Efficacy of Cobalt-Incorporated Mesoporous Silica towards Photodegradation of Azodyes and Its Kinetic Study for Advanced Application

Abstract

Along with MCM-41, cobalt-incorporated mesoporous silica (Co-MCM-41) has been created. Powder X-ray diffraction, scanning electron microscopy, and nitrogen adsorption-desorption studies were used to describe the materials. It has been discovered that the Co-MCM-41 has less surface area (SBET, m2 g−1), pore volume (cc·g−1), and pore size (Å) than the MCM-41. The SEM-EDAX analysis has also unmistakably demonstrated the existence of the appropriate elements in the materials. The photoactivity was significantly impacted by the extremely distributed Co3+ species present on the MCM-41 structure. A theoretical loading of 3.5 wt% permitted an AO7 degradation percentage of about 70% for the samples that were simply treated with Co. Increased Co3+ inactive species, such as clusters or −Co2O3 nanoparticles, are present at higher loadings, but the photoactivity is not noticeably increased. By using the Kubelka-Munk function to the UV-Vis DRS results, it was discovered that the band gap (eV) in the Co-MCM-41 was also substantially smaller than in its parent template. The Alizarin Red S dye was successfully photodegraded employing the materials as photocatalysts, and pseudo-first order kinetics was carried out using the Langmuir-Hinshelwood kinetic model. The necessary experimental setups were all optimised.

1. Introduction

A growing focus on heterogeneous catalysts has emerged in recent years due to economic and environmental factors. These catalysts typically have low cost, high reactivity, environmental friendliness, high selectivity, easy setup, and recoverability of catalysts [1]. Mesoporous MCM-41 materials have come to light as highly stable compounds with a significant surface area. They have been widely used as catalysts or catalyst supports in a variety of processes. This was explained by the reaction mixture’s higher reusability and straightforward recoverability [2]. Due to their increased oxidative or acidic character, the introduction of metal ions including Ti⁴⁺, Al³⁺, Co³⁺, and Fe³⁺ into the framework has demonstrated improved catalytic activity [3].

Due to contaminated ground water and dangerous industrial effluents, the entire world is currently experiencing environmental issues [4, 5]. These highly coloured effluents harm the environment when they are dumped into water systems because they impede light penetration and hinder aquatic life’s ability to photosynthesize [6, 7]. Intense colour is imparted by the presence of dyes in the effluent at very low concentrations (1 mg L⁻¹) but it is discovered that they are harmful to the environment [8, 9]. In order to effectively remove colour from waste fluids, physical or chemical procedures must be used [10]. The majority of dyes used on an industrial basis are derivatives of azo, anthraquinone, indigo, triphenylmethane, xanthene, and others [11, 12]. Due to their advantageous qualities—bright colour, easy application, and low energy consumption—these dyes are widely utilised in the textile industry. They are, however, typically the most poisonous and mutagenic substances found in nature [13, 14].

One of the anthraquinone class of dyes, Alizarin Red S (ARS), is widely used in the textile, woven fabric, wool, and cotton industries [15, 16, 17]. The paint, plastics, leather, and cosmetics sectors all utilise anionic dye extensively [18]. However, when industrial effluents are released into aquatic environments at amounts exceeding what is permitted, the aquatic life is negatively impacted [19, 20]. It was shown that traditional aerobic digestion techniques were ineffective at degrading these resistant compounds [21]. Photo catalysis has become a green technique for gathering solar energy and degrading organic pollutants due to the global energy crisis and environmental challenges [22, 23, 24, 25]. ARS was selected as the test molecule to undergo photodegradation in the presence of visible light as a result.

MCM-41 and its metal-included derivatives have a wide range of photocatalytic uses, so cobalt metal ion (Co⁺³) was successfully inserted into the structure of MCM-41. In order to research the photodegradation of ARS under ideal experimental settings, such as effect of photocatalyst, effect of photocatalyst dose, effect of dye concentration, and effect of pH, the materials were characterised and used as photocatalysts. To identify the active species participating in the photodegradation phenomenon, a scavenger experiment was done in addition to these research. To assess how well the outcomes matched each other, the kinetic research was carried out.

2. Results and discussion

2.1 Powder XRD studies

Figure 1 displays the XRD patterns (2°≤2θ ≤ 10°) of the MCM-41 and Co-MCM-41 samples. The patterns only display one low-angle peak for the d100 plane, which corresponds to the mesophase at a value of 2 approximately 2.2o (d-spacing: 32.54623 A, wall thickness: 2.712 A). This is typical of MCM-41’s long-range hexagonal structure [26]. Planes that mirror the characteristics of mesoporous nature as in MCM-41. Co-diffraction MCM-41’s pattern has a lesser intensity of the low angle peak than MCM-41, which suggests that the metal ions are obstructing the structure that directs the template’s action in the materials’ regular ordering.

Figure 1.

Powder XRD patterns of MCM-41 and CoMCM-41.

The diffraction planes d110 (d-spacing = 19.48761 A, wall thickness = 4.531 A), and d200 (16.95948 A, wall thickness = 5.207 A), which reflect the hexagonal array of MCM-41, are responsible for the less intense and broader peaks in the 2 of 4.0°–5.5°. Three diffraction peaks suggest that the mesopores are ordered crystallographically. The reason for the low value of 2 is primarily the template’s long carbon chain, which was employed to synthesise MCM-41 [27]. Co-MCM-41 materials, in contrast, exhibit one large peak at 2 = 2.5o, which corresponds to the mesoporous phase, and two succeeding, less intense peaks at (110) and (200) crystal planes, which mirror the mesoporous characteristic of MCM-41. Co-diffraction MCM-41’s pattern has a lesser intensity of the low angle peak than MCM-41, which suggests that the metal ions are obstructing the structure that directs the template’s action in the materials’ regular ordering.

2.2 Nitrogen adsorption-desorption studies

It was discovered that the synthetic materials followed a standard type-IV adsorption isotherm without hysteresis. This demonstrates how mesoporous these materials are [28, 29]. By using the BET (Brunauer, Emmet, and Teller) method to determine the specific surface area of the materials from adsorption isotherms, it can be demonstrated that the insertion of metal ions reduces the materials’ surface area. This might be explained by metal ions filling part of the pores. By using the BJH (Barrett-Joyner-Halenda) method, the pore size and pore volume of the materials are assessed. Table 1 lists the textural characteristics of MCM-41 and Co-MCM-41 materials.

Material	SBET (m^2g^−1)	Pore size (Å)	Pore volume (cc g^−1)
MCM-41	1023.50	17.20	0.28
CoMCM-41	698.22	16.90	0.22

Table 1.

Textural characteristics of the materials.

2.3 SEM-EDAX studies

Figures 2 and 3 show the SEM-EDAX micrographs of MCM-41 and Co-MCM-41, respectively. All of the materials have spherical morphologies similar to those of MCM-41, according to SEM micrographs of the materials. The functionalization of materials with metal ions (Co⁺³) was also validated by EDAX analysis. It was discovered that adding metal ions to the framework has no effect on the morphology of the materials.

Figure 2.

SEM-EDAX micrographs of MCM-41.

Figure 3.

SEM-EDAX micrographs of Co-MCM-41.

2.4 UV-Vis diffuse reflectance spectra & Kubelka-Munk function curve

The UV-Vis diffuse reflectance spectra were taken in order to comprehend the coordination between the Cobalt and MCM-41. Pure MCM-41 was found to lack a distinctive absorption peak in the 200–800 nm range, which suggests that it was not sensitive in the UV-Visible range [30]. However, a strong absorption peak was seen in Co-MCM-41 in the 400–450 nm (430 nm) range, which is consistent with the octahedral geometry of the Co⁺³ crystal field transition.

⁵T_2g(D)→⁵E_g(D) [(t_2g⁴e_g²)→(t_2g³e_g³)]. By using the Kubelka-Munk (KM) function, the produced mesoporous materials have also been described for their band gap values (in electron volts, eV). Figure 4 showed the findings and a plot of the band gap energy values (eV) vs. the modified Kubelka-Munk function [F(R)hv]2 [31]. In MCM-41 and Co-MCM-41 materials, the band gap was discovered to be 2.9 eV and 2.7 eV, respectively. It was clear from the analysis of these results that immobilising Cobalt caused a sizable reduction in the band gap as well as proper coordination of the Cobalt (+3) ion in the zeolite framework. A necessary component of the phenomena of photocatalysis, the photocatalytic activity in the visible light can be improved by the smaller band gap [31].

Figure 4.

KM function curve of MCM-41 and Co-MCM-41 materials.

Adsorption studies: Adsorption isotherm experiment was carried out in the dark to investigate the maximum ARS adsorption capacity on the surface of the MCM-41 and Co-MCM-41 materials. The following statement [32] inevitable factor for the phenomena of photocatalysis [31] was used to determine the amount of ARS absorbed per unit mass of the adsorbent at time, t (min).

$${\mathrm{q}}_{\mathrm{t}}=\frac{\left(\mathit{Co}-\mathit{Ct}\right)V}{m.}$$E1

Co and Ct are the dye solution concentrations (mol/L) before and after adsorption, respectively. V is the dye solution’s volume (L) in the photoreactor, and m is the photocatalyst’s mass (g).

Figure 5 shows the amount of ARS adsorption as a function of time. In the photoreactor, it was seen that a fast adsorption developed after 15 min of contact. However, the adsorption propensity started out very mildly and only became saturated after 15 min. It demonstrates that the addition of Cobalt to the MCM-41 framework increased the dye’s tendency to be removed. Based on these findings, the equilibration time was optimised to be 15 min with both mesoporous materials in darkness, and the same was fixed as the equilibration duration for additional research.

Figure 5.

Adsorption studies of ARS with MCM-41 and Co-MCM-41 materials. Catalyst load = 10 mg, [ARS] = 2.92 × 10⁻⁵ M, ARS volume = 100 mL.

Effect of photocatalyst: The synthesised materials were used to investigate the impact of deterioration on the 2.92 × 10^–5 M aqueous solutions of ARS. The results are depicted in Figures 6 and 7. Co-MCM-41 (10 mg) achieved about 98% degradation efficiency in 90 min, whereas MCM-41 only destroyed 68% of ARS during the same period (Figure 6). This might be because Co-MCM-41 material has a higher capacity for absorbing light due to its smaller pore size and volume, which could boost its activity for efficient photocatalytic degradation.

Figure 6.

Effect of photocatalyst on degradation of ARS with the photocatalysts (each 10 mg). (triangles) MCM-41; (stars) Co-MCM-41.

Figure 7.

Effect of Co-MCM-41 (10 mg) on absorbance of ARS at λ _max = 597 nm with respect to time (min).

Studying the effect of Co-MCM-41 on ARS absorbance, it was found that the intensity of absorbance significantly decreased within 90 min, as shown in Figure 7. The dye’s efficient degradation may be to blame for the decrease in ARS absorption.

Effect of photocatalyst dose: Figure 8 shows the results of research on the effect of photocatalyst (Co-MCM-41) on ARS degradation. From 2 mg to 12 mg of catalyst were added to 100 mL of 2.92 × 10^–5 M ARS solution. The findings indicated that increasing the catalyst dose increased the degradation efficiency, and that increasing the catalyst dose over 10 mg had no effect on the degradation. Because there are more active sites on the surface of the catalyst as catalyst concentration increases, the rate of dye adsorption also increases. High catalyst doses, however, cause opacity in aqueous solutions and limit visible light penetration while slowing the rate of breakdown. Therefore, a catalyst dose of 10 mg/100 mL of ARS solution was determined to be optimal for the tests.

Figure 8.

Effect of catalyst load on degradation of ARS with Co-MCM-41 (10 mg). (squares) 15 min; (pentagon) 30 min; (left in triangles) 45 min; (right in triangles) 60 min; (stars) 75 min; (diamonds) 90 min.

Effect of dye concentration: By adjusting the dye’s concentration from 1.16 × 10^–5 M to 4 × 10^–5 M, the impact of initial dye concentration on the photocatalytic degradation of ARS was calculated. According to Figure 9, the percentage degradation efficiency was high at 2.92 × 10^–5 M concentration and subsequently declined as concentrations increased. The dye’s rate of adsorption on the active sites of the photocatalyst increases with increasing dye concentration while concurrently decreasing the dye’s adsorption inclination. Additionally, excessive dye concentrations reduce the number of photons and their adsorption on the catalyst’s surface, which ultimately reduces the degradation efficiency [33].

Figure 9.

Effect of dye concentration on degradation of ARS with Co-MCM-41. Catalyst load = 10 mg/100 mL of ARS. (a = 2.92 × 10⁻⁵ M, b = 2.33 × 10⁻⁵ M, c = 1.75 × 10⁻⁵ M, d = 3.5 × 10⁻⁵ M, e = 1.16 × 10⁻⁵ M, f = 4 × 10⁻⁵ M).

Effect of pH: At different pH levels, the effluents discharged by the textile industry include hazardous colours. As a result, the effect of the dye medium’s pH on its degradation was assessed by altering pH values between 2.0 and 12.0 for a constant dye concentration (2.92 × 10^–5 M) and catalyst load (10 mg). The breakdown efficiency of ARS increased at pH 4.0 and peaked at pH 6.0, as shown in Figure 10. The effectiveness of the degradation has diminished when pH levels have increased to 12.0. Since it controls the charge on the particle surface, a solution’s pH is a crucial variable in photocatalytic reactions. Therefore, at pH levels below 6.8, the negatively charged dye molecules and surface of Co-MCM-41 may interact chemically strongly and adsorb. At a pH of between 4.0 and 6.0, this situation may have caused ARS dye to breakdown. Repulsion between the negatively charged dye and the catalyst surface may account for the extremely low degradation rate at higher pH levels.

Figure 10.

Effect of pH on degradation of ARS with Co-MCM-41. [Dye] = 2.92 × 10⁻⁵ M, catalyst load = 10 mg/100 mL of ARS.

Role of scavengers in the photocatalysis of ARS: Studies on the photocatalytic degradation of ARS were conducted in both the absence and presence of scavengers. Figure 11 illustrates how the dye’s C/Co varied as a function of time (in minutes).

Figure 11.

Variation of C/Co against time (min) in the scavenger experiment. (squares): Benzoquinone; (circles): Isopropyl alcohol; (upward triangles): No scavengers.

Because it prevents the release of a significant amount of superoxide radicals (O2•), benzoquinone limits the degradation of dye (78.5%). The elimination of ARS (91.2%) from its aqueous solution, which primarily regulates the activity of the hydroxyl radicals (OH•), is also slightly constrained by the presence of iso-propyl alcohol [34]. In contrast, when these scavengers were absent, the degradation efficiency was over 98.8%. Superoxide radical and hydroxyl radical were therefore the two main active species in the redox process of the current investigation.

Plausible mechanism for photocatalytic degradation of ARS: The electrons in the photocatalyst’s valence band would excite into the conduction band and generate electron-hole pairs when the dye solution and photocatalyst were exposed to visible light. The degradation efficiency would be decreased as a result of these charged pairs recombining either directly or through the catalyst surface. A good photocatalyst with increased visible light absorption (reduced band gap, Eg) might transport the electron to oxygen, generating the active superoxide radical ion, and therefore limit the tendency to recombine. According to the UV-Vis DRS investigation, Cobalt added MCM-41 has a lowered bang gap of 2.7 eV, which would support the visible light driven photocatalytic activity. The electron can be excited from the valence to conduction band by the photocatalyst when it interacts with visible light and proceeds in the manner described above. In subsequent steps, the superoxide radical ions might produce hydroxyl radicals, which ultimately efficiently degraded the ARS dye (Figure 12).

Figure 12.

Photocatalytic degradation mechanism of ARS using Co-MCM-41.

Kinetic analysis: The kinetic study was performed for the degradation of ARS dye using Langmuir-Hinshelwood kinetic model [35].

$$r=\mathit{dC}/\mathit{dt}=\mathit{kKC}/\left(1+\mathit{KC}\right)$$E2

On neglecting the value of K_C in the denominator (K_C < <1) and integrating with respect to time t, the above equation accords to the pseudo-first order equation.

$$\mathit{ln}\frac{\mathit{Co}}{C}=\mathit{kKt}=k_{\left(\text{appt}\right)}$$E3

where Co denotes the starting concentration and C denotes the current concentration of the ARS solution. K is the adsorption coefficient of the ARS dye onto the photocatalyst, and t. k is the rate constant.

For the rate of degradation using MCM-41 and Co-MCM-41, respectively, the rate constants (k) were estimated as 1.526 × 10^–2 min⁻¹ (2.543 × 10^–4 s⁻¹) and 9.20 × 10^–2 min⁻¹ (15.33 × 10^–4 s⁻¹). Compared to the MCM-41, the rate constant has grown six times with the Co-MCM-41. It demonstrates how adding Cobalt to MCM-41 increases the rate of reaction.

3. Experimental

Materials: Cetyltrimethylammonium bromide [CH₃-(CH₂)₁₅-N(CH₃)₃Br, CTMAB], tetraethylorthosilicate [(C₂H₅O)₄Si, TEOS], Cobalt acetate [Co(CH₃COO)₂.2H₂O], Ethyl alcohol, were procured with AR grade quality (99% pure) from Sigma Aldrich and Merck. Alizarin Red S (Merck) with molecular formula, C₁₄H₇NaO₇S (M.Wt = 342.26 g.mol⁻¹, λ _max = 597 nm). Scavenger experiments were carried out using benzoquinone and iso-propyl alcohol. None of the compounds were further purified before usage. Utilising double distilled water, all solutions needed for the experimental work were created.

Synthesis of MCM-41: MCM-41 A straightforward room-temperature co-precipitation technique was used to create the material [26]. In a typical synthesis, a clear, homogenous solution was created by dissolving 2.40 g of the surfactant CTMAB in 50.00 mL of distilled water and stirring continuously. This homogenous solution received 76.00 mL of ethyl alcohol and 13.00 mL of 25 weight percent aqueous ammonia while being stirred. The aforementioned mixture received a dropwise addition of 10.00 mL of TEOS. The solution turned milky and a gel was formed due to the hydrolysis of TEOS. The resultant mixture was stirred for about 2 h to completely hydrolyze TEOS. White precipitate thus formed was centrifuged, washed consecutively with distilled water and methanol. The product was dried overnight at 110°C. Solid product thus obtained was calcined at 550°C in air atmosphere for 5 h to remove the trapped surfactant.

Synthesis of Cobalt incorporated MCM-41 (Co-MCM-41): The in-situ approach was used to create MCM-41 with metal incorporation. These were made using the same procedure as MCM-41, with the exception that the addition of the appropriate amounts (Si/M ion ratio of 100:1, M = metal) of the corresponding metal precursors was made 20 min after the addition of TEOS. Using cobalt acetate as a precursor material, cobalt (Co) integrated MCM-41 materials were created.

Characterisation: Step scanning on the Rigaku D/MAX-2500 diffractometer (Rigaku Co., Japan) with Cu-K -radiation (k = 0.15406 nm), operated at 40 kV and 100 mA, was used to analyse the produced MCM-41 and its Cobalt doped derivative. A scanning electron microscope model Philips XL 30 ESEM was used to capture SEM images of the materials (FEI-Philips Company, Hillsboro). The surface areas of the catalysts were measured using a Quantachrome Nova 2000e surface area and pore size analyser by nitrogen adsorption-desorption in a liquid nitrogen atmosphere (77 K). With the help of the Single Monochromator UV-2600 (optional ISR-2600Plus, up to 1400 nm), UV-Vis diffuse reflectance spectra were captured.

Photocatalytic measurements: The efficiency of the photocatalysts in the photodegradation of ARS was examined using a UV-Vis Spectrophotometer (UV-2550, Shimadzu, wavelength range: 180–1100 nm). With a known weight (10 mg) of the photocatalyst per 100 mL of a variety of diluted ARS solutions, adsorption-desorption equilibrium tests were carried out. The chosen equilibration period was 15 min. The solutions were exposed to a 300 watt tungsten halogen lamp in a photocatalytic chamber with an electrical supply to conduct the photocatalytic study. A UV-Vis spectrophotometer was used to evaluate the translucent solution after centrifuging 5 mL aliquots at regular intervals of 5 min. Eq. (4) was used to get the percentage of dye degradation.

$$\text{Photocatalytic degradation}\%=\left(\mathrm{Co}-\mathrm{Ct}\right)/\mathrm{Co}\times 100$$E4

where C₀ and C_t are the initial concentration and concentration of the dilute ARS solution at a time interval, t respectively.

Scavenger experiment: The purpose of the in-situ scavenger experiment was to look for active species produced by the efficient photocatalyst during the photocatalytic breakdown of ARS. Scavengers were added to the photocatalytic process to capture holes, or superoxide radicals (O₂•) and hydroxyl radicals (OH•), such as benzoquinone (1 mmol/L) and iso-propyl alcohol (0.1 mmol/L).

4. Applications

There are many benefits to using this Co-MCM-41 heterogeneous catalyst in sophisticated oxidation processes [36]. Co-MCM-41 mesoporous materials have been synthesised, and their high specific surface area, well-ordered mesoporous structure, large surface area, and flexible framework make it easy to incorporate metal active sites in silica materials [37, 38]. These materials also make good dispersions for reaction and product molecule molecules. The host material’s adsorption characteristics are crucial for the purification of biomolecules. According to a publication, MCM-41 with Co inserted is a strong contender for use as a high surface area adsorbent for biological compounds like amino acids [39]. This has undergone testing as a method of regulated drug release for medications like ibuprofen [40]. These heterogeneous catalyst Co-MCM-41 based mesoporous materials have successfully immobilised or absorbed a number of proteins, including cytochrome C [41], lysozym [42], and trypsin [43], and their activities have been investigated [44]. Well-ordered mesoporous silica nanoparticles have recently been demonstrated to be useful as cell markers by Lin et al. [45].

5. Conclusions

The derivative of MCM-41 with cobalt was created and described. Under ideal circumstances, the materials have produced effective results in the photodegradation of the anthroquinone dye Alizarin Red S. The fact that Co-MCM-41 degrades more effectively than MCM-41 is proof of the metal’s influence on the microporous material’s framework when serving as a photocatalyst.

Acknowledgments

The author is thankful to Sophisticated analytical instrumentation facility (SAIF), India and National Institute of Technology, Warangal (NIT-W), India for providing the characterisation results. Also thankful to the Management of NS Raju Institute of Technology, Visakhapatnam, India for providing the funding and laboratory facilities.