SO2 Adsorption on Pt(111) and Oxygen Precovered Pt(111): A Combined Infrared Reflection Absorption Spectroscopy and Density Functional Study

We have performed a combined molecular beam (MB)/infrared reflection absorption spectroscopy (IRAS) and density functional (DF) study on the adsorption of sulfur dioxide (SO2) on clean and oxygen precovered Pt(111). The adsorbate species formed as a function of coverage and their structural and chemical transformations upon subsequent annealing were followed systematically by IRAS. It is shown that, upon adsorption on clean Pt(111) at 100 K, SO2 adsorbs molecularly in two geometries. Calculations reveal that these geometries correspond to molecules oriented parallel or perpendicular to the surface. Upon heating up to 250 K, conversion between the two species occurs with the fraction of upright standing SO2 increasing with increasing temperature. Between 300 and 350 K, all surface species desorb without decomposition. This is in sharp contrast to oxygen precovered Pt(111), on which, in addition to the formation of molecular SO2 in flat and perpendicular geometry, partial formation of SO3 is observed even at low temperature. Upon annealing, part of the surface SOx species are converted to SO3 and, subsequently, to SO4, which remains stable up to 450 K. DF calculations on the stable adsorption structures of SOx species provide characteristic IR frequencies, which can be correlated with the present IRAS data and photoelectron spectroscopy (PES) data previously published.(1)