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Kinetic Modeling Dynamic Oxygen Storage Capacity in Fresh and Aged Three-way Catalysts: Surface and Sub-surface Oxygen Storage
Jian Gong, Di Wang, Junhui Li, Krishna
Kamasamudram, Neal Currier, Aleksey Yezerets
Advanced Chemical Systems and Integration
Presented at CLEERS 10/5/2017
Oxygen storage components play a critical role in TWC’s performance and diagnosis
Fundamentals
Critical to oxidation and reduction chemistry in TWC
– e.g., oxidation state; promoter to WGS and SR;
Applications
Lambda control to achieve low emissions
Oxygen storage capacity is a descriptor of catalyst aging
– Monitoring OSC is the primary OBD method for TWC
Objectives
Investigate aging impact on OSC through experiments
and modeling
Optimal operating window[1]
[1] John J. Mooney, The 3-Way Catalytic Converter,2007
2
Oxygen storage in TWC
Oxidized ceria
CeO2 (Ce4+)Reduced ceria
Ce2O3 (Ce3+)
+ O
(Lean)
- O
(Rich)
The amount of O2 storage at lean depends
on reduced Ce2O3 at rich
Introducing two sites:
S1 (PGM and surface ceria interface)
S2 (sub-surface ceria)
3
At rich: H2/CO/CH4 + 2CeO2<-->Ce2O3 + H2O/CO2
At lean: Ce2O3 +0.5O2<-->2CeO2
O2 storage process in TWC
route 1: O adsorbs at
PGM and surface ceria
route 2: O
diffuse to sub-
surface ceria
O
O
Dual-site oxygen storage capacity (OSC) model
[2] J. Gong, D. Wang, J. Li, N. Currier, A. Yezerets, Appl.
Catal. B Environ. 203 (2017) 936-945
NO. Sites Reaction
1 S1 Ce2O3(s) +0.5O2<-->2CeO2(s)
2 S1 H2 + 2CeO2(s)<-->Ce2O3(s) + H2O
3 S1 CO + 2CeO2(s)<-->Ce2O3(s) + CO2
4 S2 Ce2O3(s) +0.5O2<-->2CeO2(s)
5 S2 H2 + 2CeO2(s)<-->Ce2O3(s) + H2O
6 S2 CO + 2CeO2(s)<-->Ce2O3(s) + CO2
7 PGM CO+H2O <-->CO2+H2
8 PGM CH4+H2O<-->CO+3H2
Reactions in the dual-site OSC model [2]
Two types of OSC sites
– Fast storage site: surface site 𝜽𝟏
– Slow storage site: sub-surface site 𝜽𝟐
Shrinking-core diffusion applied on sub-
surface site
– Capture diffusion-controlled oxygen storage
Thermodynamically consistent kinetics
4
OSC Measurement and different OSC Definitions
H2+2CeO2(s) H2O+Ce2O3(s)
H2+CO2 H2O+CO (rWGS)
1% H2, 5% H2O, 5% CO2,
balanced by N2
0.5% O2, 5% H2O, 5% CO2,
balanced by N2
5
Ce2O3(s) +0.5O2<-->2CeO2(s)
OSC measurement with rich (120s)/lean (120s) cycling
Cumulative H2-OSC on a fresh TWC A
Two distinct regimes of oxygen storage
The model is capable of predicting the dynamic OSC
Cumulative oxygen storage with time [2]
6
𝑂𝑆𝐶𝑡 =𝑆𝑉
𝑚𝑐𝑎𝑡 𝑡=𝑡sw𝑡𝑖𝑐ℎ
𝑡
2 xO2,𝑖𝑛 − xO2,𝑜𝑢𝑡𝑃𝑉𝑐𝑎𝑡𝑅𝑇𝑟𝑒𝑓
𝑑𝑡
OSC with different reductants on a fresh TWC A
At T<450 °C, H2-OSC>CO-OSC>CH4-OSC
At T> 450 °C, similar amount of OSC regardless of the reductants
200 300 400 500 600 7000
20
40
60
80
100
120
140
Temperature [oC]
OS
C [ m
ole
O/g
ca
t]
H2
CO
CH4
OSC with H2, CO, or CH4 as reductant
exp: solid
model: dashed
7
Cumulative oxygen storage on (one fresh and one aged) TWC B at 400 °C
SBC aging at 955 °C
for 57 hours
8
Fresh vs. Aged – apparent oxygen storage rates
Oxygen storage rates in “fast storage” regime declined more severely
– Oxygen storage rates in “fast storage” regime are about 20 times higher than that in the slow
diffusion regime9
Fresh vs. Aged – total OSC
After aging
– OSC greatly reduced at
temperature below 400 °C
• OSC decreased by about
43.75% at 400 °C
– Relationships between OSC
and temperature were altered
and why?
– OSC are different for H2, CO or
CH4 at 500 °C above
• H2-OSC>CO-OSC>CH4-OSC
• Different case in fresh catalyst A
Solid line: exp
Dashed: model
10
Fresh vs. Aged – breakthrough OSC
Breakthrough OSC is
closely correlated to
monitored OSC on-board
PGM and surface ceria
interface (site S1) was
greatly destructed
– PGM sintering
Kinetic restrictions were
extended towards higher
temperatures
11
Fresh vs. Aged – sub-surface OSC
Different OSC at 600 °C with
different reductants
– Deactivated WGS & SR reforming at
600 °C
OSC decreased with temp on
the fresh TWC
– Thermodynamic properties of the
sub-surface ceria was modified after
aging
12
Temperature dependence: OSC increases with temperature
The amount of reduced ceria (Ce2O3) increases as reduction temperature
increases
– More oxygen lattice defects from sub-surface ceria and bulk ceria
13
CO TPR on a commercial NOx absorber catalyst (containing CeZrOx)
PGM and
surface ceria
Sub-surface
ceria
Bulk ceria
Temperature dependence: why OSC decreases at high temperature on fresh TWC?
Number of reduced ceria (Ce2O3) will decreases at high temperatures with the
presence of water
– H2O splitting (H2O+Ce2O3H2+2CeO2) activity is higher as temperature increases
Amount of H2 generation from H2O splitting [3]
Feeding H2O to
Rh/Ce2O3(pre-reduced at 850 °C)
[3] F. Sadi, D. Duprez, F. Gerard, A. Miloudi, J. Catal. 213 (2003) 226.14
CeZrOx Thermodynamics on fresh and aged TWCs
Calibrated thermodynamics of CeZrOx are significantly different from
bulk ceria but close to CeZrOx or PGM supported CeZrOx
Composition∆𝑟𝐻0
[kJ/mol O]
∆𝑟𝑆0[J/mol O-K]
References
Bulk ceria -381.2 -26 Wagman et al.
Ce0.81Zr0.19O2-x (x=0.04-0.17) -273~-253 -117.5 ~ -31 Zhou et al.
Ce0.5Zr0.5O2-x (x=0.04-0.2) -263.5~-238 -39.50~-27.5 Zhou et al.
Ce0.14Zr0.86O2-x (x=0.06-0.08) -248.5~-225.5 -12~-6.5 Zhou et al.
PGM supported on CeZrOx -283.8~-243.8 -70.6~9.4 Gong et al.
PGM supported on CeZrOx -289+54𝜃12 5 Moller et al.
PGM supported on CeZrOx -270~-240 ±15 Rink et al.
Surface (fresh) -247.8 29.4 this work
Sub-surface (fresh) -252.8 29.4 this work
Surface (aged) -252.8 -0.6 this work
Sub-surface (aged) -243.8 -0.6 this work
15
Change of oxidation enthalpy and entropy for CeZrOx
Fresh vs. Aged – CeZrOx thermodynamics
The values of ∆𝐺𝑅0 on the aged TWC move towards more negative
numbers after aging backward reaction (H2O splitting) becomes lower
Gibbs energy of H2+2CeO2 H2O+Ce2O3
16
Summary
After SBC aging at 955 °C for 57 hours, OSC was significantly reduced
– More severely declined PGM and surface ceria interface compared to sub-surface ceria
– Kinetic restrictions of OSC reductions were extended towards higher temperatures
– The relationship between OSC and temperature was altered after aging
• Attributed to the change of the thermodynamic properties of ceria
With the dual-site OSC model, the dynamic OSCs on the fresh and aged TWCs
were correctly predicted
– Dynamic OSC (breakthrough OSC and sub-surface OSC)
– Temperature dependences
17
Thank You!
Questions?
18
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