Adsorption of molecular species at low and high index surface planes of platinum electrodes: In situ studies by infrared spectroscopy.

Abstract

This dissertation describes applications of infrared spectroscopy in the study of cyanide and carbon monoxide adsorption at low and high index surface planes of single crystal platinum electrodes. Experiments examined structural aspects of surface-adsorbate and adsorbate-adsorbate interactions in aqueous electrochemical environments. Cyanide adsorption studies were performed at low index surface planes of platinum (Pt(111), Pt(110), and Pt(100)) and employed a special cyanide monolayer preparation procedure which eliminated interferences from solution phase cyanide. Spectral features associated with the cyanide adlayer indicated the relationship between the metal crystallographic orientations and the cyanide coordinate environment. For the cyanide adsorbed at Pt(111), a single potential dependent spectral feature, which was ascribed to the C-N stretching vibrational mode of surface cyanide bound linearly through the carbon atom, was observed in agreement with LEED structures. For cyanide adsorbed at Pt(110) and Pt(100), an additional spectral feature appeared, which was attributed to the (Pt(CN)$\sb2$) $\sp{2-}$ complex. The formation of the complexes indicated the dissolution of surface atoms and the likelihood that surface disordering occurs under these experimental conditions. A series of studies focused on probing site-dependent aspects of adsorption by using a stable high index surface plane of single crystal platinum and carbon monoxide as a probe molecule. The surface employed was Pt(335). The coverage and potential dependence of the CO spectral features were studied as a first part of the work. The experiments defined the conditions under which vibrational bands for molecules adsorbed at step sites and at terrace sites could be detected independently. The effect of hydrogen co-adsorption was examined in subsequent experiments using intermediate CO coverages, where it was possible to discern spectral features for CO bound terminally (atop) at edge sites and at terrace sites. The results indicated that co-adsorbed hydrogen induces structural alterations of the adlayers, and that the adsorption of hydrogen occurs preferentially at step sites on the Pt(335) surface plane. In concluding studies, isotopic mixture experiments were employed to probe qualitative structural features of CO adlayers at step sites and on terrace planes of Pt(335) electrodes. From the spectral features of CO adlayers formed from $\sp{13}$CO/$\sp{12}$CO mixtures, values were determined for the dynamic dipole-dipole coupling parameter ($\Delta\nu\sb{\rm d}$) and the chemical (static-dipole) shift parameter ($\Delta\nu\sb{\rm s}$), which were used to derive information about the local adlayer structures. Values for $\Delta\nu\sb{\rm s}$ were generally small at all coverages. Values for $\Delta\nu\sb{\rm d}$ were small ($<$8 cm$\sp{-1}$) at low surface coverages, where only the edge sites were occupied, which supported a structural model of the edge-CO adlayer as a one-dimensional array of coupled oscillators. For higher coverages, where a majority of the molecules were adsorbed at terrace sites, dipole coupling parameters increase to large values ($\sim$42 cm$\sp{-1}$), which indicated increased interactions of adsorbates due to the growth of two-dimensional structures on the terrace planes. The findings presented in this dissertation have brought some understanding of processes at "real world" liquid-solid interfaces and may prove to be helpful for future studies in mechanistic surface chemistry relating to the influence of defect structures on the overall rate and specificity of chemical transformations.