N₂O, a greenhouse gas like CO₂ or CH₄, has 268 times greater global warming potential (GWP) than CO₂. Even though the volumetric portion of N₂O among the greenhouse gases is merely 6%, the potential of N₂O to exert a greenhouse effect is equivalent to 3 billion ton of CO₂, which is similar to 50% of total car emissions worldwide. In addition, N₂O plays a significant role in depleting ozone layer. The concentration of such N₂O has been increasing, due to manufacturing, transportation, and agricultural activities [1].

Various chemical methods such as a thermal decomposition, a high/medium temperature catalytic decomposition, and a selective catalytic reduction have been adopted in order to reduce N₂O [2]. They have been applied to industrial fields: massive production of nitric acid, adipic acid, and caprolactam because wasted heat could be used to elevate temperature up to a point, whereupon thermal decomposition of N₂O takes place. However, such high-temperature methods are inadequate for reduction of N₂O, which is typically emitted from room-temperature process, such as biological or medical process. Furthermore, given that the high-temperature methods require an enormous amount of energy, the successful electrochemical reduction of N₂O at room temperature with a simple apparatus could be an effective alternative.

Although N₂O is very stable in the atmosphere, it could be electrochemically reduced into N₂ in an aqueous electrolyte solution [3]. The rate of electrochemical N₂O reduction could be enhanced by adopting adequate catalysts and gas-dissolution system. In this study, Couette-Taylor flow (CTF) mixer with electrochemical system was developed for effective N₂O reduction. It consists of a stationary outer cylinder and a rotating inner cylinder, which are working and counter electrodes used for an electrochemical reaction. The gas injected into the solution present between two cylinders could form micro bubbles with great gas pressure and gas-liquid interfacial area than bulk gas due to the Taylor vortices in the CTF mixer [4]. The application of the CTF mixer resulted in the enhanced N₂O solubility and rapid dissolution. Based on the improved N₂O dissolution, high N₂O conversion over 90% was obtained.

In this presentation, we will focus on the optimization of operating condition of CTF mixer such as catalyst, rotational speed of inner cylinder, gas flow rate, and liquid circulation rate for efficient N₂O reduction. The optimized system was also applicable to electrochemical reduction of CO₂ not only N₂O.

References

[1] Hayhurst, A. N. et al. Emissions of nitrous oxide from combustion sources. Prog. Energ. Combust. 18, 529-552 (1992).

[2] Pophal, C. et al. Selective catalytic reduction of nitrous oxide over Fe-MFI in the presence of propene as reductant. Appl. Catal. B-Environ. 16, 177-186 (1998).

[3] Aziznia, A. et al. Electroreduction of nitrous oxide on platinum and palladium: Toward selective catalysts for methanol–nitrous oxide mixed-reactant fuel cells. Electrochim. Acta 56, 5238-5244 (2011).

[4] Mura, Y. et al. Frictional drag reduction by bubble injection. Exp. Fluids 1773, 1-28 (2014).