NOx Reactivity Studies of Prototype Catalysts for a Plasma–Catalyst Aftertreatment System

The reactivity of NOx over two prototype catalysts has been measured in a new bench reactor for characterizing plasma–catalyst systems that allows for in–situ post–analysis of any species which may have adsorbed on the catalyst. In these initial studies without a plasma, NO₂ was used to mimic the NOx output of a plasma reactor in a blended feedstream that mimics diesel exhaust. The baseline performance of the catalysts was measured as a function of temperature, hydrocarbon concentration, hydrocarbon type, and water content, usually at a space velocity of 29,000 h^–1. Performance was assessed in terms of the percent conversion of the incoming NO₂ to desirable non–NOx N–containing species. For the better of the two catalysts the conversion without water present peaked in the 30–40% range between 125°C and 175°C using a propene/propane mixture of hydrocarbons in a 10:1 C₁:N ratio. Experiments with NO as the NOx component yielded very poor activities. In studies at 150°C without water using nine different hydocarbons that may be found in diesel exhaust, propene (C₃H₆) and normal butene (C₄H₈) gave the best NOx conversion activity. However, in the presence of water, this activity was largely suppressed. This work makes clear that the synergistic adsorption and reactivity of NO₂ with active hydrocarbons must compete effectively with water adsorption in any catalyst that will be successful in a plasma–catalyst system.