Peroxymonosulfate activation by brownmillerite-type oxide Ca2Co2O5 for efficient degradation of pollutants via direct electron transfer and radical pathways

[Display omitted] •Ca2Co2O5 catalyst with multiple Co oxidation states was prepared.•The catalyst showed high activity and stability via PMS activation.•Both radicals and direct electron transfer process contribute to the reaction.•The role of Co species with different oxidation states was discussed. The development of mixed metal oxides with excellent activity and stability for pollutants degradation via peroxymonosulfate (PMS) activation is highly desired in recent years. In this work, the brownmillerite-type oxide, Ca2Co2O5 (CCO) with unique Ca2CoO3 and CoO2 layer structure and multiple oxidation states such as Co(II), Co(III) and Co(IV), was prepared and used to degrade organic pollutants with PMS as oxidant for the first time. It was found that CCO exhibited higher activity than other Co-based catalysts such as Co2+ and Co3O4, and more than 93% of acid orange 7 (50 μM) was degraded after 7.5 min under mild reaction conditions. The catalysts were also stable during five successive runs. The easy oxidation of Co(II) species, the strong interactions among CCO, PMS and the pollutants, and the synergistic effect between Co and Ca were identified by several technologies and contributed to the high performance of the catalyst. Moreover, different from the conventional mechanism, radicals produced from low valent Co species oxidation by PMS, as well as the direct electron transfer process through the high valent Co species contributed the degradation reaction simultaneously. These findings provide an ideal bimetallic heterogeneous catalyst and a new mechanism for PMS-mediated reaction.