Comparison of thermal and photochemical behavior of O2 chemisorbed on Pt(335)

The thermal and photochemical behavior of O2 chemisorbed on a stepped Pt(335) surface have been compared using temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Molecular O2 adsorbs preferentially at 85 K on step sites, followed by adsorption on terrace sites. On heating below 250 K, depopulation of terrace sites occurs first followed by depopulation of step sites. Both desorption and dissociation of O2(a) occur below 250 K. Photolysis of O2 occurs preferentially on step sites, with a cross section ≈3×10−21 cm2. The chemisorbed O(a) produced by either thermal dissociation or photodissociation of O2(a) is identical, exhibiting a Pt–O stretching mode at 480 cm−1 and a frustrated translation mode at 330–350 cm−1. Two regimes of photodepletion are observed, dominated by photolysis of step bound-O2 (regime I) and of terrace bound-O2 (regime II). Regime II is characterized by the presence of interactions of photoproduced electronegative O(a) with remaining O2(a), causing an increase of the O–O mode frequency due to electron withdrawal from the O2(a)π* orbital.