Extended investigation of electrocatalytic generation of dihydrogen using [(mu-1,2-benzenedithiolato)][Fe(CO)3]2 has revealed that weak acids, such as acetic acid, can be used. The catalytic reduction producing dihydrogen occurs at approximately -2 V for several carboxylic acids and phenols resulting in overpotentials of only -0.44 to -0.71 V depending on the weak acid used. This unusual catalytic reduction occurs at a potential at which the starting material, in the absence of a proton source, does not show a reduction peak. The mechanism for this process and structures for the intermediates have been discerned by electrochemical and computational analysis. These studies reveal that the catalyst is the monoanion of the starting material and an ECEC mechanism occurs.
In aqueous solution above pH 2.4 with 4% (vol/vol) CH 3 CN, the complex [Ru II (bda)(isoq) 2 ] (bda is 2,2′-bipyridine-6,6′-dicarboxylate; isoq is isoquinoline) exists as the open-arm chelate, [Ru II (CO 2 -bpy-CO 2 − )(isoq) 2 (NCCH 3 )], as shown by 1 H and 13 C-NMR, X-ray crystallography, and pH titrations. Rates of water oxidation with the open-arm chelate are remarkably enhanced by added proton acceptor bases, as measured by cyclic voltammetry (CV). In 1.0 M PO 4 3-, the calculated half-time for water oxidation is ∼7 μs. The key to the rate accelerations with added bases is direct involvement of the buffer base in either atom-proton transfer (APT) or concerted electron-proton transfer (EPT) pathways. [Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy is 2,2′-bipyridine; Fig. 1], both in solution and on surfaces, reveal mechanisms in which stepwise oxidative activation of aqua precursors to Ru V =O is followed by rate-limiting O-O bond formation (10-15). The results of kinetic and mechanistic studies have revealed the importance of concerted atom-proton transfer (APT) in the O-O bond-forming step. In APT, the O-O bond forms in concert with H + transfer to water or to an added base (11,12,(16)(17)(18)(19). APT can promote dramatic rate enhancements. In a recent study on surface-bound [Ru(Mebimpy)(4,4′-((HO) 2 OPCH 2 ) 2 bpy)(OH 2 )] 2+ [4,4′-((HO) 2 OPCH 2 ) 2 bpy is 4,4′-bis-methlylenephosphonato-2,2′-bipyridine] stabilized by atomic layer deposition, a rate enhancement of ∼10 6 was observed with 0.012 M added PO 4 3− at pH 12 compared with oxidation at pH 1 (20).Sun and coworkers (21, 22) have described the Ru single-site water oxidation catalysts, [Ru II (bda)(L) 2 ] (H 2 bda is 2,2′-bipyridine-6,6′-dicarboxylic acid, HCO 2 -bpy-CO 2 H; L is isoquinoline, 4-picoline, or phthalazine). They undergo rapid and sustained water oxidation catalysis with added Ce IV . A mechanism has been proposed in which initial oxidation to seven coordinate Ru IV is followed by further oxidation to Ru V (O) with O-O coupling to give a peroxo-bridged intermediate, Ru IV O-ORu IV , which undergoes further oxidation and release of O 2 (21, 22). We report here the results of a rate and mechanistic study on electrochemical water oxidation by complex , [Ru II (CO 2 -bpy-CO 2 )(isoq) 2 ] (isoq is isoquinoline) (Fig. 1). Evidence is presented for water oxidation by a chelate open form in acidic solutions. The chelate open form displays dramatic rate enhancements with added buffer bases, and the results of a detailed mechanistic study are reported here. − /HPO 4 2− phosphate buffer, I = 0.5 M (NaClO 4 )] in 4% (vol/vol) CH 3 CN at a glassy carbon electrode (GC) (0.071 cm 2 ). The Ag/AgCl [3 M NaCl, 0.21 V vs. normal hydrogen electrode (NHE)] reference electrode was isolated with an electrolyte filled bridge to avoid chloride ion diffusion into the anode compartment. The sample was purged with argon to remove O 2 before each scan, with only O 2 freshly produced in oxidative scans detected on reverse scans at -0.3 V vs. NHE, a...
Water oxidation is a key half reaction in energy conversion schemes based on solar fuels and targets such as light driven water splitting or carbon dioxide reduction into CO, other oxygenates, or hydrocarbons. Carrying out these reactions at rates that exceed the rate of solar insolation for the extended periods of time required for useful applications presents a major challenge. Water oxidation is the key ''other'' half reaction in these schemes and it is dominated by PCET given its multi-electron, multi-proton character, 2H 2 O / O 2 + 4e À + 4H + . Identification of PCET was an offshoot of experiments designed to investigate energy conversion by electron transfer quenching of molecular excited states. The concepts ''redox potential leveling'' and concerted electron-proton transfer came from measurements on stepwise oxidation of cis-Ru II (bpy) 2 (py)(OH 2 ) 2+ to Ru IV (bpy) 2 (py)(O) 2+ . The Ru ''blue dimer'', cis,cis-, was the first designed catalyst for water oxidation. It undergoes oxidative activation by PCET to give the transient (bpy, O-atom attack on water to give a peroxidic intermediate, and further oxidation and O 2 release. More recently, a class of single site water oxidation catalysts has been identified, e.g., Ru(tpy)(bpm)(OH 2 ) 2+ (tpy is 2,2 0 :6 0 ,2 00terpyridine; bpm is 2,2 0 -bipyrimidine). They undergo stepwise PCET oxidation to Ru IV ¼O 2+ or Ru V (O) 3+ followed by O-atom transfer with formation of peroxidic intermediates which undergo further oxidation and O 2 release. PCET plays a key role in the three zones of water oxidation reactivity: oxidative activation, O/O bond formation, oxidation and O 2 release from peroxidic intermediates. Similar schemes have been identified for electrocatalytic water oxidation on oxide electrode surfaces based on phosphonated derivatives such as [Ru(Mebimpy)(4,4 0 -(PO 3 H 2 CH 2 ) 2 bpy)(OH 2 )] 2+ . A PCET barrier to Ru III -OH 2+ / Ru IV ¼O 2+ oxidation arises from the large difference in pK a values between Ru III -OH 2+ and Ru IV (OH) 3+ . On oxide surfaces this oxidation occurs by multiple pathways. Kinetic, mechanistic, and DFT results on single site catalysts reveal a new pathway for the O/O bond forming step (Atom-Proton Transfer, APT), significant rate enhancements by added proton acceptor bases, and accelerated water oxidation in propylene carbonate as solvent with water added as a stoichiometric reagent. Lessons learned about water oxidation and the role of PCET and concerted pathways appear to have direct relevance for water oxidation in Photosystem II (PSII) with PSII a spectacular example of PCET in action. This includes a key role for Multiple Site-Electron Proton Transfer in oxidative activation of the Oxygen Evolving Complex (OEC) in the S 0 / S 1 transition in the Kok cycle.
Enhancing the surface binding stability of chromophores, catalysts, and chromophore-catalyst assemblies attached to metal oxide surfaces is an important element in furthering the development of dye sensitized solar cells, photoelectrosynthesis cells, and interfacial molecular catalysis. Phosphonate-derivatized catalysts and molecular assemblies provide a basis for sustained water oxidation on these surfaces in acidic solution but are unstable toward hydrolysis and loss from surfaces as the pH is increased. Here, we report enhanced surface binding stability of a phosphonate-derivatized water oxidation catalyst over a wide pH range (1-12) by atomic layer deposition of an overlayer of TiO 2 . Increased stability of surface binding, and the reactivity of the bound catalyst, provides a hybrid approach to heterogeneous catalysis combining the advantages of systematic modifications possible by chemical synthesis with heterogeneous reactivity. For the surface-stabilized catalyst, greatly enhanced rates of water oxidation are observed upon addition of buffer bases −H 2 PO − 4 /HPO 2− 4 , B(OH) 3 /B(OH) 2 O − , HPO 2− 4 /PO 3− 4 − and with a pathway identified in which O-atom transfer to OH − occurs with a rate constant increase of 10 6 compared to water oxidation in acid.electrocatalysis | surface stabilization H eterogeneous catalysis plays an important role in industrial chemical processing, fuel reforming, and energy-producing reactions. Examples include the Haber-Bosch process, steam reforming, Ziegler-Natta polymerization, and hydrocarbon cracking (1-8). Research in heterogeneous catalysis continues to flourish (9-15) but iterative design and modification are restricted by limitations in materials preparation and experimental access to surface mechanisms. By contrast, synthetic modification of molecular catalysts is possible by readily available routes; a variety of experimental techniques is available for monitoring rates and mechanism in solution for the investigation of homogeneous catalysis (16-23). Transferring this knowledge and the reactivity of homogeneous molecular catalysts to a surface could open the door to heterogeneous applications in fuel cells, dye sensitized photoelectrochemical cells, and multiphase industrial reactions.Procedures are available for immobilization of organometallic and coordination complexes on the surfaces of solid supports. Common strategies include surface derivatization of metal oxides by carboxylate, phosphonate, and siloxane bindings (24-27), carbongrafted electrodes (28-30), and electropolymerization (31-33). These approaches provide a useful bridge to the interface and a way to translate mechanistic understanding and ease of synthetic modification of solution catalysts to heterogeneous applications with a promise of higher reactivity under milder conditions. A significant barrier to this approach arises from the limited stability of surface binding. Surface-bound carboxylates are typically unstable to hydrolysis in water, whereas phosphonates are unstable in neutral or basic...
The six-coordinate Fe(III)-aqua complex [Fe(III)(dpaq)(H2O)](2+) (1, dpaq is 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamido) is an electrocatalyst for water oxidation in propylene carbonate-water mixtures. An electrochemical kinetics study has revealed that water oxidation occurs by oxidation to Fe(V)(O)(2+) followed by a reaction first order in catalyst and added water, respectively, with ko = 0.035(4) M(-1) s(-1) by the single-site mechanism found previously for Ru and Ir water oxidation catalysts. Sustained water oxidation catalysis occurs at a high surface area electrode to give O2 through at least 29 turnovers over an 15 h electrolysis period with a 45% Faradaic yield and no observable decomposition of the catalyst.
The oxidative stability of the molecular components of dye-sensitized photoelectrosynthesis cells for solar water splitting remains to be explored systematically. We report here the results of an electrochemical study on the oxidative stability of ruthenium(II) polypyridyl complexes surface-bound to fluorine-doped tin oxide electrodes in acidic solutions and, to a lesser extent, as a function of pH and solvent with electrochemical monitoring. Desorption occurs for the Ru(II) forms of the surface-bound complexes with oxidation to Ru(III) enhancing both desorption and decomposition. Based on the results of long-term potential hold experiments with cyclic voltammetry monitoring, electrochemical oxidation to Ru(III) results in slow decomposition of the complex by 2,2'-bipyridine ligand loss and aquation and/or anation. A similar pattern of ligand loss was also observed for a known chromophore-catalyst assembly for both electrochemical water oxidation and photoelectrochemical water splitting. Our results are significant in identifying the importance of enhancing chromophore stability, or at least transient stability, in oxidized forms in order to achieve stable performance in aqueous environments in photoelectrochemical devices.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
334 Leonard St
Brooklyn, NY 11211
Copyright © 2023 scite Inc. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.