N2 solar activation: ammonia as a hydrogen vector for energy storage

Abstract

From the plethora of energy-intensive synthetic processes, ammonia production has a particularly negative impact due to the high-energy consumption caused by the Haber–Bosch process and the high greenhouse gas (GHG) emission rate. Thus, new and effective ways to activate N₂ and synthesise NH₃ are crucial to reduce production costs and the anthropogenic footprint derived from the current harsh reaction conditions. In this study, two-dimensional materials have been employed in the photoactivation of nitrogen in an aqueous medium; M^I(II)M^II(III) (with M^I = Cu or CuNi, and M^II = Cr or Al) layered double hydroxides have been synthesised using a simple, economical and scalable co-precipitation/filtration method. The structural and functional properties were systematically investigated by XRD, SEM, TPR and BET; the results indicate that the prepared LDHs were successfully synthesised, possess high surface areas and, in the case of CuAl LDH, the material showed a nanoplate-like structure, thus confirming the two-dimensional nature of this class of catalyst. The N₂ fixation performances were evaluated using a scalable, cost-effective and low-energy-consuming setup; from the catalytic tests, a NH₃ production rate of 99 μmol g⁻¹ h⁻¹ was observed, demonstrating LDHs’ high potential and the scalability of the overall process.