反位效应对钌催化剂催化水氧化活性的影响

TRANS EFFECT ON CATALYTIC ACTIVITY OF RUTHENIUM-BASED WATER OXIDATION CATALYSTS

杨扬

11849420

硕士

化学

段乐乐

2020-05-30

2020-06-08

哈尔滨工业大学

深圳

通过太阳能光解水制氢，使用氢能替代逐渐短缺的化石燃料，是当今社会面临的最紧迫和最具挑战性的问题之一。水的裂解反应主要涉及到水氧化和质子还原两类反应，其中水氧化反应涉及了多步的电子和质子转移以及多个化学键的断裂和生成。水氧化反应在热力学上需要很高的能量，是整个水裂解反应的瓶颈，所以需要高效、稳定的催化剂来降低水氧化所需的过电位，降低能量损耗以便高效利用太阳光分解水。在以往的研究中发现带负电荷的配体由于能够提高配体交换速度，从而显著提高钌催化剂的性能。因此，为了充分发挥有机配体对过渡金属中心活性的影响,本论文提出利用经典的反位效应来活化与金属中心配位的水分子，提高配体交换速度，从而提高催化剂的催化性能。 本论文设计了两个催化剂体系，在体系一中，使用4-羟基吡啶-2,6-二羧酸（hdc2-）作为骨架配体来制备钌基水氧化催化剂（WOC）。其中hdc2-配体中的羟基基团由于p-π共轭成为强供电子基团，提高配体的供电子能力并增强反位效应。本文合成并且表征了一系列带有供电子基团和吸电子基团的Ru-hdc化合物[Ru(hdc)(py-R)3]（py-R = 4-吡啶基配体；4-甲氧基吡啶，Ru1；4-甲基吡啶，Ru2；吡啶，Ru3；4-吡啶甲酸甲酯，Ru4）。电化学水氧化实验表明（i）RuIII/II的氧化还原电位对取代基较敏感，供电子能力较强的基团则会导致较低的氧化电势；（ii）水氧化的催化电流与其配体上4-取代基的电子效应密切相关，供电子能力 -OMe > -Me > -H > -COOCH3，相应化合物的催化电流Ru1 > Ru2 > Ru3 > Ru4。在CeIV驱动的化学法催化水氧化的性能测试中也观察到了相似的现象。在体系二中,设计合成了含酰胺的三齿配体配位的钌基水氧化催化剂，通过改变配合物中负电配位原子N酰胺与配位水分子的位置来分析反位效应对钌催化剂催化活性的影响。论文以N-8-喹啉基-2-吡啶甲酰胺配体（L1）为赤道配体合成了化合物Ru5和Ru6，以N-苯基[2,2'-联吡啶]-6-甲酰胺配体（L2）为赤道配体合成了化合物Ru7。在CeIV为氧化剂的均相体系中，测得化合物的TON值Ru6 > Ru5 > Ru7，证实反位效应可以降低催化剂的氧化电位，加快配体交换速度，提高催化剂的催化性能，这也为开发新一代的钌基催化剂奠定了基础

The use of solar energy to split water into hydrogen and using hydrogen energy to replace fossil fuels is one of the most pressing and challenging issues today. Water splitting reaction includes mainly two half reactions: water oxidation and proton reduction. Water oxidation involves multi-step electron transfer, proton transfer and the chemical bond cleavage/formation. The difficulty of water splitting is that water oxidation reaction requires large energy in thermodynamics. Therefore, an efficient and stable catalyst is needed to reduce the overpotential in order to efficiently use sunlight to split water. In previous studies, it was found that negatively charged ligands can increase the rate of ligand exchange, thus increasing the activity of the ruthenium water oxidation catalyst. In order to fully exert the influence of the ligand on the activity of the transition metal center we propose, in this thesis, to use the classical trans effect to activate water molecules bound to the metal center, thereby accelerating the ligand exchange rate and improving the catalytic performance.Herein, two catalyst systems were designed. In system one, 4-hydroxypyridine-2,6-dicarboxylic acid (hdc2-) was used as a ligand to prepare Ru-based water oxidation catalyst (WOC) where the hydroxyl group of hdc2 endows strong electron-donating ability to the pyridine ring due to the p- conjugation, which improves the electron donating ability of the ligand and enhances the trans effect. A family of Ru-hdc complexes, [Ru(hdc)(py-R)3] (py-R = 4-pyridyl ligands; R = OMe, Ru1; Me, Ru2; H, Ru3; COOMe, Ru4) with electron-donating and -withdrawing groups were prepared and fully characterized. Electrochemical water oxidation experiments demonstrated that (i) the redox potentials of RuIII/II are sensitive to the substituents, and stronger electron-donating groups give lower oxidation potentials and (ii) the catalytic current for water oxidation correlates well with their electron-donating ability of pyridyl ligands, Ru1 > Ru2 > Ru3 > Ru4. A similar trend for the performance of CeIV-driven water oxidation was also observed. In system two, a series of Ru-based water oxidation catalysts with tridentate ligands containing amides was synthesized. By changing the position of the negatively charged amide N , we studied the trans effect on the catalytic activity of ruthenium catalysts. Complexes Ru5 and Ru6 were synthesized using N-(quinolin-8-yl)picolinamide (L1) as equatorial ligand. Complex Ru7 was synthesized using N-phenyl-[2,2'-bipyridin]-6-amine (L2) as equatorial ligand. In a homogeneous system with CeIV as the oxidant, the TON of the complex Ru6 > Ru5 > Ru7 was measured, confirming that the trans effect can reduce the oxidation potential of the catalysts, accelerate the exchange rate of the ligand, and improve the catalytic performance of the catalyst.

钌配合物 水氧化 水裂解 电化学 均相催化

ruthenium complex water oxidation water splitting electrochemistry homogeneous catalysis

中文

联合培养

南方科技大学

杨扬. 反位效应对钌催化剂催化水氧化活性的影响[D]. 深圳. 哈尔滨工业大学,2020.