Function Of Electrochemical Potential Research Articles

Charge-Transfer Intermediates in the Electrochemical Doping Mechanism of Conjugated Polymers.

We address the nature of electrochemically induced charged states in conjugated polymers, their evolution as a function of electrochemical potential, and their coupling to their local environment by means of transient absorption and Raman spectroscopies synergistically performed in situ throughout the electrochemical doping process. In particular, we investigate the fundamental mechanism of electrochemical doping in an oligoether-functionalized 3,4-propylenedioxythiophene (ProDOT) copolymer. The changes embedded in both linear and transient absorption features allow us to identify a precursor electronic state with charge-transfer (CT) character that precedes polaron formation and bulk electronic conductivity. This state is shown to contribute to the ultrafast quenching of both neutral molecular excitations and polarons. Raman spectra relate the electronic transition of this precursor state predominantly to the Cβ-Cβ stretching mode of the thiophene heterocycle. We characterize the coupling of the CT-like state with primary excitons and electrochemically induced charge-separated states, providing insight into the energetic landscape of a heterogeneous polymer-electrolyte system and demonstrating how such coupling depends on environmental parameters, such as polymer structure, electrolyte composition, and environmental polarity.

Real-Time Monitoring of Electrochromic Memory Loss of Layered α-MoO3 Nanoplates

Combined in situ cyclic electrochemical and UV–vis spectroscopic methods were employed to monitor the memory loss of electrochromic properties of layered α-MoO3 nanoplates. The time-resolved characteristics of this in situ study allowed for the quantification of changes in charge and optical densities as a function of electrochemical potential over time. Lithium ions trapped in the crystalline lattice of α-MoO3 during the bleaching process, along with the irreversible reduction of Mo6+ to Mo5+, govern the memory loss responsible for the degradation of the electrochromic properties. These experiments demonstrated the existence of a saturation limit of the structural charge insertion that effectively contributes to the electrochromic performance of α-MoO3 nanoplates. The study improves the understanding of electrochromic memory loss and the degradation mechanism and suggests a two-step electrochemical reaction that controls the electrochromic activity of the α-MoO3 phase.

Temperature Effect of CO2 Reduction Electrocatalysis on Copper: Potential Dependency of Activation Energy

Abstract The electrochemical CO2 reduction reaction (CO2RR) has gathered widespread attention in the past decade as an enabling component to energy and fuel sustainability. Copper (Cu) is one of the few electrocatalysts that can convert CO2 to higher-order hydrocarbons. We report the CO2RR on polycrystalline Cu from 5 °C to 45 °C as a function of electrochemical potential. Our result shows that selectivity shifts toward CH4 at low temperature and H2 at high temperature at the potential values between −0.95 V and −1.25 V versus reversible hydrogen electrode (RHE). We analyze the activation energy for each product and discuss the possible underlying mechanism based on their potential dependence. The activation barrier of CH4 empirically obeys the Butler–Volmer equation, while C2H4 and CO show a non-trivial trend. Our result suggests that the CH4 production proceeds via a classical electrochemical pathway, likely the proton-coupled electron transfer of surface-saturated COad, while C2H4 is limited by a more complex process, likely involving surface adsorbates. Our measurement is consistent with the view that the adsorbate–adsorbate interaction dictates the C2+ selectivity.

(Invited) Spectroscopy Techniques Applied to Operando Characterization of PGM-Free Catalysts for Fuel Cells

Metal-N-C catalysts (Metal = Fe, Co primarily) are being developed and studied as alternative catalysts to platinum for catalyzing the O2 reduction reaction (ORR), in acidic and alkaline media. The nature of their active sites is still a topic of intense discussions and advanced studies, this class of materials possibly comprising N-doped sites, Metal-Nx moieties and various crystallographic structures. The latter often result from excess metal relative to the number of defect sites in the carbon matrix that may host Metal-Nx moieties. Improved understanding has been gained first through the combination of controlled synthesis, ex situ spectroscopic characterization and ex-situ structure / activity correlations. More recently, operando and/or in-situ spectroscopic techniques have been applied on novel Metal-NC catalysts to better understand their site formation during pyrolysis, their electronic, spin-state and coordinative environment as a function of electrochemical potential and/or time scales. Operando spectroscopy on such catalysts may also offer a way to distinguish the metal-based sites that are electrochemically accessible from those that aren’t. This is a recognized issue for pyrolysed Metal-N-C materials. This presentation will focus on the application of X-ray absorption techniques to the operando characterization of Fe and Co in Metal-NC catalysts in various electrochemical conditions, and of what could be learnt additionally to ex situ or post mortem studies. Initial results acquired with operando 57Fe Mössbauer spectroscopy will also be presented and discussed.

The effects of electrolyte on the capacitive behavior of nanostructured molybdenum oxides

AbstractBACKGROUNDNanostructured metal oxides have shown capabilities in the application of energy conversion and storage. Among a variety of metal oxide semiconducting materials, molybdenum oxide recently became a promising candidate as an electrochemical supercapacitor for storing electrical energy. Supercapacitors, which are storage devices for electrical energy, benefit from the separation of opposite charges in electrochemical double‐layer capacitors and charge transfer as a function of electrochemical potential in pseudocapacitors.RESULTSA thin film of nanostructured molybdenum oxide was prepared using potentiodynamic electrodeposition on the surface of a stainless‐steel electrode. Its electrochemical capacitive behavior was investigated using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). To investigate the effects of the electrolyte ion type, two solutions were investigated in a comparative way: 0.005 mol L–1 H2SO4 and 0.005 mol L–1 H2SO4 + 0.095 mol L–1 Na2SO4 mixture where each solution has the same initial concentrations of hydronium ions but with different ionic strengths. The EIS studies gave rise to the specific capacitances of 340 and 160 F g–1 for H2SO4 and mixed solutions, respectively. An appropriated electrical circuit was used to interpret these EIS results.CONCLUSIONSThe present studies show that the electrolyte cations contribute to the capacitive behavior of the nanostructured molybdenum oxide in different ways. Sodium ions only participate in the process of charging the double layer from the outer Helmholtz and diffusion layers, whereas hydronium ions contribute to the Faradaic process of electrosorption by penetrating into the inner Helmholtz layer. © 2019 Society of Chemical Industry

Cyclic Voltammetry of Ge and Te in Basic Conditions: An Investigation of Surface Coverages.

Cyclic Voltammetry of Ge and Te precursors in basic solution can lead to a better understanding of the surface limited reactions taking place at the substrate-solution interface. GeTe is a phase-change chalcogenide material with excellent stability for the crystalline and amorphous phases. The stoichiometry of the film is very important as the 1:1 compound is shown to have the highest switching rates.1 Controlling the stoichiometry has proven to be difficult using conventional methods, so in this study, deposition conditions for stoichiometric GeTe are sought using Electrochemical Atomic Layer Depostion (E-ALD). EALD is a process developed by this author’s lab which makes use of underpotential deposition (UPD) to put down fractions of a monolayer of the desired material. This process can then be repeated for as many cycles as are needed to grow films of the desired thickness. Before a deposition sequence can be made, the interactions of the individual elements must be investigated, both on the substrate, and on each other. This is achieved through the use of cyclic voltammetry, which measures the current generated as each element is either deposited or stripped from the surface as a function of electrochemical potential. By quantifying the current, the number of deposited or stripped atoms can be calculated, giving an estimate of the surface coverage. Investigations have identified the surface limited and bulk potentials for each element on the substrate as well as their substrate coverage. The next step will be to develop a sequence using these potentials to develop high quality, epitaxial films.

Electrochemical Partial Oxidation of Alkenes: Understanding Selectivity and Reaction Mechanisms

With a deepening electrification of society, electrochemical conversion from one chemical to another will be essential for the chemical plants of the future. While there is intense focus on reduction reactions (i.e. CO2, water, N2), partial oxidation reactions have been relatively underinvestigated. This talk will discuss our recent work on electrochemical partial oxidation of propene. With propene having both a double bond and an allylic carbon, this is an optimal test molecule to create a broad spectrum of products following a variety of mechanisms. By using a Pd catalyst, we were able to produce acrolein, acrylic acid, allyl alcohol, propylene glycol, acetone with only small amounts of CO2 formation. Variations is oxidation potential allowed us to see changes in product distribution, which we then used to theorize potential reaction mechanisms. With acrolein being one of our largest products, this meant the allylic carbon was oxidized rather the double bond. DFT calculations showed that the acrolein mechanism was feasible, provided surface coverage of other species (oxygen species and flat lying propene) dominated the surface coverage, thus allowing for a small amount of allylic carbon to bind to the surface due to steric advantages. Electrochemical Sniffer Chip technology (1) was then used to measure submonolayer gas absorption/desorption as a function of electrochemical potential, which provided supporting evidence to the aforementioned mechanism. The talk will show our process in analyzing this reaction and mechanisms as well as how these discoveries are applicable to the broader field of partial oxidation reactions. (1) Trimarco, D. et. al, Review of Scientific Instruments 86, 075006 (2015) Figure 1

Stability of metallo-porphyrin networks under oxygen reduction and evolution conditions in alkaline media.

Transition metal atoms stabilised by organic ligands or as oxides exhibit promising catalytic activity for the electrocatalytic reduction and evolution of oxygen. Built-up from earth-abundant elements, they offer affordable alternatives to precious-metal based catalysts for application in fuel cells and electrolysers. For the understanding of a catalyst's activity, insight into its structure on the atomic scale is of highest importance, yet commonly challenging to experimentally access. Here, the structural integrity of a bimetallic iron tetrapyridylporphyrin with co-adsorbed cobalt electrocatalyst on Au(111) is investigated using scanning tunneling microscopy and X-ray absorption spectroscopy. Topographic and spectroscopic characterization reveals structural changes of the molecular coordination network after oxygen reduction, and its decomposition and transformation into catalytically active Co/Fe (oxyhydr)oxide during oxygen evolution. The data establishes a structure-property relationship for the catalyst as a function of electrochemical potential and, in addition, highlights how the reaction direction of electrochemical interconversion between molecular oxygen and hydroxyl anions can have very different effects on the catalyst's structure.

Surface Defect Chemistry and Electronic Structure of Pr0.1Ce0.9O2−δ Revealed in Operando

Understanding the surface defect chemistry of oxides under functional operating conditions is important for providing guidelines for improving the kinetics of electrochemical reactions. Ceria-based oxides have applications in solid oxide fuel/electrolysis cells, thermo-chemical water splitting, catalytic convertors, and red-ox active memristive devices. The surface defect chemistry of doped ceria in the regime of high oxygen pressure, pO2, approximating the operating conditions of fuel cell cathodes at elevated temperatures, has not yet been revealed. In this work, we investigated the Pr0.1Ce0.9O2−δ (PCO) surface by in operando X-ray photoelectron and absorption spectroscopic methods. We quantified the concentration of reduced Pr3+, at the near-surface region of PCO as a function of electrochemical potential, corresponding to a wide range of effective pO2. We found that the Pr3+ concentration at the surface was significantly higher than the values predicted from bulk defect chemistry. This finding indicat...

Prevention of surface recombination by electrochemical tuning of TiO2-passivated photocatalysts

We present a systematic study of photoluminescence (PL) spectroscopy of TiO2-passivated GaAs as a function of electrochemical potential in an ionic liquid solution. We observe a 7X increase in the PL intensity as the GaAs transitions from accumulation to depletion due to the applied potential. We attribute this to the excellent control over the surface Fermi level enabled by the high capacitance of the electrochemical double layer and TiO2. This allows us to control the surface carrier concentration and corresponding non-radiative recombination rate. In addition to photoluminescence (PL) spectroscopy, we also measured the capacitance-potential (i.e., C-V) characteristics of these samples, which indicate flat band potentials that are consistent with these regimes of ion accumulation observed in the photoluminescence measurements. We have also performed electrostatic simulations of these C-V characteristics, which provide a detailed and quantitative picture of the conduction and valence band profiles and charge distribution at the surface of the semiconductor. These simulations also enable us to determine the range of potentials over which the semiconductor surface experiences depletion, inversion, and accumulation of free carriers. Based on these simulations, we can calculate the Shockley-Read-Hall recombination rate and model the PL intensity as a function of voltage. We show that this approach allows us to explain our experimental data well.

First-Principles Calculation of Pt Surface Energies in an Electrochemical Environment: Thermodynamic Driving Forces for Surface Faceting and Nanoparticle Reconstruction.

Platinum is a widely used catalyst in aqueous and electrochemical environments. The size and shape of Pt nanoparticles and the faceting (and roughness) of extended Pt surfaces change during use in these environments due to dissolution, growth, and reconstruction. Further, many Pt nanoparticle synthesis techniques are carried out in an aqueous environment. The surface structures formed are impacted by the relative surface energies of the low index facets in these environments. Density functional theory is used to calculate the surface energy of the low index facets of platinum as a function of electrochemical potential and coverage of adsorbed hydrogen, hydroxide, oxygen, and the formation of surface oxide in an aqueous environment. Whereas Pt(111) is the lowest energy bare surface in vacuum, the strong adsorption of hydrogen to Pt(100) at low potentials and of hydroxide to Pt(110) and oxygen to Pt(100) at high potentials drives these surfaces to be more stable in an electrochemical environment. We experimentally conditioned a polycrystalline platinum electrode by cycling the potential and find a growth in the total area as well as in the fraction of 110 and 100 sites, which are lower in energy at potentials where dissolved Pt is deposited or surface oxide is reduced. Further, we find that the lower surface energy of Pt(100) at low potentials may play a role in the growth of tetrahexahedral nanoparticles seen on square wave cycling of spherical Pt nanoparticles. Wulff constructions are presented as a function of Pt electrode potential.

Full Atomistic Kinetic Monte Carlo and First Principles Study on Electromotive Force of SOFC with Direct Counting Approach

The outstanding developments of computers and theoretical methods especially on first principles electronic structure calculations enable us to investigate the stability and reactivity of bulk materials and hetero interfaces in atomistic scale with high precision, although we still have limitations on the time domain and system size. If we can extensively extend the accessible regions about time domain and system size in atomistic simulations at the same time, it will be a further development in computer simulations for materials designing. In order to realize such an extended computational tool, we have been developing a full atomistic kinetic Monte Carlo code which can easily incorporate kinetic date from first principles calculations. The characteristic points in the kMC code are 1) parallelization in domain decomposition scheme, 2) easy constructions of hetero structures and reactions, 3) boundary conditions including open-boundary, 4) Poisson solver, and 5) direct counting of flowing current. Points 1, 2, and 3 (partly) were already described in our previous paper [1]. The standard cases for the developed kMC execution in a single computer are roughly μs-to-ms time domain for 1 million atoms. Although the size in the kMC might seem to be not so large, the atomistic system represented in kMC cell is defined as a function of electrochemical potential, together with thermodynamic parameters (i.e., temperature and pressure) and chemical conditions such as dopant conditions. That is, the kMC cell can be regarded as a semi-infinite system, and thus the problems on the size limitation is technically avoided. To explain this point in more detail, let’s consider a whole SOFC system composed of anode (A), solid electrolyte (SE), and cathode (C). The chemical composition in deep inside of SE can be regarded as a stoichiometric one even at a working situation with no degradation, and this is of course true at around equilibrium conditions, OCV. On the other hand, the compositions of interfaces A/SE and C/SE can be significantly deviated from the stoichiometry, depending on the thermodynamic conditions. Our kMC can handle the hetero interfaces A/SE and C/SE smoothly connected with stoichiometric SE in the kMC computational cell. That’s why the size limitation cannot be so problematic; it should be mentioned that we can also handle the whole system of A-SE-C in one kMC computational cell when the thickness of SE is too small to realize an equilibrium composition at the inside of SE. To check the reliability of the developed kMC code, we applied it to the calculations of electromotive force (EMF) of oxide concentration cells, and compared the calculated EMF with that obtained from Nernst equation. We adopted 8mol% doped YSZ as SE. In the EMF calculations, the direct counting approach of ionic current, one of the characteristic points in our kMC, was used to find OCV condition depending on gas partial pressures. As a result, we found the excellent agreements between simulated EMF and ideal EMF from Nernst equation. This means that the reliability of our kMC dynamics in the atomistic model is guaranteed in terms of thermodynamics because the equilibrium situations and steady states in the concentration cells are recognized using ionic motions (i.e., direct counting) in kMC. The details of calculated results and computational algorisms especially for direct counting and open-boundary condition will be explained in the conference. Reference [1] T. Tada and N. Watanabe, ECS Transactions 57(1), 2437-2447 (2013).

Electrochemical Photocurrent X-Ray Spectroscopy: A Novel Analytical Surface Technique

X-ray absorption spectroscopy (XAS) is a powerful tool for characterizing the chemical nature and environment of atoms in molecules. However, in situ techniques are limited to transmission and fluorescence techniques and are not surface sensitive. A novel in situ technique that uses synchrotron radiation to produce photoelectrons was developed to provide information on the surface and near surface regions. The results obtained from this technique, Electrochemical Photocurrent X-ray Spectroscopy (EPXS) were compared transmission and fluorescence X-ray absorption spectroscopy (XAS). Surface sensitive information was obtained by collecting X-ray induced photocurrent from a polarized electrode. An Au working electrode in 0.1 N H2SO4 was polarized to 300 mVSCE and exposed to high energy X-rays. The photocurrent response was recorded during Au L3 edge scans while X-ray fluorescence yield data was collected concurrently. The photocurrent data showed good agreement with fluorescence data and transmission data collected on Au foils (Figure 1). Investigations using electrochemical impedance spectroscopy and cyclic voltammetry we conducted to insure the photocurrent response was not due to double layer charging or solution radiolysis. Minor differences in peak characteristics were observed between photocurrent and fluorescence data. We believe this to be due to the surface sensitivity of the photocurrent technique. A comparison of the surface sensitivity of fluorescence and photocurrent techniques was accomplished by evaluating signal response to X-ray beam intensity changes. The results show that the photocurrent technique, as expected, was more surface sensitive than fluorescence. Current work is focusing on EXAFS analysis of photocurrent spectra to quantify the techniques surface sensitivity. This novel technique has the potential to provide information on electron distribution on conductive surface atoms and changes in charge of surface atoms as a function of electrochemical potential. It could significantly impact fields where measurements of surface electron distribution during electrochemical charge transfer are of primary importance, such as corrosion, catalysts and semiconductor research. Figure 1- XAS spectrum of Au foil using transmission (left) and a spectrum of Au in 0.1N H2SO4 polarized to 300 mVSCE using the photocurrent technique. Figure 1

Evidence for a hopping mechanism in metal|single molecule|metal junctions involving conjugated metal-terpyridyl complexes; potential-dependent conductances of complexes [M(pyterpy)2]2+ (M = Co and Fe; pyterpy = 4'-(pyridin-4-yl)-2,2':6',2''-terpyridine) in ionic liquid.

Extensive studies of various families of conjugated molecules in metal|molecule|metal junctions suggest that the mechanism of conductance is usually tunnelling for molecular lengths < ca. 4 nm, and that for longer molecules, coherence is lost as a hopping element becomes more significant. In this work we present evidence that, for a family of conjugated, redox-active metal complexes, hopping may be a significant factor for even the shortest molecule studied (ca. 1 nm between contact atoms). The length dependence of conductance for two series of such complexes which differ essentially in the number of conjugated 1,4-C6H4- rings in the structures has been studied, and it is found that the junction conductances vary linearly with molecular length, consistent with a hopping mechanism, whereas there is significant deviation from linearity in plots of log(conductance) vs. length that would be characteristic of tunnelling, and the slopes of the log(conductance)-length plots are much smaller than expected for an oligophenyl system. Moreover, the conductances of molecular junctions involving the redox-active molecules, [M(pyterpy)2]2+/3+ (M = Co, Fe) have been studied as a function of electrochemical potential in ionic liquid electrolyte, and the conductance-overpotential relationship is found to fit well with the Kuznetsov-Ulstrup relationship, which is essentially a hopping description.

Modeling the Chloride Ion Attack of Aluminum Oxide by XANES Analysis

This paper is a summary of previously published work. We have generated molecular simulations of Cl¯ located at different sites in several aluminum oxide models and carried out local l-projected density of states (LDOS) calculations to obtain the theoretical spectra. These are compared to our earlier experimental X-ray absorption near edge structure (XANES) data in order to study the interactions of chloride ions with the passive oxide film on aluminum as a function of electrochemical potential at the Cl K-edge. We show that the chloride first attacks the hydroxyl components of the aluminum oxide, penetrates the oxide film and finally attacks the metal surface. This has led to a number of new insights in the mechanism of the breakdown of the passive oxide film in chloride solutions.

Properties of Porphyrin- and Phthalocyanin-Monolayers at Metal-Electrolyte Interfaces

The adsorption of organic layers on solid surfaces is a promising route to produce surfaces of designed functionality. In this lecture an overview will be given about state-of-the-art investigations of the structural and chemical properties of porphyrin- and phthalocyanin-monolayers adsorbed electrochemically in the form of their molecular cations on anion-modified single-crystal metal electrode surfaces as a function of the nature of the metal (copper, gold), the kind of the pread-sorbed anions (chloride, iodide, sulfate), the symmetry of the surface ((100), (111)), and the electro-chemical potential, using besides classical electrochemical methods Electrochemical Scanning Tunneling Microscopy (EC-STM), and ex-situ Synchrotron X-Ray Photoelectron Spectroscopy (S-XPS). Particular attention will be paid to the self-assembly- vs. template-effects in the structure formation process, to structural phase transitions as a function of electrochemical potential, as well as to the chemical reactivity of selected porphyrins and phthalocyanins. Fig. 1b shows 5,10,15,20-Tetrakis(4-trimethylammonio-phenyl)porphyrin molecules adsorbed on a Cl-precovered Cu(111) surface. Instead of the threefold symmetry of the substrate (Fig. 1a) the molecules self-assemble into a square molecular superlattice (Fig.1b) [1]. Fig.2 shows coexisting square and hexagonal structure domains of 3-pyridyloxy-appended phthalocyanine on iodide precovered Cu(100) [2].

Dependence of Conductivity on Charge Density and Electrochemical Potential in Polymer Semiconductors Gated with Ionic Liquids

We report the hole transport properties of semiconducting polymers in contact with ionic liquids as a function of electrochemical potential and charge carrier density. The conductivities of four different polymer semiconductors including the benchmark material poly(3-hexylthiophene) (P3HT) were controlled by electrochemical gating (doping) in a transistor geometry. Use of ionic liquid electrolytes in these experiments allows high carrier densities of order 1021 cm–3 to be obtained in the polymer semiconductors and also facilitates variable temperature transport measurements. Importantly, all four polymers displayed a nonmonotonic dependence of the conductivity on carrier concentration. For example, for P3HT in contact with the ionic liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMI][FAP]), the hole conductivity reached a maximum of 85 S/cm at 6 × 1020 holes cm–3 or 0.16 holes per thiophene ring. Further increases in charge density up to 0.35 holes per ring produced a revers...

Structure of Chlorine K-Edge XANES Spectra During the Breakdown of Passive Oxide Films on Aluminum

We generated molecular simulations of Cl– interacting at different sites in aluminum oxide models and carried out FEFF8 calculations to obtain the local l-projected density of states (LDOS) spectra. These are compared to our earlier experimental X-ray absorption near-edge structure (XANES) data in order to study the interactions of chloride ions with the passive oxide film on aluminum as a function of electrochemical potential at the Cl K edge. This led to a number of new insights in the mechanism of the breakdown of the passive film in chloride solutions. Importantly, we show the chloride first attacks the hydroxyl components of the aluminum oxide, penetrates the oxide film, and finally attacks the metal surface.

Electrochemically Driven Reorientation of Three Ionic States of p-Aminobenzoic Acid on Ag(111)

Broad band sum frequency generation spectroscopy is used to probe the orientation of three pH accessible ionic states of p-aminobenzoic acid (PABA) on an Ag(111) electrode surface as a function of electrochemical potential. Experimentally measured phase differences between resonant and nonresonant SFG signals confirm features in electrochemical capacitance and cyclic voltammograms correspond to ordering and reorientation of the PABA molecule on the electrode surface as the surface potential is varied. While the zwitterionic form rotates 180° after the potential is moved through the potential of zero charge (PZC), such is not the case for the cationic or anionic forms. In the former, the molecule rotates only 90°. In the later, the molecule does not rotate at all (because the PZC is not reached). Additionally, optical methods are utilized to show that the PZC of the surface changes dramatically following adsorbate adsorption. The surface potential at which PABA reorients is quite different from the PZC determined in neat electrolyte solutions. The measured reorientation potentials for zwitterionic PABA (ca. -300 mV) and anionic PABA (-800 mV) reflect a substantial shift of the PZC of the electrode with the adsorbed dipolar organic species.

Voltammetric characterization of the aerobic energy-dissipating nitrate reductase of Paracoccus pantotrophus: exploring the activity of a redox-balancing enzyme as a function of electrochemical potential

Paracoccus pantotrophus expresses two nitrate reductases associated with respiratory electron transport, termed NapABC and NarGHI. Both enzymes derive electrons from ubiquinol to reduce nitrate to nitrite. However, while NarGHI harnesses the energy of the quinol/nitrate couple to generate a transmembrane proton gradient, NapABC dissipates the energy associated with these reducing equivalents. In the present paper we explore the nitrate reductase activity of purified NapAB as a function of electrochemical potential, substrate concentration and pH using protein film voltammetry. Nitrate reduction by NapAB is shown to occur at potentials below approx. 0.1 V at pH 7. These are lower potentials than required for NarGH nitrate reduction. The potentials required for Nap nitrate reduction are also likely to require ubiquinol/ubiquinone ratios higher than are needed to activate the H(+)-pumping oxidases expressed during aerobic growth where Nap levels are maximal. Thus the operational potentials of P. pantotrophus NapAB are consistent with a productive role in redox balancing. A Michaelis constant (K(M)) of approx. 45 muM was determined for NapAB nitrate reduction at pH 7. This is in line with studies on intact cells where nitrate reduction by Nap was described by a Monod constant (K(S)) of less than 15 muM. The voltammetric studies also disclosed maximal NapAB activity in a narrow window of potential. This behaviour is resistant to change of pH, nitrate concentration and inhibitor concentration and its possible mechanistic origins are discussed.