(629q) Determination of Controlling Regimes for Various SCR Reactions On Zeolite Based Monolithic Catalysts

The selective catalytic reduction (SCR) of NOx with NH₃ is considered to be a highly promising technique for the efficient reduction of highly detrimental NOx (to N₂) emitted from diesel engine vehicles. Fe and Cu-based zeolite catalysts are considered to be highly stable and efficient towards NOx conversion over a broader temperature range. Cu-zeolite has been found to be a very good low temperatures (< 350 ^oC) NOx conversion catalyst. However, Fe-zeolite has been found to be a better catalyst at higher temperatures (> 350 ^oC).

An experimental study coupled with the kinetic modeling of key SCR reactions is presented here. This study is focused on the investigation of extent of mass transfer limitations in various SCR reactions. The experiments included NO and NH₃ oxidation, standard SCR (NO+NH₃), fast SCR (NO+NO₂+NH₃), and NO₂ SCR (NO₂+NH₃). 

Monolith catalysts with different washcoat loadings, washcoat thicknesses and lengths were synthesized to study the mass transfer limitations in zeolite catalysts. A steady state experimental study of various SCR reactions showed the presence of washcoat diffusion limitations at intermediate temperature range and hence these diffusion limitations should be considered in any kinetic model and catalyst designs. A detailed analysis of the effect of temperature on the transitions between the controlling regimes (for various SCR reactions) is presented. We investigated the effect of catalyst loading, catalyst aging, monolith dimensions, and space velocity, on the transition between various controlling regimes for all the above reactions.

A kinetic model which includes the effect of washcoat diffusion limitations is developed on the basis of all the above experimental observations. A good agreement is obtained between the experimental observations and the model predictions.

Keywords: Selective Catalytic Reduction, Cu-zeolite, Fe-zeolite, Washcoat diffusion, Mass transfer, Kinetic model.