Distinguishing Plasma Contributions to Catalyst Performance in Plasma-Assisted Ammonia Synthesis

Plasma-assisted catalysis is the process of electrically activating gases in the plasma-phase at low temperatures and ambient pressure to drive reactions on catalyst surfaces. Plasma-assisted catalytic processes combine conventional heterogeneous surface reactions, homogeneous plasma phase reactions, and coupling between plasma-generated species and the catalyst surface. Herein, we perform kinetically controlled ammonia synthesis measurements in a dielectric barrier discharge (DBD) plasma-assisted catalytic reactor. We decouple contributions due to plasma phase reactions from the overall plasma-assisted catalytic kinetics by performing plasma-only experiments. By varying the gas composition, temper- ature, and discharge power, we probe how macroscopic reaction conditions affect plasma-assisted ammonia synthesis on three different γ-alumina-supported transition metal catalysts (Ru, Co, and Ni). Our experiments indicate that the overall reaction and plasma-phase reaction are first-order in both N2 and H2. In contrast, the rate contributions due to plasma-catalyst interactions are first-order in N2 but zeroth order in H2. Furthermore, we find that the tuning of the plasma discharge power is more effective in controlling catalytic performance than the increasing of bulk gas temperature in plasma-assisted ammonia synthesis. Finally, we show that adding a catalyst to the DBD reaction alters the way that productivity scales with the specific energy input (SEI). KEYWORDS: