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

Introduction 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 requires a filtration step loss of metal precursor in excess of monolayer adsorption capacity Ru/Al2O3 DI Ru/Al2O3 SEA

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

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

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

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 Rigaku MiniFlex II with a high sensitivity Si slit detector

2.8 mL metal solution with pH=11.5 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)

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

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)

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?

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

CEDI with Pd(NH3)4Cl2 unwashed washed 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

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

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

Summary of metal particles

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

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.

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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