Extended X-ray Absorption Fine Structure Results Research Articles

Mixed (Oxygen Ion and n‐Type) Conductivity and Structural Characterization of Titania‐Doped Stabilized Tetragonal Zirconia

By using X‐ray diffraction lattice parameter measurements and Raman spectroscopy studies, both the solid solubility limit of titania in yttria tetragonal zirconia polycrystalline solid solutions and the tetragonal solid solution field in the system have been established. Valence state, site symmetry, and changes in local structures of Ti ions in Y‐TZP with 5 and 10 mol % are studied for the first time using EXAFS (extended X‐ray absorption fine structure), XANES (X‐ray absorption near edge structure), and X‐ray photoelectron spectroscopy. The total electrical conductivity in air of the tetragonal solid solution decreases with increasing titania content. XANES results show that as the dissolves into the tetragonal zirconia Y‐TZP matrix, a displacement of ions from the center of symmetry seem to take place which leads to a nonrandom substitution of ions on lattice sites. Ti‐O bond distances derived from EXAFS results indicate that ion can be in a square‐pyramidal arrangement, i.e., fivefold oxygen‐coordinated. As a consequence, two kind of cation‐oxygen vacancy associations ( and ) with different diffusion dynamics are created. This results in a decrease of the global concentration of moving oxygen vacancies and therefore, a decrease of ionic conductivity. Electronic conductivity, n‐type, only appeared at oxygen partial pressure lower than atm and above 800°C in Y‐TZP containing 10 mol % titania. Such an n‐type electronic conduction was attributed to a hopping of electrons between and cations by a small polaron hopping mechanism. © 1999 The Electrochemical Society. All rights reserved.

EXAFS characterization of bimetallic Pt-Pd/SiO2-Al2O3 catalysts for hydrogenation of aromatics in diesel fuel

Bimetallic Pt–Pd/SiO2–Al2O3 catalysts exhibited much higher activities in aromatic hydrogenation of distillates than monometallic Pt/SiO2–Al2O3 and Pd/SiO2–Al2O3 catalysts. The studies of extended X‐ray absorption fine structure (EXAFS) indicated that there was an interaction between Pt and Pd in the Pt–Pd/ SiO2–Al2O3 catalyst. Furthermore, from the EXAFS, it was assumed that the active metal particle on the Pt–Pd/SiO2–Al2O3 catalysts is composed of the “Pd dispersed on Pt particle” structure. Regarding both the activities of aromatic hydrogenation and the EXAFS results, it was concluded that the Pd species dispersed on Pt particles were responsible for the high activity of the bimetallic Pt–Pd/SiO2–Al2O3 catalysts.

EXAFS studies of Cu + ion conducting and semiconducting copper chalcogenide and chalcohalide glasses

The short-range order in semiconducting xCu 2Se · (1 − x)As 2Se 3 and mixed conducting xCuI · (1 − x)As 2Se 3 glasses has been investigated using Extended X-ray Absorption Fine Structure (EXAFS) measurements at the Cu, As and Se K-edges. A non-tetrahedral local coordination of copper, either by Se or by Se and I, has been found in these glassy systems. The pyramidal AsSe 3 units remain essentially intact on introducing Cu 2Se or CuI. However, the average selenium coordination number, N Se–Y, where Y=As or Cu, increases with x from 2.0 to 2.5–2.6. Simple stoichiometric relationships account for the observed changes in N Se–Y for the Cu 2Se–As 2Se 3 glasses, which suggest that, at a nearest neighbour level, the average selenium environment consists of two relatively weakly interacting components coming from vitreous As 2Se 3 and a hypothetical glassy Cu 2Se. On the contrary, the EXAFS results indicate that CuI is dispersed in the glass network on a molecular level and interacts with the host matrix. Favourable conductivity pathways, associated with CuI, are suggested to appear in the glass structure, which explain the improved Cu + ion conduction and copper diffusion.

Surface contamination of single-crystal PuSb

Pu M-edge X-ray absorption experiments were performed on a single crystal of PuSb in order to understand incongruous X-ray resonant exchange scattering (XRES) results from the same material. Analysis of the extended X-ray absorption fine structure (EXAFS) reveals that there is an amorphous or polycrystalline surface phase on the PuSb. The Pu at the surface of the crystal is coordinated primarily to a first-row main-group element, probably oxygen. In addition, the surface may be slightly deficient in antimony. These EXAFS results are sufficient to explain the absence of an XRES response in this PuSb sample.

The position of the `one-eighth' anomaly within the high-temperature superconductivity problem: matters concerning stripes and domains, Jahn-Teller effect and negative- U behaviour

The subject of mixed-valence charge segregation in HTSC materials is further examined. The self-organized microstructural details are explored including 1D/2D crossover. Neutron and extended x-ray absorption fine-structure results provide the basis for this analysis. The Jahn-Teller effect is seen as a crucial element in the `charge/spin'-separation structuring. On cooling, the emergent resonating-valence-bond-type magnetic organization of the divalent subsystem assists greatly in preparing (if not pre-pairing) the overall system for high-temperature superconductivity. Under the action of the negative-U centres of the high-valence subsystem established in the domain boundaries, the system can evolve naturally with doping and cooling from towards greater s-wave-type content of the HTSC order parameter.

Structural defects and electrochemical reactivity of β-Ni(OH) 2

Electrochemical reactivities and structural properties of several nickel hydroxide powders were analysed by X-ray diffraction, Raman spectroscopy and extended X-ray absorption fine structure (EXAFS). It is shown that the electrochemical efficiency of β-Ni(OH) 2 is associated with the amount of proton vacancies included in the crystal lattice. The number of those proton vacancies increases when the crystallite size decreases or when the ratio of co-precipitated cobalt increases. Proton vacancies shift the oxidation potential of β-Ni(OH) 2 towards less anodic values and, therefore, improve the chargeability and the electrochemical efficiency of nickel hydroxide. It is shown that both Raman spectroscopy and X-ray diffraction techniques can be used to predict effectively the electrochemical efficiency of β-Ni(OH) 2 hydroxide. EXAFS results indicate also that the oxidation level of nickel atoms inside the hydroxide is not modified by the existence of proton vacancies. It means probably that to maintain the electroneutrality in the whole crystal induces others singularities. Finally, the influence of co-precipitated additives such as cadmium and cobalt on the rate of defects has been investigated.

MgO-Supported Platinum–Tungsten Catalysts Prepared from Organometallic Precursors: Platinum Clusters Isolated on Dispersed Tungsten

MgO-supported Pt and Pt–W catalysts were prepared from organometallic precursors and characterized to determine how the nature of the precursors and the treatment conditions affected the Pt–W interactions and the catalytic activity for toluene hydrogenation. Samples were prepared from {Pt[W(CO)3(C5H5)]2(PhCN)2} (Ph=phenyl) and from [PtCl2(PhCN)2]+[W(CO)6] and characterized by infrared spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, transmission electron microscopy (TEM), and chemisorption of H2, CO, and O2. The samples were treated in H2at 400°C prior to most of the characterizations. Incorporation of W reduced the chemisorption of CO and of H2, with the reduction in chemisorption being greater for the catalyst prepared from {Pt[W(CO)3(C5H5)]2(PhCN)2} than for the catalyst prepared from the combination of monometallic precursors. EXAFS spectra measured at both the PtLIIedge and the WLIIIedge showed substantial Pt–W contributions for the former catalyst (but not the latter), with a Pt–W coordination number of about 2 and an average Pt–W distance of 2.71 Å. TEM and EXAFS results indicate that interactions between Pt and W cations in the sample made from {Pt[W(CO)3(C5H5)]2(PhCN)2} maintained the Pt in a highly dispersed form, with supported Pt clusters being smaller than about 10 Å. The catalyst prepared from the bimetallic cluster was more than an order of magnitude less active for toluene hydrogenation at 1 atm and 60°C than the catalyst prepared from the two monometallic precursors; the comparison indicates that the W acted like an isolated dispersed support, with its influence on the catalysis maximized by its proximity to the Pt.

Wide angle X-ray scattering (WAXS) study of “two-line” ferrihydrite structure: Effect of arsenate sorption and counterion variation and comparison with EXAFS results

Wide angle X-ray scattering (WAXS) measurements have been made on a suite of “two-line” ferrihydrite (FHY2) samples containing varying amounts of coprecipitated arsenate. Samples prepared at pH 8 with counter ions chloride, nitrate, and a mixture of both also were examined. The raw WAXS scattering functions show that “two-line” ferrihydrite actually has a large number of non-Bragg (i.e., diffuse scattering) maxima up to our observation limit of 16 Å −1. The type of counter ion used during synthesis produces no significant change in this function. In unarsenated samples, Radial Distribution Functions (RDFs) produced from the scattering functions show a well-defined Fe-O peak at 2.02 Å in excellent agreement with the mean distance of 2.01 Å from extended X-ray absorption fine structure (EXAFS) analysis. The area under the Fe-O peak is consistent with only octahedral oxygen coordination about iron, and an iron coordination about oxygen of 2.2, in agreement with the EXAFS results, the sample composition, and XANES measurements. The second peak observed in the RDFs is clearly divided into two populations of correlations, at 3.07 and 3.52 Å, respectively. These distances are close to the EXAFS-derived Fe-Fe subshell distances of 3.02–3.05 and 3.43–3.46 Å, respectively, though this is misleading as the RDF peaks also include contributions from O-Fe and O-O correlations. Simulated RDFs of the FeOOH polymorphs indicate how the observed RDF structure relates to the EXAFS pair-correlation function, and allow comparisons with an ordered ferrihydrite structure. The effect of increasing arsenate content is dramatic, as the RDF peaks are progressively smeared out, indicating a wider range of interatomic distances even at moderate surface coverages, and a loss of longer range correlations. At an As/Fe ratio of 0.68, the surface saturation level of arsenate, the RDF shows little order beyond what would be expected from small pieces of dioctahedral Fe oxyhydroxyl chains or small “sheet” units. Analysis of the first RDF peak yields components due to As-O and Fe-O correlations. As the As-O component at 1.67 Å increases in size, the Fe-O component decreases, reflecting a decrease in Fe coordination about the average oxygen. This reduction is consistent with a decrease in mean crystallite size as suggested by EXAFS studies. Analysis of the second RDF peak components shows the progressive decrease in Fe-Fe correlations, and the enhancement of As-Fe correlations, as arsenate level increases. Comparison of the experimental RDF from coprecipitated arsenate-saturated FHY2 with simulated RDFs of model iron oxyhydroxyl structures further constrains possible sizes and geometry for the precipitates, and is consistent with sorbed complexes of the bidentate binuclear (apical oxygen sharing) type.

An EXAFS study on oxidic and sulfided K-MoO3/?-Al2O3 catalysts

By analyzing the extended X-ray absorption fine structure (EXAFS) of the Mo K-absorption edge, structural information for both oxidic and sulfided K-MoO3/γ-Al2O3 catalysts with different potassium content was obtained. The oxidic samples show two backscatterer peaks in the radial distribution function (RDF), which correspond to the Mo-O coordinations in the nearest Mo-O shell. The nearest oxygen atoms are present with large configurational disorder. The RDF for the K/Mo = 0 sample is significantly different from that for crystalline MoO3 and ammonium heptamolybdate. The RDFs for potassium promoted samples are, in some extent, similar to that for ammonium heptamolybdate. The sample with K/Mo = 0.8 and that with K/Mo=1.5 do not show obvious difference in their local Mo-O structures. The EXAFS results support the earlier conclusions from Raman spectroscopy studies on identical samples [7]. When the samples are sulfided, a rearrangement of the local neighbors around Mo atoms takes place, to form small MoS2-like crystallites. The Mo-S and Mo-Mo coordination distances on these catalysts are the same as those in crystalline MoS2, but the coordination numbers are significantly lower than in MoS2. The EXAFS results indicate that Mo species on the K/Mo=0 catalyst mainly consist of Mo-S-Mo units (the basic building units of MoS2), which are highly dispersed and show a higher level of disorder than in MoS2. With the modification by the potassium promoter, Mo species are significantly aggregated and their local neighbors are more similar to those in MoS2, but the Mo species still exist in a state of high dispersion.

Structural Relaxation around Substitutional Cr3+ Ions in Sapphire

The local structural arrangement of ions around a substitutional Cr3+ ion in sapphire (ruby) has been studied experimentally using extended X‐ray absorption fine structure (EXAFS) in the vicinity of the Cr absorption edge. The findings are compared with an ionic model (Mott‐Littleton) computation using two sets of pairwise potentials. Both the EXAFS results and the computations reveal that when Cr substitutes for an Al ion in sapphire, the surrounding ions relax to an arrangement similar to that for Cr in α‐Cr2O3. Also, most of the structural relaxation is accommodated by the first shell of oxygen and aluminum ions around the substituted Cr3+ ion. The computations also indicate that with applied pressure (tensile and compressive) the ionic positions change self‐similarly and in proportion to the macroscopic strain.

Dinitrogen Cleavage by Three-Coordinate Molybdenum(III) Complexes: Mechanistic and Structural Data1

The synthesis and characterization of the complexes Mo[N(R)Ar]3 (R = C(CD3)2CH3, Ar = 3,5-C6H3Me2), (μ-N2){Mo[N(R)Ar]3}2, (μ-15N2){Mo[N(R)Ar]3}2, NMo[N(R)Ar]3, 15NMo[N(R)Ar]3, Mo[N(t-Bu)Ph]3, (μ-N2){Mo[N(t-Bu)Ph]3}2, and NMo[N(t-Bu)Ph]3 are described. Temperature-dependent magnetic susceptibility data indicate a quartet ground state for Mo[N(R)Ar]3. Single-crystal X-ray diffraction studies for Mo[N(R)Ar]3 and NMo[N(t-Bu)Ph]3 are described. Extended X-ray absorption fine structure (EXAFS) structural studies for Mo[N(R)Ar]3, (μ-N2){Mo[N(R)Ar]3}2, and NMo[N(R)Ar]3 are reported. Temperature-dependent kinetic data are given for the unimolecular fragmentation of (μ-N2){Mo[N(R)Ar]3}2 to 2 equiv of NMo[N(R)Ar]3 and for the fragmentation of (μ-15N2){Mo[N(R)Ar]3}2 to 2 equiv of 15NMo[N(R)Ar]3. The temperature dependence of the 15N2 isotope effect for the latter N2 cleavage process was fitted to a simple harmonic model, leading to a prediction for the difference in NN stretching frequencies for the two isotopomers. The latter prediction was consistent with the Raman spectroscopic data for (μ-N2){Mo[N(R)Ar]3}2 and (μ-15N2){Mo[N(R)Ar]3}2. The Raman spectroscopic data and EXAFS results are both consistent with an NN bond order of approximately 2 in (μ-N2){Mo[N(R)Ar]3}2. Temperature-dependent magnetic susceptibility data consistent with a triplet ground state are given for (μ-N2){Mo[N(t-Bu)Ph]3}2.

X-Ray Absorption Spectroscopy of Cobalt(II) Multinuclear Surface Complexes and Surface Precipitates on Kaolinite

Abstract We have examined the local atomic structure of multinuclear complexes and surface precipitates of Co(II) sorbed to kaolinite (Al2Si2O5(OH)4), a common clay mineral, using extended X-ray absorption fine structure (EXAFS) spectroscopy. Structural information from quantitative analysis of EXAFS spectra is used to examine differences, on a molecular level, among Co species sorbed on kaolinite at different coverages from solutions under- and over-saturated with respect to solid Co(OH)2 phases, and between sorbed species and Co hydroxide solids precipitated from solutions in the absence of kaolinite. The EXAFS results indicate formation of oxide- or hydroxide-bridged multinuclear surface complexes on kaolinite at surface coverages well below a statistical monolayer from undersaturated solutions. As Co surface loading on kaolinite is increased, the number of Co second-neighbor atoms at 3.10-3.12 A increases from ≈2 to 6, the number of second-neighbor Co backscatterers in crystalline Co (OH)2(s), and interatomic distances determined by EXAFS remain constant. These observations suggest that hydroxide-like multinuclear complexes form at low surface coverages in addition to monomeric surface complexes, perhaps as precursors to surface precipitation. At high surface coverages and apparent solubilities (i.e., solution ion activity products) both below and above saturation with respect to stable Co(OH)2(s), the spectra of the sorption products differ from the spectra of Co solids precipitated in the absence of kaolinite, including dry, crystalline Co(OH)2(s) (pink form) and wet, freshly precipitated Co(OH)2(s) (blue and pink forms). Compared to the precipitates, sorption samples have Co-Co distances shorter by 0.050.07 (±0.02) A, lack small beat patterns in normalized spectra, and have reduced backscattering amplitudes from multiple scattering among Co atoms at ≈ 3 and 6 A. Long-term equilibration of oversaturated sorption samples (21 or 45 days) resulted in a decrease in backscattering amplitudes for both single- and multiple-scattering paths among neighboring Co atoms relative to short-term (24-36 h) equilibration. These differences are attributed to a decrease in the average number of Co atoms in a hydroxide-like complex and/or to an increase in disorder in the local atomic structure. This is not expected in typical aging of a freshly precipitated solid phase, where average particle size and degree of long-range order normally increase with time, suggesting that kaolinite influences the sorption product. The contraction in interatomic distances and disruption of long-range structure indicated by EXAFS for sorption samples can be accounted for by small particle sizes of the new surface phase, structural misregistry on an atomic scale between a Co hydroxide phase and kaolinite, or dissolution of the kaolinite substrate and substitution of minor impurities into a Co hydroxide phase. The apparent reduced solubilities of the sorption products relative to bulk solubilities of Co(OH)2(s) phases may be related to Ostwald-step processes, the formation of an impure Co hydroxide phase, or a lower near-surface solvent dielectric constant.

Evidence for Site Isolation in Rh-Mo Bimetallic Catalysts Derived from CpRhMo(Co)3(PPh3)2

MgO-supported Pt and Pt–W catalysts were prepared from organometallic precursors and characterized to determine how the nature of the precursors and the treatment conditions affected the Pt–W interactions and the catalytic activity for toluene hydrogenation. Samples were prepared from {Pt[W(CO)3(C5H5)]2(PhCN)2} (Ph = phenyl) and from [PtCl2(PhCN)2] + [W(CO)6] and characterized by infrared spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, transmission electron microscopy (TEM), and chemisorption of H2, CO, and O2. The samples were treated in H2at 400°C prior to most of the characterizations. Incorporation of W reduced the chemisorption of CO and of H2, with the reduction in chemisorption being greater for the catalyst prepared from {Pt[W(CO)3(C5H5)]2(PhCN)2} than for the catalyst prepared from the combination of monometallic precursors. EXAFS spectra measured at both the PtLIIedge and the WLIIIedge showed substantial Pt–W contributions for the former catalyst (but not the latter), with a Pt–W coordination number of about 2 and an average Pt–W distance of 2.71 Å. TEM and EXAFS results indicate that interactions between Pt and W cations in the sample made from {Pt[W(CO)3(C5H5)]2(PhCN)2} maintained the Pt in a highly dispersed form, with supported Pt clusters being smaller than about 10 Å. The catalyst prepared from the bimetallic cluster was more than an order of magnitude less active for toluene hydrogenation at 1 atm and 60°C than the catalyst prepared from the two monometallic precursors; the comparison indicates that the W acted like an isolated dispersed support, with its influence on the catalysis maximized by its proximity to the Pt.

Local structure and concentration in Al-Mn alloy electrodeposits

The structures of f.c.c. crystalline and amorphous phases coexisting in Al-Mn alloy electrodeposits containing 11–30 wt% were examined by using the grazing-incidence X-ray diffraction (GIXD) method, high-resolution transmission electron microscopy combined with energy dispersive X-ray spectrometry (HRTEM/EDS), and the manganese K-edge extended X-ray absorption fine-structure (EXAFS) technique. The amorphous and f.c.c. phases were homogeneously distributed in the through-thickness direction of the electrodeposits, although the amorphous phase increased and the f.c.c. phase decreased with increasing manganese total concentration. The decrease of Mn-content in the f.c.c. crystalline phases with increasing manganese total concentration was revealed by the lattice constant measurement of the f.c.c. crystalline phase. This result was also supported by the microarea elemental analysis by HRTEM/EDS. The EXAFS result has indicated that the manganese local structures in the amorphous phases are almost identical in different manganese total concentration electrodeposits. This suggests that the Mn-content in the amorphous phases hardly changes with increasing manganese total concentration. These results have revealed that a peculiar elemental partition occurs in the amorphous and crystalline phases of Al-Mn alloy electrodeposits.

Structural study of Zr 0.8Sn 0.2TiO 4

The crystal structure of Zr 0.8Sn 0.2TiO 4 has been investigated using extended X-ray absorption fine structure (EXAFS) spcctroscopy, neutron powder diffraction and Raman spectroscopy techniques. It possesses an orthorhombic α-PbO 2; type structure [space group Pbcn]. EXAFS spectroscopy reveals that its zirconium ions possess 8-fold coordination as noted in Zr 5Ti 7O 24, while its tin ions possess 6-fold coordination. X-ray diffraction and EXAFS results indicate that Zr 0.8Sn 0.2TiO 4 exists in a structural state between that of high-temperature ZrTiO 4 and Zr 5Ti 7O 24. Consistent with the EXAFS interpretation, an empirical analysis of the Raman spectra indicates that the tin ions prefer to be on titanium sites. A superstructure is observed in Zr 0.8Sn 0.2TiO 4 which is caused by the non-stoichiometric nature of the sample. A small concentration of tin ions are probably in the reduced Sn 2+ state, creating defects which undergo short-range ordering in the crystal.

The Near Neighbor Atomic Arrangement in GaAsxP1-x Solid Solution

Extended X-ray absorption fine structure (EXAFS) technique is employed to probe the local atomic structure of III-V pseudobinary solid solution GaAsXP1-X, with the composition from X=0 to X=1. The first and second near neighbor bond lengths and bond angles are obtained and the results imply distortion of the near neighbor atomic structure. The atomic scale structure based on a random distribution of anions can be described by using five special coordination tetrahedra, the application on GaAsXP1-X is proved to be excellent agreement with the EXAFS results.

In situ EXAFS studies of modifications to supported metallic catalysts under reactive atmospheres

Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to study the structural evolution of monometallic (Pt/Al2O3) and bimetallic (Pt-Re/Al2O3) catalysts with metal loadings representative of those used industrially under conditions close to those seen during preparation (calcination and reduction), during catalysis (under a hydrogen-hydrocarbon mixture), and during regeneration (simulated by cycles of reduction and oxidation). During the catalytic reforming ofn-heptane, the formation of bonds to carbon species is observed directly by EXAFS. Despite the fact that only platinum-carbon bonds are observed, bimetallic systems show significant differences compared with monometallic systems with, in the case of Pt-Re, a reduction in the temperature range over which carbon bonding is observed, and structural modifications of the metallic particles. Under the same conditions, no carbon bonding is observed for the Pt-Sn system. In the case of Pt-Sn/Al2O3, the EXAFS results, confirmed by transmission electron microscopy, demonstrate the high resistance to sintering of this bimetallic system compared to the monometallic system.

EXAFS Analysis of Dilute Magnetic Semiconductor Thin Films Synthesized by the Ion Beam Technique

We have synthesized dilute magnetic semiconductor (DMS) thin films of CdMnTe and ZnMnSe using the ion beam technique. High doses of Mn ions (∼2–5×1016/cm 2) were implanted into single crystal CdTe and into ZnSe epilayers on GaAs, forming subsurface layers of Cdl.xMnxTe and Znl-xMnxSe alloys, respectively, with x∼0.15–0.22. Fluorescence extended x-ray absorption fine structure (EXAFS) measurements on these materials reveal that the Mn atoms in the CdMnTe and ZnMnSe layers, both as-implanted and annealed, have local environments similar to their corresponding bulk-grown DMS alloys. While the anion-cation distances (Ra-c) in the annealed samples are equivalent to those in the corresponding bulk-grown DMS, the Ra-c in the asimplanted samples are slightly larger (∼0.01Å) than those in the bulk-grown DMS. This is most likely due to the implantation damage in the as-implanted materials. Our results on the Ra-c of the ion beam synthesized layers deviate significantly from Vegard's law, but are consistent with the bimodal distribution model. The EXAFS results are corroborated with results from ion beam analysis and Raman spectroscopy.

ChemInform Abstract: Supported Metals and Supported Organometallics

Abstract Organometallic chemistry and heterogeneous catalysis are developing into common territory, illustrated by catalysts incorporating structures ranging from mononuclear metal complexes to metal aggregates (crystallites) on metal oxide supports. Reactions of precursors such as metal carbonyls and metal allyls with metal oxide surfaces have been understood on the basis of solution organometallic chemistry combined with functional group chemistry of the supports. The supported species are transformed by processes including reductive carbonylation (whereby metal complexes and salts are converted into metal clusters) and oxidative fragmentation (whereby metal clusters and aggregates are converted into mononuclear metal complexes). Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used in conjunction with other techniques to characterize bonds between mononuclear metal complexes and metal oxide surfaces and also to provide structural information about metal-support interfaces in highly dispersed supported metal aggregates. EXAFS results are also helping to define the limitations of standard methods for estimating surface areas of highly dispersed metals by measurements of chemisorption of H 2 and CO. Some supported ‘molecular’ organometallics are structurally and catalytically novel, indicating possibilities for design of surface catalytic sites. Ensembles consisting of three Re complexes on MgO are active for the structure-sensitive cyclopropane hydrogenolysis, whereas a catalyst consisting of the same complexes (evidently isolated on the support surface) is inactive. ‘Molecular’ osmium carbonyl clusters stabilized on the basic MgO surface and in basic zeolites catalyze CO hydrogenation, and 10-atom aggregates of osmium, formed by removal of CO ligands from a ‘molecular’ precursor, are active for n-butane hydrogenolysis. These structurally well-defined catalysts offer good opportunities for understanding metal-support interactions and structure sensitivity in metal catalysis.

EXAFS studies of doped-ZrO 2 systems

Extended X-ray absorption fine structure (EXAFS) spectroscopy measurements have been carried out on a number of 5% rare earth-doped ZrO 2 materials, where the dopant cations used were Er 3+, Gd 3+ and La 3+. It has been found that the nature of the dopant cation influences the location of the promoted oxygen vacancies. In the case where a relatively small dopant cation is used, e.g. Er 3+, the vacancies are located in the vicinity of the host Zr 4+ cation. However, in the situation where a much larger impurity cation was used, e.g. La 3+, the oxygen vacancies are situated in the proximity of the dopant cation. In conjunction with these EXAFS results, conductivity measurements made on these systems provide further evidence that the nature of the dopant cation can play a major role in determining the conductivity characteristics of these materials.