Synthesis of novel hexamolybdenum cluster-functionalized copper hydroxide nanocomposites and its catalytic activity for organic molecule degradation

ABSTRACT A novel heterogeneous catalytic nanomaterial based on a molybdenum cluster-based halide (MC) and a single-layered copper hydroxynitrate (CHN) was first prepared by colloidal processing under ambient conditions. The results of the elemental composition and crystalline pattern indicated that CHN was comprehensively synthesized with the support of the MC compound. The absorbing characteristic in the ultraviolet and near-infrared regions was promoted by both of the ingredients. The proper chemical interaction between the materials is a crucial reason to modify the structure of the MCs and only a small decrease in the magnetic susceptibility of CHN. The heterogeneous catalytic activity of the obtained MC@CHN material was found to have a high efficiency and excellent reuse when it is activated by hydrogen peroxide (H2O2) for the degrading reaction of the organic pollutant at room temperature. A reasonable catalytic mechanism was proposed to explain the distinct role of the copper compound, Mo6 compound, and H2O2 in the production of the radical hydroxyl ion. This novel nanomaterial will be an environmentally promising candidate for dye removal.


Introduction
The controlled assembling of several inorganic phases enables the creation of nanocomposites with versatile physical and chemical properties for possible integration in various multi-property devices [1,2]. Material engineering has developed a set of methodologies to design the various families of functional materials by applying solid-state chemistry, molecular chemistry, colloidal processing, or biochemistry. The octahedral molybdenum cluster halides with the general formula of A 2 [{Mo 6 X i 8 }X a 6 ] (A = Cs, K, or alkylammonium cations; X = Cl, Br, or I) constitute a representative family of polyversatile integrable compounds into inorganic nanocomposites [3]. A 2 [{Mo 6 X i 8 }X a 6 ] is based on the unique structured [{Mo 6 X i 8 }X a 6 ] 2cluster unit that is used as functional building blocks that are synthesized by solid-state chemistry and wet solution chemistry [4]. The latter contains the Mo 6 clusters with the Mo-Mo bonds that are stabilized by facecapping ligands (X i ) and terminal ligands (X a ). From an electronic point of view, the [{Mo 6 X i 8 }X a 6 ] 2cluster unit exhibits a closed 24 valence electron shell. The understanding of the intrinsic natures of redox transitions, photochemistry, or electrochemistry has motivated the research by several groups around the world [5][6][7][8][9]. The discovery of the unique photoactive and oxidative characteristics has broadened the potential applications to the fields of catalysis [10,11], optoelectronics [12,13], and biology [14][15][16]. The photoluminescence of the [{Mo 6 X i 8 }X a 6 ] 2cluster unit in a large red-NIR window originates from the important geometrical relaxations occurring in the triplet states. These geometric relaxations follow two possible deformations when the octahedral clusters are photo-excited. These deformations correspond to either the elongation of the Mo 6 octahedron along the pseudo-4 fold axis or to the elongation of a Mo -Mo bond. One or the other of these deformations are preferred depending on the environment of the [{Mo 6 X i 8 }X a 6 ] 2blocks (counterions, crystal packing) [9]. As a phosphorescent dye, another characteristic of the [{Mo 6 X i 8 }X a 6 ] 2cluster units is their capability to produce singlet oxygen ( 1 O 2 ) [8,17]. The catalytic characteristic of the halide [{Mo 6 X i 8 }X a 6 ] 2cluster units was approached following two pathways, i.e. i) a photoactive catalyst based on the photoexcited cluster generating a pair of holes and electrons [10,18] and ii) redox catalyst based on the oxidized [{Mo 6 X 8 }X 6 ] 1− state as a strong and powerful oxidant [6,11]. For instance, the mixed halide [{Mo 6 I 8 }Cl 6 ] 2cluster unit can be reversibly oxidized [6]. Based on this behavior, the recycling ability of the heterogeneous catalyst based on the Mo 6 cluster has been developed. The physicochemical interactions between the metal clusters and inorganic matrix in the multi-component material are essential to control the structural characteristics and the catalytic mechanisms. The catalytic properties of the Mo-based MC have been developed in recent and relevant studies by utilizing the Ouzo effect originating from nanomarbles for increasing the HER activity [19] or incorporated with graphene for photocatalytic water reduction [20] and hydrogen evolution [21] with a high efficiency. The prominent photoactive and oxidation performance of the Mo 6 cluster anchored on a layered double hydroxide activated by the hydrogen peroxide oxidizer has been revealed with a proper catalytic efficiency [11]. However, the reuse of the catalyst has not met the requirement.
For the first time, we now propose a new matrix based on the single-layered hydroxide salt (LHS) to promote the redox property of the Mo 6 cluster-based halide. The obtained nanocomposites were synthesized by using a colloidal process under ambient conditions. The general formula of LHS is M(OH) 2 , Cl − and acetate) with a negative charge n − [22]. The advantageous LHS makes this compound useful as a strategic precursor for developing copper oxide nanostructures [23,24]. Indeed, a few LHS have been reported as promising catalysts for dye removal by an advanced oxidation process using the H 2 O 2 oxidizer to produce the radical species; Zn 5 [OH] 8 (counterion) 2 /H 2 O 2 or Cu 2 [OH] 3 (counterion)/H 2 O 2 systems resulting in a high catalytic efficiency for organic dye bleaching [25,26] or layered copper hydroxynitrate (CHN) and CHN nanosheets/H 2 O 2 for the recyclable enzymemimicking colorimetric sensor of biothiols [27]. In addition, the incorporations of LHS and supporting material systems have been studied for different application purposes such as CHN/ZnO [28] and anionic iron(III) porphyrins/Zn 5 [OH] 8 (counterion) 2 [29] for dye degradation, CHN/Cu 2 O for photocatalysis [30], or CHN for conductive copper thin-films [31]. Besides the huge potential for designing a heterogeneous catalyst due to its low toxicity, high catalytic characteristic, and reusability, the copper compound has attracted attention as a promising UV-NIR adsorbing pigment for smart window applications [32].
To the best of our knowledge, only the efficient photo-redox heterogeneous hybrid composite catalyst constituting the [{Re 6 S i 8 }(CN) a 6 ] 4cluster compounds and copper oxide-modified TiO 2 have already been reported. It has been demonstrated by Kumar et al. that such an association leads to a good nanocomposite material for the reduction of CO 2 under visible light irradiation [33]. The important point was noted that the hexanuclear rhenium clusters act as a sensitizer to the copper-modified TiO 2 during the catalytic reactions.
Besides the improved catalytic activity, the reuse of catalysts also plays an important role in industrial applications. The incorporation of the different redox cluster units with a 1D, 2D or 3D structured material has progressed not only to understand the tunability of their redox and catalytic properties followed by the compositions and structural arrangements, but also to enhance their reuse. This study aimed to assemble a copper hydroxynitrate (CHN) and a cluster compound (MC) (i.e. A 2 [{Mo 6 X i 8 }X a 6 ] (A = Cs, or alkylammonium cations; X = Cl, Br, or I) in the nanocomposite to improve the optical and catalytic properties for the heterogeneous catalysts in water treatment. Interestingly, the A 2 [{Mo 6 X i 8 }X a 6 ] halide acted as a catalytic agent for the formation of the crystalline copper hydroxynitrate when wet solution chemistry has been coupled with heat treatment. The elemental composition, morphology, and optical and magnetic properties of the obtained materials were comprehensively investigated. Moreover, the proper catalytic property activated by the hydrogen peroxide (H 2 O 2 ) and their reuse in the dye removal reaction of the obtained nanomaterial has been investigated.

Preparation of the MC-functionalized copper hydroxynitrate nanocomposites
Copper hydroxynitrate, abbreviated as CHN, was fabricated in the presence of the A 2 [{Mo 6 X i 8 }X a 6 ] compounds (MCs) (A = Cs, or alkylammonium cations; X = Cl, Br or I) by the colloidal process under ambient conditions. Cu(NO 3 ) 2 •3H 2 O dissolved in ethanol (17 g/L) and the solution of MC in acetone (0.7 g/L) were separately obtained by using a magnetic stirrer with sonication for 1 h at room temperature. The MC solution was then slowly poured into the copper aqueous solution and stirred for 24 h to obtain a slight cloudy suspension. The four weight ratios of the different MCs and Cu(NO 3 ) 2 •3H 2 O in the aqueous solutions were 1:5, 1:10, 1:15 and 1:20, respectively, denoted as the 15, 110, 115 and 120 numbers after CHN (Table 1). Next, these solutions were put in a hot water bath, followed by thermal treatment to remove the solvent at 80°C until a dried green crystalline powder was obtained ( Figure 1). Generally, the advantages of the present procedure include two steps: i) a simple mechanical mixture of the cluster and Cu(NO 3 ) 2 .3H 2 O to first form tiny CHN crystals at RT, and ii) growth of the CHN crystal and performance of the nanocomposite with the cluster during  stirring at 80°C in air. The MC@CHN products were completely dried at 100°C for 6 h, then washed several times for removing any residual components. A referenced CHN was synthesized from the Cu(NO 3 ) 2 •3H 2 O compound using an ammonia solution as a catalytic agent. A homogeneous Cu(NO 3 ) 2 •3H 2 O solution in ethanol (30 g/L) was first treated at a temperature of 70°C for 30 min, then 0.2 ml of the NH 3 solution was slowly dropped into the aqueous solution. The reaction to fabricate the CHN occurred for 2 h at 70°C. The powder was collected by centrifugation and washing several times with ethanol, then dried at 100°C for 24 h in air. All the powders were kept in a dry condition at room temperature for the next characterizations.

Sample characterization
X-ray powder diffraction (XRD) patterns were recorded at room temperature in the 2 theta range of 10°-60° using the Rigaku Smart Lab (Rigaku Corporation, Japan) 3 diffractometer (Cu Ka radiation) with a step size of 0.02° and a scan speed of 2° min −1 . Le Bail fittings were performed using the FullProf program included in the WinPLOTR software. The zero-point shift, asymmetry parameters, and lattice parameters, and β angle were systematically refined, and the background contribution was manually estimated. The reflectance spectra of the powders and absorbance of the degraded dye solutions were measured by UV-Vis-NIR spectroscopy (V570, Jasco Corp., Japan) in the wavelength range of 220 to 2000 nm at the scan rate of 400 nm/min. The luminescent emitting spectra of the powders were measured by high-performance fluorescence spectroscopy (JASCO FP8500; Jasco Corporation, Japan) connected to a Xenon lamp at the scan rate of 500 nm/ min. The surface morphology and elemental composition were analyzed by field emission scanning electron microscopy (FE-SEM, SU8000, Hitachi High-Technologies Corp., Japan) at 10 kV coupled with an energy-dispersive X-ray (EDX) analysis device. Highresolution observations of the powder were performed by HR-TEM (JEOL JEM 2100 F, JEOL Ltd., Japan) equipped with an EDX analysis device. The typical chemical vibration of the powder was verified by Fourier transform infrared spectroscopy (FTIR) (Thermo scientific Nicolet 4700, Thermo Fisher Scientific Inc., Japan) in the wavenumber range from 4000 to 400 cm −1 using a KBr pellet. The electron binding energy spectra within the MC and MC@CHN materials were measured by X-ray photoelectron spectroscopy (XPS) (PHI Quantera SXM (ULVAC-PHI), Inc., Japan) using Al Kα radiation at 20 kV and 5 mA and the taken-off the angle of 45°. All the binding energies were calibrated concerning the C 1s peak of the adventitious carbon at 285 eV. The magnetic property of the nanocomposite powders carried out in medical capsules was measured by using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design (MPMS-XL), USA). The temperature in the range from 2 to 300 K was used to determine the dependence on the susceptibility in a magnetic field of 5kOe.

Catalytic degradation procedure
The catalytic activities of the MCs, CHN110 and MC@CHN110 compounds were evaluated by the oxidation degradation of methylene blue (MB) in an aqueous solution. All the catalytic reactions were performed using a 0.03 g catalyst in 20 ml of the MB aqueous solution (20 mg/L) without and with a specified dose of the activating H 2 O 2 oxidizer (5, 10, and 15 mM). The weight of the MC catalyst (0.003 g) used as a reference corresponded to 10 wt% of the MC@CHN110. All the reactions were performed under magnetic stirring for 2 hours at room temperature in the dark. Afterward, 5.0 mL of the degraded dye solution was filtered by a hydrophobic plastic membrane (0.22 μm). The filtered MB was determined at the absorption peak of 664 nm, which is characteristic of MB in the UV-Vis absorption spectrum at the given time intervals of 30 min. The catalyst reusability was determined for the catalysts with an excellent removal efficiency through 4 cycles. The color removal efficiency (η, %) of MB was calculated using Eq. 1: where C 0 is the initial concentration of MB and C t is the concentration of MB after t min.

The characterization of the MC@CHN powder
A new preparation of the copper hydroxynitrate compound in the presence of the Mo 6 cluster was investigated. Powder X-ray diffractograms of the MC@CHN nanocomposites with various MC and CHN weight ratios are displayed in Figure 2(a). The major reflection of the CHN structure (space group n°4, P1211, a ≈ 5.605 Å, b ≈ 6.087 Å, c ≈ 6.929 Å, β ≈ 94.48°) can be observed. A Le Bail refinement of the whole samples has been performed and reasonably reliable factors have been obtained (Table 2). It confirmed that the MC@CHN nanocomposites mainly contain the monoclinic CHN crystal phase (Figure 2b). An example of refinement is displayed in Figure SI1 in order to illustrate the good correspondence between the experimental and simulated XRD pattern. These results are in agreement with the monoclinic CHN crystal phase from previous studies [30,31]. The crystallinity of the A 2 [{Mo 6 X i 8 }X a 6 ] cluster almost disappears in the XRD diagram of the nanocomposite that suggests an amorphous MC phase mixed with the CHN crystals. The increase in the Cu precursor: MC ratio did not affect the crystallinity of CHN. Figure 2 (c) shows the investigation to understand the influence of the chemical composition of the A 2 [{Mo 6 X i 8 } X a 6 ] precursor (i.e. X and A) on the crystallinity of CHN. There is no significant difference in the CHNassigned crystal peak whatever the A 2 [{Mo 6 X i 8 }X a 6 ] starting precursor chemical composition. The Mo 6 metallic core will be an important factor that determines the crystalline form of CHN without the effect of the inner and apical ligands.
For a better understanding of the performance of CHN, the FT-IR spectra of the MC@CHN powders with different MC: CHN ratios were recorded ( Figure  SI2). As seen in the IR spectrum of (1)@CHN15 without washing, the vibrational peaks at 883, 784 and 676 cm −1 can be assigned to the hydrogen bonding frequencies related to the Cu-O-H bonding. In addition, the vibrational peaks contributed by the OH group also appear at 1663, 3443, and 3535 cm −1 in agreement with a previous publication. The IR peaks specific to the common NO 3 − group are 813 (υ2), 1048  (υ1), and 1384 cm −1 , while the peaks at 1334 and 1422 cm −1 can originate from the symmetric and asymmetric stretching mode of the NO 3 − group occupied between the copper layer hydroxide. All the vibrational bands assigned to Cu 2 (OH) 3 NO 3 are in agreement with previous reports [28,30,31]. The IR peaks that could be assigned to the Mo 6 metallic core in the nanocomposite could not be assigned. However, the existence of the n-C 4 H 9 counter cation in the (1) @CHN15 without washing was confirmed by the peaks at the frequencies of i) 2962, 2933 and 2877 cm −1 contributed by the stretching mode of C-H; ii) 1378 and 1466 cm −1 contributed by the bending mode of C-H, and iii) 750 and 1163 cm −1 contributed by the bending mode of C-C. These characteristic peaks disappear after the (1)@CHN15 nanocomposite was purified. The IR spectra showed no difference in the chemical bonding caused by the various compositions of the nanocomposite. This result proved that the counterion of the A 2 [{Mo 6 X i 8 }X a 6 ] cluster unit was separated from the [{Mo 6 X i 8 }X a 6 ] metallic core during the mixing with CHN.
The Cu 2 (OH) 3 NO 3 nanocrystalline was recognized in the SEM image of (1)@CHN110 (Figure 3a) presenting the visible pattern as seen in the HR-TEM image (Figure 3b). This is in agreement with the X-ray diffractograms. After the first step of the reaction at RT for 24 h, the nanocomposite is in the form of a porous structure and a flower of nanometric size (Figure 3c). However, the efficiency to synthesize CHN at room temperature is poor, and difficult to collect the crystal product. For this reason, the thermal treatment at 80°C was applied for the slurry. The crystalline and amorphous phase mixed nanocomposite was recognized in Figure 3(d) with the CHN crystal blended with the amorphous M 6 clusters. The element composition spectrum of the CHN and Mo 6 cluster was confirmed again through the use of the STEM-EDX mapping, following the intercalation of the Cu 2 (OH) 3 NO 3 and Mo 6 cluster phases (Figure 3e and f). Similarly, the use of the (3) and (4) MC precursors to fabricate the nanocomposite also result in the amorphous and crystalline mixed-phases (Figure 3g and h).
Aiming to confirm the existence of the integrity of the Mo 6 octahedral structure after the thermal and chemical treatments, the optical absorption and emission spectra were studied. In the first investigation, the dependence of the optical property on the composition ratio between the MC and Cu precursor was studied ( Figure SI3). The optical spectrum of CHN shows a strong absorption in the UV range below 300 nm, while the characteristic absorption of MC is observed below 400 nm. CHN also presents the relative absorption in the near-infrared range (NIR) from 600 to 900 nm ( Figure SI3a). The spectrum of MC@CHN110 is composed of the optical absorption of both the MC and CHN components at the various weight ratios. However, the luminescent emission efficiency in the NIR range of the (1)@CHN decreases when the concentration of the Cu precursor increases ( Figure SI3b). Considering this result, a large CHN crystal could increase the light absorption in the NIR range as seen in Figure SI3a (Figure 4b), respectively [17]. However, the shape of the spectra that had changed consisted of an asymmetric structured broadband centered roughly at 622 nm for all the MC@CHN110 powders. This is explained by a significant exchange of the apical ligand of MC during the synthesizing steps. As is well known, the intensity of the photoluminescence of the Mo 6 cluster decreased when the apical halogen ligands are partially replaced by ethanol, H 2 O molecules, or hydroxyl anions. Based on quantum chemical studies, Costuas et al. reported that the NIR photoluminescence originated from the important geometrical relaxations of the [{Mo 6 X i 8 } X a 6 ] 2based system occurring at the triplet state, depending on the outstretching of the Mo 6 octahedron and the elongation of one Mo-Mo bond [9]. In addition, the external environment (counter-ions, crystal packing) of the cluster has a noticeable impact on its relaxation processes [9]. This is in agreement with the explanation that the separation of the counterion and the exchange of apical ligands by a water molecule causes modification of the NIR photoluminescence peak of the [{Mo 6 X i 8 }X a 6 ] 2cluster unit (X = Br, Cl, I). Table 3  The element component ratio of the nanocomposite was determined by using an HR-SEM coupled EDX device with a penetrated depth of about 1 μm ( Figure SI5). The Mo/ligand ratios of the different Mo 6 clusters (the theoretical value of 6/14) were confirmed in Figure SI5 and Table SI1. The Mo/ligand ratios determined for Mo/Cl(1), Mo/Cl(2), Mo/Br(3), and Mo/I(4) were 6/13.5 ± 0.5, 6/13.3 ± 0.7, 6/11.4 ± 0.4, and 6/7.93 ± 0.5, respectively. The Mo/ligand ratio was calculated from the atom percentage of the Mo and ligand atoms indicated in the EDX spectrum. The error values of the ligand atom were calculated from 3 measurements at different positions on the surface of the samples. The apical Br and I ligands seem not to be stable in comparison to the Cl ligand. The Mo/Cl ratio is almost similar to the theoretical value (6/14) suggesting the stability of the Cl ligand of the [Mo 6 Cl 14 ] 2precursor during treatment of solvent and thermal conditions. These data are in agreement with the visible modification of the optical absorption and emission properties followed by the exchange of the apical ligand [2,9].
For a deeper discussion about the intrinsic interaction between the Mo 6 cluster and CHN as well as the confirmation of the CHN structure, X-ray photoelectron spectroscopy was performed for the Cs 2 [{Mo 6 Cl i 8 }Cl a 6 ] (2) and (2)@CHN110 powders. All the binding energies were calibrated concerning the C1s peak of the adventitious carbon at 285 eV. The chemical confirmation of CHN was first determined based on the results of the binding energy and element compositions from the XPS survey scan spectrum of the (2) @CHN110 powder ( Figure SI6). The Cu 2p binding energy peak at 935.30 (2p 1/2 ) and 954.5 (2p 3/2 ) was assigned to the Cu-O bonding of the CuOH group ( Figure 5 and Table SI2). In addition, the measured Cu/N atomic ratio of about 2/1 seen in Table 4, the N1s binding energy peak at 407.2 eV assigned to NO 3 − , and the O1s binding energy peak at 531.4 eV assigned to OH seen in Tab. SI2 proves the existence of the Cu 2 (OH) 3 NO 3 structure.
As reported in a previous study of the Mo 6 @layer double hydroxide (Zn 2 Al), the appearance of the Mo-O-Zn or Mo-O-Al bonding between the Mo atom from the Mo 6 cluster and O atom from the hydrotalcite layers of the LDH was confirmed by one new peak at 234 eV (3d 3/2 ) indicating Mo-O bonding [11]. A similar result indicating the Mo-O bonding in this study was determined at 232.8 eV (Mo 3d 5/2 ) and 235.6 eV (Mo 3d 3/2 ). On the other hand, the Mo 6 cluster could be partially oxidized during the synthesis of CHN to form the MoO 3 compound. The Mo-O bonding is suggested to belong to the MoO 3 compound or Mo-O-Cu new linking that is transformed from the Mo-Cl bonding (229.7 and 232.8 eV) of the Mo 6 cluster ( Figure 5 and Table SI2). Following the result of the total element compositions ( Figure 5 and Table 4), the Cs atom is almost separated from the product during the washing procedure that was determined by a peak at 724.5 eV. The analysis of the inner and apical ligand is also important to confirm the stability of the Mo octahedron structure. The deconvolution spectrum of the Cl 2p region of the Mo 6 cluster and in the nanocomposite similarly shows four peaks at 198. 4, 200.0, 200.4 and 202.0 eV that indicate Cl i 2p 3/2 , Cl i 2p 1/2 , Cl a 2p 3/2, and Cl a 2p 1/2 , respectively ( Figure 5 and Table SI3). The Table 4 Figure 6 presents the temperature dependence on the magnetic susceptibilities of the CHN and M 6 @CHN110 nanocomposite in the magnetic field of 5kOe. All the curves show the coincident shape at all temperatures without any significant difference between the CHN and Mo 6 @CHN110 powders. However, the T max position at 11.6 K is assigned to CHN in agreement with previous studies [37,38]. This peak slightly shifts to a lower temperature at 10.3 K when the Mo 6 cluster is added. In addition, the maximum value of the magnetic susceptibility χ M of CHN decreases from 0.043 emu/mol to 0.039 emu/mol due   (1)@CHN is approximately about 2.5 while the other ones are slightly lower than the expected value (Tab. SI4). The magnetic susceptibility illustrates how much a material will become magnetized in an applied magnetic field with χ M > 0 indicating the paramagnetic characteristic of CHN that is also obtained for the MC@CHN110 powders. The Mo-O-Cu possible covalent bonding will limit the magnetic moments of the electrons of the copper atom resulting in the reduction of the magnetic susceptibility. The reason for this interesting phenomenon should be determined in the future.

Study of the catalytic properties of the nanocomposite
The decolorization performances of the aqueous methylene blue (MB) solution, as the model of an organic pollutant, using the MC, CHN, and MC@CHN110 catalysts were evaluated for 2 h at RT in the dark (Figure 7a). The colour removal efficiency was calculated from Eq.  40, and 20%, respectively, that shows a better dye removing possibility than CHN. A similar tendency of dye removal efficiency caused by the MCs also occurred in combination with CHN. The photos of the MB solutions after reacting with a specialized catalyst are presented in Figure SI7. In Figure 7(b), the blue-colored MB existing on the surface of (2) @CHN110 was recognized and it is significantly reduced for (3)@CHN110 and (4) @CHN110. It means that the MB-concentration reduction depends on both adsorption and catalytic processes in the case of the nanocomposite. As is known, the catalytic characteristic of the MCs was partially affected by the nature of the ligands, and it is reduced from the Cl, Br and I ligand that contributes to the possibility of generating a pair of holes and electrons on the cluster [9,17]. Even though the catalytic property of itself cluster is relative, the reuse of the MCs faces a big difficulty. For this reason, finding suitable supporting material like CHN to improve the reuse of the MCs is one of the main goals of this study. Interestingly, the MB color visually observed completely disappears when it reacts with the MC@CHNb catalyst activated by H 2 O 2 resulting in a brown-colored powder, i.e. a product containing Cu (I) and Cu (II) elements. This result confirmed that the H 2 O 2 activated nanocomposite completely reduced the blue-colored MB or the catalytic reaction plays a crucial role to degrade the dye without an adsorption mechanism.
An investigation of the activation of H 2 O 2 on the MC@CHN110 compounds was then performed at different H 2 O 2 concentrations as presented in Figure 7(c). The dye removal efficiency values caused by the (2)@CHN110 and (4)@CHN110 are impressive at more than 98% in comparison with another one even for the minimum H 2 O 2 concentration (5 mM). These values are in agreement with the photos of the visually degraded MB color. The (3) @CHN110 /H 2 O 2 system presents the catalytic possibility with the saturated removal efficiency higher than 85% at the maximum H 2 O 2 concentration (15 mM). It is interesting to obtain a high removal efficiency by using Cs 2  The study of the MB removal efficiency as a function of the reacting time with an interval time of 30 min was carried out for the MC@CHN110/H 2 O 2 (10 mM) catalyst systems as seen in Figure 7(d). The calculated results were based on the optical absorption spectra of the filtered-MB solution after reacting with the catalysts shown in Figure SI8. Interestingly (Figure 7a).
The activity and stability of the MC@CHN110 nanocomposite were further investigated by four continually recycling runs. The catalyst reuse was carried