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

2. 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

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.
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

5. 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

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 Rigaku MiniFlex II with a high sensitivity Si slit detector

7. 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)

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 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

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