Cobalt aluminate (CoAl₂O₄) is one of the most common transition metal catalysts and is widely used in the persulfate advanced oxidation process. To improve the dispersion of the cobalt aluminate (CoAl₂O₄) catalyst and enhance its catalytic activity, exfoliated montmorillonite nanosheets (EMMT) as a support were prepared by a freeze–thaw–ultrasonic method. The CoAl₂O₄/EMMT composite material was synthesized by a sol–gel method, and then CoAl₂O₄/EMMT was reduced to the Co–CoAl₂O₄/EMMT composite material. The degradation performance and cycle stability of Co–CoAl₂O₄/EMMT for methyl orange were investigated, and the mechanism of catalytic degradation of methyl orange was discussed.

An appropriate quantity of montmorillonite (MMT) was uniformly dispersed in 50 mL of deionized water. After continuous stirring for 12 h, a well-dispersed MMT suspension was obtained. Subsequently, the MMT suspension was transferred into a refrigerator at –20 ℃ for 12 h. In the freezing process, the frozen MMT suspension was placed at an ambient temperature for 8 h to ensure complete thawing. After thawing, the suspension was thoroughly stirred to achieve a homogeneous state, and then ultrasonic treatment was carried out for 8 min to yield an exfoliated montmorillonite nanosheet (EMMT) suspension. Afterwards, 750 mg of aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O) and 353 mg of cobalt nitrate hexahydrate (Co(NO₃)₃·6H₂O) in a molar ratio of 2:1 were added to the EMMT suspension. The mixture was vigorously stirred for 40 min to guarantee the well-dispersion. 384 mg of citric acid and 4.5 mg of sodium dodecyl benzene sulfonate (SDBS) as organic additives were introduced into the above-mentioned solution. The solution was then heated in a water-bath at 70 ℃ for 4 h to form a sol, and the heating process was continued for 2 h to transform the sol into a wet gel. The wet gel was subsequently freeze-dried for 24 h to obtain a dry gel, which was then finely ground. An appropriate amount of the dry–gel powder was weighed and placed in a muffle furnace. The temperature increased at a heating rate of 5 ℃/min, and followed by calcination for 3 h to synthesize CoAl₂O₄/EMMT composite material. The microstructure of the CoAl₂O₄/EMMT composite material was optimized via precisely regulating the calcination temperature. Finally, the Co–CoAl₂O₄/EMMT composite material was synthesized through reduction roasting.

At a ultrasonic power of 600 W, the lamellar thickness of the exfoliated EMMT decreases from 303 nm to 7.6 nm. The Co–CoAl₂O₄/EMMT composite material synthesized at a calcination temperature of 650 ℃ and a reduction temperature of 670 ℃ exhibits the optimal catalytic performance. Its degradation rate of methyl orange reaches 87.5% within 70 min, which is greater than the catalytic performance of Co–CoAl₂O₄ (79.3%) or CoAl₂O₄/EMMT (76.8%) composite materials. Co–CoAl₂O₄/EMMT composite material has a good stability, and the degradation rate of methyl orange still remains at 77.8% after four cycles.

Exfoliated montmorillonite nanosheets (EMMT) as a carrier were prepared by a freezing-thawing-ultrasonication method, and then Co–CoAl₂O₄/EMMT composite was synthesized by a sol–gel reduction approach. Co–CoAl₂O₄/EMMT material prepared at a calcination temperature of 650 ℃ and a reduction temperature of 670 ℃ exhibited the optimal catalytic performance. After activating peroxydisulfate (PDS), the degradation rate of Co–CoAl₂O₄/EMMT for 50 mg/L methyl orange could reach 87.5% within 70 min, accompanied by a superior cycle stability. The results of radical quenching experiments confirmed that SO₄^•⁻ and •OH radicals were the dominant reactive species contributing to methyl orange degradation.