Metal-organic frameworks (MOFs) are known as porous material, and the crystalline structure consists of a metal cluster connected by organic linker molecules. In order to synthesize single-site heterogeneous catalysts, one of the simplest methods is to anchor catalytically active metal atoms, cations, or complexes directly to high surface area solid supports. These exposed identical site-isolated metal centers can easily bond and react with reactant molecules in solution. Among these above-mentioned heterogeneous metal catalysts, metal complexes are known as single-site heterogeneous catalysts which have attracted increasing attention in recent years.
The active metal sites are normally located at crystal corners, edges, and facets which exhibit the diverse catalytic properties. Metal oxides, metal nanoparticles, and metal complexes can be employed as promising candidates in a wide range of heterogeneous catalytic fields. Thus, many efforts have been devoted to the design of the suitable catalyst structure, which can significantly improve the reduction efficiency, providing excellent catalytic sustainability and recoverability.
In particular, the catalytic reduction can be considered as an effective and facile route to remove these organic pollutants. Recently, many various technologies such as adsorption, photocatalytic degradation, chemical oxidation, and membrane filtration have been applied to remove these compounds with the aim of reducing the risks. ĭue to this reason, it is necessary to develop the simple, efficient method for removal of these organic compounds in wastewaters. It is noticeable that methylene blue is also a high-toxicity carcinogenic organic compound and can be a cause of water environmental pollution. In addition to this, methylene blue (MB), known as organic dye, is broadly studied because of its many potential applications in industries such as printing, textile, paper, paints, and plastics. It is worth noting that the exposure of 4-NP would threaten human health such as damage to the nervous central system, blood system, and primary organs. This compound released from above industrial sectors can impact negatively on the ecological system and pose serious environmental pollution. However, 4-nitrophenol (4-NP), known as the toxic and highly hazardous contaminants, is found in agricultural and industrial wastewaters. Nitroaromatic compounds, especially 4-nitrophenol (4-NP), are extensively employed in the fields of pigments, pharmaceuticals, dyes, explosives, plastics, pesticides, wood, or leather preservative. The and TOF for the reduction of MB using UiO-66/Ni1.0 were 0.787 min −1 and molecules g −1 min −1, respectively. In addition, methylene blue (MB) was also chosen as the organic dye model for catalytic reduction reaction. Specifically, the as-prepared UiO-66/Ni1.0 exhibited the excellent catalytic activity and stability for 4-nitrophenol reduction in terms of low activation energy ( ), high turnover frequency (76.19 molecules g -1 min -1), and high apparent rate constant ( ). It was found that single-atom Ni heterogeneous catalysts supported by the UiO-66 structure, UiO-66/Ni1.0, showed a sphere-like morphology with a high specific surface area as well as good thermal stability. The structure properties of the catalysts were characterized using powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), N 2 adsorption-desorption isotherms (BET), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy (PL). Herein, the single-atom Ni site heterogeneous catalysts supported by the UiO-66 structure (University of Oslo-66 metal organic framework) were successfully synthesized by a postsynthetic metalation method, where Ni ions are covalently attached to the missing-linker defect sites at zirconium oxide clusters (Zr 6O 4(OH) 4) in as-prepared UiO-66 structure, (BDC (benzene-1,4-dicarboxylate), DMF (dimethylformamide)).
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