Chemisorption of Benzene on Pt(111), Pd(111), and Rh(111) Metal Surfaces: A Structural and Vibrational Comparison from First Principles

摘要

The adsorption mode of aromatic molecules on transition metal surfaces has an important implication in their catalytic transformation. As platinum, palladium, and rhodium metals are some of the most employed in heterogeneous catalytic reactions, and benzene C6H6 is the smallest aromatic molecule, these systems are good models for understanding their interactions. Those are approached in this study by first-principles density functional periodic calculations on (111) metal surfaces. The energetic results show that benzene can adsorb predominantly in a bridge position on Pt(111), while on Pd(111) and Rh(111), benzene molecules on bridge and hollow positions have similar adsorption energies. These conclusions are confirmed by the comparison of vibrational spectra from experiment (HREELS) and simulation: most peaks of the spectra can be assigned with a bridge site molecule, but for each case some peaks or shoulders can be understood only if a fraction of hollow sites is supposed. The adsorption exothermicity increases from palladium to platinum and to rhodium. An electronic analysis, via the projected densities of states and the differential electron density isosurfaces, helps to understand how this interaction can be related to the shape and filling of the d-band. The adsorption process is described as a compromise between a destabilizing distortion of the molecule and surface, and a stabilizing interaction between them. Pd corresponds to weak distortion and interaction terms, while Rh and Pt are associated with large contributions.