Catalysts and Electrodes

Binder et al. (1965) tested noble metals (Ru, Rh, Pd, Os, Ir, Pt) and their alloys as half-cell electrodes in alkaline and acidic electrolytes. In potassium hydroxide (KOH, 5 M = 5 N), the most active Raney-type catalyst at 50 mA/cm2 and 80°C was Pt, followed by Pd, Ru and Rh, Ir, and Os. The performance was different when using sulfuric acid (H2SO4, 2.25 M = 4.5 N), with the most active Raney-type catalyst being Os, followed by Ru and Ir, Pt, Rh, and Pd (50 mA/cm2 and 80°C). The polarization of the Pd electrode in acid could be reduced by alloys, and the addition of Ru (50 wt%) caused a drop in polarization from 800 (Pd) to 459 mV (PdRu alloy) at 25°C and from 570 to 290 mV at 80°C. Platinum alloyed with ruthenium gave the best performance of all alloys in H2SO4, having a polarization of 230 mV at 50 mA/ cm2 and 80°C. In a test of durability for 600 hours, the PtRu in acid supplied 2000 mA/cm2 with a constant polarization of about 420 mV. (In a 1968 paper, Liebhafsky and Cairns noted that the metal loading was nearly 200 mg/cm2, a detail covered in an earlier paper by the research group.) Binder et al. hypothesized that the high activity of the alloy was attributable to the magnetic susceptibility that allowed the optimum sorption of all reactants, noting, though, that magnetic susceptibility data were unavailable on the catalysts. However, they acknowledged that the methanol oxidation mechanism was still unclear.

The methanol oxidation mechanism on platinum in acid was reviewed by McNicol (1981). It involves adsorption of the methanol on the platinum catalyst, dehydrogenation, and oxidation of the molecule into carbon dioxide. The intermediate step of dehydrogenation produces a molecule that is a poison to the catalyst, slowing down the next step of oxidation. Therefore, to improve the performance of the catalyst, it should have improved resistance to poisoning by the dehyrogenized molecule or have improved oxidation ability of the molecule.

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