Adsorption of Small Au Particles on Goethite via Density Functional Theory

LEANDRO FORTUNATO; C E ZUBIETA; PATRICIA BELELLI; RICARDO M. FERULLO

Santiago

Congreso; 10 th Triennial Congress of the World Association of Theoretical and Computational Chemists; 2014

Iron oxides and oxohydroxides are materials of technological importance in corrosion and catalysis. They have been used as support of gold-based catalysts in reactions such as NO reduction, CO oxidation, selective hydrogenation and water gas shift. It is generally accepted that the activity of Au particles depends on different characteristics as their size, shape and charge. Although it is still controversial whether Au0 or Auδ+ are the active species, some models propose that both of them can act simultaneously [1]. In this sense, the support can play a crucial role by affecting the electronic structure of the metal particle. Recently, it was found that Au grows on goethite, α-FeOOH, forming particles in the nanometric range [2]. In this work we study the geometric and electronic structure of Au particles from 1 to 5 atoms on two types of (110) goethite surfaces, one obtained by cutting the bulk structure (the ?clean? surface), and the other generated by its hydratation (the ?hydrated? surface). The clean face presents unsaturated oxygen and iron ions which represent reactive surface sites. The calculations were performed at the Density Functional Theory (DFT) level with the Vienna Ab-Initio Simulation Package (VASP). On the clean FeOOH(110) surface, Au1 and Au2 present adsorption energy values around 2.5 eV. However, the interaction with the support is much stronger for Au3, Au4 and Au5, with values between 4.1 and 4.5 eV. The metal particles are anchored mainly with the oxygen ions of the oxide. In all the cases, there is a charge transfer from Au to the substrate acquiring the metal particle a net charge from 0.4 to 0.7e. However, it is not possible to establish a correlation between charge transfer and adsorption energy. For trimer, tetramer and pentamer, the planar structures are particularly stable. Calculations indicate that the stronger interaction with the surface for these particles can be attributed to polarization effects. Indeed, the planar particles tend to polarize toward the external gold atoms producing an important electrostatic attraction with the surface oxygen ions. On the other hand, on the hydrated surface gold particles link with the oxide with energy values between 0.7 and 1.7 eV, acquiring the particle a net charge around -0.1e.