Enhanced photodegradation of dyes and mixed dyes by heterogeneous mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposites: nanoparticles formation, semiconductor behavior and mesoporosity

Abstract

In situ loading of mono and bimetallic nanoparticles in the framework of mesoporous Al₂O₃–MCM-41 and its effect on the photo-Fenton degradation of dyes and mixed dyes has been reported in the present study. The nanocomposites are synthesized by in situ sol–gel cum hydrothermal method where oleic acid has been used as capping agent for mono and bimetallic nanoparticles. Materials were characterized by high and low angle XRD, N₂ sorption, and HRTEM to evaluate mesoporosity, morphology and textural properties. The photoluminescence (PL) study and band gap energy measurement reveals suppression of e⁻ and h⁺ recombination and semiconductor behaviour of bimetallic/Al₂O₃–MCM-41 in visible region. Both the processes of photo-Fenton and photocatalysis takes place over mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite, which is found to be an efficient material with 100% efficiency for the degradation of dyes and mixed dyes (100 mg L⁻¹) at pH 10 in just 60 minutes. Framework mesoporosity, nanoparticle morphology of the nanocomposite, semiconductor behavior, lowering of the electron–hole recombination and the formation of a large number of ˙OH radicals are the crucial factors for swift degradation of dyes and mixed dyes by mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite.

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1. Introduction

Mesoporous materials with regular geometry have been recently paid much attention owing to their great potential in practical applications such as catalysis, adsorption, separation, sensing, medical usage, ecology and nanotechnology. These applications are tailored because of its unique three dimensional structures, high surface area, tuneable and uniform pore channel and wide volumes.^1–6 Among the mesoporous material, mesoporous silica (MCM-41) and metals/metal oxides modified MCM-41 have been treated as an efficient support and catalyst. The MCM-41 has been engineered by isomorphous substitution of metals such as Al, Fe, Ti, Cu, Zn, V, Co (ref. 7–9) and extra-framework modification by mesoporous Al₂O₃ and mesoporous MnO₂NPs.^10,11 The substitution and extra-framework modification of metals and metal oxides leads to MCM-41 as an excellent catalyst. The catalytic activity depends upon the morphology of the incorporated metals and metal oxides. The best option is zero dimensional nanoparticles which has large surface-to-volume ratio and high surface atomic activity.¹² Fabrication of iron based nanoparticles is an important task. Nanoparticles/nanostructured Fe₂O₃ have been synthesized by hydrothermal method, precipitation method etc.^13–15 But fabrication of Fe₂O₃/Fe nanoparticles by sol–gel method is difficult. Single metal nanoparticles have small surface area that cause poor photocatalytic performances due to insufficient contact with the reactants.¹⁶ In order to avoid this problem, now researchers are interested to fabricate metal nanoparticles (MNPs) within the mesoporous support network. But it is also difficult to fabricate MNPs within the mesoporous framework in one pot synthesis. The combination of sol–gel and hydrothermal process in a one pot method by using suitable template/structure-directing agent may be helpful to achieve nanoparticles within mesoporous support strategy. CTAB has been used as a structure directing agent for synthesis of mesoporous Al₂O₃–MCM-41 because creation longer micelle by the formation of NH₄OH during synthesis in aqueous medium.¹⁷ Oleic acid has been treated as a capping agent for the synthesis of metal nanoparticles due to good ability and higher affinity to bind with the surfaces of the metal precursor forming metal–oleates that in turn regulate the nucleation and growth of the nanoparticles in organic medium.¹⁸ In the combination of aqueous and organic medium, the growth of metal particles in organic medium may slower and restrict to form nanoparticles. This phenomenon will be an innovative approach for synthesis metal nanoparticles into the mesoporous framework. Fabrication of Fe nanoparticle (in situ) into the mesoporous Al₂O₃–MCM-41 by using oleic acid as capping agent, cetyltrimethylammonium bromide (CTAB) as a surfactant and NH₃ as a soft template could be a novel approach. Not merely Fe nanoparticles, the other monometallic (Co and Mn) and bimetallic (Co–Fe, Mn–Co and Fe–Mn) nanoparticles could be formed by the same method.

It has been noted that Fe/iron oxide nanoparticles have been used as a heterogeneous photo-Fenton and photocatalyst for the organic dyes degradation.^14,15 The heterogeneous photo-Fenton activity of Fe/iron oxide nanoparticles can be enhanced by incorporating it with another mesoporous support material. For an example, mesoporous Fe₃O₄@SiO₂ composite and Fe₃O₄@rGO@TiO₂ have been treated as a photo-Fenton catalyst for methylene blue degradation.^19,20 The photo-Fenton degradation of rhodamine B has been achieved by Fe supported bentonite and graphene oxide–amorphous FePO₄.^21,22 Fe–MIL-101 has been treated as a heterogeneous photo-Fenton catalyst for reactive dyes.²³ Shao et al. synthesized α-Fe₂O₃–TiO₂ for an efficient photo-Fenton degradation of organic pollutant.²⁴ Iron modified Al₂O₃ nanoparticles pillared montmorillonite nanocomposite has been treated as a brilliant photo-Fenton catalyst for degradation phenolic compounds.²⁵ Lam et al. investigated the catalytic photo-Fenton oxidation of organic II dyes on Fe/MCM-41.²⁶ Mesoporous Fe/Al₂O₃–MCM-41 has been considered as an efficient photo-Fenton catalyst for degradation of dyes.¹¹ The catalytic activity could be increased by modification of Fe nanoparticles into the mesoporous Al₂O₃–MCM-41. Firstly, increase in surface active sites by the presence of Fe nanoparticles. The presence of Fe nanoparticles in the framework of Al₂O₃–MCM-41 and H₂O₂ in the reaction media will facilitate the photo-Fenton activity efficiently. Secondly, the band gap energy of FeO is 2.4 eV,^27,28 which can function as semiconducting oxides under visible light. By incorporation of Fe nanoparticles into the Al₂O₃–MCM-41, the band gap of Fe/Al₂O₃–MCM-41 nanocomposite may be reduced. So Fe/Al₂O₃–MCM-41 composite may be treated as an efficient semiconducting photocatalyst for any photocatalytic applications under visible light. Thirdly, Al₂O₃–MCM-41 has high surface area, it could act as an efficient catalytic support material. The combination of Al₂O₃ and MCM-41 is chemically favourable because the ionic radii of Al³⁺ (53.5 pm) and Si⁴⁺ (40 pm).²⁹ It has been reported that in Al–MCM-41/Al₂O₃–MCM-41 the Al³⁺ ion either substitutes or coordinates with the Si⁴⁺ ion in MCM-41 through oxygen ions.¹¹ These points are necessary and important for making Fe nanoparticles modified mesoporous Al₂O₃–MCM-41 as a heterogeneous photo-Fenton catalyst/photocatalyst for the degradation of dyes such as methylene blue (MB), Congo red (CR) and mixed dyes (methylene blue and Congo red). These organic dyes have adverse effect even at trace amount.^30,31 The heterogeneous photo-Fenton/photocatalytic process is a very quick, low cost and environment friendly process for the degradation of organic dyes and mixed dyes.^32,33 The degradation of dyes (MB, CR) and mixed dyes (MB + CR) on mesoporous Fe/Al₂O₃–MCM-41 nanocomposite by heterogeneous photo-Fenton/photocatalytic process will be more advantage. Hence not merely Fe, but Fe-like metals such as monometallic (Mn, Fe, Co) and bimetallic (Co–Fe, Mn–Co, Fe–Mn) nanoparticles modified Al₂O₃–MCM-41 nanocomposite can be considered as an effective heterogeneous photo-Fenton catalyst/photocatalyst for degradation of dyes and mixed dyes.

Herein, we report the in situ growth of mono and bimetallic nanoparticles on mesoporous Al₂O₃–MCM-41 by novel sol–gel cum hydrothermal method. The control concentration of oleic acid is responsible for formation of mono and bimetallic nanoparticles. A possible formation mechanism has been proposed based on the systematic investigation of the morphological evolution and growth processes of Fe nanoparticles into the mesoporous Al₂O₃–MCM-41 framework. Fabrication and semiconductor behavior of Mn, Fe, and Co/Al₂O₃–MCM-41 and Co–Fe, Mn–Co, Fe–Mn/Al₂O₃–MCM-41 nanocomposites are also highlighted in the present study. Furthermore, to demonstrate their potential application for dyes degradation, the as-synthesized materials are used as heterogeneous photo-Fenton catalyst and photocatalyst for degradation of dyes and mixed dyes at pH = 10 under visible light.

2. Materials and methods

2.1 Synthesis of MCM-41

MCM-41 is prepared by sol–gel cum hydrothermal method. 2.4 g of CTAB was dissolved in 120 mL of deionized water under ambient conditions. After complete dissolution, 8 mL of 32% aqueous NH₃ was added. Stoichiometry amount of tetraethyl orthosilicate (TEOS, C₈H₂₀O₄ Si, Aldrich, 99%) was added to the solution under vigorous stirring for 1 h. The precipitate was transferred into stainless steel autoclave and placed in a furnace for 20 h at 120 °C. The final product was filtered and dried at 70 °C for 12 h. The surfactant was removed from the product by calcining at 550 °C in air for 5 h.

2.2 In situ fabrication of mesoporous Al₂O₃–MCM-41

Mesoporous Al₂O₃–MCM-41 composite was synthesized by incorporating powder mesoporous Al₂O₃ into MCM-41 during the synthesis of MCM-41 by in situ, just before the addition of NH₃. The sample is prepared at Si/Al ratio of 10. The precipitate was transferred into stainless steel autoclave and placed in a furnace for 20 h at 120 °C. The final product was filtered and dried at 70 °C for 12 h. The composite material is calcined at 550 °C for 5 h. The fabricated powder material is denoted as mesoporous Al₂O₃–MCM-41 composite.

2.3 In situ sol–gel cum hydrothermal fabrication of mono and bimetallic/Al₂O₃–MCM-41 nanocomposite

In a typical synthesis, 2.5 g of CTAB was added to 120 mL of H₂O and stirred for 1 h and then 0.377 g of mesoporous Al₂O₃ was added to it. The mixture was continuously stirred by adding 10 mL aqueous NH₃ after an hour. Then 8 mL of TEOS as silica source was added to the mixture and stirred for 2 h. Then 1.0 mmol of FeSO₄·7H₂O dissolved in ethanol and oleic acid was added. The total mixture was transferred into a stainless steel autoclave and put into a furnace for 20 h at 120 °C. The gel was washed with distilled water and ethanol, and further dried in an oven at 70 °C for 12 h. The powder was calcined at 500 °C for 5 h in air. This material is denoted as mesoporous Fe/Al₂O₃–MCM-41 nanocomposite. Similar procedure was adopted to synthesize Co/Al₂O₃–MCM-41 and Mn/Al₂O₃–MCM-41 samples by taking 1.0 mmol of cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O) and 1.0 mmol of manganese acetate tetrahydrate (C₄H₆MnO₄·4H₂O), respectively.

For synthesis of bimetallic/Al₂O₃–MCM-41, 0.5 mmol of Co (NO₃)·6H₂O and 0.5 mmol of FeSO₄·7H₂O are mixed in ethanol and oleic acid. The above synthesis procedure was followed to obtain Co–Fe/Al₂O₃–MCM-4, Fe–Mn/Al₂O₃–MCM-41 and Mn–Co/Al₂O₃–MCM-41 samples.

2.4 Characterization of materials

Transmission electron microscopy (TEM) images were obtained from Philips CM 200 transmission electron microscope with a LaB₆ filament and equipped with an ultrathin objective lens and CCD camera. HRTEM images were recorded by using JEOL 3010 machine. Diffuse reflectance UV-visible (DRUV-Vis) spectra of the materials were performed by JASCO V-660 spectrophotometer equipped with 60 mm integrating sphere. The measurements were carried out at a band width of 5 nm and in the wavelength range of 200–800 nm at a scanning speed of 100 nm min⁻¹. The powder X-ray diffraction patterns (PXRD) of the synthesized materials were obtained employing Bruker AXS D8 Advance diffractometer and Cu Kα (λ = 0.15408 nm) radiation. A scan rate of 2° min⁻¹ was used to record higher angle reflections from 10 to 80°, and 0.01° s⁻¹ scan rate for low angle reflections from 0.5 to 10°. The specific surface area, pore size and pore volume were measured by N₂ sorption method at liquid nitrogen temperature (−196 °C) using Micromeritics ASAP 2020. The specific surface area and pore size distribution were estimated based on Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. X-ray photoelectron spectroscopy (XPS) measurements were performed using ESCA Probe TPD of Omicron Nanotechnology with polychromatic Mg Kα as the X-ray source (hν = 1253.6 eV). All the binding energies were calibrated by using the adventitious carbon (C 1s at 285 eV) as a reference. The photoluminescence spectra were measured on a JASCO FP-6300 fluorescence spectrometer with an excitation at 350 nm light. The FTIR spectra of the samples were recorded with JASCO FTIR-4100 spectrophotometer in the range of 400–4000 cm⁻¹ at room temperature using KBr as reference. The formation of ˙OH radical was studied by replacing phenol with 5 × 10⁻⁴ M terephthalic acid (TPA) and 2 × 10⁻³ M NaOH with the same amount of catalyst.

2.5 Photo-Fenton degradation process

Methylene blue (MB), Congo red (CR) and mixed dyes (MB + CR) used in this study were from Merck, India. A stock solution of 100 mg L⁻¹ was prepared and suitably diluted to the required initial concentration. The photo-Fenton degradation experiment has been carried out by taking 20 mL of 100 mg L⁻¹ dyes solution. The reaction was carried out in 0.02 g catalyst in 20 mL dye solution (1 g L⁻¹) in the presence of visible light (40 W) in 60 minutes with addition 1.0 × 10⁻⁵ mole of H₂O₂ at pH = 10. The pH of the solution was monitored by Systronics μ pH system 361 with proper addition of 0.01 M HNO₃ and/or NH₄OH. The solution was centrifuged and analysed by JASCO V-660 UV-visible spectrophotometer. The maximum absorbance of methylene blue (MB) and Congo red (CR) occurs at 664 nm and 498 nm, respectively. The absorbance of mixed dyes (MB + CR) follows both at 664 and 498 nm. The photocatalytic mixed dyes degradation occurred in a similar way as photo-Fenton process, without H₂O₂. TOC (total organic carbon) analysis has been carried out by TOC analyzer (ANATOC).

3. Results and discussion

3.1 Electron microscopy

The representative HRTEM micrographs of MCM-41 and Al₂O₃–MCM-41 are shown in Fig. 1. The mesoporous silica exhibited highly ordered hexagonal array of pore structure with amorphous nature of typical MCM-41 materials,³⁴ which is clearly seen in Fig. 1(a)–(c). The HRTEM micrographs of Al₂O₃–MCM-41 also show similar well-ordered hexagonal pores with amorphous nature, which are shown in Fig. 1(d)–(f). The in situ incorporation of mesoporous Al₂O₃ into the MCM-41 matrix, did not affect the orders of hexagonal pores. The sustainability of amorphous nature in Al₂O₃–MCM-41 is due to the presence of very less amount of crystalline Al₂O₃ in the highly amorphous MCM-41 matrix. The TEM and HRTEM images of Fe/Al₂O₃–MCM-41, Co/Al₂O₃–MCM-41 and Co–Fe/Al₂O₃–MCM-41 nanocomposites are shown in Fig. 2. The well-order Fe nanoparticles (particle size 8 nm) have been observed in TEM and HRTEM image of Fe/Al₂O₃–MCM-41 nanocomposite (Fig. 2(a) and (b)). The fringes with lattice spacing of 0.280 nm can be indexed to the (200) planes of FeO with the cubic structure. The selected area electron diffraction (SAED) pattern shown in Fig. 2(d) exhibits the diffraction rings (111), (200) and (220) of a cubic structure of FeO in Fe/Al₂O₃–MCM-41 nanocomposite.^35,36 The Fig. 2(e) and (f) shows Co nanoparticles having particle size 9 nm in Al₂O₃–MCM-41 nanocomposite. In Fig. 2(g), the HRTEM lattice fringes show the d-spacing of 0.250 nm from the (111) plane of cubic CoO phase.^37,38 The SAED pattern (Fig. 2(h)) of three diffraction peaks can be assigned to (111), (200), and (220) lattice planes, which are in good agreement with those of the corresponding standard pure rock salt CoO phase (JCPDS card 48-1719). The Fig. 2(i) and (j) shows the TEM and HRTEM micrographs of Co–Fe/Al₂O₃–MCM-41 nanocomposite. The Fe and Co nanoparticles (4 nm) are shown in Fig. 2(j). The lattice spacing of 0.255 nm between adjacent lattice planes in the image corresponds to the distance between two (111) crystal planes of CoO, while the lattice spacing of 0.285 nm should be assigned to the (200) plane of FeO present in the Co–Fe/Al₂O₃–MCM-41 nanocomposite (Fig. 2(k)). The SAED pattern (Fig. 2(l)) also revealed the presence of both FeO and CoO in Co–Fe/Al₂O₃–MCM-41 nanocomposite. The TEM, HRTEM and SAED pattern of MnO in Mn/Al₂O₃–MCM-41 nanocomposite is shown in the Fig. S1(a)–(c).† This result indicates that the presence of Mn nanoparticles of size 12 nm in the network of Al₂O₃–MCM-41. The observed (111) and (200) planes correspond to the cubic MnO structure.³⁹ Fig. S1(d)–(f)† shows TEM, HRTEM and SAED image of Fe–Mn/Al₂O₃–MCM-41 nanocomposite, respectively. The Fe and Mn nanoparticles of size 6 nm have been observed. The SAED pattern also supports the presence of FeO and MnO nanoparticles in Fe–Mn/Al₂O₃–MCM-41 nanocomposite. The Mn and Co nanoparticles (particle size 7 nm) have been observed from HRTEM image and the planes (111), (200), (220) and (311) correspond to MnO and CoO in Mn–Co/Al₂O₃–MCM-41 nanocomposite, shown in Fig. S1(g)–(i).† The formation of mono and bimetallic nanoparticles in the framework of Al₂O₃–MCM-41 has been well established.

Fig. 1 The HRTEM images of MCM-41 (a) honeycomb structure, (b) well-ordered, and (c) SAED pattern. HRTEM images of Al₂O₃–MCM-41 (d) honeycomb structure, (e) hexagonal ordered and (f) SAED pattern of Al₂O₃–MCM-41.

Fig. 2 Characteristics micrographs of Fe/Al₂O₃–MCM-41 nanocomposite (a) TEM image, (b, c) HRTEM images and (d) SAED pattern. The characteristics images of Co/Al₂O₃–MCM-41 material (e) TEM image, (f, g) HRTEM images and (h) SAED pattern. The representatives micrographs of Co–Fe/Al₂O₃–MCM-41 (i) TEM image, (j–k) HRTEM images and (l) SAED pattern.

3.2 Optical analysis

Fig. 3 delineates the UV-Vis diffuse reflectance spectra Fe, Co and Mn/Al₂O₃–MCM-41 and the Co–Fe, Fe–Mn and Mn–Co/Al₂O₃–MCM-41 nanocomposite. All the materials showed strong absorption band at 260 nm due to the ligand to metal charge transfer (LMCT) between surface oxygen and isolated metal ions such as Co²⁺, Mn²⁺ and Fe²⁺.⁴⁰ Moreover, a strong absorption in the visible range is due to the red shift in the band gap transition of the samples. This ascribes the electronic interaction of Al₂O₃–MCM-41 with metallic center to the influence of charge delocalization. Thus, mesoporous Al₂O₃–MCM-41 with monometallic and bimetallic system is more responsive for the visible light. The red shifting is shifted from monometallic to bimetallic/Al₂O₃–MCM-41 is due to the high electronic interaction of more charged metallic center. The band gap energy of a semiconductor can be calculated by using the following equation.⁴¹

αhν = A(hν − E_g)ⁿ

where α, ν, A, and E_g are the absorption coefficient, light frequency, proportionality constant, and band gap, respectively. The band transition depends upon the value of n, it is ½ for direct and 2 for indirect transition, respectively. The method to evaluate the value of n is given somewhere else.⁴² It has been found that all the material shows direct allowed transitions. The band gap energies of the all samples can be estimated from the plots of (αhν)² versus photon energy (hν). The intercept of the tangent to the X axis would give a good approximation of the band gap energies for the synthesized products, as shown in Fig. 4. It is observed that the band gap energy of the Co/Al₂O₃–MCM-41 and Mn/Al₂O₃–MCM-41 nanocomposite is 3.2 eV and 3 eV, respectively (Fig. 4(a) and (b)). It has been also observed that the band gap energy of neat CoO and MnO is 5 eV and 4 eV, respectively.^29,43 The band gap of Co/Al₂O₃–MCM-41 decreases to 3.2 eV while Co nanoparticles incorporate to the framework of Al₂O₃–MCM-41. This might be due to the formation of a localized state by mixing of Co 2p, Al 2p and Si 2p. Similarly, the band gap energy of Mn/Al₂O₃–MCM-41 nanocomposite decreases to 3 eV, which might be due to the formation of a localized state by mixing of Mn 2p, Al 2p and Si 2p. The band gap energy of Fe/Al₂O₃–MCM-41 nanocomposite is 2.05 (Fig. 4(d)) which is narrower than the band gap energy of pure FeO (2.4 eV) due the mixing of Fe 2p, Al 2p and Si 2p.²⁷ That means the formation and combination of localized metallic orbitals has great impact to narrow the band gap energy. Tang et al. observed that the band gap of TiO₂ decreases by incorporating ZnFe₂O₄–TiO₂ nanoparticles within mesoporous MCM-41.⁴⁴ But the bimetallic/Al₂O₃–MCM-41 nanocomposite system, the band gap energy decreases drastically as compared to monometallic system which resulting a red shift. The most interesting thing is that the band gap energy of bimetallic Mn–Co/Al₂O₃–MCM-41 nanocomposite (Fig. 4(c)) is 2.70 eV which is in visible region. This band gap narrowing (BGN) of 2.70 eV as compared to individual monometallic system Mn/Al₂O₃–MCM-41 (3 eV) and Co/Al₂O₃–MCM-41 (3.2 eV) is due to the mixing and overlapping of the impurity Mn and Co, which resulting an impurity states in between Mn and Co energy band.⁴⁵ The presence of the charge impurities (Mn²⁺ and Co²⁺) leads to the formation of the localized state in the energy band gap. The Mn 2p and Co 2p localized states are formed close to the conduction band of Mn–Co/Al₂O₃–MCM-41 nanocomposite. The high concentration of Mn 2p and Co 2p states causes the band structures to perturb, resulting in the formation of BGN in Mn–Co/Al₂O₃–MCM-41 nanocomposite. In the Fig. 4(e) and (f), band gap narrowing of Fe–Mn/Al₂O₃–MCM-41 (1.93 eV) and Co–Fe/Al₂O₃–MCM-41 (1.89 eV) as compared to individual monometallic system is due to the similar reason as in Mn–Co/Al₂O₃–MCM-41 nanocomposite. Hence, the formation of semiconductor mono and bimetallic/Al₂O₃–MCM-41 nanocomposite has been established.

Fig. 3 Diffuse reflectance UV-visible absorption spectra of (a) Co/Al₂O₃–MCM-41, (b) Mn/Al₂O₃–MCM-41, (c) Mn–Co/Al₂O₃–MCM-41, (d) Fe/Al₂O₃–MCM-41, (e) Fe–Mn/Al₂O₃–MCM-41, (f) Co–Fe/Al₂O₃–MCM-41 nanocomposites.

Fig. 4 Plots of (αhν)² vs. photon energy (hν) for the band gap energy of (a) Co/Al₂O₃–MCM-41, (b) Mn/Al₂O₃–MCM-41, (c) Mn–Co/Al₂O₃–MCM-41, (d) Fe/Al₂O₃–MCM-41, (e) Fe–Mn/Al₂O₃–MCM-41, (f) Co–Fe/Al₂O₃–MCM-41 nanocomposites.

3.3 X-ray diffraction studies

The representative low angle XRD pattern for MCM-41, Al₂O₃–MCM-41 and Co–Fe/Al₂O₃–MCM-41 samples are shown in the Fig. 5. The three samples exhibited high intense d₁₀₀ diffraction peak at low angle indicating the mesoporous nature of the materials.⁴⁶ It is observed that the peak intensity (d₁₀₀) slightly decreases from MCM-41 to Al₂O₃–MCM-41 and extensively decreases from Al₂O₃–MCM-41 to Co–Fe/Al₂O₃–MCM-41 nanocomposite. This is due to the incorporation of the mesoporous Al₂O₃ into the extra-framework of the MCM-41 and incorporation of the (Co & Fe) onto the surface of the Al₂O₃–MCM-41. The other three reflections indexed as d₁₁₀, d₂₀₀ and d₂₁₀ are comparatively less intense. This suggests the presence of a periodic hexagonal arrangement of the channel.⁴⁷ MCM-41 shows all above said reflections, indicating well-ordered hexagonal mesoporous channel. The (100) and (110) diffraction peaks related to Al₂O₃–MCM-41 are intact, indicating an addition of mesoporous Al₂O₃ powder into the MCM-41 which does not affect the mesoporosity and periodic hexagonal arrangement of the channel. This phenomenon is due to (i) the isomorphous of substitution of Si⁴⁺ by Al³⁺ in MCM-41 without affecting its mesoporosity and periodicity.⁴⁸ and (ii) the coordination of Al³⁺ ions with Si⁴⁺ through oxygen leading to extra-framework modification of MCM-41.¹¹ Incorporation of Fe and Co onto the mesoporous Al₂O₃–MCM-41 nanocomposite leads to the degeneration of structural order as shown by weak d₁₀₀ plane and disappearance of reflections (110), (200) and (210) in Fig. 5(c). This is because some Co and Fe particles tend to block the pores of Al₂O₃–MCM-41 network. Similar behaviour can be predicted in all monometallic and bimetallic/Al₂O₃–MCM-41 nanocomposites.

Fig. 5 Low angle X-ray diffractograms of (a) MCM-41, (b) Al₂O₃–MCM-41 and (c) Co–Fe/Al₂O₃–MCM-41 materials.

Fig. 6 shows the high angle XRD pattern of mesoporous Al₂O₃, mono and bimetallic/Al₂O₃–MCM-41 samples. Mesoporous γ-Al₂O₃ is crystalline in nature showing diffraction peaks at (440), (400) and (311).⁴⁹ However, the diffraction peaks of γ-Al₂O₃ phase are very feeble in monometallic and bimetallic/Al₂O₃–MCM-41 systems. The metal incorporated Al₂O₃–MCM-41 samples are substantially amorphous in nature and do not show any X-ray reflections of the respective metal oxide phases. The broad band centred at 2θ = 22° can be assigned to the characteristic refection from amorphous SiO₂ (JCPDS 29-0085). The XRD results indicate that Al₂O₃–MCM-41 matrix is highly amorphous in which cluster-like Fe, Co, Mn, Co–Fe, Mn–Co, Fe–Mn oxide species are embedded.

Fig. 6 High angle X-ray diffractograms of (a) Al₂O₃, (b) MCM-41, (c) Co/Al₂O₃–MCM-41, (d) Mn/Al₂O₃–MCM-41, (e) Fe/Al₂O₃–MCM-41, (f) Mn–Co/Al₂O₃–MCM-41, (g) Fe–Mn/Al₂O₃–MCM-41, (h) Co–Fe/Al₂O₃–MCM-41 nanocomposites.

3.4 N₂ sorption studies

The N₂ sorption isotherms of monometallic (Fe, Co, Mn) and bimetallic (Co–Fe, Fe–Mn, Mn–Co)/Al₂O₃–MCM-41 nanocomposite systems are presented in Fig. 7. The entire materials show type-IV isotherm with H1 hysteresis. This isotherm represents the mesoporous behaviour of these materials.⁵⁰ The adsorption step of all the materials centred in the relative pressure (P/P₀) region from 0.1 to 0.5. This phenomenon indicates the presence of framework-confined mesopores or framework mesoporosity/intra-particle mesoporosity.^51,52 The pore size distribution curves shown in Fig. 8 are mono modal and they occur in the narrow mesoporous range between 2–3 nm. This indicates the presence of framework mesoporosity in mesoporous mono and bimetallic/Al₂O₃–MCM-41 nanocomposite.

Fig. 7 N₂ sorption isotherms of Mn/Al₂O₃–MCM-41, Co–Fe/Al₂O₃–MCM-41, Fe– Mn/Al₂O₃–MCM-41, Fe/Al₂O₃–MCM-41, Mn–Co/Al₂O₃–MCM-41 and Co/Al₂O₃–MCM-41 nanocomposites.

Fig. 8 Pore size distributions of Fe–Mn/Al₂O₃–MCM-41, Co–Fe/Al₂O₃–MCM-41 Fe/Al₂O₃–MCM-41, Co/Al₂O₃–MCM-41, Mn/Al₂O₃–MCM-41 and Mn–Co/Al₂O₃–MCM-41 nanocomposites.

The surface area, pore diameter, pore volume of samples are summarized in Table 1. These values are obtained from N₂ sorption isotherms. The high specific surface area of Al₂O₃–MCM-41 as compared to MCM-41 indicates that the meso-Al₂O₃ is incorporated into the extra-framework of the MCM-41. In other words, the high texture mesoporous Al₂O₃ has been incorporated into the extra-framework of MCM-41 by coordination of Al³⁺ to Si⁴⁺. After incorporation of monometallic system such as Fe, Co and Mn nanoparticles into the surface of mesoporous Al₂O₃–MCM-41, the surface area decreases as compared to pure Al₂O₃–MCM-41. This is because some metals nanoparticles are blocking the mesoporous network. The bimetallic system, Co–Fe, Fe–Mn and Mn–Co nanoparticles incorporated Al₂O₃–MCM-41 nanocomposite show much less surface area as compared to monometallic/Al₂O₃–MCM-41. The pore blocking is more effective in the case of bimetallic materials compared to monometallic materials.

Table 1 Textural property of Al₂O₃, MCM-41, Al₂O₃–MCM-41, monometallic/Al₂O₃–MCM-41 nanocomposite and bimetallic/Al₂O₃–MCM-41 materials measured by BET method

Samples	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)	Average particle size (nm)
Al2O3	268	0.67	3.6	—
MCM-41	800	1.10	2.2	—
Al2O3–MCM-41	870	1.78	2.1	—
Fe/Al2O3–MCM-41	379	0.96	2.2	8.0
Co/Al2O3–MCM-41	321	0.95	2.2	9.0
Mn/Al2O3–MCM-41	308	0.94	2.2	12
Co–Fe/Al2O3–MCM-41	300	0.92	2.3	4.0
Mn–Co/Al2O3–MCM-41	275	0.84	2.2	7
Fe–Mn/Al2O3–MCM-41	265	0.79	2.3	6

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3.5 XPS studies

XPS is an excellent technique to understand the oxidation state of the transition metal ion and the relative composition of the synthesized material. Fig. 9(a)–(e) shows the core level XPS spectra of Fe 2p, Co 2p, Si 2p, Al 2p and O 1s in mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite. The electronic states of samples were investigated by XPS study. The binding energy of Al 2p in pure Al₂O₃ and Si 2p in pristine SiO₂ is 75 and 103.5 eV, respectively.^53,54 Incorporation of meso-Al₂O₃ into the framework of MCM-41 (mesoporous Al₂O₃–MCM-41) and the incorporation of Fe and Co into the mesoporous support Al₂O₃–MCM-41 is due to the higher shifting of Si 2p binding energy from 103.5 eV to 104.45 eV. The BEs of Si 2p shift towards higher value by nearly 1 eV might be due to the strong interaction between Al³⁺ and Si⁴⁺ through oxygen atom. The higher shift of BEs for Si 2p and lower shift for Al 2p (74.30 eV) might be due to the transfer of electrons from Si⁴⁺ to Al³⁺ and also suggesting some Si⁴⁺ ions is replaced by Al³⁺. It is concluded that mesoporous support Al₂O₃–MCM-41 consists of Si–O–Al network. The present study depicts that the binding energies (BEs) of Fe 2p_3/2 and Al 2p_3/2 at 709.41 eV and 74.18 eV, suggesting the presence of Fe in 2+ oxidation state and Al in 3+ oxidation state. It has been reported that the binding energy of Fe 2+ is 709 eV.^55,56 The BEs of Fe 2p_3/2 shifted towards higher value and the Al 2p_3/2 lower values indicates that a strong metal–support interaction. The strong interaction might be due to transfer of electron from Fe²⁺ to Al³⁺. Moreover, the binding energy of CoO is 780.7 eV.³⁷ But the binding energy of CoO is 781.8 eV in mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite. The higher shifting of binding energy is due to strong interaction between Co²⁺ and Al³⁺. The electronic interaction leads to transfer of electron from Co²⁺ to Al³⁺. Conclusively, the mesoporous nanocomposite Co–Fe/Al₂O₃–MCM-41 nanocomposite consists of Fe–Co–Al–O–Si linkage. The BEs of O 1s was 532.63 eV which might be due close interaction of oxygen atom with Si compared to Al atoms.

Fig. 9 XPS of (a) Fe 2p core-level spectrum, (b) Co 2p core level spectrum, (c) Si 2p spectrum, (d) representative Al 2p spectrum and (e) O 1s core-level spectrum of mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite.

3.6 Growth mechanism of mesoporous Fe/Al₂O₃–MCM-41 and Co–Fe/Al₂O₃–MCM-41 nanocomposite

Size control of metallic nanoparticles depends on the growth mechanism. Ostwald ripening, phase transfer phenomena, hydrothermal, and orientation attachment are the vital processes that have been highlighted.^57,58 These process require suitable capping agent, reaction environment and temperature for nucleation and growth of the nanoparticles. In that context, mesoporous Fe/Al₂O₃–MCM-41 and Co–Fe/Al₂O₃–MCM-41 nanocomposite have been synthesized via in situ sol–gel cum hydrothermal process. The role of oleic acid as capping agent, CTAB and optimum temperature are key factors for the formation of Fe and Co–Fe nanoparticles in the framework of mesoporous Al₂O₃–MCM-41. The entire growth mechanism is shown in Scheme 1. At the initial step, mixing of CTAB with H₂O formed a micelle which is susceptible for allowing powder Al₂O₃ and TEOS, resulting in aggregation of the micelle with Al and Si. On the other hand Fe–oleate formed by mixing of oleic acid and ethanol. The oleic acid has hydrophilic –COOH functional group with long hydrophobic alkyl moiety. Nucleation and growth takes place in both cases of Fe-oleate and –Al–O–Si– (sol form) placed in a stainless steel autoclave. The carboxylate groups of oleic acid may form stable complexes with Fe²⁺, which prevent the fast growth of Fe particle.³⁶ The replacement of sulphate to oleate may be the key step in slowing down particle growth. The high decomposition temperature Fe–oleate also stabilizes the primary growth of nanoparticles and prevents the Fe nanoparticles growth.⁵⁹ This is because the oleate molecules with abundant hydroxyl groups are favorably adsorbed on the surface of the generated primary nanoparticles, forming large steric barriers and slowing down the growth rate of primary nanoparticles.⁶⁰ Moreover, Fe–oleate and sol –Al–O–Si– may generate both organic interface and aqueous interface, respectively. The Fe particle in organic interface may fear the aqueous interface which enable the slow growth Fe particle and prevent the agglomeration. This may be called as “fearing nanoparticles” mechanism. These phenomenon aids to prevent the agglomeration of Fe nanoparticles as well as sol–gel –Al–O–Si–. The Fe nanoparticles are formed in the framework mesoporous Al₂O₃–MCM-41 by removing oleate molecule and CTAB through calcination. Likewise Co–Fe nanoparticles/Al₂O₃–MCM-41 is formed by mixing of both precursors (Scheme 1). It has been investigated that the particle size of bimetallic Co–Fe is smaller as compared to monometallic Fe (Table 1). This may be due to the repulsion of similar charge Fe²⁺ and Co²⁺ which helps to restrict the growth of nanoparticles. This phenomenon may be called as “similar charge bimetallic repulsion”. Thus, other mono and bimetallic/Al₂O₃–MCM-41 nanocomposite formed by the similar procedure.

Scheme 1 Growth mechanism of mesoporous Fe/Al₂O₃–MCM-41 and Co–Fe/Al₂O₃–MCM-41 nanocomposites.

3.7 Photo-Fenton achievement of dyes and mixed dyes

The photo-Fenton degradation of methylene blue (MB), Congo red (CR) and mixed (MB + CR) has been investigated by using mesoporous mono and bimetallic/Al₂O₃–MCM-41 nanocomposite. The reaction is carried out in the presence of visible light (400 W Xeon source), pH 10 and 1.0 × 10⁻⁶ mole of H₂O₂. The concentration of methylene blue (MB), Congo red (CR) and mixed dyes (MB + CR) are 100 mg L⁻¹ and catalyst amount is 1.0 g L⁻¹. The photo-Fenton degradation of methylene blue is proceeded very quickly and achieving 100% methylene blue degradation within 60 minute. The methylene blue degradation with different catalysts is shown in Fig. 10. The photo-Fenton degradation order is Co–Fe/Al₂O₃–MCM-41 > Fe–Mn/Al₂O₃–MCM-41 > Mn–Co/Al₂O₃–MCM-41 > Fe/Al₂O₃–MCM-41 > Co/Al₂O₃–MCM-41 > Mn/Al₂O₃–MCM-41 > Al₂O₃–MCM-41> MCM-41. The high degradation of methylene blue (MB) indicates the decrease in absorbance intensity. The similar trend appeared in the degradation of Congo red (CR) and mixed dyes (MB + CR), which is shown in the Fig. 11 and 12, respectively. The percentage of photo-Fenton degradation of methylene blue, Congo red and mixed methylene blue and Congo is described in Table 2.

Fig. 10 UV-Vis spectra of the solutions recorded after photo-Fenton degradation methylene blue (MB). The experiment is carried out on different catalyst in visible light. The concentration of the standard methylene blue solution used is 100 mg L⁻¹. The degradation process is carried out by using 1.0 g L⁻¹ of catalyst, 1.0 × 10⁻⁶ mole of H₂O₂ for 60 minutes.

Fig. 11 UV-Vis spectra of the solutions recorded after photo-Fenton degradation of Congo red (CR). The degradation process is carried out by using different 1.0 g L⁻¹ of catalyst in a 100 mg L⁻¹ Congo red solution. The reaction is accelerated by adding 1.0 × 10⁻⁶ mole of H₂O₂ in visible light for 60 minute.

Fig. 12 UV-Vis spectra of the solutions recorded after photo-Fenton degradation of mixed methylene blue and Congo red (MB + CR). The degradation process is carried out by using different 1.0 g L⁻¹ of catalyst in a 100 mg L⁻¹ (MB + CR) solution. The reaction is accelerated by adding 1.0 × 10⁻⁶ mole of H₂O₂ in visible light for 60 minutes.

Table 2 Photo-Fenton degradation (%) of methylene blue (MB), Congo red (CR), mixed dyes (MB + CR) and photocatalytic degradation of mixed dyes. The (%) of TOC removal in photo-Fenton mixed dyes (MB + CR) process has also entrenched

Catalysts	Photo-Fenton methylene blue (MB) degradation (%)	Photo-Fenton Congo red (CR) degradation (%)	Photocatalytic mixed dyes (MB + CR) degradation (%)	Photo-Fenton mixed dyes (MB + CR) degradation (%)	TOC removal (%) in photo-Fenton mixed dyes (MB + CR)
MCM-41	40	30	40	44	15
Al2O3–MCM-41	50	40	53	55	25
Mn/Al2O3–MCM-41	85	66	75	88	70
Co/Al2O3–MCM-41	87	75	79	90	78
Fe/Al2O3–MCM-41	89	78	81	93	82
Mn–Co/Al2O3–MCM-41	95	85	85	97	87
Fe–Mn/Al2O3–MCM-41	97	90	88	99	88
Co–Fe/Al2O3–MCM-41	100	95	90	100	92

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The photocatalytic degradation of mixed dyes has been studied (Fig. 13) in order to compare with the photo-Fenton activity. The photocatalytic process is progressed without H₂O₂ whereas the photo-Fenton process with H₂O₂. It has been observed from Fig. 13 and Table 2, the photocatalytic mixed dyes degradation is low as compared to photo-Fenton process. Hence, the photocatalytic process progressed solely whereas the photo-Fenton process occurring via photocatalysis.

Fig. 13 UV-Vis spectra of the solutions recorded after photocatalytic degradation of mixed methylene blue and Congo red (MB + CR). The degradation process is carried out by using different 1.0 g L⁻¹ of catalyst in a 100 mg L⁻¹ (MB + CR) solution. The reaction is accelerated by adding 1.0 × 10⁻⁶ mole of H₂O₂ in visible light for 60 minutes.

The TOC removal of mixed dyes (MB + CR) in the case of photo-Fenton degradation is shown in the Table 2. It is observed that 92% of TOC removal occurred in Co–Fe/Al₂O₃–MCM-41 nanocomposite, indicating that the reaction intermediates are converted into CO₂ and H₂O. The highest mineralization is due to the presence of nanoparticles Co and Fe in mesoporous Al₂O₃–MCM-41 support, H₂O₂ and visible light. This trends decreases from bimetallic/Al₂O₃–MCM-41 to monometallic/Al₂O₃–MCM-41 which is due to the unavailability of reacting surface. The very low mineralization occurred in Al₂O₃ and Al₂O₃–MCM-41 which showing TOC removal 15% and 25%, respectively. This is because of the absence of metallic surface, the photo-Fenton process will not operate. In absence of metallic surface only adsorption process may occur in presence of H₂O₂ and visible light. This percentage may be small if it is pure adsorption process in absence of H₂O₂ and visible light. The presence of H₂O₂ and visible light, less decomposition of mixed dyes happened due to least decomposition H₂O₂ by visible light.

3.7.1 Dual mechanism for degradation of dyes and mixed dyes. The degradation of dyes (MB, CR) and mixed dyes (MB + CR) are proceeding by both photo-Fenton and photocatalysis process (dual process). This is because the catalysts exhibits both metallic center as well as semiconductor behavior. The dual mechanism process is initiated by the action of hydroxyl radicals formed during the reaction, which is schematically shown in Scheme 2. In the photo-Fenton system, the degradation of dyes and mixed dyes is progressed in the presence mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite, H₂O₂ and visible light. The oxidation and reduction reaction is occurred from Co²⁺–Fe²⁺/Al₂O₃–MCM-41 to Co³⁺–Fe³⁺/Al₂O₃–MCM-41 nanocomposite which helps in generating ˙OH radicals. The ˙OH radicals are reacted with the dyes molecule and produced degradation product. It has been observed that the catalyst possesses semiconductor property. The electron and hole are generated when catalytic surface is exposed to visible light. The active species ˙OH radicals are formed when hole (h⁺) reacts with H₂O and photogenerated electron reacts with dissolved O₂. The generated ˙OH radicals reacts with dyes and mixed dyes produced degraded product. Hence, the dual mechanism is operating in the dyes and mixed dyes degradation process. Moreover, photocatalytic process is progressed in photo-Fenton system.

Scheme 2 Mechanistic pathways of dyes and mixed dyes degradation by mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite in visible light.

3.7.2 Factor affecting the photo-Fenton activity. The degradation of dyes such as methylene blue (MB), Congo red (CR) and mixed dyes (MB + CR) has been achieved by (a) the role of particle size and surface morphology, (b) formation of a high amount of hydroxyl radicals, and (c) lowering the electro-hole recombination.
3.7.2.1 Role of particle size and surface morphology. Particle size has a great deal of importance in the field of catalysis. The small particle (nanoparticle) will act as an efficient catalyst because of high surface to volume ratio. Generally, the surface area increases with decreasing particle size, which generate more active site. This phenomenon aids to increase the catalytic activity. Another thing is that if the nanoparticles are in supported with highly porous network, then the surface area increases as compared to neat nanoparticles. These properties of material help to increase the greater active site for the accommodation of a substrate molecule. In the present study, the photo-Fenton degradation of dyes and mixed dyes strongly depends on the particle size of mono and bimetallic nanoparticles in the framework of mesoporous Al₂O₃–MCM-41. The particle size of the mono and bimetallic nanoparticles has been calculated from a HRTEM image by ImageJ software is shown in Table 1. It is observed that the particle size of the Co and Fe is very less in mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite which lead to increase the surface area (Table 1). Hence, the dyes degradation activity is more (100%) as compared to other bimetallic/Al₂O₃–MCM-41 nanocomposite. Moreover, this may be due to the small particle size of Co and Fe (4 nm) of Co–Fe/Al₂O₃–MCM-41 nanocomposite. This leads to formation of more reactive surface and acceleration of the mass transfer which resulting in more catalytic activity.⁶¹ The migrating time of photoinduced charge carriers from inner to surfaces takes much less time which helps to increase the catalytic activity. The monometallic/Al₂O₃–MCM-41 nanocomposite has high surface area than the bimetallic system, but the photo-Fenton and photocatalytic activity is less as compared to bimetallic/Al₂O₃–MCM-41 nanocomposite. This is due to the large particle size monometallic system as compared to bimetallic system. It has been observed that the mesoporous Al₂O₃, MCM-41 and Al₂O₃–MCM-41 have high surface as compared to mono and bimetallic/Al₂O₃–MCM-41 nanocomposite. But due to lack of metallic active site, the dyes degradation activity is very less. Mesoporosity which leads to increase the surface area and metallic nanoparticles which leads to increase the catalytic active sites are vital factors for an efficient dyes degradation.
3.7.2.2 Generation of the hydroxyl radical. The hydroxyl radical has an important role for the decomposition of the dyes, and it is necessary to investigate the amount of hydroxyl radicals produced by each catalyst. This analysis is performed by using terephthalic acid (TA) as a probe molecule.⁶² In this method, TA was attacked by a ˙OH radical, forming 2-hydroxyterephthalic acid (TAOH), which gives a fluorescence signal at 426 nm.^63,64 Fig. 14 depicts the fluorescent signal of bimetallic/Al₂O₃–MCM-41 nanocomposite after reacting with TA solution. It has been observed that generation of the hydroxyl radical is proportional to the PL intensity.⁶⁵ Hence, higher the generation of hydroxyl radicals indicates the higher yield of TAOH and resulting more intense fluorescence peak. It has been observed that mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite has highest intensity which confirm the high production of ˙OH radical compared to other bimetallic system. The dyes degradation performance of a mesoporous nanocomposite follows the order: Co–Fe/Al₂O₃–MCM-41 > Fe–Mn/Al₂O₃–MCM-41 > Mn–Co/Al₂O₃–MCM-41, which is strongly consistent with fluorescence intensity.

Fig. 14 Photoluminescence responses of mesoporous bimetallic/Al₂O₃–MCM-41 nanocomposite under visible light irradiation for 60 min in 5 × 10⁻⁵ M basic solution of terephthalic acid.

3.7.2.3 Lowering of electron–hole recombination. It has been investigated that the mono and bimetallic/Al₂O₃–MCM-41 nanocomposite have semiconductor property. In this study, it is understood that photocatalytic reaction is progressed in photo-Fenton system. Hence, efficient photocatalytic reaction leads to high photo-Fenton process. The photoluminescence emission results from the radiative recombination of excited electrons and holes. Hence, it is an essential requirement of a good photocatalyst to have minimum electron–hole recombination. To learn the recombination of charge carriers, PL studies of synthesized materials have been undertaken. PL emission intensity is directly related to recombination of excited electrons and holes.⁶⁶ The photoluminescence spectra of synthesized photocatalyst are shown in Fig. 15. The result shows that bimetallic system has less PL intensity as compared to monometallic, which is due to the less electron–hole recombination in bimetallic system. This is because of small size and more metallic charged species are available in bimetallic system which leads to accumulate extra electrons onto it and forget to recombine with the hole. In bimetallic system, mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite has the lowest PL intensity. The lowest intensity is due to the least electron–hole recombination. The least electron–hole recombination is because of some generated electrons are engaged to reduce the highly charge species Co²⁺ and Fe²⁺ in Co–Fe/Al₂O₃–MCM-41. Hence, the maximum number of holes can be take part in the photocatalytic system which leads to high production of ˙OH radicals for photocatalytic dyes and mixed dyes degradation. Furthermore, the lowest PL intensity of mesoporous Fe–Co/Al₂O₃–MCM-41 nanocomposite is due to the small size of the Fe and Co within Al₂O₃–MCM-41 as compared to other bimetallic system.

Fig. 15 Photoluminescence spectra of mesoporous mono and bimetallic/Al₂O₃–MCM-41 nanocomposite, MCM-41 and Al₂O₃–MCM-41.

3.7.2.4 Evidence and stability of photo-Fenton degradation of methylene blue. The FTIR spectra of pure methylene blue, Co–Fe/Al₂O₃–MCM-41 (after MB degradation) and neat Co–Fe/Al₂O₃–MCM-41 are shown in the Fig. 16(a), (b) and (c), respectively. The two peaks appear at 2816, 2720 cm⁻¹ which represent the stretching vibration of –CH– aromatic and –CH₃ methyl groups of pure methylene blue. The spectra range from 1591 to 1363 cm⁻¹, are assigned to the aromatic ring structures in methylene blue.⁶⁷ The peak at 1170 cm⁻¹ is related to the C=C skeleton of the aromatic rings. The absence of methylene blue signature peaks in Fig. 16(b), indicating the complete mineralization of methylene blue on the surface of mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite. The IR spectrum of pure mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite is shown Fig. 16(c) for comparison. The IR spectrum of Co–Fe/Al₂O₃–MCM-41 after MB degradation (Fig. 16(b)) is exactly similar to neat Co–Fe/Al₂O₃–MCM-41 (Fig. 16(c)), indicating the stability of the Co–Fe/Al₂O₃–MCM-41 nanocomposite catalyst after methylene blue degradation. The Fig. 16(d) shows the 100% degradation methylene blue (MB), Congo red (CR) and mixed dyes (MB + CR) on mesoporous Co–Fe@Al₂O₃–MCM-41 nanocomposite leading to colourless solution at pH = 10.

Fig. 16 FTIR spectra of (a) pure methylene blue, (b) methyl blue degraded on Co–Fe/Al₂O₃–MCM-41 in comparison with (c) Co–Fe@Al₂O₃–MCM-41 material; (d) colour change after degradation of 100 mg L⁻¹ methylene blue dye using 0.02 g of Co–Fe@Al₂O₃–MCM-41 at pH = 10.

4. Conclusions

The novelty of the present investigation is the formation of Fe, Co, Mn, Co–Fe, Mn–Co and Fe–Mn nanoparticles in the framework of mesoporousAl₂O₃–MCM-41 by using oleic acid and CTAB as size controlling (capping agent) and morphology controlling (surfactant) agents, respectively. Both photo-Fenton and photocatalysis (dual) process is operating for the efficient degradation of dyes and mixed dyes over all the mono and bimetallic nanocomposite system. Mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite showed 100% dyes and mixed dyes degradation in 60 min. The high activity of mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite is ascribed to the presence of highly active mono and bimetallic nanoparticles and semiconductor behavior and mesoporosity in mono and bimetallic/Al₂O₃–MCM-41 nanocomposite. Moreover, (a) nanostructure morphology of catalysts, (b) high surface area due to intraparticle mesoporosity, (c) lowering of electron–hole recombination and (d) swift generation of a large amount of hydroxyl radicals are the key factors which make mesoporous Co–Fe/Al₂O₃–MCM-41 nanocomposite an efficient versatile photo-Fenton catalyst. It is concluded from the study that Co–Fe/Al₂O₃–MCM-41, a multifunctional nanocomposites material can be explored for treatment of wastewater containing dyes, phenolic compounds and other organic pollutants by photo-Fenton process.

Acknowledgements

Amaresh Chandra Pradhan thanks IIT Madras for Postdoctoral Fellowship. The instrumental facilities established under the FIST Scheme of SERC division of DST, Ministry of Science and Technology, New Delhi, have been very helpful to carry out this work. Mr A. Narayanan and Mrs S. Srividya carried out BET and XRD measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19923b

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