Oxygen reduction reaction on a 68-atom-gold cluster supported on carbon nanotubes: theoretical and experimental analysis

Abstract

68-Atom gold nanoparticles (Au₆₈) were adsorbed on fractured carbon nanotubes by acid treatment to obtain an active catalyst (Au₆₈–CNT) for the oxygen reduction reaction (ORR). TEM images show the importance of acid treatment to create defects on the walls of the CNT, where Au₆₈ would then adsorb. X-Ray photoelectron spectroscopy confirmed that the Au₆₈ surface is exclusively composed of metallic gold when at the surface of CNT. FTIR characterization showed the presence of 4-mercapto-benzoic acid residues, which are necessary for the synthesis and are then removed through electrochemical reduction. The electrocatalytic studies were conducted in an alkaline medium, where Au₆₈–CNT proved to be an efficient catalyst for the ORR with the transfer of 3.9 electrons for direct water production. In acidic medium, poor performance and stability were observed. Durability tests were conducted by performing 5000 accelerated cyclic voltammograms, during which the half-wave potential of the polarization curve for the ORR decreased only by 20 mV. The DFT calculations were performed to simulate the interactions between O₂ and Au₆₈–CNT, and energy barriers, using the NEB method for O₂ dissociation and contrasting values with Au (111), Au (110), Au (100), and Pt₆₈–CNT. Large energy barriers indicating low ORR activity were obtained at surfaces different from Au₆₈–CNT and Pt₆₈–CNT. O₂ would bind to four Au atoms in Au68–CNT with a binding energy of −0.56 eV. The reactivity of Au₆₈–CNT for ORR would be associated with the irregular geometry of the Au₆₈ surface.