1. Supported, Homogeneously Alloyed Bimetallic Nanoparticles By Electrostatic Adsorption
Andrew P. Wong, Qiuli Liu, John R. Regalbuto
The University of South Carolina
AIChE Conference 2016

2. Introduction
The addition of a secondary metal can significantly improve the performance of a monometallic catalyst
In the cases were alloyed formations are preferred, limited synthesis techniques are available
Difficult to synthesize
Limited bimetallic interactions
Low dispersion
Application of Strong Electrostatic Adsorption to bimetallic systems by the co-adsorption of two metals

3. Motivation
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Furfuryl AlcoholCyclopentanone
co-SEA catalysts have a higher dispersion after ageing treatments
co-SEA catalysts have 8X higher activity and conversion of furfural compared to DI (poorly-mixed) and ED (core-shell) compositions

4. Strong Electrostatic Adsorption
- 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 OH
anionic complexes:
[PtCl6]2-
[PdCl4]2-
pH<PZC
OH2+
By varying the pH of the solution in contact with support, the interaction between precursor and support is promoted. The acidity or basicity of the solution can charge the surface of the support by protonation/deprotonation and a metal precursor complex of the opposite charge can then be electrostatically adsorbed on that charged support surface. By optimizing the condition to obtain the strongest interaction between support and precursor, where maximum uptake of precursor occurs, the migration of metal is lessened during thermal treatment, which results in smaller catalysts.
Increasing catalyst activity by enhancing dispersion would only be effective to a certain extent. Size dependence of reactions can be encountered. By adding a second metal component, a bimetallic catalyst can be made (NEXT SLIDE)
- decreased mobility of metal atoms result in smaller catalyst particles (compared to simple impregnation)
support
H2O
- resulting close packed monolayer of ionic complex (retaining hydration sheaths) with strong interaction with support
H2 , Δ M0
support
H2O
- resulting smaller catalyst particles and close intimacy between two metal particles
Mx-My
Mxn+
Myn+
Mxn+

5. Materials/Methods
Evonik AEROSIL® 300 fumed silica BET Surface Area: 280 m2/g PZC: 3.4 Surface Loading of support in solution : 1000 m2/L Cationic ammine metal precursors used NH4OH and HNO3 were used to adjust the pH Metal concentrations measured by ICP-OES TPR analysis with an Micromeritics ASAP 2920 XRD analysis with Rigaku MiniFlex II

6. Uptake Surveys
Pt-Pd
Pd-Cu
Ni-Co
Uptake surveys were performed for all combinations of Pt, Pd, Co, Cu, and Ni to determine adsorption behavior
Electrostatic adsorption was observed for noble-noble, noble-base, and base-base metal combinations
Maximum adsorption density was 1.2 umol/m2 for this series shown.

7. TPR
Peak shift observed for the alloys indicate an alloy formation and spillover of H2 to the secondary metal.
More peaks in the DI catalysts suggest differ reducible species
Lower reduction temperature of the DI catalysts indicates less metal support interaction and bigger particles

8. TPR SEA catalysts show stronger binding to the surface
Smaller nanoparticles are more difficult to reduce (>Treduction)
co-DI catalysts showed various compositions on the surface indicating poor alloy formation and incomplete mixing

9. X-ray Diffraction (XRD)
XRD was used to determine the particle size and the degree of mixing of the bimetallic catalysts

10. Pd-Cu Characterization
SEA
XEDS Scans #
Cu wt%
Pd wt% 1
20.7
79.3 2
14.9
85.1 3
31.6
68.4 4
31.0
69.0 5
31.4
68.6 6
19.8
80.2 7
33.4
66.6
10 nm
SEA
5 nm Pd Cu
Pd-Cu
Cu-Pd
SEA
dn= 1.1±0.2 nm
SEA catalysts are highly dispersed, dn =1.1 nm
SEA particles are bimetallic and homogenous
DI catalysts show clear XRD peaks with different Pd-Cu phases

11. Pd-Co Characterization
SEA
SEA
dn= 1.0±0.2 nm
Pd-Co
Pd-Co Pd Co Pd
5 nm DI DI
10 nm
20 nm
XEDS Scans #
Co wt%
Pd wt% 1
66.6
33.5 2
74.4
25.6 3
58.1
41.9 4
13.6
86.4 5
57.9
42.1 6
43.8
56.2 7
35.0
65.0
SEA catalysts are highly dispersed, dn = 1.0 nm
DI catalysts show clear XRD peaks indicating larger particles
DI catalyst have a number of very large particles in the STEM images
SEA particles are bimetallic, but vary slightly in composition

12. Pd-Ni Characterization
SEA
SEA
dn= 1.1±0.2 nm Pd Ni Pd
5 nm DI DI DI
10 nm
20 nm
SEA catalysts are highly dispersed, dn = 1.1 nm
DI catalysts show clear XRD peaks indicating larger particles
DI catalyst have a number of very large particles in the STEM images
EDXS line scan shows bimetallic composition Pd Ni

13. Al2O3 and Carbon We can also extend co- SEA to other supports silica
alumina
ox-carbon
Other mixed-oxides
Avoid the agglomeration often observed in DI catalysts
X-rays indicate well mixing and alloy formation
Cho, H., Regalbuto, J. Catalysis Today 246 (2015) 143–153

14. Summary
co-SEA can be used for many bimetallic systems over various support materials
co-SEA catalysts were smaller in size and more homogenous than the DI catalysts
TPR
XRD
STEM
Preliminary chemical reactions favored SEA catalysts

15. Questions? Acknowledgements
Center for Renewable Fuels and The University of South Carolina for funding.
Dr. Alan Nichols at The University of Illinois for the electron microscopy images
Regalbuto group at The University of South Carolina.
Questions?

16. Outline Introduction/Motivation SEA technique
Co-adsorption Uptake Surveys
Temperature Programmed Reduction (TPR)
XRD/STEM/MAPS
Summary