Role of nitrogen metastable states in the removal of formaldehyde by non-thermal high pressure N$_{2}$ and N$_{2}$/O$_{2}$ mixture plasmas

Formaldehyde is one of the most toxic VOC coming from human activities. Few works have been performed in order to understand mechanisms involved in the removal of this molecule in air using non-thermal plasmas generated by pulsed discharges, most often dielectric barrier (DBD) or corona ones. Even in the N$_{2}$/CH$_{2}$O mixture, a comprehensive kinetic interpretation of measurements remains not so easy because it requires a self-consistent modelling of both the streamer physics and the strongly reactive plasma chemistry. This difficulty can be overcome in case of a spatially homogeneous plasma. So our study deals with the removal of CH$_{2}$O (max. 900 ppm) in N$_{2}$/CH$_{2}$O mixtures using : i/ a DBD energised by a pulsed high voltage power supply, at 1 bar, and ii/ a photo-triggered discharge at 400 mbars. The removal efficiency is the highest for the homogeneous plasma. Effect of the addition of a small amount of O$_{2}$ is examined in case of the photo-triggered reactor, in order to get more consistent information on the role of the nitrogen metastable states in the decomposition of the pollutant molecule. It is shown that quenchings of N$_{2}$ states by CH$_{2}$O play an important role, which is progressively counterbalanced by the oxidation kinetic when O$_{2}$ is added to the mixture.