A Supportive Support: The Important Role of Porous Supports in Plasma-Catalytic Ammonia Synthesis

Plasma catalysis is a promising method for synthesizing ammonia, utilizing dielectric barrier discharge (DBD) to supply the energy required for NN bond cleavage. Although efficient, the process is hindered by ammonia dissociation in the plasma zone, which must be prevented. Micro- and mesoporous frameworks can mitigate this issue by protecting the formed ammonia from immediate decomposition. However, their effects extend beyond porosity, as the presence of acidic sites significantly improves ammonia yields, refuting the idea of a simple spectator or inert support. Here, we systematically evaluate five different MCM materials—MWW zeolites MCM-22, MCM-36, and MCM-56, as well as mesoporous silicas MCM-41 and MCM-48—both as pristine materials and as supported NiO catalysts for plasma-catalytic ammonia synthesis. To quantify the importance of the acidic properties of the catalyst, experiments were conducted at constant discharge power and under a power-swing regime. In power-swing experiments, NiO/MCM-56, which has the highest amount of moderate acidic sites, achieved 1.5 times higher ammonia concentration than the best-performing catalysts at constant discharge power. We show that tailored acidic properties of the support further enhance the shielding effect, enabling efficient sorption-enhanced plasma-catalytic ammonia synthesis by using plasma power modulation rather than external heating for ammonia desorption. Three distinct properties required of a “good catalyst” are identified: porous structure, active phase location and, most importantly, surface acidity.