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The influence of uncompensated resistance (also called the I[R.sub.u] effect, where I is current and [R.sub.u] is uncompensated resistance) and slow electrode kinetics have been assessed for the dc and first five ac harmonics derived from Fourier transformed large-amplitude ac voltammetry. Resistance and rate constant conditions emphasized correspond to those where separation of effects attributable to either parameter is essentially impossible under conditions of dc cyclic voltammetry. Results derived from simulations and experiments demonstrate that it is relatively easy to discriminate and quantify contributions from these two effects over a wide range of values using the fourth and fifth harmonic ac components derived from single large-amplitude ac voltammetry measurement. Furthermore, these ac components also are essentially devoid of background charging current. Concepts developed initially from simulations are confirmed by experimental studies on the following: (a) the oxidation of ferrocene, in moderately resistive C[H.sub.3]CN and highly resistive C[H.sub.2][Cl.sub.2] (represents examples of I[R.sub.u] effect on a reversible electron-transfer process); (b) the reduction of a low 0.2 mM concentration of [[Fe[(CN).sub.6]].sup.3-] in the highly conductive 3 M KCl electrolyte media (case of slow kinetics with negligible I[R.sub.u] effect); (c) and reduction of a high 10 mM concentration of [[Fe[(CN).sub.6]].sup.3-] in less conductive aqueous 0.5 M KCl electrolyte media (example where the simultaneous effects of both I[R.sub.u] and slow kinetics need to be resolved).

A study was conducted in which a novel dinuclear Pt(II) and Pt(III) complexes containing cyclometalated teritary arsine ligands were prepared. The electrochemical data implicate the presence of mixed-valent one-electron-oxidized or -reduced species as being important intermediates in the overall two-electron chemical oxidation/reduction pathways.

Large-amplitude sinusoidal ac voltammetric techniques, when analyzed in the frequency domain using the Fourier transform--inverse Fourier transform sequence, produce the expected dc and fundamental harmonic ac responses in addition to very substantial second, third, and higher ac harmonics that arise from the presence of significant nonlinearity. A full numerical simulation of the process, Red [arrow right] [left arrow] Ox + [e.sup.-], incorporates terms for the uncompensated resistance ([R.sub.u]), capacitance of the double layer ([C.sub.dl]), and slow electron transfer kinetics (in particular, the reversible potential (E.sub.o]), rate constant ([k.sub.0]), and charge transfer coefficient ([alpha]) from the Butler--Volmer model). Identification of intuitively obvious patterns of behavior (with characteristically different sensitivity regimes) in dc, fundamental, and higher harmonic terms enables simple protocols to be developed to estimate [R.sub.u] [C.sub.dl], [E.sub.o], [k.sub.0], and [alpha]. Thus, if large-amplitude sinusoidal cyclic voltammograms are obtained for two concentrations of the reduced species, data obtained from analysis of the recovered signals provide initial estimates of parameters as follows: (a) the dc cyclic component provides an estimate of [E.sub.o] (because the [R.sub.u] and [k.sub.0] effects are minimized); (b) the fundamental harmonic provides an estimate of [C.sub.dl] (because it has a high capacitance-to-faradaic current ratio); and (c) the second harmonic provides an estimate of [R.sub.u], [k.sub.0], and [alpha] (because the [C.sub.dl] effect is minimized). Methods of refining the initial estimates are then implemented. As a check on the fidelity of the parameters (estimated on the basis of an essentially heuristic approach that solely utilizes the dc, fundamental, and second harmonic voltammograms), comparison of the predicted simulated and experimental third (or higher) harmonic voltammograms can be made to verify that agreement between theory and experiment has been achieved at a predetermined level. The use of the heuristic pattern recognition approach to evaluate the oxidation of ferrocene at a platinum electrode (a reversible process) in the very high resistance solvent dichloromethane (0.1 M [Bu.sub.4]NP[F.sub.6]) and the reduction of [[Fe(C[N.sub.6])].sup.3-] at a glassy carbon electrode (a quasi-reversible process) in much lower resistance but higher capacitance conditions found in aqueous (0.5 M KCl) media is described and verifies the inherent advantages of employing large-amplitude sinusoidal techniques in quantitative studies of electrode processes.

Significant advantages of Fourier transformed large-amplitude ac higher (second to eighth) harmonics relative to responses obtained with conventional small-amplitude ac or dc cyclic voltammetric methods have been demonstrated with respect to (i) the suppression of capacitive background currents, (ii) the separation of the reversible reduction of [[Ru[(N[H.sub.3]).sub.6].sup.3+] from the overlapping irreversible oxygen reduction process under conditions where aerobic oxygen remains present in the electrochemical cell, and (iii) the kinetic resolution of the reversible [[Ru[(N[H.sub.3]).sub.6]].sup.3+/2+) process in mixtures of [[Fe[(CN).sub.6]].sup.3-] and [[Ru[(N[H.sub.3]).sub.6]].sup.3+] at appropriately treated boron-doped diamond electrodes, even when highly unfavorable [[Fe[(CN).sub.6]].sup.3-] to [[Ru[(N[H.sub.3]).sub.6]].sup.3+] concentration ratios are employed. Theoretical support for the basis of kinetic discrimination in large-amplitude higher harmonic ac cyclic voltammetry is provided.

The electrochemistry of the water oxidation catalyst, Rb8K2[{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2] (Rb8K2-1(0)) has been studied in the presence and absence of potassium cations in both hydrochloric and sulfuric acid solutions by transient direct current (dc) cyclic voltammetry, a steady state dc method in the rotating disk configuration and the kinetically sensitive technique of Fourier transformed large-amplitude alternating current (ac) voltammetry. In acidic media, the presence of potassium ions affects the kinetics (apparent rate of electron transfer) and thermodynamics (reversible potentials) of the eight processes (A′/A to H/H′) that are readily detected under dc voltammetric conditions. The six most positive processes (A′/A to F/F′), each involve a one electron ruthenium based charge transfer step (A′/A, B′/B are RuⅣ/Ⅴ oxidation and C/C′ to F/F′ are RuⅣ/Ⅲ reduction). The apparent rate of electron transfer of the ruthenium centers in sulfuric acid is higher than in hydrochloric acid. The addition of potassium cations increases the apparent rates and gives rise to a small shift of reversible potential. Simulations of the Fourier transformed ac voltammetry method show that the B′/B, E/E′, and F/F′ processes are quasi-reversible, while the others are close to reversible. A third RuⅣ/Ⅴ oxidation process is observed just prior to the positive potential limit via dc methods. Importantly, the ability of the higher harmonic components of the ac method to discriminate against the irreversible background solvent process allows this (process I) as well as an additional fourth reversible ruthenium based process (J) to be readily identified. The steady-state rotating disk electrode (RDE) method confirmed that all four Ru-centers in Rb8K2-1(0) are in oxidation state IV. The dc and ac data indicate that reversible potentials of the four ruthenium centers are evenly spaced, which may be relevant to understanding of the water oxidation electrocatalysis. A profound effect of the potassium cation is observed for the one-electron transfer process (G/G′) assigned to RuⅢ/Ⅱ reduction and the multiple electron transfer reduction process (H/H′) that arise from the tungstate polyoxometalate framework. A significant shift of E°′ to a more positive potential value for process H/H′ was observed on removal of K+ (∼100 mV in H2SO4 and ∼50 mV in HCl). [ABSTRACT FROM AUTHOR]

Graphene modified electrodes have been fabricated byelectrodepositionfrom an aqueous graphene oxide solution onto conducting Pt, Au, glassycarbon, and indium tin dioxide substrates. Detailed investigationsof the electrochemistry of the [Ru(NH3)6]3+/2+and [Fe(CN)6]3-/4-andhydroquinone and uric acid oxidation processes have been undertakenat glassy carbon and graphene modified glassy carbon electrodes usingtransient cyclic voltammetry at a stationary electrode and near steady-statevoltammetry at a rotating disk electrode. Comparisons of the datawith simulation suggest that the transient voltammetric characteristicsat graphene modified electrodes contain a significant contributionfrom thin layer and surface confined processes. Consequently, interpretationsbased solely on mass transport by semi-infinite linear diffusion mayresult in incorrect conclusions on the activity of the graphene modifiedelectrode. In contrast, steady-state voltammetry at a rotating diskelectrode affords a much simpler method for the evaluation of theperformance of graphene modified electrode since the relative importanceof the thin layer and surface confined processes are substantiallydiminished and mass transport is dominated by convection. Applicationof the rotated electrode approach with carbon nanotube modified electrodesalso should lead to simplification of data analysis in this environment. [ABSTRACT FROM AUTHOR]

Under voltammetric conditions, the neutral decamethylferrocene ([Me 10 Fc]) to cationic ([Me 10 Fc] + ) Fe II/III process is a well-known reversible outer-sphere reaction. A companion cationic [Me 10 Fc] + to dicationic [Me 10 Fc] 2+ Fe III/IV process has been reported under direct current (DC) cyclic voltammetric conditions at highly positive potentials in liquid SO 2 at low temperatures and in a 1.5:1.0 AlCl 3 /1-butylpyridinium chloride melt. This study demonstrates that in room-temperature ionic liquids containing the hard to oxidize and hydrophobic tris(pentafluoroethyl)trifluorophosphate anion, the [Me 10 Fc] +/2+ process can be detected as a quasi-reversible reaction at glassy carbon (GC) and boron-doped diamond (BDD) electrodes. Large amplitude Fourier-transformed alternating current (FT-AC) voltammetry minimizes background current contributions occurring at potentials similar to those of the Fe III/IV process in the second and higher-order harmonics. This enables a straightforward determination of the thermodynamics and kinetics for both the Fe II/III and Fe III/IV processes. Unlike the ideal outer-sphere Fe II/III process, the parameters of the Fe III/IV process may be impacted by ion-interaction effects. For the faster Fe II/III process, heterogeneous rate constants are approximately 10 times smaller at BDD than those at GC electrodes. This electrode dependence is less pronounced for the slower Fe III/IV process. The slower BDD kinetics may be attributed in part to a density of states lower than that at GC.

Vanadium is accommodated in both the framework (V out V ) and central positions (V in V ) in the Keggin-type polyoxometalates (POMs) [V in V V out V M 11 O 40 ] 4- (M = Mo, W; V in V V out V M 11 ) and in the central position in [V in V M 12 O 40 ] 3- (V in V M 12 ). The structures of the V in V V out V M 11 class have been determined by X-ray crystallography and compared to those of V in V M 12 reported previously. A major feature of interest with POMs is their capacity for very extensive reduction, particularly when protonation accompanies the electron transfer step. With V in V V out V M 11 and V in V M 12 POMs, knowledge as to whether reduction occurs at V or M sites and the concomitant dependence on acidity has been obtained. Frozen solution EPR spectra obtained following bulk electrolysis showed that the one-electron reduction of V in V Mo 12 occurs at the molybdenum framework site to give V in V Mo V Mo 11 . In contrast, EPR spectra of one-electron reduced V in V W 12 at <30 K are consistent with the electron being accommodated on the central V atom in a tetrahedral environment to give V in IV W 12 . In the case of V in V V out V M 11 , the initial reduction occurs at the framework V out V site to give V in V V out IV M 11 . The second electron is delocalized over the Mo framework in two-electron reduced V in V V out IV Mo V Mo 10 , whereas it is accommodated on the central V site in V in IV V out IV W 11 . The distance between V in IV and V out IV in V in IV V out IV W 11 estimated as 3.5 ± 0.2 Å from analysis of the EPR spectrum is consistent with that obtained in V in V V out V W 11 from crystallographic data. Simulations of the cyclic voltammograms as a function of CF 3 SO 3 H acid concentration for the initial reduction processes provide excellent agreement with experimental data obtained in acetonitrile (0.10 M [ n -Bu 4 N][PF 6 ]) and allowed acid association constants to be estimated and compared with the literature values available for [XV out V M 11 O 40 ] n - (X = S ( n = 3), P and As ( n = 4); M = Mo, W). The interpretation of the voltammetric data is supported by 51 V NMR measurements on the oxidized V V forms of the POMs.

Photoelectrocatalytic (PEC) CO 2 reduction to value-added chemicals is a promising solution to address the energy and environmental issues we face currently. Herein, a unique photocathode Bi@ZFO NTs (Bi and α-Fe 2 O 3 co-modified ZnO nanorod arrays) with high utilization of visible light and sharp-tips effect are successfully prepared using a facile method. Impressively, the performance of Bi@ZFO NTs for PEC CO 2 reduction to HCOOH included small onset potential (-0.53 V vs RHE), Tafel slope (101.2 mV dec -1 ), and a high faraday efficiency of 61.2% at -0.65 V vs RHE as well as favorable stability over 4 h in an aqueous system under visible light illumination. Also, a series of experiments were performed to investigate the origin of its high activity, indicating that the metallic Bi and α-Fe 2 O 3 /ZnO nanojunction should be responsible for the favorable CO 2 adsorption/activation and charge transition/carrier separation, respectively. Density functional theory calculations reveal that the Bi@ZFO NTs could lower the intermediates' energy barrier of HCOO* and HCOOH* to form HCOOH due to the strong interaction of Bi and α-Fe 2 O 3 /ZnO.

The development of potassium-ion batteries (PIBs) is challenged by the shortage of stable cathode materials capable of reversibly hosting the large-sized K + (1.38 Å), which is prone to cause severe structural degradation and complex phase evolution during the potassiation/depotassiation process. Here, we identified that anionic doping of the layered oxides for PIBs is effective to combat their capacity fading at high voltage (>4.0 V). Taking P2-type K 2/3 Mn 7/9 Ni 1/9 Ti 1/9 O 17/9 F 1/9 (KMNTOF) as an example, we showed that the partial substitution of O 2- by F - enlarged the interlayer distance of the K 2/3 Mn 7/9 Ni 1/9 Ti 1/9 O 2 (KMNTO), which becomes more favorable for fast K + transition without violent structural destruction. Meanwhile, based on the experimental data and theoretical results, we identified that the introduction of F - anions effectively increased the redox-active Mn cationic concentration by lowering the average valence of the Mn element, accordingly providing more reversible capacity derived from the Mn 3+/4+ redox couple, rather than oxygen redox. This anionic doping strategy enables the KMNTOF cathode to deliver a high reversible capacity of 132.5 mAh g -1 with 0.53 K + reversible (de)intercalation in the structure. We expect that the discovery provides new insights into structural engineering for pursuing stable cathodes to facilitate the future applications of high-performance PIBs.

ConspectusElectrochemical reduction of the greenhouse gas CO 2 offers prospects for the sustainable generation of fuels and industrially useful chemicals when powered by renewable electricity. However, this electrochemical process requires the use of highly stable, selective, and active catalysts. The development of such catalysts should be based on a detailed kinetic and mechanistic understanding of the electrochemical CO 2 reduction reaction (eCO 2 RR), ideally through the resolution of active catalytic sites in both time (i.e., temporally) and space (i.e., spatially). In this Account, we highlight two advanced spatiotemporal voltammetric techniques for electrocatalytic studies and describe the considerable insights they provide on the eCO 2 RR. First, Fourier transformed large-amplitude alternating current voltammetry (FT ac voltammetry), as applied by the Monash Electrochemistry Group, enables the resolution of rapid underlying electron-transfer processes in complex reactions, free from competing processes, such as the background double-layer charging current, slow catalytic reactions, and solvent/electrolyte electrolysis, which often mask conventional voltammetric measurements of the eCO 2 RR. Crucially, FT ac voltammetry allows details of the catalytically active sites or the rate-determining step to be revealed under catalytic turnover conditions. This is well illustrated in investigations of the eCO 2 RR catalyzed by Bi where formate is the main product. Second, developments in scanning electrochemical cell microscopy (SECCM) by the Warwick Electrochemistry and Interfaces Group provide powerful methods for obtaining high-resolution activity maps and potentiodynamic movies of the heterogeneous surface of a catalyst. For example, by coupling SECCM data with colocated microscopy from electron backscatter diffraction (EBSD) or atomic force microscopy, it is possible to develop compelling correlations of (precatalyst) structure-activity at the nanoscale level. This correlative electrochemical multimicroscopy strategy allows the catalytically more active region of a catalyst, such as the edge plane of two-dimensional materials and the grain boundaries between facets in a polycrystalline metal, to be highlighted. The attributes of SECCM-EBSD are well-illustrated by detailed studies of the eCO 2 RR on polycrystalline gold, where carbon monoxide is the main product. Comparing SECCM maps and movies with EBSD images of the same region reveals unambiguously that the eCO 2 RR is enhanced at surface-terminating dislocations, which accumulate at grain boundaries and slip bands. Both FT ac voltammetry and SECCM techniques greatly enhance our understanding of the eCO 2 RR, significantly boosting the electrochemical toolbox and the information available for the development and testing of theoretical models and rational catalyst design. In the future, it may be possible to further enhance insights provided by both techniques through their integration with in situ and in operando spectroscopy and microscopy methods.

Polyoxometalates (POMs) have been proposed as electromaterials for lithium-based batteries because they provide access to multiple electron transfer reactions coupled to fast lithium ion transport processes and high stability over many redox cycles. Consequently, knowledge of reversible potentials and Li + cation-POM anion interactions provides a strategic basis for their further development. In this study, detailed cyclic voltammetric studies of a series of [XV V M 11 O 40 ] n - (XVM 11 n - ) POMs (where X (heteroatom) = P ( n = 4), As ( n = 4), and S ( n = 3) and M (addenda atom) = Mo, W) have been undertaken in CH 3 CN in the presence of LiClO 4 , with n -Bu 4 NPF 6 also present when required to keep the ionic strength close to constant value of 0.1 M. An analysis of the data has allowed the impact of the POM charge, and addenda and hetero atoms on the reversible potentials and the interaction between Li + and the oxidized XV V M 11 n - and reduced XV IV M 11 ( n +1)- forms of the V V/IV redox couple to be determined. The SV V / IV M 11 3-/4- process is independent of the Li + concentration, implying the absence of the association of this cation with either SV V M 11 3- or SV IV M 11 4- redox levels. However, lithium-ion association constants for both V V and V IV redox levels were obtained from a comparison of simulated and experimental cyclic voltammograms for the reduction of the more negatively charged XV V M 11 4- (X = P, As; M = Mo, W), since the Li + interaction with these more negatively charged POMs is much stronger. The interaction between Li + and the oxidized, XV V M 11 n - , and reduced, XV IV M 11 ( n +1)- , forms was also investigated by 51 V NMR and EPR spectroscopy, respectively, and it was confirmed that, due to their lower charge density, SV V M 11 3- and SV IV M 11 4- interact significantly less strongly with the lithium ion than XV V M 11 4- and XV IV M 11 5- (X = P, As). The lithium-POM association constants are substantially smaller than the corresponding proton association constants reported previously, which is attributed to a smaller surface charge density. The much stronger impact of Li + on the W VI/V - and Mo VI/V -based reductions that occur at more negative potentials than the V V/IV process also has been qualitatively evaluated.

A model is presented for the chemomechanical coupling of kinesin motors, which proposes that the rate constants of the chemical reaction are independent of the external force. On the basis of the model, we study theoretically the movement dynamics of the motors under varying external force and ATP concentration, such as the forward to backward stepping ratio, velocity, dwell time between two mechanical steps, stall force, and so on. The theoretical results reproduce quantitatively the diverse and even contradictory available single-molecule experimental data for different species of the motors. Furthermore, we study the dependence of the chemomechanical coupling ratio on ATP concentration and external force, with both ATP concentration and external force having large effects on the chemomechanical coupling.

Electrochemical reduction of CO 2 is a promising method to convert CO 2 into fuels or useful chemicals, such as carbon monoxide (CO), hydrocarbons, and alcohols. In this study, nanostructured Ag was obtained by electrodeposition of Ag in the presence of a Keggin type polyoxometalate, [PMo 12 O 40 ] 3- (PMo). Metallic Ag is formed upon reduction of Ag + . Adsorption of PMo on the surface of the newly formed Ag lowers its surface energy thus stabilizes the nanostructure. The electrocatalytic performance of this Ag-PMo nanocomposite for CO 2 reduction was evaluated in a CO 2 saturated dimethylformamide medium containing 0.1 M [ n-Bu 4 N]PF 6 and 0.5% (v/v) added H 2 O. The results show that this Ag-PMo nanocomposite can catalyze the reduction of CO 2 to CO with an onset potential of -1.70 V versus Fc 0/+ , which is only 0.29 V more negative than the estimated reversible potential (-1.41 V) for this process and 0.70 V more positive than that on bulk Ag metal. High faradaic efficiencies of about 90% were obtained over a wide range of applied potentials. A Tafel slope of 60 mV dec -1 suggests that rapid formation of *CO 2 •- is followed by the rate-determining protonation step. This is consistent with the voltammetric data which suggest that the reduced PMo interacts strongly with CO 2 (and presumably CO 2 •- ) and hence promotes the formation of CO 2 •- .

The structure of the Keggin-type β-[PW 12 O 40 ] 3- (PW 12 ) polyoxometalate, with n-Bu 4 N + as the countercation, has been determined for the first time by single-crystal X-ray analysis and compared to data obtained from a new determination of the structure of the α-PW 12 isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CH 3 CN (0.1 M [n-Bu 4 N][PF 6 ]) reveals that the reversible potential for the β-PW 12 isomer always remains ca. 100 mV more positive than that of the α-PW 12 isomer on addition of the acid CF 3 SO 3 H. Simulations of the cyclic voltammetry as a function of acid concentration over the range 0-5 mM mimic experimental data exceptionally well. These simulation-experiment comparisons provide access to reversible potentials and acidity constants associated with α and β fully oxidized and one- and two-electron reduced systems and also explain how the two well-resolved one-electron W(VI)/W(V) processes converge into a single two-electron process if sufficient acid is present. 183 W NMR spectra of the oxidized forms of the PW 12 isomers are acid dependent and in the case of β-PW 12 imply that the bridging oxygens between the W I and W II units are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced β-[PW V W VI 11 O 40 ] 4- indicate that either the W I or the W III unit in β-PW 12 is reduced in the β-[PW VI 12 O 40 ] 3- /β-[PW V W VI 11 O 40 ] 4- process. In the absence of acid, reversible potentials obtained from the α- and β-isomers of PW 12 and [SiW 12 O 40 ] 4- exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index.

The electronic properties of metal surfaces can be modulated to weaken the binding energy of adsorbed H-intermediates on the catalyst surface, thus enhancing catalytic activity for the hydrogen evolution reaction (HER). Here we first prepare PdCu alloy nanocubes (NCs) by coreduction of Cu(acac) 2 (acac = acetylacetonate) and Na 2 PdCl 4 in the presence of oleylamine (OAm) and trioctylphosphine (TOP). The PdCu NC coated glassy carbon electrode is then anodized at a constant potential of 0.51 V vs Ag/AgCl at room temperature in 0.5 M H 2 SO 4 solution for 10 s, which converts PdCu NCs into core@shell PdCu@Pd NCs that show much enhanced Pt-like activity for the HER and much more robust durability. The improvements in surface property and HER activity are rationalized based on strain and ligand effects that enhance the activity of the edge-exposed Pd atoms on core@shell PdCu@Pd structure. This work opens up a new perspective for simultaneously reducing metal Pd cost and achieving excellent performance toward the HER.

Eight new members of a family of mixed-metal (Mo,W) polyoxometalates (POMs) with amino acid ligands have been synthesized and investigated in the solid state and solution using multiple physical techniques. While the peripheral POM structural framework is conserved, the different analogues vary in nuclearity of the central metal-oxo core, overall redox state, metal composition, and identity of the zwitterionic α-amino acid ligands. Structural investigations reveal site-selective substitution of Mo for W, with a strong preference for Mo to occupy the central metal-oxo core. This core structural unit is a closed tetrametallic loop in the blue reduced species and an open trimetallic loop in the colorless oxidized analogues. Density functional theory calculations suggest the core as the favored site of reduction and reveal that the corresponding molecular orbital is much lower in energy for a tetra- versus trimetallic core. The reduced species are diamagnetic, each with a pair of strongly antiferromagnetically coupled Mo V centers in the tetrametallic core, while in the oxidized complexes all Mo is hexavalent. Solution small-angle X-ray scattering and circular dichroism (CD) studies indicate that the hybrid POM is stable in aqueous solution on a time scale of days within defined concentration and pH ranges, with the stability enhanced by the presence of excess amino acid. The CD experiments also reveal that the amino acid ligands readily exchange with other α-amino acids, and it is possible to isolate the products of amino acid exchange, confirming retention of the POM framework. Cyclic voltammograms of the reduced species exhibit an irreversible oxidation process at relatively low potential, but an equivalent reductive process is not evident for the oxidized analogues. Despite their overall structural similarity, the oxidized and 2e-reduced hybrid POMs are not interconvertible because of the respective open- versus closed-loop arrangement in the central metal-oxo cores.

Highly efficient electrocatalytic oxidation of ethanol and methanol has been achieved using the ruthenium-containing polyoxometalate molecular catalyst, [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2](10-) ([1(γ-SiW10O36)2](10-)). Voltammetric studies with dissolved and surface-confined forms of [1(γ-SiW10O36)2](10-) suggest that the oxidized forms of 1 can act as active catalysts for alcohol oxidation in both aqueous (over a wide pH range covering acidic, neutral, and alkaline) and alcohol media. Under these conditions, the initial form of 1 also exhibits considerable reactivity, especially in neutral solution containing 1.0 M NaNO3. To identify the oxidation products, preparative scale bulk electrolysis experiments were undertaken. The products detected by NMR, gas chromatography (GC), and GC-mass spectrometry from oxidation of ethanol are 1,1-diethoxyethane and ethyl acetate formed from condensation of acetaldehyde or acetic acid with excess ethanol. Similarly, the oxidation of methanol generates formaldehyde and formic acid which then condense with methanol to form dimethoxymethane and methyl formate, respectively. These results demonstrate that electrocatalytic oxidation of ethanol and methanol occurs via two- and four-electron oxidation processes to yield aldehydes and acids. The total faradaic efficiencies of electrocatalytic oxidation of both alcohols exceed 94%. The numbers of aldehyde and acid products per catalyst were also calculated and compared with the literature reported values. The results suggest that 1 is one of the most active molecular electrocatalysts for methanol and ethanol oxidation.

The motion of adsorbate molecules across surfaces is fundamental to self-assembly, material growth, and heterogeneous catalysis. Recent Scanning Tunneling Microscopy studies have demonstrated the electron-induced long-range surface-migration of ethylene, benzene, and related molecules, moving tens of Angstroms across Si(100). We present a model of the previously unexplained long-range recoil of chemisorbed ethylene across the surface of silicon. The molecular dynamics reveal two key elements for directed long-range migration: first 'ballistic' motion that causes the molecule to leave the ab initio slab of the surface traveling 3-8 Å above it out of range of its roughness, and thereafter skipping-stone 'bounces' that transport it further to the observed long distances. Using a previously tested Impulsive Two-State model, we predict comparable long-range recoil of atomic chlorine following electron-induced dissociation of chlorophenyl chemisorbed at Cu(110).

A method has been developed for the efficient electrodeposition of cobalt and nickel nanostructures with the assistance of the Lindqvist ion [Nb6O19](8-). Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry, and a range of electrochemical techniques have been used to characterize the morphology, composition, catalytic water oxidation activity and stability of the films in alkaline solution. SEM images show that films consisting of nanoparticles with diameters of ca. 30 to 40 nm are formed after 40-50 potential cycles of deposition. Nb and Co/Ni are detected in the films by EDX. ICP-MS results show an elemental ratio of 1:1 for Co:Nb and 1:3 for Ni:Nb, respectively. Raman spectra reveal the presence of both [Nb6O19](8-) and Co(OH)2/Ni(OH)2. The films exhibit excellent stability and efficiency for electrocatalytic water oxidation in alkaline solution. Turnover frequencies of 12.9 and 13.2 s(-1) were determined by rotating ring disk electrode voltammetry at an overpotential of 480 mV for Co and Ni films, respectively. Fourier transformed large amplitude alternating current (FTAC) voltammetry reveals an additional underlying oxidation process for Co under catalytic turnover conditions, which indicates that a Co(IV) species is involved in the efficient catalytic water oxidation reactions. FTAC voltammetric data also suggest that the Ni films undergoes a clear phase transformation upon aging in aqueous 1 M NaOH and the electrogenerated higher oxidation state Ni from β-NiOOH is the more active form of the catalyst.

The water insoluble complex Rb4[Ru(II)(bpy)3]5[{Ru(III)4O4(OH)2(H2O)4}(γ-SiW10O36)2], ([Ru(II)bpy]5[Ru(III)4POM]), was synthesized from Rb8K2[{Ru(IV)4O4(OH)2(H2O)4}(γ-SiW10O36)2] and used for electrocatalytic water oxidation under both thin- and thick-film electrode conditions. Results demonstrate that the [Ru(II)bpy]5[Ru(III)4POM] modified electrode enables efficient water oxidation to be achieved at neutral pH using thin-film conditions, with [Ru(bpy)3](3+)([Ru(III)bpy]) acting as the electron transfer mediator and [Ru(V)4POM] as the species releasing O2. The rotating ring disc electrode (RRDE) method was used to quantitatively determine the turnover frequency (TOF) of the catalyst, and a value of 0.35 s(-1) was obtained at a low overpotential of 0.49 V (1.10 V vs Ag/AgCl) at pH 7.0. The postulated mechanism for the mediator enhanced catalytic water process in a pH 7 buffer containing 0.1 M LiClO4 as an additional electrolyte includes the following reactions (ion transfer for maintaining charge neutrality is omitted for simplicity): [Ru(II)bpy]5[Ru(III)4POM] → [Ru(III)bpy]5[Ru(V)4POM] + 13 e(-) and [Ru(III)bpy]5[Ru(V)4POM] + 2H2O → [Ru(III)bpy]5[Ru(IV)4POM] + O2 + 4H(+). The voltammetry of related water insoluble [Ru(II)bpy]2[S2M18O62] (M = W and Mo) and [Fe(II)Phen]x[Ru(III)4POM] materials has also been studied, and the lack of electrocatalytic water oxidation in these cases supports the hypothesis that [Ru(III)bpy] is the electron transfer mediator and [Ru(V)4POM] is the species responsible for oxygen evolution.

A microporous amorphous-based titania composite photocatalysts has been fabricated using a facile low-temperature (120 °C) synthetic method. Notably, we have successfully prepared the various stages of the amorphous/crystalline heterostructure by simply adjusting the pH value. The high-pH sample favors the formation of amorphous based titania composite structure. Additionally, the BET surface area of the sample increases with the increasing of the pH value, reaching a maximum of 358 m(2) g(-1) when the pH value is 12. Unexpectedly, the H2 productivity of amorphous-based composite photocatalyst without noble metal co-catalyst increases significantly with the increasing pH value, which is attributed to the quickly increasing amorphous, and the highly active catalytic centers created by the synergistic effect between crystalline TiO2 and amorphous ZnO. This study demonstrates that it is possible to improve the properties of the amorphous-based composite photocatalyst by properly modifying the synthesis conditions. The approach presented herein can be applied to the research of controlled amorphous-based composite photocatalytic systems.

Voltammetric studies of the Ru-containing polyoxometalate water oxidation molecular catalyst [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2](10-) ([1(γ-SiW10O36)2](10-) where 1 represents the {Ru4O4(OH)2(H2O)4} core and 1(0) stands for its initial form with all ruthenium centers in the oxidation state IV) have been carried out in aqueous media over a wide range of pH (2-12 using Britton-Robinson buffer) and ionic strength. Well-defined voltammograms in buffered media are only obtained when Frumkin double layer effects are suppressed by the presence of a sufficient concentration of additional supporting electrolyte (LiNO3, NaNO3, KNO3, Ca(NO3)2, Mg(NO3)2, MgSO4, or Na2SO4). A combination of data derived from dc cyclic, rotating disk electrode, and Fourier transformed large amplitude ac voltammetry allow the assignment of two processes related to reduction of the framework and the complete series of Ru(III/IV) and Ru(IV/V) redox processes and also provide their reversible potentials. Analysis of these data reveals that K(+) has a significantly stronger interaction with 1(1) (the number inside bracket stands for the number of electrons removed from 1(0)) than found for the other cations investigated, and hence its presence significantly alters the pH dependence of the 1(0)/1(1) reversible potential. Comparison of experimental data with theory developed in terms of equilibrium constants for process 1(0)/1(1) reveals that both H(+) and K(+) interact competitively with both 1(0) and 1(1). Importantly, reversible potential data reveal that (i) proton transfer does not necessarily need to be coupled to all electron transfer steps to achieve catalytic oxidation of water, (ii) the four-electron oxidized form, 1(4), is capable of oxidizing water under all conditions studied, and (iii) under some conditions, the three-electron oxidized form, 1(3), also exhibits considerable catalytic activity.

The reaction of [Co(II)(NO3)2]·6H2O with the nitroxide radical, 4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(•)), produces the mononuclear transition-metal complex [Co(II)(L(•))2](NO3)2 (1), which has been investigated using temperature-dependent magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, density functional theory (DFT) calculations, and variable-temperature X-ray structure analysis. Magnetic susceptibility measurements and X-ray diffraction (XRD) analysis reveal a central low-spin octahedral Co(2+) ion with both ligands in the neutral radical form (L(•)) forming a linear L(•)···Co(II)···L(•) arrangement. This shows a host of interesting magnetic properties including strong cobalt-radical and radical-radical intramolecular ferromagnetic interactions stabilizing a S = (3)/2 ground state, a thermally induced spin crossover transition above 200 K and field-induced slow magnetic relaxation. This is supported by variable-temperature EPR spectra, which suggest that 1 has a positive D value and nonzero E values, suggesting the possibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radical π*NO orbitals with a small orbital overlap leading to radical-radical ferromagnetic interactions while the cobalt-radical interaction is computed to be strong and ferromagnetic. In the high-spin (HS) case, the DFT calculations predict a weak antiferromagnetic cobalt-radical interaction, whereas the radical-radical interaction is computed to be large and ferromagnetic. The monocationic complex [Co(III)(L(-))2](BPh4) (2) is formed by a rare, reductively induced oxidation of the Co center and has been fully characterized by X-ray structure analysis and magnetic measurements revealing a diamagnetic ground state. Electrochemical studies on 1 and 2 revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with that of the Fe analogues.

Electrochemical studies of the salts [cat](4)[Pt(2)(mu-pop)(4)] (cat(+) = Bu(4)N(+) or PPN(+) [Ph(3)P=N=PPh(3)](+); pop = pyrophosphite, [P(2)O(5)H(2)](2-)) have been carried out in dichloromethane. In agreement with published studies of K(4)[Pt(2)(mu-pop)(4)] in water and [Ph(4)As](4)[Pt(2)(mu-pop)(4)] in acetonitrile, the [Pt(2)(mu-pop)(4)](4-) anion is found to undergo an initial one-electron oxidation under conditions of cyclic voltammetry to a short-lived trianion, [Pt(2)(mu-pop)(4)](3-). However, in the more weakly coordinating solvent dichloromethane, [Pt(2)(mu-pop)(4)](3-) appears to undergo oligomerization instead of solvent-induced disproportionation; thus the overall process remains a one-electron reaction rather than an overall two-electron oxidative addition process, even under long time-scale, bulk electrolysis conditions. Chemical oxidation of [cat](4)[Pt(2)(mu-pop)(4)] with [NO][BF(4)] or AgBF(4) gives mainly a dark, insoluble, ill-defined solid that appears to contain Pt(III) according to X-ray photoelectron spectroscopy (XPS). In the case of [NO][BF(4)], a second reaction product, an orange solid, has been identified as a nitrosyl complex, [cat](3)[Pt(2)(mu-pop)(4)(NO)]. The X-ray structure of the PPN(+) salt shows the anion to consist of the usual lantern-shaped Pt(2)(mu-pop)(4) framework with an unusually large Pt-Pt separation [2.8375(6) A]; one of the platinum atoms carries a bent nitrosyl group [r(N-O) = 1.111(15) A; angle(Pt-N-O) = 118.1(12) degrees] occupying an axial position. The nitrosyl group migrates rapidly on the (31)P NMR time-scale between the metal atoms at room temperature but the motion is slow enough at 183 K that the expected two pairs of inequivalent phosphorus nuclei can be observed. The X-ray photoelectron (XP) spectrum of the nitrosyl-containing anion confirms the presence of two inequivalent platinum atoms whose 4f(7/2) binding energies are in the ranges expected for Pt(II) and Pt(III); an alternative interpretation is that the second platinum atom has a formal oxidation number of +4 and that its binding energy is modified by the strongly sigma-donating NO(-) ligand. Reduction of [Pt(2)(mu-pop)(4)X(2)](4-) (X = Cl, Br, I) in dichloromethane corresponds to a chemically reversible, electrochemically irreversible two-electron process involving loss of halide and formation of [Pt(2)(mu-pop)(4)](4-), as is the case in more strongly coordinating solvents.

A detailed study is reported of the influence of protons, metal cations, and media on the redox chemistry of lacunary anions [alpha-SiW11O39]8- and [alpha-PW11O39]7- of high formal negative charge. Each anion displayed a single chemically reversible one-electron reduction process in carefully dried aprotic CH3CN solution. This process was detected at very negative potentials just prior to the solvent limit. Addition of 0.3 equiv of acid gave rise to a new reduction process at considerably less negative potentials, which is attributed to formation of the protonated species [SiW11O38(OH)]7- and [PW11O38(OH)]6-. Voltammograms derived from simulations based on a double-square scheme are in excellent agreement with experiment. Previous data reported the presence of several processes in CH3CN and appear to have been influenced by the presence of protons and/or adventitious water. Not surprisingly, protonation reactions coupled to charge transfer contribute significantly to the voltammetry of these lacunary anions in buffered aqueous media over the pH range 2-6. A multi-square-scheme mechanism allowed the essential thermodynamic and kinetic features of this system to be captured and an assessment of the relative significance of possible individual pathways. The high formal anionic charges of [SiW11O39]8- and [PW11O39]7- appear to provide highly basic reduced forms that are able to abstract protons from water to produce protonated species which are reduced at potentials more than a volt less negative than those for the processes [SiW11O39]8-/9- and [PW11O39]7-/8- found in dry aprotic media.

The one-electron reduction of [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-) at a glassy carbon electrode was investigated using cyclic and rotating-disk-electrode voltammetry in buffered and unbuffered aqueous solutions over the pH range 3.45-7.50 with an ionic strength of approximately 0.6 M maintained. The behavior is well-described by a square-scheme mechanism P + e(-) <--> Q (E(1)(0/) = -0.275 V, k(1)(0/) = 0.008 cm s(-1), and alpha(1) = 1/2), PH(+) + e(-) <--> QH(+) (E(2)(0/) = -0.036 V, k(2)(0/) = 0.014 cm s(-1), and alpha(2) = 1/2), PH(+) <--> P + H(+) (K(P) = 3.02 x 10(-6) M), and QH(+) <--> Q + H(+) (K(Q) = 2.35 x 10(-10) M), where P, Q, PH(+), and QH(+) correspond to [alpha(2)-Fe(III)(OH)P(2)W(17)O(61)](8-), [alpha(2)-Fe(II)(OH)P(2)W(17)O(61)](9-), [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-), and [alpha(2)-Fe(II)(OH(2))P(2)W(17)O(61)](8-), respectively; E(1)(0)' and E(2)(0)' are the formal potentials, k(1)(0)' and k(2)(0)' are the formal (standard) rate constants, and K(P) and K(Q) are the acid dissociation constants for the relevant reactions. The analysis for the buffered media is based on the approach of Laviron who demonstrated that a square scheme with fully reversible protonations, reversible or quasi reversible electron transfers with the assumption that alpha(1) = alpha(2), can be well-described by the behavior of a simple redox couple, ox + e(-) <--> red, whose formal potential, E(app)(0)', and standard rate constant, k(app)(0)', are straightforwardly derived functions of pH, as are the values of E(1)(0)', k(1)(0)', E(2)(0)', k(2)(0)', and K(P) (only three of the four thermodynamic parameters in a square scheme can be specified). It was assumed that alpha(app) = 1/2, and the simulation program DigiSim was used to determine the values of E(app)(0)' and k(app)(0)', which are required to describe the cyclic voltammograms obtained in buffered media in the pH range from 3.45 to 7.52 (buffer-related reactions which effect general acid-base catalysis are included in the simulations). DigiSim simulations of cyclic voltammograms obtained in unbuffered media yielded the values of E(1)(0)' and k(1)(0)'; K(Q) was then directly computed from thermodynamic constraints. These simulations included additional reactions between the redox species and H(2)O. The value of the diffusion coefficient of the [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-), 2.92 x 10(-6) cm(2) s(-1), was determined using DigiSim simulations of voltammograms at a rotating disk electrode in buffered and unbuffered media at pH 3.45. The diffusion coefficients of all redox species were assumed to be identical. When the pH is greater than 6, instability of P (i.e., [alpha(2)-Fe(III)(OH)P(2)W(17)O(61)](8-)) led to the loss of the reactant and precluded lengthy experimentation.

Fourier transform analysis of ramped square-wave voltammograms indicates the availability of a novel form of kinetic selectivity for surface-confined electron-transfer processes. Thus, for all the even harmonic components, quasi-reversible processes are sensitive to the surface coverage, the reversible potential, the electron-transfer rate constant (k(0')), and the electron-transfer coefficient (alpha), as well as to the amplitude (DeltaE) and frequency (f) of the square wave and dc scan rate. Additionally, it is insensitive to background capacitance current. In contrast, reversible processes and background currents are predicted to be absent from the even harmonics and only detectable in the odd harmonic components. The square-wave voltammetry of the surface-confined quasi-reversible azurin process azurin[Cu(II)] + e(-) right arrow over left arrow azurin[Cu(I)] at a paraffin-impregnated graphite electrode has been employed as a model system to test theoretical predictions. Most voltammetric characteristics of the even harmonic components obtained from the Fourier analysis are consistent with electrode kinetic values of k(0') = 90 s(-1) and alpha = 0.48, although some nonideality possibly due to kinetic dispersion also is apparent. Conditions also have been determined under which a readily generated waveform constructed from the Fourier series of sine waves produces voltammograms that are essentially indistinguishable from those predicted when an ideal square wave is employed.

Detailed electrochemical studies in dichloromethane (0.1 M Bu4NPF6) on the oxidation of the half-lantern [Pt2(kappa2As,C-C6H3-5-Me-2-AsPh2)2(mu-kappaAs,kappaC-C6H3-5-Me-2-AsPh2)2] (1) and full-lantern [Pt2(mu-kappaAs,kappaC-C6H3-5-Me-2-AsPh2)4] (2) complexes reveal the presence of an exceptionally stable dinuclear Pt cation 2+. Thus, oxidation of 1 occurs on the voltammetric time scale via a ladder-square scheme to give 2+, whereas 2 is directly converted to 2+. Electrochemically informed chemical synthesis enabled the isolation of solid [2+][BF4-] to be achieved. Single-crystal X-ray structural analysis showed that 2+ also has a lantern structure but with a shorter separation between the Pt centers [2.7069(3) A (2+), 2.8955(4) A (2)]. EPR spectra of 2+ provide unequivocal evidence for axial symmetry of the complex and are noteworthy because of an exceptionally large, nearly isotropic hyperfine coupling constant of about 0.1 cm(-1). Spectroscopic data support the conclusion that the unpaired electron in the 2+ cation is distributed equally between the two Pt nuclei and imply that oxidation of 2 to 2+ leads to the establishment of the metal-to-metal hemibond. Results of extended Huckel molecular orbital and density functional calculations on 2 and 2+ lead to the conclusions that s, p, dz2 mixing of orbitals contributes to the large EPR Pt hyperfine coupling and also that the structural adjustments that occur upon removal of an electron from 2 are driven by the metal-metal bonding character present in 2+.

Reaction of 2-Li-4-MeC6H3AsPh2 with [PtCl2(SEt2)2] gives two isomeric dinuclear platinum(II) complexes, one containing a half-lantern structure with two chelating and two bridging C6H3-5-Me-2-AsPh2 ligands, [Pt2(kappa2As,C-C6H3-5-Me-2-AsPh2)2(mu-kappaAs,kappaC-C6H3-5-Me-2-AsPh2)2], and the other, a full-lantern with four bridging C6H3-5-Me-2-AsPh2 ligands, [Pt2(mu-kappaAs,kappaC-C6H3-5-Me-2-AsPh2)4]. The lantern structure of the latter is retained in the dihalodiplatinum(III) complexes that are formed by oxidative addition of chlorine, bromine, or iodine to the isomeric mixture. The dichloro derivative undergoes metathesis reactions with silver or sodium salts, yielding the corresponding cyano, thiocyanato, cyanato, and fluoro species. The structures of all complexes have been determined by single-crystal X-ray analysis. The oxidative addition products have Pt-Pt distances in the range 2.65-2.79 A (cf. 2.89 A in the lantern diplatinum(II) structure), consistent with the presence of a Pt-Pt bond. Electrochemical data lead to the conclusion that an initial rapid one-electron process and subsequent chemical reactions produce the dihalodiplatinum(III) lantern structure when mixtures of the lantern and half-lantern complexes are oxidized by halogens. The electrochemical data also explain why chemical reduction of the dihalodiplatinum(III) complex produces only the lantern diplatinum(II) complex.