Oxidation of Pt(1 1 1) by gas-phase oxygen atoms

Abstract

The oxidation of Pt(1 1 1) by gas-phase oxygen atoms was investigated in ultrahigh vacuum using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (ELS) and low energy electron diffraction (LEED). Atomic oxygen coverages as high as 2.9 ML (monolayers) could be generated on Pt(1 1 1) using an atomic oxygen beam, and both the adsorption of oxygen atoms and the progression of surface oxygen phases with increasing atomic oxygen coverage are found to be relatively insensitive to the surface temperature over the range from 250 to 450 K. The results show that oxidation involves the development of a chemisorbed layer for oxygen coverages up to about 0.75 ML, and that the average binding energy of an oxygen atom chemisorbed on Pt(1 1 1) decreases significantly (∼100 kJ/mol) with increasing oxygen coverage, in agreement with previous observations [D.H. Parker, M.E. Bartram, B.E. Koel, Surf. Sci. 217 (1989) 489, N.A. Saliba, Y.-L. Tsai, C. Panja, B.E. Koel, Surf. Sci. 419 (1999) 79]. Long-range order in the chemisorbed layer generally diminishes as the oxygen coverage increases above the 0.25 ML saturation coverage of the p(2 × 2) layer, though the persistence of a (2 × 2) LEED pattern up to about 0.50 ML is consistent with the formation of domains of a new, high-density ordered phase. Disordering within the chemisorbed layer becomes more pronounced with continued atomic oxygen adsorption to coverages greater than 0.50 ML. Distinct features in the O 2 thermal desorption traces at 650 K and 560 K are attributed to the desorption of oxygen from a high-density ordered phase and disordered domains, respectively, which suggests that the binding energy is lowest for oxygen atoms chemisorbed in the disordered domains. Increasing the atomic oxygen coverage above approximately 0.75 ML is shown to result in the growth of Pt oxide particles and disordering of the Pt surface. Decomposition of the Pt oxide particles produces explosive desorption of O 2 that is characterized by a shift of the desorption peak to higher temperature and a dramatic increase in the maximum desorption rate with increasing oxygen coverage. The main characteristics of the explosive desorption are well reproduced by a kinetic model in which oxygen atoms are assumed to migrate from oxide domains onto regions of the surface containing chemisorbed oxygen atoms, and with oxygen atoms desorbing only from the chemisorbed layer. The implication of this mechanism is that Pt oxide is more thermodynamically favorable than high concentrations of chemisorbed oxygen atoms on Pt(1 1 1), and hence that the growth of Pt oxide on Pt(1 1 1) is kinetically limited.