Effective enhancement of performances on photo-assisted dye degradation using a Zn coordination polymer and its post-modified Cu/Zn bimetallic analogue under natural environments

A highly stable condensed three-dimensional framework, [Zn(2,6-ndc)(H2O)] (Zn-ndc, 2,6-ndc = naphthalene-2,6-dicarboxylate, CSD entry: MIMRAB; CCDC no: 182255), and its post-modified Cu/Zn bimetallic analogue [CuxZn1−x(2,6-ndc)(H2O)] (CuZn-ndc, x = 0.45−0.49) showed remarkably catalytic activity to greatly improve the degradation performances of organic dyes including malachite green (MG), methylene blue (MB), and methyl orange (MO) in the presence of H2O2 without and with natural light illumination. The dye degradation over Zn-ndc and CuZn-ndc both followed the order MG >> MB > MO. In addition, CuZn-ndc displayed greater photocatalytic degradation activity compared to Zn-ndc, resulting in faster degradation rate and higher degradation efficiency. Zn-ndc and CuZn-ndc both retained the crystallinity and photocatalytic activity to degrade MG, MB, and MO dyes over five consecutive usages, implying long-term stability and reusability. Of note, pH exerted an effect on the surface charge of Zn-ndc and CuZn-ndc to decrease dye degradation performances of MB and MO but not MG as the decrease of pH. Further, CuZn-ndc had more negative charges on the surface than Zn-ndc, which provided higher affinity toward dyes and thus facilitated dye degradation. Explorations on mechanism provened that the dye degradation was a H2O2-trigged oxidative degradation process, thus more H2O2 dosage led to higher short-term degradation performances. Moreover, incorporation of Cu(II) would significantly reduce band gap energy of photo-assisted catalysts and also lead to a Cu-based photo-Fenton redox cycle, both of which would produce more hydroxyl radicals and photo-generated holes and electrons as active species. This greatly enhanced the photocatalytic activity to improve the performance of dye degradation process.