Photoinduced hydrogen evolution by a pentapyridine cobalt complex: elucidating some mechanistic aspects
Photoinduced hydrogen evolution by a pentapyridine cobalt complex: elucidating some mechanistic aspects
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DOI：
10.1039/c4dt02269f
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发表时间：
2014-11-21
影响因子：
4
通讯作者：
Scandola, Franco
中科院分区：
文献类型：
--
作者：
Deponti, Elisa;Luisa, Alessandra;Scandola, Franco
合成并表征了一种基于五吡啶配体的新型钴析氢催化剂1。它的光催化活性在Ru（bpy）（3）（2+）敏化剂和抗坏血酸作为牺牲电子供体的存在下，已被筛选在纯缓冲水溶液中显示TONs和TOF强烈依赖于催化剂的浓度和pH值与最佳结果在50 μ M-1和pH 4（TON = 187，TOF = 8.1分钟（-1））。通过闪光光解揭示的光化学机理涉及激发的敏化剂与抗坏血酸反应产生Ru（bpy）（3）（+）作为主要的光生还原剂，能够以显著的速率（双分子速率常数k = 5.7（+/-0.7）x 10（9）M-1 s（-1））将电子转移到1。对于氢的产生，需要沿着氢化物形成和质子化的两个1的单电子光化学还原步骤。在所用的实验条件下，析氢主要受到敏化剂和催化剂的部分分解的限制。此外，观察到抗坏血酸供体的氧化产物脱氢抗坏血酸的积累强烈降低氢产生产率。如闪光光解所示，该物质能够淬灭还原的钌物质（k = 4.4（+/- 0.5）x 10（7）M-1 s（-1）），从而与电子转移到催化剂竞争。
A new hydrogen evolving cobalt catalyst 1 based on a pentapyridine ligand has been synthesized and characterized. Its photocatalytic activity in the presence of a Ru(bpy)(3)(2+) sensitizer and ascorbic acid as a sacrificial electron donor has been screened in purely buffered aqueous solutions showing TONs and TOFs strongly dependent on both catalyst concentration and pH with the best results obtained at 50 mu M 1 and at pH 4 (TON = 187, TOF = 8.1 min(-1)). The photochemical mechanism, as revealed by flash photolysis, involves reaction of the excited sensitizer with ascorbic acid to yield Ru(bpy)(3)(+) as a primary photo-generated reductant, capable of electron transfer to 1 with a remarkable rate (bimolecular rate constant k = 5.7 (+/- 0.7) x 10(9) M-1 s(-1)). For hydrogen generation, two one-electron photochemical reduction steps of 1 are needed along with hydride formation and protonation. Under the experimental conditions used, hydrogen evolution is mainly limited by partial decomposition of both the sensitizer and the catalyst. Moreover, accumulation of the oxidation product of the ascorbic acid donor, dehydroascorbic acid, is observed to strongly decrease the hydrogen production yield. As shown by flash photolysis, this species is capable of quenching the reduced ruthenium species (k = 4.4 (+/- 0.5) x 10(7) M-1 s(-1)) thus competing with electron transfer to the catalyst.