Ru Oxidation State Research Articles

Partial oxidation of methane over supported ruthenium catalysts

Partial oxidation of methane (POM) to synthesis gas was studied over Ru catalysts (1% (w/w)) supported on silica, alumina and ceria–zirconia. Catalyst samples were prepared by incipient wetness and characterized by BET area, XRD, ESEM–EDS, and TPR–TPO analyses. Ru on silica deactivated very fast, while Ru supported on alumina has good activity and selectivity. The mixture CeO2–ZrO2 led to low selectivity towards POM, with a higher selectivity towards complete combustion, common to all the catalysts at lower temperature. Both reduced and non-reduced catalysts were tested resulting in different behaviour in the same temperature range. We investigated the effect of different GHSVs, heating rates and also sequences of heating and cooling cycles. This allowed gaining insight into the sequence of reactions taking place in the reactor and revealed hysteresis for all reaction conditions. This can be explained through a cycling between Ru oxidation states on the surface.

Reactivity of Ruthenium(II) and Copper(I) Complexes that Possess Anionic Heteroatomic Ligands: Synthetic Exploitation of Nucleophilicity and Basicity of Amido, Hydroxo, Alkoxo, and Aryloxo Ligands for the Activation of Substrates that Possess Polar Bonds as well as Nonpolar C–H and H–H Bonds

AbstractThe preparation and reactivity of late transition‐metal complexes in low oxidation states with nondative‐heteroatomic ligands (e.g., amido, hydroxo, alkoxo, and aryloxo ligands) are described. For such complexes the disruption of ligand‐to‐metal π‐donation because of a filled dπ manifold can enhance the nucleophilic and/or basic reactivity at the nondative‐heteroatomic ligand relative to transition‐metal complexes in high oxidation states. The chemistry of five‐ and six‐coordinate Ru complexes with amido, hydroxo, methoxo, and aryloxo ligands is described including Brønsted acid/base reactions, coordination, and activation of polar substrates toward N–C bond forming reactions and metal‐mediated activation of dihydrogen and C–H bonds. The impact of ancillary ligands and Ru oxidation states (RuII versus RuIII) are discussed. In addition, the preparation and reactivity of well‐defined monomeric two‐ and three‐coordinate Cu complexes with amido, alkoxo, hydroxo, and aryloxo ligands are presented including examples of reactions to probe fundamental reactivity patterns as well as catalytic addition of N–H and O–H bonds across C=C bonds of olefins. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Improved Performance of Direct Methanol Fuel Cells by Anodic Treatment

Anodic treatment of direct methanol fuel cell single cells was carried out at anode potentials of 0.1, 0.4, 0.7, and 1.0 V. An increase in maximum power density by 48% was achieved after anodic treatment at 0.7 V. Electrochemical impedance spectroscopy showed that the methanol electro-oxidation activity of the PtRu catalyst was promoted by the anodic treatment. Oxidation states of Ru changed from Ru metal (43%) and RuO 2 (57%) to RuO 2 (33%) and RuO x H y (67%), which were studied by X-ray photoelectron spectroscopy. The results showed that the formation of RuO x H y occurred at a potential range of 0.4-0.7 V.

Highly Oxidized Ruthenium Organometallic Compounds. The Synthesis and One-Electron Electrochemical Oxidation of [Cp*RuIVCl2(S2CR)] (Cp* = η5-C5Me5, R = NMe2, NEt2, OiPr)

[Cp*RuIVCl2(S2CR)] (R = NMe2, NEt2, and OiPr) were synthesized by the reaction of [Cp*RuIIICl2]2 with [RC(S)S]2. One-electron electrochemical oxidation of [Cp*RuCl2(S2CR)] produces paramagnetic [Cp*RuCl2(S2CR)]+, which are stable in CH2Cl2 solution for at least several hours at 233 K. EPR experiments performed at 293 K show isotropic signals (g ≈ 2.035) with clearly defined hyperfine coupling to 99Ru and 101Ru of 25 G and with peak-to-peak line widths of 15 G. At temperatures below 153 K, axial-shaped EPR spectra were obtained with g-values close to 2 (2.050−2.008) and narrow peak-to-peak line widths (15 G). Results from DFT calculations indicate that approximately 70% of the spin density in [Cp*RuCl2(S2CNMe2)]+ is located on the ruthenium, although there is an increase of only 0.06 in the positive charge of the metal ion as a result of the oxidation. The high spin density on Ru supports the assignment of a formally Ru(V) oxidation state, which is unprecedented in organometallic chemistry. Chemical oxidat...

Probing the Ru Oxidation State during the CH3OH Electro-Oxidation Reaction and Pt to Ru Catalyst Surface Site Distribution: A Simple Electrochemical In-situ Method

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A Theoretical Investigation into the Photophysical Properties of Ruthenium Polypyridine‐Type Complexes

Excited states of ruthenium polypyridine-type complexes have always attracted the interest of chemists. We have recently found evidence of a remarkable long-lived excited state (30 micros) for a Ru(II) complex containing a heteroditopic ligand that can be viewed as a fused phenanthroline and salophen ligand.1 To unravel this intriguing electronic property, we have used density functional theory (DFT) calculations to understand the ground-state properties of [(bpy)(2)Ru(LH(2))](2+), where LH(2) represents N,N'-bis(salicylidene)-(1,10-phenanthroline)diamine. Excited singlet and triplet states have been examined by the time-dependent DFT (TDDFT) formalism and the theoretical findings have been compared with those for the parent complex [Ru(bpy)(3)](2+). The outstanding result is the presence of excited states lower in energy than the metal-to-ligand charge-transfer states, originating from intraligand charge transfer (ILCT) from the phenolic rings to the phenanthroline part of the coordinated LH(2). The spin density distribution for the lowest triplet state provides evidence that it is in fact the lowest triplet state of the free ligand. Correlation between the energy level diagram of orbitals for the ground state and that for the (3)ILCT state clearly establishes that the ruthenium retains its formal Ru(II) oxidation state. The quenching of the luminescence and the evidence of the long-lived excited state observed for [(bpy)(2)Ru(LH(2))](2+) are discussed in the light of the computational results.

Electrochemical properties of dinuclear [Ru([n]aneS4)] complexes of 2,3-bis(2-pyridyl)pyrazine

The syntheses of three new dinuclear [Ru([n]aneS(4))] complexes, where n = 12, 14, 16, bridged by the ligand 2,3-bis(2-pyridyl)pyrazine, (dpp) are reported. The absorption spectra of the complexes show changes in the energy of the MLCT bands within the series, indicating that the thiacrown ligands stabilise the Ru(II) oxidation state to different degrees. Electrochemical studies are also consistent with these observations, and reveal that the pi-acceptor properties of [n]aneS(4) ligands lead to metal based oxidation couples occurring at potentials that are more anodic than those observed in the analogous dinuclear [Ru(bpy)(2)](2+) complex. Despite the back-bonding properties of the thiacrown ligands leading to a reduction in ligand-bridge mediated metal-metal coupling, electrochemical interactions between the metals are still considerable.

Synthesis, structure and spectral and redox properties of new mixed ligand monomeric and dimeric Ru(ii) complexes: predominant formation of the “cis-α” diastereoisomer and unusual3MC emission by dimeric complexes

The tetradentate ligands 1,8-bis(pyrid-2-yl)-3,6-dithiaoctane (pdto) and 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (bbdo) form the complexes [Ru(pdto)(mu-Cl)](2)(ClO(4))(2) 1 and [Ru(bbdo)(mu-Cl)](2)(ClO(4))(2) 2 respectively. The new di-mu-chloro dimers 1 and 2 undergo facile symmetrical bridge cleavage reactions with the diimine ligands 2,2'-bipyridine (bpy) and dipyridylamine (dpa) to form the six-coordinate complexes [Ru(pdto)(bpy)](ClO(4))(2) 3, [Ru(bbdo)(bpy)](ClO(4))(2) 4, [Ru(pdto)(dpa)](ClO(4))(2) 5 and [Ru(bbdo)(dpa)](ClO(4))(2) 6 and with the triimine ligand 2,2':6,2''-terpyridine (terpy) to form the unusual seven-coordinate complexes [Ru(pdto)(terpy)](ClO(4))(2) 7 and [Ru(bbdo)(terpy)](ClO(4))(2) 8. In 1 the dimeric cation [Ru(pdto)(mu-Cl)](2)(2+) is made up of two approximately octahedrally coordinated Ru(II) centers bridged by two chloride ions, which constitute a common edge between the two Ru(II) octahedra. Each ruthenium is coordinated also to two pyridine nitrogen and two thioether sulfur atoms of the tetradentate ligand. The ligand pdto is folded around Ru(II) as a result of the cis-dichloro coordination, which corresponds to a "cis-alpha" configuration [DeltaDelta/LambdaLambda(rac) diastereoisomer] supporting the possibility of some attractive pi-stacking interactions between the parallel py rings at each ruthenium atom. The ruthenium atom in the complex cations 3a and 4 exhibit a distorted octahedral coordination geometry composed of two nitrogen atoms of the bpy and the two thioether sulfur and two py/bzim nitrogen atoms of the pdto/bbdo ligand, which is actually folded around Ru(II) to give a "cis-alpha" isomer. The molecule of complex 5 contains a six-coordinated ruthenium atom chelated by pdto and dpa ligands in the expected distorted octahedral fashion. The (1)H and (13)C NMR spectral data of the complexes throw light on the nature of metal-ligand bonding and the conformations of the chelate rings, which indicates that the dithioether ligands maintain their tendency to fold themselves even in solution. The bis-mu-chloro dimers 1 and 2 show a spin-allowed but Laporte-forbidden t(2g)(6)((1)A(1g))--> t(2g)(5) e(g)(1)((1)T(1g), (1)T(2g)) d-d transition. They also display an intense Ru(II) dpi--> py/bzim (pi*) metal-to-ligand charge transfer (MLCT) transition. The mononuclear complexes 3-8 exhibit dpi-->pi* MLCT transitions in the range 340-450 nm. The binuclear complexes 1 and 2 exhibit a ligand field ((3)MC) luminescence even at room temperature, whereas the mononuclear complexes 3 and 4 show a ligand based radical anion ((3)MLCT) luminescence. The binuclear complexes 1 and 2 undergo two successive oxidation processes corresponding to successive Ru(II)/Ru(III) couples, affording a stable mixed-valence Ru(II)Ru(III) state (K(c): 1, 3.97 x 10(6); 2, 1.10 x 10(6)). The mononuclear complexes 3-7 exhibit only one while 8 shows two quasi-reversible metal-based oxidative processes. The coordinated 'soft' thioether raises the redox potentials significantly by stabilising the 'soft' Ru(II) oxidation state. One or two ligand-based reduction processes were also observed for the mononuclear complexes.

XPS and reactivity study of bimetallic nanoparticles containing Ru and Pt supported on a gold disk

We report a new method of immobilization of catalytic metal/alloy nanoparticles on a gold disk for transfer from an electrochemical cell to UHV (without sample exposure to air) for XPS analyses. Using this immobilization approach, several samples were examined: a core-shell Pt-on-Ru catalyst prepared from Ru black onto which Pt was spontaneously deposited, commercial Pt/Ru 50:50 nanoparticle alloy, as well as single metal Ru and Pt nanoparticle samples. The catalysts were characterized for the Ru oxidation state and for the methanol electrooxidation activity (as Pt was always metallic). For all bimetallic samples, we found that the reduced nanoparticles were more active towards methanol oxidation than the fully or partially oxidized samples. Regardless the Ru oxidation state however, the activity was lower than that previously reported for Ru decorated Pt nanoparticle catalysts (Ru-on-Pt). Possible reasons for the reactivity differences are discussed.

Active Form of Ru for the CH3OH Electro-oxidation Reaction: Introduction of a Simple Electrochemical In Situ Method

In this work, unsupported catalysts are used to address the issue of Ru oxide involvement during the methanol oxidation reaction, from the point of view of both Ru “oxidation state” (metal vs oxide) and amount of Ru oxide. It is shown that a simple electrochemical method is useful as in situ probe for the Ru oxidation state. The method also yields insight into the Pt to Ru site distribution of the catalyst surface. The results indicate that the electrochemical formation of reversibly reducible Ru oxides is effectively suppressed in solutions, most probably because of its involvement in the anodic oxidation reaction to . In the case of PtRu alloys that consist of surfaces of well-distributed Pt to Ru sites, Ru oxides are not detected on this surface for concentrations greater than . It is also shown that the electrochemical formation of Ru oxides at the potential of interest prior to the oxidation reaction deenhances the catalyst activity for oxidation. However, the experiments suggest the deenhancement is low ( at 0.6 and at vs the reversible hydrogen electrode) and only observed at potentials equal to or more positive than .

Influence of Ru Doping on the Structure, Defect Chemistry, Magnetic Interaction, and Carrier Motion of the La1-xNaxMnO3+δ Manganite

In this work, we report a structural, electrical, and magnetic characterization of the La(1-x)Na(x)Mn(1-y)Ru(y)O(3+delta) (LNMRO) system with x = 0.05 and 0.15 and y = 0, 0.05, and 0.15, also comprising an investigation of the role of the oxygen content on the related redox properties. The experimental investigation has been realized with the aid of X-ray powder diffraction, electron microprobe analysis, thermogravimetry, electrical resistivity and magnetization measurements, and electron paramagnetic resonance. We demonstrate that the effect of ruthenium doping on the studied LNMRO compounds is not only directly related to the Ru/Mn substitution and to the Ru oxidation state but also indirectly connected to the oxygen content in the sample. Our data show that ruthenium addition can "improve" electrical and magnetic properties of nonoptimally (low) cation-doped manganites, causing an increase of the T(C) value and the insurgence of magnetoresistance effect, as observed for the x = 0.05 and y = 0.05 sample (MR congruent with 60% at 7 T and at approximately 260 K).

Magnetism and electronic transport in Sr4−xLaxRu2−xMnxO9: Interplay of Mn and Ru redox chemistry

Sr 4 Ru 2 O 9 has a P6¯2C space group and differs from the Ruddelsden–Popper series of the strontium ruthenate family where Ru is distinctly present in Ru+5 state with no Jahn–Teller activity. A significant increase in magnetic moment and coercivity is observed along with lowering of resistivity, especially around a critical concentration of x=0.2 in Sr4−xLaxRu2−xMnxO9. Such an unusual effect is correlated to the mixed magnetic pair effect arising out from the variable oxidation states of Ru as suggested by x-ray photoelectron spectroscopic studies. Our results substantiate the long-range ferromagnetism observed for Ruthenium-substituted three-dimensional manganites.

Evolution of the Crystal and Electronic Structures of Magnetic RuSr2-xLaxGdCu2O8 Superconductors upon La Substitution

We have investigated systematically the effects of La substitution on the chemical bonding nature and physical properties of magnetic RuSr(2-x)La(x)GdCu2O8 superconductors. X-ray diffraction and energy dispersive spectroscopic microprobe analyses reveal that a fraction of Sr ions can be successfully replaced by La ions with the contraction of unit cell volume. According to electrical resistance and dc magnetization measurements, the La substitution gives rise to a significant reduction of superconducting transition temperature (T(c)) but to an increase of magnetic ordering temperature with depressed remanent magnetization. Ru K- and Cu K-edge X-ray absorption spectroscopic results clarify that average Ru and Cu oxidation states decrease upon the La substitution. On the basis of the spectroscopic evidences presented here, we are able to attribute the T(c) reduction upon the La substitution to the depletion of the hole density in CuO2 layers and the accompanying variation of magnetic coupling behavior to the change of Ru oxidation state.

RuSr2GdCu2O8초전도 화합물 내의 구리와 루테늄 이온의 전자구조에 대한 X선 흡수분광 연구

The electronic structure of Cu and Ru ions in magnetic RuSr2GdCu2O8 superconductor has been investigated by using X-ray absorption spectroscopy (XAS). X-ray diffraction analysis and electrical resistance measurement reveal that the prolonged sintering process under Ar and oxygen atmosphere leads to the formation of single phasic RuSr2GdCu2O8 with Tc=32 K. According to the comparative Cu Kand Ru K-edge XAS studies on RuSr2GdCu2O8 and several references, it becomes certain that the ruthenocuprate compound has a mixed Cu oxidation state of +2.08 corresponding to underdoped hole density and a mixed Ru oxidation state of ~+4.76. Taking into account the fixed oxygen stoichiometry of this phase, we have concluded that the mixed oxidation state of Ru provides an evidence of the role of RuO2 layer as a charge reservoir for superconductive CuO2 layer.

Electrocatalytic oxidation of carbon monoxide and methanol on platinum surfaces decorated with ruthenium

Ruthenium is the most active and stable promoter of platinum known for low-temperature fuel cell anode reactions such as the oxidation of methanol and CO. The mechanism of promotion by Ru, including bifunctional and ligand effects, is discussed in this selective review. Also examined are effects of surface structure, Ru distribution, and Ru oxidation state. For this purpose, some review of the mechanism of CO and methanol oxidation on Pt/Ru surfaces is provided. The review concentrates on pure Pt nanoparticles and bulk electrodes modified by deposition of Ru, although some discussion of Pt/Ru alloys and Pt-modified Ru electrodes is included in order to address and discuss all aspects of promotion by Ru.

Remote Fluorescence Imaging of Dynamic Concentration Profiles with Micrometer Resolution Using a Coherent Optical Fiber Bundle

Dynamic concentration profiles within the diffusion layer of an electrode were imaged in situ using fluorescence detection through a multichannel imaging fiber. In this work, a coherent optical fiber bundle is positioned orthogonal to the surface of an electrode and is used to report spatial and temporal micrometric changes in the fluorescence intensity of an initial fluorescent species. The fluorescence signal is directly related to the local concentration of a redox fluorescent reagent, which is electrochemically modulated by the electrode. Fluorescence images are collected through the optical fiber bundle during the oxidation of tris(2,2'-bipyridine)ruthenium(II) to ruthenium(III) at a diffusion-limited rate and allow the concentration profiles of Ru(II) reagent to be monitored in situ as a function of time. Tris(2,2'-bipyridine)ruthenium(II) is excited at 485 nm and emits fluorescence at 605 nm, whereas the Ru(III) oxidation state is not fluorescent. Our experiments emphasize the influence of two parameters on the micrometer spatial resolution: the numerical aperture of optical fibers within the bundle and the Ru(II) bulk concentration. The extent of the volume probed by each individual fiber of the bundle is discussed qualitatively in terms of a primary inner-filter effect and refractive index gradient. Experimentally measured fluorescence intensity profiles were found to be in very good agreement with concentration profiles predicted upon considering planar diffusion and thus validate the concept of this new application of imaging fibers. The originality of this remote approach is to provide a global view of the entire diffusion layer at a given time through one single image and to allow the time expansion of the diffusion layer to be followed quantitatively in real time.

Mixed-valence properties of Ruthenium–Polypyridine dimers bridged by Imidazolate and Triazolate Ligands

The dinuclear complex cis,cis-[(bpy)2ClRu(μ-bim)RuCl(bpy)2] n + (bpy = 2,2′-bipyridine; bim = benzimidazolate; n = 1, 2, or 3) was synthesized, isolated as a hexafluorophosphate salt, and investigated in organic solutions by cyclic voltammetry and UV/visible/NIR spectroelectrochemistry. The mixed-valent species (n = 2) displays significant metal–metal electronic coupling in the ground state but exhibits localized Ru(III) and Ru(II) oxidation states, as deduced from its intervalence charge transfer (IVCT) band and redox parameters. On the basis of the resonance energy (H AB) estimated in the context of Hush's semiclassical theory, the extent of intermetallic communication was found to be larger than that recently reported for the bta-bridged analog (bta = benzotriazolate). Some differences between the IVCT features of these systems have been rationalized in terms of the degree of σ,π-basic character of the bridging ligands, according to an electron superexchange mechanism of the “hole-transfer” type. The stabilization of the mixed-valent complexes is attributed mainly to cooperative metal-to-ligand/ligand-to-metal charge-transfer effects. The combined π-acceptor and σ,π-donor abilities of the ancillary (bpy) and bridging (bim or bta) ligands, respectively, are also responsible for the high stability of the fully oxidized (RuIII–L–RuIII) and fully reduced (RuII–L–RuII) isovalent species.

Interpretation of the Electronic Spectra of Ruthenium Nitrosyl Complexes with Nitrogen-containing Heterocyclic Ligands

The electronic absorption spectra of ruthenium nitrosyl complexes with nitrogen-containing heterocyclic ligands were analyzed on the basis of ab initio and CINDO/CI semiempirical calculations of free ligands L and complexes trans-[Ru(NO)(NH3)4(L)]3+ (L = pyridine, pyrazine, nicotinamide, isonicotinamide, l-histidine, imidazole). Spectral manifestations of a strong covalent Ru-NO bond were observed to conclude that the oxidation states of Ru and NO in the RuNO3+ group are expedient to represent as Ru(III) and NO0. Introduction of a nitrosyl group into the inner coordination sphere of Ru(II) complexes with nitrogen-containing heterocyclic ligands much affects the entire spectral patterns and denudes these ligands of the capacity to exhibit chromophoric properties.

Effect of Redox State of PtRu Electrocatalysts on Methanol Oxidation Activity

PtRu is a promising catalyst for methanol oxidation in direct methanol fuel cells. However, the most active Pt:Ru ratio and oxidation state of the Ru component are still under investigation. PtRu black was obtained from Johnson Matthey, and the as-received catalyst was treated with either hydrogen or oxygen at elevated temperatures to alter the oxidation state. The samples were characterized by cyclic voltammetry (CV), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and X-ray diffraction (XRD) to confirm their redox states and for correlation with their methanol oxidation activity. All of the characterization techniques support successful oxidation and reduction of the PtRu catalyst. The methanol oxidation activity was measured, and the sequence at 25 °C was found to be reduced > as-received > oxidized ≫ strongly oxidized. The effect of the drying regime and the dispersing agent for the catalysts was also investigated, and it was found that samples supported using acetic acid were more active than those supported by Nafion, but were less stable and more susceptible to change in catalyst state from heat gun drying.

Synthesis, structure, and acid-base and redox properties of a family of new Ru(II) isomeric complexes containing the trpy and the dinucleating Hbpp ligands.

Three pairs of mononuclear geometrical isomers containing the ligand 3,5-bis(2-pyridyl)pyrazole (Hbpp) of general formula in- and out-[RuII(Hbpp)(trpy)X](n+) (trpy=2,2':6',2' '-terpyridine; X=Cl, n=1, 2a,b; X=H2O, n=2, 3a,b; X=py (pyridine), n=2, 4a,b) have been prepared through two different synthetic routes, isolated, and structurally characterized. The solid state structural characterization was performed by X-ray diffraction analysis of four complexes: 2a-4a and 4b. The structural characterization in solution was performed by means of 1D and 2D NMR spectroscopy for complexes 2a,b and 4a,b and coincides with the structures found in the solid state. All complexes were also spectroscopically characterized by UV-vis which also allowed us to carry out spectrophotometric acid-base titrations. Thus, a number of species were spectroscopically characterized with the same oxidation state but with a different degree of protonation. As an example, for 3a three pKa values were obtained: pKa1(RuII)=2.13, pKa2(RuII)=6.88, and pKa3(RuII)=11.09. The redox properties were also studied, giving in all cases a number of electron transfers coupled to proton transfers. The pH dependency of the redox potentials allowed us to calculate the pKa of the complexes in the Ru(III) oxidation state. For complex 3a, these were found to be pKa1(RuIII)=0.01, pKa2(RuIII)=2.78, and pKa3(RuIII)=5.43. The oxidation state Ru(IV) was only reached from the Ru-OH2 type of complexes 3a or 3b. It has also been shown that the RuIV=O species derived from 3a is capable of electrocatalytically oxidizing benzyl alcohol with a second-order rate constant of kcat=17.1 M(-1) s(-1).