Mechanism of the Quenching of the Tris(bipyridine)ruthenium(II) Emission by Persulfate: Implications for Photoinduced Oxidation Reactions

A revised mechanism for the oxidation of the excited state of Ru­(bpy)₃²⁺ with the persulfate anion is described in this work. The formation of the precursor complex in the electron transfer reaction involves ion pairing between the metal complex in ground and excited states and S₂O₈^2‑. The equilibrium constant for the ion-pair formation (<i>K</i>_IP = 2.7 M^–1) was determined from electrochemical measurements and analysis of thermal reaction between Ru­(bpy)₃²⁺ and persulfate. It was found to be consistent with the calculated value estimated from the Debye–Hückel model. The analysis of rate constants for reactions between persulfate and various metal complexes indicates that thermal and photochemical reactions most likely proceed through a common pathway. Extremely high reorganization energy (ca. 3.54 eV) for the electron transfer obtained from fitting experimental data with the Marcus equation is indicative of significant nuclear reorganization during the electron transfer step. In view of these results the electron transfer can be described as dissociative probably involving substantial elongation or complete scission of the O–O bond. The proposed model accurately describes experimental results for the quenching of *Ru­(bpy)₃²⁺ over a wide range of persulfate concentrations and resolves some discrepancies between the values of <i>K</i>_IP and <i>k</i>_et previously reported. The implications of various factors such as the ionic strength and dielectric constant of the medium are discussed in relation to measurements of the quantum yields in photodriven oxidation reactions employing the Ru­(bpy)₃²⁺/persulfate couple

Enhancement of the photocatalytic hydrogen production with the exfoliation degree of Nb2C cocatalyst

Niobium-carbide, NbC, a member of MXene family, has recently drawn considerable attention in photocatalytic H production as potential candidate to replace Pt as co-catalyst owing to its favorable physical and chemical properties. The present study explored the influence of the exfoliation level and surface area in suspension on the photocatalytic activity toward hydrogen production of NbC as cocatalyst and Eosin Y as photosensitizer. It was found that a NbC sample obtained by chemical etching and 4 h sonication (NbC-4h) with a high specific surface area in aqueous suspension of 161 m ×g measured by the methylene blue method showed the highest hydrogen generation rate (10,290μmolhg), 3.2 (1 mg m L) or 1.7 (0.7 mg m L) times higher than that of NbC obtained by chemical etching without post-synthetic sonication that has 55 m ×g of specific surface area in suspension. The increased performance of NbC-4h surpassing many reported photocatalysts was attributed to the beneficial influence of the exposed surface area and level of exfoliation of the MXene sheets. Our results demonstrate the importance of exfoliation to achieve a high surface area in suspension, diminishing the number of layers of the MXene flakes on photocatalytic applications.Financial support by the Spanish Ministry of Science and Innovation (Severo Ochoa and PID2021-126071-OB-CO21) and Generalitat Valenciana (Prometeo 2021-038) are gratefully acknowledged. R.R.-G. also thanks the Ministry of Science and Innovation for a postgraduate scholarship. A.L-A thanks for funding under the project at AMU “Initiative of Excellence - Research University” (proposal no. 038/04/NŚ/0012)

Working the other way around: Photocatalytic water oxidation triggered by reductive quenching of the photoexcited chromophore

A detailed photophysical investigation of the photocatalytic water oxidation system based on the tetraruthenium polyoxometalate [Ru₄(μ-O)₄(μ–OH)₂(H₂O)₄(γ-SiW₁₀O₃₆)₂]^10– (<b>1</b>) as the catalyst, the tetranuclear ruthenium dendrimer [Ru­{(μ-2,3-dpp)­Ru­(bpy)₂}₃]⁸⁺ (<b>2</b>) as the light-harvesting photosensitizer, and persulfate (S₂O₈^2–) as the sacrificial electron acceptor has shown that the water oxidation mechanism proceeds through an unusual, “anti-biomimetic” pathway: The first photochemical event is indeed a reductive quenching of the excited photosensitizer by the catalyst, followed by electron scavenging by the sacrificial electron acceptor, both occurring on the picosecond time scale within ion-paired species. As an unprecedented photoreaction mechanism for molecular water oxidation systems, these results suggest a new way to combine photosensitizers and catalysts, taking advantage of suitable chemical interactions and alternative photoinduced processes for the construction of efficient water-splitting devices

Electrochemical and spectroscopic methods for evaluating molecular electrocatalysts

© 2017 Macmillan Publishers Limited. Modern energy challenges have amplified interest in transition metal-based molecular electrocatalysts for fuel-forming reactions. The activity of these homogeneous electrocatalysts, and the mechanisms by which they operate, can be uncovered using state-of-the-art electrochemical methods. Catalyst performance can be benchmarked according to metrics obtainable from cyclic voltammograms by analysis of catalytic plateau currents and peak potentials, as well as by foot-of-the-wave analysis. The application of complementary spectroscopic techniques, including spectroelectrochemistry, stopped-flow rapid mixing and transient absorption, are also discussed. In this Review, we present case studies highlighting the utility of these analytical methods in the context of renewable energy. Alongside these examples is a discussion of the theoretical underpinnings of each method, outlining the conditions necessary for the analysis to be rigorous and the type of information that can then be extracted