Author: Shyam Deo

Introduction

Platinum (Pt) is often used in many catalytic reactions including electrochemical applications as electrode surfaces for reduction of oxygen [1]. During these reduction processes, the coverage of oxygen over the Pt electrode is an important descriptor, dictating the metal activity towards oxygen reduction reaction (ORR) [2]. This study will determine the preferred binding site for atomic oxygen (O) on Pt (111) surface. Next, the adsorption trend for these atomic O will be studied as the surface coverage of O is increased.

Methods

Spin polarized plane-wave Density Functional Theory (DFT) calculations were carried out using the Vienna Ab Initio Simulation Package (VASP), version 5.4.4. The electron-electron exchange and correlation energies were computed using the Perdew, Burke, and Ernzerhof functional (PBE) [3] with  dispersion corrections, PBE-D3 [4]. The projector augmented-wave (PAW) [5] method was used to represent the ion-core electron interactions. The structural convergence criteria were 0.05 eV Å^-1 for all unconstrained atoms, while the convergence criteria defining self-consistency of the electron density was 10^-5 eV.

A Pt FCC experimental bulk lattice constant of 3.92 Å was used for building Pt (111) surface [6]. A five layered 3 x 3 unit slab of Pt (111) was used for surface calculations where the bottom three layers were frozen during the optimization and the top two layers were unconstrained (Figure 1a). A plane wave energy cutoff of 450 eV and Monkhorst-Pack [7] k-point mesh of 4 x 4 x 1 was used for all surface slab calculations after proper convergence tests described in the next section. To minimize spurious interslab dipole interactions between the periodic slabs, a vacuum space of 15 Å was used and dipole corrections were added in the direction perpendicular to the surface. The adsorption energy for atomic oxygen was calculated as:

E_ads,O = EO^_* – 1/2Eo₂ – E^*                                                                          (1)

where E_ads,O is the adsorption energy of single O, EO^_* is the energy of atomic O bound to the surface, EO2 is the energy of free oxygen molecule and E^*  is the energy of the bare surface. The gas phase energy of oxygen molecule was calculated by isolating the molecule in a large unit cell sampled with 1x1x1 k-points mesh.

Figure 1: (a) Pt(111) 3×3 unit cell with 5 layers and vacuum. (b) Convergence test for k-points. (c) Convergence test for ENCUT

Convergence Tests 

Convergence test for K-points mesh sampling the brilliouin zone was done at a plane wave energy cut off of 450 eV by varying the k-points against the single point energy of Pt (111) surface. The relative energies are plotted against the number of irreducible k-points as shown in Figure 1b. The difference in energy considered for convergence was 0.03 eV.  Then for ENCUT convergence, the cut-off energy for plane wave was varied from 350 eV to 550 eV as plotted in Figure 1c. Again, the energy tolerance considered for the convergence was 0.03 eV. Finally, the k-points mesh of 4x4x1 and a plane wave energy cut off of 450 eV was considered suitable for all subsequent surface calculations of adsorption energies at different coverage.

Results and Discussions

Preferred Binding Site of Atomic Oxygen

A Pt (111) surface has four types of binding sites available for adsorption by an atomic oxygen – atop, bridge, hollow-fcc and hollow-hcp sites shown in Figure 2a. We probed the adsorption of a single atomic O over 3×3 Pt (111) unit cell over all these sites. The adsorption energies shown in Figure 2b were reported relative to the energy of ½ oxygen molecule in the gas phase and bare Pt (111) surface according to Equation 1. On the basis of adsorption energy values, the three fold hollow-fcc site was observed to be the optimal site for binding of an atomic O (more negative adsorption energy value indicates more favorable adsorption).

Figure 2: (a) Different binding sites for atomic oxygen over Pt(111). (b) Adsorption energy of atomic oxygen at different binding sites

Coverage Effect

Coverage is defined in terms of monolayer (ML) where e.g. 1/9 ML refers to 1 oxygen atom adsorption per 9 Pt atoms (3×3 unit cell has 9 surface Pt atoms). Hollow fcc site was also the optimal site for binding at higher coverage of 2/9 ML. Hence the effect of higher coverage of atomic O on the adsorption energy (per O atom) was extensively studied for its binding at the hollow-fcc sites. Higher coverages such as 2/9, 3/9 and 1 ML of O adsorption was studied and the binding energy per atom was reported (Figure 3b). It was found that as the coverage increases, the adsorption energy per atomic O decreases which is expected since higher adsorbate-adsorbate repulsive interactions at higher coverage lowers the binding energy per atom.  At 1 ML, the adsorption energy per O drops down to less than half the value at the lowest coverage of 1/9 ML.

Figure 3: (a) Binding of atomic oxygen at hollow fcc sites at different coverages over Pt(111). (b) Adsorption energy per atom of atomic oxygen at different coverage

Conclusion

DFT calculations of atomic O binding over Pt (111) performed in this study agree well with the earlier studies that preferred binding site of oxygen on Pt (111) is the three-fold hollow-fcc site [8]. Adsorption of atomic oxygen atom weakens at higher coverage due to repulsive adsorbate-adsorbate interactions. However, for electochemical applications, the binding of atomic O over the surface and hence, the subsequent coverage, will be affected by the external electrode potential of the Pt(111) surface (which was not covered here). These need to be studied by DFT calculations through imposing electric field effects across the surface.

References

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