Jiao, Haimiao; (2023) Continuous production of green H₂ by photocatalytic flow membrane reactors. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

H2 production from the low carbon medium of methanol water mixture has emerged as a promising strategy to realise sustainable H2 release on demand, in particular driven by renewable and abundant solar energy. Although much progress has been made in photocatalytic H2 production from methanol aqueous solution using conventional batch reactors, the efficiency is still quite limited. Both efficient photocatalysts with long-term stability and an optimised photocatalytic reactor are indispensable to achieve a green and sustainable H2 production under mild conditions. This thesis aims to develop efficient photocatalysts together with a novel photocatalytic flow membrane reactor to overcome these barriers for photocatalytic H2 production from methanol water mixture. To address the issues of the limited efficiency in photocatalytic H2 production from methanol water mixture, highly dispersed CuOx nanoparticles on TiO2 (PC50) were synthesised and evaluated for continuous H2 production from aqueous methanol solution by photocatalysis in a flow reactor at a low temperature and under atmospheric pressure. The H2 yield rate is improved by 1.63 times in the flow membrane reactor as compared to that achieved in a widely used batch reactor due to the enhanced mass transfer. Besides, the optimised 1% Cu/PC50 shows a superior H2 production rate of 33, 702 μmol g-1 h-1, which is 17 times higher than that on pristine PC50. It is also found that the apparent activation energy on 1% Cu/PC50 is greatly reduced to be as low as 4.0 kJ mol 1, which is far less than those in the other methanol reforming processes. The CuOx nanoparticles on TiO2 were found to be as electron acceptors to effectively promote the charge separation and work as active sites for protons reduction, together with the boosted mass transfer in the flow membrane reactor, resulting in improved photocatalytic performance. Although the above studies have shown that CuOx loaded TiO2 integrated with the flow membrane reactor is beneficial for photocatalytic methanol dehydrogenation and reforming due to their favourable properties, e.g., the improved charge separation and enhanced mass transfer, the photocatalytic efficiency is still not high enough. Moreover, it is still challenging to utilise the solar light for TiO2 based photocatalysts because they are only responsive to the UV light due to their large bandgap (3.0-3.2 eV). To address this concern, the bimetallic PdOx-CuOx loaded oxygen-vacancy-rich TiO2 was synthesised for efficient H2 production by photocatalytic methanol decomposition and reforming in the pressurised flow membrane reactor under visible light irradiation. The optimal 0.5Cu2Pd/Ov-P25 realises high H2 yield rates of 450.3 mmol g-1 h-1 and 1507.7 mmol g-1 h-1 under visible light (λ> 420 nm) and full arc irradiation, respectively, nearly 45 times higher than previous Cu/PC50 photocatalyst under full arc illumination, together with the AQE of 29.6% at 365 nm and 0.47% at 450 nm. The apparent activation energy on 0.5Cu2Pd/Ov-P25 is reduced by around 40% to 8.5 kJ mol-1 under visible light irradiation, which is much smaller than those in other methanol reforming processes. It is proposed that PdOx-CuOx nanoparticles could serve as electron acceptors to promote the charge separation, together with the enhanced mass transfer in the pressurised flow membrane reactor, resulting in improved photocatalytic activity. Although the above two catalysts have showed excellent activity in photocatalytic H2 production from methanol water mixture, the relatively poor stability of 1% Cu/PC50 and 0.5Cu2Pd/Ov-P25 limits their long time usage. Thus, it is imperative to develop a photocatalyst with high performance, excellent stability and low cost for photocatalytic H2 production from methanol water solution. To meet this objective, a noble-metal-free CuNi-MOF/TiO2 composite was prepared by the electrostatic self-assembly method. A superior and stable photocatalytic H2 generation rate of 92.4 mmol g-1 h-1 up to 48 h from methanol water solution is achieved on the optimal 8% CuNi-MOF/P25 in the pressurised flow membrane reactor, together with the AQE of 26.5% at 365 nm, thus achieving a highly stable photocatalyst with a similar AQE to the best example in the literature. The stability on 8% CuNi-MOF/P25 has been improved by 6 times as compared to that on 1% Cu/PC50 and 0.5Cu2Pd/Ov-P25. Besides, the H2 yield can be further increased to 135.8 mmol g 1 h-1 when temperature rises to 70 ℃. The apparent activation energy on 8% CuNi-MOF/P25 can be decreased to 3.7 kJ mol-1, much lower than those on 1% Cu/PC50 and 0.5Cu2Pd/Ov-P25. It has been found that the bimetallic CuNi-MOF over TiO2 could effectively promote the charge transfer and function as active sites, thus leading to enhanced photocatalytic activity and stability.

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