G. Robert Millward

Evidence for semi-regularly ordered sequences of mirror and inversion symmetry planes in ZSM-5/ZSM-11 shape-selective zeolitic catalysts

The combined use of high-resolution electron microscopic imaging, electron diffraction, optical diffraction and computer graphics for characterizing intergrowth structures of ZSM-5 and ZSM-11 is described. In particular, a specific preparation is shown to consist mainly of the ZSM-5 framework structure but with single insertions of strips of ZSM-11 at an average spacing of ca. 66 A along [100]. Evidence is also presented for the occurrence of an intergrowth in which one slab of ZSM-5 is related to an adjoining slab such that one of these is rotated through 90° around a [001] axis. An idealized atomic model for this intergrowth is proposed.

New methods for the structural characterization of shape-selective zeolites

Two fundamentally new and powerful methods of studying zeolites have recently become available. High-resolution electron microscopy records structure, of both perfect and imperfect zeolites, directly in real space. This technique can reveal information about the presence or lack of structural integrity in situations where X-ray studies are not helpful; and it has, for example, proved possible to detect embryonic regions of ZSM-5 in an amorphous precursor and to pin-point structural faults of various kinds in otherwise well-ordered systems. To date this technique has not revealed the presence of previously postulated intermediate structural variants between ZSM-11 and ZSM-5 and members. It has also shown that when faujasite is dealuminated by gaseous SiCl4, thereby enhancing the Si/Al ratio by factors of 10–25, the structure remains intact at the atomic level. Magic-angle-spinning nuclear magnetic resonance (MASNMR) results, based on 29Si, afford information pertaining to Si,Al ordering. We show that dealuminated faujasite (Si/Al of ca. 2.45) originally consists of predominantly Si—(OAl)3, Si—(OAl)2 and Si—(OAl) groups, but upon dealumination the environment is almost exclusively Si—(O-Si)4. The selective behaviour of this highly dealuminated faujasite appears promising. Other long-standing problems in Si,Al ordering and catalysis are solvable using MASNMR, in combination with electron and neutron diffraction.

On ‘seeing’ the stacking sequence in graphite–iron(III) chloride intercalates by high-resolution electron microscopy

Lattice images of iron(III) chloride intercalate residue compounds, when recorded with their layer planns parallel tothe electron beam, reveal the occurrence of coherent intergrowths of the so-called first-, second-, fifth-, and sixth-stage intercalates in which a sheet of FeCl3, is inserted between, respectively, all, every second, every fifth, and every sixth individual sheet of carbon.

Microstructure and stability of iron chloride graphites

Two methods have been used to prepare iron chloride intercalates of natural graphite. Ferric chloride intercalates are demonstrated to be both extremely air sensitive and also to undergo gradual physical and chemical changes upon storage, solvent treatment, heating and washing. Gradual interconversion to higher staging or direct de-intercalation are the two observed reaction pathways.The ferrous chloride intercalates (obtained by H2 reduction) are found to be even more air sensitive than their ferric precursors, decomposing to yield disordered graphite and crystalline iron chloride hydrates and oxyhydrates. The Mossbauer spectrum of FeCl3 graphite was found to consist of two singlets (δ1= 0.47 mm s–1 and δ2= 0.61 mm s–1), giving rise to two different quadrupole doublets in the FeCl2 intercalate. High-resolution transmission electron microscopy confirms earlier results and regular first-stage intercalate is imaged for the first time. The discussion of the results is based on an island-structured intercalate and a stabilising influence of chemisorbed Cl2 and/or HCl.The limitations of different techniques to the study of graphite intercalates (including Mossbauer spectroscopy, X-ray powder diffraction, thermogravimetric analysis, weight-uptake measurements and electron microscopy) are discussed.

Direct, real-space determination of intergrowths in ZSM-5/ZSM-11 catalysts

The performance of shape-selective catalysts is thought to be a function of the nature of the intergrowths present in the structure, but hitherto no direct methods of identifying isolated or ordered intergrowths have been available; high resolution electron microscopy is shown to be capable of solving such problems, and in particular single strips of ZSM-11 may be directly identified in ZSM-5.

Ultra-fine particles of aluminium formed by electron-beam-induced decomposition of aluminium hydride

Abstract The electron-beam-induced decomposition of AlH3, within a transmission electron microscope, results in the formation of ultra-fine particles of aluminium. The product is of two types: the first consists of a conglomerate of individual and discrete particles combined in a manner such that the overall morphology of the parent hydride is retained but with a random orientation between individual crystallites. They are of various sizes and are supported on, and surrounded by, an oxide film and result from direct beam damage and decomposition of the hydride to the metal and hydrogen gas. The second type is widely distributed over the support carbon film and results from the beam-induced dispersion of the initial product. They are of varying size depending principally upon the beam flux. Particles of sizes between 4 and 20 nm diameter are readily manufactured, occasionally being only a few layers thick. Microstructural information is presented for the particles as provided by HREM.

On the ultrastructure and morphology of colloidal cobalt ferrite

Electron–optical and related techniques show that both unannealed and annealed samples of Co0.73Fe2.27O4 of colloidal dimensions are highly crystalline and of well-defined morphology: samples annealed at 600 °C have particles of mean diameter ca. 220 A, with 〈332〉 preferred zones and {022} and {311} preferred faces.

Probing the structure of zeolites by Fourier transform electron microscopy: zeolite-L as a test case

The structures of zeolites cannot readily be determined by X-ray methods chiefly because only microcrystalline samples are available: a new method of obtaining projected structures, based on low-dose electron microscopic imaging, is described and illustrated for the [00.1] projection of zeolite-L.

Direct evidence to support the proposal that ZSM-23 is a recurrently twinned variant of zeolite theta-1

High resolution electron microscopy reveals the presence of [110] twin boundaries in the zeolite Theta-1; this reinforces the view that ZSM-23 is a recurrently twinned variant of Theta-1.

ZSM-23: a suggested structure

X-Ray powder diffraction (Rietveld profile refinement) using a model derived from electron diffraction and adsorption data and space group limitations has yielded a structure for ZSM-23; it is a recurrently twinned version of theta-1 (ZSM-22).