Vapor Phase Transport Research Articles

Vapor-phase transport (VPT) modification of ZSM-5/SiC foam catalyst using TPAOH vapor to improve the methanol-to-propylene (MTP) reaction

Tetrapropylammonium hydroxide (TPAOH) was introduced in the vapor phase to perform the vapor-phase transport (VPT) modification of the structured ZSM-5 supported on SiC foam (ZSM-5/SiC foam) catalyst. An optimum precursor concentration of 0.5M TPAOH could effectively convert the amorphous aluminosilicate binder to the zeolitic phase with improved intracrystal mesopores, nanosized crystals (ca. 100nm), high concentration of acidity sites (83mmolg−1) as well as a high value of the relative acidity (0.7). Combined with the intrinsic property of macroscopic SiC foams such as the low pressure drop and the high thermal conductivity (14Wm−1K−1 at 773K), TPAOH VPT modified ZSM-5/SiC foam catalyst demonstrated an excellent activity in the catalytic methanol-to-propylene (MTP) reaction, surpassing the state-of-the-art hierarchal ZSM-5 monolith catalyst. The catalyst showed an extended activity for ca. 970h (>95% methanol conversion) with the high selectivity to the propylene (>45%). The coke formation was significantly retarded (ca. 2.1×10−2wt.%h−1) due to the enhanced transport phenomena within the developed structured catalyst.

Grain-growth engineering and mechanical properties of physical-vapour-deposited InSe platelets

The present work demonstrates a novel use of physical vapour deposition for grain-growth engineering by optimizing supersaturation, which led to the evolution of stoichiometric indium monoselenide crystals, employing a custom-fabricated dual-zone furnace. The growth zone was kept at a constant temperature for different experimental runs (673–883 K), while the source zone was kept at a stable temperature of 1123 K. In this way, the temperature difference ΔT= 240–450 K resulted in a significant increase of the mass transport between the zones so as to accomplish bulk crystallization. At comparatively low supersaturation (ΔT= 240 K), the presence of nodules and flakes was observed. When ΔT= 250 K, multiple grains were formed owing to temperature asymmetry at the rough vapour–solid interface. A further increase in supersaturation (ΔT= 330 K) facilitated polyhedral grain growth, with distinct grain boundaries. A subsequent increment in ΔT(400 K) led to evolution of the polycrystalline morphology to well developed hexagonal platelets owing to adsorption of atoms on surface steps and kinks in accordance with the leading-edge growth mechanism. Energy-dispersive analysis by X-rays and X-ray diffraction experiments were carried out to confirm the structure and phase of crystals. Microindentation studies were done to assess the hardness and mechanical stability of the as-grown crystals in response to external loads in order to explore their suitability for solar cell applications. The investigations of bulk vapour phase transport, morphology and strengthening of InSe platelets provide pathways for the production of crystalline textures with versatile properties.

Lasing in cuprous iodide microwires

We report on the observation of lasing in cuprous iodide (CuI) microwires. A vapor-phase transport growth procedure was used to synthesize CuI microwires with low defect concentration. The crystal structure of single microwires was determined to be of zincblende-type. The high optical quality of single microwires is indicated by the observed series of excitonic emission lines as well as by the formation of gain under optical excitation. Lasing of triangular whispering-gallery modes in single microwires is demonstrated for fs- and ns-excitation from cryogenic temperatures up to 200 K. Time-resolved micro-photoluminescence studies reveal the dynamics of the laser process on the time scale of several picoseconds.

Optical Field Confinement Enhanced Single ZnO Microrod UV Photodetector**Supported by the National Natural Science Foundation of China under Grant Nos 61475035 and 61275054, the Science and Technology Support Program of Jiangsu Province under Grant No BE2016177, and the Collaborative Innovation Center of Suzhou Nano Science and Technology.

ZnO microrods are synthesized using the vapor phase transport method, and the magnetron sputtering is used to decorate the Al nanoparticles (NPs) on a single ZnO microrod. The micro-PL and I–V responses are measured before and after the decoration of Al NPs. The FDTD stimulation is also carried out to demonstrate the optical field distribution around the decoration of Al NPs on the surface of a ZnO microrod. Due to an implementation of Al NPs, the ZnO microrod exhibits an improved photoresponse behavior. In addition, Al NPs induced localized surface plasmons (LSPs) as well as improved optical field confinement can be ascribed to an enhancement of ultraviolet (UV) response. This research provides a method for improving the responsivity of photodetectors.

Regenerative trapping: How Pd improves the durability of Pt diesel oxidation catalysts

Pt is an active component in diesel oxidation catalysts (DOCs) but it ripens to form large particles at elevated temperatures due to significant vapor phase transport of PtO2. Adding Pd improves the durability of the catalyst but there is no clear consensus in the literature for the responsible mechanism. In this study, we examined the role of PtO2 vapor pressure on sintering. The vapor pressure of PtO2 was determined by measuring the emission of Pt using planar model catalysts and electron probe microanalysis (EPMA). Significant emission of Pt to the vapor phase was observed in a Pt/SiO2 model catalyst that was aged in flowing air at 800°C. Adding Pd lowered the rate of emission, and so did decreasing the oxygen concentration. Lowering of the vapor pressure of PtO2 led to a decreased rate of sintering. However, the most significant decrease in sintering rate was observed in the presence of excess PdO. The excess PdO serves to trap mobile Pt species, resulting in significantly smaller Pt-Pd particles after aging in air at 800°C for 10h. When PtO2 is emitted from a metallic Pt-Pd particle it can result in the formation of a separate PdO phase. Hence, in these catalysts the PdO is regenerated in the course of catalyst sintering. We propose that regenerative trapping of mobile Pt species by PdO plays an important role in improving the durability of Pt-Pd bimetallic catalysts.

Direct and fast growth of highly single crystalline α-MoO3 nanosheets on various substrates and its electrochemical activities

We describe the direct and fast growth process of molybdenum trioxide (α-MoO3) nanosheets on a Si wafer, the carbon papers and the vertically aligned carbon nanotubes (CNTs) by a simple vapor phase transport using the MoO3 source at 700°C. We have carefully explored the electrochemical performances of α-MoO3/CNTs hetero-nanostructures, resulting in enhancing relevant electrochemical responses as an effective electrode material for supercapacitor.

Two-photon absorption induced photoluminescence from ZnS nanowires

In this work, the ZnS nanowires was synthesized on Au-coated Si substrate by a vapor phase transport (VPT) method. The morphology and structure of the as-synthesized ZnS nanowires were studied through scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). Under excitation of the femtosecond (fs) pulsed laser at 650 nm, a strong ultraviolet (UV) emission centered at 343 nm was observed. The peak position and the relationship between the emission intensity and the excitation intensity demonstrated that the UV photoluminescence (PL) be induced by the two-photon absorption (2PA) process. The characteristics and the generation process of the above nonlinear optical phenomena were investigated in detail.

Effect of Si Growth Temperature on Fabrication of Si-ZnO Coaxial Nanorod Heterostructure on (100) Si Substrate

The realization and application of optoelectronics, photonics, and sensing, such as in solar diode sensors and photodiodes, which are potentially useful from ultraviolet to infrared light sensing, is dramatically advanced when ZnO is integrated into semiconductor nanostructures, especially when compatible with mature silicon technology. Here, we compare and analyze the fundamental features of the Si-ZnO coaxial nanorod heterostructures (Si@ZnO NRs) grown on semi-insulating (100)-oriented Si substrates at growing temperatures of 500°C, 600°C, 650°C, and 700°C of the Si layer for device applications. ZnO NRs were grown by a vapor phase transport, and Si layers were made by rapid thermal chemical vapor deposition. X-ray diffraction, field emission scanning electron microscopy (FESEM), energy-dispersive x-ray spectroscopy, and Raman experiments showed that ZnO NRs were single crystals with a wurtzite structure, while the Si layer was polysilicon with a zincblende structure. Furthermore, FESEM revealed that Si shell thickness of the Si@ZnO NRs increases with increasing growing temperatures of Si from 500°C to 700°C.

Vapor-Phase CMAS-Induced Degradation of Adhesion of Thermal Barrier Coatings

An investigation of the interface degradation and failure of EB-PVD thermal barrier coatings (TBCs) during burner rig cyclic oxidation in the presence of calcium–magnesium–alumino-silicate (CMAS) deposits has been conducted. The formation of two compounds of major CMAS constituents, specifically MgAl2O4 (spinel) and CaAl2Si2O8 (anorthite), was observed at the interface between the alumina scale and the TBC. Spallation occurred as a result of cracking and delamination primarily through the spinel layer. The tests showed more than a tenfold decrease in TBC life relative to the oxidation test without CMAS at the same temperature. It is demonstrated that this particular form of coating degradation occurs below the melting temperature of this particular CMAS composition and hence does not involve TBC infiltration by molten silicates. The likely mechanism of interface degradation, which involves vapor-phase transport of CMAS constituents as hydroxides, is proposed and discussed.

Crystal growth of Dirac semimetal ZrSiS with high magnetoresistance and mobility

High quality single crystal ZrSiS as a theoretically predicted Dirac semimetal has been grown successfully using a vapor phase transport method. The single crystals of tetragonal structure are easy to cleave into perfect square-shaped pieces due to the van der Waals bonding between the sulfur atoms of the quintuple layers. Physical property measurement results including resistivity, Hall coefficient (RH), and specific heat are reported. The transport and thermodynamic properties suggest a Fermi liquid behavior with two Fermi pockets at low temperatures. At T = 3 K and magnetic field of Hǁc up to 9 Tesla, large magneto-resistance up to 8500% and 7200% for Iǁ(100) and Iǁ(110) were found. Shubnikov de Haas (SdH) oscillations were identified from the resistivity data, revealing the existence of two Fermi pockets at the Fermi level via the fast Fourier transform (FFT) analysis. The Hall coefficient (RH) showed hole-dominated carriers with a high mobility of 3.05 × 104 cm2V−1s−1 at 3 K. ZrSiS has been confirmed to be a Dirac semimetal by the Dirac cone mapping near the X-point via angle resolved photoemission spectroscopy (ARPES) with a Dirac nodal line near the Fermi level identified using scanning tunneling spectroscopy (STS).

A facile growth process of highly single crystalline Ir1−xVxO2 mixed metal oxide nanorods and their electrochemical properties

We introduce a facile growth process for mixed metal oxide nanorods of Ir1−xVxO2 in the broad range of metal contents between iridium and vanadium via a vapor phase transport process and their electrochemical performance. The growth process of Ir1−xVxO2 nanorods was performed by modulating the ratios of two precursors of VO2 and IrO2, the growth temperature, the position of metal oxide precursors in a furnace, and the flow rate of carrier gases. The results indicated that quasi one-dimensional nanostructures of Ir1−xVxO2 can be obtained of a single crystalline nature with the tetragonal rutile crystal structure by randomly incorporating Ir4+ and V4+ metal ions into the same lattice unit of the tetragonal structure owing to similar ionic sizes. In addition, the fundamental electrochemical performances for Ir1−xVxO2 nanorods grown directly on an Au microelectrode of 25 μm in diameter were carefully tested in order to explore the nonenzymatic amperometric sensing capacity of ascorbic acid as a biologically important molecule.

Investigation of the Mode Structures of Multiphoton Induced Ultraviolet Laser in a ZnO Microrod

Hexagonal wurtzite structural ZnO microrods were fabricated by vapor-phase transport method. Under the excitation of a pulse laser with 1200 nm wavelength, the multiphoton induced ultraviolet (UV) laser was observed in a microrod. The dependence of the laser mode structures on pump intensity was investigated. The result indicates that the laser belongs to whispering gallery mode (WGM) at low pump intensity and Fabry-Perot (FP) mode at high pump intensity. The corresponding positive feedback mechanisms were discussed.

Vapor-phase transport synthesis of microfibrous-structured SS-fiber@ZSM-5 catalyst with improved selectivity and stability for methanol-to-propylene

Microfibrous-structured SS-fiber@HZSM-5 catalyst prepared by cost-effective and high-efficiency VPT method delivers remarkable improvement in selectivity and stability for the MTP reaction due to the improved diffusion in zeolite shell.

Growth of 18O isotopically enriched ZnO nanorods by two novel VPT methods

We have developed two novel vapour phase transport methods to grow ZnO nanorod arrays isotopically enriched with 18O. Firstly, a three-step process used to grow natural and Zn-enriched ZnO nanorods has been further modified, by replacing the atmospheric O2 with enriched 18O2, in order to grow 18O-enriched ZnO nanorods using this vapour-solid method on chemical bath deposited buffer layers. In addition, 18O-enriched ZnO nanorods were successfully grown using 18O isotopically enriched ZnO source powders in a vapour-liquid-solid growth method. Scanning electron microscopy studies confirmed the success of both growth methods in terms of nanorod morphology, although in the case of the vapour-liquid-solid samples, the nanorods’ c-axes were not vertically aligned due to the use of a non-epitaxial substrate. Raman and PL studies indicated clearly that O-enrichment was successful in both cases, although the results indicate that the enrichment is at a lower level in our samples compared to previous reports with the same nominal enrichment levels. The results of our studies also allow us to comment on both levels of enrichment achieved and on novel effects of the high temperature growth environment on the nanorod growth, as well as suggesting possible mechanisms for such effects. Very narrow photoluminescence line widths, far narrower than those reported previously in the literature for isotopically enriched bulk ZnO, are seen in both the vapour-solid and vapour-liquid-solid nanorod samples demonstrating their excellent optical quality and their potential for use in detailed optical studies of defects and impurities using low temperature photoluminescence.

Non-isothermal two-phase transport in a polymer electrolyte membrane fuel cell with crack-free microporous layers

Liquid water breakthrough events were observed in crack-free microporous layers (MPLs) of operating polymer electrolyte membrane (PEM) fuel cells through in situ synchrotron X-ray radiography. The measured water thicknesses were used as inputs into a one-dimensional (1D) heat and mass transport model. This 1D model was developed to describe the coupled relationship between liquid and vapour transport through the cathode GDL and the temperature distributions in the fuel cell that accompany the in operando measurements. The current density was increased from 1.4 to 2.4Acm−2, during which MPL breakthrough was observed. Immediately following MPL breakthrough events, the fraction of vapour-phase transport of the total water flux in the MPL increased by up to 5%. Post MPL breakthrough, further increases in current density resulted in increased thermal and water vapour concentration gradients in the MPL, and vapour transport was further enhanced. A temperature gradient at the cathode catalyst layer (CL)–MPL interface resulted in decreased threshold capillary pressures, and as a result higher liquid water saturations were observed near this interface. These findings suggest that the temperature gradient should be considered in two-phase flow modeling and pore network modeling due to its impact on liquid water distributions.

Enhanced detection of nitrogen dioxide via combined heating and pulsed UV operation of indium oxide nano-octahedra.

We report on the use of combined heating and pulsed UV light activation of indium oxide gas sensors for enhancing their performance in the detection of nitrogen dioxide in air. Indium oxide nano-octahedra were synthesized at high temperature (900 °C) via vapour-phase transport and screen-printed onto alumina transducers that comprised interdigitated electrodes and a heating resistor. Compared to the standard, constant temperature operation of the sensor, mild heating (e.g., 100 °C) together with pulsed UV light irradiation employing a commercially available, 325 nm UV diode (square, 1 min period, 15 mA drive current signal), results in an up to 80-fold enhancement in sensitivity to nitrogen dioxide. Furthermore, this combined operation method allows for making savings in power consumption that range from 35% to over 80%. These results are achieved by exploiting the dynamics of sensor response under pulsed UV light, which convey important information for the quantitative analysis of nitrogen dioxide.

MEASUREMENT OF CRITICAL HEAT FLUX IN A CANDU END SHIELD CONSISTING OF A VERTICAL SURFACE ABUTTING A PACKED BED OF STEEL SHIELDING BALLS

The critical heat flux (CHF) was measured in an experimental apparatus representative of a Canada Deuterium Uranium (CANDU) reactor end-shield porous medium heated by a vertical surface. The understanding of peak heat transfer rates in this geometry is crucial to the demonstration of in-vessel retention of corium during a postulated severe accident in a CANDU reactor. In this paper, a large-scale experimental apparatus is described, which is capable of measuring CHF from a vertical surface to a packed bed of steel balls representative of a CANDU end shield. Data are presented from a large matrix of tests investigating the impact of heated surface height on CHF. Preliminary analysis of a subset of these data is also presented as well as the derivation of an empirical correlation for CHF as a function of elevation along the vertical wall. Qualitative comparisons are made between the results of the tests and predictions of peripherally related work carried out in the past, with reasonable agreement being observed in vapour-phase transport in the porous medium and in trends of CHF. It is expected that future work including further analysis of the current data will allow for the development of an improved mechanistic understanding of the phenomenon, leading to a more broadly applicable correlation for CHF that is subject to smaller uncertainties and could be incorporated into severe accident codes such as MAAP-CANDU.

Synthesis and Photoluminescence Studies of ZnO Films Using Vapor Phase Transport Combined With Rapid Heating

The vapor phase transport (VPT) process is usually used for the growth of one-dimensional nanostructures rather than for film deposition. In this study, for the first time, we report on the fabrication and optical propeties of ZnO film produced by VPT combined with rapid heating. The X-ray diffraction results showed that the intensity of the ZnO (002) peak was the largest, which indicated that the c-axis orientation of the ZnO grains was perpendicular to the substrate. In the photoluminescence (PL) spectra of the ZnO film at 12 K, PL peaks were observed at 3.374, 3.362, 3.319, and 3.242 eV, which were attributed to free excitons (FXs), excitons bound to neutral donors, donor-acceptor pairs (DAPs), and first-order longitudinal optical phonon replicas of DAPs (DAPs-1LO), respectively. The energy interval between DAPs and DAPs-1LO was almost the same as the longitudinal optical (LO) phonon energy of bulk ZnO of ~76 meV. The PL spectra in the temperature range from 12 to 300 K shifted to lower energy with an increasing temperature. The activation energy of FX was estimated to be 61 meV. This value is similar to the exciton binding energy of 60 meV. (Received September 4, 2015; Accepted December 11, 2015)

Dry‐gel conversion synthesis of SAPO‐11 molecular sieves and their use in hydroisodewaxing of hydrocracking recycle oil

AbstractSilicoaluminophosphate (SAPO‐11) molecular sieves were synthesized by the dry‐gel conversion [i.e. steam‐assisted conversion and vapor‐phase transport (VPT)] method using di‐n‐propylamine as a structure‐directing agent. The influence of the bottom water content, the crystallization time, the drying time and the pH value was investigated. The properties of the as‐synthesized samples were characterized by X‐ray powder diffraction, scanning electron microscopy, N2‐adsorption and NH3‐temperature‐programmed desorption. The results show that the crystallization process of SAPO‐11 was closely related to the pH value and the drying time of the gel; the amount of water in the wet gel plays an important role for SAPO‐11 formation. It can be observed that when the crystal time is within 48 h, and the pH value of the gel is between 0 and 5, the longer the crystal time and the higher the pH value, the higher the crystallinity of the samples. The presence of NH4+ leads to the formation of an amorphous phase in the samples synthesized by the VPT method. Of these two methods, the steam‐assisted conversion method exhibits better Si incorporation into the AlPO4 framework than the VPT method. The results from hydroisodewaxing of hydrocracking recycle oil show that the Pt/SAPO‐11 catalysts synthesized by these methods possess high activity and selectivity of isomerization. The pour point of the lubricating base oils can be lowered to below −15 from 42 °C. Copyright © 2016 Curtin University of Technology and John Wiley & Sons, Ltd.

Hydroxylation of phenol over MeAPO molecular sieves synthesized by vapor phase transport

In this study, MeAPO-25 (Me = Fe, Cu, Mn) molecular sieves were first synthesized by a vapor phase transport method using tetramethyl guanidine as the template and applied to hydroxylation of phenol. The zeolites were characterized by XRD, SEM, FT-IR, and DR UV–Vis. As a result, MeAPO-21 and MeAPO-15 were synthesized by changing the Me/Al ratio. UV–Visible diffuse reflectance study suggested incorporation of heteroatoms into the framework and FT-IR study also supported these data. Effects of heteroatoms, contents of Me in MeAPO-25, reaction temperature, phenol/H2O2 mole ratios, reaction time and concentration of catalyst on the conversion of phenol, as well as on the selectivity were studied. FeAPO-25 exhibited a high catalytic activity at the mole ratio of FeO and Al2O3 equal to 0.1 in the synthesis gel, giving the phenol conversion of 88.75% and diphenols selectivity of 66.23% at 60°C within 3 h [n(phenol)/n(H2O2) = 0.75, m(FeAPO-25)/m(phenol) = 7.5%]. Experimental results indicated that the FeAPO-25 molecular sieve was a fairly promising candidate for the application in hydroxylation of phenol.