Application of hollow fiber electrodes for green ammonia formation

Abstract

The nitrogen cycle is within the most important biogeochemical cycles on Earth due to nitrogen being the key element for all forms of life in the form of nucleic acids, amino acids, vitamins, and hormones. Although N2 is widely available in the atmosphere, it cannot be easily used due to its high stability. Therefore nitrogen needs to be activated to become more accessible to consecutive chemical processes, and usually, nitrogen is activated to form ammonia. In nature, microorganisms containing enzyme nitrogenase can do that, which results in 120 million tons of fixed nitrogen becoming available to the biosphere. [4] However, natural fixation does not meet the demand for nitrogen in our developing world.Recently, ammonia is also considered as a possible energy carrier. [12] It contains 17.6 wt.% hydrogen which in combination with easier storage compared to H2, can work as carbonfree hydrogen storage. [13] Additionally, it was proposed to use NH3 as fuel in combustion engines where only water and nitrogen can be formed as end products. [14] Also, the use of ammonia in fuel cells for electricity generation was proposed. [15] Ammonia production on an industrial scale Currently, industrial ammonia synthesis is based on the Haber-Bosch process. The possibility of ammonia formation from N2 and H2 was demonstrated by Fritsch Haber in 1908 and the idea was transformed into an industrial process by Carl Bosch. In 1913, the first plant was launched by BASF in Germany with a capacity of 30 ton/day of NH3. [16] 3𝐻 2 + 𝑁 2 ⇋ 2𝑁𝐻 3 Δ𝐻 0 = −92.4 𝑘𝐽 𝑚𝑜𝑙 −1(1.1)The reaction (1.1) requires relatively harsh conditions to achieve sufficient performance. A high temperature is required to break the very stable N≡N bond which allows for nitrogen dissociation. This step is considered the rate-determining step in ammonia synthesis. However, since the reaction is exothermic, a high temperature favors ammonia decomposition (based on Le Chatelier's principle). For this reason, high pressure is applied which allows to shift the reaction equilibrium towards ammonia. The operational temperature and pressure depend on the catalyst used but typically are in the range of 350 to 550 °C and 100 to 300 bar. [17] The energy of binding nitrogen to the catalyst surface determines its activity for ammonia synthesis. A high binding strength/energy will promote nitrogen dissociation but will limit ammonia desorption after the reaction. On the other hand, when the metal binds nitrogen too weakly, limitations in N2 dissociation can occur which will result in low activity. Based on the volcano plot, metals with intermediate binding strength can be found. [18] The most common catalyst commercially used in the Haber-Bosch process is an iron-based catalyst with the addition of multiple promoters in the form of Al2O3, MgO, and SiO2, which act as structural promoters as well as CaO and K2O as electronic promoters. [19] Other, less common but still used on the industrial scale, is the ruthenium-based catalyst on a carbon support. Although Ru shows ...