Single-Atom Ni Heterogeneous Catalysts Supported UiO-66 Structure: Synthesis and Catalytic Activities

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 (Zr6O4(OH)4) in as-prepared UiO-66 structure, [Zr6O4(OH)4(BDC)(DMF)10(OH)10] (BDC (benzene-1,4-dicarboxylate), DMF (dimethylformamide)). 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), N2 adsorption-desorption isotherms (BET), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy (PL). It was found that single-atom Ni heterogeneous catalysts supported by the UiO-66 structure, UiO-66/Ni1.0 [Zr6O4(OH)4(C8H4O4)(DMF)10(OH)8Ni2(OH)2(Cl)2], showed a sphere-like morphology with a high specific surface area as well as good thermal stability. 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 (Ea = 23:15 kJmol −1), high turnover frequency (76.19 molecules g min), and high apparent rate constant (kapp = 0:956min−1). In addition, methylene blue (MB) was also chosen as the organic dye model for catalytic reduction reaction. The kapp and TOF for the reduction of MB using UiO-66/Ni1.0 were 0.787 min and 33:89 × 1020 molecules g min, respectively.


Introduction
Nitroaromatic compounds, especially 4-nitrophenol (4-NP), are extensively employed in the fields of pigments, pharmaceuticals, dyes, explosives, plastics, pesticides, wood, or leather preservative [1][2][3][4][5][6][7]. However, 4-nitrophenol (4-NP), known as the toxic and highly hazardous contaminants, is found in agricultural and industrial wastewaters [8][9][10]. This compound released from above industrial sectors can impact negatively on the ecological system and pose serious environmental pollution. 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 [11][12][13]. 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 [14]. It is noticeable that methylene blue is also a high-toxicity carcinogenic organic compound and can be a cause of water environmental pollution [15,16].
Due to this reason, it is necessary to develop the simple, efficient method for removal of these organic compounds in wastewaters. 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 [17][18][19][20]. In particular, the catalytic reduction can be considered as an effective and facile route to remove these organic pollutants [21][22][23][24]. 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.
Metal oxides, metal nanoparticles, and metal complexes can be employed as promising candidates in a wide range of heterogeneous catalytic fields [25][26][27]. The active metal sites are normally located at crystal corners, edges, and facets which exhibit the diverse catalytic properties [28]. Among these above-mentioned heterogeneous metal catalysts, metal complexes are known as single-site heterogeneous catalysts which have attracted increasing attention in recent years [29][30][31][32]. These exposed identical site-isolated metal centers can easily bond and react with reactant molecules in solution. 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.
Metal-organic frameworks (MOFs) are known as porous material, and the crystalline structure consists of a metal clus-ter connected by organic linker molecules [33][34][35][36]. The abundance of MOFs' structure was derived from the diverse combination of metal clusters and organic components which has been intensively investigated for a wide range of application such as gas adsorption, catalysis, energy storage, sensing, and drug delivery [37][38][39][40]. Recently, many works have demonstrated that the existence of missing-linker defects on MOFs has significantly influenced the material properties [41][42][43]. In this case, an outstanding example is UiO-66 which has attracted considerable attention due to its chemical, mechanical, and thermal stability [44]. The crystal structure of UiO-66 consists of zirconium oxide clusters (Zr 6 O 4 (OH) 4 ) connected to six benzene-1,4-dicarboxylate (H 2 BDC) linkers, leading to the formation of the 3D framework. However, the UiO-66 structure has been identified to contain defect sites at zirconium oxide clusters, where a linker is missed such as water and hydroxide [45][46][47]. By utilizing the postsynthetic metalation method, the UiO-66 structure with a high surface provides a highly tunable platform to design single-site heterogeneous catalysts.
In this study, we report a simple route to synthesize the single-site heterogeneous catalyst UiO-66/Ni, where Ni atoms are to anchor to the missing-linker defect sites at     3 Journal of Nanomaterials 2.2. Instrumental Characterization. Powder X-ray diffraction (PXRD) analysis was carried out on a D8 Advanced Bruker anode X-ray diffractometer with CuKα (λ = 1:5406 Å) radiation. The morphology of the samples was observed by scanning electron microscopy (Hitachi S-4800). The transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2100 transmission electron microscope at 200 kV. Fourier transformation infrared analyses were recorded on a Shimadzu IRPrestige-21 (Japan). Nitrogen adsorption/desorption isotherms were measured using a Micromeriti CMS 2020 volumetric adsorption analyzer system. The samples were degassed by heating under vacuum at 393 K for 24 h. The specific surface area of the samples was calculated using the Brunauer-Emmett-Teller (BET) model. X-ray photoelectron spectroscopy (XPS) was recorded with a Kratos Analytical spectrometer. All binding energies were referenced to the contaminant C 1s peak (at 284.6 eV) of adventitious carbon. Thermogravimetric analysis (TGA) was carried out using a TG-DTA instrument (DTG-60H Shimadzu) under atmospheric pressure at a heating rate of 10°C min −1 . Photoluminescence spectroscopy (PL) was measured at room temperature with a photoluminescence spectrophotometer (Horiba FL3). The concentration of 4-nitrophenolate was measured using a UV-vis spectrophotometer (V-630 Jasco) at λ max 400 nm.

Synthesis of Materials
2.3.1. Synthesis of UiO-66. In a typical process, ZrCl 4 (0.53 g, 2.274 mmol) and H 2 BDC (0.38 g, 2.287 mmol) were dissolved into N,N ′ -dimethylformamide (60 mL) to form homogeneous solution at room temperature. Next, the obtained solution was transferred into a Teflon-lined autoclave and maintained at 120°C for 36 h. The white solids were then collected and washed with N,N ′ -dimethylformamide and methanol, respectively. Finally, the resulting products were dried at 60°C for 24 h in the oven to obtain UiO-66.

Catalytic
Activity of UiO-66/Ni. The reduction reaction of 4-nitrophenol with the presence of NaBH 4 solution was used to evaluate the catalytic activity of the UiO-66/Ni catalyst. In this study, the reaction process was performed in a quartz cuvette, and UV-vis spectroscopy was intermittently recorded at room temperature. Initially, 3.0 mL of aqueous 4-NP solution 20 mg L -1 and 5.0 mg of NaBH 4 were alternately transferred into a quartz cuvette. The obtained solu-tion mixture color changed immediately to deep yellow. Then, 5.0 mg of UiO-66/Ni was dispersed in 1.0 mL of deionized water with the assistance of sonication for 1 hour to yield a stable suspension. Subsequently, 5 μL of the obtained UiO-66/Ni dispersion liquid (5 mg mL -1 ) was dropped into the above yellow solution. UV-vis absorption spectroscopy was recorded to determine the reaction progress. 4-NP conversion was calculated as the following equation: where C 0 is the initial concentration of 4-NP and C t is the concentration of 4-NP at time t. The catalytic reduction of methylene blue (MB) by NaBH 4 with the presence of the UiO-66/Ni catalyst was also investigated similarly to the procedure for the catalytic reduction of 4-NP.  with the high degree of network connection. This can be explained for its high stability in most chemical solvent and stable up to 500°C in air atmosphere [49]. It is worth noting that the UiO-66 structure demonstrated the existence of linker vacancies which are located in terminate -OH/OH 2 groups of the Zr 6 O 4 (OH) 4 metal center. The amount of defect sites depends on the synthetic protocol. In this study, the stoichiometric formula of the as-prepared UiO-66 was hypothesized as follows: The structure and crystal phase of the as-prepared UiO-66 and UiO-66/Ni with different concentrations of the nickel precursor were determined by the PXRD analysis, as shown in Figure 1. It is clearly seen that all of the diffraction peaks of UiO-66 and UO-66/Ni were well matched with the simulated PXRD pattern of hydroxylated UiO-66, indicating that the as-prepared UiO-66 was successfully synthesized with a high degree of crystallinity. It is noticeable that the characteristic diffraction peaks of NiO and other nickel atom contained compound nanoparticles not observed in Figure 1. This result can be explained to be the reason for its low degree of crystallinity and very low proportion and quantum size.

Results and Discussion
The PXRD patterns of Ni single-atom-supported UiO-66 show that the crystalline structure of as-prepared UiO-66 did not change after metalation of the nickel atoms with their Intensity (a.u.) (c) Figure 6: XPS survey scan of UiO-66/Ni1.0 (a) and core scan spectra of (b, c) C1s, O1s, Ni 2p, and Zr 3d. 6 Journal of Nanomaterials framework, demonstrating that the chemical structure of the as-prepared UiO-66 with the presence of nickel atoms cannot be destroyed. However, it is difficult to obtain the structure of the Ni single-atom site using the single X-ray diffraction patterns. Thus, in order to investigate the existence of a nickel single atom in the as-prepared UiO-66 structure, the EDX analysis of the as-prepared UiO-66 and UiO-66/Nix (x = 0:3, 0:5, 0:7, and 1:0) samples was performed and is shown in Table 1 and Figures 2(a) and 2(b). These results show that the incorporation of Ni atoms in the UiO-66 structure can be controlled by the amount of NiCl 2 ·6H 2 O used in the synthesis. The Ni·:·Zr atomic ratio increased with the increase in the amount of the NiCl 2 ·6H 2 O precursor from 0.3 to 0.5 mmol and remained constant from 0.7 to 1.0 mmol, demonstrating that the number of defect sites is limited in the as-synthesized UiO-66 structure. TG-DTA analysis of UiO-66 and UiO-66/Ni1.0 catalysts was also performed to investigate the amount of the missing-linker defects in the UiO-66 structure. As can be seen in Figure 2(   The morphology of as-prepared UiO-66 and UiO-66/ Ni1.0 was clearly observed by using SEM images, as shown in Figures 4(a) and 4(b). The SEM images represented that the as-prepared UiO-66 before and after the incorporation of Ni single atoms were sphere-like particles with uniform size and smooth surface.
In order to further investigate the morphology of the samples and the existence of impurity nanoparticles on the   Figure 5. Two isotherms of UiO-66 and UiO-66/ Ni1.0 have the similar shape, exhibited typical type I with a H4 hysteresis loop, and possessed mesoporous structure [50].
The pore size distributions and the BET-specific surface area for the samples were obtained by using desorption data by the BJH method. The pore sizes for the samples do not change considerably, about 4.4 and 4.8 nm, respectively.
The specific surface area of as-prepared UiO-66 was 855.06 m 2 g -1 . After metalation with Ni single atoms, the S BET of UiO-66/Ni1.0 catalysts is slightly decreased, about 10 m 2 g -1 compared to that of as-synthesized UiO-66. These results evidenced that the as-prepared UiO-66 structure is unchangeble after metalation with the Ni single atoms.
The chemical composition and the chemical state of the elements in the sample UiO66/Ni1.0 were determined by using XPS spectra and are presented in Figure 6. As the extensive scan data in Figure 6(a), all the peaks corresponding to the Zr, Ni, O, and C elements have been detected. In addition, the high-resolution spectrum of the sample shows two peaks at 182.74 and 185.03 eV, which could be attributed to Zr 3d 5/2 and Zr 3d 3/2 of zirconium in the Zr 6 O 4 (OH) 4 metal center (Figure 6(b)). The bending energy of Ni 2p 3/2 and Ni 2p 1/2 was 855.28 eV and 872.48 eV, respectively. The energy binding distance of Ni 2p 3/2 and Ni 2p 1/2 peaks was around 17 eV, indicating that Ni2p belongs to the Ni(II) oxidation state in the UiO-66/Ni structure [51]. Those results demonstrated that Ni single-atom metalation with the UiO-66 structure was successfully synthesized via chemical bonding.  The catalytic performance of single-atom Ni-supported UiO-66 was investigated using the model of reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH 4 solution. It is clear that the maximum adsorption peak of 4-NP was observed at 317 nm and shifted to 400 nm with the presence of NaBH 4 solution (Figure 8(a)). This can be explained by an increase in alkalinity upon addition of NaBH 4 , leading to the formation of 4-nitrophenolate ions which corresponds to a color change from light yellow to deep yellow. As can be seen in Figure 8(b), the maximum adsorption peak at 400 nm remained steady over 60 min after the addition of NaBH 4 , demonstrating that the reduction reaction of 4-NP did not occur with the absence of the catalyst. After the addition of 5 μL of the UiO-66/Ni1.0 dispersion liquid, the intensity of characteristic adsorption peak at 400 nm decreased significantly over the reaction process, while the novel maximum adsorption peak was observed at 300 nm and the deep yellow of solution mixture was changed

10
Journal of Nanomaterials into colorless solution within 4 min, corresponding to the formation of the product of 4-AP (Figure 8(c)). Figure 8(d) shows the convention of reduction reactions toward 4-NP at different times without a catalyst and the presence of the as-prepared UiO-66 and UiO-66/Nix (x = 0:3, 0:5, 0:7, and 1:0) catalyst, respectively. The 4-NP convention with the presence of NaBH 4 without a catalyst or 0.01 M Ni 2+ remained stable over 20 min, approximately 5% indicating that NaBH 4 without UiO-66/Ni or NaBH4 with Ni 2+ could not reduce 4-NP. This result means that UiO-66/Ni plays an important role as a heterogeneous catalyst for 4-NP reduction. However, in the presence of the as-prepared UiO-66 catalyst, the reduction of 4-NP into the corresponding 4-AP took 20 min for the conversion of 44%; this figure was 56% for UiO-66/Ni0.3. It is noticeable that it took around 9 min for UiO-66/Ni0.5 for 99% conversion, while the UiO-66/Ni0.7 and UiO-66/Ni1.0 catalysts needed only 3 min and 4 min, respectively, to complete the reduction of 4-NP. The results demonstrated that the presence of nickel single-atom-supported UiO-66 structure plays a crucial role in the excellent catalytic performance toward 4-NP reduction. Specifically, the optimal amount of the NiCl 2 ·6H 2 O precursor, introduced in the UiO-66 structure, was 1.0 mmol (UiO-66/Ni1.0 catalyst) indicating the best catalytic performance. Figure 9(a) illustrates the plot of C t /C 0 versus reaction time of each catalyst, where C t and C 0 represent the concentration of 4-NP at the time of t and 0 min, respectively. The catalytic performance of all as-prepared samples was monitored by UV-vis spectrometry at the interval time of 1 min. The plots of ln ðC t /C 0 Þ versus reaction time for the reduction of 4-NP are shown in Figure 9(b). The excess of concentration of NaBH 4 can be considered a constant during the reaction progress. Thus, the pseudo-first-order kinetics was employed to investigate the reduction reaction of 4-NP. The linear fit with a coefficient of determination very close to unity also supports the assumption of the pseudo-firstorder kinetics. The apparent rate constant values (k app ) were found to be 0.026, 0.036, 0.198, 0.794, and 0.95 min −1 for the UiO-66, UiO-66/Ni0.3, UiO-66/Ni0.5, UiO-66/Ni0.7, and UiO-66/Ni1.0 catalysts, respectively, using the following equation: where C t was the concentration of 4-NP at time t and k app was the apparent rate constant. The detailed results are represented in Table 1 and Figure 9(c). The apparent rate constant values (k app ) increase significantly with an increase in the amount of Ni in the UiO-66 structure. The UiO-66/Ni1.0 catalyst showed the highest k app compared to as-prepared UiO-66 and different UiO-66/Nix (x = 0:3, 0:5, and 0:7). The TOF values were calculated, and the results are shown in Figure 9(d) and Table 2. The catalytic reduction reaction toward 4-nitrophenol was also evaluated by using the TOF values. As can be seen in Figure 9(d) and Table 2 In order to determine the apparent activation energy (E a ) of the reduction reaction of 4-NP using UiO-66/Ni1.0 as the catalyst, the reaction kinetics was conducted over the temperature range of 303-343 K ( Figure 10). The extraction of the apparent activation energy E a from the Arrhenius equation (3) is determined to be 23.15 kJ mol -1 for the UiO-66/Ni1.0 catalyst (Table 3).

Journal of Nanomaterials
The obtained results show that the UiO-66/Ni1.0 catalyst can significantly decrease the activation energy and increase the apparent rate constants of 4-NP catalytic reduction.
The comparison of the various catalysts in previously published papers for the reduction performance toward 4nitrophenol is listed in Table 4. It is clear that the UiO-66/ Ni1.0 catalyst exhibits the highest catalytic activity for 4-NP reduction. For example, k app is 0.529 min −1 for Pd/C NPs, 0.660 min −1 for porous prisms Co@C NPs, and 0.335 min −1 for core-shell nanoparticles Pt@Ag, which are lower than 0.956 of UiO-66/Ni1.0.

The Mechanism of the 4-Nitrophenol Reduction
Reaction Using the UiO-66/Ni1.0 Catalyst. The excellent catalytic performance of the UiO-66/Ni1.0 can be ascribed to the high activity of the nickel single-atom-supported UiO-66 network and the excellent permeability of the porous structure. In this study, the mechanism of the 4-nitrophenol reduction reaction can be described in Scheme 2.
Initially, the 4-nitrophenol can easily penetrate and diffuse into the inner pore spaces of the UiO-66/Ni1.0 catalytic structure. Then, 4-nitrophenol molecules were rapidly absorbed on the Ni(II) active sites which are present in the UiO-66 structure, thanks to the high electrical transport of the Ni(II) active sites, which can facilitate the electron transfer from BH 4 − to 4-NP and reduce the activation energy. At last, 4-AP can be formed efficiently through the hydrogenation reduction of 4-NP and diffused conveniently from the nanoporous structure via the abundant mesopores.   Figure 11(a), during the 5 cycles, the conversion of 4-nitrophenol was more than 95% within 4 min of the reaction process. The XRD pattern of the 5 th reused UiO-66/Ni1.0 catalyst is shown in Figure 11 To further evaluate the catalytic performance of UiO-66/Ni1.0, the catalytic reduction of MB in the presence of NaBH 4 as the reduction agent was also investigated and is shown in Figure 12 and Table 5.
In the presence of 5μL of the obtained UiO-66/Ni dispersion liquid (5 mg mL -1 ), the color of MB solution with the 13 Journal of Nanomaterials intensity of the characteristic adsorption peak at 664 nm disappeared swiftly within 4 min (Figure 12(a)). The MB decolorization kinetics followed the pseudo-first-order kinetics model (Figure 12(b)). Moreover, the k app and TOF for the reduction of MB using UiO-66/Ni1.0 at room temperature are 0.787min −1 and 33:89 × 10 20 molecules g −1 min −1 , respectively. The apparent activation energy E a is determined to be 28.72 kJ mol −1 (Table 5 and Figure 12(d)). The apparent kinetic rate constant for MB decolorization is found to be several ten to hundred times greater than those for photocatalysts in previous publications [52][53][54] and is much higher than that for MB decolorization with NaBH 4 as a reduction agent on the CuNPs@Gelatin catalyst [55], Fe 3 O 4 /SiO 2 /Au/ por-SiO 2 catalyst [56], Ag/PSNM nanocomposite spheres [57], and Ag, Au nanoparticles [58]. These results show that UiO-66/Ni exhibits excellent catalytic performance for the hydrogenation of MB.
Similarly, the recyclability of the synthesized catalyst for the purpose of MB degradation was also assessed. The MB reduction reaction was repeated for five cycles, and the convention remained almost constant, more than 97% within 4 min, as can be seen in Figure 13.

Conclusion
In this research, a novel single-atom Ni heterogeneous catalyst-supported UiO-66 structure has been successfully synthesized through a postsynthetic metalation method. This structure possesses the well-defined active site with a high specific surface area, where single Ni atoms are anchored to the oxygen atoms of -OH/OH 2 capping the defect sites on the Zr oxide clusters of UiO-66. The as-synthesized catalyst was employed to investigate the reduction reaction of 4nitrophenol or methylene blue with the presence of NaBH 4 . It was found that this catalyst exhibits excellent catalytic activity and high stability. It is expected that this research would pave the way for the construction of other single-site heterogeneous catalysts for wide applications.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.  14 Journal of Nanomaterials