Catalytic bromate reduction in water by Ru/C via borohydride-based hydrogenation: A comparative study

Abstract

While H2 gas is conventionally purged during hydrogenation of bromate in the presence of catalysts, the low solubility of H2 gas leads to wasteful consumption of H2 gas. Reductants in solid forms, which act as storage media of H2, would be preferable, and borohydrides, including NaBH4, and NH3BH3 are promising candidates. Although ruthenium (Ru) is an efficient catalyst for releasing H2 from these borohydrides, almost no studies have looked into usage of Ru with borohydrides for reducing bromate, and differences between NaBH4, and NH3BH3 for reducing bromate in the presence of Ru. Therefore, the goal of this study aims to compare hydrogenation of bromate using different borohydrides as well as H2 gas purge under different conditions in the presence of Ru/C as the most representative Ru catalyst. While Ru/C could slowly reduce bromate to bromide, the addition of NaBH4 or NH3BH3 would tremendously accelerate bromate removal and bromide production. Nevertheless, the H2 purge might cause interferences with the entire reaction, thereby leading to ineffective bromate reduction. Moreover, Ru/C+NaBH4 would lead to an even faster bromate reduction because adsorption of BH4– to Ru/C was much more favorable according to the DFT calculation. The Ea of bromate reduction by Ru/C+NaBH4 is also lower than that by Ru/C+NH3BH3, validating the promising advantage of NaBH4 over NH3BH3. Ru/C+NaBH4 could be also reusable for reducing bromate to bromide, demonstrating that borohydrides might be promising alternatives to H2 purge for reducing bromate to bromide in order to achieve even faster and more efficient bromate reduction.