Oxidized few layer graphene and graphite as metal-free catalysts for aqueous sulfide oxidation

Abstract

Few layer graphene and natural graphite were chemically modified by oxidation with HNO₃ at 80 and 140 °C and used to promote the oxidation of sulfide ions in aqueous solutions. TEM, potentiometric titration, XRD, BET and Raman spectroscopy show that HNO₃ treatment even at 140 °C did not modify the graphene and graphite morphologies but produced significant amounts of different oxygen surface groups. The presence of these groups on few layer graphene and graphite strongly increased the S_aq²⁻ oxidation, showing activities comparable to a high surface area activated carbon with a similar amount of oxygen surface groups. The obtained results suggest that the sulfide oxidation efficiency requires a balance between two important effects, i.e. the presence of oxygen functionalities to initiate the S_aq²⁻ oxidation and the electrical conductivity that is important to further transfer the electrons removed from sulfide.