Theoretical study of the O₂ interaction with a tetrahedral Al₄ cluster

Density Functional Theory for Molecule-Metal Surface Reactions: When Does the Generalized Gradient Approximation Get It Right, and What to Do If It Does Not

While density functional theory (DFT) is perhaps the most used electronic structure theory in chemistry, many of its practical aspects remain poorly understood. For instance, DFT at the generalized gradient approximation (GGA) tends to fail miserably at describing gas-phase reaction barriers, while it performs surprisingly well for many molecule-metal surface reactions. GGA-DFT also fails for many systems in the latter category, and up to now it has not been clear when one may expect it to work. We show that GGA-DFT tends to work if the difference between the work function of the metal and the molecule's electron affinity is greater than ∼7 eV and to fail if this difference is smaller, with sticking of O₂ on Al(111) being a spectacular example. Using dynamics calculations we show that, for this system, the DFT problem may be solved as done for gas-phase reactions, i.e., by resorting to hybrid functionals, but using screening at long-range to obtain a correct description of the metal. Our results suggest the GGA error in the O₂ + Al(111) barrier height to be functional driven. Our results also suggest the possibility to compute potential energy surfaces for the difficult-to-treat systems with computationally cheap nonself-consistent calculations in which a hybrid functional is applied to a GGA density.

Redefining the Mechanism of O2 Etching of Aln- Superatoms: An Early Barrier Controls Reactivity, Analogous to Surface Oxidation

New insights into aluminum anion cluster reactivity with O₂ were obtained through temperature-dependent kinetics measurements. Overall reactivity is controlled by a barrier at an avoided crossing where charge is transferred from the cluster to the O₂, mechanistically similar to what occurs as O₂ approaches a bulk Al surface. Contrary to prior interpretations, spin conservation does not inhibit the reaction of clusters with an odd number of Al atoms. In fact, the only spin constraint in these systems is on the reactivity of even clusters due to repulsive surfaces, not previously recognized. Although the superatom nature of Al₁₃^- is manifest in its high electron binding energy (EBE), the mechanism of its reactivity is not special; it reacts with O₂ as if it were a small piece of bulk Al. These experiments highlight the sensitivity of Al cluster reactivity with O₂ to temperature and EBE, uncovering routes to industrial scale use of aluminum superatom-based materials.

Dissociative Chemisorption of O2 on Al(111): Dynamics on a Correlated Wave-Function-Based Potential Energy Surface

Dissociative chemisorption of O₂ on the Al(111) surface represents an extensively studied prototype for understanding the interaction between O₂ and metal surfaces. It is well known that the experimentally observed activation barrier for O₂ dissociation is not captured by conventional density functional theory. The interpretation of this barrier as a result of spin transitions along the reaction path has been challenged by recent embedded correlated wave function (ECW) calculations that naturally yield an adiabatic barrier. However, the ECW calculations have been limited to a static analysis of the reaction pathways and have not yet been tested by dynamics simulations. We present a global six-dimensional potential energy surface (PES) for this system parametrized with ECW data points. This new PES provides a reasonable description of the site-specific and orientation-dependent activation barriers. Quasi-classical trajectory calculations on this PES semiquantitatively reproduce both the observed translational energy dependence of the sticking probability and steric effects with aligned O₂ molecules.

Dissociative Adsorption of O2 on Al(111): The Role of Orientational Degrees of Freedom

The interaction between O2 molecules and Al surfaces has long been poorly understood despite its importance in diverse chemical phenomena. Early experimental investigations of adsorption dynamics indicated that abstraction of a single O atom by the surface, instead of dissociative chemisorption, dominates at low O2 incident kinetic energies. Abstraction of the closer O atom suggests low barrier heights at perpendicular incidence. However, recent measurements suggest that parallel O2 orientations dominate sticking at low energies. We resolve this apparent contradiction by a systematic ab initio embedded correlated wavefunction study of the stereochemistry of O2 reacting with Al(111). We identify two important new details: (i) initially, roughly parallel oxygen molecules tend to tilt upright while approaching the surface, suggesting that the abstraction channel does dominate at low energies and (ii) the reaction channel with the lowest barrier indeed corresponds to a parallel orientation, which ultimately evolves either into dissociative chemisorption or toward abstraction.

Origin of the energy barrier to chemical reactions of O2 on Al(111): evidence for charge transfer, not spin selection

Dissociative adsorption of molecular oxygen on the Al(111) surface exhibits mechanistic complexity that remains surprisingly poorly understood in terms of the underlying physics. Experiments clearly indicate substantial energy barriers and a mysteriously large number of adsorbed single oxygen atoms instead of pairs. Conventional first principles quantum mechanics (density functional theory) predicts no energy barrier at all; instead, spin selection rules have been invoked to explain the barrier. In this Letter, we show that correct barriers arise naturally when embedded correlated electron wave functions are used to capture the physics of the interaction of O(2) with the metal surface. The barrier originates from an abrupt charge transfer (from metal to oxygen), which is properly treated within correlated wave function theory but not within conventional density functional theory. Our potential energy surfaces also identify oxygen atom abstraction as the dominant reaction pathway at low incident energies, consistent with measurements, and show that charge transfer occurs in a stepwise fashion.