(658e) Development of Electroless Deposition Bath for Pt-Ru System and Characterization for Pt-Ru Bimetallic Catalysts

Development of electroless deposition bath
for Pt-Ru system and characterization for Pt-Ru bimetallic catalysts

Weijian Diao, John Meynard M. Tengco, John
R. Regalbuto and John R. Monnier

Department of Chemical Engineering,
University of South Carolina, Columbia SC 29208 USA

Bimetallic catalysts have replaced monometallic
catalysts for a wide range of catalytic applications. Bimetallic catalysts
often exhibit enhanced selectivity, stability, and/or activity relative to
their corresponding monometallic components. Many different methodologies are
used for bimetallic catalysts preparation [1]. However, the typical methods of
co-impregnation and successive impregnation have poor control of metal-metal
interaction and surface composition. Electroless Deposition (ED) has been used
for the preparation of true bimetallic catalysts with controlled, bimetallic
surface compositions [2, 3]. We have recently studied platinum-ruthenium
bimetallic catalysts that are widely used in fuel cells and bio-mass conversion
[4]. My research has focused on the preparation and characterization of both platinum
deposited on carbon-supported ruthenium catalysts (Ru@Pt/C) as well as Ru
deposited on carbon-supported platinum catalysts (Pt@Ru/C).

Eletroless deposition is the catalytic or
autocatalytic process for deposition of metals by the pre-existing metal
(catalysis) or the metal which is being deposited (auto-catalysis). Figure 1
shows a typical electroless deposition process.

Two series of Pt@Ru/C and Ru@Pt/C bimetallic catalysts
have been prepared by electroless deposition (ED) method.  For Pt@Ru/C
preparation, a new ED bath was developed using Ru(NH₃)₆Cl₃
as Ru precursor and HCOOH as reducing agent. Temperature and pH effects were
studied by varying temperature from 70°C to 130°C and pH from 2 to 4. A
deposition temperature of 110°C (to minimize effects of CO poisoning on Pt
surface during deposition) and pH 3 (to avoid strong electrostatic adsorption)
were chosen to synthesize Pt@Ru/C catalysts with variable and controlled Ru weight
loadings. For Ru@Pt/C preparation, a standard bath using H₂PtCl₆
and DMAB as Pt precursor and reducing agent, respectively, was employed. 
Several Ru@Pt/C catalysts with different Pt weight loadings were synthesized by
controlling initial Pt concentrations in the ED bath at the preferred
conditions of 70°C and pH 10.

The Pt@Ru/C and Ru@Pt/C bimetallic catalysts have been
characterized by temperature programmed reduction (TPR), selective
chemisorption, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction
(XRD) and scanning transmission electron microscopy (STEM). TPR data showed
that for Ru@Pt/C catalysts, where Ru was the major component, the peak for the
reduction of oxygen pre-covered Ru shifted from 180°C (for monometallic 20 wt%
Ru/C) to temperatures between 60°C and 100°C.  However, for Pt@Ru/C
catalysts, where Ru was the minor component, TPR spectra resembled that for
monometallic 20 wt% Pt/C; both oxygen-covered Pt and Ru surface sites underwent
reduction at 40°C. Selective chemisorption (H₂ titration of oxygen
pre-covered surfaces) experiments also confirmed the existence of strong
surface interactions between Pt and Ru, which are explained as hydrogen
spillover (Pt-assisted reduction of oxygen pre-covered Ru). XPS analyses in
Figure 2 showed that binding energies (BE) shifted to lower values for the Ru
3d_5/2 peak, and to higher values for the Pt 4d_7/2 peak.
The directions of the binding energy shifts indicate e^- transfer
from Pt to Ru on the bimetallic surface, again indicating strong surface
interactions between Pt and Ru. There were no obvious differences between the
XRD patterns in Figure 3 for the ED catalysts and their corresponding base
catalysts, revealing that deposition of the second metal by ED bath formed only
thin overlayers of the secondary metal, and not three-dimensional
aggregates.  In addition, the peaks observed in the XRD patterns were not
shifted relative to the standard positions of the primary metals; the similar
lattice parameters remain the same, suggesting no alloy formation. Finally, The
STEM and XEDS images provided strong, visual evidence of targeted deposition of
the secondary metal on the primary metal. The XEDS images confirmed that individual
nanoparticles of the catalysts prepared by ED were bimetallic, with excellent
association between the primary and the secondary metals.  No monometallic
Pt or Ru particles were detected for either of the families of bimetallic
particles.

References

[1].
Regalbuto, J.R. in "Catalyst Preparation: Science and Engineering" CRC Press,
Boca Raton, 2007.

[2].
Schaal, M.T., Pickerell, A.C., Williams, C.T., and Monnier, J.R. J. Catal. 254,
131 (2008).

[3].
Zhang, Y., Diao, W., Williams, C.T., and Monnier, J.R. Appl. Catal. A 469, 419
(2014).

[4].
Hamnett, A. Catal. Today 38, 445 (1997).