Size-Dependent Underpotential Deposition of Copper on Palladium Nanoparticles

The underpotential deposition (UPD) of copper on palladium nanoparticles (NPs) with sizes in the range 1.6–98 nm is described. A dependence of the UPD shift on size of the nanoparticle is observed, with the UPD shift decreasing as the particle size decreases. This size dependence is consistent with the known dependence of UPD shift on work function difference between the substrate metal (Pd) and the depositing metal (Cu). The shift suggests the work function of the NPs decreases with decreasing size as expected (i.e., the smaller nanoparticles are more easily oxidized and therefore have lower work functions than larger NPs). For the smallest nanoparticles, the UPD shift does not follow the expected trend based solely on predictions of work function changes with size. On the basis of preliminary competitive anion adsorption experiments, it is speculated that strong chloride absorption on the smallest nanoparticles may be responsible for this deviation

Comparison of Oxygen Reduction Reaction at Silver Nanoparticles and Polycrystalline Silver Electrodes in Alkaline Solution

Adenosine 5′-triphosphate capped silver nanoparticles (ATP-Ag NPs) with diameters of 4.5 ± 1.1 nm were synthesized using a one-pot chemical reduction route. After removal of the ATP capping ligands, the electrochemical activity of these NPs for the oxygen reduction reaction (ORR) in aqueous alkaline solution (0.1 M NaOH) was studied in a single layer-by-layer (1 L LbL) film of NPs and compared with bulk polycrystalline Ag using cyclic voltammetry (CV) and rotating disk electrode (RDE) experiments. For the NPs in the 1 L LbL film, the active area of catalyst available for the ORR was calculated using the charge for the underpotential deposition (UPD) of lead (Pb) on the NPs. RDE data were analyzed to determine the ORR rate constant and the number of electrons (<i>n</i>-value) involved in oxygen reduction. Analysis of Koutecky–Levich (K–L) plots indicates an <i>n</i>-value between 3 and 4 with a higher <i>n</i>-value under some conditions for the NPs than for bare polycrystalline Ag. Possible origins of the variation of <i>n</i>-values for the Ag NPs compared to bulk Ag are discussed

Electrochemical Solid-State Phase Transformations of Silver Nanoparticles

Adenosine triphosphate (ATP)-capped silver nanoparticles (ATP–Ag NPs) were synthesized by reduction of AgNO₃ with borohydride in water with ATP as a capping ligand. The NPs obtained were characterized using transmission electron microscopy (TEM), UV–vis absorption spectroscopy, X-ray diffraction, and energy-dispersive X-ray analysis. A typical preparation produced ATP–Ag NPs with diameters of 4.5 ± 1.1 nm containing ∼2800 Ag atoms and capped with 250 ATP capping ligands. The negatively charged ATP caps allow NP incorporation into layer-by-layer (LbL) films with poly­(diallyldimethylammonium) chloride at thiol-modified Au electrode surfaces. Cyclic voltammetry in a single-layer LbL film of NPs showed a chemically reversible oxidation of Ag NPs to silver halide NPs in aqueous halide solutions and to Ag₂O NPs in aqueous hydroxide solutions. TEM confirmed that this takes place via a redox-driven solid-state phase transformation. The charge for these nontopotactic phase transformations corresponded to a one-electron redox process per Ag atom in the NP, indicating complete oxidation and reduction of all Ag atoms in each NP during the electrochemical phase transformation

Oxygen Reduction Reaction in Ionic Liquids: The Addition of Protic Species

The effect of proton donors on the mechanism of the electrochemical oxygen reduction reaction (ORR) is examined in the supporting ionic liquid 1-butyl-2,3-dimethylimidazolium trifluoromethanesulfonate (C4dMImTf). ORR in aqueous media is contrasted with that in aprotic media and in aprotic ionic liquid (IL) systems with the addition of protic species. This study elucidates the effect of proton activity encompassing almost thirty orders of magnitude for both platinum (Pt) and glassy carbon (GC) electrodes. In neat aprotic C4dMImTf for both platinum and glassy carbon electrodes, ORR proceeds entirely through a one electron process as expected. In ILs with protic additives, ORR approaches a four-electron pathway regardless of the identity of the protic additive on Pt, whereas ORR on GC is limited to a two-electron process due to a lack of H_ads and O_ads species

Electrochemical Capture and Release of Carbon Dioxide Using a Disulfide–Thiocarbonate Redox Cycle

We describe a new electrochemical cycle that enables capture and release of carbon dioxide. The capture agent is benzylthiolate (RS^–), generated electrochemically by reduction of benzyldisulfide (RSSR). Reaction of RS^– with CO₂ produces a terminal, sulfur-bound monothiocarbonate, RSCO₂^–, which acts as the CO₂ carrier species, much the same as a carbamate serves as the CO₂ carrier for amine-based capture strategies. Oxidation of the thiocarbonate releases CO₂ and regenerates RSSR. The newly reported <i>S</i>-benzylthiocarbonate (IUPAC name benzylsulfanylformate) is characterized by ¹H and ¹³C NMR, FTIR, and electrochemical analysis. The capture–release cycle is studied in the ionic liquid 1-butyl-1-methylpyrrolidinium bis­(trifluoro­methyl­sulfonyl)­imide (BMP TFSI) and dimethylformamide. Quantum chemical calculations give a binding energy of CO₂ to benzyl thiolate of −66.3 kJ mol^–1, consistent with the experimental observation of formation of a stable CO₂ adduct. The data described here represent the first report of electrochemical behavior of a sulfur-bound terminal thiocarbonate

Reversible Electrochemical Trapping of Carbon Dioxide Using 4,4′-Bipyridine That Does Not Require Thermal Activation

Sequestering carbon dioxide emissions by the trap and release of CO₂ via thermally activated chemical reactions has proven problematic because of the energetic requirements of the release reactions. Here we demonstrate trap and release of carbon dioxide using electrochemical activation, where the reactions in both directions are exergonic and proceed rapidly with low activation barriers. One-electron reduction of 4,4′-bipyridine forms the radical anion, which undergoes rapid covalent bond formation with carbon dioxide to form an adduct. One-electron oxidation of this adduct releases the bipyridine and carbon dioxide. Reversible trap and release of carbon dioxide over multiple cycles is demonstrated in solution at room temperature, and without the requirement for thermal activation