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.

(S1) More details of the ECW theory and convergence tests can be found in Refs 1, 2 and 3. Because ECW calculations are much more expensive than conventional DFT, the number of points is limited. As a result, two widely applied approaches, i.e., neural networks (NNs) [6][7][8][9] and the corrugation reducing procedure (CRP) 10

B. Potential Energy Surface
where the U i (Q i ) represent the Coulomb (exchange) integrals for two-body interactions ( where L is the nearest-neighbor distance between two surface atoms, and the expansion coefficients P 0 , P 1 , P 2 are obtained by solving a set of linear equations once we find the Morse parameters for three high symmetry sites (top, bridge, and hollow). Following Martin-Gondre et al., 12,13 we include a dependence on the bond length r i in α i to better describe the one- where the two additional parameters 0 and 1 are also optimized.
The choice of the Sato parameters is of essential importance in representing the global PES.
In the FPLEPS function, the periodicity is also incorporated into the Sato parameters, thus offering more flexibility: the site-dependent Sato parameters at the top, bridge, and hollow sites differ and the Fourier expansion in Eq. (S5) again yields the correct periodicity. In addition, there is a shallow physisorption well in the entrance channel in this system, which is not intrinsically described in the LEPS framework. We therefore add a negative Gaussian function with respect to the height of the molecule in the final form of the PES, 12, 13 where A, σ, and z 0 are the amplitude, width, and center of the Gaussian, which represent the well depth, shape and location, respectively. Note that the Gaussian function is also site-dependent just like the adsorption well. These parameters shaping the FPLEPS function are all optimized using the non-linear Levenberg-Marquardt algorithm 18 which minimizes the discrepancies between the energy values predicted by the FPLEPS function and the original CW data. The optimized parameters are reported in Table S1.

C. Dynamics
To validate the FPLEPS PES, extensive quasi-classical trajectory (QCT) calculations have been performed to simulate the dissociative chemisorption dynamics of O 2 on Al(111). The trajectories were initiated at 6.0 Å above the surface with the molecular center randomly chosen to cover the 1×1 unit cell. The diatomic molecule is treated as a rotating oscillator, whose In all calculations, a trajectory is terminated when the molecular bond length increases to 2.5 Å (dissociation), or when the molecule-surface separation increases beyond 6.0 Å (reflection). Note that the influence of substrate motion is neglected here within the frozen surface approximation.

S7
This is partially justified by a very weak dependence of the sticking probability on the surface temperature observed in experiment. 20 A small number of ab initio molecular dynamics trajectories also revealed that surface movements only become prominent when the O 2 molecule is quite close to the surface after dissociation. 22 In addition, low energy electron-hole pair excitations are also neglected here, which only play a minor role in the activated dissociation of molecules on metal surfaces with significant barriers. 23, 24

II. Additional results
We further investigate the anisotropy of the PES at the hollow site in both the entrance and product channels (see Figure S2). Following Cheng et al., 3  In the product channel, the PES cuts are plotted as a function of θ and r at the fcc site for // 3 geometry (φ=-30°) (see Figures S2e-f). Interestingly, the adsorbate tends to rotate from the parallel to a tilted geometry after overcoming the lowest barrier, as evidenced by the local minima θ=~70° in Figures S2e-f with the extended O-O distance. As suggested by some of the S8 current authors, 3 this offers the possibility of an abstraction channel, which yields a single adsorbed O atom and an oxygen atom emitted to the gas phase. 25 Overall, our FPLEPS PES 2D cuts demonstrate a strong dependence of the barrier on the angular coordinates, which are in general accord with the earlier analyses based on the original ECW data. 3 The preference of certain parallel orientations over the perpendicular ones could serve as the origin of the experimentally observed steric effect, as discussed in the main text.