(Invited) Study of the Impact of Vibrational Excitations and Plasma-Generated Radicals on Catalytic Conversion Processes

Abstract

We developed microkinetic models to investigate the plasma-assisted ammonia synthesis and non-oxidative coupling of methane on transition metal catalysts. These models aim at gaining insight in the reaction mechanisms of plasma-activated species at the catalytic surface. We focus on the effects of vibrational excitations as well as plasma-generated radicals, on the total rate of conversion, the product selectivity and the catalyst coverage. Based on the results of DFT calculations, we built scaling relations for the catalytic conversion of N2 and H2 towards NH3 and CH4 coupling towards C2-hydrocarbons on 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 for ammonia and CH3 for methane). 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.Ultimately, we make suggestions for the optimal catalysts and reaction conditions for the ammonia synthesis and the conversion of methane to ethane, ethylene and acetylene.