The C atalytic B ehavior of Pt- - PowerPoint Presentation

Slide1

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

Slide2

OutlineIntroduction to diesel exhaust treatmentsBimetallic Catalyst SynthesisReactor Setup (ORNL)ResultsCatalyst Performance Data (alumina and silica catalysts)Particle Morphology (XRD, STEM, Elemental Maps) Conclusions

Future Research2

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Introduction

Diesel operated vehicles require exhaust treatmentsExhaust 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

3

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Strong Electrostatic AdsorptionBimetallic 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

PtO

2

– pH1.0

PdO

–

pH 4 - 7

4

Slide5

Uptake Surveys of NM’s

Noble Metal Oxide PZCs

PtO2 – pH 1.0 PdO

–

pH 4 - 7

Uptake Survey on PtO

2

Uptake Survey on

PdO

Uptake Survey on Alumina (co-SEA)

PHC –

Chloroplatinic

acid

PdTC

– Sodium tetrachloropalladium

5

Slide6

Characterization: Fresh

a

) Pd@Pt

/silica

b

)

Pt@Pd

/alumina

c

)

Pd@Pt

/alumina

Co-SEA samples are more dispersed than co-DI

Core-shell nanoparticles are also highly dispersed

Support

Silica

Alumina

Method

co-SEA

co-DI

co-SEA

co-DI

XRD

1.3

20

2.0

3.0

STEM

1.1

v. large

1.7

aglom.

Summary of Particle Sizes (nm)

6

Core@Shell

:

Co-SEA

: homogenously alloyed nanoparticles

Cho, H., Regalbuto, J. Catalysis

Today 246 (2015) 143–153

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Flow Reactor at ORNL

Feed: 1500 ppm CO, 87 ppm C3H6, 87 ppm C3H

8, 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 & agingHold at 750°C for 8 hour hydrothermal agingRamp to 500°C: evaluation of aged sampleAnalysis instruments: mass spectrometer and chemiluminescence NOx analyzer7

Conversion is a measurement of all CO and HC reductants to CO

2

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Hydrothermal agingCONDITION 1 – Pretreatment 1% CO, 10% H2

O, and 10% O2 in N2Ramp 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

1% CO, 10% H

2

O, and 10% O

2

in N

2

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)

TC2

TC3

TC4

gas flow

Water bath

8

Slide9

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

9

High Pt

wt

% catalysts had good NO to NO

2

conversions

Slide10

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 PdAddition of a small amount of Pt on a Pd

-core does not seem to help HC oxidation performanceLarger amounts of Pt on Pd return the HC oxidation performanceThe addition of Pt is necessary for NOx conversion

10

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Results- Alumina (co-SEA)Homogenously Alloyed co-SEA catalyst

Alloyed co-SEA catalyst exhibited good hydrothermal stability, with virtually no changes in light-off temperaturesNO to NO2 conversion is good

Co-SEA sample

XRD Particle Size (nm)

Initial

2.0

Aged @750C, 8hr

~13

11

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Characterization: Aged Al

2

O

3

Elemental Pd-Pt maps after aging at 750

°

C:

Pt-

Yellow

, Pd-

Red

Pt heavy catalysts are still mostly alloyed for SEA and DI samples

c

o-DI sample is poorly alloyed

Some particles have enriched Pd shells

seq

-SEA

Pd@Pt

sample is mostly alloyed, but has a few particles with enriched Pd outer shells

co-SEA sample

c

o-DI sample

Pt:Pd

> 1

Pt:Pd

< 1

seq

-SEA

Pd@Pt

sample

c

o-DI sample

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XRD Patterns13

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11 nm

26 nm

30 nm

< 3



15

nm

All Pt-heavy alumina supported catalysts end as mostly alloyed

Co-SEA particles were the most resistance to sintering

PdO

disappears at higher temperatures

Aged Al

2

O

3

Catalyst Characterization

14

< 3



13

nm

< 3

 20

nm

< 3

 24

nm

100 nm

Pt





Pt/

Pd AlloyPoorly AlloyedPt@PdAlloyAlloy

Slide15

Aging affects the Pt-only catalysts more than the

bimetallics

All the

bimetallics

had similar T

50

’s after aging at 750

°C

.

DI sample had the largest particles

Al

2

O

3

Catalyst Activity

15

T

50

Slide16

7 nm

44 nm

Aged Al

2

O

3

Catalyst Characterization

PdO

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

16

2

0 nm

100 nm







Poorly Alloyed

Poorly Alloyed

< 3



8

nm

< 3



11

nm< 3  16 nm++Pd@PtAlloy

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Al

2

O

3

Catalyst Activity

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 T

50

compared to the same wt. loading co-DI catalyst

17

T

50

Slide18

18

Characterization: Aged SiO

2

Elemental Pd-Pt maps after aging at 750

°

C:

Pt-

Yellow

, Pd-

Red

Pt:Pd

< 1

4

nm

seq

-SEA

Pd@Pt

co-SEA sample

co-DI

sample

s

eq

-SEA contained a mixture of small

Pd@Pt

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

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8 nm21 nm

8 nm

8 nm (oxide)

28 (metallic)

8 nm (oxide)

24 (metallic)

14 nm (oxide)

34 (metallic)

6 nm (oxide)

Only

PdO

is observed in Pd

only catalyst

SEA catalyst were much smaller than DI

Some

Pd

-cores remain in the

Pd@Pt

catalyst

co-DI contained various

Pt:Pd

ratios with Pt cores

SiO

2

Catalyst Characterization

PdO

19

100 nm

50 nm

50 nm





++Poorly AlloyedPd@PtAlloyAlloy

Slide20

Pd@Pt

SiO

2

Catalyst Activity

Pd catalyst on SiO

2

deactivated more than on Al

2

O

3

Pd

-core/Pt-shell retained on the SiO

2

seq

-SEA catalysts

Pd

-core/Pt-shell catalyst was very stable

c

o-DI catalyst had

PdO

migration to the outside, which is more active in some HC reactions

20

T

50

Slide21

Conclusions and Future WorkThe addition of Pd aids in the stability of Pt catalysts

After high temperature agingall alumina catalysts were mostly alloyed, with some Pt cores surrounded by Pd on the lower Pt:Pd catalysts

silica SEA catalyst showed some Pd@Pt remainingsilica 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

21

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AcknowledgementsA 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

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Thank you!!!Questions?

Slide24

Results- Core StructuresPt-core or Pd-core?

Pd-core/Pt-shell catalysts is more thermally stableBeing Pd heavy could also aid stabilityHigher loading wt

% catalyst is expected to have better activity

24

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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

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Pt on Silica8nm @500C17nm @750C

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VGL-25 Alumina27