Effect of surface porosity on plasma assisted ammonia synthesis*

We have investigated the effect of surface porosity of catalytic support particles on plasma assisted ammonia synthesis from nitrogen and hydrogen.  The experiments were performed using an AC coaxial dielectric barrier discharge (DBD) plasma reactor at room temperature and near atmospheric pressure (550 torr). Reactor performance was evaluated using support particles of porous silica (SiO₂) and non-porous soda lime glass beads of equal diameter. N₂ conversion, ammonia synthesis rate, and energy yield measured at several applied voltages were found to be higher in the case of porous silica compared to non-porous glass. The effect of these different catalytic supports on the physical properties of the discharge was negligible. High resolution optical emission spectra (OES) were used to explore the evolution of reactive gas phase species N₂⁺, N, H_α, H_β, and N₂ in the presence of both support particles. The relative concentration of these reactive gas phase species was higher in the case of the non-porous glass supports regardless of applied voltage, which suggests an increased role for these species in gas phase reactions for this support.  However, the higher formation rate of ammonia for porous silica supports indicate the importance of these support surfaces in the plasma-catalytic synthesis of ammonia.