Surface and catalytic properties of stable Me(Ba, Ca and Mg)SrCoO for the degradation of orange II dye under dark conditions

Abstract

This work investigates the surface and catalytic properties of Co containing metal oxides MeSrCoO by partially substitution of Sr with another alkaline metal Me (Ba, Ca and Mg). The catalysts were used for the degradation of a textile dye orange II (OII) under dark conditions and without the addition of any chemical additive, light irradiation or energy input. Although all catalysts were prepared under the same conditions, BaSrCoO formed a pure perovskite phase, while MgSrCoO and CaSrCoO resulted in a mixture of perovskite and metal oxide phases. All these catalysts were characterised by a non-porous materials with low surface areas (<∼1 m2 g−1). A total value of ∼80% OII degradation percentage was reached in 4 h and ∼90% in 8 h, though their surface properties resulted in different reaction kinetics. For instance, BaSrCoO and MgSrCoO reaction kinetics were faster and fitted a second order reaction whilst CaSrCoO was slower and fitted a first order reaction. OII degradation was mainly attributed to the catalytic surface properties of these metal oxides, as sorption was not significant except for CaSrCoO which explains the lower reaction kinetics. All catalysts demonstrated good stability over 7 cycle testing (56 h), which was also confirmed by XRD and XPS analysis of pristine and spent samples. Interestingly, non-substituted SrCoO resulted in a similar OII degradation rate, but decayed cycling stability. Hence, the partial substitution of Sr with alkaline metals (Ba, Ca and Mg) conferred increased surface stability. Due to the catalytic surface property of MeSrCoO, the primary reaction mechanism was the contact of OII with the catalyst, leading to the generation of electrons. Subsequently, the electrons reacted with dissolved O2 in the solution, and in a series of reactions, formed hydroxyl radicals and singlet oxygen, leading to OII further degradation.