Electrochemical Reduction of Greenhouse Gas with Couette-Taylor Flow (CTF) Mixer

Abstract

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.