Photocatalytic Degradation of Methylene Blue (MB) and Methyl Orange (MO) by the Highly Oxidative Properties of SnO₂–Sb₂O₃ Particles

Abstract

SnO₂–Sb₂O₃ particles, with various Sb₂O₃ mole fraction (x) ranging from 0 to 1, have been synthesized by a sol-gel method. Tin oxalate and antimony chloride were used as the metal precursors, water as the solvent, citric acid as the pH modifier, and triethanolamine as a stabilizing agent. The effect of the chemical composition of the synthesized catalyst on the photocatalytic degradation of methylene blue (MB) and methyl orange (MO) was evaluated. It was established that a SnO₂–Sb₂O₃ (Sb₂O₃ mole fraction of x = 0.15) catalyst presents a highly oxidative power to degrade a 20 µM MB solution until 98.60%, using a UV lamp of 254 nm and 6 W. The kinetics of the SnO₂–Sb₂O₃ (x = 0.15) photocatalytic oxidation was monitored as a function of the initial concentration of MO (100, 200, 300, and 400 µM) at different reaction times. It was found that the reaction probably presents a pseudo-first-order kinetic model. The effect of the intensity of the power lamp was studied, finding an increment from 16.32 to 98.42%, for UV lamps of 6 W and 72 and 1200 mW cm⁻², respectively. Finally, the effect of temperature (298, 318, 328 and 338 K) was analyzed, and the activation energy from Ea = 7.2 kcal mol⁻¹ was determined.

1. Introduction

Industrial wastewater often contains several organic pollutants. Colorants are the most common, since they are often used in cosmetics, pharmaceuticals, textile, foods, etc.^1–3) It is estimated that the global annual production of colorants is 70,000 tons, allowing access to approximately 100,000 commercialized colorants, of which about 10 to 15% are released into wastewater only in textile processes.⁴⁾ Also, the increasing development of the textile industry produces wastewater with dyes, organic pollutants which are non-biodegradable and persistent and affect the ecological balance.⁵⁾ These pollutants usually absorb a significant amount of sunlight, thus reducing the development of aquatic species and decreasing dissolved oxygen in the water, further limiting aquatic species.⁶⁾

Since most of the colorants contain aromatic rings in their chemical structures, they are hardly biodegradable and even carcinogenic. Therefore, it is important that the wastewater that carries them is correctly treated.⁷⁾ The colorants can be classified into three large groups: anionic, with a negative charge given mainly by a sulfate group; cationic, whose positive charge is due to a protonated amino group; and non-ionic that depends on the dissociation behavior in aqueous solutions. Azo-compound dyes can be both anionic and cationic, they are usually stabilized with N=N bonds which are stable to light, heat and aerobic digestion.^8,9) Conventional wastewater treatment methods, such as adsorption, ion exchange, coagulation and flocculation, cannot remove a large part of the organic pollutants. The associated separation operations generate solid waste with high toxicity.^10,11)

An alternative approach to improve wastewater treatment quality is to use advanced oxidation processes (AOPs). These processes are unit operations that deal with pollutants that are difficult to degrade. In these operations, oxidizing agents, such as hydrogen peroxide (H₂O₂), are produced to oxidize organic molecules, which partially decompose into reactive oxygen species which break the pollutant molecule.¹²⁾ The most common oxidizing agent produced in AOPs is the hydroxyl radical (·OH). This chemical species appears typically in processes such as; ozonation, Fenton, electrooxidation, photocatalysis, among others, and is very reactive, unstable, and highly oxidizing.¹³⁾ For this reason, organic molecules that are stable, such as the aforementioned dyes, can be degraded.

In this study, a new and highly effective catalyst for a heterogeneous photocatalysis process is proposed. Heterogeneous photocatalysis is a common method for oxidizing organic pollutants in water, whereby a catalyst adsorbs the undesirable compounds on its surface, increasing degradation by photocatalysis.¹⁴⁾ This is a low-cost, continuous operation and is efficient in degrading different organic compounds in water. A semiconductor is used as a catalyst to release electrons into the water. These electrons oxidize organic pollutants^15,16) by producing free hydroxyl radicals (·OH) at higher concentration and faster by using UV light.¹⁷⁾

Photocatalyst materials have been used for environmental treatment^18,19) and, over recent decades, several inorganic semiconductors have been studied due to their high oxidative performance without requiring chemicals or high temperatures.²⁰⁾ They have been used in photovoltaic and photodegradation of organic pollutants in air and water.²¹⁾ Some commonly proposed materials are ZnO,²²⁾ TiO₂²³⁾ or CeO₂²⁴⁾ nanoparticles, due to being non-toxic and biocompatible materials. Another alternative is the use of nanotubes, or magnetic nanocomposites, to get a high superficial area without dealing with nanoparticle remotion afterwards.^25,26)

The use heterogeneous photocatalysis to degrade organic pollutants in water is mentioned in many papers. The most common model molecule used as an organic pollutant is methylene blue (MB).^27–29) This molecule is widely used in the textile, leather and pharmaceutical industries. It is a biochemical dye whose exposure can produce nausea, dizziness, vomiting, mutations in prolonged contact, and even cancer.³⁰⁾ Heterogeneous photocatalysis results for the oxidation of MB in the last 4 years are shown in Table 1. It presents the maximum elimination of a certain dye concentration during a specific time, and an exact UV irradiation power. It is observed that those experiments use concentrations much lower than 100 µM, and require long exposure times to achieve degradations over 90% This reduces the viability of continuous flow reactors, an important trend for industrial wastewater treatment.

Table 1 Examples of photo catalysts used in the degradation of model molecule; MB.

To increase the efficiency of the process, the power of the UV lamp used to promote the photocatalytic reactions is often increased, increasing the energy costs of operation. Therefore, the search for better ceramic materials which could be used as catalysts continues, to increase the efficiency of the degradation process, as well as diminishing the time required for treatment.

The present work analyzes the use of the ceramic system of SnO₂–Sb₂O₃. This is a ceramic semiconductor used mainly in conductivity studies,³¹⁾ and is mixed with other materials in the synthesis of electrodes for electrooxidation of organic pollutants,^32,33) photovoltaic cells,³⁴⁾ etc. SnO₂–Sb₂O₃ presents great potential as a semiconductor because antimony (Sb) and tin (Sn) have valence numbers +4 and +3, respectively. This material presents similar properties to nano-ceria, which has Ce⁺³/Ce⁺⁴ species; these have demonstrated self-oxidizing and the production of peroxides. These peroxides can oxidize ceria from +3 to +4, allowing the regeneration of the compound.³⁵⁾ It is also known that SnO₂–Sb₂O₃ has a very good performance transporting electrons,³⁴⁾ so in water, this would help to generate more hydroxyl radicals (·OH) for oxidation.

To date, no study explores the photocatalytic efficiency of SnO₂–Sb₂O₃ particles as a function of chemical composition. Also, there is no information about the kinetic model which controls the process.

The present study presents the synthesis of SnO₂–Sb₂O₃ from tin oxide and antimony oxide, MB and MO degradation by heterogeneous photocatalysis and the kinetic model.

2. Experimental Procedure

The experimentation is divided into two steps; 1) synthesis of the tin and antimony oxide by sol-gel which is divided into two operations; obtaining the tin and antimony precursors, and subsequently obtaining the sol precursor of SnO₂–Sb₂O₃ ceramic, and 2) heterogeneous photocatalysis of organic compounds methylene blue (MB) and methyl orange (MO) by means of a photocatalytic process.

2.1 Synthesis of SnO₂–Sb₂O₃ ceramic system obtained by sol-gel method

Tin chloride (SnCl₂) causes the chloride anion to generate random n-type doping in the tin oxide preparation, diminishing the photocatalytic efficiency. On the other hand, a practical method to improve the electron generation of the SnO₂ surface is to add antimony, which generates the best impurities with the lowest electrical resistance.³⁶⁾

To minimize the presence of chlorine ions, tin oxalate (SnC₂O₄) was prepared as an Sn precursor, starting off common tin chloride (SIGMA ALDRICH, 98%), and oxalic acid (H₂C₂O₄, SIGMA ALDRICH, 99.5%). First, 0.252 M tin chloride and 0.264 M oxalic acid solutions were prepared separately in distilled water and stirred for 1 h to homogenize. Subsequently, the oxalic acid solution was added dropwise to the thin chloride solution, and both solutions were stirred for 15 min. A white precipitate, tin oxalate, was obtained. Subsequently, the precipitate was washed 10 times with hot distilled water to remove excess chloride, rinsed 3 times with ethanol (CH₃CH₂OH, 99%) to displace the remanent water, and finally dried at 100°C for 24 h and pulverized. The overall reaction is:   

\begin{equation} \text{SnCl}_{2} + \text{H$_{2}$C$_{2}$O$_{4}$} \to \text{SnC$_{2}$O$_{4}$} + 2\text{HCl} \end{equation}

To obtain the Sb sol, antimony chloride III (SbCl₃, SIGMA ALDRICH, 99%) was dissolved in distilled water for 1 h and, after evaporation, a white precipitate was obtained. This was washed 10 times with distilled water and 3 times with ethanol to eliminate all chloride residues.

2.2 Sol-gel SnO₂–Sb₂O₃ particles synthesis

10 mL of tin oxalate solution at 0.5 M was prepared using distilled water and stirred for 1 h. A citric acid (C₆H₈O₇, MEYER, 99.5) solution was prepared at 1.16 M, 10 mL of which was added to the oxalate solution and stirred for 12 h, with a final tin oxalate/citric acid molar relation of 0.43. Later, 6 mL of triethanolamine (TEA, N(CH₂CH₂OH)₃, 99.4%) was added dropwise to the tin sol (kept under stirring) until a clear solution was obtained. Finally, 5 mL of ethanol were added to form tin precursor in the sol. To obtain different Sb₂O₃ mole fractions x = 0 to 1 (where 0 represents pure SnO₂ and 1 represents pure Sb₂O₃) in the SnO₂–Sb₂O₃ ceramic, the corresponding amount of antimony oxide was added to the tin sol and stirred vigorously for 24 h. Subsequently, the sol was dried for 24 h at 100°C, and the xerogel formed was crushed. Finally, the ceramic powders obtained were heat-treated at 600°C for 2 h.

2.3 Photocatalysis studies. Methylene-blue (MB) degradation

The photocatalytic activity of the ceramics samples was determined using the recommendations of the standard ISO 10678:2010 “Determination of the photocatalytic activity of surfaces in an aqueous medium by degradation of methylene blue”.³⁷⁾ First, a 20 µM MB (C₁₆H₁₈ClN₃S, FERMONT) solution was prepared in distilled water. Later, 20 mL volume of MB solution was placed in a flask, and the photocatalyst was added at a fixed concentration of 25 mg mL⁻¹. The setup was placed in a dark box, to avoid sun-light exposure, and it was stirred for 30 min in complete darkness to avoid the measurement of adsorption between powder and dye. After this, the photocatalytic process was started on the same sample, stirring vigorously for 10 min under a 365 nm UV lamp (Analytikjena, UVL-56) with power of 6 W and light intensity of 12,000 mW cm⁻² at a fixed 10 cm distance.

The degradation efficiency of the MB with the photocatalyst was determined by comparing the initial and final absorbances of the sample at 656 nm. The UV spectra measurement was carried out in an UV-vis spectrometer (JEANWAY model 6300) collecting data from 400 to 700 nm. To analyze the effect of the Sb₂O₃ mole fraction (x) on the photocatalytic activity, the degradation tests were carried out at 30 min and at a higher MB concentration of 200 µM. All measurements were carried with 3 independent samples, to diminish the error via the natural variability of the experiments.

2.4 Photocatalysis studies. Methyl-orange (MO) degradation

Once the sample with the chemical composition that maximizes degradation was determined, we proceeded to analyze the effect of dye concentration and temperature on the degradation, and thus determine the kinetic model that applies to the system. The effect of the power-output of the lamp was analyzed, using the 365 nm UV lamp (Analytikjena, UVL-56) with power of 6 W and light intensity of 12,000 mW cm⁻², and another 365 nm UV lamp (LANCETA HG, 105002) with 72 mW cm⁻² of light intensity. Once the MB degradation results had been analyzed, the dye was changed to MO, a dye whose chromophore is an azo bond, which links two aromatic rings. It has proven to be an exceptionally stable molecule,³⁸⁾ more difficult to degrade than MB,^6,29) and is commonly used in textile, pharmaceutical, food industries, etc.^39,40)

The set-up experiment was like the previous study. To analyze the effect of concentration, the experiments were carried out with 20 mL of aqueous MO solution at 100, 200, 300 and 400 µM (C₁₄H₁₄N₃NaO₃S, Fermont) and the kinetics were monitored from 0 to 120 min. The UV spectra were monitored from 200 to 600 nm, and the degradation was calculated at 456 nm. To analyze the effect of temperature on the reaction kinetics, the system was maintained at fixed temperatures of 298, 318, 328 and 338 K. Other parameters, catalyst concentration (25 mg mL⁻¹) and lamp distance (10 cm) remained constant. All experiments were carried out in triplicate.

2.5 Structural and morphological studies

Ceramic powders were characterized microstructurally by X-ray diffraction (XRD) with a BRUKER diffractometer (model ECO 08 ADVANCE) with copper tube at λ = 1.54 Å using a high-speed SSD160 detector. Chemical assays, to quantify precisely the Sn and Sb content, were carried out by Atomic Absorption Spectroscopy (AAS) in a Perkin Elmer Aanalyst 200 spectrometer. The samples were analyzed 3 times, and the uncertainty of the measurement was calculated. Finally, the morphological characterization was performed in a scanning electron microscope (SEM) with 15 keV and tilt angle of 0°.

3. Results and Discussion

3.1 Methylene blue degradation: effect of the chemical composition of the catalyst

Figure 1 shows the UV-vis spectrum for photocatalytic degradation of MB SnO₂–Sb₂O₃ Sb₂O₃ mole fraction of x = 0.18, using the conditions recommended by ISO 10678:2010 of MB 20 µM, where 25 mg mL⁻¹ of powder were placed in 20 mL for 10 min under UV exposure. As can be seen, the degradation was 98.20%, when analyzing band at 664 nm, corresponding to MB. Degradation is calculated using the following equation:   

\begin{equation} \mathit{Efficiency} = \left(1 - \frac{\mathit{Abs}_{t0} - \mathit{Abs}_{ft}}{\mathit{Abs}_{t0}} \right) \times 100 \end{equation}

Where Abs_t0 is the absorbance before UV exposure (also 30 min stirred in complete darkness to eliminate errors of absorbance between powder and dye), and Abs_ft is the absorbance after UV exposure.

Fig. 1

MB degradation after 10 min of reaction. SnO₂–Sb₂O₃ (Sb₂O₃ mole fraction x = 0.18).

The MB degradation was very fast and occurs in the first 10 minutes. The high oxidative power of the photocatalyst, compared with several recent studies, is apparent in Table 1. Therefore, the subsequent experiments were carried out at a concentration of 200 µM, ten-fold greater than the established standard and much higher than that reported in other studies. The use of this concentration allowed the oxidative power to be accurately determined. Samples of 200 µM MB were reacted with 15 different compositions of SnO₂–Sb₂O₃ ceramics powder (Fig. 2) for 30 min and the final degradation was measured. The MB degradation reached a maximum of 99.71% after 30 min for the SnO₂–Sb₂O₃ (x = 0.15) sample. This composition could lead to the maximum production of hydroxyl radicals (·OH) due to the vacancies promoted by Sb₂O₃. Therefore, as the amount of Sb₂O₃ increases in the system, vacancies increase until a maximum at x = 0.15, after that point, as there is an excess of Sb₂O₃, vacancies are reduced as the system is now mostly Sb₂O₃.

Fig. 2

(a) MB degradation as function of Sb₂O₃ mole fraction (x), (b) 30 min degradation kinetics of MB.

A similar increase of vacancies as a function of composition has been proposed for CeO₂–Y₂O₃⁴¹⁾ and ZrO₂Y₂O₃.⁴²⁾ Figure 2(b) presents the reaction kinetics of SnO₂–Sb₂O₃ x = 0.15 sample. As can be observed, the reaction rate is very high, since in just 5 min it is possible to achieve almost 90% of the reduction of the 200 µM methylene blue solution, and finally achieve another 10% degradation from 5 to 30 min, with only a 6 W commercial lamp. Therefore, the composition SnO₂–Sb₂O₃ (x = 0.15) was selected to degrade MO.

3.2 Structural studies: XRD

X-ray diffraction spectra of SnO₂–Sb₂O₃ (x = 0, 0.13, 0.15, 0.18 and 1.0) are shown in Fig. 3. For those samples containing more SnO₂, their XRD spectra match with the tetragonal phase of the tin oxide (ICSD 90609, cassiterite lattice parameters: a = 4.738 Å, c = 3.118 Å). On the other hand, the sample of Sb₂O₃ matches with the ICSD 036145 for antimony oxide in cubic structure and lattice parameters of a = 11.14 Å. Samples with Sb₂O₃ mole fraction x = 0.02, 0.04, 0.06, 0.08, 0.1, 0.12 (not showed) and until x = 0.13 shows no evidence of the Sb₂O₃, suggesting that a fraction of Sb could be dissolved in the cassiterite structure. The x = 0.15, in addition to the crystalline structure associated with SnO₂, present small diffraction peaks around 2teta = 30°, probably from the presence of Sb₂O₄. Later, for the x = 0.18 sample, a different compound is formed, the Sn_0.9Sb_0.1O₂ compound (ICSD 155959, tetragonal structure). This suggests a partial solid dissolution between Sb₂O₃ and SnO₂. Similar findings were reported by Haineng Bai⁴³⁾ and Xiupei Yang³³⁾ where Sb₂O₃ was dissolved at 10% into the SnO₂ matrix. Both tin oxide and antimony oxide fulfill the two Hume-Rothery rules for a total solid solution; similar atomic radius and electronegativity. Since SnO₂ and Sb₂O₃ have different valence and crystalline structure, they form a partial solid solution, i.e., Sb₂O₃ is partially diluted in SnO₂. The partial solid solution can generate vacancies due to the different crystalline structures and the different values of valence, +4 and +3 for SnO₂ and Sb₂O₃, respectively. This could help transport electrons from the ceramic to the water, allowing a greater generation of hydroxyl radicals (·OH) that allows fast oxidation.

Fig. 3

XRD spectrum, showing compositions of SnO₂, SnO₂–Sb₂O₃ Sb₂O₃ mole fraction x = 0.13, 0.15 and 0.18; and Sb₂O₃.

On the other hand, when Sb₂O₃ is predominant, there is a greater amount of valence +3, and there are not enough vacancies to allow the transport of electrons from the system to the water, reducing the number of hydroxyl radicals (·OH) and consequently reducing the degradation of the colorant.

A similar effect has been reported for the Sb₂O₃–PbO system, where an increment of the Sb₂O₃ phase increases the photocatalytic activity.⁴⁴⁾ An excess of Sb ions on the structure could lead to the formation of recombination centers, thereby reducing photocatalytic performance.⁴⁵⁾ It also can be observed that the Sb₂O₃ presence makes the peaks of the SnO₂ displace to higher 2θ angles, probably due to an excess of Sb, which will promote a deformation of the SnO₂ crystal lattice, which also allows us to assume the formation of the solid solution. To confirm the chemical composition of the samples, a quantitative analysis has been carried out for the analyzed samples. The results in weight % are (the theoretical content is listed in parenthesis): For the x = 0.13 sample, Sn% = 60.5 ± 0.12 and Sb% = 19.05 ± 0.17 (Sn% = 61.10 and Sb% = 18.73); for the x = 0.15 sample, Sn% = 59.10 ± 0.25 and Sb% = 20.15 ± 0.55 (Sn% = 21.20 and Sb% = 58.72); and, for the x = 0.18 sample, Sn% = 54.26 ± 1.12 and Sb% = 23.89 ± 0.55 (Sn% = 55.29 and Sb% = 24.90). As can be observed, the experimental results are in good agreement with the theoretical ones.

3.3 Morphological studies: SEM

Figure 4(a) shows the morphology of the x = 0.15 powders at a magnification of 3000 X; powders mainly consist of irregular form. The particle size distribution was obtained by measuring 150 particles from several micrographs. The particle size ranges from 0.16 to 221.32 µm. Larger particles present a smooth surface, in contrast to the smaller ones. It is important to notice that the smaller particles present some porosity, which could enhance photocatalytic efficiency. Figures 4(b), 4(c) and 4(d) present the elemental Sb, Sn and O distribution of the sample. As can be observed, the elemental distribution is almost homogenous, with no clear indication of Sn or Sb segregation. Since the homogeneous distribution is observed, the idea is reinforced that, at least in part, the SnO₂ phase dissolves Sb. Only two areas slightly enriched in Sn are observed, but none enriched in Sb.

Fig. 4

(a) SEM micrograph of SnO₂–Sb₂O₃ (x = 0.15) particles at 3000 X. (b) Sb distribution, (c) Sn distribution, (d) O Distribution, (e) Weibull particle size distribution of SnO₂–Sb₂O₃ Sb₂O₃ mole fraction x = 0.13, 0.15 and 0.18.

Figure 4(e) shows the particle size distribution for SnO₂–Sb₂O₃ (x = 0.18, 0.15 and 0.13) samples that presented the highest photocatalytic capacity. To determine if a significant difference in particle size exists, the average particle size was calculated, assuming a typical Gaussian distribution, finding values of 19.3, 14.5, and 13.0 µm for SnO₂–Sb₂O₃ (x = 0.18, 0.15 and 0.13) samples, respectively. It is observed that the average size is similar in the three cases, so there is no evidence of a significant change in this property by increasing the Sb content. However, because, as observed in Fig. 4(e), the distribution cannot be considered Gaussian, but the mode is found in the lower sizes, it was decided to consider a Weibull-type distribution. This is used to describe the size distribution of a particle population formed by fragmentation,⁴⁶⁾ which is frequently the case of sol-gel processes, where the xerogel fragmentation stage exists, and which explains why there are very large particles in the sample.

As can be seen in Fig. 4(e), the distribution is similar for the three samples (the line in the histogram). The values of the scale parameter (scale Alpha) and the shape factor (shape beta) were calculated. The first determines the size for 63.2% of the particles, this being 15,414, 12,820 and 9,292 µm respectively, for the aforementioned samples which are also of the same order of magnitude. The second determines that the distribution effectively shifts towards smaller sizes, where values less than 1 indicate that the distribution is asymmetric to the left. This is the case in the present study, with values of 0.721, 0.807 and 0.663, respectively, for the aforementioned samples, which shows that, although there are some large particles, the vast majority will be in the range of up to 20 µm.

3.4 Methyl orange degradation

Since the SnO₂–Sb₂O₃ (x = 0.15) sample achieved the highest efficiency in MB degradation, experiments using MO were performed for comparison (Table 2).

Table 2 Comparison of the efficiency of various systems in the degradation of MO.

From Table 2, it is observed that the ZnO addition seems to have the highest efficiency; however, high power consumption UV lamp, low concentration of MO or long-time reactions were employed.

3.4.1 Effect of the concentration of methyl orange

In a similar way to the MB, a concentration of 25 mg mL-1 of the SnO₂–Sb₂O₃ (x = 0.15) sample, was used in 20 mL of MO, using the same lamp power of 6 W. However, reactions were carried out with concentrations of MO of 100, 200, 300 and 400 µM, with 120 min time reaction. Since there is no standard that establishes degradation parameters for MO, the aforementioned concentrations were chosen because the system proved to be very fast to degrade MB, even at 200 µM in 30 min. Consequently, ensuring several points of different kinetics, required high concentrations.

Figure 5 shows UV spectra for the MO degradation kinetics of a 100 µM solution at selected times. Average absorbance values indicate a loss of azo bond (N=N) of 97.16% in a reaction of 90 minutes.

Fig. 5

Degradation UV-Spectra of methyl orange with a concentration of 100 µM as function of time. Only selected times are showed.

The band located around 250–300 nm increases as the reaction increases, suggesting that MO decomposes into other subproducts, probably aromatic. Additionally, this band shows a hypsochromic shift which might be related to the azo bond break. This would leave two amino groups (auxochromes) generating a hypsochromic effect and intensity increase. That is, the MO molecule would first break at the azo bond, generating two aromatic molecules, one with two amino groups in the para position, and the other with one amino group and one sulfate in the para position. The presence of these aromatics increases at 110 minutes of reaction, however, at 120 minutes it seems to decrease to 4.26%, which indicates how much it can begin to stabilize and/or start the aromatics oxidation process.

Figure 6(a) shows the kinetics for MO degradation as a function of initial concentration. It is observed that, even with a high dye concentration of 200 µM, the MO decreases, giving a loss of colorant of 92.83%, less degradation due to the fact that the MO has twice the concentration. Despite that, the dye is decomposed efficiently with 300 µM concentration. It can be seen that MO is degraded up to 89.25%, and finally, even at a concentration of 400 µM, the dye was decomposed up to 89.04%, a very high degradation efficiency, specially considering such a high MO concentration.

Fig. 6

(a) Kinetic curves of methyl orange degradation with 100, 200, 300 and 400 µM concentration after 120 min reaction. (b) Linear adjustment pseudo-first order.

3.4.2 Kinetic modelling

The kinetic modeling for the degradation efficiency of MO at different concentrations with the ceramic sample of SnO₂–Sb₂O₃ (x = 0.15) was performed, the results are shown in Fig. 6(b). A pseudo-first-order kinetic and a second-order kinetic model were tested, since both models have been used to describe the photodegradation kinetics of different organic compounds and using several different ceramic particles, e.g., TiO₂,^47,48) ZnO,^49,50) Fe₃O₄⁵¹⁾ and SnO₂.⁵²⁾ The pseudo-first-order kinetic model is presented in (eq. (1)):   

\begin{equation} \ln C = -k_{\textit{app}}t + \ln C_{0} \end{equation}

The second-order kinetic describing the photocatalytic process for MO is presented in (eq. (2)):   

\begin{equation} \frac{1}{C} = kt + \frac{1}{C_{0}} \end{equation}

Where C and C₀ are the reaction concentration and the initial concentration of MO, respectively, t (min) the reaction time, k_app (min⁻¹) is the pseudo-first-order kinetic constant, and k (%⁻¹ min⁻¹) is the second-order constant. For both cases, the k values were obtained by applying least-squares regression analysis, Table 3.

Table 3 Parameters obtained for the of the SnO₂–Sb₂O₃ (x = 0.15) sample photocatalytic activity with MO at different concentrations, fitting to a pseudo-first and second kinetic model.

Experimental data obtained from the correlation achieved a better fit with the pseudo-first-order kinetic model. Also, as expected, the k_app values decrease as a function of MO concentration. The explanation of such behaviour is because, as the initial concentration of MO was increased, the dosage of generated hydroxyl radical (·OH) was not correspondingly increased, since the same dosage of photocatalyst was added to the system resulting in a lower hydroxyl radical (·OH/MO) ratio. Similar results have been observed for MB degradation with dumbbell-shaped ZnO photocatalyst.⁵³⁾

3.4.3 Effect of the intensity power of the lamp

The photocatalytic efficiency of the SnO₂–Sb₂O₃ (x = 0.15) sample strongly depends on the intensity of the lamp used for irradiation; it has a direct involvement on the degradation of the organic compounds.⁵⁴⁾ This is because an increase in light intensity increases the number of photons that interact with the catalyst, which then produces more hydroxyl radicals (·OH) to the solution. The greater the number of photons interacting with the photocatalyst, the greater the number of dye molecules oxidizing in the solution.

A high light intensity seems to provide more photons to the reaction, this might increase the performance of the catalyst and, in the end, the oxidation.⁵⁵⁾ Then, using different UV light intensities, the degradation changes. If UV intensity is low, degradation will fall,⁵⁶⁾ because light intensity directly influences the photocatalytic reaction.⁵⁷⁾ Since high degradation was obtained using the 1200 µW cm⁻² lamp, a lower intensity lamp was employed to examine the temperature effect. The difference in degradation is observed in detail using 100 µM of MO concentration. In this case, Fig. 7 displays a reduction to 16.12% in degradation efficiency using a UV lamp with an intensity of 72 mW cm⁻², compared with the 97.16% efficiency obtained using the 1200 µW cm⁻² lamp.

Fig. 7

Degradation efficiency with two 6 W power lamps, but different light intensity, 1200 mW cm⁻² and the other of 72 mW cm⁻².

3.4.4 Effect of temperature: activation energy

To analyze the effect of temperature on the rate of the reaction, the photocatalytic activity was analyzed for the degradation efficiency of MO at 100 µM, with the ceramic sample of SnO₂–Sb₂O₃ (x = 0.15) as function of the temperature, Fig. 8(a). Table 4 shows the values ok k_app and k, for the pseudo-first-order and second order kinetics of this experiment. The fit is better for the former, and the rate constant k_app increases as 1.41 × 10⁻³ min⁻¹ (T = 298 K) < 3.37 × 10⁻³ min⁻¹ (T = 318 K) < 4.21 × 10⁻³ min⁻¹ (T = 328 K) < 6.16 × 10⁻³ min⁻¹ (T = 338 K). The rate of the reaction increases 4.3 times (436%) from 298 to 338 K (Fig. 8(b)).

Fig. 8

(a) Kinetic curves of MO degradation with temperatures 298, 318, 328 and 338 K after 120 min of reaction. (b) Linear adjustment to n = 1.

Table 4 Parameters obtained for the photocatalytic activity fitting to a pseudo-first and second kinetic models, as function of temperature using different temperatures.

For reactions where the rate is high, it is possible to calculate the apparent activation energy through the Arrhenius equation:   

\begin{equation} \ln k_{\textit{app}} = -\frac{E_{a}}{RT} + \ln A \end{equation}

Where k_app is the rate constant of pseudo-first-order, R is the ideal gas constant, T is the absolute temperature of the reaction, E_a is the apparent activation energy and A is the exponential factor. The graph of the T⁻¹ vs. ln k_app allows the activation energy to be calculated (Fig. 9), being the calculated value for the present investigation of E_a = 7.2 kcal mol⁻¹ (30.3 KJ mol⁻¹). This establishes that the reaction is slightly influenced by the transport of MO to the surface of the ceramic particles and its subsequent reaction. The calculated value is in the range of the other photocatalytic process for MO, 12.1 kJ mol⁻¹ (2.89 kcal mol⁻¹) for 5Cu/ZnO particles⁶⁾ or 6.69 kcal mol⁻¹ (28.01 kJ mol⁻¹) for FeZ (Fe nanoparticles supported on zeolite).⁵⁸⁾

Fig. 9

Arrhenius Plot.

4. Conclusions

The sol-gel method is a simple process for obtaining semiconductor ceramics of SnO₂–Sb₂O₃. Obtaining powders of different compositions allowed us to determine that the mixture with SnO₂–Sb₂O₃ (x = 0.15 mole fraction) presents the best behavior for the degradation of organic compounds. Also, it has been stated that this composition is capable of degrade stable molecules such as MO with high concentrations, in relatively short time reaction, with a high possibility of oxidizing even aromatic compounds. Structural results shows that probably a partial dissolution of Sb atoms is possible on the SnO₂ structure until at least x = 0.18, and, since the valence difference of both metals, the formation of great number of vacancies can be expected, which later participate in the formation of (·OH), that allows fast oxidation of organic compounds.

Acknowledgments

The authors acknowledge to SNI-CONACYT and SIP-IPN (20221369 & 20220462) for supporting this work. The authors are also indebted to Laboratory CREA-CIITEC-IPN.

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