TERRACE WIDTH EFFECT ON ADSORBATE VIBRATIONS - A COMPARISON OF PT(335) AND PT(112) FOR CHEMISORPTION OF CO

The adsorption of CO on the stepped Pt single crystals Pt(335) (four-atom wide terraces) and Pt(112) (three atom wide terraces) has been studied by infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD). Pt(335) and Pt(112) have similar terrace and step site structures (terrace-(111); step-(100)). Similar CO adsorption and desorption kinetics related to step and terrace sites were observed in the TPD studies from the two crystals. In contrast, IRAS shows different CO adsorption behavior. On Pt(335), four absorbance bands of CO were observed at high coverages. They are terminal CO on the step and terrace sites (nu(co) approximate to 1900 cm(-1)), and bridge CO on the step and terrace sites (nu(co) approximate to 1800 cm(-1)). In contrast, on Pt(112), step bound and terrace bound terminal CO can not be resolved easily by infrared spectroscopy. In addition, on Pt(112): based on relative IRAS intensities, only a small surface coverage of bridge CO develops compared to Pt(335), Rather different properties of Pt(335) and Pt(112) single crystals which expose four and three-atom wide terraces, respectively, are thus revealed by IRAS studies using CO as a probe. These differences result from a number of factors including the electrostatic field difference, the terrace width difference, and the differences in average CO CO coupling distances for CO molecules on step and terrace sites. The Pt(335) crystal with the wider terraces more closely resembles Pt(111) than the more highly stepped Pt(112) crystal, insofar as CO chemisorption is concerned. The results give insight into the properties of the step defect sites and the effect of step spacing on adsorbates.