Adsorption and reaction of SO2 on clean and oxygen precovered Pd(100)-a combined HR-XPS and DF study

We studied the adsorption and reactivity of SO2 on clean and oxygen precovered Pd(100) with high resolution X-ray photoelectron spectroscopy and density functional calculations. Upon adsorption at 120 K two different SO2 species were detected, which were identified as upright-standing and flat-lying molecules by comparing the calculated core level shifts. In agreement with the relative stabilities determined by the calculations the intensities of the photoelectron spectra indicate that the majority species are upright-standing SO2. Upon heating the quantitative analysis of the data indicates desorption of SO3 and formation of atomic sulfur. On the oxygen precovered surface small amounts of SO3 are formed already upon SO2 adsorption at low temperatures. Upon heating stepwise oxidation of SO2 to SO3 and, eventually, to SO4 is found. Two different SO4 species were detected, which are assigned to SO4 bound in the proximity of or remote from oxygen adatoms, according to core level shift estimates.