Gas Reaction Cell Research Articles

AMS of 36Cl with the VERA 3 MV tandem accelerator

Recent progress with compact ionization chambers has opened new possibilities for isobar suppression in accelerator mass spectrometry (AMS). Separation of 36Cl (t1/2=0.30Ma) at natural isotopic levels from its stable isobar 36S became feasible at particle energies of 24MeV, which are also accessible for medium-sized tandem accelerators with 3MV terminal voltage like VERA (Vienna Environmental Research Accelerator). Investigations with an ionization chamber revealed how physics favors isobar separation even at energies below the maximum of the Bragg curve. The strong energy focusing effect at high energy losses reduces energy straggling significantly and isobar separation steadily increases up to almost full energy loss. With an optimized detection setup, sulfur suppression factors of 2×104 have been achieved.Refraining from the additional use of degrader foils has the benefit of high transmission to the detector (∼16%), but requires a low sulfur output from the ion source. Therefore several backing materials have been screened for sulfur content. The dependence of the sulfur output on the AgCl sample size has been investigated as well.Precision and accuracy have been thoroughly assessed over the last two years. Since drifts in the spectra are efficiently corrected by monitoring the position of the 36S peak, the reproducibility for high ratio samples (36Cl/Cl>10−12) is better than 2%. Our blank value of 36Cl/Cl≈(5±5)×10−16 is competitive to other labs. 36Cl has become a routine AMS-isotope at VERA.Recently we also explored novel techniques for additional sulfur suppression already in the ion source. While results with a small gas reaction cell in front of the sputter target were discouraging, a decrease in the sulfur/chlorine ratio by one order of magnitude was achieved by directing 300mW continuous wave laser beam at 445nm towards the cathode in the ion source.

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In situ XANES cell used for the study of lanthanum strontium cuprate deNO x catalysts

The potential NO x -reduction electro catalyst lanthanum strontium cuprate has been characterized with an in situ X-ray spectrochemical gas reaction cell. In a series of samples with increasing substitution of trivalent lanthanum by divalent strontium ions, the oxidation state of copper increased, maintaining charge neutrality, with the concentration of oxygen vacancies likely increasing at substitution ratios larger than Sr/La > 0.08. During heating in air, the valence of copper ions in the structure increased. Upon exposure to NO at 500 °C the valence of copper ions in a lanthanum strontium cuprate sample decreased, whereas it remained unchanged in the strontium-free lanthanum cuprate sample.

Fast- and ultra-fast laser pulse induced reactions between carbon dioxide and methane

The direct excitation of CO 2 using fast (nanosecond) and ultrafast (femtosecond) pulsed lasers was investigated. A gas reaction cell was used to excite CO 2 in a 50 : 50 mixture of CO 2 and CH 4 using nano- and femtosecond laser systems, to induce a reaction between these two compounds. FT-IR spectra showed that CO was formed using the nanosecond and femtosecond laser systems. It was also found that hydrocarbons, containing C-C bonds were formed. For both the nanosecond and femtosecond laser, it was found that more C-C higher hydrocarbons were formed after 5 h compared to 3 h of irradiation. Irradiation at pressures of 250, 350 and 500 kPa with the femtosecond laser system showed the expected increase in the amount of CO formed with an increase in pressure. Results from this study showed that carbon dioxide and methane can be activated successfully using nanosecond laser pulses at 2000 nm and femtosecond laser pulses at 795 or 2000 nm and that these activated species react to form CO and C-C containing products.

Non-isothermal decomposition kinetics, thermal behavior and computational detonation properties on 4-amino-1,2,4-triazol-5-one (ATO)

The thermal behavior of 4-amino-1,2,4-triazol-5-one (ATO) was studied under non-isothermal condition by DSC method in a sealed cell of stainless steel. The melting enthalpy and melting entropy of ATO are 21.34 ± 0.49 kJ mol −1 and 46.54 ± 0.30 J mol −1 K −1, respectively. The kinetic parameters were obtained from the analysis of DSC curves by Kissinger method, Ozawa method, the differential method and the integral method. The main exothermic decomposition reaction mechanism of ATO is classified as nucleation and growth, and the kinetic parameters of the reaction are E a = 119.50 kJ mol −1 and A = 10 9.03 s −1. The gas products and condensed phase products of the thermal decomposition of ATO were studied on two simultaneous devices of the fast thermolysis reaction cell (gas reaction cell) in situ in conjunction with rapid scan transform infrared spectroscopy (RSFT-IR) and the solid reaction cell in situ. The heat of formation (HOF) for ATO was evaluated by G3 theory. The detonation velocity ( D) and detonation pressure ( P) were estimated by using the well-known Kamlet–Jacobs equation, based on the theoretical HOF and the determined crystal density.

3D atom probe study of gas adsorption and reaction on alloy catalyst surfaces I: Instrumentation

A versatile gas reaction cell has been developed for a 3D atom probe system to provide a new method for studying the reaction mechanisms of heterogeneous catalysis. We describe the features and advantages of this new system. The reaction cell enables exposure of metal specimens in high temperature (⩽873 K) and high pressure (⩽1 bar) environments to a wide range of single gases or gas mixtures. Following treatment, specimens are transferred into the UHV analysis chamber of the 3D atom probe, which provides atomic-scale positional and chemical information for adsorbed gas and metal atoms/oxides from selected regions of the specimen apex. Results demonstrating the effects of heating alloy catalysts in vacuo and in air are presented, which validate the instrument design, alongside a sample of data for exposing Pt and Pt-Rh alloys to NO, revealing the power of the 3D atom probe technique in this field.

In Situ Oxidation of Nb22O54 in a Gas Reaction Cell High-Resolution Transmission Electron Microscope

A nonstoichiometric block structure oxide, Nb22O54 (NbO2.455), with monoclinic symmetry was characterized following in situ oxidation in a controlled environment, high-resolution electron microscope. This instrument is based on a JEOL 4000EX electron microscope equipped with a gas reaction cell. The oxidation reaction was carried out by introducing ca. 20 mb of oxygen gas to the specimen region and heating the specimen by means of a focussed electron beam. The resulting structures obtained after the oxidation reaction were not totally homogeneous. (1) Mainly microdomains of Nb10O25 were found, which consists of [3×3] blocks of octahedra linked through tetrahedrally coordinated sites—such a structure is isostructural with PNb9O25. Nb10O25 seems to be a metastable phase in the Nb–O system, and was found here for the first time as extended domains. (2) A highly disordered structure was observed in some areas of the crystal after oxidation, with a corresponding electron diffraction pattern similar to an “X” phase interpreted as two-dimensionally disordered H-Nb2O5. Very clear lamellar defects were also found after oxidation.

Solid–gas reactions of complex oxides inside an environmental high-resolution transmission electron microscope

In a gas reaction cell (GRC), installed in a high-resolution transmission electron microscope (HRTEM) (JEOL 4000EX), samples can be manipulated in an ambient atmosphere (p<50 mbar). This experimental setup permits not only the observation of solid–gas reactions in situ at close to the atomic level but also the induction of structural modifications under the influence of a plasma, generated by the ionization of gas particles by an intense electron beam. Solid state reactions of non-stoichiometric niobium oxides and niobium tungsten oxides with different gases (O 2, H 2 and He) have been carried out inside this controlled environment transmission electron microscope (CETEM), and this has led to reaction products with novel structures which are not accessible by conventional solid state synthesis methods. Monoclinic and orthorhombic Nb 12O 29 crystallize in block structures comprising [3×4] blocks. The oxidation of the monoclinic phase occurs via a three step mechanism: firstly, a lamellar defect of composition Nb 11O 27 is formed. Empty rectangular channels in this defect provide the diffusion paths in the subsequent oxidation. In the second step, microdomains of the Nb 22O 54 phase are generated as an intermediate state of the oxidation process. The structure of the final product Nb 10O 25, which consists of [3×3] blocks and tetrahedral coordinated sites, is isostructural to PNb 9O 25. Microdomains of this apparently metastable phase appear as a product of the Nb 22O 54 oxidation. The oxidation reaction of Nb 12O 29 was found to be a reversible process: the reduction of the oxidation product with H 2 results in the formation of the starting Nb 12O 29 structure. On the other hand, the block structure of Nb 12O 29 has been destroyed by a direct treatment of the sample with H 2 while NbO in a cubic rock salt structure is produced. This in situ technique has also been applied to niobium tungsten oxides which constitute the solid solution series Nb 8− n W 9+ n O 47 with 0≤ n≤4. All of these phases crystallize in the threefold tetragonal tungsten bronze (TTB) superstructure of Nb 8W 9O 47 ( n=0). In the main reaction, these phases decompose in a gas plasma (O 2, H 2 or He, p=20 mbar) into WO 3− x , which evaporates and solidifies again near the irradiated crystallite, and (Nb,W) 24O 64, which crystallizes in a 2 a superstructure of the TTB type observed here for the first time in the system Nb–W–O. Nb 8W 9O 47, Nb 7W 10O 47 and Nb 6W 11O 47 always react in this way, independent of the applied gas. On the other hand, the treatment of Nb 5W 12O 47 ( n=3) and Nb 4W 13O 47 ( n=4) in an oxygen atmosphere often caused a different reaction: these phases have been oxidized and a heavily disordered bronze-type structure has been formed. The oxygen excess in these products is largely accommodated in segregated domains of WO 3.

From Nb12O29 to Nb22O54 in a Controlled Environment High-Resolution Microscope

A nonstoichiometric Nb12O29(NbO2.417) block structure oxide with monoclinic symmetry was slightly oxidized in a controlled environmental high-resolution electron microscope. This instrument is based on a JEOL 4000 EX electron microscope equipped with a unique gas reaction cell in which the oxide could be oxidized by introducing O2 at pressures up to 30 mbar, while promoting local reaction by electron beam heating. The oxidation process was seen to start in the regions where some stacking faults perpendicular to the c axis appear (in which the structure has local orthorhombic symmetry). The initial movement of atoms, involving a formation of lamellar defects, could be traced with great certainty. Such defects provide empty rectangular tunnels, similar to those found in the NaNb13O33 structure, which is a (4×3) block structure with Na cations situat ed in the rectangular tunnels. This stage of the oxidation reaction implies a minimal rearrang ement, usually involving a simple unit jump (the primitive diffusion step) by atoms in certain sites. Then, the cooperative unit jumps along rows of atoms propagate the transformation through the crystal. A further stage in the oxidation gives rise to the Nb22O54(NbO2.455) structure in some areas of the crystal, which could be identified in the high-resolution transmission electron microscopy image as well as in the electron diffraction patterns.

A New Niobium Tungsten Oxide as a Result of anin SituReaction in a Gas Reaction Cell Microscope

Nb7W10O47was heated by an electron beam in a gas atmosphere (O2, He, or H2) of the gas reaction cell installed in a JEOL 4000 EX high-resolution transmission electron microscope (HRTEM). This treatment generated a new niobium tungsten oxide (Nb, W)12O32, crystallizing in2a×2asuperstructure of the tetragonal tungsten bronze (TTB) type. While the TTB substructure remains unaltered by this reaction, the proportion of pentagonal tunnels occupied with metal–oxygen strings has increased. The simultaneous occupation of all four pentagonal tunnels around a central square of octahedra is the characteristic structural feature of (Nb,W)12O32. This arrangement is observed here for the first time in the ternary system Nb/W/O. During the reaction, a part of the tungsten oxide is etched away and condenses as small particles near the irradiated crystallite.

In SituChemical Reaction of Nb12O29in a High Resolution Gas Reaction Cell Microscope

Solid-state oxidation–reduction of a nonstoichiometric Nb12O29(NbO2.417) with monoclinic cell was studied using a JEOL 4000 EX HRTEM equipped with a gas reaction cell in which the oxide was oxidized and then reduced by sequentially introducing O2and H2. The oxidation yielded other block structures with higher oxygen content. The first step of the oxidation mechanism, involving formation oflamellardefects of composition Nb11O27(NbO2.450), could be elucidated with great certainty. Nb10O25(a metastable polymorph of NbO2.500) was formed as a final stage of the oxidation process. Subsequent reduction led to a nearly ordered Nb12O29phase suggesting that such an oxidation-reduction reaction is a reversible process. When Nb12O29was directly reduced, NbO was found to grow on the surface of the crystal.

Observations of solid–gas reactions by means of high-resolution transmission electron microscopy

A controlled environment specimen chamber, or gas reaction cell, has been constructed for a 400 kV JEOL 4000EX transmission electron microscope. It allows high-resolution (better than 0.26 nm) observation of specimens while they are interacting at elevated temperatures (typically 600 °C) with low pressures of gas (typically 500 Pa). The application of the cell to the study of small metal particles, of interest in catalysis, is described. In particular, the behaviour of conventional supported particles is compared with that of particles produced within carbon nanotubes. Preliminary studies of the reduction of oxide particles to metal have indicated possible epitaxial relationships between the particles and the constraining carbon walls in certain cases, which suggest that it may be possible to tailor specific indexed surfaces to be preferentially available to reactant gases. Interactions between the electron beam and the gaseous environment, which may in turn affect the specimen, are then discussed. In particular, sputtering processes can both contaminate and damage the sample that is under investigation, although at higher temperatures, the damage is usually self-healing. In the special case of carbonaceous materials, the impact of inert gas atoms can promote fullerene nucleation.

The mechanism of formation of ammonia synthesis catalyst

The formation of ammonia synthesis catalyst by the reduction of magnetite by hydrogen at 450 °C has been studied using electron microscopy, X-ray energy dispersive and electron energy loss analysis, and including in situ reduction experiments in a gas reaction cell in an electron microscope. The reaction is characterised by a cellular mechanism of reduction leading to the formation of a periodic network of iron crystallites and pores without any significant volume change. The iron nucleates epitaxially on the oxide, and minor elements, notably aluminium, initially in solution in the oxide, partition to the iron crystallite surfaces. Elements present in intercrystalline ferrite and glassy phases remain essentially immobile during reduction with the exception of potassium which migrates rapidly to the iron surfaces.

Microstructural changes in vanadium pentoxide in controlled environments

Abstract Direct observations have been made of the reduction of V2O5 between room temperature and ∼600°C in reducing gases such as H2, balanced by He, CO and C2H4, using a gas-reaction cell in a high-voltage electron microscope. The systematic study has revealed microstructural changes relevant to structural and catalytic properties of the system. The reduction of the surface begins at low temperatures of ∼80–100°C. The defect structures formed are analysed, and it is shown that these are important in understanding redox reactions. The results could also perhaps explain chemical studies of oxygen desorption from V2O5.

Morphological Development of Hydrating Tricalcium Silicate as Examined by Electron Microscopy Techniques

This study of C3S hydrates used a high‐voltage electron microscope equipped with a gas reaction cell that allows the specimen to be kept moist, and high‐resolution scanning and scanning transmission microscopes. Mechanical thinning and ion‐beam thinning coupled with fractography allowed this study of microstructural development with time, enabling both the morphologies and the spatial relations of the morphologies to be observed. The combination of several microscopic techniques allowed the hydration products of C3S to be classified and compared with those of Diamond for portland cement hydrates.

Applications of In-Situ Electron Microscopy in Catalysis

Controlled reduction and oxidation reaction experiments on metal oxide catalysts, sulphides, and iron-titanium oxide minerals have been conducted in an environmental (or gas reaction) cell fitted to an AEI-EM7 high voltage electron microscope operating at 1MV. With this technique it is possible to observe dynamic heterogeneous reactions using gas pressures in the range from a few torr to an atmosphere, and temperatures from room temp, to ∼1000°C. A full dynamic description of the behaviour of the solid with respect to temperature, pressure, gaseous environments can therefore be obtained. These observations are useful in understanding mechanisms of heterogeneous catalysis, redox processes, and migration of the reduced species, which affect catalyst selectivity and activity. Examples of some in-situ reactions have been reported in the literature.

The use of a high-voltage electron microscope and gas reaction cell for the microstructural investigation of wet Portland cement

The use of a high-voltage electron microscope and gas reaction cell for the microstructural investigation of wet Portland cement

In situ oxidation of Fe: Cr: Ni foils in the H.V.E.M

Continuous observation of oxidation processes on the surface of austenitic stainless steels has been achieved byin situoxidation of electrochemically thinned foils in the AEI EM7 H.V.E.M. using the gas reaction cell designed by Swann (1)Figure 1 shows the sequence of oxidation of a Fe: 18% Cr: 10% Ni foil at 700°C in air at 3.5 kPa recorded with an accelerating potential of 700 kV. Initially the surface reaction is characterised by the rapid formation of a transparent layer of minute randomly oriented crystals of the cubic oxide (Fe, Cr)3O4which is revealed by the appearance of diffuse rings on the single crystal γFe diffraction pattern. After approximately 30 seconds at 700γC the interface between the cubic oxide and the base alloy can be observed progressing inwards from the thinnest edges of the foil, and the formation of nuclei of α(Fe, Cr)2O3commences within the cubic oxide layer over both sides of the foil.

Direct observation of the reduction of moo 3 at low temperatures

An electron microscope study has been carried out of the reduction of MoO 3 by hydrogen and carbon monoxide in a gas reaction cell in an AEI EM7 1 MeV electron microscope. At temperatures below 80 °C, line defects deviating a few degrees from [101] are generated at a depth of about 200 Å below the crystal surface; the presence of platinum was found to be essential for the effects to be observed. The line defects are found to be partial screw dislocations bounding stacking faults extending from the surface. The nature of these dislocations is such that they tend to accommodate the misfit between the reduced surface layer and the underlying matrix. The reduction is thought to be accompanied by the generation of anion vacancies which diffuse slowly into the crystal; by identifying the depth of the dislocation with the diffusion front (and in other ways), the activation energy for migration of anion vacancies is estimated to be about 0.85–0.9 eV.

In situ observations of the nitriding of tantalum

SUMMARYA gas reaction cell developed for the AEI EM7 has been used to observe directly the structural changes which occur when tantalum is heated in nitrogen. It is demonstrated that sharp images of the microstructure can be obtained from specimens at temperature while surrounded by a 5.5 mm thick layer of the reaction gas at a pressure of 35 torr. The usefulness of the in situ technique for studying gas/metal reactions is assessed.