Interaction of NO2 with a Pt(111) Surface

The interaction of NO2 with a Pt(111) surface was studied by molecular beam and thermal desorption techniques. At 300 K NO2 adsorbs dissociatively into NOad + Oad with an initial sticking coefficient so = 0.97. so decreases slightly with increasing surface temperature; the variation of the sticking coefficient with coverage exhibits a marked “precursor” state behavior. The rate of formation of gaseous NO is limited by desorption of this molecule, i.e. there is no evidence for a direct process NO2 ŕ NOg + Oad. At ≈ 400 K NO desorption is complete and only Oad is building up on the surface. The maximum coverage Θo,max ≈ 0.77 is much higher than that so far reported for exposure to O2 (0.25), which is ascribed to kinetic effects, i.e. a much higher sticking coefficient in the case of NO2. Two additional states with desorption energies of about 32 and 38 kcal/mol characterize the high oxygen coverage situation. Desorption of O2 starts above 450 K.