Time filter

Source Type

Almelo, Netherlands

Hong J.-I.,Georgia Institute of Technology | Choi J.,Georgia Institute of Technology | Jang S.S.,Georgia Institute of Technology | Gu J.,California State University, Long Beach | And 4 more authors.
Nano Letters | Year: 2012

It is known that bulk ZnO is a nonmagnetic material. However, the electronic band structure of ZnO is severely distorted when the ZnO is in the shape of a very thin plate with its dimension along the c-axis reduced to a few nanometers while keeping the bulk scale sizes in the other two dimensions. We found that the chemically synthesized ZnO nanoplates exhibit magnetism even at room temperature. First-principles calculations show a growing asymmetry in the spin distribution within the distorted bands formed from Zn (3d) and O (2p) orbitals with the reduction of thickness of the ZnO nanoplates, which is suggested to be responsible for the observed magnetism. In contrast, reducing the dimension along the a- or b-axes of a ZnO crystal does not yield any magnetism for ZnO nanowires that grow along c-axis, suggesting that the internal electric field produced by the large {0001} polar surfaces of the nanoplates may be responsible for the distorted electronic band structures of thin ZnO nanoplates. © 2012 American Chemical Society.

Cui M.,Dalian University of Technology | Zhang Y.,Dalian University of Technology | Liu X.,Dalian University of Technology | Wang L.,PANalytical | Meng C.,Dalian University of Technology
Microporous and Mesoporous Materials | Year: 2014

Changes of medium-range structure during the crystallization of zeolite omega from magadiite were characterized. It is found that although the long-range order of magadiite is collapsed in the initial stage, parts of 5-member rings and 6-member rings are still preserved as secondary building units. The fraction of 5-member rings and 6-member rings increases as the crystallization progresses. The 4-member ring chains are formed at a stage later than that of 5-member rings and 6-member rings. © 2014 Elsevier Inc. All rights reserved.

Vermeulen A.C.,PANalytical
Materials Science Forum | Year: 2014

The alignment of a 1/4-circle Eulerian cradle is discussed. The method is based on diffraction and uses a special alignment tool and a stress-free powder sample. A new profile shape function is introduced to describe better distorted diffraction peaks. © (2014) Trans Tech Publications, Switzerland.

SAS method for preparing georgeite and zincian georgeite. Copper(ii) acetate monohydrate (4 mg ml−1) and zinc(ii) acetate dihydrate (2.16 mg ml−1) (Sigma Aldrich ≥ 99% Puriss) were dissolved in ethanol (reagent grade, Fischer Scientific) containing 0 vol%, 5 vol% or 10 vol% deionized water. Smithsonite ZnCO was prepared with zinc(ii) acetate dehydrate (2.16 mg ml−1) in a 10 vol% water and ethanol solution. SAS experiments were performed using apparatus manufactured by Separex. CO (from BOC) was pumped through the system (held at 110 bar, 40 °C) via the outer part of a coaxial nozzle at a rate of 6 kg h−1. The metal salt solution was concurrently pumped through the inner nozzle, using an Agilent HPLC pump at a rate of 6.4 ml min−1. The resulting precipitate was recovered on a stainless steel frit, while the CO –solvent mixture passed down stream, where the pressure was decreased to separate the solvent and CO . The precipitation vessel has an internal volume of 1 litre. Precipitation was carried out for 120 minutes, and followed by a purge of the system with ethanol–CO for 15 minutes, then CO for 60 minutes under 110 bar and 40 °C. The system was then depressurized and the dry powder collected. Recovered samples were placed in a vacuum oven at 40 °C for 4 hours to remove any residual solvent. Approximately 1.5 g of georgeite is prepared during the 120-minute duration of solution precipitation. Co-precipitation method for preparing malachite and zincian malachite precursors for the standard methanol-synthesis catalysts. The procedure was performed via a semi-continuous process, using two peristaltic pumps to maintain pH. Copper(ii) nitrate hydrate and zinc(ii) nitrate hydrate solutions in deionized water were prepared with copper/zomc molar ratios of 1/0, 1/1 and 2/1. The premixed metal solution (5 l, ~0.5 g ml−1) was preheated along with a separate 5 l solution of 1.5 M sodium carbonate. The mixed metals were precipitated by combining the two solutions concurrently at 65 °C, with the pH being held between 6.5 and 6.8. The precipitate would spill over from the small precipitation pot into a stirred ageing vessel, held at 65 °C. The precipitate was aged for 15 minutes after all precursor solutions had been used. The precipitate was than filtered and washed to minimize sodium content. The sample was washed with 6 l of hot deionized water, and the sodium content monitored using a photometer. The washing process was repeated until the sodium content showed no change. The sample was then dried at 110 °C for 16 hours. Samples were calcined for 6 hours at 300 °C or 450 °C in a tube furnace under static air. The ramp rate used to reach the desired temperature was 1 °C min−1. Catalyst testing was performed with 0.5 g of the calcined catalyst, pelleted and ground to a sieve fraction of 0.6–1 mm for both the methanol-synthesis and the LTS test reactions. The catalysts were reduced in situ using a 2% H /N gas mixture at 225 °C (ramp rate 1 °C min–1), before the reaction gases were introduced. Data reported for the industrial standards are the mean value from four repeat experiments. Error bars are based on the standard deviation from the mean values. Methanol synthesis. Testing was carried out in a single-stream, six-fixed-bed reactor with an additional bypass line. After reduction, the catalysts were then subjected to synthesized syngas (CO/CO /H /N  = 6.9/2/67/17.8) at 3.5 l h−1, 25 bar pressure and 195 °C. In-line gas analysis was performed using an FT-IR spectrometer, which detected CO, CO , H O and CH OH. Downstream of the catalyst beds, knockout pots collected effluent produced from the reaction. The contents were collected after each test run and analysed using gas chromatography to evaluate the selectivity of catalysts. The total system flow was maintained using the bypass line. LTS. Testing was performed in six parallel fixed-bed reactors with a single stream feed and an additional bypass line. After reduction, the catalysts were subjected to synthetic syngas (CO/CO /H /N  = 1/4/13.75/6.25) at 27.5 bar pressure and 220 °C. The reactant gas stream was passed though vaporized water to give a water composition of 50 vol%. This gives a total gas flow of H O/CO/CO /H /N  = 50/2/8/27.5/12.5. The standard mass hourly space velocity (MHSV) used for testing was 75,000 k h−1 kg−1. In-line IR analysis was carried out to measure CO conversion. Selectivity was determined by the methanol content within the knockout pots downstream of the reactors. Space and mass velocity variation tests were performed at 65 hours’ time-on-line by altering the flow for each catalyst bed. Relative activities were calculated by altering the flow for each catalyst bed in order to achieve 75% CO conversion after 75 hours’ time-on-line. The total system flow was maintained using the bypass line. Powder X-ray diffraction (XRD). Measurements were performed using a PANalytical X’pert Pro diffractometer with a Ni-filtered CuK radiation source operating at 40 kV and 40 mA. Patterns were recorded over the range of 10–80° 2θ using a step size of 0.016°. All patterns were matched using the International Centre for Diffraction Data (ICDD) database. An in situ Anton Parr XRK900 cell (with an internal volume of ~0.5 l) was used to monitor the formation of metallic copper during the reduction of the CuO/ZnO materials. A flow of 2% H /N (60 ml min-1) was passed through the sample bed while the cell was heated to 225 °C (ambient temperature to 125 °C, ramp rate10 °C min−1; 125–225 °C, ramp rate 2 °C min−1). The sample was then cooled to room temperature and a 20–100° 2θ scan performed. Copper crystallite size was estimated from a peak-broadening analysis of the XRD pattern using Topas Academic30, and the volume-weighted column height (L ) was calculated according to ref. 31. X-ray absorption fine-edge spectroscopy (XAFS). K-edge XAFS studies of copper and zinc were carried out on the B18 beamline at the Diamond Light Source, Didcot, UK. Measurements were performed using a quick extended (QE) XAFS set-up with a fast-scanning silicon (111) double-crystal monochromator. The time resolution of the spectra reported herein was 1 minute per spectrum (k  = 14), and on average three scans were acquired to improve the signal-to-noise ratio of the data. All ex situ samples were diluted with cellulose and pressed into pellets to optimize the effective-edge step of the XAFS data and measured in transmission mode using ion-chamber detectors. All transmission XAFS spectra were acquired concurrently with the appropriate reference foil (copper or zinc) placed between I and I XAS data processing, and EXAFS analysis were performed using IFEFFIT32 with the Horae package33 (Athena and Artemis). The FT data were phase-corrected for oxygen. The amplitude-reduction factor, S2 , was derived from EXAFS data analysis of a known copper reference compound, namely tenorite (CuO). For the fitting of the local coordination geometry of georgeite and malachite, the Jahn–Teller distorted copper–oxygen bond was difficult to observe because of thermal disorder, and so was fixed in the model. X-ray pair distribution function (PDF) analysis. Synchrotron X-ray PDF data were collected on the 11-ID-B beamline at the Advanced Photon Source, Argonne National Laboratory, and on the I15 beamline at the Diamond Light Source. Powder samples were packed into kapton capillaries with an internal diameter of 1 mm. Room-temperature powder X-ray diffraction data were collected at a wavelength of 0.2114 Å (11-ID-B) and 0.1620 Å (I15) using the Rapid Acquisition PDF method34. The scattering data (0.5 ≤ Q ≤ 22 Å−1) were processed into PDF data using the program GudrunX35. Two notations of PDF data are presented in this manuscript: G(r) and D(r) (ref. 36). The total radial distribution function, G(r), is the probability of finding a pair of atoms, weighted by the scattering power of the pair, at a given distance, r; it is the Fourier transform experimentally determined total structure factor. D(r) is re-weighted to emphasize features at high r, such that D(r) = 4πrρ G(r), where ρ is the average number density of the material (in atoms Å−1). Fourier transform-infrared spectroscopy (FT-IR). Analysis was carried out using a Jasco FT/IR 660 Plus spectrometer in transmission mode over a range of 400–4,000 cm–1. Catalysts were supported in a pressed KBr disk. Raman spectroscopy. Raman spectra were obtained using a Renishaw inVia spectrometer equipped with an argon ion laser (λ = 514 nm). Thermal gravimetric analysis (TGA). TGA measurements were performed using a SETARAM Labsys analyser with sample masses of about 20 mg, at 1 °C min−1 under air with a flow rate of 20 ml min−1. Evolved gas analysis (EGA). EGA experiments were performed using a Hiden CATLAB under the same conditions used in the TGA experiments. Copper surface area analysis. This was determined by reactive frontal chromatography. Catalysts were crushed and sieved to a particle size of 0.6–1 mm and packed into a reactor tube. The catalyst was purged under helium for 2 minutes at 70 °C before being heated under a 5% H reduction gas to 230 °C (8 °C min−1) for 3 hours. The catalyst was then cooled to 68 °C under helium before the dilute 2.5% N O reaction gas was added with a flow rate of 65 ml min−1. The formation of N from the surface oxidation of the copper catalyst by N O was measured downstream using a thermal conductivity detector (TCD). Once the surface of the copper is fully oxidized, there is a breakthrough of N O, detected on the TCD. From this, the number of oxygen atoms that are chemisorbed on the copper surface can be determined. The number of exposed surface copper atoms and the copper surface area can then be derived. Quoted surface areas are calculated using discharged sample mass. Note that recent work37, 38 has shown that, if the catalyst is exposed to partial pressures of H exceeding 0.05 bar, then partial reduction of ZnO at the copper interface can occur. This will affect copper surface area results, owing to N O oxidizing both copper and ZnO . In these cases, alternative techniques, such as a H thermal conductivity detector, will give more accurate data with respect to copper surface area37, 38. Copper surface area analysis after exposure to LTS conditions. Copper surface area analysis of the fresh catalysts were carried out on a Quantachrome ChemBET 3000. Sample (100 mg) was packed into a stainless-steel U-tube and purged with high-purity helium for 5 minutes. Samples were reduced using 10% H /Ar (30 ml min−1) heated to 140 °C at 10 °C min−1, before heating further to 225 °C at 1 °C min−1. The resulting catalyst was held at this temperature for 20 minutes to ensure that complete reduction took place. Note that, under this partial pressure of H , it has been reported that ZnO species in contact with copper can partially reduce37, 38. Residual H was flushed from the system by switching the gas line back over to helium (80 ml min−1), while holding the sample at 220 °C for another 10 minutes. The temperature was then reduced to 65 °C for N O pulsing (BOC AA Grade). A programme of 12 pulses of 113 μl N O with a 5-minute stabilization time between pulses was carried out, followed by three pulses of N for calibration. Unreacted N O was trapped before reaching the detector using a molecular sieve 5A (pelleted, 1.6 mm, Sigma Aldrich) trap. The copper surface area was determined from the amount of N emitted and the post-reaction analysis of catalyst mass, as follows: where N is the Avogadro constant, 6.022 × 1023 (atoms). The key assumptions are that the amount of N emitted amounts to half a monolayer’s coverage of oxygen, and that the surface density of copper is 1.47 × 1019 (atoms m−2). The volume of N produced was quantified using a TCD. After copper surface area analysis, the samples were kept under N and transferred to an LTS reactor. Ageing was carried out in a single fixed-bed reactor equipped with a bypass line. CO, N , CO and H were introduced to the catalyst bed via mass-flow controllers (Bronkhorst). Water of high-performance liquid chromatography (HPLC) grade was passed through a liquid-flow controller (Bronkhorst) and then into a controlled evaporator mixer (Bronkhorst) that was heated to 140 °C. N was fed through the vaporized water to give a dilute syngas mixture (H O/CO/CO /H /N  = 25/1/4/13.75/56.25). This mixture was introduced at 220 °C after re-reduction of the catalyst. The gas flows were controlled to achieve an MHSV of 30,000 l h−1 kg−1. After ageing for 40 hours, the samples were transferred back to the Quantachrome ChemBET Chemisorption analyser, whereby, after re-reduction, the copper surface areas of the aged catalysts were measured, as described above. Scanning transmission electron microscopy (STEM). Samples for STEM characterization were dry-dispersed onto holey carbon TEM grids. They were examined in an aberration-corrected JEOL ARM-200CF scanning transmission electron microscope operating at 200 kV in bright-field and dark-field STEM imaging modes. Reliable electron microscopy results could be obtained only from the set of zincian-georgeite- and zincian-malachite-derived materials, as these were found to be stable under the vacuum environment of the microscope, and largely unaffected by electron-beam irradiation. By way of contrast, the copper-only georgeite precursor materials were highly unstable under vacuum conditions (even without electron-beam irradiation), turning from a blue to dark-green colour, probably owing to the loss of occluded water. The corresponding zincian-georgeite materials showed no such colour transformation under vacuum. Environmental transmission electron microscopy (ETEM). Samples for reduction during characterization by ETEM were dry-dispersed on heater chips (DensSolution trough hole) and then mounted on a DensSolution SH30 heating holder. The holder with sample was inserted into an FEI Titan 80-300 environmental transmission electron microscope operated at 300 kV (ref. 39) . The reduction of the samples was performed in situ as follows, for both the georgeite and the malachite precursors: a flow of H was let into the microscope, building up a pressure of 2 mbar. The sample was heated from room temperature to 150 °C using a heating ramp rate of 10 °C min−1. The final heating to 225 °C was done at 1 °C min−1. The oxidation state of copper was monitored by electron energy-loss spectroscopy (EELS) during the reduction treatment using a Gatan Tridiem 866 spectrometer attached to the microscope. After an extended in situ treatment of several hours at 225 °C, phase-contrast lattice imaging was performed at elevated temperature in an H atmosphere for both the georgeite and malachite samples, and recorded using a Gatan US 1000 charge-coupled-device camera. X-ray photoelectron spectroscopy (XPS). A Kratos Axis Ultra DLD system was used to collect XPS spectra, using a monochromatic Al K X-ray source operating at 120 W. Data were collected with pass energies of 160 eV for survey spectra, and 40 eV for the high-resolution scans. The system was operated in the Hybrid mode, using a combination of magnetic immersion and electrostatic lenses and acquired over an area approximately 300 × 700 μm2. A magnetically confined charge-compensation system was used to minimize charging of the sample surface, and the resulting spectra were calibrated to the C(1 s) line at 284.8 eV; all spectra were taken with a 90° take-off angle. A base pressure of ~1 × 10−9 Torr was maintained during collection of the spectra. Gas treatments were performed in a Kratos catalysis cell, which mimics the conditions of a normal reactor vessel, allowing the re-creation of reactor conditions and analysis of the chemical changes taking place on the catalyst surface. Briefly, the cell consists of a fused quartz reactor vessel contained within a stainless-steel vacuum chamber (base pressure ~10−8 Torr after baking). Samples were heated at a controlled ramp rate of 2 °C min−1 to a temperature of 225 °C using a eurotherm controller. The catalysts were exposed to an atmosphere of 2% H in nitrogen with a flow rate of 30 ml min−1, controlled using MKS mass flow controllers during the heating ramp, during the 20-minute isotherm at 225 °C, and also while the catalyst was cooled to 25 °C. The samples were analysed before and after gas treatment without breaking the vacuum. Inductively coupled plasma mass spectrometry (ICP-MS) and carbon–hydrogen–nitrogen (CHN) analysis. These analyses were provided as a commercial service by Warwick Analytical Services. Helium pycnometry. This analysis was provided as a commercial service by MCA Services.

Van Der Meer R.,MESA Institute for Nanotechnology | Kozhevnikov I.,Russian Academy of Sciences | Krishnan B.,MESA Institute for Nanotechnology | Huskens J.,MESA Institute for Nanotechnology | And 7 more authors.
AIP Advances | Year: 2013

We demonstrate single-order operation of Lamellar Multilayer Gratings in the soft x-ray spectral range. The spectral resolution was found to be 3.8 times higher than from an unpatterned multilayer mirror, while there were no significant spectral sideband structures adjacent to the main Bragg peak. The measured spectral bandwidths and peak reflectivities were in good agreement with our theoretical calculations. Copyright © 2013 Author(s).

Discover hidden collaborations