Highly Dispersed Pt-Pd Bimetallic Catalysts for Diesel Exhaust Treatment

1. Highly Dispersed Pt-Pd Bimetallic Catalysts for Diesel Exhaust Treatment Andrew P. Wong1, Todd J. Toops2, and John R. Regalbuto1 1Department of Chemical Engineering - University of South Carolina 2Fuels, Engines, and Emissions Research Center - ORNL ACS Fall 2017 August 23, 2017

2. Background • Diesel Oxidation Catalysts (DOC): role is to oxidize CO, HC’s, and NOx. • Low-temperature activity • High-temperature stability • Catalyst deactivation • Typical catalysts incorporate combinations of Pt/Pd supported on Al2O3and other modified supports. • Our goal is to improve DOC activity/stability by focusing on creating highly-dispersed bimetallic Pt-Pd systems.

3. Strong Electrostatic Adsorption (SEA) - Inducing a surface charge on the support by adjusting the pH of the solution cationic complexes: [Pt(NH3)4]2+ [Pd(NH3)4]2+ [Cu(NH3)4]2+ [Co(NH3)6]3+ [Ni(NH3)6]2+ pH>PZC O- Support pH@PZC OH anionic complexes: [PtCl6]2- [PdCl4]2- pH<PZC OH2+ - resulting smaller catalyst particles and close intimacy between two metal particles - resulting close packed monolayer of ionic complex (retaining hydration sheaths) with strong interaction with support

4. Materials • 2 wt% total of Pt-Pd. • Pt:Pd wt ratios of 1:0, 3:1, 1:1, 1:3, and 0:1. • Tetraammineplatinum (II) hydroxide hydrate - Pt(NH3)4 (OH)2 • Tetraamminepalladium (II) chloride monohydrate - Pd(NH3)4Cl2 10 nm 2 nm

5. XRD • High sensitivity XRD Si slit detector. • Particle size detection limit ~1.5 nm. • All SEA synthesized Pt-Pd bimetallics show no XRD peaks. • Dry Impregnation (incipient wetness) catalysts have larger particles, detected with XRD. (111)

6. Evaluation • USDRIVE low-temperature combustion of diesel protocol (2015) • Degreening conditions used to screen catalyst activity and stability • Catalyst ageing performed at 800°C for 25 hours Ramp-up @ 2°C/min Degreened activity Hold @ 700°C for 4 hours Degreening process Ramp-up @ 2°C/min Fresh activity 500° C Ramp-down @ 2°C/min 100°C 100°C • Gas characterization performed using a GC, Mass Spectrometer, and chemiluminescence NOx analyzer.

7. Feed Conditions Flow Reactor at ORNL Total Flowrate: 333 sccm Sample: 100 mg GHSV: 200 L/g-hr (~100,000 hr-1) 6% CO2 12% O2 6% H2O 2000 ppm CO 100 ppm NO 400 ppm H2 835 ppm C2H4 333 ppm C3H6 111 ppm C3H8 Balanced Ar

8. 001 (70Al-30Si) 160m2/g • Pt:Pd (1:0) Degreened Degreened Total HC

9. 001 (70Al-30Si) 160m2/g • Pt:Pd (3:1) • Light-off curves are less steep. • No activity towards propane until 250°C. Degreened Degreened Total HC

10. 001 (70Al-30Si) 160m2/g • Pt:Pd (1:1) • Propane activity starts ~275°C. Degreened Degreened Total HC

11. 001 (70Al-30Si) 160m2/g • Pt:Pd (1:1) IWI • Incipient wetness catalyst showed improvements to low-temperature activity after degreening. • Difficulty towards complete HC conversion. Degreened Degreened Total HC

12. 001 (70Al-30Si) 160m2/g • Pt:Pd (1:3) • Pd-rich catalyst appears to be better than Pt:Pd 1:1. Degreened Degreened Total HC

13. 001 (70Al-30Si) 160m2/g • Pt:Pd (0:1) • Pd-only catalyst has good CO activity. • Poor HC activity. Degreened Degreened Total HC

14. HC Conversion Summary Higher Pd content • In general, activity towards total HC conversion decreased with Pd addition for catalysts pretreated at 700°C.

15. 005 (88Al-12Si) 249 m2/g • Pt:Pd (1:0) • Excellent activity for this Pt-only catalyst. Degreened Degreened Total HC

16. 005 (88Al-12Si) 249 m2/g • Pt:Pd (1:0) 800C/25hr • Catalyst deactivated after ageing treatment. • Propane conversion was less than 50%. Degreened Degreened Total HC

17. 005 (88Al-12Si) 249 m2/g • Pt:Pd (1:3) 800C/25hr • Catalyst still deactivated; however propane conversion was improved. • Compared to the Pt-only catalyst, the T50 was lower, but T90 was higher. Degreened Degreened Total HC

18. Summary

19. XRD Characterization • All catalysts showed signs of sintering after 700°C. • The Pd rich catalysts existed as partly PdO. • Greater PdO content observed in the IWI compared to SEA catalysts; in agreement that co-SEA catalysts exists as homogenously alloyed.

20. EDS Characterization Support 001 Pt:Pd 3:1 700°C • Particles remained alloyed after 4 hours at 700°C. • STEM agrees with XRD particle size of 9 nm. • Pd-rich catalysts show phase segregation after 25 hours at 800°C. 200 nm Support 006 Pt:Pd 1:3 800°C Pt Pd 200 nm

21. Trends for Pt-only catalysts The best catalysts have ~10-30 wt% Si. Higher surface areas improve catalyst function.

22. Summary We can synthesize highly-disperse, homogenously alloyed bimetallic nanoparticles across mixed oxide supports. Pt is required to oxidize heavier HC’s and NOx. After 800°C ageing, Pt-only catalysts were still the most active (lowest T90’s) . However, Pt-Pd catalysts had better T50’s. Pt-Pd phase segregation occurs for Pd-rich catalysts Support optimization is key.

23. Acknowledgements Thank you Questions? Colleagues at ORNL and USC. Solvay Chemicals for providing supports.

24. 001 (70Al-30Si) 160m2/g • Only a small amount of Pt is needed for NO oxidation • Very little NO to NO2 oxidation over Pd

25. Previous Work • Previous studies have shown improvements using co-SEA versus other bimetallic syntheses for some reactions. Pt-Pd Diesel Exhaust Catalysts: CO+HC+NOxCO2+H2O+NOx Pd-Cu Furfural Conversion Catalysts: FurfuralFurfurylAlcoholCyclopentanone • co-SEA catalysts have a higher dispersion after ageing treatments compared with IWI catalysts • co-SEA catalysts have 8X higher activity and conversion of furfural compared to DI (poorly-mixed) and ED (core-shell) compositions Wong, A.P., et al. Catalysis Today267, (2016).

26. Support 005 (88Al-12Si) 249 m2/g Pt:Pd (1:0) Degreened

27. 006 (80Al-20Si) 292 m2/g • Pt:Pd (1:0)

28. 006 (80Al-20Si) 292 m2/g • Pt:Pd (1:0) 800C/25hr

29. 006 (80Al-20Si) 292 m2/g • Pt:Pd (1:3) 800C/25hr

30. 008 (100Al-0Si) 166 m2/g Pt:Pd (1:0)

31. SiO2 (0Al-100Si) 282 m2/g Pt:Pd (1:0)

32. 002 (95Al-05Si) 105m2/g • Pt:Pd (1:0)

33. 003 (88Al-11Si-4La) 171 m2/g • Pt:Pd (1:0)