Hydrogen Production by Photoreforming of Organic Waste Using Cd-ZnS Solid Solution Photocatalyst via MOF Precursor
Authors: Kumai Yusei , Nagata Morio 
Published: December 2024

Currently, global energy consumption continues to increase, but the depletion of energy resources due to the decrease in the amount of fossil fuels that can be mined and carbon dioxide emissions due to the use of fossil fuels are becoming an issue. Under these circumstances, hydrogen has been attracted a great deal of attention as a next-generation energy source for moving away from fossil fuels. Hydrogen is eco-friendly energy that does not emit carbon dioxide during combustion. However, most of the current hydrogen production methods are obtained by reforming fossil fuels, which releases carbon dioxide. Therefore, it is desirable to establish eco-friendly hydrogen production technology that does not use fossil fuels and does not emit carbon dioxide.
Photocatalyst is an eco-friendly hydrogen production technology that does not emit carbon dioxide during hydrogen production. Since the Honda-Fujishima effect announced in 1972, there have been many studies on hydrogen generation by water splitting using photocatalysts. Among them, hydrogen production by photoreforming of organic waste [1] in artificial photosynthesis using photocatalysts allows the use of waste, which has been increasing in recent years, as a resource. In addition, the use of waste reduces the environmental burden caused by landfill or incineration. Furthermore, cost reduction can be achieved because sacrificial reagents used in general photocatalytic hydrogen production are not required.
There have been reports of the use of sulfide photocatalysts in the hydrogen production by photoreforming of organic wastes. For example, CdS quantum dots [1] and CdS/SiC composite photocatalysts [2]. We are also aiming to improve the hydrogen production activity using sulfide photocatalysts. Among them, Cu and In-doped zinc sulfide (ZnS) photocatalysts prepared using a metal organic framework (MOF) as a precursor have been reported to have higher hydrogen production activity and higher crystallinity than those of the conventional solvothermal synthesis [3]. However, ZnS is not visible-light responsive, and thus cannot utilize most of the solar energy. Therefore, we will design a photocatalyst that has higher hydrogen production activity by combining a photocatalyst that is responsive to visible light and can form MOFs.
In this study, the preparation of cadmium sulfide (CdS) and ZnS solid solution photocatalysts via cadmium (Cd) and zinc (Zn) MOF precursors is used to improve the hydrogen production activity. We designed a photocatalyst that can maintain visible light responsivity by having it create a solid solution with visible light responsive CdS as opposed to ZnS, which does not have visible light responsivity. In addition, MOF was introduced as a precursor to improve the hydrogen production activity by reducing the number of defects, which are the recombination centers of electrons and holes, by improving the crystallinity.
The MOF precursor was prepared by an existing method and the solid solution photocatalyst was obtained by solvothermal synthesis by mixing it with a sulfide source. The solid solution photocatalyst was then used to measure hydrogen production under artificial sunlight. The measurement was performed using gas chromatography to check the change in hydrogen production over time. The results showed that the amount of hydrogen production was significantly higher than that of the CdS photocatalyst. The XRD measurements of the fabricated solid solution photocatalyst also showed a shift in the peaks. Furthermore, differences in the hydrogen production activity and the shift ratio of the XRD peaks were confirmed depending on the type of MOF used as a precursor.
References：

[1] David W. Wakerley, Moritz F. Kuehnel, et al, NATURE ENERGY, 2017, 2, 17021.

[2] H. Nagakawa, M. Nagata, ACS Appl. Mater. Interfaces, 2021, 13, 47511–47519.

[3] H. Nagakawa, M. Nagata, Adv. Mater. Interface, 2022, 9, 2101581.

Figure 1