The role of proton coupled electron transfer in water oxidation
The role of proton coupled electron transfer in water oxidation
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DOI：
10.1039/c2ee03311a
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发表时间：
2012-07-01
影响因子：
32.5
通讯作者：
Meyer, Thomas J.
中科院分区：
文献类型：
--
作者：
Gagliardi, Christopher J.;Vannucci, Aaron K.;Meyer, Thomas J.
水氧化是基于太阳能燃料和目标的能量转换方案中的关键半反应，如光驱动水分解或二氧化碳还原为CO，其他含氧化合物或碳氢化合物。在有效应用所需的较长时间内，以超过太阳日晒率的速率进行这些反应是一项重大挑战。在这些方案中，水氧化是关键的“另一半”反应，由于PCET具有多电子、多质子的特性，它以2H(2)O -> O-2 + 4e + 4H（+）为主。ppet的鉴定是通过电子转移猝灭分子激发态来研究能量转换的一个分支。“氧化还原电位平衡”和协同电子-质子转移的概念来自于顺式ru - ii (bpy)(2)(py)(OH2)（2+）逐步氧化到Ru-IV (bpy)(2)(py)(O)（2+）的测量。Ru“蓝色二聚体”cis, cis(bpy)(2)(H2O) RuORu(OH2)(bpy)(2)（4+）是第一个设计的水氧化催化剂。它经过PCET的氧化活化，产生瞬态（bpy）(2)(O) Ru-V ORuV (O)(bpy)(2)(4+)， O原子攻击水产生过氧化中间体，并进一步氧化和释放O-2。最近，一类单位点水氧化催化剂已被确定，如Ru(tpy)(bpm)(OH2)(2+) （tpy为2,2':6',2 " -三吡啶；bpmis为2,2'-联嘧啶）。它们经过逐级的epet氧化(u(IV) = O2+或ruv (O)(3+)，然后O原子转移，形成过氧化中间体，再经过进一步氧化和O2释放。PCET在水氧化反应的三个区域中起关键作用：氧化活化，O…O键形成，氧化和O-2从过氧化物中间体释放。基于膦化衍生物[Ru(Mebimpy)(4,4'-(PO3H2CH2)(2)bpy)(OH2)](2+)，已经确定了类似的氧化电极表面电催化水氧化方案。由于Ru-III-OH2+和Ru-IV(OH)（3+）之间pKa值的巨大差异，Ru-III-OH2+/Ru-IV = O2+氧化的PCET屏障产生。在氧化物表面，这种氧化通过多种途径发生。单位点催化剂的动力学、机理和DFT结果揭示了O…O键形成步骤（原子质子转移，APT），通过添加质子受体碱显著提高速率，以及在碳酸丙烯酯作为溶剂中添加水作为化学计量试剂加速水氧化。关于水氧化和PCET的作用以及协同途径的经验教训似乎与光系统II （PSII）中的水氧化直接相关，PSII是PCET作用的一个壮观例子。这包括多位点电子质子转移在S-0 -> S-1转变过程中氧络合物（OEC）的氧化活化中的关键作用。
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 + 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)(OH2)(2+) to Ru-IV (bpy)(2)(py)(O)(2+). The Ru "blue dimer'', cis, cis( bpy)(2)(H2O) RuORu(OH2)(bpy)(2)(4+), was the first designed catalyst for water oxidation. It undergoes oxidative activation by PCET to give the transient (bpy)(2)(O) Ru-V ORuV (O)(bpy)(2)(4+), 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)(OH2)(2+) (tpy is 2,2':6',2 ''-terpyridine; bpmis 2,2'-bipyrimidine). They undergo stepwisePCET oxidation toRu(IV) = O2+ orRuV(O)(3+) followed by O-atom transfer with formation of peroxidic intermediates which undergo further oxidation and O2 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'-(PO3H2CH2)(2)bpy)(OH2)](2+). A PCET barrier to Ru-III-OH2+/Ru-IV = O2+ oxidation arises from the large difference in pKa values between Ru-III-OH2+ 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.