The Effect of Bimetallic Surface Composition for Methanol Oxidation

A series of Ru-Pt bimetallic catalysts prepared by the electroless deposition of controlled and variable amounts of Ru on the Pt surface of a commercially-available 20 wt% Pt/C catalyst has been characterized and evaluated for the oxidation of methanol at the anode of a PEM fuel cell.  The activity of each Ru-Pt catalyst was determined as a function of surface composition; all bimetallic catalysts exhibited performances greater than either of the monometallic counterparts.  For the Ru-Pt bimetallic catalysts, activity passed through a maximum at approximately 50% (atomic) theoretical, monodisperse Ru coverage. The activity of this composition for methanol oxidation was much higher than for either monometallic Pt or Ru catalysts or the commercially-available Ru-Pt/C catalyst.

                Direct methanol fuel cells (DMFC) are a good candidate for a power source in portable electronic devices for several reasons, including high energy density and ease of handling (liquid vs high pressure H₂) [1].  Platinum catalysts are effectively able to catalyze the oxidation of methanol to carbon dioxide; however, the surface Pt sites become rapidly poisoned by strongly adsorbed CO that is also produced during the oxidation process to limit the overall rate of Methanol oxidation [1-4]. Studies have shown that carbon-supported, bimetallic catalysts composed of Pt and Sn, or more preferably Pt and Ru, are more active for the oxidation of Methanol and Ethanol compared to catalysts composed solely of Pt [5-11]. Supported Pt-Ru catalysts are currently recognized as the most active catalyst formulations for methanol oxidation in acidic media [12]. The mechanism by which these bimetallic catalysts alleviates CO poisoning presumably involves the adsorption of H₂O on Ru sites to form a Ru-OH species which oxidizes CO strongly adsorbed on Pt to form CO₂ and H+. Thus, it is important for Ru and Pt to co-exist in close, proximal contact on the surface for the Ru-assisted oxidation of CO to occur. It follows that a bimetallic catalyst where the Ru is evenly distributed in a controlled amount on the Pt surface should exhibit enhanced performance.

We have shown that electroless deposition can be used to develop catalysts with higher performance than commercial counterparts on a Pt mass basis. We are able to systematically vary the surface composition as seen in Table I. ED methodologies could permit the development of bimetallic catalysts that may replace the corresponding monometallic Pt catalysts by lowering the costs of fuel cell fabrication. This is due to the fact that less Pt has to be used in a Pt-Ru catalyst compared to a monometallic Pt catalyst to obtain the same performance. Consequently, higher performance will be seen with the same amount of Pt in a Pt-Ru catalyst when compared to a monometallic Pt catalyst.

Figure 1. Direct methanol fuel cell performance of Ru-Pt/C catalysts prepared by ED and comparison to a commercial Ru-Pt/C composition of 6.8 wt% Ru-13.2 wt% Pt/C. 

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