(Invited) Nitrogen Fixation Using Non Thermal Atmospheric Plasma (over water): Challenges and Perspectives on Nitrogen Fixation Using Non Thermal Atmospheric Plasma (over water): Challenges and Perspectives

Abstract

Nitrogen fixation, i.e. the transformation of molecular nitrogen into nitrates or ammonia is a major industrial process. The Haber-Bosch and the Ostwald processes are at the origin of the massive increase of yield of food production, contributing to the development of modern agriculture. However, despite the fact that these processes are exothermic, the need to operate at high temperature for kinetic reasons, the necessity to start (most often) from natural gas as a source of hydrogen, and the industrial plants located far away from the end user, make these industrial processes huge producers of CO2.Historically, the Birkeland-Eyde process, however, allow to fix nitrogen using arc plasma. The energy yield of this process being small, it was replaced by the Haber-Bosch process. With the development of low temperature, non thermal plasmas processes, there is nowadays a hope to develop local, low energy cost, nitrogen fixation reactors, with only air as reactant. For food and agriculture applications, especially for urban agriculture, a possible approach is to generate nitrates directly inside the water that feeds the plants. The talk will present a few non thermal atmospheric plasma setups than can be used for nitrogen fixation. In a first setup, nitrogen oxides are generated directly in a dielectric barrier discharge (DBD) filled with various proportions of nitrogen and oxygen. In a second setup, one of the dielectrics is replaced by water. In a third setup, a DC microplasma is lighted up between a metallic electrode and a water solution, containing an electrolyte. In this last setup, a counter electrode closes the electrical circuit, making it resembling a electrochemical cell. In this last setup, two configurations are tested, by changing the polarity of the two electrodes.For all the setups, the amount of various nitrogen oxides formed in the gas phase as a function of the experimental parameters is studied using optical emission spectrometry, infrared spectrometry and mass spectrometry. Similarly, the composition of the liquid phase is analyzed using ionic chromatography and specific reagents tests. Correlations between a) the gas phase composition and b) the liquid phase composition are established. The role of OH radicals (in the gas phase) and hydrogen peroxide (in the liquid phase) on the relative amounts of nitrites and nitrates is evidenced. For the microplasma experiments, it is shown that inverting the polarity of the electrodes lead to drastic changes in the yields of the species produced. From all these results, potential mechanisms will be presented, and future challenges will be open to discussion.