronic 123 as a surfactant in an acidic medium at a hydrochloric acid concentration of 1.72 M. The X-ray diffraction data have indicated that the obtained sample has a highly ordered hexagonal structure, which agrees with the large specific surface area and sorption pore volume (S _sp = 880 m²/g, V _s = 1.28 cm³/g) typical of SBA-15 materials. However, transmission electron microscopy data show that this material contains two phases, with one of them being similar to SBA-15 mesoporous silica and the other belonging to socalled “honeycomb foams.” An ion-exchange reaction has been employed to immobilize phosphomolybdic acid H₃[P(Mo₃O₁₀)₄] on the pore surface, and the fact of immobilization has been confirmed by IR and TGA data. This functionalization significantly reduces the S _sp and V _s values of the sample but has no effect on isotherm pattern, thereby attesting to retaining the silica structure after the modification.     

Molecular Mechanisms of the Effect of Water on CO<Subscript>2</Subscript>/CH<Subscript>4</Subscript> Mixture Adsorption in Slitlike Carbon Pores

The grand canonical ensemble Monte Carlo method has been used to study adsorption of carbon dioxide, methane, and their mixtures with different compositions in slitlike carbon pores at a temperature of 318 K and pressures below 60 atm. The data obtained have been used to show the effect of fixed amounts of pre-adsorbed water (19, 37, and 70 vol %) on the adsorption capacity and selectivity of carbon micro- and mesopores. The presence of water reduces the adsorption capacity throughout the studied pressure range upon adsorption of gaseous mixtures containing less than 50% CO₂, as well as in narrow micropores (with widths of 8?12 Å). Upon adsorption of mixtures with CO₂ contents higher than 50%, the adsorption capacity of pores with low water contents appears, in some region of the isotherm, to be higher than that in dry pores. In the case of wide pores (16 and 20 Å), this region is located at low and moderate pressures, while for mesopores it is located at high pressures. The analysis of the calculated data has shown that the molecular mechanism of the influence of preadsorbed water on the adsorption capacity is based on the competition between the volume accessible for adsorption (decreases the capacity) and the strength of the interaction between carbon dioxide molecules and water molecules (increases the capacity). Therewith, the larger the surface area of the water–gas contact, the stronger the H₂O–CO₂ interactions.     

Characterization of ZrO2- Al2O3mixed oxides support prepared by urea hydrolysis

The characteristics of Al₂O₃, ZrO₂ and three binary mixtures of ZrO₂-Al₂O₃ were studied by determining their BET surface areas, micropore surface area, total pore volume, adsorption-desorption isotherms, the X-ray diffractogram, surface acidity and catalytic functionality for cumene cracking. The XRD results show that the incorporation of alumina into the zirconia from 50% and beyond renders it amorphous. Furthermore, the mixed oxide containing 50% alumina and 50% zirconia had the highest BET surface area of 199.9 m²/g whilst pure zirconia had the lowest BET surface area of 37.19 m²/g. The pores for all the mixed oxides were found to be monomodal and zirconia pores were more open. The results of the acidity measurements and cumene cracking functionality indicates that whilst pure zirconia has low total acidity, the incorporation of alumina increases its acidity through a synergistic effect. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ceramic membranes modified with catalytic oxide films as ensembles of catalytic nanoreactors

Hybrid catalytic membrane systems have been produced by modifying porous ceramic membranes with metal oxide films. A two-layer cermet membrane consisting of a flexible stainless steel layer and an overlying porous TiO₂ ceramic layer and a ceramic titanium carbide membrane are examined. The membrane surfaces have been modified by the alkoxide method using colloidal organic solutions of metal complex precursors. Producing a tetragonal single-phase ZrO₂/Y₂O₃ coating on the cermet surface increases the abrasion strength of the ceramic layer. CO oxidation and the oxidative conversion of methane into synthesis gas and light hydrocarbons can be markedly intensified by modifying the membrane channels with Cu_0.03Ti_0.97O_2±δ and La + Ce/MgO catalysts, respectively. A method has been developed for depositing, onto the geometrical surface of a membrane, a film of the new single-phase oxide P_0.03Ti_0.97O_2±δ with an anatase structure and uniform pores of mean diameter 〈d〉 ～ 2 nm. Blocks of zeolite-like silicalite can be formed on the surface of the phosphorus-titanium oxide film. The resulting hybrid membrane is characterized by an anisotropic permeability depending on the flow direction. This property has an effect on conversion and selectivity in the nonoxidative dehydrogenation of methanol.     

Preparation and characterization of activated carbon fibers from liquefied wood

Wooden activated carbon fibers (WACFs) were prepared from phenolated Chinese fir (Cunninghamia lanceolata) using CO₂ activation; microstructure characterization, the adsorption capacity, BET-specific surface area, and pore distribution of WACFs were investigated by SEM and X-ray analysis. Results showed that WACFs have a smooth surface and round or elliptical cross-section. The (002) crystal plane diffraction peak of the WACFs was obviously heightened, also showing an apparent (100) diffraction peak. With increased activation temperature, the value of d ₍₀₀₂₎ gradually decreased, whereas the values of the crystallite sizes L _a and L _c initially decreased and then increased. The L _c/d ₍₀₀₂₎ and g values corresponding to the degree of change in the graphitization structure increased. WACFs mainly have micropores as well as a few macro- and mesopores. The micropore diameter of WACFs has a narrow range (0.3–0.5 nm). With increased activation temperature, the single-point surface area, Brunauer-Emmett-Teller surface area, micropore area, single-point total pore volume, and micropore volume of WACFs increased, while the pore diameter decreased. At 900 °C, the iodine adsorption and yield rate of WACFs were 779.22 mg/g and 51.48 %, respectively.     

Sol–gel synthesis,characterization and catalytic activity of mesoporous γ-alumina prepared from boehmite sol by different methods

In this study, boehmite sols were used as alumina precursors for preparing mesoporous γ-aluminas by two different methods. In one case polyethylenimine was used as a structure-directing agent, and in another case ultrasound treatment was applied. Nitrogen physisorption showed that aluminas that had been prepared by these methods demonstrated different porous structures. The sample obtained without additional treatment had closely packed spherical particles and pores had ink-bottle neck morphology. Ultrasound treatment led to the transformation of ink-bottle pores into cylindrical form and to the increase in surface area and pore volume. Aluminas prepared using polyethylenimine as a template showed larger cylindrical wormhole-like mesopores with a broader pore size distribution, high surface area and pore volume. Catalytic tests showed that textural properties as well as crystallite size were very important parameters of synthesized samples which affected the catalytic activity in the methanol dehydration reaction. It was found that γ-Al₂O₃ prepared by ultrasound treatment had large crystallite size and demonstrated high catalytic activity.     

The diffusion of methane in inorganic membrane pores under Knudsen conditions: the kinetics and depth of pyrocarbon deposition and its influence on transport properties

The kinetics of the topochemical reaction of methane dehydrogenation was studied using TRUMEM ultrafiltration membranes (TiO₂ + Cr₂O₃ on porous steel, the size of transport pores 50 nm). The depth of the deposition of pyrocarbon nanocrystallites (PNC) into pores was determined. The depth of PNC deposition was estimated using scanning electron microscopy and high-resolution energy-dispersion spectrometry. The deposition of PNC at a 4.9 kPa methane pressure in the reaction zone created Knudsen conditions for methane diffusion in pores. The deposition of PNC therefore occurred over the whole area of pore surfaces. Studies of the kinetics of PNC formation on the surface of pores showed that reaction rate V and its constant k substantially depended on reaction duration. The influence of PNC deposition on the electrosurface properties and permeability of the membranes to ethanol and dodecane was studied. After the deposition of PNC with L _c = 1.0–1.1 nm on the surface of pores, the ζ-potential and surface charge density (σ) decreased; simultaneously, the efficiency with respect to ethanol increased.     

A credible role of copper oxide on structure of nanocrystalline mesoporous titanium dioxide

Copper oxide-titania catalysts with nanocrystalline mesoporous structure were prepared by sol-gel technique using tetra isopropyl ortho titanate (TiPT) as the inorganic precursor and amino-2 ethanol as the swelling agent. Characterization was performed using X-ray diffraction (XRD), fourier transformed infrared spectra (FTIR), scanning electron microscopy (SEM), diffuse reflectance UV-Vis spectroscopy (DRS), and N₂ adsorption-desorption measurements. It was found that CuO (0.025–0.1 mol ratio) has some effect on the particle size, surface area, pore-volume, pore-diameter, crystallinity of the particles, and crystalline phase of TiO₂ nanocrystalline. The results indicated that 0.1 CuO-TiO₂ had higher surface area and total pore volume among all CuO-TiO₂ samples. The SBET value of 0.1 mol ratio CuO-load TiO₂ sample is approximately similar to that of Degussa P25 while its pore volume (0.1198 cm³ g^?1) is larger than Degussa P25 due to production of large number of pores. Therefore, the physical property of 0.1 CuO-TiO₂ catalyst is comparable with Degussa P25.At 823K, the 0.1 mol ratio CuO-load TiO2 sample shows the phase transformation from anatase to rutile in the ratio of 1:1.1. The synthesized CuO-TiO₂ nanocrystalline will be able to show photocatalytic reaction under visible light.     

Combustion synthesis and characterization of porous perovskite catalysts

Porous perovskite-type complex oxides LaCoO₃ and La_0·95Sr_0·05Ni_0·05Co_0·95O₃ were produced by combustion method. The properties of these porous materials such as crystal structures, particle sizes, surface patterns, pore size, surface area and pore volume were characterized by X-ray diffraction( XRD), scanning electron microscopy(SEM) and BET measurements. The results indicated that all porous materials are of the perovskite-type complex oxides. Doping Sr²⁺ ions on site A and doping Ni²⁺ ions on site B entered the crystal lattices of LaCoO₃ in the place of La³⁺ and Co³⁺, respectively, and the maximum peak of XRD patterns of doping sample was weaken and broaden. Morphological microscopy demonstrated agglomerates involved mostly thin smooth flakes and layers perforated by a large number of pores and its lamella decreased with the introduction of Sr²⁺ and Ni²⁺. Hysteresis loop in the N₂ adsorption-desorption isotherm of samples indicated its porous structures and the doping effect on its pore size, surface area and pore volume were improved. The porous catalysts have been tested for methane catalytic combustion and the results showed that these catalysts possessed high catalytic activity.     

Enhanced physical and chemical adsorption of carbon dioxide using bimetallic copper–magnesium oxide/carbon nanocomposite

Mixed Cu and Mg oxides on nitrogen-rich activated carbon (AC) from Nypha fruticans biomass were characterized and their CO₂ adsorption performance was measured. Highly dispersed CuO and MgO nanoparticles on AC was obtained using an ultrasonic-assisted impregnation method. The optimum adsorbent is 5%CuO–25%MgO/AC having good surface properties of high surface area, pores volume and low particles agglomeration. The higher content of MgO of 5%CuO–25%MgO/AC adsorbent contributes to less metal carbide formation which increases their porosity, surface area and surface basicity. XPS analysis showed some amount of nitrogen content on the surface of the adsorbent which increased their surface basicity towards selective CO₂ adsorption. The presence of moisture accelerated the CO₂ chemisorption to form a hydroxyl layer on the surfaces. The 5%CuO–25%MgO/AC adsorbent successfully adsorbed CO₂ via physisorption and chemisorption of 14.8 and 36.2 wt%, respectively. It was significantly higher than fresh AC with better selectivity to CO₂.     

The kinetics of phase transitions in vitreous chalcogenide semiconductors As10.2Se89.8 and As9Se90Bi in early stage of physical ageing process

The kinetics of glass transition in selenide glasses As_10.2Se_89.8 and As₉Se₉₀Bi in early stage of physical ageing process has been investigated by parallel differential scanning calorimetry (DSC) and exoelectron emission (EEE). It has been found that the glass transition process occurring in investigated glasses is evidenced by peaks on EEE intensity and DSC curves. Admixture of bismuth causes a distinct lowering of the temperature of glass transitions process both in the surface layer and in the volume. The addition of Bi causes a decrease in the value of the activation energy for glass transition process in both the volume and in the surface layer, thus reducing the thermal stability of investigated glasses. Physical ageing in Se-rich chalcogenide glasses leads to a significant increase of endothermic peak area A, temperature of glass transition T _g and decrease of the activation energy value E. All these effects are strongly dependent on glass composition.     

Comparison of molecular simulation of adsorption with experiment

Experimental measurements of adsorption yield the surface excess. The Gibbs surface excess is the actual or absolute amount of gas contained in the pores less the amount of gas that would be present in the pores in the absence of gas-solid intermolecular forces. Molecular simulation of adsorption yields the absolute amount adsorbed. Comparison of simulated adsorption isotherms and heats of adsorption with experiment requires a conversion from absolute to excess variables. Molecular simulations of adsorption of methane in slit pores at room temperature show large differences between absolute and excess adsorption. The difference between absolute and excess adsorption may be ignored when the pore volume of the adsorbent is negligible compared to the adsorption second virial coefficient (V?B _1s ).     

Preparation of porous TiO2/silica composites without any surfactants

TiO₂-SiO₂ composites, with high specific surface area (up to 308 m²/g), large pore volume, and narrow distribution with average pore sizes of 3.2 nm, have been synthesized from wollastonite and titanium sulfate in the absence of any surfactants. Calcium sulfate, a microsolubility salt, plays an important role in the formation of pores in this porous TiO₂/silica composite. The microstructure and chemical composition of composite were characterized by X-ray diffractometry (XRD), transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectrometer (XPS) and N₂ adsorption and desorption analysis. The as-prepared porous titanium dioxide-silicon dioxide composites with high specific surface area and well-crystallized anatase contents were used as an efficient photocatalyst.     

Photochemical properties of photosystem 1 immobilized in a mesoporous semiconductor matrix

The pigment-protein complex of photosystem 1 (PS1) isolated from cyanobacterium Synechocystis sp. PCC 6803 has been adsorbed on a solid mesoporous film made from TiO₂ nanoparticles. was on The TiO₂ film supported on a glass substrate with a surface area of 1 cm² adsorbs up to 0.045 nmol of PS1. PS1 molecules are distributed in the pores of the mesoporous support. Immobilization has an insignificant effect on the optical and photochemical properties of PS1. A reversible photoinduced EPR signal from the oxidized primary electron donor P700 of immobilized PS1 has been detected. It has been shown by photoelectrochemical methods that the photoexcitation of PS1 results in electron injection from PS1 to the conduction band of TiO₂.     

The influence of the Si(OC2H5)4/(CH3O)3Si(CH2)3SH ratio on the structure-adsorption characteristics of xerogels formed and accessibility of functional groups in their surface layers

It was shown for the example of the Si(OC₂H₅)₄/(CH₃O)₃Si(CH₂)₃SH system that successively increasing the fraction of tetraethoxysilane in it (from 1: 1 to 5: 1 (mol)) successively decreased the content of 3-mercaptopropyl groups in xerogels synthesized by the sol-gel method (in the presence of methanol as a solvent and fluorine ions as a catalyst) from 5.0 to 1.9 mmol/g, whereas the specific surface area of such xerogels simultaneously increased from 13 to 631 m²/g. The sorption volume of pores also increased, their mean diameter varying insignificantly. The mean diameter of pores (2.2–2.5 nm) was close to the boundary between meso-and micropores, which was in agreement with the form of nitrogen adsorption isotherms (type I according to the IUPAC classification). It was shown by scanning electron microscopy that virtually nonporous xerogels formed at a 1: 1 ratio between alkoxysilanes consisted of spherical partially united particles 2.5–3 μm in diameter. All the 3-mercaptopropyl groups of this and other samples were, however, accessible to silver(I) ions. It follows that these groups are situated in the surface layer of xerogels. The number of thiol groups per 1 nm² of the surface of nonporous xerogels was 1.7–7.0 groups/nm² and depended on the ratio between reacting alkoxysilanes and s _sp.     

Surface characterization and heats of adsorption of chromatographic alumina gel

The porous nature of chromatographic alumina gel has been investigated by adsorption/condensation processes and electron microscopy. Having 63% porosity, the gel is very porous. Total pore volume as determined by the fluid-displacement method is 0.497 cm³ g^–1. Its specific surface area, as determined by water vapor adsorption, is 225 m² g^–1. Micropore volume, as determined by utilizing Gurwitsch's rule, turns out to be 0.262 cm³ g^–1. The greater portion of the surface area and pore volume occurs in small and transitional pores, with average pore radii (hydraulic) less than 2.1 nm.Organic vapors, such as methyl ethyl ketone, acetone, methyl acetate, and methyl alcohol, were adsorbed on the gel between 0 and 36°C under vacuum, and the data were recorded on a Cahn-1000 electrobalance device. Isosteric heats of adsorption were calculated by applying the Clausius Clapeyron equation to the adsorption isosters at different surface coverages. Two types of adsorption processes, one with low activation energy and other with high activation energy can be distinguished. The increase in values ofq _st indicates that increasing temperature changes physical adsorption into chemisorption.     

Comparison of deconvoluted plasma DSC curves on patients with solid tumors

Melting of crystalline compounds inside the nanopores of open-morphology porous systems was studied on a model system, consisted of 1-octadecene and silica gels with different pore sizes, by means of thermogravimetry, differential scanning calorimetry and powder X-ray diffraction. The parameters of silica gels porous structure (surface area, pore size and volume) were calculated using N₂ adsorption data. To describe the experimental results, a new thermodynamic model of crystallites melting inside the nanopores of irregular shape was established. This model allows an analytical prediction for the shift of phase transition temperature and melting enthalpy (latent heat of melting) due to the surface tension effects. To a first approximation, both parameters must linearly depend on the specific ratio of the total surface of pores to their total volume, and experimental studies have mostly confirmed this result for the melting of 1-octadecene confined inside the pores of a wide range of various silicas (with the pores of different sizes and geometry).

     

Controllable Synthesis of Mesoporous Iron Oxide Nanoparticle Assemblies for Chemoselective Catalytic Reduction of Nitroarenes

Iron(III) oxide is a low‐cost material with applications ranging from electronics to magnetism, and catalysis. Recent efforts have targeted new nanostructured forms of Fe₂O₃ with high surface area‐to‐volume ratio and large pore volume. Herein, the synthesis of 3D mesoporous networks consisting of 4–5 nm γ‐Fe₂O₃ nanoparticles by a polymer‐assisted aggregating self‐assembly method is reported. Iron oxide assemblies obtained from the hybrid networks after heat treatment have an open‐pore structure with high surface area (up to 167 m² g^?1) and uniform pores (ca. 6.3 nm). The constituent iron oxide nanocrystals can undergo controllable phase transition from γ‐Fe₂O₃ to α‐Fe₂O₃ and to Fe₃O₄ under different annealing conditions while maintaining the 3D structure and open porosity. These new ensemble structures exhibit high catalytic activity and stability for the selective reduction of aryl and alkyl nitro compounds to the corresponding aryl amines and oximes, even in large‐scale synthesis.