Theoretical study of hydrogen adsorption on Co clusters

Abstract

We present results of a systematic study of the adsorption of one to five hydrogen atoms on Conn=1–5 neutral clusters. The density functional (DFT) formalism has been adopted to perform this study with the use of the hybrid functional B3LYP. We present results for their binding, vertical and adiabatic detachment energies, magnetic moments, HOMO–LUMO gaps and their infra red spectra. The results show that by the use of an hybrid functional we obtain different ground state geometries as well as magnetic moments than some of the previous theoretical studies. We test the methodology comparing the calculated Infra Red (IR) spectra for the cationic clusters: Co4Ar+, Co5Ar+, Co4H2+ and Co5H6+ with the available Far Infra Red Multiple Adsorbed Photon (FIR-MPD) experimental spectra, and we obtained a good agreement. We found the adsorption of H2 exclusively dissociative for all neutral ConHmn=1–5, m=1–5 clusters. The twofold bridge site is identified to be the most favorable for adsorption on neutral Con (n>2). Therefore hydrogen adsorption in clusters is different than it is on surfaces, where it is known that hydrogen adsorbs in high coordination sites. We found that as we add H atoms: first, a structural transition from three to two dimensions occur in the cluster geometry, second an enhanced but non-monotonic change in their magnetic moments is observed, and finally the stability of the system is greatly increased.