Comparison of the dynamics of n-hexane in ZSM-5 and 5A zeolite structures

The translational and rotational dynamics of n-hexane adsorbed in ZSM-5 and 5A zeolites has been studied by neutron scattering and deuterium solid-state NMR, at various temperatures. The dynamics of n-hexane is quite different in the two zeolites. In the ZSM-5 structure, the molecule sits in channel segments, the energy barrier between adjacent adsorption sites is small and fast anisotropic motions are observed. In the 5A zeolite, the molecule is adsorbed in -cages; the barrier between adjacent cages is larger so that the molecule spends a longer time exploring the volume of an -cage, leading to a more isotropic motion. The diffusion coefficient of the molecule is reduced by more than 4 orders of magnitude in 5A zeolite compared with ZSM-5.Received: 1 January 2003, Published online: 14 October 2003PACS: 82.75.Jn Measurements and modeling of molecule migration in zeolites - 61.12.Ex Neutron scattering techniques (including small-angle scattering) - 76.60.-k Nuclear magnetic resonance and relaxation     

Nanocrystalline zeolite beta and zeolite Y as catalysts in used palm oil cracking for the production of biofuel

Nanocrystalline zeolites with crystal size smaller than 100 nm are potential replacement for conventional zeolite catalysts due to their unique characteristics and advantages. In this study, the synthesis of nanocrystalline zeolite Y (FAU) and nanocrystalline zeolite beta (BEA) under hydrothermal conditions is reported. The effect of crystal size on the physico-chemical characteristics of the zeolite, Y (FAU), and beta (BEA) is reported. The properties of nanocrystalline zeolites Y and Beta with crystal size around 50 nm are compared with the microcrystalline zeolite Y and microcrystalline zeolite beta, respectively. The performance of the nanocrystalline zeolite as a catalyst was investigated in the cracking of used palm oil for the production of biofuel. The nanocrystalline zeolite catalytic activity was compared with the activity of microcrystalline zeolite in order to study the effect of crystal size on the catalytic activity. Both nanocrystalline zeolites gave better performance in terms of conversion of used palm oil as well as selectivity for the formation of gasoline fraction. The increase in surface area and improved accessibility of the reactant in nanocrystalline zeolites enhanced the cracking activity as well as the desired product selectivity.     

Iron ions in ZSM-5 zeolite: Fe3+ in framework, Fe2+ in extra-framework positions in catalytic N2O decomposition

Fe-ZSM-5 samples containing a combination of ⁵⁷Fe³⁺ in framework (FW) and regular iron in extra-framework (EFW) sites were prepared by introducing ⁵⁷Fe in hydrothermal synthesis, then exchanging Fe²⁺ of natural isotope composition into the lattice. The stability for one part of Fe²⁺ and Fe²⁺ ? Fe³⁺ reversibility for the other part in catalytic decomposition of N₂O is demonstrated by in situ Mössbaer measurements. Formation of dinuclear Fe_FW–O–Fe_EFW pairs is not prevailing.     

Morphological effect of lanthanum-based supports on the catalytic performance of Pt catalysts in crotonaldehyde hydrogenation

Rod-like and particle-like La₂O₂CO₃ and La₂O₃ were obtained via morphology-preserved thermal transformation of the La(OH)₃ precursors. La₂O₂CO₃- and La₂O₃-supported Pt catalysts were prepared by impregnation method and tested in the liquid-phase crotonaldehyde hydrogenation reaction. The textural and physicochemical properties of the samples were studied by a series of techniques including XRD, TG-DSC, N₂ adsorption–desorption, TEM and HRTEM, IR spectrum, H₂-TPD, and H₂-TPR. Even after 600 °C reduction, Pt particles of about 0.8–2.8 nm interplayed with support surface to form Pt-doped interface, thereby preventing the catalysts from migration and affording a high dispersion of platinum. The specific exposed crystal-facets and surface oxygen species depending on the shape of the support affected the preferential deposition of Pt species and the metal-support interaction. Thus, Pt catalysts performed different physicochemical properties and catalytic performance relying on the morphology and structure of the supports. During the cycle experiment, severe deactivation was observed for NP-supported catalysts with an increased selectivity due to the aggregation and growth of Pt particles. Meantime, the NR-supported catalysts retained relatively high reactivity as a consequence of the crystal-facet confinement of rod-shaped lanthanum supports.     

Transport properties of modified Nafion membranes: Effect of zeolite and precursors

Fe-silicalite/Nafion composite membranes with high relative selectivity (as defined by the proton conductivity to methanol permeability ratio) of 5.4 and proton conductivity of 11 mS cm^− 1 were prepared by in situ hydrothermal synthesis of the zeolite within the pores of Nafion membranes. The effects of the zeolite structure and precursor structure were evaluated in terms of transport properties and acidity levels for a series of Nafion membranes modified with silica and tetrapropylammonium (TPA) and tetrabutylammonium (TBA) cations. Introduction of up to 40% (w/w) of silica vs. pure Nafion shows little effect on the transport and acidity properties of the composite membranes. Introduction of tetraalkylammonium (TAA) cations reduces water uptake of the membranes, and results in the appearance of protons that are inaccessible for titration in water media. The selectivity of the composite membranes increases in the order: SiO₂/Nafion < TAA/Nafion < Fe-silicalite/Nafion.     

ZSM-5 zeolite synthesis in the presence of a copper-phthalocyanine complex characterization by ESR and Xe NMR

In the presence of a copper phthalocyanine complex (CuPc), it is possible to synthesize ZSM-5 zeolite containing the complex, and the crystallinity is good, provided that the amount of CuPc trapped is lower than the number of channel intersections where it can be located without too much distortion. The crystallinity is retained after decomposition of the CuPc in a stream of air at 500 °C. The copper-ZSM-5 system is characterized by the combined application of xenon adsorption, ESR and ¹²⁹Xe NMR spectroscopy.     

Carbon-supported iron catalysts: Iron-57 Mössbauer spectroscopic studies of phase formation and dispersion properties which influence catalytic performance

The treatment of carbon-supported iron in hydrogen and carbon monoxide gives the formation of iron carbide phases, the nature of which depends on the degree of dispersion of the supported iron phase. The carbon-supported iron catalysts show high activity, low conversion to methane, and good selectivities for the formation of gaseous olefins and liquid hydrocarbons. The catalytic performance is associated with strong interactions between the carbon support and the metallic iron particles as revealed by⁵⁷Fe Mössbauer spectroscopy. The results imply that the carbon may not only function as a support for the dispersion of iron, but that electron movement to the iron/carbon interface influences catalytic performance.     

Investigation of a traditional catalyst by contemporary methods: Parallel electron spectroscopic and catalytic studies on Pt black

Results of electron spectroscopy (XPS and UPS) of platinum black catalyst measured in various states of the catalyst have been summarized. XPS showed up to almost 50% carbon and up to 20% oxygen on a sample stored in air. These, however, had almost no influence on the chemical state of Pt, except for the appearance of minor surface oxide. A Pt purity of ～90% could be reached by regeneration with O₂ and H₂. The C 1s peak contained several components from individual C atoms to graphitic and polymeric hydrocarbon layers. Thus, the active catalyst was not clean Pt but metallic Pt; the impurities exerting little influence on catalytic activity. Regeneration and deactivation led also to slight structural rearrangement, as detected by XRD. Intentional deactivation with hydrocarbon–hydrogen mixtures was monitored by XPS, UPS and catalytic tests. Correlation was found between catalytic activity and selectivity in hexane reaction and the amount – and also the chemical state – of carbon accumulated during deactivating runs. A short summary of electron spectroscopy of supported Pt catalysts is also given. The main underlying idea regards solid catalyst and reactants as a dynamic system, including also solid-state changes of the former.     

Synthesis and photochemical properties of HTaWO6/ (Pt,TiO2) nanocomposite under visible light irradiation

A new layered nanocomposite, HTaWO₆/(Pt, TiO₂) was synthesized using n-type semi-conductor HTaWO₆ as a host material. HTaWO₆ and HTaWO₆/TiO₂were white, while both HTaWO₆/Pt and HTaWO₆/(Pt, TiO₂) were yellow. The yellow color might be attributed to H_1-xTaWO_6-x/2 formed by the photo-induced phase transformation promoted by Pt. Although HTaWO₆/Pt showed absorption in visible light region (λ > 400 nm), the hydrogen evolution activity was negligibly small. On the other hand HTaWO₆/(Pt, TiO₂) showed excellent photocatalytic activities even under visible light irradiation. The sample containing rutile type titania such as TiO₂(P-25) also showed hydrogen evolution activity, but the activity was smaller than that of HTaWO₆/(Pt, TiO₂). These results suggested that rutile type titania, which can be excited by visible light of wavelength less than 413 nm, played an important role in the visible light-induced photocatalytic activity. The improvement of the hydrogen evolution activity of rutile type titania by intercalating into HTaWO₆/Pt may be due to the depression of the recombination of photoinduced electrons and holes by the heterogeneous electron transfer from rutile type TiO₂ to HTaWO₆/Pt.     

Infrared spectra and stability of CO and H2O sorption over Ag-exchanged ZSM-5 zeolite: DFT study

The infrared spectra and stability of CO and H₂O sorption over Ag-exchanged ZSM-5 zeolite were investigated by using density function theory (DFT). The changes of NBO charge show that the electron transfers from CO molecule to the Ag⁺ cation to form an σ-bond, and it accompanies by the back donation of d-electrons from Ag⁺ cation to the CO (π*) orbital as one and two CO molecules are adsorbed on Ag-ZSM-5. The free energy changes ΔG, −5.55 kcal/mol and 6.52 kcal/mol for one and two CO molecules, illustrate that the Ag⁺(CO)₂ complex is unstable at the room temperature. The vibration frequency of C-O stretching of one CO molecule bonded to Ag⁺ ion at 2211 cm⁻¹ is in good agreement with the experimental results. The calculated C-O symmetric and antisymmetric stretching frequencies in the Ag⁺(CO)₂ complex shift to 2231 cm⁻¹ and 2205 cm⁻¹ when the second CO molecule is adsorbed. The calculated C-O stretching frequency in CO-Ag-ZSM-5-H₂O complex shifts to 2199 cm⁻¹, the symmetric and antisymmetric O-H stretching frequencies are 3390 cm⁻¹ and 3869 cm⁻¹, respectively. The Gibbs free energy change (ΔGH₂O) is −6.58 kcal/mol as a H₂O molecule is adsorbed on CO-Ag-ZSM-5 complex at 298 K. The results show that CO-Ag-ZSM-5-H₂O complex is more stable at room temperature.     

Influence of pretreatment methods on adsorption and catalytic characteristics of toluene over heterogeneous palladium based catalysts

The adsorption and catalytic characteristics of heterogeneous palladium based catalyst and its modified catalysts with gases (air and hydrogen) and acidic aqueous solution (HCl) were studied for evaluating the influence of pretreatment methods for toluene. The structural and energetic adsorption properties of the parent and pretreated catalysts were analyzed by means of nitrogen adsorption isotherms and gravimetric methods. The light-off curve and the XPS investigation were used for analyzing the catalytic activity and the surface state of palladium. It was clearly shown from the experimental results that hydrogen pretreated catalysts having metallic surface state exhibited the highest adsorption capacity and catalytic activity compared to that of parent and modified catalysts. The adsorption equilibrium data for toluene were obtained at three different temperatures and correlated successfully with the two-site Langmuir molecular isotherm model (L2m). It was also found that the palladium phase has more adsorption affinity for toluene molecules than the alumina support. The isosteric heat of adsorption calculated by using the Clausius-Clapeyron equation significantly changed with the coverage and the lateral interactions between the adsorbate-adsorbate molecules control the system. Furthermore, comparative analysis of the adsorption energy distribution revealed that the parent and its modified catalysts have different types of surface energetic heterogeneities.     

Water irradiation by high-frequency ultrasonic wave: effects on properties of passive film formed on stainless steel

In this paper an aqueous solution was irradiated with a 1.63MHz ultrasonic wave. It is shown that if stainless steel can passivate under dynamic polarization in this medium, under static polarization, the latter does not show any repassivation behaviour with time. This is attributed to a diminution of the diffusion layer thickness that is developed at the electrode/electrolyte interface, which is associated with a production of H(2) species by sonolysis and which maintains reductive conditions at the interface. The oxide film formed under ultrasonic irradiation for 1h at a passive potential of+0.2V(SCE) shows an early stage of passivation and an increased disordered state, which implies a considerable decrease in the corrosion resistance behaviour of the sample. The polarization resistance of the stainless steel R(p) is divided by a value of 4.5 under ultrasonic conditions.     

Effects and mechanism of ultrasonic irradiation on solidification microstructure and mechanical properties of binary TiAl alloys

In spite of their high temperature and reactivity, the binary TiAl alloys are successfully imposed by the ultrasonic irradiation and the microstructure evolution, solidification behaviors and mechanical properties are elaborately investigated. After ultrasonic irradiation, a high quality ingot without shrinkage defects and element segregation is obtained and the coarse dendrite structure is well modified into fine non-dendrite globular grains. The coarse lamellar colony and lamellar space of Ti44Al alloy is refined from 685 μm to 52 μm and 1185 nm to 312 nm, respectively (similarly, 819 μm to 102 μm and 2085 nm to 565 nm for Ti48Al alloy). For Ti48Al alloy, the α peritectic phase is simultaneously precipitated from the melt as well as the β primary phase before the peritectic reaction and the solidification is transformed into the mixed α-solidifying and β-solidifying. Ultrasonic irradiation promotes the peritectic reaction and phase transformation completely and the phase constituent becomes more close to the equilibrium level. The compressive strength of Ti44Al and Ti48Al alloys are increased from 623 MPa to 1250 MPa and 980 MPa to 1295 MPa, respectively. The grain refinement and dendrite transformation enhance the grain boundary sliding improving the plastic deformation ability. Ultrasonic irradiation significantly accelerates the melt flow and solute redistribution and the main grain refinement mechanism is the cavitation-enhanced nucleation by inclusion activation and heightened supercooling.     

Effects of ultrasonic irradiation on the properties of coatings obtained by electroless plating and electro plating

This paper deals with the effects of ultrasonic irradiation on electroless copper coating i.e. metallic deposition on non-conductive substrates and on electroplating on metallic substrates. Ultrasonic irradiation was both applied during activation (surface preparation for the electroless coating) and during plating steps in both cases. Several parameters were monitored, such as plating rates, practical adhesion, hardness, internal stress versus varying acoustic powers and frequencies. Optimum conditions for irradiation time, frequency and power were determined for each step. It appears clearly that ultrasound use affects deposit properties. Then, changes in the coating mechanisms can be discussed, and several parameters will be explored in this paper, to explain enhancement of deposit properties: increase in catalyst specific area, stirring dependence, surface energy evolution, dihydrogen desorption, structure of coating.     

The influence of ion irradiation on the structure and properties of amorphous carbon films

Presented in this work are the results of investigation of the structure and electrophysical properties of amorphous carbon films. The films were produced by sputtering of graphite by ion beam and usin ion irradiation (E=0–200 eV) during condensation process. The structure of i-C films has been studied by means of transmission electron microscope. The electron diffraction data have been interpretated by employing the calculated interference function of carbon clusters. The structure of V-band was obtained from AES by deconvolution method. Experimental data shows that under ion irradiation the transformation of short range order and electron bonds is an oscillating function of ion energy E. This paper presents a theoretical calculation of tunneling neutralization cross-section of Ar⁺ ions on carbon surface. The process also has an oscillating dependence on ion energy. A significant importance of inelastic processes in carbon phase transformation has been revealed.     

Improvement in properties of coal water slurry by combined use of new additive and ultrasonic irradiation

Coal water slurry (CWS) was prepared with a newly developed additive from naphthalene oil. The effects of ultrasonic irradiation on coal particle size distribution (PSD), adsorption behavior of additive in coal particles and the characteristics of CWS were investigated. Results showed that ultrasonic irradiation led to a higher proportion of fine coal in CWS and increased the saturated adsorption amount of additive in coal particles. In addition, the rheological behavior and static stability of CWS irradiated by ultrasonic wave were remarkably improved. The changes on viscosity of CWS containing 1% and 2% additive are qualitatively different with the increasing sonication time studied. The reason for the different effect of sonication time on CWS viscosity is presented in this study.     

Effect of Ag promoter on redox properties and catalytic performance of Ag-Mo-P-O catalysts for oxidative dehydrogenation of propane

The influence of addition of sliver ions on the redox properties and catalytic performance of molybdenum-phosphate catalyst in oxidative dehydrogenation (ODH) of propane has been investigated. MoO₃ and AgMoO₂PO₄ have been detected in Ag-doped Mo-P-O catalysts. The redox properties have been characterized by H₂-temperature-programmed reduction (TPR), electronic paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). The catalytic performance of MoO₃, Ag₂O, AgMoO₂PO₄ and Ag-doped Mo-P-O catalysts has also been examined. The incorporation of Ag improved the number of Mo⁵⁺ species and the reducibility of the catalysts. The Ag-doped catalysts favor activity and selectivity in ODH of propane higher than the undoped catalyst. The best catalyst was obtained with Ag/Mo ratio of 0.3 and MoO₃/AgMoO₂PO₄ ratio of 1.1. The influence of promoter Ag⁺ on redox properties and catalytic performance is due to forming redox couple “Ag⁰+Mo⁶⁺Ag⁺+Mo⁵⁺” and the synergetic effect originating from “coherent interface” between MoO₃ and AgMoO₂PO₄.     

Direct methanol fuel cells: The influence of methanol on the reduction of oxygen over Pt/C catalysts with different particle sizes

Two Pt/C catalysts with different particle sizes (Pt/C: 2.5 nm, Pt/C-700Ar: 5.1 nm) were investigated by applying a half-cell configuration —rotating disk electrode (RDE) technique in H₂SO₄ aqueous solutions in the absence of or in the presence of methanol with different concentrations. Pt/C catalyst exhibited higher mass activity in H₂SO₄ aqueous solution without methanol and slightly lower mass activity in H₂SO₄ plus 0.1 mol/L CH₃OH in comparison with that of Pt/C-700Ar catalyst. On the contrary,single direct methanol fuel cell (DMFC) tests showed that Pt/C exhibited higheroxygen reduction reaction (ORR) activity and better cell performance, mainly due to the different kinds of electrolyte properties. Furthermore, it suggested that a better single DMFC performance could be obtained with a smaller particle size Pt-based cathode catalyst. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 — 18, 2004.     

UV-CVD deposited SiNH films on InP: Optimization of the physico-chemical and interface properties

High quality silicon nitride films are deposited at low temperature on InP substrates by direct photolysis at 185 nm of a NH₃-SiH₄ gas mixture. The composition of the films is measured by nuclear analysis. The thickness and refractive index are obtained by ellipsometry at 632.8 nm. As-deposited and post annealed samples are electrically characterized: quasi-static I(V) at 5×10^–4 Hz and C(V) characteristics at 1 MHz are performed on InP MIS diodes structures in order to optimize bulk and interface properties. At 250° C and 4 Torr, it is found that the highest critical field (measured for a leakage current density of 10^–9 A/cm²) is obtained for the injected ratio [SiH₄]/[NH₃]=2%. For these conditions, the film is stoichiometric, the critical field is 4 MV/cm and the resistivity is 6×10¹⁵ cm. The interface state density (N _ss) on InP is deduced from Terman analysis. The annealing conditions and the surface cleaning of InP have been optimized in order to reduce the N _ss which is, for our best conditions, as low as 2×10¹¹ eV^–1 cm^–2.     

The influence of ligands on the preparation and optical properties of water-soluble CdTe quantum dots

In a systematic investigation has been found that ligands play an important role in both the water-phase preparation and optical properties of CdTe quantum dots. Experiments were performed using three typical thioalkyl acids as ligands, namely mercaptoacetic acid (MAA), l-cysteine (Cys) and reduced glutathione (GSH). The growth rate and size-distribution of CdTe quantum dots (QDs) are shown to depend on the type of ligands. A proper choice of ligand enables to make lager nanocrystals with narrower size-distribution. The effects of pH (buffer solution), illumination, heating and cations on the spectroscopic properties of CdTe QDs for the three ligands are reported. In addition, three same-size CdTe QDs were individually characterized by micellar electrokinetic capillary electrophoresis with laser-induced fluorescent detection, which proved their monodisperse size-distribution and different electric charge distribution on the surface for each of the three different type of QDs.