Abstract: Complexes in which a sigma-H--E bond (E=H, B, Si, C) acts as a two-electron donor to the metal center are called sigma complexes. Clues that it is possible to interconvert sigma ligands without a change in oxidation state derive from C--H activation reactions effecting isotope exchange and from dynamic rearrangements of sigma complexes (see Frontispiece). Through these pathways, metathesis of M--E bonds can occur at late transition metals. We call this process sigma-complex-assisted metathesis, or sigma-CAM, which is distinct from the familiar sigma-bond metathesis (typical for d(0) metals and requiring no intermediate) and from oxidative-reductive elimination mechanisms (inherently requiring intermediates with changed oxidation states and sometimes involving sigma complexes). There are examples of sigma-CAM mechanisms in catalysis, especially for alkane borylation and isotope exchange of alkanes. It may also occur in silylation and alkene hydrogenation.

Abstract: Different sized platinum nanoparticles on carbon supports have been prepared using PtCl4 (catalyst Ia) and H2PtCl6 (catalyst IIa) as starting materials and 1-hexanethiol as a surfactant and then heated to 200 °C (catalyst Ib and catalysts IIb), 300 °C (catalyst Ic and catalysts IIc), and 400 °C (catalyst Id and catalysts IId) for 4 h under argon gas. All the catalysts showed a face-centered cubic (fcc) crystal structure as determined by X-ray diffraction. X-ray diffractograms and transmission electron microscopy results reveal that the platinum nanoparticles are homogeneously dispersed on the carbon support, exhibit narrow particle size distribution, and show no appreciable aggregation. The average platinum particle size as determined from XRD data was found to be ∼2.00, ∼2.56, ∼4.23, ∼4.52, ∼2.13, ∼2.77, ∼4.29, and ∼4.62 nm for catalysts Ia−d and IIa−d, respectively. X-ray photoelectron spectra of all the catalysts indicated that most (>70%) of the platinum nanoparticles have an oxidation state of zero a...

Abstract: The mechanism of selective production of methyl chloride by a reaction of methane, hydrogen chloride, and oxygen over lanthanum-based catalysts was studied. The results suggest that methane activation proceeds through oxidation-reduction reactions on the surface of catalysts with an irreducible metal-lanthanum, which is significantly different from known mechanisms for oxidative chlorination. Activity and spectroscopic measurements show that lanthanum oxychloride (LaOCl), lanthanum trichloride (LaCl3), and lanthanum phases with an intermediate extent of chlorination are all active for this reaction. The catalyst is stable with no noticeable deactivation after three weeks of testing. Kinetic measurements suggest that methane activation proceeds on the surface of the catalyst. Flow and pulse experiments indicate that the presence of hydrogen chloride is not required for activity, and its role appears to be limited to maintaining the extent of catalyst chlorination. In contrast, the presence of gas-phase oxygen is essential for catalytic activity. Density-functional theory calculations suggest that oxygen can activate surface chlorine species by adsorbing dissociatively and forming OCl surface species, which can serve as an active site for methane activation. The proposed mechanism, thus, involves changing of the formal oxidation state of surface chlorine from -1 to +1 without any changes in the oxidation state of the underlying metal.

Abstract: The support effect on the low temperature catalytic combustion of methane over palladium catalysts was studied by using a series of metal oxides as the support. The catalytic activity varied with the support, and support oxides with moderate acid strength gave maximum methane conversion. The oxidation state of palladium after the catalytic run, characterized by XRD and XAFS, also varied with the support, and it was higher on more basic supports. The relation between the catalytic activity and the oxidation state derived from these results did not agree with that obtained by changing oxygen partial pressure. The catalytic activity increased with oxygen partial pressure even up to that corresponding to 30 times as high as the stoichiometric ratio, indicating that the activity was higher at higher oxidation state. This agreed with lower activity of palladium supported on acidic support, but not with that on basic support. The lower activity of palladium supported on basic support in spite of higher oxidation state was attributed to excess stabilization of palladium oxide cluster anion such as (PdO)nδ− by electrophobic cation in a similar way to the formation of binary oxide from PdO and basic support at the boundary of them. Lower reactivity of oxidized form of palladium was confirmed by TPR by methane and TPD of oxygen, and the formation of binary oxide containing palladium and magnesium in Pd/MgO catalyst was confirmed by Pd K-edge XAFS study.

Abstract: The use of IR spectroscopy of adsorbed CO for determination of gold speciation on supported gold catalysts is reviewed. Different ionic and metallic sites (Au 3+ , Au + , Au°, and Au - ) can be differentiated on the basis of the frequency and stability of their surface carbonyl species formed after CO adsorption. In addition, the study of CO adsorption and reaction with other gases can provide unique information on the mechanisms of catalytic reactions with participation of CO. Finally, results from complementary techniques such as X-ray absorption spectroscopy, also useful for determination of gold oxidation state, are briefly considered.

Abstract: The monovalent oxidation state of nickel has received a growing amount of attention in recent years, in part due to its suggested catalytic role in a number of metalloprotein-mediated transformations. In coordination chemistry, nickel(I) is suitable for reductive activation of dioxygen, provided ligands are used that stabilize this less common oxidation state against disproportionation reactions. Two distinct molecular systems have been explored, which have provided access to new nickel–dioxygen structure types, namely, monomeric side-on and end-on superoxo and trans-μ-1,2-peroxo–dinickel complexes. The geometric and electronic structures of the complexes have been established by advanced spectroscopic methods, including resonance Raman and X-ray absorption spectroscopies, and augmented by density functional theory analyses.

Abstract: A Colombian bentonite was modified by intercalation with polyhydroxocationic solutions of Al 3+ containing Fe 3+ and Ce 3+ . Addition of these solutions led to the formation of pillared clays with important catalytic properties in three environmental impact reactions: phenol oxidation in diluted aqueous medium, oxidation of CO and 2-propanol oxidation in gas phase. The catalytic tests revealed the activity of the introduced iron species and the promoter effect of the cerium on the catalytic activity of these species. X-ray diffraction and the transmission electron microscopy results indicated, respectively, the formation of oxide nanoparticles inside and outside of the interlayer spaces of the clay mineral structure. Mossbauer spectroscopy analysis revealed the presence of iron oxides with oxidation state +3 and hematite structure on the solids.

Abstract: Ag−hollandite nanofibers were synthesized through a simple hydrothermal process by oxidizing Mn(NO3)2 with AgMnO4 in aqueous solution. The temperature was found to play an essential role in determining both the crystalline structure and the morphology of the Ag−hollandite materials, whereas the AgMnO4/Mn(NO3)2 molar ratio affected only the morphology of the product. Ag−hollandite nanofibers with diameters of 20−40 nm and lengths of 0.5−4 μm were prepared at 160 °C and a 2/3 AgMnO4/Mn(NO3)2 molar ratio. X-ray photoelectron spectroscopy surface analysis revealed that the silver species presented as Ag+ and the average oxidation state of manganese was 3.9. More promisingly, the Ag−hollandite nanofibers showed quite high catalytic performance for ethanol oxidation, with ethanol conversion of 75% and acetaldehyde selectivity of 95% at 230 °C for 200 h time-on-stream. The high activity, selectivity, and stability were attributed to the stable presence of Ag+ species and the unique morphology of the Ag−hollandit...

Abstract: The catalytic activity of gold-based catalysts for CO oxidation is influenced by gold particle size, dispersion, and redox properties of the support. The nature of the active site (gold oxidation state) and its modification along the course of the reaction are under discussion. In this work, we studied the modifications of a Au/CeO2 catalyst in the presence of gas-phase CO by simultaneous in situ diffuse reflectance infrared Fourier transform mass spectrometry (DRIFT-MS) in isothermal conditions. Redox processes involving surface hydroxyl groups, gold atoms, and gas-phase CO molecules play a determinant role in surface gold dynamics. The interaction of CO with the catalyst surface results in the evolution of CO2 and H2 through both the decomposition of the formate species and the formation of the [Au(CO)2]+ species, which accounts for the redispersion of gold atoms. The identification of the involved surface species by DRIFT lets us state that a deep reduction of the surface (oxygen vacancy creation) chan...

Abstract: Various ionic liquids (ILs), together with Pd-metal species were immobilized on a high-surface area, structural active carbon cloth. The resulting SILCA (supported ionic liquid catalyst) catalysts were studied in the production of fine chemicals, i.e. in the hydrogenation of unsaturated aldehydes, citral and cinnamaldehyde. These model molecules are challenging ones, due to the possibility of several parallel and consecutive reactions that can occur, depending on the experimental conditions and the nature of the catalyst. In this paper we illustrate the feasibility of altering the selectivity profiles, not only by means of altered reaction conditions but also by variation of the nature of the IL present in the catalytic layer. The catalysts were characterized by means of, e.g. nitrogen physisorption, XPS as well as FESEM and EFTEM. The results revealed that Pd derived from a Pd(acac)2-precursor, initially dissolved into the ionic liquid, undergoes a change of oxidation state from Pd2+ to Pd4+, regardless of the ionic liquid in question. However, upon decomposition of the precursor, at 373 K, under H2-flow, a transition to either Pd+ or Pd0 occurred, indicating the formation of catalytically active Pd complexes or nano particles, respectively.

Abstract: Mono and multi-noble metal particles on Al2O3 were prepared in one step by flame spray pyrolysis (FSP) of the corresponding noble metal precursors dissolved in methanol and acetic acid (v/v 1:1) or xylene. The noble metal loading of the catalysts was close to the theoretical composition as determined by WD-XRF and LA-ICP-MS. The preparation method was combined with high-throughput testing using an experimental setup consisting of eight parallel fixed-bed reactors. Samples containing 0.1‐5 wt% noble metals (Ru, Rh, Pt, Pd) on Al2O3 were tested in the catalytic partial oxidation of methane. The ignition of the reaction towards carbon monoxide and hydrogen depended on the loading and the noble metal constituents. The selectivity of these noble metal catalysts towards CO and H2 was similar under the conditions used (methane: oxygen ratio 2:1, temperature from 300 to 500 8C) and exceeded significantly those of gold and silver containing catalysts. Selected catalysts were further analysed using XPS, BET, STEM-EDXS and XANES/EXAFS. The catalysts exhibited generally a specific surface area of more than 100 m 2 /g, and were made up of ca. 10 nm alumina particles on which the smaller noble metal particles (1‐2 nm, partially oxidized state) were discernible. XPS investigation revealed an enrichment of noble metals on the alumina surface of all samples. The question of alloy formation was addressed by STEM-EDXS and EXAFS analysis. In some cases, particularly for Pt‐Pd and Pt‐Rh, alloying close to the bulk alloys was found, in contrast to Pt‐Ru being only partially alloyed. In situ X-ray absorption spectroscopy on selected samples was used to gain insight into the oxidation state during ignition and extinction of the catalytic partial oxidation of methane to hydrogen and carbon monoxide. # 2006 Elsevier B.V. All rights reserved.

Abstract: We address the structure gap between surface science and catalysis studies of the activity of oxide supported Au clusters. Reviewing the recent literature we find that surface science investigations often deal with highly reduced systems that have anionic Au clusters and oxygen vacancies in the support. The catalysis studies on the other hand consistently report on oxidized samples with traces of cationic Au. Performing density functional theory calculations we show that the effect of oxidation of oxide supported Au clusters, Au8/MgO, Au7/TiO2 and Au10/TiO2, is a strong increase in the Au/support adhesion energy and a great structural transformation of the clusters. Some of the Au atoms become positively charged (cationic) in the oxidation process as evidenced indirectly by calculated vibrational stretch frequency shifts of adsorbed CO.

Abstract: A series of Cr complexes varying in oxidation state, ligand and geometry were studied with Cr K‐edge XANES. The main absorption edge energy shift for an oxidation state change from Cr0 to Cr6+ is found to be similar to that for a series of Cr3+ complexes with different ligands. Theoretical XANES and density of states calculations using FEFF8.0 provided detailed insights in the origin of the XANES features for the series of distorted octahedral CrCl3L complexes. The geometry of the CrCl3L complex governs the position of the main absorption edge. Hard versus soft donor effects are overruled by the chlorine ligand for complexes with a facial geometry, whereas the chlorine ligand does not play a significant role in meridional geometry. The combined results call for a redefinition of generally used concepts like oxidation state.

Abstract: The present invention relates to a copper-manganese mixed oxide cathode material, which is suitable for use in a cathode of an electrochemical cell, and which has the formula Mn x Cu y O z .nH 2 O, wherein the oxidation state of Cu is between about +1 and about +3, the oxidation state of Mn is between about +2 and about +7, x is equal to about 3-y, y is less than about 3, z is calculated or experimentally determined, using means known in the art, based on the values of x and y, as well as the oxidation states of Mn and Cu, and nH 2 O represents the surface and structural water present in the mixed oxide material. The present invention further relates to an electrochemical cell comprising the noted cathode material.

Abstract: UO2, UO3, Nd2O3, Eu2O3 and Pr6O11 powders were successfully dissolved in the ionic liquid 1-methyl-3-butyl-imidazolium bistriflimide (BumimTf2N) with the help of small amounts of HNO3. The uranyl species have been characterised through UV-vis and EXAFS spectroscopies. The uranium dissolution leads to the formation of UO2(NO3)3−, even in the presence of water. The kinetics of UO2 dissolution/oxidation is intricate, due to the transient existence of NO2−, which is absent in the case of the UO3 dissolution, occurring with no change in the oxidation state of U.

Abstract: Direct synthesis of H 2 O 2 from its elements was carried out in an acidic aqueous reaction medium over halide-modified oxidized and reduced Pd/Al 2 O 3 catalysts under very mild conditions (at 27 °C and atmospheric pressure). The halide ions were introduced into the catalyst by incorporating halide ions into supported Pd/γ-Al 2 O 3 catalyst or via depositing halide ions on the support (γ-Al 2 O 3) prior to Pd deposition. The H 2 O 2 decomposition and hydrogenation over the corresponding catalysts were also carried out under the reaction conditions similar to those employed for the H 2 O 2 synthesis in order to elucidate the factors strongly affecting the H 2 O 2 yield/selectivity in the direct H 2 O 2 process. The performance of halide-modified Pd/Al 2 O 3 catalysts in the direct H 2 O 2 synthesis revealed that halide insertion in the catalyst system prior to or after Pd deposition on the support had comparable qualitative effect on the H 2 O 2 formation. Both the Pd oxidation state and the nature of the halide ions had strong influences on the H 2 conversion (in direct H 2 O 2 synthesis process) and H 2 O 2 decomposition and/or hydrogenation reaction. While the effect of Pd oxidation state on the H 2 O 2 formation was significant for the catalytic system containing F − and Cl − ions, the influence of the Pd oxidation state was found less important for the catalyst system containing Br − ions; the H 2 O 2 formation selectivity increased significantly due to the presence of Br − ions, irrespective of the Pd oxidation state. The nature of the H 2 O 2 destruction pathway (i.e. hydrogenation and/or decomposition) in the presence of hydrogen over halide-modified Pd/Al 2 O 3 catalysts was found to be strongly dependent upon the nature of the halide ions incorporated in the catalyst during halide-modification of the catalyst.

Abstract: The catalytic partial oxidation of methane to synthesis gas over ruthenium catalysts was investigated by thermogravimetry coupled with infrared spectroscopy (TGA-FTIR) and in situ X-ray absorption spectroscopy (XAS). It was found that the oxidation state of the catalyst influences the product formation. On oxidized ruthenium sites, carbon dioxide was formed. The reduced catalyst yielded carbon monoxide as a product. The influence of the temperature was also investigated. At temperatures below the ignition point of the reaction, the catalyst was in an oxidized state. At temperatures above the ignition point, the catalyst was reduced. This was also confirmed by the in situ XAS spectroscopy. The results indicate that both a direct reaction mechanism as well as a combustion-reforming mechanism can occur. The importance of knowing the oxidation state of the surface is discussed and a method to determine it under reaction conditions is presented.