Cu, Ag and Au adsorption on hematite: A comparative density functional study

CAROLINA ZUBITA; SILVIA A. FUENTE; LUIS G. AQUINO LINAREZ; PATRICIA G. BELELLI; RICARDO M. FERULLO

Santa Fe

Congreso; VI San Luis Congress of Surfaces, Interfaces and Catalysis; 2018

Universidad Nacional del Litoral

We have investigated the adsorption of very small Mn clusters (M = Cu, Ag and Au, with n = 1-5) on the Fe-terminated (0001) surface of alpha-Fe2O3 (hematite). To model the atom-by-atom nucleation until obtaining Cu5, Ag5 and Au5 clusters on the hematite surface, we have made a detailed study of the atomic preferential adsorption site for each metal. Cu and Ag atoms prefer to link on an O-hollow site (that one with an Fe ion lying at the fourth layer); the metal atom interactions occur through three O ions at 2.00 A and 2.34 A for Cu and Ag, respectively. For Au, the most stable site was on-top of Fe, with an Au-Fe distance of 2.51 A. These results are as expected because Cu and Ag are more oxidizable metals than Au. The adsorption energies indicate that Cu atom binds strongly to the hematite surface with a value of -2.32 eV, Ag with a lower value of -1.38 eV, and Au atom with the weakest interaction (-0.97 eV). From these adsorbed M1 states, the sequential growth was modeled by exploring the different possible isomers for each case and selecting the most stable one. Calculations evidenced that for the three metals, dimers and trimers (triangle-shaped) adsorbed tilted with respect to the hematite surface. Whereas Au4 adopts a planar structure, Cu4 and Ag4 are tetrahedron-shaped. For pentamers, the three metals take flattened geometries with all the M atoms directly interacting with surface ions. Concerning the total charge of adsorbed Mn clusters, Aun aggregates result in null or slightly negative values; for their part, Cun and Agn acquire positive values, with higher values for n=3-5 (0.8-1.3e). With respect to the total spin of adsorbed Mn, Cun and Agn present low values (< 0.05 B), except for the case of Cu2. Aun show an even-odd zigzag behavior as it was observed previously on goethite.1 According to these results, the Enucl variations seem to be correlated mainly with the spin of each particular aggregate. Indeed, when an adsorbed Mn particle with an odd number of atoms interacts with an M atom coming from the gas phase a strong interaction is expected. This indeed is the case for Au. For its part, Ag shows almost constant values of Enucl in line with the corresponding states of spin. Finally, Cu presents a maximum absolute value of Enucl at Cu3, when adsorbed Cu2 (with 0.3 B) interacts with an incoming Cu atom.