The role of proton coupled electron transfer in water oxidation

• Published in: Energy & environmental science 2012-01, Vol.5 (7), p.7704-7717
• Main Authors: Gagliardi, Christopher J, Vannucci, Aaron K, Concepcion, Javier J, Chen, Zuofeng, Meyer, Thomas J
• Format: Article
• Language: English
• Subjects: Activation; Catalysts; Direct power generation; Electron transfer; Energy conversion; Oxidation; Oxides; Pathways
• ISSN: 1754-5692; 1754-5706
• DOI: 10.1039/c2ee03311a

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 sub(2)O arrow right O sub(2) + 4e super(-) + 4H super(+). 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 super(II)(bpy) sub(2)(py)(OH sub(2)) super(2+) to Ru super(IV)(bpy) sub(2)(py)(O) super(2+). The Ru "blue dimer", cis,cis-(bpy) sub(2)(H sub(2)O)RuORu(OH sub(2))(bp y) sub(2) super(4+), was the first designed catalyst for water oxidation. It undergoes oxidative activation by PCET to give the transient (bpy) sub(2)(O)Ru super(V)ORu super(V)(O)(bpy) sub(2) super(4+), O-atom attack on water to give a peroxidic intermediate, and further oxidation and O sub(2) release. More recently, a class of single site water oxidation catalysts has been identified, e.g., Ru(tpy)(bpm)(OH sub(2)) super(2+) (tpy is 2,2':6',2''-terpyridine; bpm is 2,2'-bipyrimidine). They undergo stepwise PCET oxidation to Ru super(IV)=O super(2+) or Ru super(V)(O) super(3+) followed by O-atom transfer with formation of peroxidic intermediates which undergo further oxidation and O sub(2) release. PCET plays a key role in the three zones of water oxidation reactivity: oxidative activation, O...O bond formation, oxidation and O sub(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'-(PO sub(3)H sub(2) CH sub(2)) sub(2)bpy)(OH sub(2))] super(2+). A PCET barrier to Ru super(III)-OH super(2+) arrow right Ru super(IV)=O super(2+) oxidation arises from the large difference in pK sub(a) values between Ru super(III)-OH super(2+) and Ru super(IV)(OH) super(3+). On oxide surfaces this oxidation occurs by multiple pathways. Kinetic, mechanistic, and DFT results on single site catalysts reveal a ne