Spatially-Resolving the SCR Reaction

William  Epling, University of Waterloo

William S. Epling, Xuxian Hou, Jin-Yong Luo, Steven Schmieg and Wei Li

There have been several recent reports regarding the spatial resolution of gas species in monolith-supported automobile exhaust catalysts, with examples including characterization of DOCs (diesel oxidation catalysts) and LNTs (lean NOx traps). However, spatially resolving gas concentrations in NH3-SCR (selective catalytic reduction) applications has not been previously reported. In this study, a gas-phase FT-IR analyzer was used to spatially resolve gas concentrations in a monolith-supported SCR catalyst. The reactions analyzed include standard SCR, fast SCR and SCR with NO2, as well as NH3 adsorption.

The results show that the three SCR reactions proceed at significantly different rates, especially at temperatures below 300°C, and can be correlated to the amount of catalyst used. For the standard SCR reaction, kinetic analysis, and spatially resolved NO oxidation and SCR results consistently indicate that the rate-determining step is NO oxidation. NH3 has an inhibition effect, as it suppresses NO oxidation by competitive adsorption on the active sites. At mid-range temperatures, the outlet NOX conversion is not limited by the reaction kinetics, but by insufficient NH3 supply, since part of the NH3 is oxidized by O2. Also at higher temperatures, higher NOX conversions are attained due to significantly enhanced NO oxidation, and the resulting increase in NH3 reacting with NOX via SCR rather than O2 via NH3 oxidation. For NO2-SCR, a considerable amount of N2O was formed at 250°C but decreased with increasing temperature. The decreased N2O is due to improved selectivity in the NO2-SCR to N2, as well as N2O decomposition at the back part of the catalyst at high temperature. Finally, different SCR reaction patterns were identified when testing with NO:NO2 = 3:1 and 1:3. For NO:NO2 = 3:1, the SCR reactions proceed in series, namely through the fast reaction first, followed by standard SCR. The fast SCR and NO2-SCR reactions proceed in parallel for NO:NO2 = 1:3. The results indicate that if NO2 is the limiting reactant, fast SCR dominates, but if excess NO2 is available, the NO2 SCR reaction can proceed in parallel.

In order to characterize NH3 adsorption, pulses of NH3 were introduced and adsorption location and amount determined. Results show that as the pulse time decreased, the distribution of adsorbed NH3 along the catalyst length became sharper. Temperature was also found to affect the adsorption pattern, in terms of amount and sharpness of the surface concentration profiles. Exposing the catalyst to standard SCR conditions verified the location of adsorbed NH3, and also the effectiveness of that adsorbed NH3.

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