Electrochemical and Catalytic Hydrogen Evolution by Porphyrin-based Transition Metal Complexes

過渡金屬紫質錯合物進行電催化產氫

貝多蒙(Belete Bedemo Beyene)

指導教授 ： 洪政雄 ； 廖文峯

國立清華大學/理學院/化學系/博士(2016年)

摘要

Abstract
The rational design toward developing energy-efficient and cost-effective processes to generate H2 remains as an outstanding challenge. To come up with this challenge, various transition metal complexes of macrocycles have been extensively reported as catalysts, with little attention given to porphyrin-based systems. In line with this, we reported electrochemical H2 evolution from organic acids and neutral aqueous solution using cheap and earth abundant transition metal complexes of porphyrin as catalysts. The thesis is divided in to 6 chapters. The 1st chapter is introduction part, chapters 2-5 author’s original works and the last chapter is conclusion of thesis.
Encouraged by literature reports of H2 evolution based on cobalt(II) porphyrins in organic solvents or aqueous solution, we reported a water-soluble cobalt(II) tetrakis(p-sulfonatophenyl) porphyrin (CoTPPS) as a stable, active, and efficient electrocatalyst for H2 generation from neutral aqueous solution without any organic additives. The molecule features nearly quantitative Faradaic efficiency with a TOF of ~1.83 s-1 measured over 1 h and a TON of 1.9 x 104 moles of H2 per mole of catalyst with no loss in activity over 73 h at an applied potential of -1.29 V vs SHE in neutral phosphate buffer solution.
In the 3rd chapter, H2 evolution activity of a series of cobalt(II) porphyrins with EW groups (COOMe, COOH, and SO3H), unsubstituted phenyl and ED groups (NH2, OH and OMe) at para position of meso-phenyl rings has been investigated in DMSO using acetic acid as a proton source. The study showed that nature of substituents significantly influences catalytic performance and overpotential. Faradaic efficiencies ranging from 44 to 99 %, TONs from 1.5 to 104 (~11 h electrolysis), TOFs from 0.23 to 9.1 h-1 and onset overpotentials ranging from 25 to 445 mV were obtained by tuning the substituents. Molecules with -SO3H, -COOH and -NH2 groups showed high activity and efficiency with more positive onset potentials as compared to a parent molecule, (CoTPP) and other molecules in this study. The key factor in enhancing activity at lower overpotential was supposed to be acidity of a functional group at para position of meso-phenyl ring during catalysis process.
In chapter four, the effect of position and electronic nature of substituents were examined. Cobalt(II) porphyrins with pendant orth-amino, orth-nitro, and para-amino groups have been prepared to examine the role of substituent position and electronic nature in tuning overpotential and activity. Enhanced catalytic activity with H2 evolution rate constant of 1.12 x 105 M-1 s-1 with onset overpotential as low as 20 mV was obtained for cobalt(II) porphyrin with ortho-amino pendant group, confirming the substantial role of pendant amino group in intramoleular shuttling of proton to cobalt center thereby thermodynamically favouring proton-hydride interactions to evolve H2.
In the 5th chapter, M-(p-NH2phenyl)4porphyrins (where M = Fe, Co, Ni, Cu and Zn) were synthesized and employed for H2 evolution study in organic solvents using weak and strong acids as well as in neutral aqueous solution to explore the roles of nature of central metal ion and proton source in H2 evolution activity and overpotential. In acetic acid (weak acid), the catalytic current appeared at a potential close to M1+/M0 redox couple, implying formation of M(II)-H as reactive intermediate through protonation of M(0) and the activity order of catalysts is Fe> Co> Ni> Cu>. However, in trifluoroacetic acid (strong acid) catalysis occur close to M2+/M1+ redox couple for Co and Cu while close to M1+/M0 redox event for Fe and Ni complexes with activity order of Co>Fe>Cu>Ni. In neutral aqueous solution, high activities were noticed for Co and Fe complexes than others. Based on our results, the redox potential of central metal ion and thermodynamic reduction potential of a proton source seem to play roles in tuning catalytic activity.
In the last chapter, summary of author’s original work is presented. Thus, electrocatalytic H2 evolution studies using various metalloporphyrins has been reported in organic solvents and aqueous solution.

關鍵字

紫質金屬錯合物 ； 產氫 ； 電催化反應 ； 過電位 ； 控制電位電催化 ； 電化學研究 ； 催化效率 ； 還原電位

並列摘要

抽象
對於設計與發展出低成本，高效率的氫氣產生催化劑至今仍是科學界的一大挑戰。為了解決此問題，許多不同的大環金屬錯合物已被報導，而當中屬於紫質的系統卻不多。基於此，我們在此報導以一系列紫質錯合物，其以便宜且量多的過渡金屬元素為金屬中心在酸性有機溶劑或者是中性水溶液下進行產氫反應。本論文共分為六章節，第一章為導論，第二至五章為作者的實驗主要內容，最後一章則為結論。
受到文獻上紫質鈷錯合物用來在酸性有機溶液或者中性水裡面進行產氫的啟發，我們在此報導一磺酸化的水溶性紫質鈷錯合物其可在完全不含有機添加物的中性水中進行穩定且高效率的產氫反應。此分子幾近完全將電能轉化成化學能，而在-1.29V (相對於標準氫電級)的電壓進行電解，以一小時的測量時間計算，其TOF值約為1.83 s-1，TON則為1.9x104，並且在持續電解73小時候催化劑的活性仍無降低。

在第三章，我們在DMSO中加入醋酸，測試了一系列meso苯環上para位置未取代，或者帶有推(NH2，OH，OMe)或拉(COOMe，COOH，SO3H)電子基的紫質鈷錯合物的氫離子還原能力。結果顯示，meso苯環上的取代基會影響over potential。此些化合物的法拉第效率分布從44到99%，TON值的分布為1.5到104 (經過11小時的電解)，TOF值的分布為0.23到9.1每小時，啟動的過電位的分布則從25到445mV。和meso苯環上para位置未有取代基的紫質鈷錯合物相比，當有-SO3H，-COOH或NH2時，錯合物表現出較高的活性和效率，以及較高的啟動過電位。而在較低的過電位增加反應活性的主要因素被歸咎於meso苯環上的para位置的取代基之酸性。

在第四章，我們對於meso苯環上的取代基的位置和性質相對於催化效果做了探討。我們合成出了在meso苯環上鄰位有amino基，nitro基以及對位上有amino 基的紫質鈷錯合物以研究苯環上的取代基之性質和位置對於過電位和反應活性的影響。在meso苯環鄰位有amino基的紫質鈷錯合物，在-20 mV的低過電位的條件下，我們測量到了增強的氫氣催化反應，反應速率常數為1.12x105M-1S-1。此結果證實了meso苯環鄰位上的amino基有聚集質子於中心鈷離子附近的效應，使得熱動力學上較易於形成質子與氫負離子的配對以產生氫氣。

在第五章，我們合成出了meso苯環對位上有-NH2基的紫質，並且於中心配位不同的過渡金屬。此些錯合物被溶於有機溶劑中並加入弱或強酸，或者是在中性水溶液中進行產氫實驗以研究中心金屬，以及酸性的來源對於催化劑的活性以及過電位的影響。以醋酸(弱酸)為酸性來源時，我們可在接近M+1/M0的還原對電壓的位置觀測到催化反應所產生的電流，此暗示了催化反應的過程中M(0)的質子化形成了M(II)-H此反應中間體；此些不同金屬的中心，反應速率鐵>鈷>鎳>銅。然而，以三氟醋酸(強酸)為酸性來源時，以鈷和銅為中心時的紫質錯合物，催化反應則發生在近M2+/M1+的還原對電壓的位置；而以鐵和鎳為中心時，催化反應則仍是發生在接近M+1/M0的還原對電壓位置；而反應速率則為鈷>鐵>銅>鎳。在中性水溶液中，鈷和鐵錯合物擁有較高的催化效應。基於此結果，金屬中心的還原電位以及不同酸性質子來源的熱動力還原電位似乎在催化反應中扮演了某些角色。

最後一章，對於作者所做的研究，以不同的紫質金屬錯合物在有機溶液以及水溶液中所進行的氫氣催化反應做了總結。

並列關鍵字

Metalloporphyrin ； Hydrogen generation ； Electrocatalytic activity ； Overpotential ； Controlled-potential electrolysis ； Electrochemical studies ； Catalytic efficiency ； Redox potential

參考文獻

(71) Stoyanovsky, D. A.; Huang, Z.; Jiang, J.; Belikova, N. A.; Tyurin, V.; Epperly, M. W.;

(1) Karunadasa, H. I.; Chang, C. J.; Long, J. R. Nature 2010, 464, 1329.

(19) Bigi, J. P.; Hanna, T. E.; Harman, W. H.; Chang, A.; Chang, C. J. Chem. Commun. 2010,

(58) Chang, C. J.; Loh, Z.-H.; Shi, C.; Anson, F. C.; Nocera, D. G. J. Am. Chem. Soc. 2004,

(34) Tard, C.; Liu, X.; Ibrahim, S. K.; Bruschi, M.; Gioia, L. D.; Davies, S. C.; Yang, X.;