P. L. Gai

Dynamic studies of metal oxide catalysts: MoO3

Abstract The dynamic microstructural changes of the metal oxide catalyst MoO3 in reducing atmospheres of carbon monoxide, hydrogen and propylene diluted with helium as well as mixtures of propylene and oxygen have been studied up to temperatures of ∼600°C in a gas-reaction eell fitted to a high-voltage electron microscope. On reduction, apart from partial screw dislocations at ∼80°C, two sets of surface domains with predominantly pure shear displacement across them are generated at ∼ 120°C: and lie parallel to ⟨304⟩. Up to 380°C, the domains grow in size, appear to change their character, and extend into the bulk. At operating temperatures of ∼400°C, crystallographic shear planes (CSPs) are formed at the surface bounded by partial edge dislocations with Burgers vector b =[α/2, b/7, 0 01 bounding a stacking fault extending from the surface to a depth of ∼300 A. Observations of nucleation and growth confirm the model of collapsed vacancy discs expanding by dislocation climb. The response of the defects in p...

The reduction of MoO3 at low temperatures. II

Abstract New observations on the surface defects generated in MoO3 on reduction in H2 in situ in a high voltage electron microscope have revealed an error in the previous interpretation (Thorn and Hirsch 1976). The defects are now shown to be partial screw dislocations, with Burgers vector ½〈101〉, which accommodate the misfit between the reduced surface layer and the underlying matrix.

Direct observation of the reduction of moo3 at low temperatures

An electron microscope study has been carried out of the reduction of MoO3 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 A 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.

Cation ordering in Ln1−xSrxCoO3

Electron diffraction studies show that rare-earth and Sr2+ ions in Ln1−xSrxCoO3 (Ln=rare earth) are ordered in alternate sites if x=0.5, while they are randomly distributed if x<0.5. This observation supports Goodenoughs explanation of the novel magnetic properties of these oxides.

Electron microscopy of industrial oxidation catalysts

Abstract A combination of in situ high-voltage electron microscopy (HVEM), X-ray/ electron diffraction methods, X-ray and electron probe microanalysis as well as high-resolution phase contrast lattice imaging (<2 AS resolution) microscopy techniques has been used to investigate the reaction properties of an industrial tellurium-molybdenum oxide catalyst system used, for example, in the ammoxidation of propylene to produce organic chemicals such as acrylonitrile, acrolein and other hydrocarbon conversion reactions. In situ electron microscopy experiments conducted on the catalysts in reducing/oxidizing gaseous environments with pressures comparable to those used in the commercial reactions have revealed degradation products which have been analysed by high-resolution microscopy. In this industrial system at typical catalyst operating temperatures no evidence for crystallographic shear planes has been obtained and the implications concerning the role of such extended defects in industrial catalytic reaction...

Dynamic electron microscopy of copper-palladium intermetallic compound catalysts

Abstract In-situ electron microscopy has been used to probe fundamental aspects of the dynamic state of supported copper-palladium intermetallic catalysts in controlled-reactions conditions, relating to methanol synthesis from hydrogenation of carbon monoxide. The chemical nature of intermetallic surfaces is found to be different in the different gases and is related to gas chemisorption effects. Subnanometer overlayers of palladium have been imaged on individual small particles under reducing carbon monoxide and synthesis gas environments and characterized by a combination of high-precision analytical electron microscopy, high-resolution electron microscopy and image simulations. They are mostly equilibrated Pd-rich (100) surfaces. The nature of catalytically active centers is explored. Diffuse scattering observed in electron diffraction patterns of perfect crystallites is attributed to directional lattice vibrations, with the diffuse streaks resulting primarily from the intersections of {111} reciprocal lattice (rel) walls, and of 〈110〉 rel rods with the Ewald sphere. Coherent bremsstrahlung peaks have been observed in the microanalysis traces of the small intermetallic particles at low incident electron energies.

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.

Lattice imaging of close-packed structures by high resolution electron microscopy: ABO3 perovskite polytypes

Lattice imaging technique of high resolution electron microscopy has been employed to examine 4H, 6H and 9R ABO3 perovskite polytypes. The lattice images can be correlated with the lattice periodicity and the stacking sequence of AO3 layers and BO6 octahedra. The study shows the utility and validity of the lattice imaging technique for the study of relatively close-packed systems.

Direct lattice imaging of silicon carbide

The high-resolution electron microscopic technique of lattice imaging shows 15 and 3 AA repeat sequences in 6H SiC. The 15 AA sequence is further resolved into 2.5 AA layers corresponding to the interlayer separation.

Diffuse electron scattering and vacancy ordering in VO Possible role of charge density waves

Abstract Electron diffraction patterns of disordered VO show extensive diffuse scattering and the presence of satellite spots which are incommensurate with the Bragg reflections. Annealed samples show evidence of a superstructure due to vacancy ordering with the superstructure bearing relation to the cubic lattice of the disordered phase, but no relation to the satellite spots due to diffuse scattering. The mechanism of diffuse scattering is discussed in terms of short-range order of vacancies as well as existence of CDW (charge density waves) and the latter model seems to be reasonable.