Plasma-catalytic Ammonia Synthesis via Eley-Rideal Reactions: A Kinetic Analysis

The reaction mechanism of plasma-catalytic ammonia synthesis is not fully understood. MgO supported Ru, Co, Pt, Pd, Cu and Ag catalysts are tested in a DBD plasma at temperature between room temperature and 500 °C and plasma power between 3.8 W and 6.4 W. The resulting ammonia production in the presence of a plasma and a catalyst can be distinguished into (1) temperature-independent plasma-based ammonia synthesis, and (2) temperature-dependent plasma-catalytic ammonia synthesis. Turn-over-frequencies (TOF) are calculated based on the rate of the second pathway and chemisorption data, measuring the number of active sites. Underestimation of TOFs caused by ammonia decomposition was minimized by using exclusively observations at low ammonia concentration. The kinetic results suggest that the Eley-Rideal reaction between N radicals from the plasma with chemisorbed H atoms is the rate-determining step for plasma-catalytic ammonia synthesis on Ru/MgO, Co/MgO, Pt/MgO, Pd/MgO, and Cu/MgO, with apparent activation barriers in the range 18–24 kJ mol⁻¹. In contrast, the apparent activation barrier on Ag/MgO is significant higher at 30 kJ mol⁻¹, suggesting a shift in rate determining step. The low H coverage on Ag may induce a shift to a Langmuir-Hinshelwood pathway, via adsorption of N radicals on the metal surface.