Tuning the catalytic activity of Ag₇Au₆ cluster for oxygen reduction reaction via support interactions

We investigate the suitability as oxygen reduction reaction (ORR) catalysts of Ag₇Au₆ clusters supported by nitrogen-doped graphene (N-graphene) sheets with two different nitrogen contents (3.61% and 8.43%), as well as TiO₂(101) monolayers, using post-DFT methods. We employ two energetic descriptors (namely, the O₂ and OH binding energies and O-O bond length), to compare the effect of two different supports on the catalytic activity of the Ag₇Au₆ cluster. We find that low adsorption energies for O₂ (approximately -1 eV) and OH (below the one of its reaction on Pt), and O-O bond elongation (which reaches up to 1.34 Å in our study, corresponding to the superoxide bond length) cause the high ORR activity of our materials of interest. Our results show that the supported clusters have higher catalytic performance compared to the unsupported ones due to the lower binding energies of the O₂ and OH molecules. Moreover, the TiO₂(101) monolayer is a better support for the Ag₇Au₆ cluster, due to the O-O bond distance increasing up to 1.33 Å, facilitating O₂ dissociation. In addition, the low (-0.35 eV) OH adsorption energy decreases the difficulty of OH removal. Through free energy calculations, we prove the ORR mechanism in acidic conditions to display an over-potential of -1.76 V, competitive with to the ones featured by currently available catalysts.