Andrew J. Dent

An EXAFS study of uranyl ion in solution and sorbed onto silica and montmorillonite clay colloids

The mechanisms of sorption which control the uptake of ionic species from solution at mineral surfaces are of fundamental importance in colloid science, but are poorly understood because of a lack of knowledge of the structure of the surface species that form. EXAFS spectroscopy provides a means of studying the structure of sorbed species directly. The technique has been applied here to investigate uranyl species in solution and sorbed onto a montmorillonite clay and silica colloids. In solution at low pH (<3) the X-ray absorption fine structure (EXAFS) data are consistent with a monomeric aquo- complex containing five or six aquo- ligands (U-Oeq) in the equatorial plane of the uranyl ion. At higher pH (4–5) and degree of hydrolysis a reduction in EXAFS amplitude of the equatorial oxygen shell is observed and can be ascribed to increased structural disorder. A uranium neighbor shell at 3.82 A is observed for a 0.01 mole dm−3 solution at pH 4.3, confirming the formation of polynuclear species. EXAFS indicates that the structure of the uranyl ion is retained on sorption at a clay/water and an oxide/water interface. The EXAFS of uranyl species sorbed onto montmorillonite at pH 5 closely resembles those in hydrolyzed solutions and are consistent with the uptake of solution species by an outer-sphere, ion-exchange mechanism. The EXAFS of uranyl species sorbed onto silica from solutions at pH 3 and 5 are similar, but quite different from those sorbed on the clay, and resemble those of the uranyl hydrous oxide.

Development of Time-Resolved XAFS Instrumentation for Quick EXAFS and Energy-Dispersive EXAFS Measurements on Catalyst Systems

From its very earliest days, at which time measuring a spectrum took at least 1 h, XAFS has been used for catalysis studies. In more recent times technological advances have improved the time resolution to minutes and seconds with quick EXAFS, and subsecond to millisecond with energy-dispersive EXAFS. This paper describes the instrumentation necessary and elucidates with specific examples where this has been used. In particular the Daresbury QEXAFS monochromator will be shown and several examples of combined XAFS and diffraction, a powerful technique for in situ studies. The development of energy dispersive EXAFS at Daresbury will be described. Recent results on in situ Rh catalysis combining EDE and mass spec measurements at the ESRF are highlighted, and finally the details of the latest microstrip detector that will enable continuous measurements at 10 μs time intervals is presented.

Hydrotalcite-derived mixed oxides containing copper: catalysts for the removal of nitric oxide

Hydrotalcite structures containing copper, magnesium and aluminium in the metal hydroxide layers have been investigated for their potential as precursors to the formation of catalysts for the decomposition and reduction of nitrogen oxides. These CuIIMgIIAlIII hydrotalcites are catalytically active towards both the decomposition of nitric oxide and its reduction in the presence of an appropriate coreductant, such as propane. The copper centred active species, identified using EXAFS spectroscopy and combined EXAFS/XRD under operating conditions, have been found to be CuI for the decomposition and Cu0 for the reduction.

Energy dispersive XAFS: characterization of electronically excited states of copper(I) complexes.

Energy dispersive X-ray absorption spectroscopy (ED-XAS), in which the whole XAS spectrum is acquired simultaneously, has been applied to reduce the real-time for acquisition of spectra of photoinduced excited states by using a germanium microstrip detector gated around one X-ray bunch of the ESRF (100 ps). Cu K-edge XAS was used to investigate the MLCT states of [Cu(dmp)2]+ (dmp =2,9-dimethyl-1,10-phenanthroline) and [Cu(dbtmp)2]+ (dbtmp =2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) with the excited states created by excitation at 450 nm (10 Hz). The decay of the longer lived complex with bulky ligands, was monitored for up to 100 ns. DFT calculations of the longer lived MLCT excited state of [Cu(dbp)2]+ (dbp =2,9-di-n-butyl-1,10-phenanthroline) with the bulkier diimine ligands, indicated that the excited state behaves as a Jahn–Teller distorted Cu(II) site, with the interligand dihedral angle changing from 83 to 60° as the tetrahedral coordination geometry flattens and a reduction in the Cu–N distance of 0.03 Å.

Rapid phase fluxionality as the determining factor in activity and selectivity of highly dispersed, Rh/Al2O3 in deNOx catalysis.

The nature of oxide-supported metal catalysts may change in oxidising conditions: Studies on the correlation between the Rh phase in the structure of the Rh/AL2O3 catalyst and the catalytic performance for the reduction of NO by H2 to N2 (on reduced, metallic sites) and N2O (on oxidised sites) reveal that the phases of the supported metal species can be interconverted on time scales that can be deterministic in temrs of the activity and selectivity of the catalysts.

Synchrotron-related studies on the dynamic and structural aspects of zirconia synthesis for ceramic and catalytic applications

Abstract Zirconia is an important component material for several industrial applications. In these studies we have used intense radiation sources, primarily synchrotron radiation, to shed new light on the properties of zirconia and its synthesis via existing industrial routes. Certain “static” techniques have been used to probe the atomic structure of precursor compounds used in the synthesis of zirconia, whereas time-resolved techniques were required to dynamically capture the various calcination stages during the heating section of the synthesis cycle. Finally the important tetragonal to monoclinic zirconia transformation during cooling was studied in detail using time resolved energy dispersive diffraction. Altogether what emerges is a new unfolding story on the various stages of zirconia synthesis, in which synchrotron radiation has played a vital role.

Identification of the surface species responsible for N2O formation from the chemisorption of NO on Rh/alumina.

Energy dispersive EXAFS (EDE) and diffuse reflectance infrared spectroscopy (DRIFTS) are combined synchronously at high time resolution (17 Hz) to probe how NO(g) reacts with gamma-Al(2)O(3) supported, metallic Rh nanoparticles of an average 11 A diameter; a bent nitrosyl species is considered to be the key to the formation of N(2)O.

Application of stopped flow techniques and energy dispersive EXAFS for investigation of the reactions of transition metal complexes in solution: Activation of nickel β-diketonates to form homogeneous catalysts, electron transfer reactions involving iron(III) and oxidative addition to iridium(I)

Stopped-flow techniques of rapid mixing have been combined with energy dispersive X-ray absorption spectroscopy to monitor the reaction of Ni(dpm)2 [dpm = Bu1C(O)CHC(O)Bu1] by aluminium alkyls (AlEt2X, X = OEt and Et) to form the active species for the catalytic di- and tri-merisation of hex-1-ene. Acquisition times down to ca. 30 ms were achieved on Station 9.3 of the SRS using a photodiode array detector. The EXAFS features of the resulting solution complexes are of the form [Ni(O-O)R)(alkene)]. In the presence of PPh3, [Ni(O-O)(R)(PPh3) appears to be the redominant type of species. The reduction of aqueous Fe(III) by hydroquinone was investigated on ID24 at the ESRF by Fe K-edge energy dispersive EXAFS with a CCD camera as detector, spectra were obtained in 1 ms or longer. No intermediate inner sphere complex was detected prior to the formation of aqueous Fe(II). Finally the oxidative addition of CH3SO3CF3 to [IrI2(CO)2]- was monitored on Station 9.3 with a silicon microstrip detector. A single acquisition of 400 micros was feasible, with spectra recorded in multiples of 1.2 ms. In that time, the first stage of the reaction had been completed, with a slower stage thereafter. The results are consistent with the two-stage ionic oxidative addition mechanism.

In situ study of ceramic formation from Co2+ and Zn2+ exchanged zeolite-A using combined XRD/XAFS techniques

Abstract The production of a cobalt spinel (Co3−xAlxO4(x = 0,1,2)) based ceramic composite from the heat induced collapse of cobalt exchanged sodium zeolite-A (Co/Na zeolite-A) has been studied both in situ and ex situ using synchrotron radiation XRD and XAFS techniques. The precipitation of this phase is accompanied by the transitory presence of nepheline (NaAlSiO4) indicating the intermediate behaviour of this system between that of Na zeolite-A, which fully precipitates nepheline on heating, and Zn/Na zeolite-A which does not exhibit any nepheline production over the temperature range studied.

The structure of halogenometallate complexes dissolved in both basic and acidic room-temperature halogenoaluminate(III) ionic liquids, as determined by EXAFS

The salts [MeEtim]2[MCl4]([MeEtim]+= 1-methyl-3-ethylimidazolium; M = Mn, Co, or Ni) are shown by EXAFS to dissolve in basic [MeEtim]Cl–AlCl3 ionic liquids to yield four-co-ordinate [MCl4]2–, and to dissolve in acidic [MeEtim]Cl–AlCl3 ionic liquids to yield six-co-ordinate [M(AlCl4)3]– anions.