Nanoparticle Synthesis via Electrostatic Adsorption using Incipient Wetness Impregnation

1. Nanoparticle Synthesis via Electrostatic Adsorption using Incipient Wetness Impregnation Sonia Eskandari, John R. Regalbuto The University of South Carolina 08-31-2017

2. Introduction Ru/Al2O3 DI Incipient wetness impregnation or “dry impregnation” (DI): • most-used supported metal catalyst preparation • simple and easy, no loss of metal, no filtration but • metal nanoparticles often large with polydisperse size distributions Strong electrostatic adsorption (SEA) • usually done with large excess of solution • pH controlled to charge surface –OH groups • gives small nanoparticles with tight size distribution but • requires a filtration step • loss of metal precursor in excess of monolayer adsorption capacity Ru/Al2O3 SEA

3. How do we combine the simplicity of DI with the effectiveness of SEA? “surface loading” (SL) [=] m2/L of surface per liter or solution SEA Excess Liquid pH pHopt • Typical SEA laboratory studies: thin slurries (500 – 1000 m2/L) • - minimizes pH shifts • - ease of pH, metal concentration measurement • - pH easily controlled to pHopt • Typical DI: thick slurries (100,000 – 200,000 m2/L) • pH is buffered to PZC of support • (Park and Regalbuto JCIS 1995, 175, 239) DI pore filling pH pHPZC  There is no reason in principle why SEA can’t be done at high SL

4. Hypothesis: Charge Enhanced Dry Impregnation (CEDI) Electrostatic adsorption can occur in DI (max SL) if the impregnating solution is sufficiently basified or acidified. Thick slurries desired (≥100,000 m2/L) CEDI: Thick slurry, control initial pH Typical laboratory studies: thin slurries (500 – 1000 m2/L) Dry Impregnation Pore Filling pH PZC CEDI Pore Filling pH pHopt SEA Excess Liquid pH pHopt See Zhu et al., ACS Catal. 2013, 3, 625

5. SEA monolayer limit: [Pt(NH3)4]2+ [Pt(NH3)4]2+ on silica: 500m2/L silica  Max uptake is closed packed monolayer of precursor complexes which retain two hydration sheaths O- O- O- O- O- O- metal uptake, G (mmol/m2) H2O Gmax≈ 0.9 mmol/m2 = 1 complex/2 nm2 silica = 1 monolayer of precursor pH final

6. Experimental • Support: SiO2 (Aerosil 300), SA 280 m2/g, PV 2.8 mL/g • Precursors: • Pt(NH3)4(OH)2 and Pt(NH3)4Cl2 • Pd(NH3)4Cl2 • Co(NH3)6Cl3 • Ni(NH3)6Cl2 • pHinitial= 11.5 with NH4OH • Metal concentrations measured by ICP-OES • XRD analysis with RigakuMiniFlex II with a high sensitivity Si slit detector

7. Preparation Step 1 • Impregnation to incipient wetness with basified solution final pH = 10 1 gram silica powder Dry at 120°C overnight in air* 2.8 mL metal solution with pH=11.5 * For Pt, use vacuum drying at r.t. overnight to circumvent formation of mobil (NH3)4Pt(OH)2 species (Munoz-Paezet al., J. Phys. Chem., 1995,99: p.4193)

8. Preparation Step 2 • Washing step for precursors with chloride counterions Reduce and XRD Shake for 10 min, 120 rpm, room temperature 1 gram dried and unreduced powder Filter with 0.2 µm filter paper ICP 300 mL NH4OH solution with pH=10.5

9. CEDI with (NH3)4Pt(OH)2 XRD STEM • Small particles formed at all loadings • Small Pt particles oxidize to Pt3O4 • (see Banerjee et al., Catal. Lett 2017, 147, 1754)

10. CEDI with Pt(NH3)4Cl2 • Nanoparticles are much larger • Cl- can be used with CEDI to control particle size • (see Liu et al., Catal. Tod. 2017, 280, 246) •  Can small particles be produced if Cl- is removed by washing?

11. CEDI with Pt(NH3)4Cl2 washed unwashed • Washing Cl- from samples dramatically decreases Pt particle size • Above 1 ML, loss of Pt occurs during washing • Small particles after washing again oxidize

12. CEDI with Pd(NH3)4Cl2 washed unwashed • Larger nanoparticles seen after CEDI with chloride counterion • Washing Cl- from samples dramatically decreases Pd particle size • Above 1 ML, significant loss of Pd occurs during washing

13. CEDI with Co(NH3)6Cl3 washed unwashed • Growth of Co particles occurs with increasing Co concentration • Washing Cl- from samples dramatically decreases Co particle size • Significant loss of Co again occurs in excess of 1 ML

14. CEDI with Ni(NH3)6Cl2 unwashed washed • Growth of Ni particles occurs with increasing Ni concentration • Washing Cl- from samples dramatically decreases Ni particle size • No loss of Ni during washing

15. Summary of metal particles

16. Conclusions • A simple change in incipient wetness methodology (basifying the impregnation solution) induces electrostatic adsorption and can give very small nanoparticles with tight size distributions if: • A precursor salt with OH counterions is used (though not many out there) • Chloride counterions are washed out • The proper amount to basify the solution can be easily estimated based on the PZC of the support and the surface loading (slurry thickness) of the impregnation • Significant loss of metal occurs when metal loading is above one monolayer of precursors (except Ni) • Method can be extend to anionic precursors over high PZC supports, using acidified impregnating solutions

17. Acknowledgements • Center for Renewable Fuels and The University of South Carolina for funding. • Dr. Monnier and Dr. Regalbuto group at The University of South Carolina.

18. Thanks for your attention! Any question?

19. Drying temperature effect Washing Pt(NH3)4Cl2 after drying at 120 °C • Drying Pt samples at 120 °C leads to more than 70% Pt loss during washing with larger particles • Drying at 120 ° C forms neutral Pt(NH3)2O species* • Samples were dried at room temperature under vacuum • Drying temperatures didn’t effect on other metals loss or size during washing • * Munoz-Paez, A., and Koningsberger, D.C., Decomposition of the Precursor [Pt(NH3)4] (OH)2, Genesis and Structure of the Metal-Support Interface of Alumina Supported Platinum Particles: A Structural Study Using TPR, MS, and XAFS Spectroscopy, J. Phys. Chem., 1995,99: p.4193-4204