CO2 reduction in aqueous electrolytes suffers efficiency losses because of the simultaneous reduction of water to H-2. We combine in situ surface-enhanced IR absorption spectroscopy (SEIRAS) and electrochemical kinetic studies to probe the mechanistic basis for kinetic bifurcation between H-2 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 similar to 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 H-2 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 H-2 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.

Rational design of selective CO2-to-fuels electrocatalysts requires direct knowledge of the electrode surface structure during turnover. Metallic Cu is the most versatile CO2-to-fuels catalyst, capable of generating a wide array of value-added products, including methane, ethylene, and ethanol. All of these products are postulated to form via a common surface-bound CO intermediate. Therefore, the kinetics and thermodynamics of CO adsorption to Cu play a central role in determining fuel-formation selectivity and efficiency, highlighting the need for direct observation of CO surface binding equilibria under catalytic conditions. Here, we synthesize nanostructured Cu films adhered to IR-transparent Si prisms, and we find that these Cu surfaces enhance IR absorption of bound molecules. Using these films as electrodes, we examine Cu-catalyzed CO2 reduction in situ via IR spectroelectrochemistry. We observe that Cu surfaces bind electrogenerated CO, derived from CO2, beginning at -0.60 V vs RHE with increasing surface population at more negative potentials. Adsorbed CO is in dynamic equilibrium with dissolved (CO)-C-13 and exchanges rapidly under catalytic conditions. The CO adsorption profiles are pH independent, but adsorbed CO species undergo a reversible transformation on the surface in modestly alkaline electrolytes. These studies establish the potential, concentration, and pH dependencies of the CO surface population on Cu, which serve to maintain a pool of this vital intermediate primed for further reduction to higher order fuel products.

Hydrogen evolution reaction (HER) on Cu and Ag electrodes in neutral and alkaline media (but not in acidic media) is significantly accelerated by the addition of 4,4'-bipyridine (BiPy). The mechanism of the HER acceleration is discussed at a molecular level by using surface-enhanced infrared absorption spectroscopy (SEIRAS) coupled with electrochemical techniques and density functional theory (DFT) calculations. Simultaneous SEIRAS and cyclic voltammetry measurements reveal that the onset potential of the accelerated HER coincides with the 1e + 1H(+) reduction potential of adsorbed BiPy to yield monohydro BiPy radical (BiPy-H-center dot). Tafel plot analysis and DFT calculations suggest that BiPy-H-center dot is further reduced to N,C(3)-dihydro BiPy via a subsequent 1e + 1H(+) step and finally H-2 is evolved by the combination of two H atoms on N and C(3) atoms (BiPy-H-2 -> BiPy + H-2). Since adsorbed BiPy is readily reduced again to BiPy-H-center dot in the potential range of HER, the reaction process is catalytically cycled. The enhanced HER is not observed on Au electrodes due to the desoption of BiPy from the surface. Also the enhanced HER is not observed on Pt electrodes despite strong adsorption on Pt because the redox potential of adsorbed BiPy, the first step of the HER, is more negative than the onset of HER on bare Pt surface. (C) 2017 Elsevier Ltd. All rights reserved.

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 H2SO4 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 mu C 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.

The orientation of cinchonidine (CD) adsorbed on a Pt surface, a model system of enantioselective catalysis for hydrogenation reactions, is studied in 1,2-dichloroethane by in situ surface-enhanced infrared absorption spectroscopy in attenuated total reflection configuration. The quinoline (QN) moiety of CD is shown to be reoriented from a pi-bonded nearly parallel orientation to an N-bonded upright one by bubbling the solution with H-2. The origin of the reorientation is ascribed to the repulsive interaction of a QN pi-orbital and the negative surface charge induced by dissociative adsorption of H-2 and hydrogenation of the vinyl moiety.

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/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). (C) 2012 Elsevier B.V. All rights reserved.

Formate was found to be the only adsorbed species detected with time-resolved ATR-SEIRAS during the electrooxidation of formic acid on Au in acidic media. No adsorbed CO was detected, since even in CO-saturated acidic solutions the adsorption of CO on gold is negligible. The onset of formic acid electrooxidation coincides with that of formate electroadsorption, pointing to bridge-bonded adsorbed formate as the reaction intermediate. Furthermore, at constant potential the current increases quadratically with the formate coverage, which indicates that the rate-determining step in the oxidation of adsorbed formate to CO2 is a bimolecular reaction between adjacent formate species. The rate of this reaction was unaffected by potential changes, which unequivocally confirms its chemical character. (C) 2012 Elsevier B.V. All rights reserved.

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.=20

Electro-oxidation of CO adsorbed on a polycrystalline Pt electrode in the potential region of hydrogen absorption is examined by fast time-resolved surface-enhanced infrared absorption spectroscopy coupled to voltammetry or chronoamperometry. Oxidation dynamics at a weak preoxidation peak around 0.5 V (vs RHE) and the main oxidation peak around 0.7 V observed in stripping voltammetry are focused. IR spectra show that the shift of bridge-bonded CO to atop sites triggers the partial oxidation of CO adsorbed at atop sites to yield the preoxidation peak. It is also shown that CO on terraces becomes very mobile in the main oxidation region after some amount of CO being oxidized via a nucleation-and-growth mechanism and that terrace CO is oxidized faster than CO adsorbed at step edges. The result is interpreted in terms of a Langmuir-Hinshelwood type mechanism involving adsorbed CO and an oxygen-containing species (most likely OH) adsorbed at steps. The oxidation mechanism is essentially identical to that proposed in some earlier studies, but more convincing spectroscopic evidence of the mechanism is presented.

A seeded-growth approach has been developed to fabricate a Cu nanoparticle film (simplified hereafter with nanofilm) on Si for electrochemical ATR surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). The approach comprises an initial activation of the reflecting plane of hemicylindrical Si prism by introducing a Cu seed layer in a CUSO4-HF solution and the subsequent electroless deposition of the Cu nanofilms from an electroless Cu plating bath. The as-deposited Cu nanofilm exhibited strong SEIRA effect for the CO probe and interfacial free H2O. ATR-SEIRAS was also applied to characterize the adsorbed geometries of pyridine at the Cu/electrolyte interface. Only vibrational bands assignable to the A, symmetry modes were detected in the entire potential window investigated, suggestive of an end-on adsorption via the ring N-atom on a Cu electrode. (C) 2007 Elsevier Ltd. All rights reserved.

Three advanced IR spectroscopy techniques, surface-enhanced IR absorption spectroscopy (SEIRAS), step-scan Fourier-transform interferometry, and two-dimensional (2D) IR correlation analysis, have been applied to the study of electrochemical reactions. A combined use of SEIRAS and step-scan interferometry enables time-resolved spectral monitoring of electrochemical reactions with time-resolutions ranging from microseconds to milliseconds. 2D-IR correlation analysis highlights the dynamic information obscured in the time-resolved spectra. The basic concept of 2D-IR is somewhat analogous to that of 2D-NMR, and synchronous and asynchronous 2D spectra defined by two independent wavenumbers are generated by a correlation analysis of dynamic fluctuation of IR signals induced by a potential modulation. The synchronous and asynchronous spectra characterize the coherence and incoherence respectively of dynamic fluctuations of IR signals at two different wavenumbers. Bands arising from different transient species are clearly differentiated by their characteristic time-dependent behavior. The temporal relationship between the intensity fluctuations of different bands also becomes clear. From these data, deep insights into reaction processes are gained. The utilities of 2D-IR are demonstrated for the one-electron reduction of heptylviologen at a silver electrode surface.

The composition and structure of a binary mixed self-assembled monolayer (SAM) of 3-aminopropyltiethoxysilane (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.