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 SO(2) 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 SO(2) 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 SO(2). Upon heating the quantitative analysis of the data indicates desorption of SO(3) and formation of atomic sulfur. On the oxygen precovered surface small amounts of SO(3) are formed already upon SO(2) adsorption at low temperatures. Upon heating stepwise oxidation of SO(2) to SO(3) and, eventually, to SO(4) is found. Two different SO(4) species were detected, which are assigned to SO(4) bound in the proximity of or remote from oxygen adatoms, according to core level shift estimates.