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

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

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

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

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

Characterization: Fresh Co-SEA: homogenously alloyed nanoparticles Core@Shell: 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) Pd@Pt/silica b) Pt@Pd/alumina c) Pd@Pt/alumina Cho, H., Regalbuto, J. Catalysis Today 246 (2015) 143–153

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

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

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

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

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 Aged @750C, 8hr ~13

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 Pd@Pt sample co-DI sample Pt:Pd < 1 seq-SEA Pd@Pt 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

XRD Patterns

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 Pt@Pd < 3  20 nm  26 nm Alloy < 3  24 nm Poorly Alloyed  30 nm

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

Aged Al2O3 Catalyst Characterization < 3  8 nm  PdO Pd@Pt < 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

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

Characterization: Aged SiO2 Elemental Pd-Pt maps after aging at 750°C: Pt-Yellow, Pd-Red Pt:Pd < 1 seq-SEA Pd@Pt co-SEA sample co-DI sample 4 nm seq-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

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 Pd@Pt catalyst co-DI contained various Pt:Pd ratios with Pt cores  PdO 8 nm (oxide) 28 (metallic) Alloy  + 8 nm 100 nm Pd@Pt 8 nm (oxide) 24 (metallic)  + 8 nm 50 nm Alloy 14 nm (oxide) 34 (metallic) Poorly Alloyed  21 nm 50 nm

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 Pd@Pt

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 Pd@Pt 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

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

Thank you!!! Questions?

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

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

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

VGL-25 Alumina