Polyoxometalate-Promoted Electrocatalytic CO₂ Reduction at Nanostructured Silver in Dimethylformamide

Electrochemical reduction of CO₂ is a promising method to convert CO₂ into fuels or useful chemicals, such as carbon monoxide (CO), hydrocarbons, and alcohols. In this study, nanostructured Ag was obtained by electrodeposition of Ag in the presence of a Keggin type polyoxometalate, [PMo₁₂O₄₀]^3– (PMo). Metallic Ag is formed upon reduction of Ag⁺. Adsorption of PMo on the surface of the newly formed Ag lowers its surface energy thus stabilizes the nanostructure. The electrocatalytic performance of this Ag–PMo nanocomposite for CO₂ reduction was evaluated in a CO₂ saturated dimethylformamide medium containing 0.1 M [<i>n</i>-Bu₄N]­PF₆ and 0.5% (v/v) added H₂O. The results show that this Ag–PMo nanocomposite can catalyze the reduction of CO₂ to CO with an onset potential of −1.70 V versus Fc^0/+, which is only 0.29 V more negative than the estimated reversible potential (−1.41 V) for this process and 0.70 V more positive than that on bulk Ag metal. High faradaic efficiencies of about 90% were obtained over a wide range of applied potentials. A Tafel slope of 60 mV dec^–1 suggests that rapid formation of *CO₂^•– is followed by the rate-determining protonation step. This is consistent with the voltammetric data which suggest that the reduced PMo interacts strongly with CO₂ (and presumably CO₂^•–) and hence promotes the formation of CO₂^•–

Quenching of the Electrochemiluminescence of Tris(2,2′-bipyridine)ruthenium(II)/Tri‑<i>n</i>‑propylamine by Pristine Carbon Nanotube and Its Application to Quantitative Detection of DNA

In this study, we describe the quenching of electrochemiluminescence (ECL) of tris­(2,2′-bipyridine)-ruthenium­(II)­(Ru­(bpy)₃²⁺)/tri-<i>n</i>-propylamine­(TPA) at pristine multiwall carbon nanotube (MWNT) modified glassy carbon (GC) electrode. Even though the faradic current of the Ru­(bpy)₃²⁺/TPA system and the oxidation of TPA obtained at pristine MWNT-modified GC electrode is enhanced compared with those at the bare GC electrode, the intensity of ECL produced at MWNT electrode is smaller than that at GC electrode. For testing the possible reason of quenching, a comparison of ECL behavior of Ru­(bpy)₃²⁺/TPA at pristine MWNT and acid-treated, heat-treated, and polyethylene glycol (PEG)-wrapped MWNT-modified GC electrode is studied. The results demonstrate that the oxygen-containing groups at the surface of MWNT and the intrinsic electron properties of MWNT are considered to be the major reason for the suppression of ECL. The comparison also demonstrates that this quenching is related to the distance between MWNT and Ru­(bpy)₃²⁺/TPA. Utilizing this essential quenching mechanism, a new signal-on DNA hybridization assay is proposed on the basis of the MWNT modified electrode, where single-stranded DNA (ssDNA) labeled with Ru­(bpy)₃²⁺ derivatives probe (Ru-ssDNA) at the distal end is covalently attached onto the MWNT electrode. ECL signal is quenched where Ru-ssDNA is self-organized on the surface of MWNT electrode; however, the quenched ECL signal returns in case of the presence of complementary ssDNA. The developed approach for sequence-specific DNA detection has good selectivity, sensitivity, and signal-to-background ratio. Therefore, the quenching of the ECL of Ru­(bpy)₃²⁺/TPA system by the pristine MWNT can be an excellent platform for nucleic acid studies and molecular sensing

Direct Detection of Electron Transfer Reactions Underpinning the Tin-Catalyzed Electrochemical Reduction of CO₂ using Fourier-Transformed ac Voltammetry

Two underlying electron transfer processes that directly underpin the catalytic reduction of carbon dioxide (CO₂) to HCOO^– and CO at Sn electrodes have been detected using the higher order harmonic components available in Fourier-transformed large-amplitude ac voltammetry. Both closely spaced electron transfer processes are undetectable by dc voltammetry and are associated with the direct reduction of CO₂ species and have reversible potentials of approximately −1.27 and −1.40 V vs Ag/AgCl (1 M KCl). A mechanism involving a reversible inner-sphere one-electron reduction of CO₂ followed by a rate-determining CO₂^•– protonation step is proposed. Molecular CO₂ has been identified as the dominant electroactive species that undergoes a series of coupling electron transfer and chemical reactions to form the final products. The substantial difference in the catalytic responses of Sn­(SnO_<i>x</i>)-modified glassy carbon and Sn foil electrodes are attributed to their strongly preferred Sn (200) orientation and polycrystalline states, respectively. The Fourier-transformed ac technique should be generally applicable for predicting the performance of Sn catalysts