Inhibited proton transfer enhances Au-catalyzed CO2-to-fuels selectivity.

CO2 reduction in aqueous electrolytes suffers efficiency losses because of the simultaneous reduction of water to H2 We combine in situ surface-enhanced IR absorption spectroscopy (SEIRAS) and electrochemical kinetic studies to probe the mechanistic basis for kinetic bifurcation between H2 and CO production on polycrystalline Au electrodes. Under the conditions of CO2 reduction catalysis, electrogenerated CO species are irreversibly bound to Au in a bridging mode at a surface coverage of ∼0.2 and act as kinetically inert spectators. Electrokinetic data are consistent with a mechanism of CO production involving rate-limiting, single-electron transfer to CO2 with concomitant adsorption to surface active sites followed by rapid one-electron, two-proton transfer and CO liberation from the surface. In contrast, the data suggest an H2 evolution mechanism involving rate-limiting, single-electron transfer coupled with proton transfer from bicarbonate, hydronium, and/or carbonic acid to form adsorbed H species followed by rapid one-electron, one-proton, or H recombination reactions. The disparate proton coupling requirements for CO and H2 production establish a mechanistic basis for reaction selectivity in electrocatalytic fuel formation, and the high population of spectator CO species highlights the complex heterogeneity of electrode surfaces under conditions of fuel-forming electrocatalysis.

Biomimetic environment to study E. coli complex I through surface-enhanced IR absorption spectroscopy.

In this study complex I was immobilized in a biomimetic environment on a gold layer deposited on an ATR-crystal in order to functionally probe the enzyme against substrates and inhibitors via surface-enhanced IR absorption spectroscopy (SEIRAS) and cyclic voltammetry (CV). To achieve this immobilization, two methods based on the generation of a high affinity self-assembled monolayer (SAM) were probed. The first made use of the affinity of Ni-NTA toward a hexahistidine tag that was genetically engineered onto complex I and the second exploited the affinity of the enzyme toward its natural substrate NADH. Experiments were also performed with complex I reconstituted in lipids. Both approaches have been found to be successful, and electrochemically induced IR difference spectra of complex I were obtained.

Enzyme-catalyzed hydrolysis of the supported phospholipid bilayers studied by atomic force microscopy.

Atomic force microscopy (AFM) is employed to reveal the morphological changes of the supported phospholipid bilayers hydrolyzed by a phospholipase A(2) (PLA(2)) enzyme in a buffer solution at room temperature. Based on the high catalytic selectivity of PLA(2) toward l-enantiomer phospholipids, five kinds of supported bilayers made of l- and D-dipalmitoylphosphatidylcholines (DPPC), including l-DPPC (upper leaflet adjacent to solution)/l-DPPC (bottom leaflet) (or l/l in short), l/d, d/l, d/d, and racemic ld/ld, were prepared on a mica surface in gel-phase, to explicate the kinetics and mechanism of the enzyme-induced hydrolysis reaction in detail. AFM observations for the l/l bilayer show that the hydrolysis rate for l-DPPC is significantly increased by PLA(2) and most of the hydrolysis products desorb from substrate surface in 40 min. As d-enantiomers are included in the bilayer, the hydrolysis rate is largely decreased in comparison with the l/l bilayer. The time used to hydrolyze the as-prepared bilayers by PLA(2) increases in the sequence of l/l, l/d, ld/ld, and d/l (d/d is inert to the enzyme action). d-enantiomers in the enantiomer hybrid bilayers remain on the mica surface at the end of the hydrolysis reaction. It was confirmed that the hydrolysis reaction catalyzed by PLA(2) preferentially occurs at the edges of pits or defects on the bilayer surface. The bilayer structures are preserved during the hydrolysis process. Based on these observations, a novel kinetics model is proposed to quantitatively account for the PLA(2)-catalyzed hydrolysis of the supported phospholipid bilayers. The model simulation demonstrates that PLA(2) mainly binds with lipids at the perimeter of defects in the upper leaflet and leads to a hydrolysis reaction, yielding species soluble to the solution phase. The lipid molecules underneath subsequently flip up to the upper leaflet to maintain the hydrophilicity of the bilayer structure. Our analysis shows that d-enantiomers in the hybrid bilayers considerably reduce the hydrolysis rate by its ineffective binding with PLA(2).

Synthesis and properties of the cyano complex of oxo-centered triruthenium core [Ru3(µ3-O)(µ-CH3COO)6(pyridine)2(CN)].

The preparation and properties of a new cyano complex containing the Ru3(µ3-O) core, [Ru3(µ3-O)(µ-CH3COO)6(py)2(CN)] (1; py = pyridine), are reported. Complex 1 in CH2Cl2 showed intense absorption bands at 244, 334, and 662 nm, corresponding to a π-π* transition of the ligand, cluster-to-ligand charge transfer, and intracluster transitions, respectively. The cyclic voltammogram of 1 in 0.1 M (n-Bu)4NPF6-CH2Cl2 showed redox waves for the processes Ru3(II,II,III)/Ru3(II,III,III), Ru3(II,III,III)/Ru3(III,III,III), and Ru3(III,III,III)/Ru3(III,III,IV) at E1/2 = -1.49, -0.26, and +1.03 V vs Ag/AgCl, respectively. The first two redox potentials are more negative by ca. 0.2 V in comparison with the corresponding potentials of [Ru3(µ3-O)(µ-CH3COO)6(py)3](+). This is in sharp contrast to the positive shifts of the corresponding waves of [Ru3(II,III,III)(µ3-O)(µ-CH3COO)6(py)2(CO)]. Density functional theory (DFT) calculations of [Ru3(II,III,III)(µ3-O)(µ-CH3COO)6(py)3], [Ru3(II,III,III)(µ3-O)(µ-CH3COO)6(py)2(CN)](-), and [Ru3(II,III,III)(µ3-O)(µ-CH3COO)6(py)2(CO)] showed that the positive charge of the ruthenium is delocalized over the triruthenium cores of the first two and is localized as Ru(II)(CO){Ru(III)(py)}2 in the CO complex. The calculations explain the difference in the π interactions of the two ligands with the triruthenium cores.

Importance of acid-base equilibrium in electrocatalytic oxidation of formic acid on platinum.

Electro-oxidation of formic acid on Pt in acid is one of the most fundamental model reactions in electrocatalysis. However, its reaction mechanism is still a matter of strong debate. Two different mechanisms, bridge-bonded adsorbed formate mechanism and direct HCOOH oxidation mechanism, have been proposed by assuming a priori that formic acid is the major reactant. Through systematic examination of the reaction over a wide pH range (0-12) by cyclic voltammetry and surface-enhanced infrared spectroscopy, we show that the formate ion is the major reactant over the whole pH range examined, even in strong acid. The performance of the reaction is maximal at a pH close to the pKa of formic acid. The experimental results are reasonably explained by a new mechanism in which formate ion is directly oxidized via a weakly adsorbed formate precursor. The reaction serves as a generic example illustrating the importance of pH variation in catalytic proton-coupled electron-transfer reactions.

Real-time observation of the destruction of hydration shells under electrochemical force.

Major alteration or even destruction of the hydration shell around interacting molecules and ions in solution is an important process that determines how hydrated substances interact. Therefore, the direct observation of structural changes in hydration shells around solutes in close contact with other solutes or surfaces is important for understanding chemical processes that take place in solution. In the work described in this paper, time-resolved IR absorption measurements were performed to study the interaction of hydrated Na(+) or tetrapropylammonium cation (Pr4N(+)) with a hydrophobic CO-covered Pt surface; the adsorption force between cations and the surface was controlled by using an electrochemical system. We found that the hydrophobic hydration shell of Pr4N(+) is initially stabilized on the hydrophobic surface, but application of a strong force to the cation approaching CO destroys the water layers between them. This process is rather slow, taking a few hundred milliseconds. Hydrophilic Na(+) behaves quite differently from Pr4N(+) due to the different structure of its hydration shell. These experimental results are supported by molecular dynamics simulations.

Proton-coupled electron transfer and Lewis acid recognition at self-assembled monolayers of an oxo-bridged diruthenium(III) complex functionalized with two disulfide anchors.

A new µ-oxo-bis(µ-acetato)diruthenium(III) complex bearing two pyridyl disulfide ligands {[Ru2(µ-O)(µ-OAc)2(bpy)2(L(py-SS))2](PF6)2 (OAc = CH3CO2(-), bpy = 2,2'-bipyridine, L(py-SS) = (C5H4N)CH2NHC(O)(CH2)4CH(CH2)2SS) (1)} has been synthesized to prepare self-assembled monolayers (SAMs) on the Au(111) electrode surface. The SAMs have been characterized by contact-angle measurements, reflection-absorption surface infrared spectroscopy, cyclic voltammetry, and reductive desorption experiments. The SAMs exhibited proton-coupled electron transfer (PCET) reactions when the electrochemistry was studied in aqueous electrolyte solution (0.1 M NaClO4 with Britton-Robinson buffer to adjust the solution pH). The potential-pH plot (Pourbaix diagram) in the pH range from 1 to 12 has established that the dinuclear ruthenium moiety was involved in the interfacial PCET processes with four distinct redox states: Ru(III)Ru(III)(µ-O), Ru(II)Ru(III)(µ-OH), Ru(II)Ru(II)(µ-OH), and Ru(II)Ru(II)(µ-OH2). We also demonstrated that the interfacial redox processes were modulated by the addition of Lewis acids such as BF3 or Al(3+) to the electrolyte media, in which the externally added Lewis acids interacted with µ-O of the dinuclear moiety within the SAMs.

Structure and lateral interaction in mixed monolayers of dioctadecyldimethylammonium chloride (DOAC) and stearyl alcohol.

π-A isotherms, atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy are employed to investigate the molecular structure and lateral interactions in mixed monolayers of dioctadecyldimethylammonium chloride (DOAC) and stearyl alcohol (SA) at air/water and air/solid interfaces. To avoid possible interference between the two molecules in the SFG spectroscopic measurements, perprotonated DOAC and perdeuterated SA (dSA) were used. The thermodynamic analyses for the π-A isotherms show that DOAC is miscible with dSA. SFG observations reveal that DOAC molecules become conformationally ordered as dSA molecules are introduced into the monolayer. AFM observations demonstrate coexistence of DOAC-rich and dSA-rich domains in the mixed monolayer with ratios different from their initial composition in the subphase. The present study suggests that DOAC molecules in the mixed monolayer are condensed by mixing with dSA in which the repulsive interactions between positively charged head groups of the DOAC molecules are largely reduced along with an increase of van der Waals interactions with dSA.

Effect of functional group on the monolayer structures of biodegradable quaternary ammonium surfactants.

The monolayer structures and conformational ordering of cationic surfactants including the biodegradable quaternary ammonium molecules have been systematically characterized by π-A isotherm, surface potential, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and sum frequency generation (SFG) vibrational spectroscopy. It was found that the monolayer of the typical dialkyl dimethylammonium on the water surface was less densely packed along with many conformational gauche defects. The packing density and ordering of these monolayers were improved as halide ions were added to the subphase. A similar condensation effect was also observed when amide or ester groups are present in the alkyl tails of the surfactant. These results are discussed on the basis of the repulsive electrostatic interactions between the terminal ammonium moieties, the hydrogen bonding between the functional groups in the alkyl chains, as well as the flexibility of the alkyl chains in these surfactants. The present study is crucial to understanding the relationship between the interfacial structures and the functionalities of the biodegradable quaternary ammonium surfactants.

Structure and stability studies of mixed monolayers of saturated and unsaturated phospholipids under low-level ozone.

In the present study, stability and structure of single and binary mixed monolayers of an unsaturated phospholipid, DOPC, and a saturated phospholipid, DPPC-d75, on the water surface, were explored using the π-A isotherm, atomic force microscopy (AFM) and sum frequency generation (SFG) vibrational spectroscopy in various environments. Our results demonstrated that DOPC in the monolayers becomes unstable after the exposure to a low concentration of ozone (20 ± 10 ppb) or even to ambient laboratory air, which has a similar ozone level, but is stable in nitrogen or oxygen. DOPC can be selectively oxidized by a trace amount of ozone in the ambient environment but can be partially inhibited by the presence of DPPC in the monolayer. The present study provides useful information for understanding the physicochemical properties of the cell membranes.

First principles study of sulfuric acid anion adsorption on a Pt(111) electrode.

A first principles theory combined with a continuum electrolyte theory is applied to adsorption of sulfuric acid anions on Pt(111) in 0.1 M H(2)SO(4) solution. The theoretical free energy diagram indicates that sulfuric acid anions adsorb as bisulfate in the potential range of 0.41 < U ≤ 0.48 V (RHE) and as sulfate in 0.48 V (RHE) < U. This diagram also indicates that sulfate inhibits formations of surface oxide and hydroxide. Charge analysis shows that the total charge transferred for the formation of the full coverage sulfate adlayer is 90 µC cm(-2), and that the electrosorption valency value is -0.45 to -0.95 in 0.41 < U ≤ 0.48 V (RHE) and -1.75 to -1.85 in U > 0.48 V (RHE) in good agreement with experiments reported in the literature. Vibration analysis indicates that the vibration frequencies observed experimentally at 1250 and 950 cm(-1) can be assigned, respectively, to the S-O (uncoordinated) and symmetric S-O stretching modes for sulfate, and that the higher frequency mode has a larger potential-dependence (58 cm(-1) V(-1)) than the lower one.

Potential-dependent recombination kinetics of photogenerated electrons in n- and p-type GaN photoelectrodes studied by time-resolved IR absorption spectroscopy.

Recombination kinetics of photogenerated electrons in n-type and p-type GaN photoelectrodes active for H(2) and O(2) evolution, respectively, from water was examined by time-resolved IR absorption (TR-IR) spectroscopy. Illumination of a GaN film with UV pulse (355 nm and 6 ns in duration) gives transient interference spectra in both transmittance and reflection modes. Simulation shows that the interference spectra are caused by photogenerated electrons. We observed that recombination in the microsecond region is greatly affected by the applied potentials, the lifetime becoming longer at negative and positive potentials for n- and p-type GaN electrodes, respectively. There is a good correlation between potential dependence of the steady-state reaction efficiency and that of the number of surviving electrons in the millisecond region. We also performed potential jump measurement to examine the shift in Fermi level by photogenerated charge carriers. In the case of n-type GaN, the electrode potential jumps to the negative side by accumulation of electrons in the bulk. However, in the case of p-type GaN, the electrode potential first jumps to the negative side within 20 µs and gradually shifts to the positive side in a few milliseconds, while the number of charge carriers is constant at >0.2 ms. This two-step process is ascribed to electron transport from the bulk to the surface of GaN, because the electrode potential is sensitive to the number of electrons in the bulk. The results confirm that TR-IR combined with potential jump measurement provides useful information for understanding the behavior of charge carriers in photoelectrochemical systems.

Preferential adsorption of amino-terminated silane in a binary mixed self-assembled monolayer.

The composition and structure of a binary mixed self-assembled monolayer (SAM) of 3-aminopropyltriethoxysilane (APS, NH(2)(CH(2))(3)Si(OCH(2)CH(3))(3)) and octadecyltrimethoxysilane (ODS, CH(3)(CH(2))(17)Si(OCH(3))(3)) on a silicon oxide surface have been characterized by water contact-angle measurements, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy. XPS demonstrated that APS in the mixed SAM is significantly enriched in comparison to that in solution, indicating the preferential adsorption of APS during the SAM formation. AFM observations showed that the mixed SAM becomes rougher. SFG revealed that the coadsorption of APS induced a conformation disordering in the ODS molecules present in the mixed SAM. The surface enrichment of APS has been explained in terms of differences in the surface adsorption rates of the two components as well as in the self-congregation states of APS molecules in the bulk solution. Furthermore, the structure of the water molecules on the mixed SAM surface in contact with the aqueous solutions at different pH's has also been studied. The results indicate that the mixed-SAM modified surface is positively charged at pH < 5 and negatively charged at pH > 7.

Adsorbed formate: the key intermediate in the oxidation of formic acid on platinum electrodes.

The electrooxidation of formic acid on Pt and other noble metal electrodes proceeds through a dual-path mechanism, composed of a direct path and an indirect path through adsorbed carbon monoxide, a poisoning intermediate. Adsorbed formate had been identified as the reactive intermediate in the direct path. Here we show that actually it is also the intermediate in the indirect path and is, hence, the key reaction intermediate, common to both the direct and indirect paths. Furthermore, it is confirmed that the dehydration of formic acid on Pt electrodes requires adjacent empty sites, and it is demonstrated that the reaction follows an apparently paradoxical electrochemical mechanism, in which an oxidation is immediately followed by a reduction.

Anisotropic light absorption by localized surface plasmon resonance in a thin film of gold nanoparticles studied by visible multiple-angle incidence resolution spectrometry.

Anisotropic light absorptions via localized surface plasmon resonance in a gold-evaporated film parallel (in-plane; IP) and perpendicular (out-of-plane; OP) to the film surface are studied using visible multiple-angle incidence resolution spectrometry (Vis-MAIRS). When the thin film was aged for eighteen days, the time-dependent Vis-MAIRS IP spectra exhibited significantly different variation from that of the OP spectra: the IP spectra exhibited a large shift to the shorter wavelength side, whereas the OP spectra were explained by a linear combination of three-constituent spectra. The surface topographical analysis of the film revealed that a continuous film coalesced to form aggregates of metal particles. The intrinsic difference between the IP and OP spectra was readily elucidated by considering the surface-parallel and -perpendicular dipoles interaction depending on the topographical changes, which was confirmed by performing spectral simulation using metal particle array models.

Interference effects in the sum frequency generation spectra of thin organic films. I. Theoretical modeling and simulation.

A general theoretical calculation is described for predicting the interference effect in the sum frequency generation (SFG) spectra from a model thin-film system as a function of film thickness. The calculations were carried out for a three-layer thin film consisting of an organic monolayer, a dielectric thin film of variable thickness, and a gold substrate. This system comprises two sources of SFG, namely, a resonant contribution from the monolayer/dielectric film interface and a nonresonant contribution from the dielectric film/gold interface. The calculation shows that both the spectral intensity and the shape of the SFG spectra vary significantly with the thickness of the dielectric layer due to interference effects in the thin film. The intensity changes at a particular frequency were explained in terms of the changes in the local field factors (L factors) as a function of the dielectric film thickness. The L factor for each beam changes periodically with the thickness of the dielectric film. However, the combined L factor for the three beams shows complicated thickness dependent features and no clear periodicity was found. On the other hand, if the susceptibilities of both the resonant and nonresonant terms are fixed, changes in the spectral shape will be mainly due to changes in the phase differences between the two terms with the film thickness. The interference behavior also depends strongly on the polarization combinations of the sum frequency, visible, and infrared beams. A general method is provided for predicting changes in the spectral shapes at different film thicknesses by taking into account the relative intensities and phases of the SFG signals from the two interfaces. The model calculation provides important insights for understanding the nonlinear optical responses from any thin-film system and is an essential tool for quantitatively revealing the nonlinear susceptibilities, which are directly related to the actual structure of the interfacial molecules from the observed SFG spectra after quantitative removal of the L factors.

Interference effects in the sum frequency generation spectra of thin organic films. II: Applications to different thin-film systems.

In this paper, the results of the modeling calculations carried out for predicting the interference effects expected in the sum frequency generation (SFG) spectra of a specific thin-layer system, described in the accompanying paper, are tested by comparing them with the experimental spectra obtained for a real thin-layer film comprising an organic monolayer/variable thickness dielectric layer/gold substrate. In this system, two contributions to the SFG spectra arise, a resonant contribution from the organic film and a nonresonant contribution from the gold substrate. The modeling calculations are in excellent agreement with the experimental spectra over a wide range of thicknesses and for different polarization combinations. The introduction of another resonant monolayer adjacent to the gold substrate and with the molecules having a reverse orientation has a significant affect on the spectral shapes which is predicted. If a dielectric substrate such as CaF(2) is used instead of a gold substrate, only the spectral intensities vary with the film thickness but not the spectral shapes. The counterpropagating beam geometry will change both the thickness dependent spectral shapes and the intensity of different vibrational modes in comparison with a copropagating geometry. The influences of these experimental factors, i.e., the molecular orientational structure in the thin film, the nature of the substrate, and the selected incident beam geometry, on the experimental SFG spectra are quantitatively predicted by the calculations. The thickness effects on the signals from a SFG active monolayer contained in a thin liquid-layer cell of the type frequently used for in situ electrochemical measurements is also discussed. The modeling calculation is also valid for application to other thin-film systems comprising more than two resonant SFG active interfaces by appropriate choice of optical geometries and relevant optical properties.

Destruction of the hydration shell around tetraalkylammonium ions at the electrochemical interface.

The structure and behavior of the hydration shells around tetraethyl-, -propyl-, and -butylammonium ions (Et(4)N(+), Pr(4)N(+), and Bu(4)N(+), respectively) at a CO-covered Pt electrode was studied by surface-enhanced IR absorption spectroscopy. Selective concentration of cations at the electrochemical interface by controlling the applied potential facilitates the observation of the hydration shells without interference from hydrated anions. We report for the first time that the hydration shells around Pr(4)N(+) and Bu(4)N(+) are decomposed at sufficiently negative potentials because of the strong electrostatic attraction with the surface. On the other hand, the hydration shell around Et(4)N(+), the smallest ion, is more stable and does not decompose in the potential range examined. The difference in the stability can be ascribed to the difference in the size of the central cation. On the other hand, the hydration shells are easily deconstructed on a bare Pt surface because of the stronger interaction with the surface.

ATR-SEIRAS investigation of the Fermi level of Pt cocatalyst on a GaN photocatalyst for hydrogen evolution under irradiation.

The interaction of photogenerated carries in GaN photocatalyst with Pt cocatalyst for hydrogen evolution under irradiation was investigated from in situ ATR-SEIRAS measurement by following the CO vibrational frequency. After irradiation, the CO frequency shifted higher, indicating that the Fermi level of Pt particles was positively shifted by the photogenerated holes. Thereafter, a lower frequency peak appeared, indicating that the Fermi level of some Pt particles was negatively shifted to the hydrogen evolution potential by photogenerated electrons, which is the essential function of cocatalysis for hydrogen evolution.