Water Splitting and Organic Oxidation Catalyzed by Cobalt and Osmium Compounds


Student thesis: Doctoral Thesis

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  • Man CHEN


Awarding Institution
Award date22 Jul 2015


This thesis is divided into five parts. In Part I, coordinatively saturated cobalt complexes are used to catalyze water oxidation in chemical and photoinduced processes. In part II, special, multi-nuclear cobalt compound is used as both molecular catalyst and precatalyst for water oxidation. In part III, a series of cobalt polypyridyl complexes are investigated as catalysts for proton reduction. In part IV, electrons from anode of microbial fuel cell (MFC) are used as electron donor for hydrogen production. A microbial fuel cell is also used for desalination or alkali production to upgrade biogas. In the last part, Lewis acids are used to activate several osmium nitrido complexes towards catalytic organic oxidation.

In part I, several coordinatively saturated cobalt complexes, [Co2(spy)2](ClO4)4, [Co(tpy)2](ClO4)2 and [Co(bpy)3](ClO4)2 were synthesized and their use as catalysts for water oxidation was investigated. It was found that [Co2(spy)2](ClO4)4 is active in both chemical and light-induced water oxidation at pH 7.5-8.5, whereas both [Co(tpy)2](ClO4)2 and [Co(bpy)3](ClO4)2 are inactivity. A maximum TON of 442 was obtained by [Co2(spy)2](ClO4)4 in the presence of [Ru(bpy)3]2+/S2O82- at pH 8.5 under visible light irradiation. Several lines of evidence, DLS, ICP-AES, ESI-MS, etc., prove that [Co2(spy)2](ClO4)4 functions as a molecular catalyst and does not decompose to CoOx in photo-catalytic water oxidation.

In part II, water oxidation catalyzed by [Co2(spy)2](CoCl4)2 is described. It could effectively catalyze water oxidation in chemical and light-induced process. A maximum TOF of 27 s-1 was obtained in chemical oxidation, while a TON of 3758 was obtained in the presence of [Ru(bpy)3]2+/S2O82- at pH 8.5 under visible light irradiation. It was found that half of the cobalt in [Co2(spy)2](CoCl4)2 decomposes to CoOx, while the other half is still soluble in the solution, as evidenced by ICP-AES, DLS and SEM studies. Comparing with Co(NO3)2, both cobalt in [Co2(spy)2](CoCl4)2 are considered to contribute to oxygen generation. Therefore, [Co2(spy)2](CoCl4)2 is not only a precatalyst but also a molecular catalyst for water oxidation.

In part III, a series of cobalt polypyridyl complexes, [Co(tpy)(L)]2+ (L = 1,10-phenanthroline, 2-hydroxy-1,10-phenanthroline, 2-amine-1,10-phenanthroline and 3,4,7,8-tetramethyl-1,10-phenanthroline) and [Co2(spy)2](ClO4)4, were synthesized and studied as catalysts in proton reduction. All of [Co(tpy)(L)]2+ complexes could catalyze proton reduction, but they do not show much difference in reactivity. [Co2(spy)2](ClO4)4 could also catalyze hydrogen production under visible light irradiation when [Ir(dF(CF3)ppy)2(dmbpy)]PF6 (dF(CF3)ppy = anion of 2-(2,4-difluorophenyl)-5-tri-fluoromethylpyridine, dtbbpy = 4,4’-di-tert-butyl-2,2’ –bipyridine) is used as photosensitizer and triethanolamine (TEOA) is used as sacrificial donor. Under optimized, a TON of 1238 was obtained.

In part IV, electrons from the anode of a MFC are used as the electrons source for homogeneous photocatalytic proton reduction. Hydrogen could be detected in the cathode without any extra electric energy input. MFC also could be used for desalination. When the desalination chamber was packed with ion exchange resin (IER), the desalination rate increased to 3.9 times higher and the infinite concentration was 2.7 times lower comparing with that one without IER packing. The biogas could be upgraded to 100% using the alkali produced by MFC integrated with bipolar membrane; the highest pH of 12.0 was obtained under the 0.5 V applied voltage.

The osmium (VI) nitrido complex, [OsVI(N)(L)(CH3OH)]+ (1, L = N,N’-bis(salicylidene)-o-cyclohexyldiamine dianion) is an efficient catalyst for the oxidation of alkanes at ambient conditions using H2O2 as the oxidant. Alkanes are oxidized to the corresponding alcohols and ketones, with yields up to 75% and turnover number up to 2230. Experimental and computational studies are consistent with a mechanism that involves O-atom transfer from H2O2 to [OsVI(N)(L)]+ to generate an [OsVIII(N)(O)(L)]+ active intermediate.