Model electrodes for electrocatalysis : Ultrathin palladium films on Pt(111)

In this work the electrocatalytic activity of a bimetallic model catalyst, a pseudomorphic monolayer of palladium supported on Pt(111), is characterized. The reactions probed, which are the carbon monoxide oxidation reaction, the oxygen reduction reaction, the hydrogen evolution as well as the hydrogen oxidation reaction, and the oxidation of small organic molecules, are all strongly related to low temperature fuel cells. It is shown that in alkaline solution as well as in perchloric acid solution the CO oxidation reaction is inhibited on the palladium film compared to bare Pt(111). For alkaline solution this finding is attributed to a stronger interaction of OH- anions with the palladium film, whereas in perchloric acid solution specific adsorption of chloride anions blocks the active sites of the catalyst. For the oxidation reduction reaction in alkaline solution an exceptionally high catalytic activity is found for the thin palladium film. For the first time an increase in the catalytic activity towards this important reaction is found by modifying the highly active Pt(111) surface. For the hydrogen oxidation reaction a high catalytic activity is found as well. It is shown that for both reactions the catalytic activity of the model catalyst in alkaline solution is dependent on the same properties. The electrooxidation of formic acid is proved to proceed via two different reaction mechanisms with the faster pathway prevailing at lower temperatures