Unraveling spongy Co3O4 mediated activation of peroxymonosulfate: Overlooked involvement of instantaneously produced high-valent-cobalt-oxo

Peroxymonosulfate (PMS) activation induced by tricobalt tetroxide spinel (Co3O4) has been confirmed as a typical Haber-Weiss reaction, while free radicals were once considered as the dominated reactive species in the previous studies. However, the catalytic mechanism of the spongy Co3O4 driven PMS activation was surprisingly found as a radical/nonradical mixed process rather than a pure radical process in the present work. The important role of sulfate radical (SO4-) was confirmed through the quenching experiments. Despite the inhibition of furfuryl alcohol (FFA) and 1,4-benzoquinone (BQ) on degradation was generally accepted as the evidence to support the existence of 1O2 and O2-, additional experiments using methyl phenyl sulfoxide (PMSO) as the indicator indeed verified high-valent-cobalt-oxo rather than 1O2 and O2- dominated the very early reaction stage. Notably, instead of homogeneous Co3+, heterogeneous Co(IV) = O on catalyst surface was believed to be responsible for the oxidation of organics. Spongy Co3O4 not only possessed stronger catalytic ability than commercial Co3O4 (k[spongy Co3O4] = 0.74 min-1, k[Co3O4] = 0.08 min-1), but also owned preferable stability. The performance of catalytic system was barely affected by the solution pH under the near neutral condition. Besides, little suppression of the widely existing anions on the degradation indicated the potential application of spongy Co3O4/PMS system. This study provides a reliable oxidation technology for the removal of organic pollutants, and sheds new light on the cobalt oxide triggered PMS activation process.