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