A primary goal of the proposed work is to determine the potential of non-thermal reactant activation to modify the behavior of a catalytic system, through application to a well-defined and well-characterized thermal catalytic system (ammonia oxidation), chosen for reaction features that are amenable to careful experimental and computational interrogation. We will demonstrate the ability to connect incisive observation and high fidelity models to understand and predict reactivity, through validated microkinetic modeling approaches that capture in physically appropriate ways the influence of reaction environment on observed reactivity. We will exercise those models to predict catalyst/reaction condition combinations that optimize performance at conditions of interest. The program will be organized around four primary research pillars:
Coverage-aware microkinetics We will develop microkinetic mean-field and lattice-based Monte Carlo models, based on density functional theory (DFT) calculations and innovative approaches to incorporate coverage dependence into those models, to predict NH3 oxidation rates, rate orders, and selectivities as a function of reaction conditions, catalytic material and morphology. We will develop well-validated and efficient approaches for incorporating cover dependence in microkinetic models that are transferable to other problems. These models will inform experimental evaluations of thermal NH3 oxidation.
NH3 oxidation kinetics We will prepare well-defined, supported metal catalysts, will collect full kinetic data (rates, selectivities, reaction orders) on these materials, and develop a complete baseline NH3 oxidation reference that relates materials and conditions to reaction selectivity. These results will provide a well-defined baseline against which to evaluate the modifying influences of plasmas. We will exercise validated models to predict materials (composition, structure, reaction conditions) that have superior NH3 oxidation rates and avoid undesirable N2O production.
Characterizations We will use incisive characterization techniques to interrogate catalyst state and observe critical intermediates during NH3 oxidation reactions, in the absence and with the presence of plasma stimulation. We will design and develop an in-house in situ/operando transmission infrared spectroscopy cell capable of operating in the presence of a non-thermal plasma (NTP). These and complementary inelastic neutron scattering (INS) experiments at the Spallation Neutron Source (SNS) will allow us to observe adsorbate species at conditions representative of catalytic reactivity. We will use the DFT models to assign and interpret these spectra, and use the results as a further test of the reliability of the microkinetic model predictions of surface species.
NTP-modified NH3 oxidation We will elaborate the coverage-dependent microkinetic models to incorporate the potential influence of plasma stimulation on NH3 oxidation rates and selectivities, incorporating potential influences of the NTP on reactants and consequent influence on catalytic reactivity. We will observe the catalytic consequences of plasma stimulation over a range of conditions, to develop a rich set of high quality reaction data to quantify and confirm any influence of NTP on observed changes in reactivity, to evaluate models, and to provide a basis for identifying the primary mechanisms responsible for observed NTP-promoted NH3 oxidation.
Completion of these tasks will simultaneously enhance DFT models for catalytic activity at metal surfaces, advance the state of knowledge of environmentally significant oxidative nitrogen chemistry, and place plasma-stimulated catalysis on a firmer scientific footing.
