Abstract
The interest in analyzing the bimetallic substrates in molecule/catalyst interfaces is to show their influence in the absorptionand the corresponding chemical and geometrical effects. Specifically, it has generated great interest in the development ofalternative catalysts that they are more active on the preservation of the double carbon-carbon bond of the original organic acid,for instance, to produce unsaturated fatty alcohols.In this work, we use the Vienna Ab initio simulation package (VASP) to study bimetallic surface compounds of a monolayerof Pt on Ni(111) and their interaction with an organic acid molecule, the cis-3-hexenoic acid. A previous study shows that onePt layer system was even more stable than the pure bulk substrate because both, the chemical affinity between the two metalsin contact and, the compression in the surface plane which provides an increased electron density on the platinum sites thatbalance the lowered density due to the surface broken Pt bonds.We have modeled the bimetallic surface of one layer of Pt on top of four Ni(111) layers within the three-dimensionallyperiodic supercell. The bottom two layers of the PtNi(111) slab are kept fixed in bulk positions to represent the semi-infinitebulk crystal beneath the surface. We find that using more layers of metal substrate (and relaxing one more layer) only changesthe absorption energy within the error of the calculations. In order to take into account the magnetic properties of Pt and Ni, thecalculations were performed at the spin-polarized level. To model the molecule, we have placed it in a 20 Å cubic box. Highlysymmetrical boxes can produce wrong orbital occupancies for the isolated molecule. We have explored the molecule absorptionon the surface with a 3x3x1 k mesh (previously checked). The molecule is allowed to relax with the top three layers of the metalsubstrate. When the maximum force acting on each atom of relaxed layers are below 0.01 eV/Å, the structural relaxation isstopped.In order to analyze absorption, we have considered several possible configurations for C5H9COOH absorption above thebimetallic PtNi(111) surface; after system relaxation, we have selected for the present study the most stable geometry, it canbe seen in Fig. 1. Their complete molecular structure interacts with the bimetallic surface and the absorption results to bestrong; we have obtained an optimum energy of -2.4 eV for the relaxed system. The molecule interacts through the C=C bondbut also presents interactions via the COOH group. The carboxyl group is partial dissociate: the C-OH remains tied to themolecule interacting with the surface, while the O (of C=O group) is released from the molecule and adsorbs on the surfaceseparately; this indicates that the PtNi(111) induces the conversion from the unsaturated acid to the unsaturated alcohol. Duringabsorption, the Pt surface atoms that interact with the molecule move away from their lattice original position and it rearrangesas consequence of the strong interaction with the molecule. We can observe that the PtNi(111) surface preserves intact thedouble bond of the adsorbed molecule and the dehydrogenation of the double bond is not observed.