Synthesis of ZSM-22/SAPO-11 composite molecular sieve-based catalysts with small crystallites and superior n-alkane hydroisomerization performance

To improve oil quality,ZSM-22/SAPO-11 composite molecular sieves were synthesized by adding ZSM-22 into a synthetic gel of SAPO-11 for n-decane hydroisomerization.The mass ratios of ZSM-22/(ZSM-22+SAPO-11)in the composite molecular sieves were optimized and the optimal ZSM-22/SAPO-11 composite(ZS-9)was obtained.The electrostatic repulsions between the ZSM-22 precursors and the SAPO-11 crystalline nuclei produced small ZSM-22 and SAPO-11 crystallites in ZS-9,which increased the specific surface area and mesopore volume and thereby exposed more acid sites.In comparison with conventional SAPO-11,ZSM-22 and their mechanical mixture,ZS-9 with smaller crystallites and the optimal medium and strong Brønsted acid centers(MSBAC)content displayed a higher yield of branched C_(10) isomers(81.6%),lower cracking selectivity(11.9%)and excellent stability.The correlation between the i-C_(10) selectivity and the MSBAC density of molecular sieves indicated that the selectivity for branched C_(10) isomers first increased and then decreased with increasing MSBAC density on the molecular sieves,and the maximum selectivity(87.7%)occurred with a density of 9.6×10^(−2)μmol m^(−2).

Surfactant-aided hydrothermal preparation of La(2-x)Sr_xCuO_4 single crystallites and their catalytic performance on methane combustion

Perovskite-like oxide La2-xSrxCuO4 （x = 0, 1） single crystallites with microrod-like morphologies and tetragonal crystal structures were prepared hydrothermally at 240 ℃ with poly（ethylene glycol） （PEG） or hexadecyltrimethyl ammonium bromide （CTAB） as a surfactant and after calcination at 850 ℃. The physicochemical properties of the materials were characterized by means of XRD, BET, SEM, TEM/SAED （selected-area electron diffraction）, XPS and H2-TPR techniques. It is found that doping Sr2＋ to La2CuO4 lattice enhanced the catalytic activity for methane combustion and the LaSrCuO4 catalyst derived from PEG is the best among the tested ones. It is concluded that factors, such as adsorbed oxygen species concentration, reducibility and surface area, determined the catalytic performance of such single-crystalline materials.

Pulling out a peptide chain from β-sheet crystallite: Propagation of instability of H-bonds under shear force

Anti-parallel β-sheet crystallite as the main component of silk fibroin has attracted much attention due to its superior mechanical properties. In this study, we examine the processes of pulling a peptide chain from β-sheet crystallite using steered molecular dynamics simulations to investigate the rupture behavior of the crystallite. We show that the failure of β-sheet crystallite was accompanied by a propagation of instability of hydrogen-bonds （H-bonds） in the crystallite. In addition, we find that there is an optimum size of the crystallite at which the H-bonds can work cooperatively to achieve the highest shear strength. In addition, we find that the stiffness of loading device and the loading rates have significant effects on the rupture behavior of β-sheet crystallite. The stiff loading device facilitates the rebinding of the Hbond network in the stick-slip motion between the chains, while the soft one suppresses it. Moreover, the rupture force of β-sheet crystallites decreases with loading rate. Particularly, when the loading rate decreases to a critical value, the rupture force of the β-sheet crystallite becomes independent of the loading rates. This study provides atomistic details of rupture behaviors of β-sheet crystallite, and, therefore, sheds valuable light on the underlying mechanism of the superior mechanical properties of silk fibroin.

Tuning the crystallite size of monoclinic ZrO_(2) to reveal critical roles of surface defects on m–ZrO_(2) catalyst for direct synthesis of isobutene from syngas

The effects of crystallite size on the physicochemical properties and surface defects of pure monoclinic ZrO_(2) catalysts for isobutene synthesis were studied.We prepared a series of monoclinic ZrO_(2) catalysts with different crystallite size by changing calcination temperature and evaluated their catalytic performance for isobutene synthesis from syngas.ZrO_(2) with small crystalline size showed higher CO conversion and isobutene selectivity,while samples with large crystalline size preferred to form dimethyl ether(DME)instead of hydrocarbons,much less to isobutene.Oxygen defects(ODefects)analyzed by X-ray photoelectron spectroscopy(XPS)provided evidence that more ODefectsoccupied on the surface of ZrO_(2) catalysts with smaller crystalline size.Electron paramagnetic resonance(EPR)and ultraviolet–visible diffuse reflectance(UV–vis DRS)confirmed the presence of high concentration of surface defects and Zr3+on mZrO_(2)-5.9 sample,respectively.In situ diffuse reflectance infrared Fourier transform spectroscopy(in situ DRIFTS)analysis indicated that the adsorption strength of formed formate species on catalyst reduced as the crystalline size decreased.These results suggested that surface defects were responsible for CO activation and further influenced the adsorption strength of surface species,and thus the products distribution changed.This study provides an in-depth insight for active sites regulation of ZrO_(2) catalyst in CO hydrogenation reaction.

Effect of Mechanical Treatment Temperature on Electrical Properties and Crystallite Size of PVDF Film

Fabrication of PVDF films has been making using Hot Roll Press. Preparation of samples carried out for nine different temperatures. This condition is carried out to see the effect of temperature fabrication on electrical properties and crystallite size of PVDF films. The electrical properties like as surface resistivity are discussion focus in this paper. Surface resistivity properties of PVDF can be improved by mechanical treatment on the varying film thickness and the temperature. To obtain the diffraction pattern of sample characterization is performed using X-Ray Diffraction. Crystallite size of PVDF films calculate from broadening pattern of X-Ray Diffraction. Furthermore, from the diffraction pattern calculated β fraction and crystallite size, for calculation to determine the crystallite size of the sample by using the Scherrer equation. Has been obtained an increase piezoelectric properties of PVDF films that characterized by increasing β fraction. Have been obtained β fraction increased from 25.4% up to 44% for temperatures of 130°C up to 170°C, respectively. Resistivity value has been obtained at temperature 130°C up to 170°C, decreased from 1.23 × 104 Wm up to 0.21 × 104 Wm respectively. From the experimental results and the calculation of crystallite sizes obtained for the samples with temperature 130°C up to 170°C respectively are increased from 7.2 nm up to 20.54 nm. These results indicate that mechanical treatment caused increase β fraction and decrease surface resistivity. Increasing temperatures will also increase the size of the crystallite of the sample. This happens because with the increasing temperature causes the higher the degree of crystallization of PVDF film sample is formed, so that the crystallite size also increases.

Tuning surface active oxygen of lanthanum oxide via partial dissolution and regrowth of crystallites

A commercial lanthanum oxide was modified into X%-La2O3 samples via partial dissolution and regrowth of crystallites, wherein the X value denotes the weight percent of the initial metal oxide powder dissolved by nitric acid. BET measurements and TEM observations indicate that the 20%-La2O3 sample has an increased specific surface area, relatively thin and small crystallites in size. TPD/TPR profiles also show that the 20%-La2O3 sample has more surface active oxygen than the 0%-La2O3 sample. Among the four X%-La2O3 samples, the highest activity for methane oxidation was obtained over the 20%-La2O3 sample. Therefore, the present work proves that the method of partial dissolution and regrowth of crystallites are effective to modify surface property of lanthanum oxide.

Femtosecond Time-Resolved Exciton Recombination Dynamics in ZnO Microcrystallite Thin Films at Room Temperature

We report the femtosecond time-resolved studies of room temperature exciton recombination and ultrafast stimulated emission dynamics in ZnO microcrystallite thin films.A free exciton photoluminescence lifetime of a few tens of picoseconds and a decay time of a few picoseconds for the stimulated emission were observed.The relatively slow rise time(3ps)for the P band as the result of exciton-exciton scattering compared with the 0.8ps rise time for the N band attributed to electron-hole plasma recombination clearly distinguished the two stimulated emission processes.

Thermodynamic Equilibrium Studies of Nanocrystallite CeO_(2) Grain Boundaries by High Temperature X-Ray Photoelectron Spectroscopy and Thermal Gravimetric Analysis

Nanostructured CeO_(2) thin films and powders are studied by high temperature x-ray photoelectron spectroscopy and thermal gravimetric analysis.The results indicate that the surface composition strongly depends on temperature,the surface O/Ce ratio initially increases with increasing temperature,then decreases with the further increase of temperature,the maximum surface O/Ce ratio is at about 300℃.The variation of the surface composition with temperature arises from the ion migration,redistribution and transformation between lattice oxygen and gas phase oxygen near the grain boundaries during the thermodynamic equilibrium process.The results also show that CeO_(2) has a weakly bond oxygen,high oxygen mobility in the bulk and a high molecular dissociation rate at the surface,especially for the sol-gel prepared nanocrystallite CeO_(2).

Size Effects on the Crystallite Structure and Phase Transition in Lanthanaum-Modified Lead Titanate Nanocrystals

The size effects on the crystallite structure,Raman spectra as well as phase tian-sition in lanthanum-modified lead titanate Pb_(1-x)La_(x)Ti_(1-3/4x)O_(3)(x=0.14)(PLT14)nanocrystalline powders were investigated.The results show that with decreasing grain size,the crystallite structure changes from tetragonal perovskite phase to cubic perovskite phase;the soft mode frequency at room temperature decreases,and disappears when the grain size is as small as 19nm;the phase transition temperature from ferroelectric to paraelectric shifts towards a low temperature region.The phase transition in nanocrystalline PLT14 is a second order one,which is confirmed by its dielectric constant measurement.

Femtosecond Degenerate Four-Wave-Mixing in ZnO Microcrystallite Thin Films

Transient third-order optical nonlinearityχ^(3)of ZnO microcrystallite thin films is measured at various temperatures by using femtosecond degenerate four-wave-mixing.Room-temperature excitonic enhancement of is observed.The magnitude ofχ^(3)ranges between 10^(-4)to 10^(-6)esu from 4.2K to room temperature.The measuredχ^(3)response time ranging from 200 to 300 fs ultrafast for temperature down to 4.2K.

Temperature Characteristics Studies of Nanocrystallite SrTiO_(3) Grain Boundaries by X-Ray Photoelectron Spectroscopy

Strontium titanate(SrTiO_(3))nanocrystallite thin films have been studied by x-ray photoelectron spectroscopy at different temperature.The surface oxygen concentration and Ti/Sr ratio increase with temperature rising up to 700K,and then decrease.New O1s and Ti 3d peaks appear at the low binding energy side of the corresponding main peaks,these new peaks have some different features from the corresponding main peaks,this phenomenon was not observed in single crystal SrTiO_(3).A model is proposed to explain this interesting phenomenon.

Dislocations in the Crystallites of Nanocrystalline α-Fe—A Molecular Dynamics Study

Molecular dynamics simulations are carried out in order to Study the atomic structure of crystalline component of nanocrystalline α-Fe when it is consolidated from small grains. A two-dimensional computational block is used to simulate the consolidation process. All the preset dislocations in the original grains glide out of them in the consolidation process, but new dislocations can generate when the grain size is large enough. It shows that dislocations exist in the consolidated material rather than in the original grains. Whether dislocations exist in the crystalline component of the resultant model nano-material depends upon grain size. The critical value of grain size for dislocation generation appears to be about 9 nm. This result agrees with experiments qualitatively.

Effect of Nitriding Current Density on the Surface Properties and Crystallite Size of Pulsed Plasma-Nitrided AISI 316L

In this work, plasma-nitrided AISI 316L stainless steel samples were performed by ion nitriding process under pulsed direct current (DC) discharge at different current densities (1 to 2.5 mA/ cm2). The effect of nitriding current density on the size of crystalline coherently diffracting domains (crystallite size) and strain grade was investigated using X-ray diffraction (XRD) coupled with Williamson-Hall method. Additionally, hardness and wear resistance of the nitriding layer were characterized using a Vickers indenter and pin-on-disk technique respectively. Results showed a decrease in crystallite size from 99 nm for untreated samples to 1.4 nm for samples nitrided at 2.5 mA/cm2 promoted both: an increase in hardness from 226 HV25g to 1245 HV25g, and a considerably decrease in volume loss by wear effect.

Effect of TiO2 Crystallite Diameter on Photocatalytic Water Splitting Rate

The effect of (Pt-loaded)TiO2 crystallite diameter (i.e. Scherrer size) on the photocatalytic water splitting rate was investigated. (Pt-loaded)TiO2 powders with a wide range of crystallite diameters from about 16 to 45 nm with a blank region between about 23 and 41 nm were prepared by various annealing processes from an identical TiO2 powder. Water splitting experiments with these powders were carried out with methanol as an oxidizing sacrificial agent. It was found that the photocatalytic water splitting rate was sensitively affected by the crystallite diameter of the (Pt-loaded)TiO2 powder. More concretely, similar steep improvements of photocatalytic water splitting rates from around 15 and a little over 2 to about 30 μmol·m-2hr-1?were obtained in the two (Pt-loaded)TiO2 crystallite diameters ranging from 16 to 23 and from 41 to 45 nm, respectively.

Studies on the Effects of Crystallite Sizes and Scattering Layers on the Conversion Efficiency of Dye-Sensitized Solar Cell

We have carried out the characterization of dye-sensitized solar cell (DSSC) using various techniques including current-voltage characteristic measurements, scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-visible spectrophotometry, energy dispersive X-ray (EDX) spectroscopy and X-ray Diffraction (XRD), etc. The result we obtained from our work has clearly shown that dye adsorption capability of the DSSC is affected by the film thickness. The EDX spectroscopy verified that our samples are generally of good quality without significant inclusion of foreign elements. The XRD pattern demonstrated that all the TiO2 active layers are consisting of anatase phase. Our results also showed that smaller crystallite sizes of the TiO2 and inclusion of a scattering layer produce better efficiency for the DSSC.

Synthesis of ZSM-5 Zeolite with Small Crystallite and its Application in Aromatiration of Propane

By optimizing the orystallization condition, a procedure for the synthesis of small crystallite zeolite ZSM-5 was developed. Compared with the larger crystallite ZSM-5. the smaller one exhibits higher catalytic performance in aromatization reaction of propane.

Sublayer design and ablation resistance of CVD-TaC alternate coatings with different crystallite morphologies for C/C composites

Tantalum carbon(TaC)alternate coatings with sublayers comprised of different crystallite morphologies were prepared on carbon/carbon composites by chemical vapor deposition.Their ablative behaviors and defending mechanisms were both investigated.The specimen with the sublayer composed of columnar crystals exhibited a better ablation resistance due to the toughness enhancement induced by the lami-nated structure.However,the mechanical denudation of the sample only containing acicular crystals and the coating spallation caused by superfluous gaseous products of the sample with the sublayer composed of nanocrystals both indicate their inferior anti-ablation properties.It is believed that the results will be helpful for the structural design and practical application of chemical vapor deposition(CVD)alternate coatings.

Growth kinetics of MgH2 nanocrystallites prepared by ball milling

MgH2 is one of promising hydrogen storage materials due to its high hydrogen capacity of 7.6 wt%.However,MgH2 nanocrystallites easilygrow up during hydrogen absorption-desorption cycling,leading to deterioration of hydrogen storage properties.To clarify the growth kinetics of MgH2 crystallites,the growth characteristics of MgH2 nanocrystallites are investigated in this work.The growth exponents of MgH2 nanocrystallites in pure MgH2 and MgH2-10 wt% Pr3 Al11 samples are evaluated to be n=5 and n=6,respectively.Meanwhile,their activation energies for crystallite growth are also determined to be109.2 and 144.2 kJ/mol,respectively.The increase of growth exponent and rise of activation energy for crystallite growth in MgH2-10 wt% Pr3 Al11 composite are ascribed to the presence of nano-sized Pr3 Al11phase.