Cluster Expansion Studies of Oxygen Adsorption on Transition Metal Surfaces

Catalytic oxidation is of practical importance in many chemical processes. Model single crystal surfaces are often used to study catalytic oxidations; however, even single crystal surfaces do not behave as quintessential homogeneous models. Instead single crystal surfaces exhibit heterogenous catalytic behavior. One example of heterogenous catalytic behavior on a single crystal surface is the oxidation of NO to NO₂ on the Pt (111) surface.

NO + å_ O₂ → NO₂

Prior experiments and theory show that, at reaction conditions, the Pt (111) surface is covered in atomic oxygen, the coverage of atomic oxygen is set by the pressure ratio of NO to NO₂, dissociative oxygen adsorption (O₂+2^∗ → 2O^∗) is the rate determining step for NO oxidation, and atomic oxygen preferentially sits in fcc hollow sites at coverage up to half a monolayer. In this work we employ planewave, supercell DFT calculations, the cluster expansion model Hamiltonian, and Monte Carlo simulations, as well as micro-kinetic modeling to study dissociative oxygen adsorption and NO oxidation on transition metal (111) surfaces, including the Pt (111) surface.

On the Pt (111) surface, we find that electronic effects create strong first nearest neighbor repulsions between atomic oxygen adsorbates and that adsorbate induced surface strain reduces these first nearest neighbor repulsions but increase long-ranged repulsions and many-bodied interactions. Our micro-kinetic analysis, in contrast to prior theoretical papers, predicts rate orders and apparent activation energies consistent with experiment.

We apply our models to eight transition metal (111) surfaces, counting Pt (111), to gain a better understanding of catalytic phenomena. The effects of adsorbate induced surface strain are similar for all metals. Additionally, these metal (111) surfaces fall into two categories: catalytically active surfaces and catalytically inactive surfaces. The kinetic properties are insensitive to the choice of catalytically active transition metal. Oxygen on the catalytically active surfaces are disordered while oxygen on the catalytically inactive surfaces are ordered. This suggests that disrupting adsorbate ordering may activate catalysts at lower temperatures.