(24e) Experimental and Kinetic Modeling Study of Selective Catalytic Reduction of NOx On Fe- and Cu-Based Zeolitic 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. Amongst the various catalysts available for SCR, 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).

This is a comprehensive experimental study coupled with the kinetic modeling of key SCR reactions. A comparison is made between Fe- and Cu-zeolite catalysts and the observed differences between the two catalysts are explained with the kinetic model. These experiments included NH₃ and NOx uptake and temperature-programmed desorption (TPD), NO and NH₃ oxidation, standard SCR (NO+NH₃), fast SCR (NO+NO₂+NH₃), and NO₂ SCR (NO₂+NH₃). 

The Cu-zeolite catalyst was seen to have higher ammonia and NO₂ storage capacities. It was insensitive to NO₂/NOx ratios at lower temperatures and gave very high NOx conversions. However, Fe-zeolite catalyst was found be a better NO oxidation catalyst and NO₂ was found to have a strong inhibition on NO oxidation reaction.  NH₃ was seen to inhibit the standard SCR reaction on both Fe and Cu-zeolites at lower temperatures. But this inhibition was less pronounced for Cu and it disappeared at slightly higher temperatures. Ammonium nitrate was found to inhibit NOx reduction at lower temperatures in the presence of excess feed NO₂.

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.

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, Standard SCR, Fast SCR, Ammonia oxidation, NO Oxidation, Washcoat diffusion, Mass transfer, Kinetic model.