Adsorbate Coverage Dependence in Heterogeneous Catalysis: Kinetics of Oxygen Adsorption on Platinum Surfaces from First Principles

The rational design of more versatile, selective, and durable catalysts relies on the ability to both understand the chemistry of existing catalysts and predict the behavior of new materials. Quantum-based computational methods play an important role in this discovery by allowing accelerated screening of new materials and providing a deeper, molecular-level understanding of how chemistry happens at catalyst surfaces.

However, due to the complexity and heterogeneity of real heterogeneous catalysts, purely computational methods have difficulties predicting quantitatively accurate metrics of catalyst performance. A fundamental inconsistency exists between the kinetic models typically used to understand experimental results and the nature of first-principles data intended to parameterize such models. To overcome this 'model gap,' new kinetic models must be developed to incorporate surface heterogeneity directly into the rate expressions.

In this work, we combine DFT calculations with cluster expansion and Monte Carlo techniques to model reactivity at catalyst surfaces completely from first principles. Oxygen adsorption to transition metal surfaces, particularly platinum, is a critical step in many catalytic oxidation reactions and presents interesting modeling challenges due to the existence of strong lateral interactions between adsorbates. We first characterize the coverage-dependent adsorption thermodynamics of oxygen on the low-symmetry Pt(321) surface, identifying ground state structures and examining their relative stability when exposed to various gas-phase environments.

We then explore the coverage-dependent kinetics of O₂ dissociation on this surface in the context of Pt-catalyzed NO oxidation:
NO(g) + 1/2 O₂(g) <--> NO₂(g)
and temperature programmed desorption of O₂ from Pt:
2 O* --> O₂(g) + *

In doing so, we introduce a non-mean-field, first-principles 'reaction site' kinetic model and develop a novel combination of computational tools needed for practical application of this model. We report the first molecularly detailed, quantitatively accurate model of NO oxidation on Pt and examine how the structure insensitivity observed experimentally is tied to the coverage-induced heterogeneity in this system. We also predict oxygen TPD in agreement with experiment and provide insight into the role of adsorbate-adsorbate interactions in determining the shape and position of desorption peaks.