Plasma-Catalysis in Confined Spaces for Cold Start NOx Abatement in Automotive Exhaust
The research project investigates the interaction between physical and chemical processes inside a non-thermal plasma and the catalytic reactions of plasma generated molecules, radicals and atoms at the surface of a solid catalyst. The plasma-catalytic reduction of nitrogen oxides by methane on a Pd/Al2O3/CeO2/ZrO2 catalyst, which is a standard catalyst in automotive catalytic exhaust treatment, will be studied as test reaction. This testbed system is of technical and societal relevance because more than 80% of NOx emissions caused by automotive exhaust are released during cold start, that means when the catalyst has a temperature below 200°C and is unable to activate the rather inert hydrocarbons contained in the exhaust stream as reducing agent for nitrogen oxides. By coupling of a non-thermal plasma, inside which highly reactive atoms, molecules and radicals are formed, with a DeNOx catalyst, very unreactive hydrocarbons such methane can be activated basically at room temperature and the resulting radicals can reduce nitrogen oxide species adsorbed on the catalyst surface to N2, CO2 and H2O. Because radicals are highly reactive with lifetimes on the order of micro- to milliseconds (e.g. CH3 and OH) or milliseconds to seconds (e.g. CH3OO), efficient coupling between plasma and catalyst requires spatial confinement and transport distances in the submillimeter range. To separate and match plasma processes, transport and catalytic reactions in a well defined manner, plasma and catalyst will be connected both in series and in parallel to each other. By feeding the effluent of a non-thermal plasma into a catalyst coated capillary of up to one millimeter inner diameter, reactions can be probed in a systematic manner, which are mediated by comparably long-lived radicals such as CH3OO for example. Reactions mediated by short-lived radicals such as CH3 oder OH, which can only diffuse over very short distances, can be probed systematically by igniting the non-thermal plasma inside narrow channels of 50-1000µm wall distance with the catalyst coated as thin layer at the channel walls. By combining defined catalytic experiments with spatially resolved diagnostic measurements using methods like Laser Induced Fluorescence Spectroscopy or Molecular Beam Mass Spectrometry and with multidimensional plasma modeling, the following key questions in plasma catalysis will be answered: 1) Can confined reactor geometries overcome transport limitations impacting plasma-catalytic chemistry coupling? 2) Which are the key plasma species enabling effective coupling between plasma and catalytic processes and how can their production be controlled? 3) What are the key differences in surface species and reactions during plasma-catalysis compared to thermal catalysis?