Investigation of cyanide-based catalysts and hybrid assemblies for artificial photosynthesis

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Bilkent University
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The storage and conversion of solar energy appear to be a highly promising solution to the global energy dilemma due to the sustainability and abundance of sunlight. Among various strategies, artificial photosynthesis, in which solar energy is directly converted to chemical bonds, has gained a great deal of attraction over the past decades. Such a design requires a catalyst coupled with a semiconductor and/or a photosensitizer as light-harvesting components with proper band levels for efficient charge separation. Therefore, the development of an earth-abundant, robust, and visible-light absorbing photocatalytic assembly has been one of the bottlenecks for the advancement of scalable cells. This work aims to overcome this critical challenge by developing new cyanide-based hybrid assemblies for light-driven water splitting and CO2 reduction. CoFe Prussian blue analogues (CoFe-PBAs) have recently emerged as active water oxidation catalysts with excellent long-term stabilities. In the first study, a ZnCr layered double hydroxide (ZnCr-LDH) with a two-dimensional (2D) morphology and a CoFe-PBA are combined to afford a precious metal-free photocatalytic assembly involving a visible light-absorbing semiconductor (SC) and a water oxidation catalyst (WOC). The SC−WOC hybrid materials exhibit a threefold enhancement in activity compared to bare ZnCr-LDH, which is maintained for 6 h under photocatalytic conditions. The band energy diagram was extracted from optical and electrochemical studies to clarify the origin of the improved photocatalytic performance. The assembly is observed to have an appropriate band energy alignment that facilitates charge transfer from the valence band of ZnCr-LDH to the HOMO level of CoFe-PBA for the water oxidation process. In a follow-up study, we move one step forward by coupling exfoliated Dion−Jacobson type niobate nanosheets as a 2D semiconductor with the CoFe-PBA to construct an SC−WOC hybrid structure, which produces a p−n junction. The assembly exhibits a promoted activity (89.5 μmol g−1 h−1) with a proper band energy alignment for the photocatalytic water oxidation process, and it is stable throughout a 12 h photocatalytic study. These studies mark a straightforward pathway to developing low-cost and precious metal-free assemblies for light-driven water oxidation. Metal dicyanamides are another well-known cyanide-based materials, which can be employed as a catalyst for hydrogen evolution reaction (HER) and CO2 reduction due to the partial electron delocalization and the proper coordination environment of metal ions. Herein, we promote a cobalt dicyanamide coordination polymer, Co-dca, for the first time, as a selective catalyst to reduce CO2 to CO in the presence of a ruthenium photosensitizer (Ru PS) under visible light irradiation. A series of photocatalytic experiments under various reaction conditions were performed to reveal the role of the PS, the scavenger, and the solvent in the selectivity and the activity of the photocatalytic process. We find that Co-dca exhibits an activity of 254 µmol h−1 g−1 and a CO selectivity as high as 93%. Furthermore, cobalt dicyanamide also displayed enhanced H2 evolution activity (25000 µmol h−1 g−1), which is maintained for at least 12 h in a mixed aqueous solution containing a Ru-based photosensitizer and a sacrificial electron donor. The effect of various reaction parameters on the photocatalytic activity of the system was investigated. Overall, this thesis presents various low-cost and stable SC-WOC hybrid assemblies based on CoFe-PBA for the light-driven water oxidation process. Moreover, we suggest the use of Co-dca, for the first time, as catalysts for HER and CO2 reduction reactions.

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Photocatalytic water splitting, Artificial photosynthesis, CO2 reduction reaction, Cobalt−iron Prussian blue, Cobalt dicyanamide, Hybrid assembly, 2D semiconductors
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