Ni Oxidation State Research Articles

X-ray Crystallographic, Multifrequency Electron Paramagnetic Resonance, and Density Functional Theory Characterization of the Ni(P(Cy)2N(tBu)2)2(n+) Hydrogen Oxidation Catalyst in the Ni(I) Oxidation State.

The Ni(I) hydrogen oxidation catalyst [Ni(P(Cy)2N(tBu)2)2](+) (1(+); P(Cy)2N(tBu)2 = 1,5-di(tert-butyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane) has been studied using a combination of electron paramagnetic resonance (EPR) techniques (X-, Q-, and D-band, electron-nuclear double resonance, hyperfine sublevel correlation spectroscopy), X-ray crystallography, and density functional theory (DFT) calculations. Crystallographic and DFT studies indicate that the molecular structure of 1(+) is highly symmetrical. EPR spectroscopy has allowed determination of the electronic g tensor and the spin density distribution on the ligands, and revealed that the Ni(I) center does not interact strongly with the potentially coordinating solvents acetonitrile and butyronitrile. The EPR spectra and magnetic parameters of 1(+) are found to be distinctly different from those for the related compound [Ni(P(Ph)2N(Ph)2)2](+) (4(+)). One significant contributor to these differences is that the molecular structure of 4(+) is unsymmetrical, unlike that of 1(+). DFT calculations on derivatives in which the R and R' groups are systematically varied have allowed elucidation of structure/substituent relationships and their corresponding influence on the magnetic resonance parameters.

Combined Spectroscopic and Electrochemical Detection of a Ni(I) ⋅⋅⋅H-N Bonding Interaction with Relevance to Electrocatalytic H2 Production.

The [Ni(P(R) 2 N(R') 2 )2 ](2+) family of complexes are exceptionally active catalysts for proton reduction to H2 . In this manuscript, we explore the first protonation step of the proposed catalytic cycle by using a catalytically inactive Ni(I) complex possessing a sterically demanding variation of the ligand. Due to the paramagnetic nature of the Ni(I) oxidation state, the protonated Ni(I) intermediate has been characterized through a combination of cyclic voltammetry, electron nuclear double resonance (ENDOR) spectroscopy, and hyperfine sublevel correlation (HYSCORE) spectroscopy. Both the electrochemical and spectroscopic studies indicate that the Ni(I) complex is protonated at a pendant amine that is endo to Ni, which suggests the presence of an intramolecular Ni(I) ⋅⋅⋅HN bonding interaction. Using density functional theory, the hydrogen bond was found to involve three doubly-occupied, localized molecular orbitals: the 3dxz , 3d z 2, and 3dyz orbitals of nickel. These studies provide the first direct experimental evidence for this critical catalytic intermediate, and implications for catalytic H2 production are discussed.

Surface Oxygen Exchange Properties of Sr Doped La2NiO4+δ as SOFC Cathode: Thin-Film Electrical Conductivity Relaxation Investigation

La2-xSrxNiO4+δ dense films are prepared by a novel spray-modified pressing method. The surface reaction kinetics is investigated via electrical conductivity relaxation (ECR). The layer thickness, 5~10 μm, is much less than the characteristic length of lanthanum nickelates, resulting in surface-controlled situation and allowing more accurate fitting than the traditional pellets ECR on the surface exchange coefficient (k). k for LNO is 1.6×10-5 cm/s in 0.2 atm at 700oC. Sr doping impairs the exchange kinetics, and k of Sr40 is about one order of magnitude smaller than undoped one. Interstitial oxygen and Ni oxidation state are suggested the predominant roles in determining surface kinetics. Given the properties of the thin-film herein developed by spray-modified pressing is closer to those in practical porous electrode compared to pulsed laser deposited film in terms of preferential orientation and strain, it warrants the use of such a method in a variety of pertinent investigations.

Synthesis, characterization, and preliminary fluorescence study of a mixed-ligand bis(dicarbollyl)nickel complex bearing a tryptophan-BODIPY FRET couple

In continuation of our work on nickelacarborane-based nanomolecular devices, the design and synthesis of a bis(dicarbollyl)nickel complex, in both formal Ni(III) and Ni(IV) oxidation states, bearing two fluorophore molecules capable of fluorescence resonance energy transfer (FRET) was performed. The FRET couple with a small Förster radius (R0 of 26 Å) consisted of tryptophan and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY). Each fluorophore was connected to the nickelacarborane core by a rigid linker containing two pairs of alternating ethynyl and para-phenylene groups, which created a specific distance (l) between the fluorophore molecules depending on the conformation of the nickelacarborane core. The presence (13 Å < l < 39 Å) or absence (l > 39 Å) of energy transfer in the designed system provides insight into the conformational changes of nickelacarboranes in solution. The target nickelacarboranes were prepared via a multistep organic/organometallic synthesis. The structures and compositions of the intermediates and final products were established by a combination of spectroscopic methods and X-ray structure analysis. A preliminary fluorescence study of the prepared nickelacarboranes was performed.

The Effect of Ni Oxidation State on Li-Rich Layered Oxides

Li-rich layered oxides (LLO) are being studied intensively as an upgrade over current layered oxide cathodes for Li-ion batteries due to their increased discharge capacity.1 There are several issues, however, that need to be overcome before LLO can be adopted for commercial use. One of the largest issues is the voltage decay and the subsequent energy decrease, which is caused by the slow transformation of the layered phase into a 3 V spinel-like phase, which occurs due to Mn4+ ions being reduced to Mn3+ during cycling.2 It has recently been reported that incorporating Ni3+ ions into LLO and reducing the Mn4+ content can suppress the voltage decay.3 Our study uses a systematic approach to investigate the effect of Ni3+ ions on LLO with the series Li1.2Mn0.6-x Ni0.2+x O2 with x = 0.00, 0.02, 0.05, 0.10, 0.15, and 0.20, which produce Ni oxidation states of, respectively, 2.00+, 2.18+, 2.40+, 2.67+, 2.86+, and 3.00+. All the materials displayed the expected X-ray diffraction patterns, which consist of the R-3m structure with additional superstructure peaks due to Li/Mn ordering. The (003)/(104) peak ratio, which is a measure of cation mixing, generally decreases with increasing Ni oxidation state. This is because Ni3+ ions (0.56 Ǻ) are smaller than Ni2+ ions (0.69 Ǻ), so there is less Li/Ni cation mixing. Increasing the Ni oxidation state has several effects on the first charge-discharge cycle as well. It increases the sloping-voltage region because there are less insulating Mn4+ ions. It decreases the plateau-voltage region because a decrease in Mn4+ ions reduces the amount of Li2MnO3-like phase in the material and the decrease in Li/Ni cation mixing enhances this effect. Despite the decreased plateau-region capacity, the irreversible capacity loss (IRC) increases, which in turn causes the discharge capacity to decrease generally. The increased IRC may be caused by undesirable side reactions similar to those experienced by layered oxides with high nickel content. Figure 1(a) displays the first discharge cycle dQ/dV data, which show a large decrease in the size of the Mn4+ reduction peak at ~ 3.3 V and increase in the Ni reduction peaks at ~ 4.3 V and ~ 3.8 V, suggesting that increasing the Ni oxidation state could avoid the reduction of Mn4+. Increasing the Ni oxidation state also generally increases the cyclability of these materials. Due to the decrease in the plateau-region capacity and Mn4+ reduction that a higher Ni oxidation state causes, the conversion to the 3 V spinel-like phase is suppressed, resulting in increased cyclability. Because of these effects, the voltage decay is also reduced with increasing Ni oxidation state. Figures 1(b) and (c) show, respectively, the voltage curves for the materials at their 2nd and 40thcycles. Besides the sample with a Ni oxidation state of 2.67+, the increased Ni oxidation state decreases the voltage decay upon extended cycling. The presentation will also focus on the synthesis of another series of materials with Co in order to increase the capacity of the materials. 1. H. Yu and H. Zhou, The Journal of Physical Chemistry Letters, 4, 1268 (2013). 2. E.-S. Lee and A. Manthiram, Journal of Materials Chemistry A, 2, 3932 (2014). 3. M. N. Ates, S. Mukerjee and K. M. Abraham, Journal of The Electrochemical Society, 161, A355 (2014). Figure 1: Electrochemical data for materials with varying Ni oxidation states: (a) first discharge cycle dQ/dV data, (b) 2nd charge-discharge cycle voltage plots, and (c) 40th charge-discharge cycle voltage plots. Figure 1

Effects of Ni2+/3+ Redox Peak Potential on Oxygen Evolution Activity of Mixed-Transition-Metal-Oxides in Alkaline Electrolyte

High-performance, low-cost electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes are vital for the commercialization of water electrolysis systems. Modular water electrolyzers could be the building blocks of a “plug & play” hydrogen highway infrastructure which would allow on-demand hydrogen production for re-fueling fuel-cell electric vehicles. Current state-of-the-art water electrolysis systems are based on proton-exchange-membrane (PEM) technology. The highly acidic environment of these PEM systems requires the use of rare & expensive platinum group metals (PGMs). Recent break-throughs in anion-exchange-membrane (AEM) technology have re-ignited the development of non-PGM catalysts for operation in alkaline electrolytes.1 Recent literature has shown that PGM-free AEM-electolyzers can achieve performance on par with PEM systems.2 However the mass-loading of the catalysts in this proof-of-concept study was quite high. Further increases in HER & OER activity & stability will move these systems closer to commercial deployment. Recent studies have shown that Ni-Fe hydroxides exhibit the highest OER activity of any non-PGM electrocatalysts.3 Our research has investigated binary & ternary Ni-X-(Y) mixed metal oxides. Results indicate that amorphous (non-stoichiometric) oxy-hydroxide surfaces exhibit the greatest OER activity. Ni-Fe-Mo mixed oxide has shown OER activity on par with perovskites.4 We have identified that mixed metal oxide films exhibit shifts in the Ni2+/3+ redox peaks. In-situ XAS studies are correlated with CV data to show how mixing Ni-oxides with Co or Fe can stabilize or inhibit (respectively) formation of Ni in the 3+ oxidation state. These observations indicate that the OER enhancement from Ni-Fe-oxides is a result of charge-transfer from Fe to the Ni active sites, thus providing a lower (non-integer) Ni oxidation state. The resulting Nix+ (2<x<3) surface sites have been shown experimentally to form more amorphous surface structure5 and presumably behave as more reversible active sites. It is likely that the amorphous structure stabilizes the active oxy-hydroxide network, thus stabilizing the OER intermediates and decreasing the OER on-set potential. In addition, the more reversible Nix+ (2<x<3) active sites enable high turn-over-frequencies by decreasing the binding energy of the Ni-O2 adsreaction product. Acknowledgements: Authors acknowledge the financial support from Proton On-Site as part of an ARPA-E grant and use of the National Synchrotron Light Source (NSLS) (beamline X3B), Brookhaven National Lab (BNL).

Adiabaticity of the proton-coupled electron-transfer step in the reduction of superoxide effected by nickel-containing superoxide dismutase metallopeptide-based mimics.

Nickel-containing superoxide dismutases (NiSODs) are bacterial metalloenzymes that catalyze the disproportionation of O2(-). These enzymes take advantage of a redox-active nickel cofactor, which cycles between the Ni(II) and Ni(III) oxidation states, to catalytically disprotorptionate O2(-). The Ni(II) center is ligated in a square planar N2S2 coordination environment, which, upon oxidation to Ni(III), becomes five-coordinate following the ligation of an axial imidazole ligand. Previous studies have suggested that metallopeptide-based mimics of NiSOD reduce O2(-) through a proton-coupled electron transfer (PCET) reaction with the electron derived from a reduced Ni(II) center and the proton from a protonated, coordinated Ni(II)-S(H(+))-Cys moiety. The current work focuses on the O2(-) reduction half-reaction of the catalytic cycle. In this study we calculate the vibronic coupling between the reactant and product diabatic surfaces using a semiclassical formalism to determine if the PCET reaction is proceeding through an adiabatic or nonadiabatic proton tunneling process. These results were then used to calculate H/D kinetic isotope effects for the PCET process. We find that as the axial imidazole ligand becomes more strongly associated with the Ni(II) center during the PCET reaction, the reaction becomes more nonadiabatic. This is reflected in the calculated H/D KIEs, which moderately increase as the reaction becomes more nonadiabatic. Furthermore, the results suggest that as the axial ligand becomes less Lewis basic the observed reaction rate constants for O2(-) reduction should become faster because the reaction becomes more adiabatic. These conclusions are in-line with experimental observations. The results thus indicate that variations in the axial donor's ability to coordinate to the nickel center of NiSOD metallopeptide-based mimics will strongly influence the fundamental nature of the O2(-) reduction process.

Correlation between Chemical and Morphological Heterogeneities in LiNi0.5Mn1.5O4 Spinel Composite Electrodes for Lithium-Ion Batteries Determined by Micro-X-ray Fluorescence Analysis

The high-voltage LiNi0.5Mn1.5O4 (LNMO) spinel is a promising material for high-energy battery applications, despite problems of capacity fade. This is due in part to transition metal leaching that produces chemical and morphological inhomogeneities. Using fast micro-X-ray fluorescence spectroscopy to scan the sample at medium spatial resolution (500 nm) over millimeter ranges, effects of cycling rate and state-of-charge on the elemental distribution (Ni and Mn) for LiNi0.5Mn1.5O4/carbon composite electrodes in LNMO/Li cells are visualized. Charge distribution is imaged by mapping the Ni oxidation state by acquisition of a stack of elemental maps in the vicinity of the Ni K edge. Our results show significant effects on morphology and elemental distribution, such as formation of elemental hot-spots and material erosion, becoming more pronounced at higher cycling rates. In nickel hot-spots, we observed hampered oxidation of nickel during charging.

Effect of nickel oxidation state on the structural and electrochemical characteristics of lithium-rich layered oxide cathodes

Increasing the Ni oxidation state in Li-rich layered oxides enhances the cyclability and reduces the voltage decay due to a decreased degree of Mn4+ reduction and layered-to-spinel transformation.

Cysteinate protonation and water hydrogen bonding at the active-site of a nickel superoxide dismutase metallopeptide-based mimic: implications for the mechanism of superoxide reduction.

Nickel-containing superoxide dismutase (NiSOD) is a mononuclear cysteinate-ligated nickel metalloenzyme that catalyzes the disproportionation of superoxide into dioxygen and hydrogen peroxide by cycling between Ni(II) and Ni(III) oxidation states. All of the ligating residues to nickel are found within the first six residues from the N-terminus, which has prompted several research groups to generate NiSOD metallopeptide-based mimics derived from the first several residues of the NiSOD sequence. To assess the viability of using these metallopeptide-based mimics (NiSOD maquettes) to probe the mechanism of SOD catalysis facilitated by NiSOD, we computationally explored the initial step of the O2(-) reduction mechanism catalyzed by the NiSOD maquette {Ni(II)(SOD(m1))} (SOD(m1) = HCDLP CGVYD PA). Herein we use spectroscopic (S K-edge X-ray absorption spectroscopy, electronic absorption spectroscopy, and circular dichroism spectroscopy) and computational techniques to derive the detailed active-site structure of {Ni(II)(SOD(m1))}. These studies suggest that the {Ni(II)(SOD(m1))} active-site possesses a Ni(II)-S(H(+))-Cys(6) moiety and at least one associated water molecule contained in a hydrogen-bonding interaction to the coordinated Cys(2) and Cys(6) sulfur atoms. A computationally derived mechanism for O2(-) reduction using the formulated active-site structure of {Ni(II)(SOD(m1))} suggests that O2(-) reduction takes place through an apparent initial outersphere hydrogen atom transfer (HAT) from the Ni(II)-S(H(+))-Cys(6) moiety to the O2(-) molecule. It is proposed that the water molecule aids in driving the reaction forward by lowering the Ni(II)-S(H(+))-Cys(6) pK(a). Such a mechanism is not possible in NiSOD itself for structural reasons. These results therefore strongly suggest that maquettes derived from the primary sequence of NiSOD are mechanistically distinct from NiSOD itself despite the similarities in the structure and physical properties of the metalloenzyme vs the NiSOD metallopeptide-based models.

Synthesis and support composition effects on CH4 partial oxidation over Ni–CeLa oxides

Two series of Ni (6wt%) catalysts supported over CeO2, La2O3 and mixed CeO2–La2O3 were prepared by co-precipitation and by wet-impregnation. The effect of the two Ni loading procedures on the catalyst structural properties was investigated by XRD, TPR and XPS. The catalytic behavior of the catalysts was tested in the methane partial oxidation reaction performed at 1atm in a temperature range of 400–800°C using dilute feed gas mixture with CH4/O2=2 and gas hourly space velocity of 60,000mlg−1h−1. Total methane combustion was observed within the 450°C≤T≤650°C temperature range. Above 650°C partial oxidation of methane started to occur, reaching at 800°C a CO selectivity close to 90%. The lanthanum containing catalysts prepared by wet impregnation exhibited higher CPO activity at lower temperature as compared to the co-precipitated ones. Opposite behavior was observed with the Ni–CeO2 catalysts. During the stability test at high temperature, carbon formed only over the single oxide supported catalysts, Ni–CeO2 and Ni–La2O3. According to the characterization results carbon was not responsible for catalyst deactivation but it prevented the sintering of the nickel particle. The interplay of the crystallite sizes and the formation of various nickel–lanthanum oxide species with different Ni oxidation states were responsible for the differences in the CPO performance.

A Study on Structural and Electrochemical Properties of Co Fixed LiNi0.5+XCo0.2Mn0.3-XO2 (x=0, 0.1, 0.2) Using Synchrotron Based X-Ray Techniques

A recent attention in lithium ion battery research is focused on increasing its energy density and structural stability. As a result, the layered Li[NixCoyMnz]O2 (x+y+z=1) electrode materials have attracted a great deal of attention as an alternative of LiCoO2, due to its remarkable electrochemical properties. This three-component system shows high capacity of LiNiO2, layered character of LiCoO2, low cost and thermal stability of manganese in LiMnO2.[1,2] Recently, a lot of research is being carried out to improve the energy density by increasing the Ni content. However, there is always a trade-off between the benefits of large amount of Ni atoms and the risk of side effects, such as poor thermal stability and lower cyclability. In this study, the structural and electrochemical properties of Co fixed LiNi0.5+xCo0.2Mn0.3-xO2 (x=0, 0.1, 0.2) cathode material were investigated by using synchrotron based X-ray diffraction and absorption techniques to understand the effects of Ni and Mn components. The Rietveld refinement of diffraction data was performed by using R-3m model. Refinement results show that lattice parameters of LiNi0.5Co0.2Mn0.3O2 (x=0) are a=2.8697Å, c=14.2378Å, and by increasing the nickel content in LiNi0.5+xCo0.2Mn0.3-xO2 (x=0, 0.1, 0.2)lattice parameters “a” and “c” shrinkas shown in Table 1. Generally, it is believed that the Ni2+ content in the Li layer increase when the total amount of Ni component increase in the three component system. However, in the Co fixed LiNi0.5+xCo0.2Mn0.3-xO2 cathode material system, interestingly cation mixing decreases by increasing the nickel content. The NCM721 (x=0.2) electrode has lower Ni2+ ions in the lithium layer compared to NCM622 (x=0.1) which has even lower quantity of Ni2+ ions than that of NCM523 (x=0). In pristine electrode material, the theoretical value of average Ni oxidation state is Ni2.4+, Ni2.67+, Ni2.86+ for x=0, 0.1, 0.2 respectively and this trend is confirmed by Ni K-edge XANES spectra. The Ni K-edge XANES spectra show shift in energy position with change of nickel content in the electrode material. In NCM523, Ni K-edge position is close to that of Ni2+, while that of NCM721 is at higher energy position, indicating higher Ni valence (close to Ni3+) as shown in Figure 1. The increase of average Ni oxidation state in NCM721 pristine material suggests that Ni3+ component is predominant in its transition metal layer. Therefore, shrinking of lattice parameters by increasing the the nickel quantity can be explained on the basis of Ni3+ content in the transition metal layer. In the Co fixed system (rCo3 +=0.545Å), the unit cell dimensions in a hexagonal setting shrink by increasing the Ni content because of the difference in size between divalent and trivalent nickel ions (rNi2+=0.69Å and rNi3+=0.56Å). When the Ni3+ increases, the Ni2+ and the Mn4+ (rMn4 + =0.53Å) content decrease, net result is decrease in lattice parameters because the increment is small (rNi3+-rMn4+=0.03Å) compare to the decrement (rNi3+-rNi2+=-0.13Å). For the Ni2+ content in Li layer, we can also say that there is low opportunity for Ni2+ to go into the Li layer because the total amount of Ni2+ content decreases with increase of the Ni content. Consequently, these effects of nickel content in the layered structure would affect electrochemical properties such as capacity, cyclability and rate capability; hence their study can assist in designing safe and high performing electrode materials. The more detailed structural and electrochemical properties of Co fixed LiNi0.5+xCo0.2Mn0.3-xO2 cathode material will be presented at the time of meeting.[1] M.S. Whittingham, Chem. Rev. 104 (2004) 4271-4301[2] K.-W. Nam et al., Journal of Power Sources 189 (2009) 515-518

Aqueous solution synthesis of Pt-M (M = Fe, Co, Ni) bimetallic nanoparticles and their catalysis for the hydrolytic dehydrogenation of ammonia borane.

Platinum-based bimetallic nanocatalysts have attracted much attention due to their high-efficiency catalytic performance in energy-related applications such as fuel cell and hydrogen storage, for example, the hydrolytic dehydrogenation of ammonia borane (AB). In this work, a simple and green method has been demonstrated to successfully prepare Pt-M (M = Fe, Co, Ni) NPs with tunable composition (nominal Pt/M atomic ratios of 4:1, 1:1, and 1:4) in aqueous solution under mild conditions. All Pt-M NPs with a small size of 3-5 nm show a Pt fcc structure, suggesting the bimetallic formation (alloy and/or partial core-shell), examined by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray absorption fine structure (XAFS) analysis. The catalytic activities of Pt-M NPs in the hydrolytic dehydrogenation of AB reveal that Pt-Ni NPs with a ratio of 4:1 show the best catalytic activity and even better than that of pure Pt NPs when normalized to Pt molar amount. The Ni oxidation state in Pt-Ni NPs has been suggested to be responsible for the corresponding catalytic activity for hydrolytic dehydrogenation of AB by XAFS study. This strategy for the synthesis of Pt-M NPs is simple and environmentally benign in aqueous solution with the potential for scale-up preparation and the in situ catalytic reaction.

Insight into the Structure and Mechanism of Nickel-Containing Superoxide Dismutase Derived from Peptide-Based Mimics

Nickel superoxide dismutase (NiSOD) is a nickel-containing metalloenzyme that catalyzes the disproportionation of superoxide through a ping-pong mechanism that relies on accessing reduced Ni(II) and oxidized Ni(III) oxidation states. NiSOD is the most recently discovered SOD. Unlike the other known SODs (MnSOD, FeSOD, and (CuZn)SOD), which utilize "typical" biological nitrogen and oxygen donors, NiSOD utilizes a rather unexpected ligand set. In the reduced Ni(II) oxidation state, NiSOD utilizes nitrogen ligands derived from the N-terminal amine and an amidate along with two cysteinates sulfur donors. These are unusual biological ligands, especially for an SOD: amine and amidate donors are underrepresented as biological ligands, whereas cysteinates are highly susceptible to oxidative damage. An axial histidine imidazole binds to nickel upon oxidation to Ni(III). This bond is long (2.3-2.6 Å) owing to a tight hydrogen-bonding network. All of the ligating residues to Ni(II) and Ni(III) are found within the first 6 residues from the NiSOD N-terminus. Thus, small nickel-containing metallopeptides derived from the first 6-12 residues of the NiSOD sequence can reproduce many of the properties of NiSOD itself. Using these nickel-containing metallopeptide-based NiSOD mimics, we have shown that the minimal sequence needed for nickel binding and reproduction of the structural, spectroscopic, and functional properties of NiSOD is H2N-HCXXPC. Insight into how NiSOD avoids oxidative damage has also been gained. Using small NiN2S2 complexes and metallopeptide-based mimics, it was shown that the unusual nitrogen donor atoms protect the cysteinates from oxidative damage (both one-electron oxidation and oxygen atom insertion reactions) by fine-tuning the electronic structure of the nickel center. Changing the nitrogen donor set to a bis-amidate or bis-amine nitrogen donor led to catalytically nonviable species owing to nickel-cysteinate bond oxidative damage. Only the amine/amidate nitrogen donor atoms within the NiSOD ligand set produce a catalytically viable species. These metallopeptide-based mimics have also hinted at the detailed mechanism of SOD catalysis by NiSOD. One such aspect is that the axial imidazole likely remains ligated to the Ni center under rapid catalytic conditions (i.e., high superoxide loads). This reduces the degree of structural rearrangement about the nickel center, leading to higher catalytic rates. Metallopeptide-based mimics have also shown that, although an axial ligand to Ni(III) is required for catalysis, the rates are highest when this is a weak interaction, suggesting a reason for the long axial His-Ni(III) bond found in NiSOD. These mimics have also suggested a surprising mechanistic insight: O2(-) reduction via a "H(•)" tunneling event from a R-S(H(+))-Ni(II) moiety to O2(-) is possible. The importance of this mechanism in NiSOD has not been verified.

Organometallic Nickel(III) Complexes Relevant to Cross-Coupling and Carbon–Heteroatom Bond Formation Reactions

Nickel complexes have been widely employed as catalysts in C-C and C-heteroatom bond formation reactions. In addition to Ni(0) and Ni(II) intermediates, several Ni-catalyzed reactions are proposed to also involve odd-electron Ni(I) and Ni(III) oxidation states. We report herein the isolation, structural and spectroscopic characterization, and organometallic reactivity of Ni(III) complexes containing aryl and alkyl ligands. These Ni(III) species undergo transmetalation and/or reductive elimination reactions to form new C-C or C-heteroatom bonds and are also competent catalysts for Kumada and Negishi cross-coupling reactions. Overall, these results provide strong evidence for the direct involvement of organometallic Ni(III) species in cross-coupling reactions and oxidatively induced C-heteroatom bond formation reactions.

Crystal chemistry and valence determinations for Mn, Ni and Co oxides as cathode materials in Li batteries

Abstract Cation charge distribution is one of the basic problems for cathode materials in Li and Na batteries. This work analyzed the valence–distance relationship for a large number of Ni, Mn, and Co oxides. It uses high quality structural data obtained recently, and takes into account the influence of lattice strains on the bond lengths. The linear character of the valence–distance correlation for the Ni and Mn cations allows for an accurate valence determination from the average Mn–O and Ni–O distances in the charge range from + 2 to + 4. The correlation for Co is more complicated, and the unambiguous valence assignment is possible only in the range from + 2 to + 3. Although we used mainly the diffraction data to establish the valence–distance relationships, in many cases, especially for the mixed oxides, the assignment of Ni, Mn, and Co oxidation states should be done based on the local structure descriptions obtained by X-ray absorption spectroscopy. Important examples are presented to illustrate the use of the valence–distance correlations for detailed structural analysis, especially for the lattice strain determinations.

Pulsed laser deposition of yttria stabilized zirconia based heterostructure

Yttria stabilized zirconia (YSZ), nickel-doped yttria stabilized zirconia (NiYSZ) thin films and NiYSZ/YSZ anode–electrolyte heterostructure have been synthesized on Si(100) substrate by pulsed laser deposition technique. Atomic Force Microscopy, Scanning Electron Microscopy, and Variable Angle Spectroscopic Ellipsometry studies have been applied to determine the surface topography, film morphology and thickness. The X-ray diffraction measurements evidenced a highly textured growth along (111) direction for NiYSZ/YSZ heterostructure with crystallite dimension of 18nm. The X-ray photoelectron spectroscopy analysis showed a displacement of Ni oxidation states along the depth profile, with Ni3+ oxidation state on the surface, a very thin layer containing Ni2+ associated to NiO buried under the surface, while metallic Ni is predominant in the bulk. The presence of an intermix layer at the interface between the NiYSZ and YSZ, suggested by the ellipsometric measurements, was confirmed by Secondary Neutral Mass Spectrometry data.

Conformational Dynamics and Proton Relay Positioning in Nickel Catalysts for Hydrogen Production and Oxidation

The [Ni(PR2NR′2)2]2+ complexes (where PR2NR′2 is 1,5-R′-3,7-R-1,5-diaza-3,7-diphosphacyclooctane) are fast electrocatalysts for H2 production and oxidation. Binding of a fifth ligand (CH3CN or BF4–) or chair/boat isomerization has the potential to slow catalysis by blocking the addition of H2 or by incorrectly positioning the pendant amines. We report the structural dynamics of a series of nickel complexes characterized by NMR spectroscopy and theoretical modeling to examine the effects of the fifth ligand for the Ni(II) complexes, including CH3CN, BF4–, Cl–, and H–, as well as the differences in dynamics between the Ni(II) and Ni(0) oxidation states. A fast exchange process was observed for the [Ni(CH3CN)(PR2NR′2)2]2+ complexes, with rates ranging from 104 to 107 s–1 depending on the phosphorus and nitrogen substituents on the PR2NR′2 ligand. This exchange process was identified to occur through a multistep mechanism, which consists of dissociation of the acetonitrile, boat/chair isomerization of each of the four rings (including nitrogen inversion), and reassociation of an acetonitrile on the opposite side of the complex. The rate of the chair/boat inversion was found to be influenced by varying the substituent on the nitrogen atom, but the rate of the overall exchange process is at least an order of magnitude faster than the catalytic rate in acetonitrile, demonstrating that the structural dynamics of the [Ni(CH3CN)(PR2NR′2)2]2+ complexes do not hinder catalysis. Possible catalytic implications of the coordination of a fifth ligand to the Ni(II) complex are discussed.

In situ XAS and FTIR studies of a multi-component Ni/Fe/Cu catalyst for hydrogen production from ethanol

Multicomponent catalysts containing Ni, Fe, Cu active for ethanol reforming reactions, prepared by solution combustion synthesis are characterized by multiple techniques such as ex situ XRD, XPS, and in situ XAFS and FTIR. XRD results indicate copper to be present in the reduced state as CuNi bimetal while nickel and iron are observed to be partially in a spinel NiFe2O4 structure. In situ XANES and XAFS analysis show a change in Ni, Fe and Cu oxidation states during reaction. Cu, which was fully reduced before reaction, became partly oxidized upon exposure to ethanol and oxygen. Ni is mostly (75%) reduced and does not seem to change its oxidation state during the reaction. Fe is not present in metallic form after reduction and during the reaction, but some change in the oxidation state from Fe(II) to Fe(III) occurred during the reaction. XPS and SEM images indicate the formation of carbon filament on the spent catalyst. XPS results also indicate the enrichment of surface by Fe and Cu during the reduction of the catalyst. Based on the activity and characterization results obtained, and literature review, the role of predominant phases during ethanol decomposition reaction is proposed.

Origin of Electrochromism in High-Performing Nanocomposite Nickel Oxide

Electrochromic effects of transition metal oxides provide a great platform for studying lithium intercalation chemistry in solids. Herein, we report on an electronically modified nanocomposite nickel oxide (i.e., Li2.34NiZr0.28Ox) that exhibits significantly improved electrochromic performance relative to the state-of-the-art inorganic electrochromic metal oxides in terms of charge/discharge kinetics, bleached-state transparency, and optical modulation. The knowledge obtained from O K-edge X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) suggests that the internally grown lithium peroxide (i.e., Li2O2) species plays a major role in facilitating charge transfer thus enabling optimal electrochromic performance. This understanding is relevant to recent theoretical studies concerning conductivity in Li2O2 for lithium-air batteries (as cited in the main text). Furthermore, we elucidate the electrochromism in modified nickel oxide in lithium ion electrolyte with the aid of Ni K-edge XAS and Ni L-edge XAS studies. The electrochromism in the nickel oxide materials arises from the reversible formation of hole states on the NiO6 units, which then impacts the Ni oxidation state through the Ni3d-O2p hybridization states. This study sheds light on the lithium intercalation chemistry for general energy storage and semiconductor applications.