Lowering the overpotential for the electrochemical conversion of CO2 to useful products is one of the grand challenges in the Department Of Energy report, “Catalysis for Energy”. In a previous paper, we showed that CO2 conversion occurs at low overpotential on a 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4)-coated silver catalyst in an aqueous solution of EMIM-BF4. One of the surprises in the previous paper was that the selectivity to CO was better than 96% on silver, compared with ∼80% in the absence of ionic liquid. In this article, we use sum frequency generation (SFG) to explore the mechanism of the enhancement of selectivity. The study used platinum rather than silver because previous workers had found that platinum is almost inactive for CO production from CO2. The results show that EMIM-BF4 has two effects: it suppresses hydrogen formation and enhances CO2 conversion. SFG shows that there is a layer of EMIM on the platinum surface that inhibits hydrogen formation. CO2, however, can react with the EMIM layer to form a complex such as CO2-EMIM at potentials more negative than −0.1 V with respect to a standard hydrogen electrode (SHE). That complex is converted to adsorbed CO at cathodic potentials of −0.25 V with respect to SHE. These results demonstrate that adsorbed monolayers can substantially lower the barrier for CO2 conversion on platinum and inhibit hydrogen formation, opening the possibility of a new series of metal/organic catalysts for this reaction.

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Macroscopic properties of aqueous β-lactoglobulin (BLG) foams and the molecular properties of BLG modified air-water interfaces as their major structural element were investigated with a unique combination of foam rheology measurements and interfacial sensitive methods such as sum-frequency generation and interfacial dilatational rheology. The molecular structure and protein-protein interactions at the air-water interface can be changed substantially with the solution pH and result in major changes in interfacial dilational and foam rheology. At a pH near the interfacial isoelectric point BLG molecules carry zero net charge and disordered multilayers with the highest interfacial dilatational elasticity are formed at the air-water interface. Increasing or decreasing the pH with respect to the isoelectric point leads to the formation of a BLG monolayer with repulsive electrostatic interactions among the adsorbed molecules which decrease the interfacial dilational elasticity. The latter molecular information does explain the behavior of BLG foams in our rheological studies, where in fact the highest apparent yield stresses and storage moduli are established with foams from electrolyte solutions with a pH close to the isoelectric point of BLG. At this pH the gas bubbles of the foam are stabilized by BLG multilayers with attractive intermolecular interactions at the ubiquitous air-water interfaces, while BLG layers with repulsive interactions decrease the apparent yield stress and storage moduli as stabilization of gas bubbles with a monolayer of BLG is less effective.

The ethanol electrooxidation reaction (EOR) on polycrystalline Pt catalysts in alkaline solution was studied for the first time with broadband sum-frequency generation (BB-SFG) spectroscopy. We find that CÀC bond cleavage and CO formation occur as early as 0.05 V versus reversible hydrogen electrode (RHE), and that CO is oxidized at ∼0.45 V, which is 0.2 V lower than in acidic media. In order to track the oxidation of singlecarbon intermediates, we have monitored the oxidation of isotopically labeled ethanol ( 12 CH 3 13 CH 2 OH). Surface-adsorbed 12 CO and 13 CO are observed and show very different potential-dependent behaviors. 13 CO molecules formed from preoxidized carbon species such as ÀCH x O, show the behavior expected from studies of CO-saturated alkaline media. 12 CO, however, which is indicative of the oxidation of methyl-like species (ÀCH x ) on the catalyst surface, is observed at unusually high potentials. The strongly adsorbed ÀCH x is not oxidatively removed from the surface until the electrode potential is swept past 0.65 V.

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Surface disorder on a nanometer scale is shown to have fundamental influence on the molecular structure of R-Al 2 O 3 (0001)/water interfaces which have been studied by sum-frequency vibrational spectroscopy. On rough surfaces, we observe isolated surface hydroxyl groups which do not couple to the network of hydrogenbonded water molecules of the interface and which we assign to aluminol groups in hydrophobic nanopores of the surface. The pH dependence of O-H stretching vibrations of water molecules at the rough interface is distinctly different from that of an atomically smooth interface and points to different pK values for deprotonation of surface hydroxyl groups at defect sites and regular terrace sites.

The surface structure of Pt(111) in a 0.1 M H2SO4 electrolyte was investigated in the potential range of sulfate adsorption with electrochemical scanning tunneling microscopy (STM) and cyclic voltammetry. Two ordered anion structures were observed coexisting in the potential range between 0.49 and 0.79 V (vs RHE): the well-known (radical3xradical7)R19.1 degrees superstructure with an anion coverage of 0.20 monolayer and a new, high-density (3x1) superstructure with a coverage of 0.33 monolayer. Both superstructures undergo a reversible order-disorder transition at 0.8 V. Simultaneous imaging of the adsorbed ions and of topographic details of the Pt substrate lattice allows us to study the local adsorption geometry of the sulfate. In the (radical3xradical7)R19.1 degrees, structure the sulfate ions are adsorbed close to depressions in the STM image of the Pt substrate which may be identified with face-centered cubic (fcc) hollow sites. In addition to the sulfate ions, a coadsorbed species, possibly water molecules, is observed in the unit cell of the (radical3xradical7)R19.1 degrees superstructure. Preliminary potentiodynamic STM data indicate that the transformation of the ordered sulfate adlayer into a disordered structure at 0.8 V is not directly related to adsorption/desorption features in the voltammogram commonly attributed to the adsorption/desorption of OH, and that the sulfate adlayer remains on the surface for potentials well above the adsorption potentials of OH.

Applications of gold nanoparticles often demand that the particle's ligand shell is modified after particle formation. Obviously, there is a great need for a molecular understanding of this process which is often not accessible in situ. Here, we have applied second-harmonic light scattering (SHS) to investigate the ligand exchange at the surface of colloidal gold nanoparticles in situ and in real time. We demonstrate that the ligand exchange at the surface of citrate-covered Au nanoparticles with 3-mercapto-1-propanesulfonate (MPS) must be described by a fast (<100 s) and a slow reaction process (<23 min), which can be attributed to MPS adsorption on lowand high-coordinated Au surface sites. Using a modified Langmuir isotherm, the average Gibbs free energy of adsorption ΔG (−46 kJ/mol) and the surface coverage Γ (∼3.5 μmol/m 2 ) for MPS on Au nanoparticles were determined. The latter was found to be much smaller compared to planar gold surfaces which points to coadsorption of MPS with citrate on high-coordinated sites, i.e., Au terraces. On more reactive low-coordinated Au sites, i.e., edge sites, citrate is easily replaced by MPS. In fact, we find that a substantial portion (49%) of the surface-adsorbed MPS is present on these lowcoordinated sites.