Effect of surface porosity of catalytic supports on plasma-assisted catalysis for ammonia synthesis

Abstract

A fundamental understanding of plasma-catalyst interactions is important for understanding reaction mechanisms, optimizing the catalyst, and increasing the efficiency of plasma-assisted catalytic process for ammonia (\ce{NH3}) synthesis. We report on the effect of the surface porosity of the catalyst support on this reaction carried out in a coaxial dielectric barrier discharge (DBD) plasma reactor. The discharge was created using a variable AC applied voltage at room temperature and near atmospheric pressure (550 Torr). Two catalyst supports were compared: porous silica (\ce{SiO2}) ceramic beads and smooth, non-porous soda lime glass beads of almost equal diameter ($\sim$1.5 mm) were used. \ce{N2} conversion and the \ce{NH3} synthesis rate was increased with increasing voltage for both supports, but the energy yield for \ce{NH3} production increased for the \ce{SiO2} beads and decreased for the glass beads. All three of these parameters were always higher when using the \ce{SiO2} beads, which suggests that porosity can be a small advantage for plasma assisted \ce{NH3} synthesis. Discharge and plasma properties were estimated from Lissajous plots and using calculations with the BOLSIG+ software. The effect of different catalyst supports on the physical properties of the discharge was negligible. High resolution optical emission spectra (OES) were used to explore the evolution of gas phase active species, \ce{N2+}, atomic N, electronically excited \ce{N2}, and atomic H (H$_\alpha$, H$_\beta$), in the plasma in the presence of both supports. The relative concentration of these species was lower in the case of the porous \ce{SiO2} beads for all applied voltages, which suggests that surface reactions are more significant than gas phase reactions for the formation of \ce{NH3} in plasma assisted \ce{NH3} synthesis.