Ir Oxidation State Research Articles

N2O decomposition over ceria-promoted Ir/Al2O3 catalysts: The role of ceria

The impact of CeO2 in the Al2O3-20wt% CeO2 support prepared by the co-precipitation method on the Ir particle size, morphology and oxidation state, and in turn on the deN2O catalytic activity (1000ppmN2O) of supported Ir catalysts were investigated in the absence and presence of excess O2 (2vol%) conditions. It was demonstrated that the deN2O activity of Ir/Al2O3 is notably suppressed by the presence of oxygen in the feed stream, namely, the N2O conversion at 600°C is declined to 65% in the presence of oxygen as compared to 100% in the absence of oxygen. A similar detrimental catalytic effect was also observed for the Ir/CeO2 solid. On the contrary, the deN2O performance of CeO2-modified Ir/Al2O3 catalyst is only slightly affected by the presence of oxygen. An extensive characterization study involving surface texture analysis (N2 adsorption-desorption at −196°C), temperature-programmed reduction in H2 (H2-TPR), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption and desorption (CO-DRIFTS) was carried out to gain insight into the origin of the CeO2-induced promotional effect. The characterization results revealed the existence of IrO2 phase (H2-TPR, XRD, HRTEM, EELS and CO-DRIFTS) as well as of very small isolated particles of Ir on the Al2O3, CeO2 and CeO2-Al2O3 supports (STEM) but to a notably different extent. The coexistence of large IrO2 particles of perfect crystallite structure and very small Ir particles located at the Ir-ceria interface was revealed only in Ir/AlCe. The establishment of a certain Irδ+/Ιr0 ratio and oxygen vacant sites (VO) concentration in ceria around very small Ir particles under oxidative reaction conditions seem to largely promote N2O adsorption and subsequent decomposition into N2 and O2 over the CeO2-promoted Ir/Al catalyst. In the case of Ir/Al, a different deN2O decomposition mechanism occurs, where the site reactivity of Irδ+/Ιr0 established under oxidizing conditions is reduced significantly.

Understanding the Effect of Monomeric Iridium(III/IV) Aquo Complexes on the Photoelectrochemistry of IrO(x)·nH2O-Catalyzed Water-Splitting Systems.

Soluble, monomeric Ir(III/IV) complexes strongly affect the photoelectrochemical performance of IrO(x)·nH2O-catalyzed photoanodes for the oxygen evolution reaction (OER). The synthesis of IrO(x)·nH2O colloids by alkaline hydrolysis of Ir(III) or Ir(IV) salts proceeds through monomeric intermediates that were characterized using electrochemical and spectroscopic methods and modeled in TDDFT calculations. In air-saturated solutions, the monomers exist in a mixture of Ir(III) and Ir(IV) oxidation states, where the most likely formulations at pH 13 are [Ir(OH)5(H2O)](2-) and [Ir(OH)6](2-), respectively. These monomeric anions strongly adsorb onto IrO(x)·nH2O colloids but can be removed by precipitation of the colloids with isopropanol. The monomeric anions strongly adsorb onto TiO2, and they promote the adsorption of ligand-free IrO(x)·nH2O colloids onto mesoporous titania photoanodes. However, the reversible adsorption/desorption of electroactive monomers effectively short-circuits the photoanode redox cycle and thus dramatically degrades the photoelectrochemical performance of the cell. The growth of a dense TiO2 barrier layer prevents access of soluble monomeric anions to the interface between the oxide semiconductor and the electrode back contact (a fluorinated tin oxide transparent conductor) and leads to improved photoanode performance. Purified IrO(x)·nH2O colloids, which contain no adsorbed monomer, give improved performance at the same electrodes. These results explain earlier observations that IrO(x)·nH2O catalysts can dramatically degrade the performance of metal oxide photoanodes for the OER reaction.

Crystallographic and magnetic properties of Pb2−xBixIr2O7−δ (0 ≤ x ≤ 2)

The title series of compounds have been synthesized by solid state reaction and characterized by x-ray diffraction (PXD), neutron powder diffraction (PND), second harmonic generation (SHG), x-ray absorption spectroscopy (XAS) and magnetization measurements. The crystal structures have been refined from PND data in the centrosymmetric space group with pyrochlore structure. No sign of a break of the centrosymmetry has been observed by SHG and no change of the structure to a polar space group in any component of the series, as it was previously reported for Pb2Ir2O7−δ with non-centrosymmetric space group. We have determined by PND that the oxidation state of Ir is slightly decreasing from 5+-to-4+ in the Pb-rich to almost 4+ in the Bi-rich samples. This evolution is confirmed by XAS and also it explains the progression of the crystallographic parameters. All the samples are paramagnetic in the temperature range measured and the magnitude of the effective magnetic moment is enhanced with Bi content, correlated with the enhancement of Ir4+ compared to Ir5+; suggesting Ir5+ to be present in a trigonal antiprism crystal field splitting and therefore Ir4+ as the only magnetic cation.

Crystal and Magnetic Structure of Sr2MIrO6 (M = Ca, Mg) Double Perovskites – A Neutron Diffraction Study

AbstractSr2CaIrO6 and Sr2MgIrO6 double perovskites have been prepared as polycrystalline powders from citrate precursors treated under oxygen pressure conditions. These compounds have been studied by X‐ray and neutron powder diffraction (NPD), magnetic measurements, and X‐ray absorption spectroscopy. The crystal symmetry of both oxides is monoclinic (space group P21/n), and the unit‐cell parameters are related to the ideal simple perovskite cell as a ≈ √2a0, b ≈ √2a0, c ≈ 2a0, β ≈ 90°, where a0 is the edge of the simple cubic ABO3 perovskite. This superstructure is a result of the 1:1 ordering of Ca/Mg and Ir ions over the B site and the distortion of the oxygen sublattice, which leads to a tilted three‐dimensional network of corner‐sharing BO6 octahedra. Magnetic measurements indicate an antiferromagnetic ordering below TN = 58 K for Sr2CaIrO6 and 74 K for Sr2MgIrO6. The low‐temperature magnetic structure of Sr2CaIrO6 was determined by NPD and selected from the possible magnetic solutions compatible with the P21/n space group according to the group‐theory representation. The propagation vector is k = (1/2 1/2 0). A canted antiferromagnetic structure is observed below TN with an ordered magnetic moment of 1.33(2) μB for the Ir6+ cations. Sr2MgIrO6 does not present magnetic reflections. An X‐ray absorption spectroscopy investigation gives an insight into the Ir oxidation states and suggests a hexavalent state for the Ca compound and an admixture of Ir6+ and Ir5+ for the Mg perovskite, in agreement with the NPD data, which indicate a significant oxygen understoichiometry for this material.

Superconductivity in Ca10(Ir4As8)(Fe2As2)5 with Square-Planar Coordination of Iridium

We report the unprecedented square-planar coordination of iridium in the iron iridium arsenide Ca10(Ir4As8)(Fe2As2)5. This material experiences superconductivity at 16 K. X-ray photoemission spectroscopy and first-principles band calculation suggest Ir(II) oxidation state, which yields electrically conductive Ir4As8 layers. Such metallic spacer layers are thought to enhance the interlayer coupling of Fe2As2, in which superconductivity emerges, thus offering a way to control the superconducting transition temperature.

Interrogating the photogenerated Ir(iv) state of a water oxidation catalyst using ultrafast optical and X-ray absorption spectroscopy

Using sunlight to drive molecular water oxidation catalysts for fuel formation requires understanding the single electron transfer events involved in catalyst activation. In an effort to photogenerate and characterize the highly reactive Ir(IV) state of the Ir(III)-based water oxidation catalyst Cp*Ir(ppy)Cl (ppy = 2-phenylpyridine), we have incorporated the complex into a covalent electron acceptor–chromophore–Cp*Ir(ppy)Cl triad, in which naphthalene-1,8:4,5-bis(dicarboximide) (NDI) is the electron acceptor and perylene-3,4-dicarboximide (PMI) is the chromophore. Photoexcitation of the PMI chromophore in dichloromethane results in two competitive reactions: NDI–1*PMI–Ir(III) → NDI–PMI˙−–Ir(IV) and NDI–1*PMI–Ir(III) → NDI˙−–PMI˙+–Ir(III) that each proceed with τ < 5 ps, as determined by femtosecond transient absorption spectroscopy. Both intermediate ion pairs undergo charge shift reactions to produce NDI˙−–PMI–Ir(IV). The fully charge-separated ion pair has a lifetime of 17.2 ± 0.1 ns, and its photophysical behavior is similar in the more polar solvent benzonitrile. Time-resolved X-ray absorption measurements on the triad at 100 ps following PMI photoexcitation show a new absorption feature at the LIII-edge of Ir and a blue-shifted white-line peak, which provides direct evidence of a change in the Ir oxidation state from Ir(III) to Ir(IV), consistent with the photophysical measurements. Our work underscores the utility of ultrafast spectroscopy performed on covalent assemblies of electron donor–acceptor systems with solar fuels catalysts to generate and probe their higher valence states in ways that complement chemical or electrochemical oxidation and establish the nature of key intermediates implicated in their catalytic mechanisms.

Oxidation-state dependent electrocatalytic activity of iridium nanoparticles supported on graphene nanosheets

Nanocomposites of iridium nanoparticles (Ir NPs), supported on graphene nanosheets, are synthesized and their electrocatalytic acitivities in the oxygen reduction reaction (ORR) are studied depending on their Ir oxidation state. Graphene functionalized with poly(vinyl pyrrolidone) (pRGO) is a suitable support for Ir NPs, producing well-monodispersed Ir NPs anchored strongly on the pRGO surface (Ir NP/pRGO) with a very high density. This was confirmed by scanning electron microscopy and transmission electron microscopy. The ORR activity of the Ir NP/pRGO nanocomposites in 0.5 M H2SO4 solution was observed to be dependent on the oxidation state of the immobilized Ir NPs. In fact, the nanocomposite composed of Ir(0) metal NPs, rather than Ir oxide (IrOx) NPs, exhibits higher ORR activity, such as more positive onset potential, higher and flatter limiting current density, a greater n value, and a sharper curve shape in the rotating disk electrode voltammetry experiments. Higher ORR activity of Ir is ascribed to the stronger adsorption of oxygen on the surface of Ir compared to IrOx. The practical stability of the Ir NP/pRGO composite was also confirmed under O2 saturated/acidic conditions.

Rh2O3 versus IrO2: relativistic effects and the stability of Ir4+

Despite the wide-ranging applications of binary Rh and Ir oxides, their stability and trends in Rh and Ir oxidation states are not fully understood. Using first-principles electronic structure calculations, we demonstrate that the origin of the categorical stability of Ir4+ is the relativistic contraction of the 6s orbital and, consequently, an expansion of 5d orbitals. Relativistic effects significantly stabilize Ir4+-containing metallic rutile IrO2 over a wide range of O chemical potentials, despite the choice that Ir has of forming semiconducting corundum Ir2O3. In contrast, Rh is found to display a wider stability range for corundum Rh2O3 with Rh3+ and a greater propensity for multiple oxidation states.

Intrinsic Potential-Dependent Performances of a Sol–Gel-Prepared Electrocatalytic IrO2–TiO2 Coating of Dimensionally Stable Anodes

The electrochemical properties of a sol–gel-prepared IrO2–TiO2 coating on Ti were investigated from the standpoint of the charging/discharging behavior in 1.0°mol dm−3 H2SO4 and in 0.50°mol dm−3 NaCl, pH 2. The potential-dependent changes from low oxidation states of Ir, with low activity, to those of higher activity (enhanced pseudocapacitive performances) were analyzed at different charging/discharging rates by cyclic voltammetry and electrochemical impedance spectroscopy. The results showed the distribution of Ir oxidation states through the coating, with lower ones being placed largely in the internal, hardly accessible, parts of the coating. The transition to higher oxidation states appears as crucial for the full development of pseudocapacitive performances. This transition occurs much easier in the NaCl than in the H2SO4 solution, since the potentials of redox transitions of Ir are much closer due to lower hydrogen ion concentration in NaCl solution. As a consequence, a unique overall coating capacitance was registered in both solutions.

On the propensity for electron transfer in high-energy collisions of iridium complex anions with cesium atoms

Iridium complex anions ( IrX 6 2 - , IrX 5 - , IrX 4 - , K + · IrX 6 2 - X = Cl, Br, and K + · IrCl 5 2 - ) were subjected to high-energy collisions with Cs atoms in an accelerator mass spectrometer. The experiments probed the complex anions’ propensity for electron attachment, which was only observed for IrCl 5 - , IrBr 5 - and K + · IrCl 5 2 - . These observations indicate that electron transfer is controlled by the relative stability of the Ir oxidation state and the availability of low-lying unoccupied molecular orbitals. Formation of IrX 5 2 - is notable since these ions are amongst the smallest molecular dianions observed in the gas-phase, while the K + · IrCl 5 3 - complex contains the smallest experimentally observed trianion.

Density Functional Studies of Catalytic Alkane Dehydrogenation by an Iridium Pincer Complex with and without a Hydrogen Acceptor

AbstractFor Abstract see ChemInform Abstract in Full Text.

Density functional studies of catalytic alkane dehydrogenation by an iridium pincer complex with and without a hydrogen acceptor

Density functional calculations have been applied to models of both transfer and acceptorless alkane dehydrogenation for the iridium(III) pincer complex, ( R PCP )IrH 2 ( R PCP η 3 -C 6H 3(CH 2PR 2) 2-1,3). The iridium pincer complex is the first efficient homogeneous catalyst that does not require photons or sacrificial hydrogen acceptors to drive the reaction, although with the latter the transfer reaction can accomplished under mild conditions. There are four essential steps for the catalytic cycle in both transfer and acceptorless reactions: oxidative addition of alkane, reductive elimination, β-hydrogen (β-H) transfer, and loss of olefin. The transfer reaction can utilize a hydrogen acceptor to produce a 14-electron intermediate, ( R PCP )Ir(I), and the acceptorless reaction can produce an 18-electron intermediate, ( R PCP )Ir(V)(H) 3(alkyl) species. Because the critical barriers are well-balanced in the Ir(V) catalytic cycle for acceptorless alkane dehydrogenation, no transition state (TS) is higher in energy than the products and no intermediate is lower in energy than the reactant. This balance could explain why pincer complexes are more efficient catalysts for alkane dehydrogenation than the corresponding Cp–Ir(III) complex. Entropy contributions, which play a larger role at high temperature, open new pathways for the acceptorless reaction. Free energies of activation suggest that the dynamics of the acceptorless reaction might be sampling the entire range of pathways and Ir oxidation states from Ir(I) to Ir(V).

Ir/H-ZSM-5 Catalysts in the Selective Reduction of NOx with Hydrocarbons

Three different Ir catalysts supported on H-ZSM-5 were prepared and tested for the selective catalytic reduction of NO under net oxidizing conditions using propene as reducing agent. The preparation of highly active Ir catalysts and the elaboration of a procedure for enhancing activity by on stream conditioning was targeted. Structural changes of the catalyst during conditioning were investigated by means of XRD, TEM and activity measurements. Under reaction conditions Ir was present as Ir0 and IrO2. The presence of Ir0 was essential for high DeNOx activity. The ratio of Ir0/Ir4+ was found to depend on the size of Ir-containing crystallites. Larger crystallites contained predominantly Ir0. Crystallite size and oxidation state of Ir have been identified to be crucial for the NO reduction behaviour of Ir/H-ZSM-5.

Isotopic Homogenization of Iridium for High Sensitivity Determination by Isotope Dilution Inductively Coupled Plasma Mass Spectrometry

Iridium has been analyzed at nanogram levels in a geological sample by isotope dilution mass spectrometry employing inductively coupled plasma mass spectrometry (ICP-MS). It is well known that platinum group elements form strong anionic complexes, thus anion exchange resin was used for the preconcentration of Ir from sample matrix. Separation of Ir from elements in a complex matrix is problematic, as Ir(IV) is more efficiently extracted on the anion exchange resin than is the lower oxidation state of Ir. The total Ir content in a solution was oxidized to Ir(IV) using chlorinated lime as the oxidizing agent in order to enhance extraction of Ir on anion exchange resin in HCl medium. Its stripping was carried out with a mixture of HCl and HClO4. Prior to extraction, the most important step in isotope dilution mass spectrometric analysis of Ir is to achieve complete isotopic homogenization of natural Ir with the enriched isotope 191Ir*. In this paper, we have described a method for the isotopic homogenization of Ir in solution by microwave digestion spanning over a period of 12h. The isotope ratio values obtained by ICP-MS are sufficiently precise and accurate; from them the concentration was computed. The method has been successfully applied to determine Ir in a Cretaceous-Tertiary boundary sample from Stevns Klint, Denmark.

Structural characterization and magnetic susceptibility of Ba 2RIrI 6 distorted perovskites (R = rare earth)

The isostructural Ba 2RIrO 6 (R  La, Pr, Ho) perovskites have been characterized from X-ray diffraction data by means of the Rietveld method. They show an orthorhombic symmetry, space group Pbnm, with a random distribution of R and Ir cations in B sites. The magnetic properties of these materials have been studied via magnetic susceptibility measurements in the temperature range 4.2–300 K. Oxidation states of III were found for the rare earths and V for iridium when R  La and Ho, whereas in the case of Ba 2PrIrO 6 the oxidation state of both Pr and Ir was IV. The small value obtained for the effective magnetic moment of Pr(IV), i.e. 1.30 BM, has been explained in terms of crystal field effects; no magnetic interactions were detected.

Dioxygen and Carbon Monoxide Uptake by Iridium(I) Complexes Stabilized by Mixed N,P-Donor Ligands

The preparation of a new family of iridium(I) complexes with the mixed-donor bidentate ligands o-Ph 2 PC 6 H 4 -CH=NR (PNalkyl) (R=Et, Pr i , Bu i ) is described. In these complexes, the iridium center is coordinated by two PNalkyl ligands in a square-planar environment. All [(PNalkyl) 2 Ir] + compounds undergo reversible electron transfer to the Ir(0) oxidation state as well as irreversible oxidation to Ir(III). Only for R=Bu t is the Ir(II) oxidation state accessible through a reversible oxidation process. For comparative purposes, the electrochemical behavior of the Rh(I) congeners [(PNalkyl) 2 Rh]PF 6 has been investigated

Cl NQR Spectra of Some New Iridium Chlorochalcogenide Complexes

The 35C1 NQR spectra of the chlorochalcogenide complexes of iridium (SCl3)2 [IrCl6] 1, (TeCl3)2 [IrCl6] 2, (SeCl3) [IrCl6] 3, [IrCl3(SCl2)2]2 4, (SCl3) [IrCl4(SCl2)2] 5 and [IrCl3(SeCl2)2]2 6 have been studied. The spectra were recorded by pulse spectrometry. They consist of two mul tiplets, widely separated in frequency. The high-frequency multiplet belongs to the chlorine atoms bonded to sulfur, selenium and tellurium, respectively, in the ligand. A high-frequency shift is observed, which is typical for the coordination of the molecules ACl2(A=S, Se) and ACl4(A=S, Se, Te). The low frequencies in the multiplets of 4 and 6 are attributed to additional bonds formed between chalcogen atom and chlorine in the coordination environment of iridium. In the spectral region 18-24 MHz lie the NQR frequencies of 35Cl bonded to iridium. For 2 the frequencies are close to those of [IrCl6]2-, which confirms the Ir oxidation state IV. The unexpectedly high frequencies for IrIII in the case of 4 and 5 are explained by the influence of ACl2 molecules, which are weaker σ-donors than the chloro ligands. This promotes electron density transfer from neighbouring chlorine atoms. The 18 MHz frequency is assigned to the bridge chlorine atoms in dimeric molecule 4. This assignment is confirmed by the positive value of d v/dT = +0,2.

Crystal structure of polymeric pyrite type Ir 3Te 8

A single crystal X-ray diffraction study performed on pyrite-like Ir 3Te 8 indicates that this compound has rhombohedral symmetry. Magnetic measurements were performed to determine the formal 3 + oxidation state of Ir. This value aided in explaining the interatomic distances of our structure determination. Ir 3Te 8 has been found to belong to a polymeric-type structure, with a tridimensional network made of interlinked Te 2 pairs. The results are compared with pyrite-like MTe 2 ( M = Mn, Fe, Ni, Cu, Ru, Rh, and Os).

Esca study of the state of iridium and oxygen in electrochemically and thermally formed iridium oxide films

X-ray photoelectron spectroscopic measurements on the state of Ir and O species in electrochemically and thermally formed iridium oxide films are reported. The electrochemically formed films were grown to three thicknesses by means of potential cycling. The monolayer oxide film which exhibits electrochemical irreversibility in its formation and reduction already causes a change of electronic state of the Ir substrate surface atoms corresponding to formation of an Ir(OH) 4 species. However, if any electrochemically reversible, thick film oxide, 100×a monolayer is formed, it behaves like bulk IrO 2 powder with regard to 4 f electron shell binding energies. The state of O (1 s ) is also different in this material from that in thinner oxide films, and contains clearly distinguishalbe ionic O 2− rather than OH − , bound water, or adsorbed O 2 , although the latter type of species is also present. Appreciable SO 4 2− ion incorporation or irreversible adsorption can also be detected. No difference in the oxidation state of Ir can be detected on (i.e., over ∼1.5 nm) an oxide film that has been produced electrochemically in its oxidized state or in its reduced state at the end of an anodic or cathodic sweep, respectively, prior to introduction into the electron spectrometer. This is probably due to rapid aerial oxidation of the reduced state of the oxide surface region during the process of transfer of the sample to the spectrometer in ex situ experiments. The electronic characteristics of Ir and O in a thermally formed oxide film are found to be identical with those in the electrochemically formed film only when the latter is thick but are also almost identical with those of Ir and O in chemically prepared IrO 2 powder.

ESCA study of the state of iridium and oxygen in electrochemically and thermally formed iridium oxide films

X-ray photoelectron spectroscopic measurements on the state of Ir and O species in electrochemically and thermally formed iridium oxide films are reported. The electrochemically formed films were grown to three thicknesses by means of potential cycling. The monolayer oxide film which exhibits electrochemical irreversibility in its formation and reduction already causes a change of electronic state of the Ir substrate surface atoms corresponding to formation of an Ir(OH) 4 species. However, if any electrochemically reversible, thick film oxide, 100×a monolayer is formed, it behaves like bulk IrO 2 powder with regard to 4 f electron shell binding energies. The state of O (1 s) is also different in this material from that in thinner oxide films, and contains clearly distinguishalbe ionic O 2− rather than OH −, bound water, or adsorbed O 2, although the latter type of species is also present. Appreciable SO 4 2− ion incorporation or irreversible adsorption can also be detected. No difference in the oxidation state of Ir can be detected on (i.e., over ∼1.5 nm) an oxide film that has been produced electrochemically in its oxidized state or in its reduced state at the end of an anodic or cathodic sweep, respectively, prior to introduction into the electron spectrometer. This is probably due to rapid aerial oxidation of the reduced state of the oxide surface region during the process of transfer of the sample to the spectrometer in ex situ experiments. The electronic characteristics of Ir and O in a thermally formed oxide film are found to be identical with those in the electrochemically formed film only when the latter is thick but are also almost identical with those of Ir and O in chemically prepared IrO 2 powder.