Effect of Porous Catalyst Support on Plasma-Assisted Catalysis for Ammonia Synthesis

We report on the effect of catalyst support particle porosity on the conversion of NH₃ synthesis from N₂ and H₂ in a coaxial dielectric barrier discharge (DBD) plasma reactor. The discharge was created using an AC applied voltage with the reactor at room temperature and near atmospheric pressure (550 Torr). Two different particles of almost equal diameter (∼1.5 mm)─porous silica (SiO₂) ceramic beads (average pore size: 8 nm) and smooth, nonporous soda lime glass beads─were compared in the DBD reactor. As the pore size in the SiO₂ particles was smaller than the Debye length, penetration of the plasma into the pores of the particles was unlikely; however, reactive species generated in the plasma outside the particles could diffuse into the pores. The N₂ conversion and energy yield of NH₃ increased with applied voltage for both particle types, and these values were consistently higher when using the SiO₂ beads. Discharge and plasma properties were estimated from Lissajous plots and using calculations with the BOLSIG+ software. The effect of these two different catalyst supports on the physical properties of the discharge was negligible. High resolution optical emission spectra revealed that the concentrations of N₂⁺, atomic N, and atomic H (H_α, H_β) in the plasma discharge were lower with the porous SiO₂ beads than with the glass beads at every applied voltage tested. This indicates that these active species participate in heterogeneous reactions at support particle surfaces and that the larger surface area presented by the porous particles led to higher rates of depletion of these intermediates and a higher rate of ammonia synthesis.