(Invited) Microkinetic Models for Plasma-Catalytic NH3 Synthesis: Effects of Catalyst Material, Gas Temperature and Power Deposition on Rates and Yields

Abstract

We developed microkinetic plasma catalysis models to investigate the plasma-assisted ammonia synthesis on transition metal catalysts in dielectric barrier discharge plasmas. These models aim at gaining insight in the reaction mechanisms and kinetics of plasma-activated species at the catalytic surface. We focus on the effects of vibrational excitations as well as plasma-generated radicals, on the turnover frequencies and equilibrium conversion.Based on the results of DFT calculations, we built scaling relations for the catalytic conversion of N2 and H2 towards NH3on transition metal catalysts. These scaling relations allow the generation of ‘volcano’ plots that show the logarithm of the turnover frequencies as function of a catalyst descriptor (in our case the binding energy of N). We compare the volcano plots for thermal catalysis with vibrationally-enhanced catalysis and radical-enhanced catalysis in order to investigate how the plasma can most efficiently aid in the catalytic conversion. Careful analysis of reaction flow diagrams shows which pathways are dominant in each of the regimes. We show that Eley-Rideal (ER) mechanisms can become very important in plasmas with high densities of radical species as opposed to thermal conditions where ER is usually considered negligible compared to Langmuir-Hinschelwood. We determined on which catalysts ER mechanisms are most important and how they impact the turnover frequency.This framework was introduced in a plasma chemistry model in ZDPlasKin to study conversions as function of catalyst material, gas temperature and specific energy input. Model results show the temporal behavior, with the specific role of microdischarge filaments and afterglows, of filamentary plasmas and how catalyst material, gas temperature and specific energy input can induce conversions beyond thermal equilibrium.