Development of efficient photocatalysts for the degradation of low-concentration NO under visible light

Abstract

This PhD thesis delves into the critical challenge of enhancing the photocatalytic degradation efficiency of nitrogen oxide (NO), a persistent environmental pollutant. Despite the considerable progress in photocatalytic research, the efficiency of NO degradation is inhibited by suboptimal light utilization and inherent limitations such as rapid electron-hole recombination and inefficient charge carrier transfer, which collectively restrict the attainment of maximal degradation efficiency.
To address these limitations, our research introduces a novel multi-layer Z-scheme Co(OH)2/CeO2-g-C3N4 photocatalyst designed to facilitate enhanced electron transfer within its heterostructure. In this composite, Co(OH)2 is incorporated as an electron mediator between CeO2 and g-C3N4, accelerating electron transfer and introducing additional OH pathways for the photocatalytic oxidation of NO. Remarkably, our 50CoCe-CN composite, featuring equal mass ratios of Co and Ce, demonstrates 53.5% conversion efficiency for NO at 600 ppb concentration under visible light exposure— surpassing g-C3N4's performance by 1.82 times. Detailed density functional theory (DFT) analyses and the elemental distribution mapping of cobalt and ceria corroborate the formation of this innovative multi-layer structure, underscoring the synergistic interplay between CeO2, Co(OH)2 and g-C3N4 that significantly augments photocatalytic capabilities. Through comprehensive experimentation and analysis, this thesis not only elucidates the underlying mechanisms contributing to improved photocatalytic performance but also highlights the potential of employing advanced ternary photocatalytic systems for effective NO degradation.
Although the achieved efficiency is commendably high, it still falls short of the benchmarks necessary for real-world applications. Therefore, our attention turned towards covalent organic frameworks (COFs) as promising photocatalysts, noted for their robust performance in hydrogen evolution and carbon dioxide reduction. A notable gap was identified in the application of COF-based materials for the photocatalytic degradation of NO. Thus, our second experiment was dedicated to the synthesis and optimization of pyrene-based COF materials, with a specific focus on their application in NO degradation. Our investigation aimed to evaluate the effectiveness of pyrene based COFs as photocatalysts and to improve their performance through in-situ integration with g-C3N4. By comparing the photocatalytic capabilities of TAPPy DMTP-COF, TAPPy-TPA-COF, and TAPPy-BPDA-COF, we identified TAPPy DMTP-COF as the optimal configuration for further study. This led to the development of a covalently linked Type-II heterostructure combining TAPPy-DMTP-COF with g C3N4. The creation of this composite was enabled by the Schiff-base reaction, which was enhanced by the additional amine groups present on g-C3N4, facilitating the direct formation of TAPPy-DMTP-COF on its surface. This methodological approach resulted in the 40TAPPy-DMTP-COF composite, which showcased an outstanding 45.8% conversion efficiency for NO at low concentrations. The significant enhancement in photocatalytic activity is attributed to the synergistic effects of accelerated electron transfer through chemical bonding and the increased photocatalytic oxidation driven by photogenerated holes within TAPPy-DMTP-COF. This research not only underscores the potential of pyrene-based COFs in photocatalytic applications but also pioneers a novel method for boosting NO degradation through the construction of a covalent linked Type-II heterostructure, providing valuable insights for the advancement of sophisticated photocatalytic systems.
Upon comparing this study with our initial research, it becomes evident that TAPPy-DMTP-COF exhibits lower efficiency than the Co(OH)2/CeO2-g-C3N4 photocatalyst. It appears that, although COF materials can achieve commendable efficiency in the photocatalytic degradation of NO, surpassing certain efficiency thresholds may necessitate the inclusion of metal elements. These elements not only exhibit excellent semiconductor properties, enhancing photogenerated electron and hole availability, but also act as active sites that boost the adsorption of NO and O2, thereby accelerating the photocatalytic reaction. Motivated by this insight, our third experiment introduce a pioneering chemically bonded Pt/TP-BPY-CN/g-C3N4 composite photocatalyst, synthesized through an in-situ growth approach employing a Schiff base reaction between the amino-rich g-C3N4 and the aldehyde-functionalized TP-BPY-COF. The formation of -H-C-N-H- bonds at the interface of g-C3N4 and TP BPY-COF plays a pivotal role in promoting electron communication, thereby substantially enhancing electron transfer efficiency between the two constituents. Moreover, the incorporation of single atom platinum via coordination establishes a Pt2+/Pt4+ redox cycle, facilitating both oxidation and reduction of NO during the photocatalytic process. As a result, the Pt/40TPBPY-CN composite achieves a 65.3% conversion efficiency in photocatalytic NO degradation through both oxidation and reduction, with almost 100% oxidation selectivity towards NO3 - . In-situ X-ray photoelectron spectroscopy (XPS) analysis provides evidence of robust electron communication between TP-BPY-COF and g-C3N4 facilitated by -H-C-N-H- bonds, as well as the redox transformation between Pt2+ and Pt4+ . In addition, DFT calculation provides a deeper insight into photocatalytic oxidation and reduction pathway, confirming an inhibition of N2 desorption. This research highlights the significance of a chemically bonded binary heterostructure, which demonstrates enhanced electronic interaction capabilities compared to directly loaded heterostructures. Additionally, the effect of bifunctional single-atom platinum applied in photocatalysis is also defined.
Throughout this thesis, we present a comprehensive analysis that not only showcases the advancements in photocatalytic materials and mechanisms but also highlights the pivotal role of integrating COF materials with metal elements to achieve superior photocatalytic performance. Our findings contribute significantly to the field, offering novel insights into the design and application of advanced photocatalyst for photocatalytic degradation of NO.

Date of Award	Oct 2024
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Jun He (Supervisor), Yong Sun (Supervisor) & George Zheng Chen (Supervisor)