硅酸盐学报

为了改善铝酸钴(CoAl_2O₄)催化剂的分散性并提高其催化活性，采用冷冻–解冻–超声法制备剥离的蒙脱石纳米片(EMMT)并将其作为载体，通过形成溶胶–湿凝胶–干凝胶而后进行还原焙烧的溶胶凝胶–还原法制备Co–CoAl_2O₄/EMMT复合材料，探究焙烧温度和还原温度等制备条件对复合材料微结构的影响。结果表明在焙烧温度为650℃，还原温度为670℃时合成的Co–CoAl_2O₄/EMMT复合材料具有最佳的催化性能。其在活化过二硫酸盐(PDS)后70 min内对甲基橙的降解率达到87.5%，高于Co–CoAl_2O₄(79.3%)和CoAl_2O₄/EMMT(76.8%)复合材料的催化性能。Co–CoAl_2O₄/EMMT复合材料展现出良好的稳定性，4次循环后对甲基橙的去除率仍保持在77.8%。

Introduction Cobalt aluminate(CoAl_2O₄) 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(CoAl_2O₄) catalyst and enhance its catalytic activity, exfoliated montmorillonite nanosheets(EMMT) as a support were prepared by a freeze–thaw–ultrasonic method. The CoAl_2O₄/EMMT composite material was synthesized by a sol–gel method, and then CoAl_2O₄/EMMT was reduced to the Co–CoAl_2O₄/EMMT composite material. The degradation performance and cycle stability of Co–CoAl_2O₄/EMMT for methyl orange were investigated, and the mechanism of catalytic degradation of methyl orange was discussed. Methods 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)₃·9H_2O) and 353 mg of cobalt nitrate hexahydrate(Co(NO₃)₃·6H_2O) 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 CoAl_2O₄/EMMT composite material. The microstructure of the CoAl_2O₄/EMMT composite material was optimized via precisely regulating the calcination temperature. Finally, the Co–CoAl_2O₄/EMMT composite material was synthesized through reduction roasting. Results and discussion At a ultrasonic power of 600 W, the lamellar thickness of the exfoliated EMMT decreases from 303 nm to 7.6 nm. The Co–CoAl_2O₄/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–CoAl_2O₄(79.3%) or CoAl_2O₄/EMMT(76.8%) composite materials. Co–CoAl_2O₄/EMMT composite material has a good stability, and the degradation rate of methyl orange still remains at 77.8% after four cycles. Conclusions Exfoliated montmorillonite nanosheets(EMMT) as a carrier were prepared by a freezing-thawing-ultrasonication method, and then Co–CoAl_2O₄/EMMT composite was synthesized by a sol–gel reduction approach. Co–CoAl_2O₄/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–CoAl_2O₄/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.