Direct Mechanism of CO Tolerance Ligand or Electronic Effect

4 Direct Mechanism of CO Tolerance (Ligand or Electronic Effect) 

The consequences of the ligand effect and the bifunctional mechanism for CO oxidation on Pt/Ru have been debated extensively in the literature. The emerging consensus seems to be that the ligand effect is less important than the bifunctional mechanism [106, 107, 138-142]. Some authors have attempted to quantitatively determine the relevance of the ligand effect as compared to the bifunctional mechanism, with Masel, Wieckowski, and coworkers concluding that about 20% of the enhancement in CO oxidation afforded by Ru deposition can be attributed to the ligand effect, while the remaining 80% of the enhancement is due to the bifunctional mechanism [106, 107]. 

5 Surface Science Study and Modeling of CO-tolerance Mechanism 

CO adsorption and oxidation have been studied for many years, but a greater understanding was achieved by the development of ex situ and in situ spectroscopic and microscopic methods for application in electrochemistry [9, 143-146], together with the use of well-defined nanocrystalline electrode surfaces [147]. The opportunity to study in situ electrooxidation of carbon monoxide [148-157] under fuel cell reaction conditions has brought significant progress in understanding interfacial electrochemistry on metallic surfaces, hi combination with conventional electrochemical methods these techniques have been used to find connections between the microscopic surface structures and the macroscopic kinetic rates of the reactions. 

Pt-Ru alloy [158-160] or Ru modified Pt surfaces [161] are known to be prominently effective catalysts for methanol oxidation. Watanabe and Motoo [87] introduced a bifunctional mechanism into electrochemical interfaces in that Ru in Pt-Ru alloy surfaces supplies OH moieties to promote CO oxidation on neighboring Pt surfaces. In this context, it is crucial to elucidate water or hydroxide that are anticipated to adsorb on Ru in the alloy or modified Pt electrode surfaces. Nevertheless, there have been substantial uncertainties on this point because of experimental constraints in vibrational spectroscopy relevant for this purpose, e.g., poor sensitivity or uncompensated interruption by bulk solution species in IRAS (infrared reflection absorption spectroscopy). Watanabe and co-workers recently reported that water molecules were detected on the Pt-Ru alloy surfaces, a finding 

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