Water Splitting Catalysed By Salenco(Ⅱ) And Other Co Compounds

Development of inexpensive and renewable pollution-free energy sources that may be used on a large scale has become one of the most profound challenges of the21st century facing our society. H2is considered to be the most suitable candidate because there is only H2O generated when H2combines with O2. Splitting water to produce molecular oxygen and hydrogen by artificial photosynthesis is supposed to be one of the most promising technologies to store energy.Photolysis of water is composed of two half-reactions:the reduction of protons to H2and the oxidation of water to O2(H2O�'H2+O2). In artificial photosynthesis, the oxidation of water viewed as bottleneck of the overall process, is recognized as the vital step for supplying electron to other half-reaction. Therefore, developing efficient water oxidation catalyst (WOC) is much significant to obtain inexpensive and renewable pollution-free energy sources. In the past few decades, lots of effort has been devoted to develop various metal complexes as efficient WOC.The development of efficient WOCs remains a major scientific challenge despite the above considerable progress, especially for the high TON, TOF and high quantum yield. Herein, we present an efficient photocatalytic water oxidation reaction system using [Ru(bpy)3]2-(bpy=2,2’-bipyridine) as a photosensitizer, Na2S2O8as a sacrificial electron acceptor, borate as buffer reagent in the presence of N,N’-Bis(salicylidene)ethylenediaminecobalt (Ⅱ)(SalenCo(Ⅱ)).In this work, we mainly studied three parts as following:1. Several cobalt compounds [CoⅡ(Me6tren)(OH2)]2+, Co-Salophen,([[meso-tetra(4-carboxyphenyl)porphyrinato]cobalt(Ⅲ)]Cl)·7H20,[Co4(H2O)2(PW9O34)2]10-and [Co；ICo、(H20)W11O39]7-were synthesized and characterized. Above compounds and SalenCo(Ⅱ) were texted their water oxidation performacne, in which SalenCo(Ⅱ) showed the best activity. Variables of photocatalytic reaction conditions, including catalyst concentrations, buffer types, pHs, photosensitizers, were systemically investigated. The oxidative and hydrolytic stability of SalenCo(Ⅱ) was examined with multiple experiments (UV, DLS, CV, SEM, XPS, ESI-MS and catalyst aged experiment) within the photocatalytic water oxidation duration. All the evidence stated here supports that SalenCo(Ⅱ) is hydrolytic stable and oxidative unstable at least within the water oxidation duration. That is to say SalenCo(Ⅱ) is a precatalyst. In the water oxidation, SalenCo(Ⅱ) was decomposed to a mixture of Co(Ⅲ) containing oxide and/or Co(Ⅲ) hydroxide. These species should be the real water oxidation catalyst.2. Synthesis and characterization of SalenCo@SBA-16. After introducing a given number of SalenCo(Ⅱ) complexes in the nanocages of SBA-16, the pore entrance size was reduced through a silylation reaction. The SalenCo@SBA-16was obtained and showed no activity for the water oxidation. The main reason maybe that [Ru(bpy)3]3+is too large to get into the nanocages of SBA-16for its small pore entrance.[Ru(bpy)3]3+and SalenCo(Ⅱ) can not contact together, so no reaction happened.3. Complexes of SalenCo(Ⅱ),[CoII(Me6tren)(OH2)]2+, CoTCPP and CoSlp were tested for the water reduction. Only CoSIP and [CoII(Me6tren)(OH2)]2+showed the ability for hydrogen evolution using H2PtCl6as cocatalyst, Eosin-Y as photosencitizer and TEOA as electron donor.