Catalytic oxidation of water and alcohols by iron, cobalt and nickel compounds

鐵, 鈷以及鎳化合物用於水和醇的氧化

Student thesis: Doctoral Thesis

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Author(s)

  • Peng TAN

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date2 Oct 2015

Abstract

To convert solar energy into chemical fuels through water splitting is a promising way to meet the increasing energy demand. Water oxidation is considered the bottleneck reaction in water splitting. In the past two decades, considerable efforts have been made to develop highly active water oxidation catalysts (WOCs), which are mostly based on second- and third-row transition metals. To be economically practical, WOCs should be based on earth-abundant transition metals. However, relatively less progress has been made on developing WOCs based on first-row transition metals. In this thesis, seven macrocyclic ligands derived from cyclam (1,4,8,11-tetraazacyclotetradecane) have been synthesized and characterized, and a number of WOCs based on iron, cobalt, nickel and copper with these ligands have been reported. There are three parts. The first part describes the synthesis of macrocyclic iron complexes and their chemical-driven water oxidation activities with sodium periodate as the oxidant. Only [Fe(L3)Cl2](PF6) (L3 = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane) is proved to be a homogeneous and active WOC, with a maximum turnover number (TON) of 1030 and a highest turnover frequency (TOF) of 1.7 min-1. The active intermediate is proposed to be an iron(V) oxo species, presumably [FeV(L3)(O)(OH)]2+. On the other hand, at pH 7.5 - 8.5 under photocatalytic condition, [Fe(L3)Cl2](PF6) is only a precatalyst, iron oxide nanoparticles are formed which are the actual WOC. Moreover, [Fe(L3)Cl2](PF6) is also a highly efficient catalyst for the oxidation of alcohols by NaIO4 in water, with>98% conversions and yields. The active intermediate is also proposed to be [FeV(L3)(O)(OH)]2+. In the second part, five cobalt complexes have been synthesized, characterized and investigated for their chemical and photo-driven water oxidation properties. Among them, only [Co(L3)Cl2](PF6) is found to be a homogeneous and robust WOC, while [Co(L1)](ClO4)2 (L1 = 1-(2-pyridylmethyl)-4,8,11-trimethyl-1,4,8,11- tetraazacyclotetradecane) and [Co(L6)](ClO4) (HL6 = 1-(2-acetic acid)-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane) are shown to be precatalysts for water oxidation in borate buffer at pH 7.5 – 8.5. In the third part, six nickel complexes, [Ni(L1)](ClO4)2, [Ni(L2)](PF6)2, [Ni(L3)(NO3)](PF6), [Ni(L4)](PF6)2 (L4 = 1,4,8,11-tetraazabicyclo[6.6.2]hexadecane), [Ni(L6)](ClO4) and [Ni(L7)](ClO4), have been synthesized, characterized and preliminarily studied for their photo-driven water oxidation activities. [Ni(L1)](ClO4)2 is found to decompose to nickel oxide nanoparticles in borate buffer at pH 8.5.

    Research areas

  • Cobalt compounds, Catalysis, Nickel compounds, Oxidation, Water, Alcohols, Iron compounds