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Article

Catalytic Reductive Degradation of 4-Nitrophenol and Methyl orange by Novel Cobalt Oxide Nanocomposites

by
Hawra A. Bukhamsin
1,
Hassan H. Hammud
1,*,
Chawki Awada
2 and
Thirumurugan Prakasam
3
1
Department of Chemistry, College of Science, King Faisal University, P.O. Box 400, Al-Ahsa 31982, Saudi Arabia
2
Department of Physics, College of Science, King Faisal University, P.O. Box 400, Al-Ahsa 31982, Saudi Arabia
3
Chemistry Program, New York University Abu Dhabi (NYUAD), Abu Dhabi P.O. Box 129188, United Arab Emirates
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(1), 89; https://doi.org/10.3390/catal14010089
Submission received: 12 December 2023 / Revised: 13 January 2024 / Accepted: 16 January 2024 / Published: 21 January 2024
(This article belongs to the Special Issue Design and Synthesis of Nanostructured Catalysts, 2nd Edition)
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Versions Notes

Abstract

:
Cobalt oxide nanocomposites were synthesized and used for the catalytic degradation of 4-nitrophenol (4-NP) and methyl orange (MO). Cobalt oxide nanocomposites PyroHAB9 was prepared by heating cobalt acetylacetonate complex HAB9 at 300 °C, while PyroHAB19 was prepared by heating cobalt acetylacetonate–carboxymethyl cellulose complex at 300 °C. FTIR indicated the presence of Co3O4 species, while Raman spectrum indicated the presence of graphite in PyroHAB19. The SEM morphology of nanocomposites exhibited irregular spherical shape nanoparticles with sizes ranging between 20 to 60 nm. Additionally, nanowires were also seen in HAB19. Also, 2Ɵ peaks in PXRD revealed the formation of Co3O4 in HAB19. Cyclic voltammetry indicated enhanced electrochemical redox activity of HAB19. The structures of the nanocomposites were related to their catalytic activities. The turnover frequency (TOF) values of the catalytic reduction of p-nitrophenol (P-NP) and methyl orange (MO) were greater for HAB19 compared to HAB9 nano-catalysts. Also, the TOF values of the catalytic reduction of MO were greater than that of P-NP by both nano-catalysts. It is obvious that the rate constants of catalytic reductions for MO by metal oxide nanocomposites were greater than the corresponding rate constants for PNP. The highest rate constant was found for PyroHAB19 in MO reduction.

Graphical Abstract

1. Introduction

Industrialization and urbanization cause environmental costs, including water shortages and contamination, affecting the 21st century global community [1]. Industries release wastewater, causing water and soil pollution due to synthetic dyes used in drugs, textile, food, paper, pulp, and cosmetic industries, making conventional removal difficult [2]. Reducing dye pollution is crucial, as conventional wastewater treatment methods like chemical, physicochemical, and biological methods often result in incomplete removal and long-term energy and cost consumption. Other techniques, like filtration, reverse osmosis, electrochemical treatment, distillation, and ion exchange, require energy and expensive operation [3].
Advances in wastewater treatment involve nanotechnology through nanoscale filtration, adsorption of pollutants, and catalysis for contaminants degradation [4]. Recent trends and challenges regarding potential applications of nano-catalysts in wastewater treatment and water purification were presented and discussed [5]. Transition metal oxides show special features with catalytic properties [6]. In addition, the catalytic techniques degrade pollutants and convert them into harmless products [7]. Nanomaterials can adsorb the pollutants or decompose them by diverse catalytic methods promoted by sodium borohydride NaBH4, hydrogen peroxide H2O2, or photolysis for water purification purposes [5].
Recently, there is a growing development in the field of nanotechnology and nanoscience, involving metal nanoparticles (MNPs) and metal oxide nanoparticles (MONPs). Transition metal oxide NPs proved to be a selective and efficient catalyst for the degradation of many organic compounds, like alkenes, alkanes, alcohols, and sulfides. The size and morphology of metal oxide nanoparticles affect their catalytic activity and selectivity in oxidation reactions. Due to their great chemical activity and large surface-to-volume ratio, cobalt oxide NPs have been used as electrochemical sensors, gas sensors, and catalysts [6]. The special porous structure combined with nano- and micro-scale properties facilitate electron transfer in electrochemical performance [8]. MNPs are also important catalysts due to their great surface area per volume and contain large numbers of active sites. Cobalt oxide show exceptional magnetic, electronic, and catalytic properties [9]. Cobalt has a beneficial effect on human health [10,11]. It can form variable oxidation state Co+2, Co+3, and Co+4 that allow electrons transport. These oxides often remain stable in an alkaline environment but not in an acidic environment. The inexpensive cost, high surface-to-volume ratio, and stable chemical state [12,13] of nano metal oxides cause them to perform efficiently as catalysts [14].
Cobalt oxide nanoparticles (NPs) possess exceptional properties and are used in various industries, including catalysis, supercapacitors, micro-batteries, electrode materials, and gas sensors [15]. CoO nanoparticles (CoO NPs) are an antiferromagnetic p-type semiconductor that effectively degrades organic waste in visible light. Having large surface areas, pore sizes, and a dense structure, they can be improved with carboxymethylcellulose (CMC) for sustainability and faster reduction rates in sodium borohydride reduction reactions of methylene blue dye and 2,6-dinitrophenol [16]. Also, the ability to develop Co NPs and CoO NPs catalysts for degradation of methyl orange using NaBH4 reducing agent in an aqueous solution has been proven [17,18]. Additionally, cobalt oxide-based heterogeneous catalysts showed good oxidation performance in the epoxidation of alkene [19].
Nitrophenols pollutants in wastewater are released from agricultural and industrial sources. Metal nanoparticles (MNPs) were used as catalysts for the converting p-nitrophenol to p-aminophenol. A reduction mechanism was also proposed. The cobalt nano colloids were found to be efficient reusable reduction catalysts for p-nitrophenol. They were synthesized from the reaction of the salt (cobalt sulphate), the surfactant (tetra butyl ammonium bromide), and the reductant (NaBH4) [20]. As a nanoscale compound, several chemical and physical strategies have been applied to fabricate CoO NPs including pyrolysis, thermal decomposition, microwave-assisted, chemical spray pyrolysis, vapor deposition [21], and hydrothermal methods [22]. In another study, cobalt oxide nanoparticles were obtained by hydrothermal treatment of cobalt chloride with ammonium hydroxide. The magnetic properties of the cobalt oxide nanoparticles change with the increase in the reaction temperature. The nanoparticles were fully characterized by X-ray diffraction (XRD), UV–Visible absorption, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) [23]. Cobalt oxides’ (CoO) hollow spheres and octahedra were synthesized by the thermal decomposition of the cobalt acetylacetone complex in 1-octadecene at 260 °C. Co3O4 NPs with 13nm size have been synthesized with calcination of the precursor at 300 °C [24], while cobalt oxide (Co3O4) hollow spheres and octahedra were prepared via the calcination of CoO at 600 °C [25]. Longer decomposition times result in a higher CoO ratio whereas higher temperatures (450 and 600 °C) produce pure Co3O4 [26]. New cobalt oxide-based catalysts (CoxOy–N/C) were synthesized via a wet impregnation method of cobalt (II) acetate and different types of nitrogen donor ligands on Vulcan XC72R succeeded by heating at 800 °C. Characterization of the catalyst by XRD, TEM, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) instruments indicated the formation of cobalt oxide (Co3O4 and CoO) nanoparticles, as well as some particles with a metallic Co core and an oxidic shell [19]. Cobalt oxide nanoparticles were also prepared by pyrolysis of cobalt acetate at various temperatures [27].
In the present work, direct thermal pyrolysis of commercial cobalt acetate exhibits outstanding performance as a facile, cheap, green, and scalable approach with simple and low energy costs for the fabrication of CoO/Co3O4 nanocomposite under 300 °C. Also, the efficiency of newly prepared cobalt oxide nanostructures was evaluated as a catalyst in the reduction reactions of p-nitrophenols (PNP) and azo aryl sulfonate MO dye in an aqueous solution and at room temperature.

2. Results and Discussion

2.1. Characterization of the Precursors HAB9 and HAB19 via UV and FTIR

To justify the metallic activity of the prepared precursors, the UV–Vis absorption spectra were measured. The UV–Vis spectra depicted in Figure 1a,b featured UV-absorption peaks observed at λmax 290 and 291 nm for both HAB9 and HAB19 complexes, respectively. The HAB 19 complex showed a small red shift compared to the HAB9 complex due to that carboxy methylcellulose formed adduct with cobalt acetylacetonate complex in HAB19.
FTIR is a promising technique for characterizing the materials’ functional groups and molecular interactions. This technique is used in the present study to understand and characterize the structure of precursor 1 HAB 9, precursor 2 HAB19, and their pyrolyzed nanocomposites. From Figure 2, the wide peak at 3407 cm−1 is due to O–H stretching vibration of water molecules [28] which became weak in HAB19 IR spectra. The peak at about 1593 cm−1 is due to coordinated C=O or C=C of acetyl acetate ligand. While the peak at about 567 cm−1 is assigned to Co-O stretching or referred to the deformation of the organic moiety [29,30], the peaks at 1395 and 1018 cm−1 showed the presence of CH3 groups [31]. Table 1 lists the characteristic peaks of the two samples.

2.2. Characterization of Cobalt Oxide Nanocomposite PyroHAB9 and PyroHAB19

Figure 3 illustrates the UV–Visible spectra and gap band of cobalt oxide nanocomposites CoONCs. The as-prepared nanoparticles at calcination temperatures 300 °C were characterized by UV–Vis absorption spectrum in the range of 200–600 nm. The spectrums revealed the existence of absorption peaks at 294 and 420 nm for cobalt oxide nanocomposites Pyro HAB9, and 292 nm and 460 nm for PyroHAB19, which are expected for Co3O4 nanoparticles and indicated ligand–metal charge transfer events O2− → Co3+ and O2− → Co2+, respectively [32].
The FTIR spectra of CoO-NCs in the range 400–4000 cm−1 were shown in Figure 4a,b. The peaks obtained at 1578 cm−1 can be due to the carbonyl groups in the samples. To further validate the cobalt oxide nature of the synthesized nanoparticles, the IR absorption bands observed at 660 cm−1, 559 cm−1, and 572 cm−1 were attributed to the fingerprint stretching vibrations of the Co-O bond in Co3O4. This also depicted the presence of Co2+ (tetrahedral site) and Co3+ (octahedral site), respectively [33].
Figure 5a–c showed the SEM images of cobalt oxide nanocomposites of HAB9 at high magnification and HAB19 at medium and high magnifications, respectively. The morphology exhibited well-uniform narrow size particles with an irregular spherical shape size distribution in the range of 20 to 60 nm. The effect of CMC was evident; it aided in the formation of nanowires (100 nm) on the surface of nanocomposites in HAB19 (Figure 5b).
Raman analysis was undertaken to estimate carbon defects and the degree of graphitization. In Figure 6b, the two peaks at 1379 and 1579 cm−1 were due to defective D and graphitic G bands, respectively. These peaks are present in pyrolytic graphite and carbon nanotubes [33], which also appeared here due to the added CMC during pyrolysis. The D band is characteristic of sp3 disordered or defected graphitic carbon. The shift in the D band can be due to the presence of oxygen-containing functional groups that lead to a different carbon–carbon bond distance and thus structural distortion of graphene. The G band is characteristic of sp2 hybridized carbon in ordered graphite. The upshift in the G band is due to defects, strain, doping, and the number of layers. Figure 6a,c showed three characteristic sharp peaks of cobalt at 204 cm−1, 492 cm−1, and 693 cm−1. As seen in Figure 6, the Raman spectra of MWCNTs decorated with cobalt oxide particles exhibit the same distinctive peaks at roughly the same places. It fitted the published cobalt oxide spectrum well and validated the interaction between Co2+ and carbon nanotubes [33]. They correspond to the classical vibration modes of Co3O4. This was also supported by the observation of Co3O4 peaks in the Raman spectra of the catalysts at 492 and 693 cm−1 [33,34].
The presence of cobalt oxide nanoparticles in the composites was confirmed by powder X-ray diffraction (PXRD) analysis (Figure 7). The position of the reflexes and the calculated cell parameters are presented in Table 2. The obtained diffraction pattern for PyroHAB9 showed broad peaks, indicating that the synthesized nanoparticles have a limited crystalline nature. However, there are weak peaks at 2θ values of 31 and 36.4 (°) in the XRD pattern for PyroHAB9 and PyroHAB9+CMC. In contrast, PyroHAB19 exhibits sharper peaks at 2θ values of 3.88, 31.6, and 45.4 (°), and other smaller peaks at 56.8, 75.58, and 84.121 (°). These peaks correspond to specific crystallographic planes [25], such as 210, 311, 400, and 511 planes of Co3O4, respectively, as per ICDD:04-005-4386, ICSD:63164 data [35], and the lattice parameters were calculated to be Crystal system: Cubic, Space group: Fd-3m, Space group number: 227, a (Å): 8.0850, b (Å): 8.0850, c (Å): 8.0850, Alpha (°): 90.0000, Beta (°): 90.0000, Gamma (°): 90.0000, volume of cell (106 pm3): 528.49, respectively. The presence of sharp peaks in the XRD pattern for PyroHAB19 indicates that the nanoparticles in this sample are well crystalline and have a more defined crystal structure compared to PyroHAB9. This information from XRD analysis provides insights into the crystallinity and crystal structure of the synthesized cobalt oxide nanoparticles in different samples. However, most of the uneven baseline comes from amorphous parts of the samples [36].
The electrochemical behavior of the cobalt oxide nanocomposite was investigated via cyclic voltammetry using a Gamry potentiostat/Galvanostat [37]. The cyclic voltammogram of glassy carbon electrode modified with nanocomposites HAB9 and HAB19, versus sat. Ag/AgCl electrode in 0.1 M NaOH indicated that HAB19 is more electroactive than HAB9 (Figure 8). HAB19 showed a redox peak due to Co3O4 for a reversible scan range from 0 V to 0.7 V. The oxidation peak appeared at potential 0.3 V, then the current continued to increase. While in cathodic scan, a well-defined reduction peak is generated at 0.45 V [36,37,38]. The enhancement of electrochemical properties in HAB19 can be related to its carbonaceous content, originating from the addition of CMC during preparation of precursor.

2.3. Catalytic Activity

2.3.1. Catalytic Reduction of Para-Nitrophenol (P-NP) by Cobalt Oxide Nanocomposites PyroHAB9 and PyroHAB19

The nano-catalyst HAB9 had been applied for the catalytic reduction of P-NP. The obtained UV–Visible absorption spectrum for PyroHAB9 is shown in Figure 9a. The spectrum showed the change that occurs in the absorbency of the P-NP in the presence and absence of the nano-catalysts before and after the addition of NaBH4. Then, 0.1 mL of stock P-NP (0.28 mg/L) solution was added to 0.2 mL of the nano-catalyst solution (1 mg/mL) and diluting with water to 3.1 mL. The absorbance of the P-NP showed a peak with λmax at 318 nm, and after the addition of the nano-catalyst, the peak at 404 nm was slightly increased. The addition of 4 mg NaBH4 to the mixture led to the raise of the absorbance at 404 nm at 0 min. Then, it gradually decreased in about 5–15 min. In parallel, there was an increase in the absorbance at 235 nm due to the formation of para-aminophenol PAP.
Similarly, the UV–Visible absorption spectrum for Pyro HAB19 was shown in Figure 9b, while P-NP absorption spectra showed three different peaks, a weak peak at 228 nm, a strong peak at 318 nm, and a weak peak at 404 nm which increased after the nano-catalyst addition (Figure 9b). After adding 4 mg of NaBH4 to the solution, the peak at 404 nm increased. The first order kinetic plot ln A versus time for cycle 3 for catalyst HAB9 is shown in Figure 9c. This plot can be compared with the first order kinetic plot, ln A versus time t(min) for cycle 3 for the catalysis of P-NP by catalyst Pyro HAB19 (Figure 9d) [39,40]. The obtained rate constants were 0.0877 and 0.0925 min−1 for HAB9 and HAB19, respectively.
Figure 10a illustrated the dramatic decreases in the absorbance of P-NP in cycle 7 during 9 min by PyroHAB9 after the addition of 0.7 mL P-NP. While Figure 10b illustrated the dramatic decrease in the absorbance of the peak at 404 nm by Pyro HAB19 in cycle 4 after the addition of a total of 0.4 mL of P-NP during 14 min, and the complete formation of PAP at the end of the cycle with increase in the peak at 235 nm.
The reduction mechanism of P-NP via metal oxide nano-catalysts can be followed at the height (absorbance) of the p-nitrophenolate anion peak at 404 nm, which declined with time. A new peak appeared at 235 nm and increased with the time of the reaction, which indicated the successful reduction of P-NP to p-aminophenol. Where the nitro group is reduced to the amino group as described in reference [41,42], the color of the solution changed gradually to colorless in less than 15 min. The reduction of P-NP occurred due to hydrogen generation by NaBH4, which increased dramatically in the presence of metal oxide nano-catalysts.

2.3.2. Catalytic Reduction of Methyl orange (MO) by Cobalt Oxide Nanocomposite PyroHAB9 and PyroHAB19

The catalytic reduction of MO by nano-catalysts PyroHAB9 had been applied (Figure 11b). The MO had absorption peak at a wavelength of 222 nm (weak), 272 nm (medium), and a broad strong peak at 465 nm, as shown in Figure 11a. The UV–Visible absorption spectra for the catalyst HAB9 had a very weak peak at 293 nm. Two absorbance peaks at 272 nm and 465 nm were exhibited when adding 0.05 mL of stock MO (1.28 mg/L) to 0.2 mL of the nano-catalyst solution (1 mg/mL) and diluting with water to 3.05 mL. Also, there was a variation in the absorption spectra following the addition of 4 mg of NaBH4, where there was an improvement in the absorbance of the two peaks (Figure 11a). The reduction of MO was complete after 7 min, supported by the presence of a strong peak at 253 nm due to the formation of MO reduction products (N, N-Methyl-P-Phenylenediamine, and sulfanilic acid).
Thanks to the self-hydrolysis of NaBH4 and hydrogen generation, the catalytic efficiency of the reduction in a faster manner than the reduction in the absence of a nano-catalyst has been proven. The catalytic activities towards the reduction of MO by Pyro HAB19 had also been applied following the same procedure as in Pyro HAB9 as shown in Figure 11b. The MO showed a small absorption peak at 222 nm and 272 nm, and a wide peak at 465 nm, while the UV–Visible absorption spectra for Pyro HAB19 had a very small peak at 287 nm. Also, Figure 11b showed two absorbance peaks at 272 and 465 nm for a mixture of (0.05 mL MO (1.28 mg/mL) + 0.2 mL nano (1 mg/mL) in 2.8 mL water) = MIX1. The addition of 4 mg of the reducing agent to MIX1 reflected a decrease in the absorbance at 465 nm. Cycle 1 took place in 3 min (Figure 11b), where 0.05 mL of MO was consumed.
Thus, PyroHAB19 displayed higher catalytic activities towards the reduction of MO than PyroHAB9 by following the absorbance at 465 nm; the reduction took place in 3 min for PyroHAB19 compared to 7 min for PyroHAB9, which reflected the improvement due to the added CMC during preparation of precursor (Figure 11a,b).
Figure 11c shows the first order kinetic plot for cycle 3 for the reduction of MO by HAB9 compared to the first order kinetic plot for cycle 3 by Pyro HAB19 (Figure 11d). HAB19 had a higher rate constant of 0.8358 min−1 compared to 0.3591 min−1.
The reduction mechanism of MO by metal oxide nano-catalysts can be outlined at the height (absorbance) of the MO peak at 464 nm which declined sharply. A new peak appeared at 251 nm and increased with time of reaction which indicated the successful reduction of methyl orange (MO) to the (less toxic) aromatic amine compounds (N, N-Methyl-P-Phenylenediamine, and sulfanilic acid). Where the azo group is reduced to the amino group as described in reference [43], it is also assisted by NaBH4 reagent. The color of the solution changed gradually to colorless within 3–7 min due to the disappearance of MO and the formation of aromatic amine compounds. The presence of graphitic carbon in PyroHab19 facilitated the transfer of electrons. This can explain the increase in the observed rate constant (Table 3).
Figure 12a displayed the change in the absorption spectra of cycle 7 for PyroHAB9 during 9 min after the addition of 0.3 mL of MO, while Figure 12b showed the change in the absorption spectra by PyroHAB19 of cycle 14 during 5 min after the addition of 0.7 mL MO.
The catalytic activity of cobalt oxide nanocomposites for the reduction of P-NP and MO were compared (Table 3). PyroHAB19 is superior to PyroHAB9 as a catalyst for the reduction of P-NP, where the turnover frequency (TOF) for PyroHAB19 was 0.0114 (mg P-NP/mg nano)/min compared to 0.0078 (mg P-NP/mg nano)/min for Pyro HAB9. On the other hand, the effect of CMC is more evident in the case of the reduction of methyl orange (MO). Pyro HAB19 performed better as a catalyst than Pyro HAB9 where the reduction cycles increased to 21 cycles compared to 8 cycles, and the turnover increased by 2.63 times. The TONs were 6.72 and 2.56 (mg P-NP/mg nano-catalyst) for PyroHAB19 and PyroHAB9, respectively.
Also, the average rate constants for each experiment from all the involved cycles were calculated and presented in Table 3. It is obvious that the rate constants of catalytic reductions for MO by metal oxide nanocomposites were greater than the corresponding rate constants for PNP.
The characterization by SEM indicated nanoparticles of the size ranging from 20 to 100 nm with irregular spheres and rod shapes. The small size of nanoparticles did improve the catalytic activity, because it increased the surface area and thus the contact between catalysts and substrate. The presence of graphitic carbon in PyroHAB19, as evidenced by Raman analysis, did improve the catalytic activity towards the reduction of MO compared to PyroHAB9. The cobalt oxide nanoparticles confirmed by UV–Visible, FTIR, and powder Xrd are the active sites for the reduction reaction assisted by hydride donation from NaBH4 reagent.

3. Experimental

3.1. Characterization Instruments

The absorbance of solutions was measured via UV–Visible spectroscopy (sp-3000 plus, Optima, Tokyo, Japan). The infrared spectra of compounds were measured between 200–4000 cm−1 on a Shimadzu 8300 FTIR spectrophotometer, Kyoto, Japan. Morphological characterization was performed by Field Emission Scanning Electron Microscope (FE-SEM) model QuantaFEG450, FEI, and JSM-6460LV, Tokyo, Japan. Raman spectra were measured by DXR (Thermo scientific, Waltham, MA, USA). Powder X ray diffraction (PXRD) patterns of the nanocomposites were measured on Malvern Panalytical Empyrean diffractometer (Malvern, UK) using Cu Kα radiation consisting of a focusing elliptical mirror and a fast high resolution detector (PIXCEL) with the radiation wavelength of 0.15418 nm. The PXRD patterns of the nanocarbon samples were acquired at two theta range 5–90 with a step size of 0.053°, and a time per step of 99.45 s.

3.2. Electrochemical Measurement

The electrochemical activity of prepared nanocomposites was tested in a cell using three electrodes system: a working electrode (modified glassy carbon), saturated silver/silver chloride (sat. Ag/AgCl) as a reference electrode, and platinum Pt 1 mm wire acting as the counter electrode. The modified electrode was made by placing 0.5 mg of nanocomposite in 0.5 mL of deionized water and combining it with 25 μL of Nafion solution. After being sonicated, the suspension was drop-casted onto a polished glassy carbon electrode surface (3.0 mm diameter). The modified electrode was placed in an air oven set at 70 °C until dried. The electrochemical behavior for cobalt oxide nanocomposite was carried out using cyclic voltammetry by a Gamry potentiastat/Galvanostat/ZRA Reference 600 with software v7.

3.3. Synthesis of Cobalt Acetylacetonate HAB9

Cobalt chloride hexahydrate (2.378 gm, 10 mmol) CoCl2.6H2O was dissolved in 10 mL distilled water H2O in a flask by mixing with a magnetic stirring bar. Then, 2.11 mL, 20.5 mmol of acetylacetonate (acac) was added to the previous solution under continuous stirring for 15 min under soft heating. Then, a solution of 10 mL of H2O and 0.82 gm, 20.5 mmol of NaOH was prepared and added to the reaction flask dropwise. Heating to 70 °C resulted in the precipitation of pink powder. After filtration and leaving for two days, the resulting product was washed with distilled water and dried in the oven for a further ½ h under 80 °C (Figure 13). The total amount of the complex HAB9 was 2.4936 with a yield of 69.9% g. FTIR(ῡ): 420, 567, 657, 761, 911,1015, 1192,1266, 1386, 1505, 1593, 2990, 3258, and 3407 cm−1, UV (ʎmax): 290 nm.

3.4. Synthesis of Cobalt Acetylacetonate–Carboxy Methylcellulose HAB19 Complex

CoCl2.6H2O (4.75 gm, 19 mmol) was dissolved in 15 mL of distilled water H2O in a flask and mixed with a magnetic stirring bar. Then, 0.2 gm of carboxymethylcellulose (CMC) was added and a gel was formed. Then, with continuous stirring, 4.22 mL (4.09 gm, 40 mmol) of acetylacetonate (acac) was added to the mixture. A thin layer was formed above the solution under soft heating due to the presence of CMC. Then, dropwise, (1.64 gm, 41 mmol) of NaOH dissolved in 15 mL of H2O was added to the flask. Heating to 70 °C under stirring for approximately 1 h caused the formation of light to violate the precipitate. After that, the product was transferred to an oven for 3 h to dry under 105 °C (Figure 14). The weight of the precipitate HAB19 was 6.809 gm. FTIR(ῡ): 414, 567, 766, 925, 1018, 1259, 1395, 1519, 2164, and 2029 cm−1, UV (ʎmax): 291 nm.

3.5. Pyrolysis Preparation of Metal Oxide Nanocomposites PyroHAB9 and PyroHAB19

Cobalt oxide nanocomposite PyroHAB9 was prepared via the pyrolysis of HAB9, while PyroHAB19 was prepared via the pyrolysis of HAB19. The calcinated temperatures were varied (300 °C and 350 °C) in order to study the effect of pyrolysis conditions on the structures, while Pyro HAB19 was prepared via the pyrolysis of HAB19 under 300 °C for 1 h (Figure 15).

3.6. Catalysis Methodology

In order to demonstrate the efficacy of our catalysts in the degradation of dyes, the experiment was performed in the presence of nano-catalysts PyroHAB9 and PyroHab19 (Figure 16a,b). The methodology of degradation requires two stock solutions: the nano-catalyst stock solution (sonication of 1 mg nano in 1 mL H2O) and the dyes stock solution P-NP (7 mg/25 mL) or MO (32 mg/25 mL). In a typical P-NP experiment, 0.1 mL of P-NP stock solution and 0.2 mL of nano-catalyst stock solution were added together, then the volume was increased to 3.1 mL by adding distilled water. Thus, the initial concentration of P-NP was 9.03 mg/L, while in a typical MO experiment, 0.05 mL of MO stock solution and 0.2 mL of nano-catalyst stock solution were added together, then the volume was increased to 3.05 mL by adding distilled water. Thus, the initial concentration of MO was 20.1 mg/L. Then, we measured the UV–Visible spectra of the resulting solutions. After that, we added 4 mg of reducing agent NaBH4 and ran the absorption spectra until the degradation of all the added amounts of dyes (counted as cycle 1). We repeated the same experiment by adding dye only till the end of the reaction (cycle 2) and so on. The rate constant of the reduction reaction was estimated from the decrease in the height of the absorption peak at 404 nm. The experimental data were fitted by both the pseudo-first-order and the pseudo-second-order kinetic models in order to determine the reaction rate order. The pseudo-first-order kinetic model can be expressed via the linear relationship between ln(A) and time t (min). It can be used to directly obtain the apparent rate constants k from the slope of a straight line, where A stand for the absorbance of P-NP at λmax 404 nm after time t min. The reaction was expected to follow a pseudo-first-order kinetics reaction since the concentration of sodium borohydride is assumed to be a constant.

4. Conclusions

This study investigated the preparation, morphology, and characterization of novel cobalt oxide nanocomposites, focusing on the effect of carboxy methyl cellulose (CMC) on the preparation of nanocomposites and the pyrolysis temperatures. The efficiency of the nanomaterials, such as catalysts of organic dyes and electrode materials, was tested. Because of their extreme curvature, nanoscale particles have a high surface energy and numerous weakly bound atoms on their surface. These surface atoms are very active in the lattice because they are chemically active, physically unstable, and prone to several chemical reactions. The results show a successful preparation of the nanocomposites using solid-state pyrolysis of the cobalt acetylacetonate complex in the presence and absence of CMC. The size and shape of the nanocomposites were found to be highly dependent on pyrolysis temperatures. FTIR analysis revealed functionalized nanostructures with peaks in Pyro HAB9 and Pyro HAB19, which were not well-crystalline but corresponded to Co3O4.
PXRD also revealed the formation of Co3O4 species in HAB9 and HAB19, while Raman spectrum indicated the presence of graphite in PyroHAB19. SEM analysis showed irregular spherical shape nanoparticles with sizes of about 40 nm. Additionally, nanowires were also seen in HAB19. Cyclic voltammetry showed enhanced electrochemical redox activity for HAB19.
The efficiency of Pyro HAB19 when removing para-nitrophenol (P-NP) and methyl orange (MO) from aqueous solutions was found to be related to the use of CMC in the preparation of the precursor, where the prepared metal oxide nanocomposites became more active in reducing MO and P-NP with a high number of cycles and a higher turnover frequency (TOF). In general, the catalytic activity towards MO is greater than that of P-NP. It was found that the rate constants of catalytic reductions for MO by metal oxide nanocomposites were greater than the corresponding rate constants for PNP.

Author Contributions

Conceptualization, H.H.H.; methodology, H.H.H., H.A.B., C.A. and T.P.; software, H.H.H.; validation, H.H.H.; investigation, H.A.B., T.P. and C.A.; resources, H.H.H.; writing—original draft preparation, H.H.H. and H.A.B.; writing—review and editing, H.H.H.; supervision, H.H.H.; project administration, H.H.H.; funding acquisition, H.H.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Deanship of Scientific Research, vice Presidency for Graduate Studies and Scientific Research, King Faisal University Saudi Arabia [Grant No. 3627].

Data Availability Statement

All data are available in the main text.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

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Catalysts 14 00089 g001
Figure 1. UV–Visible absorption spectra for (a) HAB9 and (b) HAB19 complexes.
Figure 1. UV–Visible absorption spectra for (a) HAB9 and (b) HAB19 complexes.
Catalysts 14 00089 g001
Catalysts 14 00089 g002
Figure 2. FTIR absorption spectra for cobalt acetylacetonate (a) HAB9 and(b) HAB19.
Figure 2. FTIR absorption spectra for cobalt acetylacetonate (a) HAB9 and(b) HAB19.
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Catalysts 14 00089 g003
Figure 3. UV–Visible absorption spectra for cobalt oxides nanocomposites (a) Pyro HAB9 and (b) PyroHAB19.
Figure 3. UV–Visible absorption spectra for cobalt oxides nanocomposites (a) Pyro HAB9 and (b) PyroHAB19.
Catalysts 14 00089 g003
Catalysts 14 00089 g004
Figure 4. FTIR absorption spectra for (a) PyroHAB9 and (b) PyroHAB19 complexes.
Figure 4. FTIR absorption spectra for (a) PyroHAB9 and (b) PyroHAB19 complexes.
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Catalysts 14 00089 g005
Figure 5. SEM images for cobalt oxide nanocomposites (a) HAB9 high magnification (size bar = 1 nm), (b) HAB19 at medium magnification (size bar = 100 μm), and (c) HAB 19 at high magnification (size bar = 100 nm).
Figure 5. SEM images for cobalt oxide nanocomposites (a) HAB9 high magnification (size bar = 1 nm), (b) HAB19 at medium magnification (size bar = 100 μm), and (c) HAB 19 at high magnification (size bar = 100 nm).
Catalysts 14 00089 g005
Catalysts 14 00089 g006
Figure 6. Raman spectra for cobalt oxides nanocomposites (a) Pyro HAB9, (b) PyroHAB9+CMC, and (c) PyroHAB19.
Figure 6. Raman spectra for cobalt oxides nanocomposites (a) Pyro HAB9, (b) PyroHAB9+CMC, and (c) PyroHAB19.
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Catalysts 14 00089 g007
Figure 7. XRD spectra for (a) PyroHAB9, (b) PyroHAB9+CMC, and (c) PyroHAB19.
Figure 7. XRD spectra for (a) PyroHAB9, (b) PyroHAB9+CMC, and (c) PyroHAB19.
Catalysts 14 00089 g007
Catalysts 14 00089 g008
Figure 8. Comparison between electroactivity of PyroHAB9 and PyroHAB19.
Figure 8. Comparison between electroactivity of PyroHAB9 and PyroHAB19.
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Catalysts 14 00089 g009
Figure 9. Comparison of the catalytic reduction of P-NP by nanometal oxide catalyst: Absorption spectra (a) PyroHAB9 and (b) Pyro HAB19. The first order kinetics for cycle 3 by (c) Pyro HAB9 and (d) Pyro HAB19.
Figure 9. Comparison of the catalytic reduction of P-NP by nanometal oxide catalyst: Absorption spectra (a) PyroHAB9 and (b) Pyro HAB19. The first order kinetics for cycle 3 by (c) Pyro HAB9 and (d) Pyro HAB19.
Catalysts 14 00089 g009
Catalysts 14 00089 g010
Figure 10. Comparison of the absorbance spectra (a) cycle 7 by Pyro HAB9 and (b) cycle 4 by Pyro HAB19.
Figure 10. Comparison of the absorbance spectra (a) cycle 7 by Pyro HAB9 and (b) cycle 4 by Pyro HAB19.
Catalysts 14 00089 g010
Catalysts 14 00089 g011
Figure 11. Catalytic reduction of MO by nanometal oxide catalyst; Absorption spectra (a) Pyro HAB9 and (b) Pyro HAB19. The first order kinetic for cycle 3 (c) Pyro HAB9 and (d) Pyro HAB19.
Figure 11. Catalytic reduction of MO by nanometal oxide catalyst; Absorption spectra (a) Pyro HAB9 and (b) Pyro HAB19. The first order kinetic for cycle 3 (c) Pyro HAB9 and (d) Pyro HAB19.
Catalysts 14 00089 g011
Catalysts 14 00089 g012
Figure 12. Comparison of the first order kinetic (a) for cycle 7 by Pyro HAB9 (b) for cycle 14 by Pyro HAB19.
Figure 12. Comparison of the first order kinetic (a) for cycle 7 by Pyro HAB9 (b) for cycle 14 by Pyro HAB19.
Catalysts 14 00089 g012
Catalysts 14 00089 g013
Figure 13. Synthesis of cobalt acetylacetonate HAB9 complex.
Figure 13. Synthesis of cobalt acetylacetonate HAB9 complex.
Catalysts 14 00089 g013
Catalysts 14 00089 g014
Figure 14. Synthesis of cobalt acetylacetonate–CMC HAB19 complex.
Figure 14. Synthesis of cobalt acetylacetonate–CMC HAB19 complex.
Catalysts 14 00089 g014
Catalysts 14 00089 g015
Figure 15. Pyrolysis synthesis of cobalt oxide nanocomposites PyroHAB9 and PyroHAB19.
Figure 15. Pyrolysis synthesis of cobalt oxide nanocomposites PyroHAB9 and PyroHAB19.
Catalysts 14 00089 g015
Catalysts 14 00089 g016
Figure 16. Methodology of reduction by nano-catalysts of (a) P-Nitrophenol (P-NP) and (b) methyl orange (MO).
Figure 16. Methodology of reduction by nano-catalysts of (a) P-Nitrophenol (P-NP) and (b) methyl orange (MO).
Catalysts 14 00089 g016
Table 1. Characteristic FTIR wavenumber (cm−1) of the cobalt acetylacetonate HAB9, HAB19.
Table 1. Characteristic FTIR wavenumber (cm−1) of the cobalt acetylacetonate HAB9, HAB19.
HAB9HAB19Assignment
3407–3258 w ν(OH)/H2O
2990 w2164 w, 2029 wυas(CH3),υs(CH3)
1593 m1590 mν(C=C)
1505 s1519 sν(C=C) + τ(CH3)
1386 s1395 sω(CH3)
1266 m1259 mνs(C=C=C)
1015 m1018 mω(CH)-
911 w925 wδ(C=C=C)
761 m766 mγ(CH)-
657 m-ρ(CH3)-
567 m567 wρ(CH3)-
420 w414 wν(M-O)
Table 2. Powder X-ray diffraction analysis of cobalt oxide nanocarbons. Position of the reflexes and the calculated cell parameters.
Table 2. Powder X-ray diffraction analysis of cobalt oxide nanocarbons. Position of the reflexes and the calculated cell parameters.
2θ Exp.
(°)		2θ Cal.
(°)		Diff. in 2θHklIntensity
(%)		d (Å)
31.64831.6490.0012 2 028.12.858
45.41445.4120.0024 0 017.42.021
56.81256.8150.0034 2 27.61.650
75.58175.5820.0016 2 02.31.278
84.12184.1190.0027 1 11.71.132
Table 3. Comparison of Catalytic Reduction of Para-nitrophenol (P-NP) and Methyl Orange (MO).
Table 3. Comparison of Catalytic Reduction of Para-nitrophenol (P-NP) and Methyl Orange (MO).
Pollutant Nano-CatalystNo. of
Cycles 		Total
Time		Rate
Constant
(min−1)		Turnover Number (TON)
(mg P-NP/mg Nano-Catalyst)		Turnover Frequency (TOF)
(mg P-NP/mg Nano-Catalyst)
/min
P-NPPyro HAB97 cycles126 min0.11950.980.0078
Pyro HAB195 cycles61 min0.08600.700.0114
MOPyro HAB98 cycles67 min0.20582.560.0380
Pyro HAB1921 cycles90 min0.34406.720.0746
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Bukhamsin, H.A.; Hammud, H.H.; Awada, C.; Prakasam, T. Catalytic Reductive Degradation of 4-Nitrophenol and Methyl orange by Novel Cobalt Oxide Nanocomposites. Catalysts 2024, 14, 89. https://doi.org/10.3390/catal14010089

AMA Style

Bukhamsin HA, Hammud HH, Awada C, Prakasam T. Catalytic Reductive Degradation of 4-Nitrophenol and Methyl orange by Novel Cobalt Oxide Nanocomposites. Catalysts. 2024; 14(1):89. https://doi.org/10.3390/catal14010089

Chicago/Turabian Style

Bukhamsin, Hawra A., Hassan H. Hammud, Chawki Awada, and Thirumurugan Prakasam. 2024. "Catalytic Reductive Degradation of 4-Nitrophenol and Methyl orange by Novel Cobalt Oxide Nanocomposites" Catalysts 14, no. 1: 89. https://doi.org/10.3390/catal14010089

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