Catalytic reduction of toxic dyes over nickel oxide nanoparticles supported on CMK-3 catalyst

In the current paper, a NiO nanoparticles-loaded mesoporous carbon (CMK-3) catalyst, denoted as NiO/CMK-3, has been successfully synthesized using a facile strategy. The as-prepared material has been characterized through XRD, Raman spectroscopy, low-temperature N2 physisorption measurements, FTIR, FE-SEM, TEM, and XPS. The as-fabricated NiO/CMK-3 catalyst manifested a superior activity in the NaBH4-assisted reduction of methylene blue (MB) dye to its colorless leuco form. Remarkably, over 99% of 25 mg L−1 MB was reduced by 7.5 mM/L NaBH4 using 0.1 g L−1 NiO/CMK-3 within 3 min at room temperature. Furthermore, the kinetics study confirmed the appropriateness of the pseudo-first-order kinetic model for elucidating the kinetics of MB reduction by the catalyst. Importantly, the NiO/CMK-3 catalyst maintained almost constant catalytic activity even after 5 times of reuse in MB reduction, demonstrating its superior stability and reusable ability. So, NiO/CMK-5 appears as a promising heterogeneous catalyst for the effective remediation of dye-containing wastewater.


Synthesis of NiO/CMK-3 catalyst
The NiO/CMK-5, catalyst with a NiO loading of 5 wt%, was synthesized as follows: 1.0 g of as-obtained CMK-3 was placed in 50 mL of absolute EtOH and sonicated for 1 h.Subsequently, the desired quantity of the as-obtained NiO powder was gradually added to the CMK-3 dispersion under sonication.The resulting suspension was permitted to stir at ambient temperature for 12 h before being evaporated to dryness at 80 °C for 5 h.The resulting powder was then oven-dried at 100 °C overnight and ultimately calcined in flowing nitrogen at 500 °C for a period of 2 h.The final solid catalyst was referred to as NiO/CMK-3.

Catalyst characterization
To evaluate porous properties of the catalyst, N 2 -adsorption/desorption isotherms at 77 K were recorded using a Quanta Chrome Nova 3200 analyzer.A Nexus-870 FTIR spectrometer was employed to record the Fourier transform infrared (FTIR) spectra of the obtained materials.Raman spectroscopy was conducted at room temperature using SENTERRA Dispersive Raman Microscope.X-ray photoelectron spectroscopy (XPS) was utilized to identify the catalyst structure with a Thermo Scientific X-ray photoelectron spectrometer.Powder X-ray diffraction patterns were recorded using a X'Pert PRO MPD PANalytical diffractometer at the 2θ range from 10° to 90°.

Catalytic activity tests
The NaBH 4 -assisted reduction experiments were executed at 25 ℃ in a 500 mL glass beaker containing 200 mL of dye at the desired concentration under continuous mechanical stirring.In a typical experiment, 0.025 g catalyst was introduced into the dye solution, followed by the addition of NaBH 4 at 7.5 mM/L.After specific time intervals elapsed, samples were withdrawn from the reaction mixture, filtered via a syringe filter to get rid of the suspended catalyst, and analyzed for their remaining dye concentration via a UV-vis spectrophotometer (JASCO, V-750 UV-visible Spectrophotometer).Each reduction experiment was conducted three times, and the mean values were used.

Catalyst characterization
The XRD diffractograms of CMK-3 and the NiO-loaded catalyst are illustrated in Fig. 1a.For the sake of comparison, the diffractogram of NiO nanoparticles is also presented in Fig. 1b.As for the pure CMK-3 material, two broad reflections can be observed at 2θ values of about 24° and 44°, corresponding to the (110) and (200) diffraction facets of graphite, respectively, revealing the low crystalline extent of the as-obtained CMK-3 29 .Regarding the XRD diffractogram of the NiO/CMK-3 catalyst, the reflections positioned at 2θ values of approximately 37.2°, 43.29, 62.85, 75.35, and 79.15 originated from the (111), ( 200), ( 220), (311), and (11) diffraction facets of NiO (JCPDS File No. 78-0643), respectively.This affirms that the NiO species has been successfully loaded.Please note that the wideness of the reflections attributed to NiO reveals the small size of the NiO nanoparticles in the composite 38 .The average crystallite size of the NiO nanoparticles was estimated to be approximately 9.87 nm based on the value of full width at half maximum (FWHM) of the (200) diffraction peak, adopting the Debye-Scherrer equation.
Figure 2 illustrates the Raman spectra of the CMK-3 and NiO/CMK-3 samples.Considering the bare CMK-3, two prominent peaks located at wavenumbers of about 1310 cm −1 and 1660 cm −1 are evident, corresponding to the disordered (D-band) and graphitic (G-band) carbon structure, respectively [39][40][41] .The ID/IG area ratio of the CMK-3 was computed to be 0.99, revealing its low graphitization extent, in good accord with the results from the XRD study.The NiO/CMK-3 hybrid demonstrates an almost similar Raman spectrum to CMK-3, signifying that the NiO loading does not influence the structure of the pure support material.
FTIR analysis was executed to scrutinize the chemical nature of the NiO/CMK-3 catalyst.FT-IR spectra of CMK-3 and NiO/CMK-3 are depicted in Fig. 3.The FTIR spectrum of the pristine CMK-3 depicts an absorption band at around 1133.5 cm −1 , which is attributed to the stretching modes of the aromatic C-O bonds 39 .The absorption peak assigned to the C=C stretching vibrations was obviously observable at approximately 1619 cm −141 .In addition, the band emerging at 3404 cm −1 belongs to the O-H stretching vibrations 42,43 , whereas the peak at 2916 cm −1 is likely to arise from methylic vibration 39 .All these characteristic peaks distinctly insinuate that the expected mesoporous carbon material was successfully synthesized employing the adopted synthetic strategy.An identical spectrogram was observed for the NiO-loaded catalyst, revealing that the structure of the support material remained basically unaltered after the loading of NiO nanoparticles.The porous attributes of the synthesized materials were dissected through low-temperature nitrogen physisorption experiments.The resulting N 2 sorption isotherms and BJH pore size distribution plots are manifested in Fig. 4. As depicted in Fig. 4a, the isotherm of the CMK-3 material is identified as type-IV isotherms, typical for mesoporous materials with pore widths in the range from 2 to 50 nm.A pronounced hysteresis loop also appears at a relative pressure from 0.45 to 1.0, caused by the capillary condensation of N 2 in mesopores.The NiO-containing material (Fig. 4a) retained the same isotherm shape, ascertaining that the original mesoporous system of the parent support material was well conserved after loading NiO species.Please note that NiO/CMK-3 adsorbed less nitrogen compared to bare CMK-3, and the hysteresis loop becomes less obvious, signifying the successful incorporation of the NiO nanoparticles inside the porous system of CMK-3.The BJH pore size distribution plots derived from the isotherms, Fig. 4b, further demonstrate the mesoporous features of the two samples.
CMK-3 has a BET-specific surface area and total pore volume of about 495 m 2 g −1 and 0.37 cm 3 g −1 , respectively.As expected, the BET-specific surface area and total pore volume of the pristine CMK-3 obviously dropped from to 792.3 to 328.7 m 2 g −1 and 0.58 to 0.31 cm 3 g -1 , respectively, after the loading of NiO species.This implies that the porous system of the CMK-3 is partially occupied by the NiO nanoparticles.It is worth mentioning that NiO/CMK-3 catalyst still possesses a relatively high surface area and pore volume, which are critical for effective catalytic process.
The morphologies of the synthesized NiO/CMK-3 catalyst were investigated through scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques.The SEM images, in Fig. 5a,b, shows highly agglomerated nanoparticles with irregular shape and rough surfaces.The TEM image reveals that NiO NPs are uniformly distributed in the CMK nano layer with diameters ranging from 6 to 10 nm (Fig. 5c).
The XPS survey spectra of NiO@CMK-3 (Fig. 6a) confirmed the existence of C, O, and Ni elements.The chemical valence of the surface species in the NiO@CMK-3 catalyst was additionally assessed by XPS spectra.The XPS spectra of the C 1s peaks were deconvoluted into three spectral bands at 284.2 eV, 285.8 eV, and 288.8 eV.The prominent peak located at 284.  43 .In the deconvoluted Ni 2p spectra (Fig. 6d), the peaks that appeared at binding energies of 855.6 and 872.5 eV are related to Ni 2+ ; meanwhile, Ni 3+ is responsible for the small shoulder at 853.7 and 871.06 eV binding energy, revealing the existence of a small amount of Ni 2 O 3 on the surface 46 .

Catalytic reduction of toxic dyes
The catalytic performance of the as-obtained NiO/CMK-3 was scrutinized in the reductive decolorization of methylene blue (MB) dye at room temperature, adopting NaBH 4 as an efficacious and eco-friendly reductant.MB is a cationic thiazine pigment that is vastly used for coloring and printing purposes in multifarious industries.Nonetheless, MB dye poses a grave peril to mankind's health and the eco-environment, so the effectual remediation of MB-containing industrial effluents is of massive importance 7 .The change of the decolorization degree of MB (represented by Ct/Co, where C o (mg L −1 ) denotes the concentration of the MB solution before the commencement of the reduction reaction, and C t (mg L −1 ) designates the remaining concentration of the dye solution after the elapse of time t of the reaction) during the process is drawn in Fig. 7.In the "NiO/CMK-3 + NaBH 4 " system, the decolorization degree of MB is accomplished within 3 min.For the sake of comparison, the reduction of MB (25 mg L −1 ) using the "NiO/CMK-3 only", and "NaBH4 only" systems was also appraised (Fig. 7).For the "NiO/CMK-3 only" system, only slight decolorization degree of 32.1% was observed.Similarly, as can be noticed from Fig. 7, the reduction of the MB dye essentially did not occur using the "NaBH4 only" system, which is likely due to the high activation energy barrier of the reaction 7 .The direct electron transfer from the − anions and conveying them to MB molecules.Such electron transfer disrupts the conjugated chromophoric system of MB, leading to its decolorization and the generation of the colorless Leucomethylene blue (LMB) product 7,47 .

Effect of catalyst dose on the catalytic reduction of MB
The ideal catalyst amount for the reduction of MB dye was determined by executing a series of reduction experiments with four different doses: 0.005, 0.075, 0.1, and 0.15 g L −1 of NiO/CMK-3.All other operational parameters were held unaltered: 25 °C, 3 min, [MB] = 25 mg L −1 , [NaBH 4 ] = 7.5 mM/L.The variation of the decolorization degree of MB as a function catalyst dose is plotted in Fig. 8a. Figure 8a depicts that the MB decolorization degree augmented as a function of NiO/CMK-3 dose, and the highest decolorization degree was attained at 0.1 g L −1 , which is likely to arise from the increase in the number of available active centers for catalytic reductive decolorization of the MB dye 48 .Higher amounts of NiO/CMK-3 resulted in no further considerable improvement in the MB decolorization degree.Hence, 0.1 g L −1 of NiO/CMK-3 was taken as the optimum dose for maximum MB decolorization.

Effect of NaBH 4 concentration on the catalytic reduction of MB
The impact of NaBH 4 concentration on the decolorization degree of MB was further investigated.Figure 8b depicts the plot of decolorization degree MB versus NaBH4 concentration.For this purpose, different NaBH 4 concentrations of 2.5, 5, 7.5 and 10 mM/L were used.All reduction tests were accomplished at 25 ℃ for 3 min, using 0.1 g L −1 NiO/CMK-3 dose.As perceived in Fig. 8b, with the concentration of NaBH 4 increased from 2.5 www.nature.com/scientificreports/ to 7.5 mM/L, the dye decolorization degree augmented swiftly.The higher the concentration of NaBH 4 , the more BH 4 -anions are created, which results in an increase in the electron concentration on the NiO/CMK-3 surface and the final decolorization degree of the MB dye 48 .Nonetheless, when the NaBH 4 concentration surpassed 7.5 mM/L, there was no further notable increase in the degree of dye decolorization.In view of these findings, 7.5 mM/L of NaBH 4 was chosen for the ongoing experiments.

Effect of dye concentration on the catalytic reduction of MB
The impact of increasing MB concentration in the range from 25 to 100 mg L −1 on the decolorization degree was further investigated, as illustrated in Fig. 8c.Other operational variables were kept unchanged at the following levels: temperature = 25 °C, time = 3 min, NaBH 4 concentration = 7.5 mM/L, and catalyst dose = 0.10 g L −1 .As expected, the decolorization degree of MB demonstrated a trend of continued lowering with the initial concentration.Raising MB concentration from 25 to 100 mg L −1 ; the MB removal efficiency is lowered from 99.2 to 85%.With the augmentation of MB concentration, the available catalytically active sites on the catalyst surface become insufficient, giving rise to a lower MB decolorization degree 48 .Please note that even in the presence of high concentrations of MB, a high decolorization degree can also be accomplished in a short reaction time (3 min), revealing that the NiO@CMK-3 was effective over a broad range of MB concentrations.
Besides, the NiO/CMK-3 catalyst underwent examination in the reductive degradation of methyl orange dye, MO, serving as a model of anionic dyes.This reaction was conducted adopting the optimized variables derived from the experiments on MB reduction (25 ºC, 0.1 g L −1 NiO/ CMK-3, and 7.5 mM/L NaBH 4 ).As manifested in Fig. 8d, the degradation degree of MO dye reached 95.8% within 10 min.So, NiO/ CMK-3 catalyst proves to be an advantageous solid catalyst for the effective NaBH 4 -assisted reduction of both cationic and anionic dyes.

Kinetic study
Kinetic data derived from the NaBH4-assisted reduction of MB to LMB over NiO/CMK-3 was further scrutinized by adopting the pseudo-first-order kinetic model.(Eq. 1) 7 .where C o (mg L −1 ) denotes the concentration of the MB solution before the commencement of the reduction reaction, C t (mg L −1 ) designates the remaining concentration of the dye solution after the elapse of time t of reaction, and k app (min -1 ) symbolizes the pseudo-first-order rate constant.
As depicted in Fig. 9a, a perfect straight line was obtained when plotting against t, characterized by a high regression coefficient (R 2 ) of 0.977.This underscores the appropriateness of the pseudo-first-order kinetic model in elucidating the mechanism of MB reduction by the catalyst.The perfect fit of this insinuates that the reduction rate depends solely on the concentration of MB.From the slope of the line, the value of the apparent rate constant (k app ) was calculated as 2.1 min −1 .
In recent years, metal-based heterogeneous catalysts` have been intensively examined for the reduction of several organic dyes using NaBH 4 .A comparison of the reductive decolorization of MB by NaBH4 employing diverse heterogeneous catalysts is outlined in Table 1.Based on the value of the normalized first-order reaction rate constant, k nor , computed employing Eq. ( 2), the as-obtained NiO/CMK-3 material, subject to investigation in the current work, exhibits superior performance in the decolorization of MB when compared with the aforementioned catalysts.The outstanding performance of NiO/CMK-3 underscores its potential for applications in the remediation of dye-contaminated wastewater through the reductive degradation process.
where w denotes the mass of the NiO/CMK-3 catalyst.

Reusability
The stability and reusability of the catalyst in the reductive degradation of organic dyes are essential for the economic viability and environmental sustainability of the process.To assess the reusability of the NiO/CMK-3 catalyst, five cycles of the reduction reaction for MB were accomplished under identical process variables.After the completion of each reduction cycle, the used catalyst was isolated from the reaction mixture through centrifugation, rinsed copiously with distilled water, and oven-dried before being submitted to the next round of MB reduction.As depicted in Fig. 9b, the catalytic performance of the catalyst did not apparently drop after 5 times of reuse, demonstrating acceptable stability and reusable ability.Ni-leaching was also examined after the reduction reaction, unveiling only 0.1 mg/L of Ni in the solution after the catalytic cycle.This concentration is significantly below the maximum acceptable concentration of Ni in wastewater resources (900 mg/L) 54 .So, the weight loss of the catalyst during the recovery and washing processes can account for the observed slight drop in the decolorization degree upon repetitious usage.Furthermore, the used catalyst was characterized by FTIR to check the stability of the catalyst after the catalytic reduction of MB.The FT-IR spectra (see Fig. S1) of the NiO/CMK-3 was similar to that of the fresh sample, confirming that the structural integrity of the catalyst was well maintained after the catalytic run.

Conclusions
In this study, a NiO/CMK-3 catalyst has been fabricated and employed for the NaBH4-assisted reduction of methylene blue dye.The catalyst manifested exceptional activity toward the reduction of methylene blue in the presence of NaBH4, achieving almost complete decolorization within 3 min at room temperature.Moreover, the results from the kinetic study unveiled the appropriateness of the pseudo-first-order kinetics to define the reductive decolorization of MB dye, with a high normalized rate constant of 3.5 × 10 −2 s −1 mg −1 .The catalyst was also utilized for the reductive degradation of MO dye, as a representative of anionic azo dyes, and the degradation degree reached 95.8% within 10 min.This establishes NiO/CMK-3 as an advantageous solid catalyst for the effective NaBH 4 -assisted reduction of both cationic and anionic dyes.Interestingly, the NiO/CMK-3 catalyst can be successfully recycled for five times for MB reduction with no pronounced lowering in the decolorization degree.In light of its superior catalytic performance, exceptional reusability, and efficacy in treating both cationic and anionic dyes, NiO/CMK-3 emerges as a promising catalyst for the highly efficient NaBH 4 -assisted reduction of toxic dyes in contaminated industrial effluents.
2 eV was assigned to C in C-C/C=C, and the peaks at 285.8 eV and 288.8 eV corresponded to C in C-O/C-O-C and C in -C=O/O-C = O, respectively, (Fig. 6b) 44,45 .The prominence of the spectral peak at 484.2 eV means that CMK-3 is predominantly made up of C-C.The deconvolution of the XPS O 1s spectra (Fig. 6c) gives rise to two defined peaks located at 529.2 and 531.4 eV, which corresponded to Ni-O/C-O-Ni and Ni-OH/C=O, respectively

Figure 7 .Figure 8 .
Figure 7. Reduction of MB by different systems.

Figure 9 .
Figure 9. (a) Pseudo-first order kinetics for the reduction of MB by "NiO/CMK-3 + NaBH 4 " system, and (b) the recyclability of NiO/CMK-3 for five successive rounds of MB reduction.

Table 1 .
Comparison of some heterogeneous catalysts for the reduction of MB with NaBH 4 .