David G. Nicholson

In situ EXAFS study of the bimetallic interaction in a rhenium-promoted alumina-supported cobalt Fischer-Tropsch catalyst

The local environments about the rhenium atoms in a Co–Re/γ-Al2O3 catalyst after different reduction periods have been studied by X-ray absorption spectroscopy (EXAFS). The bimetallic catalyst containing 4.6 wt% cobalt and 2 wt% rhenium has been compared with a corresponding monometallic sample with 2 wt% rhenium on the same support. The rhenium LIII EXAFS analysis shows that bimetallic particles are formed after reduction at 450u2009○C with the average particle size being less than 15 Å. More than 6 h reduction at 450u2009○C is required for complete reduction of accessible rhenium.

EXAFS study of zinc coordination in bacitracin A

Bacitracin is a dodecapeptide antibiotic produced by Bacillus sp. The antibacterial activity depends upon the peptide binding a divalent metal. Hitherto, the exact coordination of the cation has not been established. In particular the role played by the sulphur and nitrogen atoms of the thiazoline ring of bacitracin A has not been clear. Here the coordination of Zn2+ by bacitracin A has been studied using extended X-ray absorption fine structure. The experimental data are consistent with a model in which zinc is coordinated by one oxygen and three nitrogen atoms with the sulphur atom of the thiazoline ring not being directly involved in the zinc coordination.

Structure of the diamminecopper(I) ion in solution. An X-ray absorption spectroscopic study

X-Ray absorption spectroscopy (XAS) using synchrotron radiation has been used to probe the structure of copper (I) species in an aqueous ammonia solution that contains hydrazine to prevent any oxidation to copper (II) species. Combining a characteristic copper (I) pre-edge feature with the extended X-ray absorption fine structure (EXAFS) of the XAS enabled the stereochemistry and bond distances to be extracted for the predominant species, which is shown to be the diamminecopper (I) ion. The structure is linear with Cu—N bond lengths of 1.88 ± 0.02 A.

X-Ray absorption spectroscopic study at the cobalt K-edge on the calcination and reduction of the microporous cobalt silicoaluminophosphate catalyst CoSAPO-34

The local environments about the cobalt atoms in as-synthesised, calcined and hydrogen-reduced CoSAPO-34 have been studied by X-ray absorption spectroscopy. Calcination of the as-synthesised material and reduction in hydrogen of the product of calcination were carried out at 450 °C and for 6 h, respectively. The results show that cobalt is sited as CoII in the framework and that calcining CoSAPO-34 with 4.4% Co leads to two-thirds of the CoII content being oxidised to CoIII thereby lending support to the view that the material is a redox catalyst. Reducing the calcined material with hydrogen leads to a material containing only CoII. Taken together, the pre-edge, XANES and EXAFS regions indicate that one-third of the original tetrahedrally coordinated divalent cobalt in the as-synthesised product is maintained in the calcined and reduced products with the remaining cobalt being in the form of trivalent hexacoordinated and divalent hexacoordinated cobalt, respectively.

Studies on the pre-edge region of the X-ray absorption spectra of copper(I) oxide and the diamminecopper(I) ion

The distinctive pre-edge feature in the X-ray absorption spectra of copper(I) oxide and the diamminecopper(I) ion at the copper K-edge has been studied using synchrotron radiation. The relative intensities of these features, which are characteristic of copper(I) compounds, have been measured via the peak areas. The intensity of the oxide peak, with regard to the diammine and other two-coordinate copper environments, is anomalous, being reduced by ca. 60%. This is attributed to the macromolecular nature of the oxide, and the formation of intense excitons. Effects due to the small degree of non-stoichiometry may play and additional part in modifying pre-edge intensity by influencing the exciton lifetime.

Simulation study of nitrogen adsorption in parallel-sided micropores with corrugated potential functions

The adsorption of 12–6 nitrogen in graphite micropores at 77.4 k has been investigated using grand ensemble computer simulation. The pores were modelled as the space between parallel basal planes. Three pore sizes, with internal widths of 2.7 σ, 3.1 σ and 3.6 σ were chosen, and the graphite adsorbent potential was corrugated. The resulting adsorption isotherms are type I in the two smaller pores, but type IV in the largest pore. Hysteresis was observed in the intermediate sized pore but not in the other two. The results are discussed and interpreted in terms of singlet distribution functions from the simulations.

Supported metallocene catalysts—interactions of (n-BuCp)2HfCl2 with methylaluminoxane and silica

Abstract The species on supported olefin polymerisation catalysts consisting of ( n -BuCp) 2 HfCl 2 , methylaluminoxane (MAO) and dehydroxylated silica were identified by EXAFS and UV-Vis spectroscopy. Whereas the reaction of ( n -BuCp) 2 HfCl 2 with silica leads to a product containing Hf⋯O and Hf⋯Si non-bonded interactions with concurrent loss of Hfue5f8Cl bonds, the reaction of ( n -BuCp) 2 HfCl 2 with silica pretreated with methylaluminoxane yields a mixture of several hafnocene species. The bonding features of ( n -BuCp) 2 HfCl 2 and ( n -BuCp) 2 HfCl 2 /SiO 2 are still present to some extent but with new interactions consistent with hafnocene cation formation. The relative proportions of these species depend strongly on the method of the catalyst preparation.

Stereochemical activity of lone pairs. The crystal and molecular structure of a complex of 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) with bismuth(III) chloride

Abstract The crystal structure of a complex formed between BiCl3 and 18-crown-6(1,4,7,10,13,16-hexaoxacyclooctadecane, L) has been determined from X-ray diffractometer data by Patterson and Fourier methods. Crystals are monoclinic, space group Pa, with Z = 2 in a unit cell of dimensions a = 15.09(1), b = 16.60(1), c = 7.987(8) A, β = 103.0(1)°. The structure, which refined to R = 0.058 for 2766 observed reflections, consists of discrete BiCl3·H2O·L (1) and BiCl3L (2) molecules. In 2 bismuth is sandwiched between two essentially parallel planes containing six oxygens and three chlorines, respectively, and in 1 between three ether and a water oxygen and three chlorines, respectively. In 1 there is intramolecular hydrogen bonding between the water molecule and two of the two ether oxygen atoms not bonded to the metal.

Selective catalytic reduction of NOx over microporous CuAPO-5: structural characterisation by XAS and XRD

The effects of copper source, copper content, Al : P ratio and silicon content for the hydrothermal synthesis of CuAPO-5 using tetraethylammonium hydroxide as template have been investigated. The copper to phosphorus ratio in the synthesis gel was varied from 0.04 to 0.2, and copper(II) oxide and copper(II) acetate were used as copper sources. The samples were characterised by XAS (XANES, EXAFS), XRD (X-ray diffraction), TGA (thermogravimetric analysis), SEM (scanning electron microscopy) and elemental analysis. Rietveld refinements of the structure of CuAPO-5 were carried out on synchrotron X-ray powder diffraction data. The catalytic properties of CuAPO-5 in selectively reducing NOx in the presence of hydrocarbons (SCR-HC) were studied. The results from EXAFS show that copper is present in a distorted octahedral environment, with 4 or 5 Cu–O distances at 1.95 A and 1 or 2 Cu–Al/P distances at about 3.14 A for the as-synthesised samples. The calcined samples still show the same four Cu–O distances at 1.95 A, but with the composite peak at 3.14 A being significantly reduced. The turquoise colour of the as-synthesised CuAPO-5 is consistent with the copper atoms being complexed by water molecules within the pores as the hexaaquacopper(II) complex. The extraframework complex cation is electrostatically bound to the anionic phosphate sites of the framework. The colour changes to olive green on calcination, indicating tetrahedrally coordinated framework copper. The selective catalytic reduction of NOx by propene is catalysed by CuAPO-5 to the extent of 18% conversion. The Rietveld analysis show that calcined CuAPO-5 belongs to the P6/mcc spacegroup in which Al and P are indistinguishable.

An X-ray absorption spectroscopic study on the local environment of copper in CuAPO-5

A copper-containing AlPO4-5 was synthesised using copper(II) oxide as the copper source and tetraethylammonium hydroxide as the template. The local environment about copper in the as-synthesised, calcined and hydrogen-reduced materials has been studied using XANES and EXAFS. The results for the as-synthesised product show that copper is sited within the AlPO4-5 framework and that its local environment is a tetragonally distorted Cu(O)6 octahedron in which the axial bonds are possibly to hydroxy groups. On calcining at 550u2006°C the environment changes to a non-tetragonally distorted octahedron for which the coordination to the framework is increased by two long bonds to framework oxygens. Copper(II) in CuAPO-5 is reduced by 5% H2 in He to copper particles of dimensions >40xa0A. The catalytic properties of two CuAPO-5 samples (derived from copper(II) oxide and, for comparison, from copper(II) acetate) in the selective catalytic reduction of NO in the presence of NH3 and O2 have been investigated, showing maximum conversions of 27% at 440u2006°C and 53% at 340u2006°C, respectively.