Simultaneous Production of Hydrogen and Chlorine from Brine with Pt-Loaded TiO2 Photocatalyst

Global warming caused by greenhouse gas emissions must be mitigated by switching from fossil fuels to renewable energy sources, which is a critical issue for modern society. Against this background, the use of particulate photocatalysts to convert solar energy from water to green and inexpensive H2 has attracted much attention.1 Freshwater is essential for ingestion and agriculture and is sometimes considered a valuable resource in developing countries and remote islands.2 Therefore, seawater and brine, which constitutes more than 97% of the water on Earth, are worth studying from this perspective. Seawater contains about 500 mM Cl– ion, which is involved in oxidation reactions. Cl– ion is oxidized to chlorine (Cl2), hypochlorous acid (HClO), and hypochlorite ion (ClO–) under acidic, neutral, and basic conditions, respectively. Therefore, the simultaneous production of H2 and Cl2 through photocatalytic seawater splitting is worth investigating as an economically and practically feasible system. In this study, we devised an overall brine splitting process for concurrently producing H2 and Cl2 with a particulate photocatalyst (Fig. 1(a)). Gas evolution reactions from brine at various pH under 365 nm light irradiation were monitored over time (Fig. 1(b))4. Under acidic condition (pH 1), H2 and Cl2 were preferentially produced, although a small amount of O2 was also produced. The e–/h+ ratio of the product, which was initially 3.4, decreased with time and eventually reached almost unity, indicating that the reaction proceeded stoichiometrically. Under neutral and basic conditions, H2 and a small amount of O2 occurred stably, while Cl2 and HClO were not detected by colorimetric measurements. Furthermore, the e–/h+ ratio of the final product in those scenarios was clearly not unity. This is likely due to photolysis of HClO or ClO–. The Pt-loaded TiO2 provided stable linear production of H2 and Cl2 for more than 10 hours under acidic conditions (Fig. 1(c)). After 14 h, the ClO– concentration exceeded 4 mM, a concentration sufficient for disinfection-related applications5. This result indicates that oxidation of Cl– ion occurred preferentially, even though O2 is thermodynamically easier to generate than Cl2. The apparent quantum yield of the overall brine splitting reaction in acidic media (about 0.6% at 365 ± 20 nm) is comparable to that previously reported for TiO2 photocatalytic water splitting reactions6. Furthermore, the calculated turnover number of H2PtCl6 (about 103) indicates that Cl2 was generated from the Cl– ion in solution. Thus, this result establishes the reliability of a new artificial photosynthesis system that can simultaneously produce H2 and Cl2 from brine. [1] J. Whitehead, P. Newman, J. Whitehead and K. L. Lim, J. Sustainable E arth Rev., 6, 1 (2023). [2] C. Klassert, J. Yoon, K. Sigel, B. Klauer, S. Talozi, T. Lachaut, P. Selby, S. Knox, N. Avisse, A. Tilmant, J. J. Harou, D. Mustafa, J. Medellı ́nAzuara, B. Bataineh, H. Zhang, E. Gawel and S. M. Gorelick, Nat Sustainability, 6, 1406-1417 (2023). [3] R. K. B. Karlsson and A. Cornell, Chem. Rev., 116, 2982-3028 (2016). [4] T. Okada, M. Korera, Y. Miseki, H. Kusama, T. Gunji, K. Sayama, Chem. Commun ., 60, 3299-3302 (2024). [5] E. W. Rice, R. M. Clark, C. H. Johnson, Emerging Infect. Dis., 5, 461–463 (1999). [6] K. Maeda, Chem. Commun., 49, 8404-8406 (2013). Figure 1