Plasma-catalytic NOx production in a three-level coupled rotating electrodes air plasma combined with nano-sized TiO2

A novel three-level coupled rotating electrodes air plasma with nano-sized TiO2 photocatalysts is developed for plasma-catalytic NOx production. The effects of plasma catalysis on NOx production with different air flow rates, different N2 fractions and different humidity levels are evaluated. Final results show that the exceptionally synergistic effect between TiO2 and three-level coupled rotating electrodes air plasma significantly increases the NOx concentration by 68.32% (from 4952 to 8335 ppm) and reduces the energy cost by 40.55% (from 2.91 to 1.73 MJ mol-1) at an air flow rate of 12 l min-1 and relative humidity level of 12%, which beats the ideal thermodynamic energy limit ~2.5MJ/mol for the thermal gas-phase process. A possible mechanism for enhanced NOx production with TiO2 is discussed: Highly energetic electrons in plasma contribute to the formations of the electron–hole pairs and oxygen vacancy (Vo) on the TiO2 catalyst surface, it may facilitate the dissociative adsorption of O2 molecules to form superoxide radical groups (like O2.-), and H2O molecules to form surface hydroxyl groups (like OH.), and thus, improving energy efficiency.