(735a) 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)