Bifunctional Mechanism of CO Tolerance

Despite the wealth of research in this area, the reasons for the effectiveness of Pt-Ru over other alloys are still hotly debated. Two widely accepted mechanisms for the oxidation of CO from the Pt-Ru surface exist [31, 106-109] (1) the 

To remove COads from the surface it is necessary to generate some oxygenated species that can react with COads to produce CO2 and release some free sites on Pt surfaces for the hydrogen oxidation reaction. The mechanism for the oxidative removal of the COads from platinum anodes has been a topic of intense investigation for the past 40 years. The overall reaction for removing COads is 

The mechanism of hydrogen oxidation in the H2/CO mixtures on platinum has been given by Equations 16.2-16.6. 

Hydrogen oxidation reaction occurs on the free sites liberated during the time between COads oxidative removal, Equation 16.4, and CO re-adsorption from solution. Equation 16.2. At the low potential (E 0.6 V) the rate constant of COb re-adsorption is much higher than the rate constant for COads oxidation, and in practical terms only an infinitely small number of platinum sites could be liberated for H2 oxidation. 

Based on early work by Watanabe and Motoo [89] (originally on the enhanced oxidation of CO and methanol at Pt electrodes with various oxygen-adsorbing adatoms [34, 39, 81-83, 90, 93, 94]), a bifunctional mechanism involving water activation by Ru (Equation 16.11) and subsequent CO electrooxidation on a neighboring Pt atom (Equation 16.12) has been postulated by a number of groups [20, 102,103, 110-112]. 

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