| Author: | Xie, Fengjia |
| Title: | Artificial photosynthesis for glucose precursor production |
| Advisors: | Zhang, Xuming (AP) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Photosynthesis Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xxvi, 153 pages : color illustrations |
| Language: | English |
| Abstract: | Food production relies heavily on natural photosynthesis of green plants. It has long been a dream to develop an artificial photosynthesis that mimics natural photosynthesis and directly converts CO2 into basic food materials, which have been reviewed in chapters 1 and 2. This thesis presents a novel artificial photosynthesis system that combines photocatalytic coenzyme regeneration with enzymatic synthesis of glucose precursor. It covers three major aspects: synthesis of key materials, development of reaction pathway, and fabrication of reactors. In addition, detailed studies have been conducted on photocatalyst characterization, coenzyme and glucose precursor yield, and reaction mechanisms. The research contents as demonstrated in chapters 3 to 5 are: First, a new nanomaterial called dual-defect g-C3N4 (DDCN) is developed for efficient photocatalytic cofactor regeneration. Although graphitic carbon nitride (g-C3N4) is very suitable for photocatalytic nicotinamide cofactor regeneration since it is metal-free, visible-light responsive and has strong binding with nicotinamide cofactor, it suffers from some intrinsic drawbacks such as low visible absorption, fast electron/hole recombination and limited active sites. Here the DDCN is developed with controllable defects of nitrogen vacancies and cyano groups via a KOH-assisted thermal polymerization by using urea as a precursor. Although DDCN is widely used for other photocatalytic applications such as organic degradation and hydrogen peroxide production, this work is original in the application to photocatalytic cofactor regeneration. Material characterizations confirm the successful introduction of nitrogen vacancies and cyano groups. Measurements of nicotinamide-cofactor generation show that the DDCN samples assisted with 0.1-g and 0.01-g KOH are 3.0 and 2.5 times that of pristine g-C3N4 in terms of nicotinamide-cofactor yield, respectively. The high yields are attributed to the synergetic effect of both enhanced light absorption and improved charge separation, achieved through the introduction of energy levels and trap states via dual defects. Secondly, we have developed a new photo-enzymatic pathway that effectively integrates photocatalytic coenzyme regeneration with the Calvin cycle for the synthesis of glucose precursor. Remarkably, the DDCN0.1 sample has exhibited exceptional performance, yielding a superior coenzyme production and consequently leading to a significant increase in glucose precursor production at a rate of 0.38 mM h-1. This represents a 1.23-fold improvement as compared to the glucose precursor production using pristine g-C3N4 as a catalyst. Additionally, our findings indicate that an optimal triethanolamine concentration of 5% and temperature of 30°C, further enhances glucose precursor production in the system. The developed photo-enzyme system for G3P production exhibits an AQY of 0.64% and a solar-to-chemical conversion efficiency of 0.0068%, which highlight the potential of the photo-enzyme system for efficient G3P production and solar energy conversion. Finally, we have successfully fabricated a microfluidic reactor coated with g-C3N4 for the purpose of photocatalytic NADH regeneration. The integration of microfluidic reactors in artificial photosynthesis ensures fast mass transfer and high photonic efficiency, resulting in a high efficiency. More specifically, this microfluidic reactor obtains an impressive NADH yield of 85.5 µmol h–1 g–1, which is 2.2 times of that achieved by conventional slurry reactors. This significant improvement in NADH yield underscores the exceptional performance and efficiency of the microfluidic reactor, making it a promising alternative for NADH production in industrial applications. In summary, this thesis has presented an artificial photosynthesis to directly convert CO2 into glucose precursor by synthesizing the key material dual-defect g-C3N4, developing the enabling photo-enzymatic pathway and fabricating the novel microfluidic reactor(Chapter 6). It has obtained an NADH yield of 85.5 µmol h–1 g–1 and a glucose precursor production rate of 1.8 mM h-1. It moves one step up toward food production using manmade materials and may provide a scientific solution to eliminate the food shortage problem. |
| Rights: | All rights reserved |
| Access: | open access |
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