Self-directing organometallic deposition of ruthenium: Preparation, characterization and evaluation of bimetallic, ruthenium adatom modified platinum surfaces for the electrooxidation of methanol in direct methanol fuel cells

State of the art direct methanol fuel cells use Pt-Ru particles as anode electrocatalysts for the oxidation of methanol. There are two longstanding difficulties with evaluating such catalysts; first, there are no proven methods to measure their specific surface areas (number of active sites); and second, there are no proven methods to measure the surface ratio of Pt to Ru on black (rough) Pt-Ru surfaces or particles. Further, the reported methods to prepare such catalysts do not have demonstrable control over these parameters, and surface segregation may occur during the high temperature stages of the preparation. As a result of these difficulties, the optimum surface composition and chemical state of such catalysts is still under investigation nearly 40 years after their discovery. An organometallic chemistry approach to this problem was chosen. It was found that black Pt (as a gauze or as a powder) effects the hydrogenation (1 atm, −40°C, in hexanes) of (1,5-cyclooctadiene)Ru(C ₃H₅)₂ to result in Ru adatoms (Ru_ad) adsorbed by the Pt surface and the concomitant formation of cyclooctane and propane. By monitoring the reaction at low temperatures using gas chromatography or UV-Vis spectrophotometry, real time control over both the number of surface equivalents of Ru_ad deposited on Pt, and over the activity of the resulting Pt-Ru_ad surfaces was realized. This reaction was used to prepare a series of blacked Pt-Ru_ad gauzes and black Pt-Ru _ad powders of reasonably defined surface areas and surface compositions. The activities of these gauzes and powders towards the electrooxidation of methanol under typical operating conditions for a direct methanol fuel cell were determined. Remarkably, the black Pt-Ru_ad powders were stable towards hot pressing into Nafion® membranes and towards use in prototype direct methanol fuel cells operating at 90°C. The optimum surface composition for operation in direct methanol fuel cells under these conditions was determined.