(216c) Lean-Rich Switching over a Modified Three-Way Catalyst:  Experiments and Modeling

Mengmeng Li, Sam Malamis, Michael P. Harold^*,
William Epling^**

Department
of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204

*
mharold@uh.edu, ** wsepling@central.uh.edu

The most recent CAFE standard for automobile
requires an average fuel economy of 54.5 miles per gallon by 2025 [1].
At
the same time, more stringent emission rules enacted by the EPA for 2017-18
will require an 80% reduction in non-methane organic gases (NMOG) plus NOx from
current Tier 2 Bin 5 levels [2]. Gasoline
vehicles that operate under both lean-burn and stoichiometric combustion hold
promise since lean-burn combustion is more fuel efficient than conventional stoichiometric
combustion. However, elimination of NOx (NO+NO₂) by the traditional three-way
catalyst (TWC) is not possible under lean conditions. Advanced NOx reduction strategies
must be applied to exploit the more efficient lean-burn engine combustion such
as the three-way catalyst with a NOx storage function (TWNSC). The TWNSC stores
NO_x storage during lean operation and reduces the trapped NOx during
stoichiometric operation. To date, there have been only a few studies that
report TWNSC.  In this study a predictive TWNSC model is developed to predict
TWNSC performance and in so doing provide operational insight and optimization.

A model of a catalytic monolith
channel was developed that incorporates a TWNSC kinetic model into a
low-dimensional model formulation [3–5]. The
convection-diffusion-reaction equations are averaged in the transverse
direction which replaces the transverse gradient term with an algebraic term
that contains an internal mass transfer coefficient. Overall heat and mass
transfer coefficients are used to represent the heat and mass transfer in the
transverse direction between the bulk of fluid and solid phase. The
TWNSC kinetics consists of a combination of elementary
steps and global reactions. Following our previous experimental study of NOx
reduction by propylene over a multifunctional NOx trap catalyst [6],
the hybrid model includes the kinetics of the oxidation of CO, H₂,
NO and hydrocarbons, of reduction of NO/NO₂ by CO, H₂ and
representative hydrocarbons, and of storage of NOx and O₂. Fig. 1
shows the reaction network including 10 main paths that described the chemistry
in qualitative terms.  

Figure
1. Simplified reaction network of NOx reduction using propylene as reductant[6]

 A step-wise approach was
used to develop kinetics for the oxidations of CO, H₂ and propylene.
First, global kinetics are fitted tuned to transient light-off experiments
following Raj et al. [5].
For example, the Langmuir-Hinshelwood expression is given by

Fig. 2 shows the fitted
light-off curve for CO oxidation and the comparison between experimental and
model data. The model shows a reasonable fit and captures the dynamical feature
of CO TPO (temperature programmed oxidation) experiment.

Figure 2. (a) Fitted light-off
curve of CO oxidation (b) comparison between experimental and model-predicted
conversions. [Conditions: 0.5% CO, 0.5% O; temperature ramp: 3˚C/min]  

The
rate expressions for the co-oxidation of CO, H₂, and C₃H₆
include coupling effects.  These are combined with a dual-site NOx storage
model and incorporated into species balances. 

We
will show how the model is effective in capturing the main trends in the
lean-rich switching data.  This study advances the understanding of TWNSC
catalyst and provides guidance for optimizing catalyst formulation and
operation strategies.

Reference

[1]      http://www.nhtsa.gov/About+NHTSA/Press+Releases/2012/Obama+Administra

tion+Finalizes+Historic+54.5+mpg+Fuel+Efficiency+Standards.

[2]      T.
Johnson, SAE Int. J. Engines. 6 (2014) 699–715.

[3]      S.Y.
Joshi, M.P. Harold, V. Balakotaiah, AIChE J. 55 (2009) 1771–1783.

[4]      D.
Bhatia, R.D. Clayton, M.P. Harold, V. Balakotaiah, Catal. Today. 147 (2009)
250–256.

[5]      R.
Raj, M.P. Harold, V. Balakotaiah, Chem. Eng. J. 281 (2015) 322–333.

[6]      M.
Li, V.G. Easterling, M.P. Harold, Catal. Today. 267 (2016) 177–191.