Nonuniform Synergistic Effect of Sn and Ru in Site-Specific Catalytic Activity of Pt at Bimetallic Surfaces toward CO Electro-oxidation

This work investigates the mechanisms by which Sn and Ru can improve or inhibit the site-specific catalytic activity of Pt (neighboring or distant from foreign atoms) at bimetallic surfaces. For this purpose, we decorated Pt stepped surfaces (nonequivalent sites) by site-selective electrodeposition of different coverages of either Sn or Ru on (110) Pt steps, forming Sn_steps/Pt(hkl) and Ru_steps/Pt(hkl) bimetallic surfaces, and we used CO adlayer electro-oxidation as a surface probe reaction, monitored by in situ FTIR and cyclic voltammetry techniques. The results showed that both Sn and Ru selectively accelerated the reaction pathway of CO electro-oxidation only at the (111) Pt terrace sites but importantly played different underlying roles in favoring activity at these active sites. In the case of Sn_steps/Pt(hkl) catalysts, the CO adlayer oxidation started at potentials lower than those on Ru_steps/Pt(hkl), but Sn only improved the activity at sites on atoms of the first rows of (111) Pt terraces, while the catalytic benefit of Ru seemed to extend further along the (111) Pt terraces. In comparison to unmodified Pt surfaces, Ru did not influence the activity at the line of the (110) Pt steps, while Sn slightly inhibited the activity there, which characterized a slight contrasting effect in catalytic activity at the (111) terraces in comparison to the (110) step sites. In this regard, the chemical modification by irreversible deposition of either Sn or Ru at lines of Pt steps on a stepped Pt surface interestingly resulted in a nonuniform synergistic effect or balancing of energies involving different site-specific catalytic activities at nonequivalent Pt surface sites. Since the electro-oxidation of CO takes place at the (111) Pt terrace sites away from Sn or Ru, and because CO_ads behaves as an immobile species during its oxidation, it is reasonable to assume that the classical bifunctional mechanism completely fails as a model to interpret the enhancement of catalytic activity toward CO electro-oxidation at Sn_steps/Pt(hkl) or Ru_steps/Pt(hkl) catalysts. The selective alteration in site-specific catalytic activity of Pt was related to (i) the type and coverage of foreign atoms (Sn or Ru) at the lines of Pt steps, (ii) the crystallographic orientation of Pt sites, whether (111) terraces or steps, and (iii) the width of the (111) Pt terraces.