Catalysis with engineered Prussian blue analogues under external bias, light, and magnetic field

Abstract

The design of robust and feasible catalysts is one of the main concerns towards a carbon emission-free world. Prussian blue analogues (PBAs), the most well-known family of cyanide-based compounds, offer diversity and facile tunability of the structural components to achieve robust catalysts with high selectivities and reproducibilities. Herein this thesis, the catalytic performances of CoFe PBAs have been investigated for glucose and water oxidation processes. The structure of the Prussian blue (PB) framework has been engineered to tune the morphological and electronic properties for enhanced catalytic activity. In this regard, my thesis could be divided into three sections: (i) Electrocatalytic glucose oxidation: In the first part of this study, a CoFe PB modified fluorine-doped tin oxide (FTO) electrode, which is prepared via an electrodeposition method, was investigated as a non-enzymatic glucose sensor under neutral conditions. The electrode exhibits a linear detection of glucose in the 0.1 − 8.2 mM range with a detection limit of 67 μM, and a sensitivity of 18.69 mA mM−1 cm−2. Its stability is confirmed with both electrochemical experiments and characterization studies performed on the pristine and post-mortem electrodes. We also conducted a comprehensive electrochemical analysis to elucidate the identity of the active site and the glucose oxidation mechanism on the PB surface. In the second part, a series of PB modified carbon cloth (CC) electrodes were prepared with different cyanoferrate groups. A sensitivity as high as 145.43 μA mM−1 cm−2 in a 0.1 – 6.5 mM concentration range is achieved with a response time below 2 s under physiological pH. The electrodes exhibit a superior selectivity of glucose in the presence of interfering agents, including sucrose, lactose, sodium chloride, ascorbic acid, and uric acid. The electrodes also show outstanding long-term stability over 15 days. Furthermore, we performed comprehensive electrochemical and characterization studies to elucidate the role of the cyanoferrate group on the morphologic and electronic properties of non-enzymatic glucose sensors. (ii) Photocatalytic water oxidation: We present a simple and easy-to-scale synthetic method to plug common organic photosensitizers into a cyanide-based network structure for the development of photosensitizer-water oxidation catalyst (PS−WOC) dyad assemblies for the photocatalytic water oxidation process. Three photosensitizers, one of which absorbs red light similar to P680 in photosystem II, were utilized to harvest different regions of the solar spectrum. Photosensitizers are covalently coordinated to CoFe PB structures to prepare PS-WOC dyads. All dyads exhibit steady water oxidation catalytic activities throughout a 6 h photocatalytic experiment. Our results demonstrate that the covalent coordination between the PS and WOC groups enhances not only the photocatalytic activity but also the robustness of the organic PS group. We find that the photocatalytic activity of these “plug and play” dyads relies on several structural and electronic parameters, including the position of the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the PS with respect to the HOMO level of the catalytic site, the intensity and wavelength of the absorption band of the PS, and the number of catalytic sites. (iii) Intermetallic charge transfer induced electrocatalysis: We report a novel route to enhance the sluggish kinetics of oxygen evolution reaction (OER) by manipulating the intermetallic charge transfer (IMCT) of PBAs. It is found that CoFe PBAs with dissimilar charge transfer abilities reveal a positive response for OER under external stimuli such as magnetic field and light illumination, in which the magnitude of enhancement can be correlated to the intensity of metal-to-metal charge transfer (MMCT) profiles rather than the catalytic activity. An enhancement of almost 57% for OER activity is observed under a 1 h light irradiation for the CoFe PBA that exhibits the strongest IMCT nature. Several control experiments are conducted correlating the direct relation of IMCT and external stimuli induced activity involving –electrochemical experiments at varying pH conditions. Overall, this thesis indicates that CoFe PBAs could be engineered to design robust catalysts for oxidation reactions. Furthermore, they could also be fine-tuned to develop catalytic assemblies, which are responsive to applied bias, magnetic field, and light irradiation. Given the previous efforts in employing PBAs for catalytic applications, this thesis pushes the limits one step forward and brings a new level to this challenge.