Sensitivity of compressed carbon monoxide adlayers on platinum(III) electrodes to long-range substrate structure: Influence-of monoatomic steps

Infrared spectra are reported for saturated CO adlayers on highly ordered P t(111) and related stepped surfaces in aqueous 0.1 M HClO4 with the objecti ve of ascertaining the importance of long-range substrate structure to the surface-binding configurations of the compressed adlayers. On Pt(111) in th e presence of CO in solution, a pair of CO-stretching (nu(CO)) bands are ob tained throughout the accessible potential region below the onset of adsorb ate electrooxidation that-are diagnostic of atop/3-fold hollow coordination , fingerprinting the (2 x 2)-3CO (theta(CO) = 0.75) compressed adlayer also characterized earlier by scanning tunneling microscopy (STM). This finding differs from some earlier reports, which indicate a structural conversion to an atop/bridging CO binding arrangement at higher potentials: this trans ition was seen to be triggered here only by the onset of adsorbate electroo xidation. The corresponding spectral fingerprint obtained in the absence of CO in solution (i.e., for the irreversibly adsorbed case) indicates a mixt ure of 3-fold and 2-fold bridging along with atop CO, suggesting the presen ce of less compressed adlayer domains, consistent with the known lower pack ing densities (theta(CO) approximate to 0.65 to 0.7). The introduction of e ven occasional (110) or (100) steps, specifically for Pt(17,17,15) and -(17 ,15,15) (i.e, n(111) x (110) and n(111) x (100) where n = 16],yields signif icantly different nu(CO) spectra, especially for the irreversibly adsorbed case which indicate the predominant presence of bridging rather than 3-fold hollow CO, most clearly at higher potentials. The presence of higher (110) step densities attenuates;as well as broadens further the multifold nu(CO) bands, eventually (for n = 2) yielding a spectral fingerprint closely akin to that of Pt(110) itself. Increasing the (100) step density (to n = 5,2) also removes the multifold nu(CO) features associated with the (111) terrac es, bridging vco bands indicative of bonding on the (100):step sites becomi ng prevalent. The importance of long-range substrate order in determining t he compressed CO adlayer arrangements on Pt(111) terraces is assessed in th e light of these findings.