1. The Catalytic Behavior of Pt-Pd Bimetallic Catalysts for Use as Diesel Oxidation Catalysts
Andrew Wong, Todd J. Toops*, and John R. Regalbuto
*Oak Ridge National Lab

2. Outline Introduction to diesel exhaust treatments
Bimetallic Catalyst Synthesis
Reactor Setup (ORNL)
Results
Catalyst Performance Data (alumina and silica catalysts)
Particle Morphology (XRD, STEM, Elemental Maps)
Conclusions
Future Research

3. Introduction Diesel operated vehicles require exhaust treatments
Exhaust treatment involves three parts:
Diesel oxidation catalyst (DOC)
Diesel particulate filter (DPF)
Lean-NOx-trap (LNT) and/or selective catalytic reduction (SCR)
Vehicle emissions are highest during a cold-start

4. Strong Electrostatic Adsorption
Bimetallic Catalysts
Surface is charged by changing the pH
Use a precursor oppositely charged from the surface
Seq-SEA prevents wasting the metal on the support
Noble Metal Oxide PZCs
PtO2 – pH1.0
PdO – pH 4 - 7

5. Uptake Surveys of NM’s Uptake Survey on PtO2 Noble Metal Oxide PZCs
PtO2 – pH 1.0
PdO – pH 4 - 7
Uptake Survey on Alumina (co-SEA)
Uptake Survey on PdO
PHC – Chloroplatinic acid
PdTC – Sodium tetrachloropalladium

6. Characterization: Fresh
Co-SEA: homogenously alloyed nanoparticles
Co-SEA samples are more dispersed than co-DI
Core-shell nanoparticles are also highly dispersed
Summary of Particle Sizes (nm)
Support
Silica
Alumina
Method
co-SEA
co-DI
XRD
1.3 20
2.0
3.0
STEM
1.1
v. large
1.7
aglom. a) b) c)
Cho, H., Regalbuto, J. Catalysis Today 246 (2015) 143–153

7. Flow Reactor at ORNL Feed: 1500 ppm CO, 87 ppm C3H6, 87 ppm C3H8,
300 ppm NO, H2O, and O2
Space velocity: 360,000 hr-1
Three ramp up temperatures (500°C, 750°C, 500°C)
Ramp to 500°C: initial evaluation & pretreatment
Ramp to 750°C: 2nd evaluation & aging
Hold at 750°C for 8 hour hydrothermal aging
Ramp to 500°C: evaluation of aged sample
Analysis instruments: mass spectrometer and chemiluminescence NOx analyzer
Conversion is a measurement of all CO and HC reductants to CO2

8. Hydrothermal aging CONDITION 1 – Pretreatment
Water bath
CONDITION 1 – Pretreatment
1% CO, 10% H2O, and 10% O2 in N2
Ramp up from 100°C to 500°C, 10°C/min (1h)
Pretreatment at 500°C, 2h (2h)
Ramp down to 50°C from 500°C, 10°C/min (1h)
CONDITION 2 – Hydrothermal aging
Ramp up from 100°C to 750°C, 10°C/min (1h)
Thermal aging at 750°C, 8h (8h)
Ramp down to 50°C from 750°C, 10°C/min (1h)
gas flow
TC3
TC2
TC4

9. Results- Alumina (Pt@Pd)
Adding a Pd-shell to a Pt-core
We can reduce the light-off temperature by increasing the Pd shell loading
Adding the second metal reduces the light-off temperature by 60°C
The bimetallic catalysts showed a reduced aging effect compared to the Pt only catalyst
High Pt wt% catalysts had good NO to NO2 conversions

10. Results- Alumina (Pd@Pt)
Adding a Pt-shell to a Pd-core
Pd-only catalyst is more stable than the Pt-only catalyst, but low initial activity likely due to unoxidized Pd
Addition of a small amount of Pt on a Pd-core does not seem to help HC oxidation performance
Larger amounts of Pt on Pd return the HC oxidation performance
The addition of Pt is necessary for NOx conversion

11. Results- Alumina (co-SEA)
Homogenously Alloyed co-SEA catalyst
Alloyed co-SEA catalyst exhibited good hydrothermal stability, with virtually no changes in light-off temperatures
NO to NO2 conversion is good
Co-SEA sample
XRD Particle Size (nm)
Initial
2.0
8hr
~13

12. Characterization: Aged Al2O3
Elemental Pd-Pt maps after aging at 750°C: Pt-Yellow, Pd-Red
co-SEA sample
co-DI sample
Pt:Pd > 1
Pt heavy catalysts are still mostly alloyed for SEA and DI samples
seq-SEA sample
co-DI sample
Pt:Pd < 1
seq-SEA sample is mostly alloyed, but has a few particles with enriched Pd outer shells
co-DI sample is poorly alloyed
Some particles have enriched Pd shells

13. XRD Patterns

14. Aged Al2O3 Catalyst Characterization
All Pt-heavy alumina supported catalysts end as mostly alloyed
Co-SEA particles were the most resistance to sintering
PdO disappears at higher temperatures
< 3  15 nm Pt 
Alloy
< 3  13 nm
Pt/Pd Alloy 
11 nm
100 nm
< 3  20 nm 
26 nm
Alloy
< 3  24 nm
Poorly Alloyed 
30 nm

15. Al2O3 Catalyst Activity T50
Aging affects the Pt-only catalysts more than the bimetallics
All the bimetallics had similar T50’s after aging at 750°C.
DI sample had the largest particles

16. Aged Al2O3 Catalyst Characterization
< 3  8 nm 
PdO
< 3  11 nm  +
7 nm
20 nm
Alloy
< 3  16 nm
Poorly Alloyed 
Poorly Alloyed +
44 nm
Only PdO is observed in Pd only catalyst
SEA catalyst were much smaller than DI
Some Pd enrichment on the surface
Particle agglomeration explains the difference in particle sizes between XRD and STEM
100 nm

17. Al2O3 Catalyst Activity T50
Pd only catalyst exhibits highest activity for HC conversion and best stability, but lacks NOx conversion
seq-SEA catalyst has smaller particles and had a lower T50 compared to the same wt. loading co-DI catalyst

18. Characterization: Aged SiO2
Elemental Pd-Pt maps after aging at 750°C: Pt-Yellow, Pd-Red
Pt:Pd < 1
seq-SEA
co-SEA sample
co-DI sample
4 nm
seq-SEA contained a mixture of small and alloyed particles
co-SEA catalysts remained small and mostly contained homogenous alloys
co-DI catalysts mostly contained poorly alloyed cores with enriched Pd shells

19. SiO2 Catalyst Characterization
6 nm (oxide)
Only PdO is observed in Pd only catalyst
SEA catalyst were much smaller than DI
Some Pd-cores remain in the catalyst
co-DI contained various Pt:Pd ratios with Pt cores 
PdO
8 nm (oxide)
28 (metallic)
Alloy  +
8 nm
100 nm
8 nm (oxide)
24 (metallic)  +
8 nm
50 nm
Alloy
14 nm (oxide)
34 (metallic)
Poorly Alloyed 
21 nm
50 nm

20. SiO2 Catalyst Activity T50
Pd catalyst on SiO2 deactivated more than on Al2O3
Pd-core/Pt-shell retained on the SiO2 seq-SEA catalysts
Pd-core/Pt-shell catalyst was very stable
co-DI catalyst had PdO migration to the outside, which is more active in some HC reactions

21. Conclusions and Future Work
The addition of Pd aids in the stability of Pt catalysts
After high temperature aging
all alumina catalysts were mostly alloyed, with some Pt cores surrounded by Pd on the lower Pt:Pd catalysts
silica SEA catalyst showed some remaining
silica co-DI catalyst had enriched Pt phase surrounded by PdO
co-SEA alumina catalyst exhibits excellent stability and activity
The addition of Pt is needed for NOx conversion
Working with Solvay to use commercially stable modified supports in order to improve catalyst activity and stability
We plan on investigating the effects of different Pt:Pd ratios of homogeously alloyed particles on these supports

22. Acknowledgements
A portion of this research was sponsored by the U.S. DOE, EERE, Vehicle Technologies Program. The authors at ORNL wish to express their gratitude to program managers Ken Howden and Gurpreet Singh for their support.
The National Science Foundation, the University of South Carolina, and the Center of Catalysis for Renewable Fuels for project funding.
Support and guidance from my co-workers at the University of South Carolina and ORNL

23. Thank you!!! Questions?

24. Results- Core Structures
Pt-core or Pd-core?
Pd-core/Pt-shell catalysts is more thermally stable
Being Pd heavy could also aid stability
Higher loading wt% catalyst is expected to have better activity

25. Results- Silica Silica Catalysts (monometallic vs bimetallic)
Bimetallic has greater hydrocarbon activity
Bimetallic has improved stability
The addition of Pt aids after-aging NOx conversion

26. Pt on Silica
@500C 8nm
@750C 17 nm

27. VGL-25 Alumina