Chemisorption of high coverages of atomic oxygen on the Pt(111), Pd(111), and Au(111) surfaces

Abstract

Studies of the interaction of oxygen with some transition metal surfaces are limited by activation barriers to adsorption and reaction of O2. These barriers can be surmounted by using oxidants more powerful than O2 to chemisorb oxygen on these surfaces under ultrahigh vacuum (UHV) conditions, but at high ‘‘effective’’ pressures of O2. We report here on the use of very reactive molecules, such as nitrogen dioxide (NO2) and ozone (O3) to study the interaction of high concentrations of oxygen atoms on the Pt(111), Pd(111), and Au(111) single crystal surfaces. Chemisorbed oxygen adatom coverages of Θ0≤0.75 monolayer (ML) on Pt(111), Θ0≤1.4 ML on Pd(111), and Θ0≤0.80 ML on Au(111) surfaces have been characterized. On the Pd(111) surface, higher concentrations (Θ0≤3.1 ML) can be formed and the initial stages of oxidation, including migration of oxygen into the subsurface region and formation of Pd oxide can be studied. This method of cleanly forming high coverages of oxygen on metal surfaces allows for new insight into the nature of oxygen–metal interactions. For example, in the case of Pt(111) and Au(111), temperature programmed desorption (TPD) can be used to determine kinetic parameters of O2 adsorption and desorption as a function of oxygen coverage and upper limits for the metal–oxygen bond strengths over a large range of oxygen coverages can be determined from the activation energy of O2 desorption. Also, the kinetics of oxidation reactions can be studied over a much larger range of oxygen coverage than previously carried out under UHV conditions on surfaces that are relatively unreactive to O2 chemisorption, such as Pt(111) and Au(111). We report preliminary results concerning transient kinetic studies of CO oxidation over Pt(111) and Au(111) surfaces.