John O. Williams

New families of catalysts for the selective oxidation of methane

The catalytic performance of 11 chemically distinct crystalline solids, all of which have layered structures of either the Sillen, perite or bipox type in which infinite sheets of [Bi2O2]2+ or [PbBiO2]+ are interleaved with other layers such as chloride ions, metal chlorides or tungsten oxide, is reported. Several kinds of well characterized Sillen structures, designated X1, X2, X3, XN, as well as regular intergrowths thereof, were tested. Some of these monophasic materials show good C2 selectivity with ratios of C2H4/C2H6 ranging from ca. unity to in excess of 30 at temperatures close to 1000 K for the oxidative coupling of methane. Scope exists for further structural modification and the design of superior catalysts of other bismuth-containing solids based on fluorites and pyrochlores.

Microscopic Studies of Enhanced Reactivity at Structural Faults in Solids

Conventional optical microscopy and, more particularly, a range of electron microscopic techniques have been adapted to identify the nature, and examine the chemical consequences, of emergent structural imperfections in single crystals of a number of anisotropic solids. Detailed studies have been carried out on graphite and MoS2, which are highly anisotropic both with respect to their reactivities and their dislocation behaviour; and which, in addition, react with oxygen to yield volatile products. Complications arising from the nucleation of new solid phases are therefore circumvented. Certain types of low-energy dislocations and stacking faults present in these solids are shown to be of little consequence as active centres in the oxidation process. High-energy dislocations are, however, extremely active. Indigeneous or created vacancies in the exterior surface also function as active centres. Their presence may be detected at concentrations greater than 1 in 1010 of the exposed, basal-plane atoms, and their generation, particularly favoured by impingement of 1D and 1S oxygen atoms, may be monitored by an etch-decoration technique capable of detecting monatomic steps. In situ electron microscopic studies of photoinduced changes at, or immediately below, the surfaces of rather less anisotropic, organic molecular crystals, which do involve solid-state nucleation of product, again reveal the importance of certain types of dislocations in facilitating chemical transformations.

Phase transitions and the fluorescence emission of p-terphenyl single crystals

Abstract The fluorescence emission of p -terphenyl (TP) molecules in solution and in the crystalline state at various temperatures in the range 300-4 K are compared. The appearance of new bands at higher energies with decreasing temperature corresponds with a phase transition in the crystalline material. The outer and central rings of the molecule are known to be inclined at angles between 16 and 25° in this low temperature modification and it is concluded that a similar molecular configuration exists in solution. Broad vibronic peaks observed at 4 K are consistent with structures exhibiting long range disorder in which the long molecular axis may adopt variable orientations and where the rings assume a number of different relative inclinations.

Electron and optical microscopic studies of a stress-induced phase transition in 1,8-dichloro-10-methylanthracene

Abstract Electron and optical microscopic studies of 1,8-dichloro-10-methylanthracene have revealed that the parent orthorhombic structure may, under certain circumstances, undergo a stress-induced phase transformation into a daughter structure. The overall process is one in which a single crystal is converted into another single crystal at an invariant plane the indices of which are shown to be (2–88, 0, 1) and the crystallographically equivalent (2–88, 0, 1). The transformation, which is akin to a martensitic process, may be regarded as the result of the operation of an invariant plane strain. The irrational habit plane is thought to be composed of stepped, close-packed planes the existence of which, in turn, suggests possible mechanisms for the formation of the interface between the two structures. Furthermore, the description of some of the properties of the interface may be formulated in terms of slip dislocations. Recent comprehensive treatments of phase transitions in organic molecular crystals are...

Triplet excitons as probes for structural imperfections in crystalline anthracene

Abstract The phosphorescene and delayed fluorescence associated with triplet exciton trapping in crystalline anthracene have been used to demonstrate the physical nature of many of the traps and to determine their depth.

The characterisation of carbon-fibre-paper-supported platinum and platinum-ruthenium catalysts using temperature-programmed reduction and cyclic voltammetry

Temperature-programmed reduction (TPR) and cyclic voltammetry have been used to investigate Pt and PtRu dispersed catalysts unsupported and supported on conducting carbon at various stages of the catalyst preparation. Differences in the TPR profiles and the corresponding uptake of H2 by unsupported and supported Pt(NH3)2(NO2)2 dried from HNO3 solution at 120 °C are attributed to an interaction via ligand exchange between the nitrato-substituted Pt complex and nucleophilic surface groups formed on the carbon support by the action of HNO3. A similar interaction was observed for carbon-supported Ru salts prepared from RuNO(NO3)x. Support interactions present after the drying stage led to an enhanced catalyst dispersion and to the elimination of the original interaction after activation in air at 300 °C. Bimetallic PtRu catalysts display reducibilities in TPR intermediate between those observed for the pure components in both unsupported and supported forms. The influence of the carbon support is absent after activation in air. At 300 °C, activation produces a high bimetallic dispersion (90 m2 g−1) whereas activation at 400 °C leads to more perfect alloy formation and consequently to an increased intrinsic activity as an electrocatalyst for methanol oxidation at the expense of a reduced dispersion (54 m2 g−1). Cathodic processes at 0.1–0.4 V (vs RHE) during the initial linear potential sweep, corresponding to oxide reduction, are observed for air-activated Pt and PtRu catalysts. Electrochemical reduction of oxide species in this way is analogous to the characteristic processes observed in TPR except that initial cathodic reduction extends ca.4 atomic layers into the oxide particles. Our conclusions concerning air-activated Pt catalysts, largely composed of PtO2, are in essential agreement with similar studies of oxide layers formed electrochemically on smooth Pt electrodes.

The characterisation of catalyst surfaces by cyclic voltammetry

Abstract Cyclic voltammetry as a technique for the characterisation of catalyst surfaces is reviewed. Comparisons are made between this technique and its gas phase analogues — the combined techniques of temperature programmed reduction, oxidation and desorption. Similarities between results obtained on catalysts independently by the different techniques are highlighted. The characterisation of bulk and supported metal and alloy catalysts in relation to surface enrichment, segregation of particular metals at alloy surfaces and surface area (particle size and metal-support interactions) effects are discussed. Cyclic voltammetry is readily applied to the detection of catalyst poisoning by species such as S, CO, electrolyte degradation products and other impurities. The modes of action of certain promoters for Pt catalysts may also be classified employing this technique. Similarly, the characterisation of surface groups on carbon supports by cyclic voltammetry complements the more conventional spectroscopic techniques such as X-ray photoelectron spectroscopy and infra-red methods. It is concluded that the technique may find applicability outwith the electrocatalysis field particularly in the characterisation of semiconducting, heterogeneous catalysts.

Effects of Crystal Growth Environment on Defect Concentrations in Anthracene Crystals

Abstract An efficient yet inexpensive crystal growing furnace has been constructed. With this furnace crystals have been grown with helium, argon, nitrogen or xenon as the inert gas heat exchanger. The nature of this inert gas affects the concentration of defects within the bulk of the crystal. Thus, though several crystal parameters are invariant under these conditions, this is not always the case, particularly at low temperatures. A low pressure of helium is found to have the least effect upon these crystal parameters.

Epitaxial growth of ZnSe on (100) GaAs by open-tube transport of elemental vapours in H2 flows

Abstract Using the technique of open tube transport of elemental vapours in hydrogen carrier flows, hetero epitaxial layers of ZnSe have been grown on (100)GaAs substrates. Crystalline layers having the zinc blende structure and ranging in thickness from 2 to 30μm have been produced which are slightly lattice mismatched ca. 0.2% with respect to the substrate. Provided the stoichiometry of Zn and Se in the vapour phase above the substrate is maintained at 1 : 1 the resulting layers exhibit blue photoluminescence. Deviations from this 1 : 1 stoichiometry in favour of Zn and Se result in deep level emission at 555 nm (green) and 620 nm (yellow) respectively. The growth technique is applicable to a wide range of II–VI materials.

Electron microscopic studies of extended defects in organic molecular crystals. Part 1.—p-Terphenyl

Transmission electron microscopy has been utilized to reveal directly the nature of the dislocations that occur in solution- and vapour-grown thin crystals of p-terphenyl. The identity of the linear defects was established chiefly by diffraction contrast using bright-field images, but it also proved possible to employ, to a limited degree, real-space crystallography. Many of the slip systems deduced to be present in simple aromatic solids by dislocation-etching and topographical studies have been more fully characterized in this study {e.g. (001)[010], (001)[110], (100)[010] and (100)[001]}. Of special interest is the identification of the hitherto undetected slip system (001)[12w](with w probably zero). At the cores of such dislocations, in aromatic solids crystallizing in the P21/a space-group, molecules within the structure adopt a space-symmetry which indicates that incipient dimers occur recurrently. This is relevant in the solid-state chemical behaviour of anthracene since such dislocations can rationally account both for the production of topochemically forbidden dianthracene upon u.v. irradiation of the cyrstalline monomer and te trapping of triplet excitons in single-crystals of anthracene.