CO adsorption and oxidation on Pt and PtRu alloys: dependence on substrate composition

Adsorption and electrooxidation of CO on polycrystalline Pt and PtRu alloys (Pt_0.5Ru_0.5, Pt_0.7Ru_0.3) has been studied by means of in situ infrared (FTIR) spectroscopy and differential electrochemical mass spectrometry (DEMS). Within the resolution limits of this study (ΔR/R ≈ 10^-3) only one adsorbed CO(a) species at ν ≈ 2070-2075 cm^-1 (Pt) and 2050-2060 cm^-1 (PtRu) was detected. No significant difference in CO frequency between Pt_0.5Ru_0.5 and Pt_0.7Ru_0.3 was found. The oxidation of CO adsorbed in the hydrogen region on PtRu shows distinct differences when compared with a polycrystalline Pt electrode. Whereas on Pt the oxidation of CO proceeds in two stages, on PtRu cyclic voltammetry shows only one oxidation peak at a less positive potential compared to Pt. The lowest peak potential for CO adsorbate oxidation was observed on Pt_0.5Ru_0.5 at about 380 mV vs. rhe. The results for CO oxidation obtained on Pt and Pt_0.5Ru_0.5 deposited onto a porous Au substrate by on line mass spectrometry were in accordance with the ir spectroscopic data. It was shown that Ru atoms at the surface of a PtRu bimetallic electrode increase the CO tolerance of the catalyst. The promoter effect of Ru is explained with the bifunctional mechanism for CO oxidation where oxygen containing species are bounded preferentially to ruthenium surface atoms.