Effective degradation of azo dyes in the dark by Cu2+ active sites in CaSrNiCu oxides

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Metal oxides containing calcium strontium nickel and copper (CSNC) were investigated for the degradation of an azo dye Orange II (OII) pollutant under dark conditions and without any external stimulant. The most active catalyst contained equimolar concentration of Ni and Cu. The reaction proceeded rapidly reaching 50% OII degradation within the first 5 min and 95% by 2 h with 54% of the organic carbon mineralized. The pristine CSNC catalyst containing Ni2+/Cu1+ underwent partial oxidization to Ni3+/Cu2+. Despite this oxidation change, the CSNC catalyst remained stable for over 15 cycles, and Cu2+ proved to be an active reaction site as the Ni3+ phase is not catalytically active. The degradation mechanism under dark conditions occurs by OII contacting the catalytic surface of CSNC where NN azo bonds were broken and electrons were donated. Concomitantly, more electrons were donated by the oxidation of Ni2+/Cu1+. Electrons led to the formation of reactive species (O2•-, HO2• and OH⋅) and hydrogen peroxide, as confirmed by the partial mineralisation of OII and radical trapping experiment.