Spatially Resolving NH3-Selective Catalytic Reduction Reactions
William Epling, University of Waterloo
William Epling, Jin-Yong Luo, Peter Hou, Prasanna Wijayakoon, Wei Li and Steve Schmeig
University of Waterloo, General Motors
Integrated emission control systems, such as the combination of DOC+DPF+SCR catalysts are effective in reducing CO, hydrocarbon, NOX and particulate matter pollutants from diesel engine exhaust. In this system, urea is injected before the SCR catalyst as an NH3 source for NOX reduction. Three SCR reactions, namely the standard-, fast- and NO2-SCR reactions, have been proposed in the literature. In order to promote low temperature NOX reduction via the fast SCR reaction, NO2 concentrations can be raised by NO oxidation on the upstream DOC catalyst. While this information is very important for kinetic modeling and catalyst design, with respect to a monolith SCR catalyst, knowledge about where and how these reactions occur along the axial length is still lacking. Another unresolved issue is that if there exists unbalanced NO:NO2 ratios (?1), which represents the situation in real application, what are the reaction patterns along the monolith? To better address such issues, spatial resolution techniques involving a capillary-inlet FTIR method have been used in this study to monitor concentration profiles along a monolithic SCR catalyst.
Spatially-resolved NO, NO2, NH3 as well as N2O concentration profiles were obtained as a function of temperature and inlet NO:NO2 ratio. Based on data obtained with 0:1, 1:1 and 1:0, NO:NO2, the reaction rate follows the sequence of fast-SCR > NO2-SCR > standard-SCR over the sample tested. As one example, at 300°C, the fast-SCR reaction occurs only in the front 0.7 cm of the monolith catalyst, NO2-SCR mainly occurs in the first 1.5 cm (>90% conversion), whereas the standard SCR is observed in the front 2.4 cm. Further results obtained at temperatures between 150 and 500°C will also be presented.