EXAFS and XANES Studies of Silica-Titania Sol-Gel Films

A series of SiO₂-TiO₂ sol-gel films with and without heat treatment were analyzed by EXAFS and XANES spectroscopies. Both techniques indicate that essentially all Ti⁴⁺ ions remain four-fold coordinated, with a Ti–O bond distance between 1.82–1.85 Å. In the glassy films produced by heat treatment at 900° C, a gradual phase separation may occur at the nanoscale, as the TiO₂ concentration of the films increases.     

Solid-state NMR and EXAFS spectroscopic characterization of polycrystalline copper(I) O,O'-dialkyldithiophosphate cluster compounds: Formation of copper(I) O,O'-diisobutyldithiophosphate compounds on the surface of synthetic chalcocite

A number of polycrystalline copper(I) O,O'-dialkyldithiophosphate cluster compounds with Cu4, Cu6, and Cu8 cores were synthesized and characterized by using extended X-ray absorption fine-structure (EXAFS) spectroscopy. The structural relationship of these compounds is discussed. The polycrystalline copper(I) O,O'-diisobutyldithiophosphate cluster compounds, [Cu8{S2P(OiBu)2}6(S)] and [Cu6{S2P(OiBu)2}6], were also characterized by using 31P CP/MAS NMR (CP = cross polarization, MAS = magic-angle spinning) and static 65Cu NMR spectroscopies (at different magnetic fields) and powder X-ray diffraction (XRD) analysis. Comparative analyses of the 31P chemical-shift tensor, and the 65Cu chemical shift and quadrupolar-splitting parameters, estimated from the experimental NMR spectra of the polycrystalline copper(I) cluster compounds, are presented. The adsorption mechanism of the potassium O,O'-diisobutyldithiophosphate collector, K[S2P(OiBu)2], at the surface of synthetic chalcocite (Cu2S) was studied by means of solid-state 31P CP/MAS NMR spectroscopy and scanning electron microscopy (SEM). 31P NMR resonance lines from collector-treated chalcocite surfaces were assigned to a mixture of [Cu8{S2P(OiBu)2}6(S)] and [Cu6{S2P(OiBu)2}6] compounds.     

Determination of the Mo surface environment of Mo/TiO2 catalysts by EXAFS,XANES and PCA

The local structure of oxidic Mo/TiO₂ catalysts (0.5 to 13.5 wt.% Mo) has been studied using EXAFS and XANES. Both EXAFS and XANES results suggest that the Mo surface phase is octahedrally coordinated for all Mo loadings. The EXAFS results were also examined using principal component analysis (PCA) to determine the number of Mo species present on the Mo/TiO₂ catalysts. Results from PCA of the Mo EXAFS spectra suggested the presence of three Mo species: two surface species and bulk MoO₃.     

Solid-state static Cu and P CP/MAS NMR, and liquid-state EXAFS studies on copper(I) O,O′-dialkyldithiophosphate cluster compounds: Formation of the copper(I) O,O′-di-iso-amyldithiophosphate cluster compound on the surface of synthetic chalcocite

Polycrystalline octa-nuclear copper(I) O,O′-di-i-propyl- and O,O′-di-i-amyldithiophosphate cluster compounds, {Cu₈[S₂P(OR)₂]₆(μ₈-S)} where R = ⁱPr and ⁱAm, were synthesized and characterized by ³¹P CP/MAS NMR at 8.46 T and static ⁶⁵Cu NMR at multiple magnetic field strengths (7.05, 9.4 and 14.1 T). The symmetries of the electronic environments around the P sites were estimated from the ³¹P chemical shift anisotropy (CSA) parameters, δ_aniso and η. Analyses of the ⁶⁵Cu chemical shift and quadrupolar splitting parameters for these compounds are presented with the data being compared to those for the analogous octa-nuclear cluster compounds with R = ⁿBu and ⁱBu. The ⁶⁵Cu transverse relaxation for the copper sites in {Cu₈[S₂P(OⁱPr)₂]₆(μ₈-S)} and {Cu₈[S₂P(OⁱAm)₂]₆(μ₈-S)} was found to be very different, with a relaxation time, T₂, of 590 μs (Gaussian) and 90 μs (exponential), respectively. The structures of {Cu₄[S₂P(OⁱPr)₂]₄} and {Cu₈[S₂P(OⁱPr)₂]₆(μ₈-S)} cluster compounds in the liquid- and the solid-state were studied by Cu K-edge EXAFS. The disulfide, [S₂P(OⁱAm)₂]₂, was obtained and characterized by ³¹P{¹H} NMR. The interactions of the disulfide and of the potassium O,O′-di-i-amyldithiophosphate salt with the surfaces of synthetic chalcocite (Cu₂S) were probed using solid-state ³¹P NMR spectroscopy and only the presence of copper(I) dithiophosphate species with the {Cu₈[S₂P(OⁱAm)₂]₆(μ₈-S)} structure was observed.     

Structural investigation of high-valent manganese-salen complexes by UV/Vis,Raman, XANES,and EXAFS spectroscopy

XANES and EXAFS spectroscopic studies at the Mn-K- and Br-K-edge of reaction products of (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride ([(salen)Mn(III)Cl], 1) and (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) bromide ([(salen)Mn(III)Br], 2) with 4-phenylpyridine N-oxide (4-PPNO) and 3-chloroperoxybenzoic acid (MCPBA) are reported. The reaction of the Mn(III) complexes with two equivalents of 4-PPNO leads to a hexacoordinated compound, in which the manganese atom is octahedrally coordinated by four oxygen/nitrogen atoms of the salen ligand at an average distance of approximately 1.90 A and two additional, axially bonded oxygen atoms of the 4-PPNO at 2.25 A. The oxidation state of this complex was determined as approximately +IV by a comparative study of Mn(III) and Mn(V) reference compounds. The green intermediate obtained in reactions of MCPBA and solutions of 1 or 2 in acetonitrile was investigated with XANES, EXAFS, UV/Vis, and Raman spectroscopy, and an increase of the coordination number of the manganese atoms from 4 to 5 and the complete abstraction of the halide was observed. A formal oxidation state of IV was deduced from the relative position of the pre-edge 1s-->3d feature of the X-ray absorption spectrum of the complex. The broad UV/Vis band of this complex in acetonitrile with lambda(max)=648 nm was consistent with a radical cation structure, in which a MCPBA molecule was bound to the Mn(IV) central atom. An oxomanganese(V) or a dimeric manganese(IV) species was not detected.     

Synthesis of 4,5-dihydroisoxazoles by condensation of primary nitro compounds with alkenes by using a copper/base catalytic system

A new procedure for the synthesis of 4.5-dihydroisoxazoles by condensation of primary nitro compounds with olefins by using a copper/base catalytic system is described. The catalytic effect of copper(II) salts is evidenced by comparison of the reaction rates. Thus, activated nitro compounds react faster than with organic catalysis by tertiary amines, whereas nitroalkanes, unable to condense with dipolarophiles in the presence of the base alone, undergo the reaction on addition of a copper(II) catalyst. The observed occurrence of induction periods in most reactions is ascribed to an equilibrium preceding the rate-determining step, and gives a hint as to the proposed reaction mechanism. The results indicate that this method might be of practical and general utility for synthetic practice.     

Structural studies of amino acid complexes-Iexafs study of glutamic acid complex of copper

The ambiguity in the assignment of stereochemistry and co-ordination to Cu(II) complexes can be resolved by using x-ray absorption spectroscopic technique. The difficulty in the structural assignment of the Cu-glutamate complex is studied in detail. The Cu-acetylacetonate complex has also been studied as an example of square planar geometry. The Cu-glutamate complex is a distorted octahedral molecule.     

Fundamental Aspects of Ceria Supported Au Catalysts Probed by In Situ/Operando Spectroscopy and TAP Reactor Studies

The discovery of the activity of dispersed gold nanoparticles three decades ago paved the way for a new era in catalysis. The unusual behavior of these catalysts sparked many questions about their working mechanism. In particular, Au/CeO₂ proved to be an efficient catalyst in several reactions such as CO oxidation, water gas shift, and CO₂ reduction. Here, by employing findings from operando X-ray absorption spectroscopy at the near and extended Au and Ce L_III energy edges, we focus on the fundamental aspects of highly active Au/CeO₂ catalysts, mainly in the CO oxidation for understanding their complex structure-reactivity relationship. These results were combined with findings from in situ diffuse reflectance FTIR and Raman spectroscopy, highlighting the changes of adlayer and ceria defects. For a comprehensive understanding, the spectroscopic findings will be supplemented by results of the dynamics of O₂ activation obtained from Temporal Analysis of Products (TAP). Merging these results illuminates the complex relationship among the oxidation state, size of the Au nanoparticles, the redox properties of CeO₂ support, and the dynamics of O₂ activation.     

Organothallium compounds. Mechanistic study of the disproportionation reaction of organothallium compounds with copper in acetonitrile

Disproportionation reactions of various phenyl- and thienylthallium dicarboxylates with copper in acetonitrile have been carried out. The reaction mechanism proposed involves the participation of the ArTl²⁺ dication and various cationic pairs of the dication and the intermediate formation of organothallium radicals.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1970–1973, November, 1993.     

On the detection by EXAFS spectroscopy of multimeric species and of complex anions in diethylether and diethylcarbonate

EXAFS spectroscopy has been shown to be a reliable tool for detecting the formation in solutions of molecular associations (LiBr)_n where n is solvent dependent. The signal Br^*...Br observed at 3.8₇ Å confirms the formation of tetrameric species (n=4) in diethylether (Et₂O) but only of dimers (n=2) in diethylcarbonate. Another signal observed at 2.4₅ Å has·been attributed to the pair Br^*...Li. The study of Li₂MBr₄–Et₂O solutions (M=Co, Cu, Zn) also affords new evidence for the formation of complex anions MBr ₄ ^2– but the spectra obtained at the bromine or metalK-edges for LiZnBr₃–Et₂O solutions seems to indicate the formation of more complex aggregates (LiMBr₃)_q. While the signal of the shell Br^*...Br is found quite intense for the reference CBr₄–Et₂O system, it does not appear for the MBr ₄ ^2– complex except for CuBr ₄ ^2– where the Jahn-Teller effect might rigidify a distorted tetrahedral structure.Presented, in part, at the VIIth International Conference on Non-Aqueous Solutions, Regensburg, West Germany, Ausgust 1980.     

Compressed octahedral coordination in chain compounds containing divalent copper: structure and magnetic properties of CuFAsF6 and CsCuAlF6

Crystal structures and magnetic investigations of CuFAsF6 and CsCuAlF6 are reported. Together with KCuAlF6, these appear to be the only examples of Jahn-Teller pure Cu(II) compounds containing only one type of ligand that exhibits a compressed octahedral coordination geometry. The Rietveld method has been used for refining the CsCuAlF6 structure based on neutron powder diffraction data at 4 K. The compound crystallizes in space group Pnma (no. 62) with a=7.055(1), b=7.112(1), c=10.153(1) A and Z=4 at 4 K. The structure is built from infinite [CuF5]n(3n-) chains of [CuF6]4- octahedra running along the [1 0 0] direction and (AlF6)3- octahedra connected by corners in the trans position, thus giving rise to chains oriented along the [0 1 0] direction. Single crystals of CuFAsF6 were prepared under solvothermal conditions in AsF5 above its critical temperature. The structure was determined from single-crystal data. CuFAsF6 crystallises in the orthorhombic space group Imma (No. 74) with a=10.732(5), b=6.941(3), c=6.814(3) A and Z=4 at 200 K. The structure can also be described in terms of one-dimensional infinite [CuF5]n(3n-) chains of tilted [CuF6](4-) octahedra linked by trans-vertices running along the b axis. The [CuF5]n(3n-) chains are connected through [AsF6]- units sharing joint vertices. The compressed octahedral coordination of CuII atoms in CuFAsF6 and CsCuAlF6 compounds at room temperature is confirmed by Cu K-edge EXAFS (extended x-ray absorption fine structure) analysis. For both compounds strong antiferromagnetic interactions within the [CuF5]n(3n-) chains were observed (theta(p)=-290+/-10 K and theta(p)=-390+/-10 K for CuFAsF6 and CsCuAlF6, respectively). The peculiar magnetic behaviour of chain compounds containing divalent copper at low temperature could be related to uncompensated magnetic moments in the one-dimensional network.     

Electronic structure and luminescent properties of antimony(III) complex compounds with nitrogen containing outer-sphere organic cations

X-Ray photoelectron (XPS) and luminescent spectroscopy have been used to investigate complex compounds of antimony(III) halides with nitrogen containing organic bases. Inequality of bonds of amine and imine groups was found by XPS to disappear when complexes of antimony(III) with N,N′-diphenylguanidine (Dphg) were formed. The appearance of N1s symmetric line when transiting from Dphg to the cation N,N′-diphenylguanidine ( Dphg⁺) in complex compounds of antimony(III) testifies to this. The study performed demonstrates that electron density increment on the antimony(III) central atom (complexing agent) results in the bathochromic shift ³P₁ → ¹ S ₀ of the luminescence band of antimony(III) ion.     

Oxidation of isobutane catalyzed by vanadyl, copper and cesium substituted H3PMo12O40

Effects of Cs⁺, H⁺ and Cu²⁺ counterions in the vanadium containing heteropoly compounds Cs_xH_1-xVO[PMo₁₂O₄₀] and Cs_yH_0.5-yCu_0.25VO[PMo₁₂O₄₀] on the catalytic oxidation of isobutane and characterization by TGA, IR and ESR spectroscopies are reported. A high selectivity of 76% for methacrylic acid and methacrolein together has been obtained with Cs_0.75H_0.25VO[PMo₁₂O₄₀] catalysts at a reactivity of 5.3x10^-1 mmol/h cm³.     

(Nb2W4O19), TMA2, Na4(OH2)14(SO4): a new layered structure with Lindqvist heteropolyanions, XAS characterization of the HPAs

The new (Nb₂W₄O₁₉),TMA₂, Na₄(OH₂)₁₄(SO₄) has been evidenced as a minor phase during the Nb₂W₄O₁₉TMA (tetramethylammonium) salt synthesis. Its crystal structure has been refined from single crystal X-ray diffraction data, system monoclinic, a=10.166(5) Å, b=17.93(1) Å, c=24.81(1) Å, β=93.057(7)°, space group (S.G.) C2/c, Z=4, R₁=3.96%, wR₁=4.50%. It shows the stacking of cationic and anionic bidimensional layers. The anionic layer of formula [(Nb₂W₄O₁₉), TMA₂ ]²⁻ is formed of isolated Lindqvist HPAs surrounded by TMA groups. The isolated layers adopt a trigonal symmetry that is lost in the crystal by the association of the cationic sheets. These later, of formula [Na₄(OH₂)₁₄(SO₄)]²⁺ form porous net-like sheets with nearly circular cavities of diameter 7.5 Å. groups host the available cavities in a disordered manner. The cohesion between the sheets is performed by both electrostatic interactions and a set of hydrogen bonds. In the cationic layers, the highly symmetrical surrounding of HPAs by TMA groups yields a homogeneous electrostatic field at their external surface leading to a statistic Nb/W disorder over the three available independent metallic positions. Then, XAS experiments at the L₁/L₃-W edge complementarily helped to highlight the preferential cis configuration of (Nb₂W₄O₁₉)⁴⁻ anions, help to the strong Nb vs W contrast in their contribution to the backscattering paths. Previously to these experiments, it was of course checked that both the two phases present in the prepared sample contain Nb₂W₄O₁₉ anions with nearly unchanged geometry.     

The A-cluster in subunit beta of the acetyl-CoA decarbonylase/synthase complex from Methanosarcina thermophila: Ni and Fe K-edge XANES and EXAFS analyses

The acetyl-CoA decarbonylase/synthase (ACDS) complex catalyzes the cleavage of acetyl-CoA in methanogens that metabolize acetate to CO(2) and CH(4), and also carries out acetyl-CoA synthesis during growth on one-carbon substrates. The ACDS complex contains five subunits, among which beta possesses an Ni-Fe-S active-site metal cluster, the A-cluster, at which reaction with acetyl-CoA takes place, generating an acetyl-enzyme species poised for C-C bond cleavage. We have used Ni and Fe K fluorescence XANES and EXAFS analyses to characterize these metals in the ACDS beta subunit, expressed as a C-terminally shortened form. Fe XANES and EXAFS confirmed the presence of an [Fe(4)S(4)] cluster, with typical Fe-S and Fe-Fe distances of 2.3 and 2.7 A respectively. An Fe:Ni ratio of approximately 2:1 was found by Kalphabeta fluorescence analysis, indicating 2 Ni per [Fe(4)S(4)]. Ni XANES simulations were consistent with two distinct Ni sites in cluster A, and the observed spectrum could be modeled as the sum of separate square planar and tetrahedral Ni sites. Treatment of the beta subunit with Ti(3+) citrate resulted in shifts to lower energy, implying significant reduction of the [Fe(4)S(4)] center, along with conversion of a smaller fraction of Ni(II) to Ni(I). Reaction with CO in the presence of Ti(3+) citrate generated a unique Ni XANES spectrum, while effects on the Fe-edge were not very different from the reaction with Ti(3+) alone. Ni EXAFS revealed an average Ni coordination of 2.5 S at 2.19 A and 1.5 N/O at 1.89 A. A distinct feature at approximately 2.95 A most likely results from Ni-Ni interaction. The methanogen beta subunit A-cluster is proposed to consist of an [Fe(4)S(4)] cluster bridged to an Ni-Ni center with one Ni in square planar geometry coordinated by 2 S + 2 N and the other approximately tetrahedral with 3 S + 1 N/O ligands. The electronic consequences of two distinct Ni geometries are discussed.     

Solvent and substituent effects on the equilibrium between mononuclear and binuclear complexes of copper(II) carboxylates in 2-methylpyridine and 2,6-dimethylpyridine solutions

Electronic spectra of Cu(RCO₂)₂ [R=H, CH₃, CH₃CH₂, (CH₃)₂CH, C₆H₅, C₆H₅CH₂] as solutions in the pure amines, 2-methylpyridine, L, 2,6-dimethylpyridine, L, and in amine-chlorobenzene mixtures have been measured and interpreted in terms of the equilibrium 2Cu(RCO₂)₂L₂=Cu₂(RCO₂)₄L₂ +2L. Formation of the binuclear species was greater when 2,6-dimethylpyridine was the solvent. The measured equilibrium constants indicated that the tendency to form binuclear complexes in the solutions increased with increasing pK_a of the parent carboxylic acid.     

Hydrogenolysis of cycloalkanes on a tantalum hydride complex supported on silica and insight into the deactivation pathway by the combined use of 1D solid-state NMR and EXAFS spectroscopies

Hydrogenolysis of cyclic alkanes is catalysed by [(triple bond)SiO)(2)Ta-H] (1) at 160 degrees C and leads to lower alkanes and cyclic alkanes including cyclopentane. The turnover number is correlated with the number of carbon atoms of the cyclic alkanes, and therefore while cycloheptane is readily transformed, cyclopentane does not give any product (<1 %). The mechanism of ring contraction probably involves carbene de-insertion as a key carbon-carbon bond-cleavage step. The reluctance of cyclopentane to undergo hydrogenolysis was further studied: under the reaction conditions cyclopentane reacts with 1 to give the corresponding cyclopentyl derivative [(triple bond)SiO)(2)Ta-C(5)H(9)] (13), which evolves towards cyclopentadienyl derivative [(triple bond)SiO)(2)Ta(C(5)H(5))] (14) according to both solid-state NMR and EXAFS spectroscopies. This latter complex is inactive in the hydrogenolysis of alkanes, and therefore the formation of cyclopentane in the hydrogenolysis of various cyclic alkanes is probably responsible for the de-activation of the catalyst by formation of cyclopentadienyl complexes.     

Influence of ionic strength and pH on the first 60 min of Pseudomonas aeruginosa attachment to ZnSe and to TiO2 monitored by ATR-IR spectroscopy

Establishing the factors which influence the attachment of bacteria to surfaces is important in both preventing and enhancing biofilm formation. The initial hour of attachment of Pseudomonas aeruginosa to ZnSe and to TiO₂ from solutions of different ionic strength and pH was studied using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. The TiO₂ surface was prepared by dip-coating a ZnSe internal reflection element, which produced a 50 nm thick, continuous flat film. At pH 6.3 attachment was found to increase with ionic strength up to 0.03 mol l⁻¹ but to decrease at 0.15 mol l⁻¹. At an ionic strength of 0.003 mol l⁻¹ attachment increased with pH from 4 to 6.3 to 10, but at ionic strength of 0.03 mol l⁻¹ attachment was greater at pH 6.3 than at pH 10. The influence of ionic strength appears to be due to charge factors and/or related changes in the degree of extension of bacterial surface polymers. The complex trends in the influence of pH on attachment can not be explained solely in terms of bacterial and substrate charge, bacterial surface polymer extension or bacterial metabolic activity.