Reaction and Transport Interactions

NO2 SCR compositions, superior NOx conversion performance was achieved with the layered architecture. The schematic shown in Fig. 11.17 explains the concept while Fig. 11.18 provides typical data for several monolith samples. The catalyst design and operating strategy was to exploit differences in the intrinsic activity and selectivity of the two catalysts through coupled reaction and diffusion. At low temperature the top layer should behave in the limit as simply as a diffusion barrier, whereas at high temperature the top layer should be sufficiently active so as to confine most of the conversion in that layer. This was of definite benefit because at low temperature, the Fe layer was much less active than the underlying Cu layer which was selective for N2, while at high temperature reaction occurred in the more selective Fe top layer. 

To illustrate, the data in Fig. 11.18 shows that a washcoat catalysts containing different fractions of Fe and Cu but a fixed total loading result in quite different 

NOx conversions. The 50 50 Fe Cu-layered catalyst achieved a NOx conversion that was essentially the arithmetic average of the individual Fe- and Cu-exchanged catalysts. In contrast, the dual layer catalyst with a thin Fe-zeolite (33 % of the total washcoat loading) layer on top of a thicker Cu-zeolite layer (67 %) resulted in a high NOx conversion over a wide temperature range and NO2/NOX feed ratio values. In the lower temperature range, the conversion approached that of the Cu-zeolite, whereas at higher temperatures the conversion approaches that of the Fe-zeolite. 

These dual layer results provide clear evidence of the existence of mass transport limitations. That the conversion for the dual-layer Fe/Cu catalyst (I, J, K) approached that for the Fe (top) layer at sufficiently high temperature indicates that significant transport limitations were present. In fact, the experiment helps to pinpoint the temperature at which the onset of diffusion limitations occurs for an Fe top layer of a prescribed loading (thickness). As the Fe top layer thickness decreases, the temperature at which the dual layer catalyst conversion is within a few percent of the single layer Fe catalyst (sample F) conversion increases. For example, the conversion for the thickest Fe top layer catalyst (sample I) approaches that of the single layer Fe catalyst at about 300 °C. For next thinner top layers (samples J), the temperature increases to 400 °C. Were diffusion limitations not present, the conversion would approach the arithmetic average of the Fe and Cu catalysts, not unlike a mixed layer catalyst. 

In conclusion, mass transport limitations cannot be ignored during SCR for moderate to high temperatures and realistic washcoat loadings. This is particularly true for more active catalysts and/or fast SCR conditions. This opens the need for 

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Reaction and Transport Interactions. The importance of the various design and operating variables largely depends on relative rates of reaction and transport of reactants to the reaction sites. If transport rates to and from reaction sites are substantially greater than the specific reaction rate at meso-scale reactant concentrations, the overall reaction rate is uncoupled from the transport rates and increasing reactor size has no effect on the apparent reaction rate, the macro-scale reaction rate. When these rates are comparable, they are coupled, that is they affect each other. In these situations, increasing reactor size alters mass- and heat-transport rates and changes the apparent reaction rate. Conversions are underestimated in small reactors and selectivity is affected. Selectivity does not exhibit such consistent impacts and any effects of size on selectivity must be deterrnined experimentally. 

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