Xiong, Lunqiao; (2023) Investigation of highly selective metal-oxide photocatalysts for glycerol oxidation into value-added chemicals. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Glycerol is an important by-product in biodiesel production and is produced in large quantities, resulting in a huge surplus flooding the market with a very low price (US $0.11 per kg). It is also a highly versatile polyol which can be transformed into a plethora of different value-added chemicals. Despite many research efforts devoted to finding active catalysts to transform glycerol into valuable products, it remains a significant challenge to develop an efficient and highly selective catalyst for the transformation of glycerol into a specific product. Therefore, glycerol transformation is not only of great scientific significance but also of economic interest. Photocatalysis has been recognised as a promising strategy in glycerol conversion under very mild conditions. However, one barrier is that most semiconductor-based photocatalysts are either non-selective (particularly with the presence of water and/or oxygen) or ineffective to activate the C-C or C-O bond because of poor oxidative activity. To address the above issues, a highly selective catalyst was first synthesised for the transformation of glycerol to the high-value product glycolaldehyde, which is composed of nickel single atoms confined on titanium dioxide. Driven by light, the catalyst operates under ambient conditions using air as a green oxidant. The optimised catalyst shows a selectivity of over 60% to glycolaldehyde, resulting in a 1.06 mmol·g-1·h-1 production rate, and nearly 3 times higher turnover number than NiOx nanoparticle-decorated TiO2 photocatalyst. Diverse operando and in-situ spectroscopies unveil the unique function of the Ni single atoms, which can significantly promote oxygen adsorption, work as an electron sink and accelerate the production of superoxide radicals, thereby improving the selectivity towards glycolaldehyde over other by-products. Next a two-step co-catalyst modification strategy was applied by loading Au and Ag onto ZnO to significantly promote charge separation/transport, meanwhile generating H2O2 and glyceraldehyde simultaneously. Under light irradiation, the optimised catalyst shows a very high glyceraldehyde yield of 3.86 mmol·g-1·h-1, nearly 6 times higher than ZnO, with a remarkable 75% selectivity to glyceraldehyde. Furthermore, H2O2 yield reaches 4.64 mmol·g-1·h-1, 13 times higher than pristine ZnO, leading to a record AQY of 25%. Spectroscopic results and structure analysis unveil that AuAg alloy can effectively act as hole acceptors, improving the charge transfer and facilitating glycerol oxidation. O2•− radicals are facilely formed over Ag single atoms supported ZnO by photogenerated electrons and further reduced to H2O2. In addition, Ag single atoms over the ZnO surface can prevent the H2O2 decomposition and thus significantly enhance the H2O2 selectivity. As a result, nearly 100% atom economy and high charge utilisation have been achieved. Finally, two valuable trioses, glyceraldehyde and dihydroxyacetone, are converted from glycerol over Cuδ+-decorated WO3 photocatalyst with a five-fold enhancement in conversion (14.52 mmol·g-1) and a selectivity of 84%. In-situ spectroscopies and isotopic analysis confirm the function of Cu+ species, which efficiently serve as hole acceptors, enhancing charge transfer which in turn improving the photocatalytic activity. Moreover, it is found that the glycerol oxidation over Cuδ+-decorated WO3 is initiated by the photogenerated holes, followed by coupling with •OH radicals, and finally dehydrated into the target products.

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