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Effect of van der Waals interactions on the chemisorption and physisorption of phenol and phenoxy on metal surfaces
Structural Adhesives: Chemistry and Technology, edited by S. R. Hartshorn (Plenum Press, New York, 1986).
F. Lu, G. N. Salaita, L. Laguren-Davidson, D. A. Stern, E. Wellner, D. G. Frank, N. Batina, D. C. Zapien, N. Walton, and A. T. Hubbard, Langmuir 4, 637 (1988).
In order to find the most stable adsorption site of any molecule on any surface, we first considered the eight high symmetry adsorption sites. The most energetic configuration is assigned as the most stable one for each molecule on a specific surface. For the AEP calculations, instead of dealing with these eight structures, we considered the most stable one among the eight. We started initially with the most stable configuration and fixed only the z-coordinate (relaxing x and y coordinates).
In the AEP curves, the effect of constraining the atomic positions has a small effect on the adsorption energy of molecules on the surfaces. For instance, for PL/Au(111) the minimum of AEP calculated with PBE-vdW is −0.80 eV and the fully optimized value is −0.89 eV. For PL/Pt(111) calculated with PBE-vdW, the chemisorption and physisorption AEP energies are −1.79 and −1.13 eV, and the fully optimized energies are −1.82 and −1.23 eV, respectively. For PX on Au and Pt(111), the minimum of AEP calculated with PBE-vdW is −1.51 and −2.73 eV, and the adsorption energies of fully relaxed systems are −1.51 and −2.87 eV, respectively.
K. Berland, C. A. Arter, V. R. Cooper, K. Lee, B. I. Lundqvist, E. Schroder, T. Thonhauser, and P. Hyldgaard, J. Chem. Phys. 140, 18A539 (2014).
As the adsorption energy of benzene and phenol on Au(111) was not particularly dependent on the adsorption site for PBE and PBE-vdW, we decided not to test systematically the molecules for revPBE-vdW and PW86-vdW2.
Even though the energy difference between B-30 and hcp-30 sites calculated by PBE-vdW is very small (0.02 eV), the molecule is distorted from its gas-phase geometry at B-30 site and keeps its planar geometry at hcp-30 site. In contrast, PL on hcp-30 site calculated by PBE is not stable and goes to B-30 site with a chemisorbed geometry. We have also explored the physisorption energies and structural features with the other functionals including vdW interactions. For B-30 site, both PW86-vdW2 and revPBE-vdW functionals yield to chemisorbed structures, while PL on hcp-30 site is stable and keeps its gas phase geometry.
NEB calculations for Au surface with eight reaction intermediates changed the activation energy by 0.03 eV. Thus, due to the computational cost, we used four reaction intermediates for Pt surface.
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The adsorption of phenol and phenoxy on the (111) surface of Au and Pt has been investigated by density functional theory calculations with the conventional PBE functional and three different non-local van der Waals (vdW) exchange and correlation functionals. It is found that both phenol and phenoxy on Au(111) are physisorbed. In contrast, phenol on Pt(111) presents an adsorption energy profile with a stable chemisorption state and a weakly metastable physisorbed precursor. While the use of vdW functionals is essential to determine the correct binding energy of both chemisorption and physisorption states, the relative stability and existence of an energy barrier between them depend on the semi-local approximations in the functionals. The first dissociation mechanism of phenol, yielding phenoxy and atomic hydrogen, has been also investigated, and the reaction and activation energies of the resulting phenoxy on the flat surfaces of Au and Pt were discussed.
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