NSF-DFG Confine: Plasma-Catalysis in Confined Spaces for Cold Start NOx Abatement in Automotive Exhaust

该研究项目研究了非热等离子体内部物理和化学过程之间的相互作用，以及等离子体在固体催化剂表面产生的分子、自由基和原子的催化反应。以汽车尾气催化处理中的标准催化剂Pd/Al2O3/CeO2/ZrO2为催化剂，研究甲烷等离子体催化还原氮氧化物的实验反应。该测试平台系统具有技术和社会意义，因为汽车尾气排放的80%以上的氮氧化物排放是在冷启动期间释放的，这意味着当催化剂的温度低于200°C时，无法激活废气流中含有的惰性碳氢化合物作为氮氧化物的还原剂。通过非热等离子体（等离子体内部形成高活性的原子、分子和自由基）与DeNOx催化剂耦合，可以在室温下基本活化甲烷等不活泼的碳氢化合物，产生的自由基可以将吸附在催化剂表面的氮氧化物还原为N2、CO2和H2O。由于自由基具有高活性，其寿命在微到毫秒（如CH3和OH）或毫秒到秒（如ch300）之间，因此等离子体和催化剂之间的有效耦合需要空间约束和亚毫米范围内的传输距离。为了以明确的方式分离和匹配等离子体过程、传输和催化反应，等离子体和催化剂将以串联和并联的方式相互连接。通过将非热等离子体的流出物注入内径达1毫米的涂有催化剂的毛细管中，可以系统地探测反应，这些反应是由寿命相当长的自由基（例如ch300）介导的。通过点燃50-1000µm壁距狭窄通道内的非热等离子体，并在通道壁上涂覆薄层催化剂，可以系统地探测由CH3和OH等短寿命自由基介导的反应，这些自由基只能在很短的距离内扩散。通过将定义的催化实验与使用激光诱导荧光光谱或分子束质谱等方法的空间分辨诊断测量以及多维等离子体建模相结合，将回答等离子体催化中的以下关键问题：1)受限的反应器几何形状能否克服影响等离子体-催化化学耦合的输运限制？2)哪些是能使等离子体和催化过程有效耦合的关键等离子体物质？如何控制它们的产生？3)与热催化相比，等离子体催化过程中表面物质和反应的主要区别是什么？

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?