Nucleation of Surface Platinum Oxide on Pt(111)

Understanding the growth and properties of oxygen phases on platinum surfaces is fundamental to many heterogeneous catalytic reactions of industrial importance. At pressures of commercial applications, O exists on Pt surfaces in a variety of states or phases, intermediate between a chemisorbed layer and the bulk oxide(s). Substantial motivation for understanding the properties and development the different O phases derives from different O phases having distinct catalytic properties. An understanding of conditions leading to different O phases, and interactions of O phases with reactant and product molecules could more accurately describe and even predicting the behavior of Pt catalysts. The ability to control the formation, size and spatial separation of oxides is important for improving Pt and other oxidizing catalysts.

Molecular O adsorbs on the Pt(111) surface below 160K, but dissociates at higher temperatures forming a p(2x2) adlayer structure with O atoms occupying fcc three-fold hollow sites. The surface saturates at a coverage of 0.25ML in ultra high vacuum, due to strong lateral repulsive interactions between the O atoms. Higher chemisorbed O coverages (0.25-0.75 ML) or oxide overlayers (0.75 ML and higher) can be obtained either by annealing in \ce{O2} or by utilizing ozone, atomic O, or \ce{NO2}. Since oxidizing NO to \ce{NO2} is of important interest to our group, understand how O occupies different surface sites (fcc and hcp hollow sites) on Pt(111) at intermediate coverages in the range of 0.25-0.75 ML could further enhance our NO oxidation studies.

The occupation of the hcp three-fold hollow site becomes thermodynamically favored at coverages above 0.42 ML and agree with experimental results indicating the occupation of fcc only sites at lower coverages. When the 1 nearest neighbor fcc-hcp pair is occupied, surface Pt atoms buckle up into the O layer to screen repulsive O-O interactions. The buckled Pt becomes geometrically and electronically similar to Pt atoms in bulk Pt oxides, further supporting the formation of surface oxide chains. Combining \ extit{ab initio} Density Functional Theory calculations and cluster expansion models we find new thermodynamically stable ground state structures with an underlying p(2x1) fcc O pattern consistent with experimental low-energy electron diffraction patterns.

The diffusion of O across the Pt(111) surface is mostly independent of surface O coverage and configuration, both in terms of energetics and diffusion barriers. However the buckling of surface Pt atoms highly depends on local surface coverage and local O configuration. The strong repulsive O-O interactions within the chemisorbed layer at moderate (0.25-0.75ML) coverages may act to overcome kinetic barriers and drive the structural changes promoting surface Pt oxide formation in continuous chains. In agreement with literature values and experimentally observed results, the barriers to buckle become lower with increased oxygen coverage. Our analysis of the energetics and kinetics reproduces the characteristics needed for describing experimentally observed temperature program desorption data.