(701c) Preparation of Pt-Ru Bimetallic Catalysts Using Electroless Deposition Methods and Applications for Upgrading of Biomass and Direct Methanol Fuel Cells

Preparation
of Pt-Ru bimetallic catalysts using electroless deposition methods and applications for
upgrading of biomass and direct methanol fuel cells

Weijian Diao, John Meynard M. Tengco, Taylor R. Garrick, John W. Weidner, John R. Regalbuto and John R. Monnier

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

Monometallic catalysts have been replaced by
bimetallic catalysts for a wide range of catalytic applications. Bimetallic
catalysts often exhibit enhanced selectivity, stability, and/or activity
relative to their corresponding monometallic components. Such catalysts may be
prepared using many different methodologies [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 expanded our program to include
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 Ru deposited on carbon-supported platinum catalysts
(Ru-Pt/C) as well as platinum deposited on carbon-supported ruthenium 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.

In catalyst synthesis part of my research, formic
acid (FA), hexammineruthenium(III) chloride (Ru(NH₃)₆Cl₃)
and commercially available 20 wt% Pt/C
were used for the preparation of Ru-Pt/C 
catalysts.  This ED bath used
formic acid as the reducing agent (which was catalytically activated on the Pt
surface) and Ru(NH₃)₆Cl₃
as the reducible Ru salt. The working pH of this bath was maintained below pH
4.8, the measured point of zero charge of the Pt/C catalyst to avoid strong
electrostatic adsorption of Ru(NH₃)₆³⁺
on the carbon surface. Dimethylamine borane (DMAB), chloroplatinic
acid (H₂PtCl₆) and commercially available 20 wt% Ru/C) were used for the preparation
of Pt-Ru/C bimetallic catalysts. 
This ED bath used DMAB as the reducing agent and H₂PtCl₆
as the reducible Pt salt. By controlling different reaction temperatures and
initial metal salt concentrations, a series of different weight loading and
coverage Pt-Ru/C and Ru-Pt/C
catalysts were synthesized, which are shown in Figure 2. 

The catalysts prepared in this study are summarized
in Table 1. Temperature Program Reduction (TPR) of O-precovered
surfaces was used to infer the surfaces of the bimetallic catalysts and
selective chemisorption was used to measure the concentration of surface sites
for both monometallic catalysts. The TPR results for a select number of the
Ru-Pt/C and Pt-Ru/C catalysts are shown in Figures 3 and 4, respectively.
Reduction of Pt-O sites occurs rapidly at 40°C, while temperatures of
approximately 180°C are required to reduce Ru-O sites. For the bimetallic
surfaces, Pt-assisted reduction of Ru-O occurs, with higher atomic ratios of
Pt/Ru giving lower reduction temperatures. The absence of Ru-O reduction peaks
at 180°C confirms the existence of Pt-Ru bimetallic surfaces, and not separate
Ru and Pt particles.

The Ru-Pt compositions
have also been examined for direct methanol fuel cell (DMFC) activity and the
results are shown in Figure 5. The Ru-Pt/C catalysts prepared by ED have much
higher activity than commercial Ru-Pt/C composition
of 6.8 wt% Ru-13.2 wt% Pt/C. The optimized composition is 1:1 atomic ratio for Pt and Ru.

The
next part of my research will be testing Pt-Ru/C and Ru-Pt/C bimetallic
catalysts prepared by electroless deposition for biomass upgrading. The initial
reaction is levulinic acid hydrogenation.

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]. Rebelli, J., Rodriguez, A.A., Ma, S.G.,
Williams, C.T., and Monnier, J.R. Catal.
Today 160, 170 (2011).

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