Sensitivity of Compressed Carbon Monoxide Adlayers on Platinum(111) Electrodes to Long-Range Substrate Structure: Influence of Monoatomic Steps

Abstract

Infrared spectra are reported for saturated CO adlayers on highly ordered Pt(111) and related stepped surfaces in aqueous 0.1 M HClO4 with the objective of ascertaining the importance of long-range substrate structure to the surface-binding configurations of the compressed adlayers. On Pt(111) in the presence of CO in solution, a pair of CO-stretching (νCO) bands are obtained throughout the accessible potential region below the onset of adsorbate electrooxidation that are diagnostic of atop/3-fold hollow coordination, fingerprinting the (2 × 2)-3CO (θCO = 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 transition was seen to be triggered here only by the onset of adsorbate electrooxidation. The corresponding spectral fingerprint obtained in the absence of CO in solution (i.e., for the irreversibly adsorbed case) indicates a mixture of 3-fold and 2-fold bridging along with atop CO, suggesting the presence of less compressed adlayer domains, consistent with the known lower packing densities (θCO ≈ 0.65 to 0.7). The introduction of even occasional (110) or (100) steps, specifically for Pt(17,17,15) and -(17,15,15) (i.e, n(111) × (110) and n(111) × (100) where n = 16], yields significantly different νCO 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 νCO 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 νCO features associated with the (111) terraces, bridging νCO bands indicative of bonding on the (100) step sites becoming prevalent. The importance of long-range substrate order in determining the compressed CO adlayer arrangements on Pt(111) terraces is assessed in the light of these findings.