Detailed Kinetic Modeling of Selective Catalytic Reduction of NO With C3H6 On Cu-Chabazite Monolithic Catalyst

Detailed Kinetic Modeling of Selective Catalytic Reduction of NO with C₃H₆ on Cu-chabazite Monolithic Catalyst

Richa Raj, Michael P. Harold and Vemuri Balakotaiah

Department of Chemical & Biomolecular Engineering, University of Houston, Houston, TX, United States.

Abstract

NO_x storage and reduction (NSR) and selective catalytic reduction (SCR) are the two commercialized aftertreatment technologies for the reduction of NO_x emissions from lean–burn engines The combined LNT-SCR systems are emerging as promising technology for NO_x reduction as NO_x can be stored during lean phase and NH₃ and hydrocarbons (C₃H_6, C₂H₄ etc.) released during rich phase can be stored on SCR catalyst and these stored reductants can reduce the NO_x coming out from LNT during lean phase. The storage of hydrocarbons and their role as co-reductants of NO_x lead to an intricate system. Hence it is important to study the HC-SCR chemistry. A systematic steady state and transient experimental studies were performed to elucidate the mechanistic pathways involved in NO_x reduction with C₃H₆ [1].

            The steady state experiments were performed for different feed compositions for CO+O₂, C₃H₆+O₂ and C₃H₆+NO+O₂ reaction systems. Fig.1a shows that C₃H₆ light-off curves for C₃H₆ oxidation for varying O₂ concentration (0.5-5%) in the feed. Similarly, fig.1b shows the C₃H₆ conversion with temperature for different C₃H₆ concentration (200-500 ppm). Fig.2a and 2b show the C₃H₆ and NO conversion in temperature range of 200-550^oC for feed with 500 ppm C₃H₆, 500 ppm NO and 1% and 5% O₂ respectively. A steady state detailed kinetic model for NO reduction with C₃H₆ on Cu-chabazite (Cu-SSZ13) monolith catalyst is developed based on bench scale flow reactor experiments The following mechanistic sequence is proposed based on experimental studies:

R1: C₃H₆ + S ↔ C₃H₆-S        

R2: O₂ + S ↔ O₂-S    

R3: O₂-S + S ↔ 2O-S

R4: C₃H₆-S + O-S → C₃H₄-S + H₂O

R5: C₃H₄-S + O-S → C₃H₄O-S

R6: C₃H₄O-S + 4O-S → 3CO-S + 2H₂O      

R7: CO –S↔ CO + S

R8: CO-S + O-S ↔ CO₂ + 2S           

R9: C₃H₄O-S + NO → C₂H₂-NCO-S + H₂O

R10: C₂H₂-NCO- S +3O-S → 3CO-S + N-S + H₂O           

R11: N-S + N-S ↔ N₂ + 2S  

. Kinetic Parameters estimated capture the experimental trends and meet the thermodynamic constraints. The spatially 2D (1D + 1D) mathematical model of the catalytic monolith reactor coupled with the detailed kinetic model is used to study the complex reaction system. The kinetic model developed capture the main reaction pathways and predicted conversion (CO, C₃H₆ and NO) and product distribution (CO, CO₂ and N₂) are in good qualitative and quantitative agreement with the experimental data. The model predicts for the light off curves for three reaction systems CO+O₂, C₃H₆+O₂ and C₃H₆+NO+O₂ for different feed compositions. The kinetic model is used to determine the trends in species surface coverages to explain the experimentally observed trends in selectivities.

Figure 1 (a) Light- off curves of C₃H₆ for different oxygen concentrations in the feed (C₃H₆ = 500 ppm, O₂ = 0.5 or 1 or 5%, Balance gas: Ar), (b) Light- off curves of C₃H₆ for different C₃H₆ concentrations in the feed (C₃H₆ = 200 or 500 ppm, O₂ = 1 %, Balance gas: Ar),

Figure 2(a) C₃H₆ and NO conversion with temperature (C₃H₆ = 500 ppm, NO = 500 ppm, O₂ = 1%, Balance gas: Ar) (b) C₃H₆ and NO conversion with temperature (C₃H₆ = 500 ppm, NO = 500 ppm, O₂ = 5%, Balance gas: Ar)

Keywords: SCR, Kinetic Modeling, Cu-chabazite, Propylene

References:

[1] R. Raj, M.P. Harold, V. Balakotaiah, Industrial & Engineering Chemistry Research, revisions pending (2013)