Environment-Dependent Oxygen Adsorption on Transition Metal Surfaces and Its Implications for Surface Reactivity

Ye Xu (CNMS Staff), Rachel Getman, and William F. Schneider
(CNMS Users, University of Notre Dame)

Achievement
With the rapid development of theoretical catalysis, the understanding of the surface reactivity of transition metals has been steadily expanding in recent years. For instance, evidence has accumulated that the surface coverage of reaction intermediates can affect not only the energetics but also the mechanism of a catalytic reaction. In this work, the interaction between oxygen and Pt(111) is used as a model system to demonstrate the thermodynamics that determine the coverage of oxygen. The most stable arrangements of O atoms are found for a number of coverages by searching many different configurations. Four adsorption regimes are identified, each spanning a range of coverages and having a distinct differential adsorption energy (the energy of adsorbing one extra O atom) that becomes weaker with increasing coverage. The chemical environment that can induce various oxygen coverages are investigated using first-principles thermodynamics (FPT). In particular, several gas-phase oxidants, including O₂, NO₂, and O₃; a mixture of O₂ and a reductant, NO; and a solid oxide (ceria-zirconia) are considered as the sources of oxygen. Phase diagrams that allow the determination of surface oxygen coverage at different temperatures and pressures are constructed for each of the oxygen sources.

Significance
The adsorption strength of an O atom is directly linked to its reactivity, because the activation barrier of a more strongly adsorbed O atom for further reaction tends to be higher. By demonstrating that distinctly different adsorption energy regimes exist even for the same adsorbate (O) on the same surface (Pt(111)), we have provided a theoretical explanation for the observed, counter-intuitive phenomenon that certain platinum group metal-catalyzed reactions proceed only when the catalyst surfaces are saturated with oxygen (e.g., NO oxidation). Previous applications of FPT only involve non-reacting gas-phase species in equilibrium with their respective surface counterparts. We have expanded the framework to include reactive gas phase as well as partially constrained gas-surface interaction, i.e., when the recombinative desorption of oxygen is kinetically hindered. Systematic theoretical underpinnings are demonstrated for the first time for two experimentally well-known phenomena: 1) NO₂ and O₃ can access higher surface oxygen coverages than is possible by O₂, and 2) ceria-zirconia functions as an oxygen storage/buffer material. This work thus provides an improved framework that incorporates often-overlooked environmental effects, and facilitates more accurate, in-depth analysis of transition metal-catalyzed reactions in future studies.

Complete details can be found in R.B. Getman, Y. Xu, and W.F. Schneider, “Thermodynamics of Environment-Dependent Oxygen Adsorption on Pt(111),” J. Phys. Chem. C, 112 (2008) 9559 (cover article).

Part of the work was performed at the Center for Nanophase Materials Sciences, ORNL, which is sponsored by DOE Division of Scientific User Facilities. Work at the University of Notre Dame was supported by the University of Notre Dame and by DOE Office of Basis Energy Sciences under the grant DE-FG02-06ER15830-001.

Temperature-pressure phase diagrams showing equilibrium oxygen coverages on Pt(111) arising from different oxygen sources: (a) O₂; (b) O₃; (c) NO₂.