Adsorption of NO and N2O on Fe-BEA and H-BEA zeolites

FT-IR (Fourier-transform infrared) spectroscopy and density function theory (DFT) methods have been applied to the investigation of the interaction of NO and N₂O with Fe³⁺ species in a beta zeolite (BEA). The geometries for H-BEA and Fe-BEA represented as 10T cluster, and NO and N₂O adsorption on them in η¹-O and η¹-N modes have been completely optimized. The results show that NOx could be adsorbed on Fe³⁺ species and Brønsted acid sites in two modes, but NOx is mainly bonded by N to H or Fe atom and the iron site is preferred. NOx adsorbed on Fe³⁺ species is more stable than on Brønsted acid sites. Adsorption energies for N₂O and NO follow the order of NO > N₂O, predicating that the affinity of NO molecule on BEA zeolite is much stronger than N₂O molecule on BEA zeolite.     

Mössbauer spectroscopy in the characterisation of new materials

The use of Mössbauer spectroscopy for the characterisation of new materials is illustrated here by examples of its application in the study of different types of solids. Firstly, the incorporation of large metal ions within the channels of the compound NbTiP₃O₁₂ which has a three-dimensional channel-type structure similar to that of the fast ion conductor called nasicon and the examination of the products by⁵⁷Fe and¹¹⁹Sn Mössbauer spectroscopy and X-ray powder diffraction is discussed. Secondly, the investigation by¹¹⁹Sn Mössbauer spectroscopy and X-ray photoelectron spectroscopy of new conducting tin dioxide pillared clays synthesised by microwave heating is described. Finally, the examination by¹²⁵Te Mössbauer spectroscopy and X-ray powder diffraction of conducting selenium-tellurium alloys prepared by precipitation techniques is discussed.     

Dehydration of isopropyl alcohol used as an indicator of the type and strength of catalyst acid centres

The subject of our studies was determination of the kind and strength of the catalyst acid centres on isopropyl alcohol conversion. The investigations were carried out for two groups of catalysts: typical Lewis acids γ-Al₂O₃ and ZrO₂ and Brønsted acids: H₃PW₁₂O₄₀ and H₃PMo₁₂O₄₀. Considerable differences between Lewis and Brønsted acids in the conversion of isopropyl alcohol were observed. The influence of Brønsted acid centres was studied for a group of catalysts with different strength: mixtures of two different heteropolyacids. It was observed that the increasing strength of Brønsted acids centres leads to higher catalytic activity but it does not significantly affect the activation energy of dehydration to propene.     

Enhancement of catalytic activity in NH3-SCR reaction by promoting dispersibility of CuCe/TiO2-ZrO2 with ultrasonic treatment

A series of CuCe-modified TiO₂-ZrO₂ catalysts synthesized by stepwise impregnation method and ultrasonic-assisted impregnation method were investigated to research the removal of NO in the simulated flue gas. Results showed that the CuCe/TiO₂-ZrO₂ catalyst prepared by ultrasonic-assisted impregnation method exhibited the superior NO conversion, in which higher than 85% NO was degraded at the temperature range of 250–400 °C and the highest NO conversion of 94% at 350 °C. It proves that ultrasonic treatment can markedly improve the performance of catalysts. The effect of ultrasonic enhancement on CuCe/TiO₂-ZrO₂ was comprehensively studied through being characterized by physicochemical characterization. Results reveal that the ultrasonic cavitation effect improves the distribution of active species and the synergistic interaction between Cu with Ce components (Cu⁺ + Ce⁴⁺ ↔ Cu²⁺ + Ce³⁺) on the catalysts significantly, thus resulting in better dispersibility as well as a higher ratio of Cu²⁺ and Ce³⁺ of the catalysts. Moreover, it was found that the CuCe/TiO₂-ZrO₂ catalyst prepared by the ultrasonic-assisted impregnation method represented a higher degree of ultrafine metal particles and evenness. The above results were described with the generalized dimension and singularity spectra in multifractal analysis and validated by the comparative test. Therefore, it can be concluded that ultrasonic treatment facilitates the particle size and distribution of active sites on the catalysts.     

Adsorption of ammonia on vanadium-antimony mixed oxides

We analyzed the adsorption of ammonia (NH₃) on the VSbO₄(1 1 0) catalyst surface using density functional theory (DFT) calculations. We followed the evolution of the chemical bonds between different atoms of the resulting NH₃/VSbO₄ system and the changes in the electronic structure of the catalyst. NH₃ preferential adsorption geometries were analyzed through the crystal orbital overlap population (COOP) concept and the density of states (DOS) curves. The VSbO₄(1 1 0) surface exhibits Lewis and Brønsted acid sites on which the ammonia molecule can interact. On the Lewis acid site, NH₃ adsorption resulted in the interaction between the N and a surface V-isolated cation. On Brønsted acid site, N interacted with a surface H coming from the chemical dissociation of water. The COOP analysis indicate that NH₃ interaction on the VSbO₄(1 1 0) surface is weak. In addition, the DOS curves show more developed electronic interactions for NH₃ adsorption on Lewis acid site than over Brønsted acid site.     

Radiative lifetimes in Ce I and Ce II

New radiative lifetime measurements based on time-resolved laser-induced fluorescence techniques are reported for 18 even-parity levels belonging to the 4f5d^26p and 4f ²5d ² configurations of Ce I and 6 even-parity levels belonging to the 5d^26s, 4f5d6p, and 4f6s6p configurations of Ce II. Free neutral and singly ionized cerium atoms were produced by laser ablation. The Ce I and Ce II levels range in energy from 26 545 to 29 102 cm^-1, and 42 573 to 48 152 cm^-1, respectively. Received 25 September 2002 Published online 4 March 2003     

Effect of crystal field on the spin density at the copper nucleus in copper complexes

The role of lower symmetry component of the crystal field in causing a mixing of excited 3d ^x−1 4s with the ground 3d ^x configuration and leading to spin density at the nucleus for iron group ions was suggested by Griffith and Orgel. This mechanism has been examined in detail for the two low-symmetry copper complexes, one square planar (D_4h symmetry) and the other distorted tetrahedron (D_2d symmetry) and the calculation has been performed using the powerful Racah method and tensor operator technique. It is found that for the two types of copper complexes, copper pthalocyanin (square planar, D_4h symmetry) and cesium copper chloride (distorted tetrahedron, D_2d symmetry) the contribution from this mechanism to the spin density at the nucleus vanishes identically.     

Microstructure,dielectric, and piezoelectric properties of Ce‐modified Na0.5Bi4.5Ti4O15 high temperature piezoceramics

The cerium modified sodium bismuth titanate (Na_0.5Bi_4.5Ti₄O₁₅, NBT) piezoelectric ceramics have been prepared by using the conventional mixed oxide method. X‐ray diffraction analysis revealed that the cerium modified NBT ceramics have a pure four‐layer Aurivillius phase structure. The piezoelectric activity of NBT ceramics was found significantly improved by the modification of cerium. The Curie temperature T_c, and piezoelectric coefficient d₃₃ for the NBT ceramics with 0.50 wt% cerium modification were found to be 655 °C, and 28 pC/N respectively. The Curie temperature gradually decreased from 668 °C to 653 °C with the increase of cerium modification. The dielectric spectroscopy showed that the samples possess stable piezoelectric properties, demonstrating practical potential that for high temperature applications. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The acid-base properties of the surface of native zinc oxide layers: An XPS study of adsorption of 1,2-diaminoethane

The acid-base properties of native zinc oxide surfaces have been studied using X-ray photoelectron spectroscopy (XPS). The native layers of zinc oxide have been obtained by ageing mechanically polished pure zinc disks in a glass dryer for 1 month. Such a treatment lead to the formation of an unstable oxide layer and dehydroxylation has been observed during storage in vacuum. By following adsorption in ultrahigh vacuum of 1,2-diaminoethane (DAE) several types of active sites have been evidenced. Zinc cations react with the probe molecule following a Lewis acid/base interaction, while the hydroxyl and the carbonate-like species react following a Brønsted acid/base reaction. Although initial interaction via the Brønsted-like mechanisms is favoured, it has been shown that the resulting complexes are not stable. Under vacuum conditions, the adsorbed DAE molecules either partly desorb or modify their interaction mode with the surface to form additional Lewis-like bonded stable complexes. In addition, a cleaning effect of the molecule has been observed which lead to partial removal of the carbonate-like contamination.     

Polymer/clay nanocomposites based on MMT/ODA intercalates

Natural Na-montmorillonite (MMT) with the formula (Na_0.67K_0.01Ca_0.02)(Al_2.90Mg_0.59 Fe³⁺ _0.49Ti_0.01)(Si_7.92Al_0.08)O₂₀(OH)₄was intercalated with octadecylamine (C₁₈H₃₉N) (ODA) with varied MMT/ODA ratios. The basal spacings d(00l) of natural and intercalated Na-MMT with ODA were determined by XRD analysis. The set of basal diffractions: d(001) = 56 Å, d(002) = 28 Å, d(003) = 18 Å, d(004) = 14 Å, d(005) = 11 Å for the saturated Na-MMT/ODA intercalate indicates existence of highly ordered periodic structure with the ODA double-layer intercalated in the silicate interlayer spacing. Molecular modeling calculations were applied to the Na-MMT/ODA saturated intercalation system. Stable structure with d-spacing 53 Å has been calculated which is in a good agreement with the experimental data. Such structure gives good conditions for the organosilicate layered structure exfoliation and nanocomposite production. Na-MMT/ODA intercalates were compounded with polypropylene. In accordance with theoretical predictions, XRD analysis applied to compounded structures have proved that a high degree of the organoclay exfoliation was achieved.     

Large orders in the 1/N perturbation theory by inverse scattering in one dimension

When one tries to compute large orders in the 1/N series à la Lipatov a complicated non-linear equation for the instanton is found in ø⁴ or non-linear sigma models.We solve here this equation in the one-dimensional case (quantum mechanics) by inverse scattering techniques. From the instanton solutions we obtain theK ^th order of the 1/N perturbation theory up to 0(K ^–1) for the 0(N) symmetric anharmonic oscillator and up to a factor 0(K ⁰) for a non-symmetric model. In the symmetric case we agree with results recently obtained in quantum mechanics by Hikami and Brézin following a different procedure. For the non-symmetric anharmonic oscillator we believe our formulae are new.     

Characterization of multilayer systems by high-resolution x-ray diffraction

The x-ray diffraction pattern of MBE-grown Ga_1–x Al _x As/GaAs superlattices has been measured withd-spacing resolutiond/d approaching 17000, corresponding to about 8500 Å^–1. The resolution was achieved by employing a highly dispersive monochromator consisting of a pair of fivefold reflecting grooved silicon crystals which delivers about 10⁴ photons s^–1 to the sample. Detailed information like the presence of buffer layers, the molar fractionx, the numberp of layers, the superlattice periodt _p , and the component layer thicknessest ₁ ,t ₂ are extracted from the measured pattern by comparing it with the pattern calculated from the dynamical theory for layer structures. In addition, the influence of disorder on the determination of the above superlattice parameters is investigated by including two different models of disorder.     

Tin-119 Mössbauer spectroscopic studies of novel tin oxide pillared clays

¹¹⁹Sn Mössbauer spectroscopy has shown that attempts to intercalate three aryltin compounds (Ph₃SnCl, (Ph₃Sn)₂O, Ph₂SnCl₂) into the synthetic smectite clay laponite under ambient conditions result in the formation of tin(IV) oxide pillared clays. The Mössbauer data indicate that the effectiveness of conversion to tin(IV) oxide pillars is in the order Ph₃SnCl> (Ph₃Sn)₂O>Ph₂SnCl₂. The organic product of the pillaring reaction has been identified by¹³C m.a.s. n.m.r. spectroscopy as benzene trapped within the pillared clay. The pillaring in these new materials is achieved via neutral precursors rather than by sacrificial reaction of the exchanged cation. Measurements by A.C. conductivity measurements (12 Hz–100 kHz) show significant precursor related increases in conductivity for the new pillared materials as compared with sodium-exchanged laponite.     

Characterization of nanoporous carbon fibrous materials obtained by chemical activation of plane tree seed under ultrasonic irradiation

An ultrasonic irradiation was applied for the impregnation by chemical agents in the chemical activation process of new type of active carbon precursor. Plane tree seed, due to the unique fibrous structure and low cost is a promising eco-friendly raw material for the preparation of activated carbon materials. Ultrasonic irradiation was used for the impregnation step allowing the chemical activation by different agents: potassium or sodium hydroxide, hydrogen peroxide and pyrogallol. The porous structures were examined by nitrogen adsorption/desorption isotherms at 77 K and electrochemically by cyclic voltammetry. The textures of these materials were observed by scanning electron microscopy. The application of ultrasonic irradiation in the impregnation step increased surface area of the final material more than two times in comparison to the material which impregnation in the activation process was by conventional stirring. Ultrasonic irradiation enhances the chemical activation process and the activated carbon fibrous materials with nanoporous structure were obtained by impregnation of seeds with alkaline hydroxides. Total surface areas of these samples were 976 m² g⁻¹ and 1130 m² g⁻¹. These fibers have total specific capacitance as high as 125 F g⁻¹ and 53 F g⁻¹ which major fraction in both cases originate from internal micropores structure.     

Cationic versus Anionic Pillared Layered Substrates: Comparison of the Pillaring Reactions and the Resulting Porosities

Cationic clays and Layered Double Hydroxides (LDHs) are both layeredion exchangers, in which a stable (micro)porosity can be induced via apillaring process. For the cationic clays, the [Zr]-pillaring of hectoritecreates a broad micropore distribution with the maximum at 1.42–2.12nm. The [Al]-equivalent exhibits a narrower distribution, with pores between0.71 and 1.06 nm being dominant. In case of the [Zr]-pillared form a surfacearea of 294 m²/g and a micropore volume of 0.118cm³/g have been obtained. The same reaction on the syntheticlaponite clay reveals a much higher surface area (606 m²/g)and porosity (µPV = 0.336 cm³/g). Forlaponite, extra pores are created in the supermicropore-small mesoporeregion due to the preferential edge-to-face and edge-to-edge stacking of itssmaller sized clay layers.For the pillaring of MgAl- and ZnAl-LDHs with polyoxometalates (POMs),using large organic anions for pre-swelling purposes forms the mostpromising method for the creation of stable pores. It avoids the formationof sidephases, and gives rise to medium(-high) µPVs. Charge density onthe layers forms the key factor, lowering it improves the porositycharacteristics significantly. [Fe(CN)₆]-MgAl-LDHs exhibitmore spectacular properties, with surface areas and µPVs exceedingthose of pillared hectorite. A variation in the charge density via theM ^II/M ^III ratio optimizes theporosity properties. A M ^II/M ^IIIratio of 3.33 results in a SA of 499 m²/g and a µPV of0.177 cc/g. For LDHs, both types of pillars create mainly small micropores,with a diameter smaller than 0.71 nm.     

Characterization and catalytic performance of pure and Li2O-doped CuO/CeO2 catalysts

Pure and Li₂O-doped CuO/CeO₂ catalysts calcined at 500 °C were prepared by impregnation method. The catalysts are characterized by DTA, TG-DTG, XRD, IR, TEM, nitrogen adsorption at −196 °C and the catalytic decomposition of hydrogen peroxide at 30 °C.The effects of molar ratio, heat treatment time and the doping on the structural, surface and catalytic properties of nanocrystalline Cu/Ce-mixed oxides system have been studied. It was found that the catalytic activity of ceria-supported copper oxide catalysts increased by increasing both the heat treatment time and dopant content. However, the pure Cu/Ce-mixed oxide solids containing 10 wt.% CuO exhibited the best performance. The characterization results indicated that the higher surface area, the formation of solid solution between copper and cerium oxides, and the high dispersion of copper species on the ceria were responsible for the high catalytic activity of the CuO/CeO₂ catalysts.     

Electron impact single ionization of copper

Electron impact single ionization cross sections of copper have been calculated in the binary encounter approximation using accurate expression for σ _ΔE as given by Vriens and Hartree-Fock momentum distribution for the target electron. The BEA calculation based on the usual procedure does not show satisfactory agreement with experiment in this case but a striking modification is found to be successful in explaining the experimental observations. The discrepancy is linked with the ionization of the 3d ¹⁰ electrons and probably effective single ionization does not take place from 3d shell of copper leading to smaller values of experimental cross sections.     

On the strength of electron correlations in high-T c superconducting copper oxides

We analyze the strength of electronic correlations in the half-filled antibonding Cu–O orbitals of high-T _c superconducting copper oxides by considering a Cu₁₂O ₁₇ ^n– cluster withn=8 or 10, respectively. The correlated ground state is calculated by the method of the local approach (LA) in a version which allows the treatment of stronger correlated electrons. As mean-field basis a semiempirical Hamiltonian of the ZDO (zero differential overlap) type has been adopted. It is found that the correlations are particularly strong in the Cu 3d _x ²–y² orbitals. The nonintegral orbital occupation allows for valence fluctuations between Cu⁺ and Cu²⁺ in spite of the remarkable correlations. According to the present model excess holes are located at the oxygen sites. The theoretical findings are compared with the results of spectroscopic investigations. The present electronic-structure analysis allows for a straighforward rationalization of previous experimental measurements. The strong connection between the importance of electronic correlations and the symmetry properties of the electronic wave function is emphasized.     

Phase mixing in the critical region of the ferroelectric phase transition in KD2PO4

A novel neutron high resolution double crystal arrangement has been used to measure changes in d-spacing, d-spread and mosaicity through the first order ferroelectric phase transition of DKDP. Coexistence of both para and ferroelectric phases was directly observed in the range T_c to T_c — 3 K. No hysteresis could be detected within the precision of the temperature measurement (10^-3K). The origin of the overshoot effect has been elucidated.     

Ultrasonic-assisted preparation and characterization of hierarchical porous carbon derived from garlic peel for high-performance supercapacitors

Ultrasonic-assisted impregnation is used to synthesize physically modified garlic peel-based 3D hierarchical porous carbons (PCs), and the effect on PCs is investigated by changing ultrasonic time. The results show that ultrasonic waves can effectively peel off surface attachments of the carbonized product, so that activator has a better mass transfer process and create more active sites. The connectivity of 3D pore network is enhanced as well, so the structure and electrochemical properties of garlic peel-based porous carbon (GBPC) are improved. The ultrasonic disperser is used as an ultrasonic generator, specific conditions are as follows: ultrasonic frequency is 40 kHz, ultrasonic power is 500 W, and ultrasonic time is 0, 3, 6, and 9 min, respectively. With the increase of ultrasonic time, impurities again block the pore structure during dynamic movement, resulting in a decrease in electrochemical performance. Specifically, the performance of GBPC-6 is the most excellent, the specific surface area (SSA) increases from 2548 m² g⁻¹ to 3887 m² g⁻¹, the specific capacitance increases from 304 F g⁻¹ to 426 F g⁻¹ at a current density of 1 A g⁻¹ in a two-electrode test system. Energy density and cycle performance are also improved at the same time, which are attributed to rational structure. In addition, the effectiveness of the strategy of ultrasonic-assisted synthesis has been confirmed on another precursor material–scallion, meaning that this work proposes a new and simple modification method for improving the performance of biomass-based PCs.