Photodegradation of Pyridine in a Fluidized Bed Photocatalytic Reactor Using Pt-ZnO Supported on Al₂O₃ as a Catalyst

Abstract

Pyridine is a recalcitrant organic compound present in industrial wastewater that causes severe effects on the environment and the health of living beings, as it is considered a toxic, mutagenic, teratogenic, and carcinogenic agent. Therefore, this research explored the efficacy of a zinc oxide catalyst, doped with platinum nanoparticles and supported alumina through the precipitation method, for the photocatalytic degradation of pyridine using a fluidized bed reactor. A Box–Behnken experimental design was used to analyze the effect of the pH (4–10), the pyridine concentration (20–300 ppm), and the amount of catalyst (20–100 g). The X-ray diffraction (XRD) characterization results confirmed the hexagonal structure of the zinc oxide and the successful incorporation of platinum. Scanning electron microscopy (SEM) revealed a nano-bar morphology upon catalyst doping, favoring the photocatalytic activity. Pyridine removal of 57.7% was achieved under the following conditions: a pH of 4, 160 ppm of pyridine, and 100 g of catalyst. The process followed a pseudo-first-order model, obtaining the reaction constant k₁ = 1.943 × 10⁻³ min⁻¹ and the adsorption constant k₂ = 1.527 × 10⁻³ L/mg. The results showed high efficiency and stability of the catalyst in the fluidized bed reactor for pyridine degradation, especially under acidic conditions, representing a promising technological alternative for treating industrial wastewater contaminated with N-heterocycles such as pyridine.

1. Introduction

Every year, millions of cubic meters of wastewater from municipal, industrial, and agricultural discharges enter waterways, which are inadequately treated or not treated at all. Water pollution severely impacts ecosystems and human health, so new alternatives are needed to reduce or eliminate substances that harm the environment and, consequently, the human species. One group comprising some of the most recalcitrant pollutants that industries use is the N-heterocycles group, members of which pose a latent threat to our environment [1,2]. N-heterocycles are highly harmful, toxic compounds introduced into the natural environment through wastewater [3,4,5].

Pyridine (C₅H₅N) is a colorless, liquid N-heterocycle compound found in industrial wastewater [3,6,7]. It boils at 115.2 °C and freezes at 41.6 °C; its density is 0.9819 g/cm³, close to that of water; it has an unpleasant pungent odor; it is volatile, toxic, and flammable; and it can be found mainly in coal tar, mineral oil, and vegetable alkaloids [8,9,10]. Unfortunately, it is a complex compound to replace, as it is indispensable in several industrial processes, serving as a raw material in the production of pesticides, herbicides, dyes, explosives, and pharmaceuticals, and it is used as a chemical solvent in the paint and rubber industries [3,8,11]. According to the United States Environmental Protection Agency (EPA) [12], pyridine is a priority pollutant because it is a toxic, mutagenic, and teratogenic agent. If it is released into water bodies, it could take days, months, or even several years to degrade completely, causing irreversible damage to living beings and the environment’s quality [3,4,13,14]. Therefore, its degradation represents a challenge for treatment processes [8,15] because physical or chemical methods are costly, and biological treatments are often inefficient [15,16].

Some of the most commonly used treatments involve adsorption on materials such as activated carbon, which offers a low cost and relative operational simplicity. However, this process does not destroy the pyridine molecule; it only transfers it to a solid phase, generating a hazardous solid residue. Heterogeneous or homogenized Fenton reactions enable pyridine’s partial or complete transformation by free radicals such as -OH. Although effective, these methods usually require high reagent concentrations (H₂O₂, O₃) and strict pH control; they also generate secondary by-products that, in some cases, may be toxic or persistent. Gamma radiation or microwaves have demonstrated high efficiency for pyridine degradation; however, these methods involve high operating costs and risks associated with radiation [8,9,10].

Advanced oxidation processes (AOPs) are an alternative for removing recalcitrant organic compounds [17,18,19]. One such process is heterogeneous photocatalysis, one of the most efficient methods for degrading organic compounds [20]. This process uses semiconductor materials with a high catalytic activity aided by a source of ultraviolet radiation. These catalysts can be recovered and reused, representing a sustainable, efficient, economical, and scalable alternative that allows for the complete mineralization of contaminants [19,21,22,23].

In the interfacial region between the excited solid and the solution, the reactions for the destruction or removal of contaminants occur without the catalyst undergoing chemical changes. The semiconductor can be excited in two ways: (1) directly, in which the photons used in the process are absorbed, or (2) through the initial excitation of the molecules adsorbed on the catalyst surface, which in turn are capable of injecting charges (electrons) during the photocatalytic process [22,24]. Three fundamental components are needed for a heterogeneous photocatalytic reaction to occur: a photon source (with an appropriate wavelength), a catalytic surface (usually a semiconductor material), and an electron acceptor, which, in many cases, is oxygen from the air. The various parameters or operating conditions that influence the rate and yield of photocatalytic degradation reactions include the temperature, the catalyst mass, the solution’s pH, the light intensity, and the oxidizing agent concentration [25,26].

A fluidized bed reactor (FBR) consists of circulating a fluid through a static bed of solid particles, with a surface velocity sufficient to suspend the particles and cause them to behave like a fluid [25,26].

Fluidized bed reactors are effective wastewater treatment technologies because they are excellent contacting devices and have the potential to improve efficiency, enhance effectiveness, and improve energy efficiency if designed and used appropriately [27]. Although applying a fluidized bed reactor in advanced oxidation processes is relatively new, FBRs have proven to be practical reactors in advanced oxidation process applications. Some of the excellent characteristics of these reactors include low operating costs, high resistance to system disturbances, high mass transfer rates, and uniform mixing [25,27].

Zinc oxide (ZnO) is a new type of semiconductor material that has attracted considerable attention in materials science due to its low cost and excellent catalytic, photochemical, and thermal stability properties [28]. Recent research has focused on evaluating changes in the properties of catalysts by modifying their structure with metallic and non-metallic materials to enhance their photocatalytic potential and sensitivity to visible and ultraviolet light [13,29,30], thereby improving their photocatalytic ability. Most studies have focused on the doping of silver and gold on ZnO, but few have used ZnO nanoparticles doped with platinum nanoparticles [29,31,32].

Platinum-group metals (PGMs), including rhodium (Rh), ruthenium (Ru), platinum (Pt), and palladium (Pd), are essential catalysts in automotive catalytic converters and advanced applications such as heterogeneous catalysis and fuel cells [33]. These materials have unique physicochemical properties, in which the catalytic cycle optimizes the efficiency through an interaction between metallic Pt nanoparticles and highly dispersed Pt oxidic centers. In PGM catalysts, Rh acts primarily as a reductant, Pd as an oxidant, and Pt as a bifunctional catalyst [34].

The morphology of Pt nanoparticles directly influences their catalytic activity. Crystalline facets exposed on the surface of the nanocrystals enhance the reactivity, depending on their orientation and structure. Pt nanoparticles with octahedral, cubic, or spherical shapes dispersed on inert supports demonstrate a variable catalytic performance due to their morphology and surface area [34].

MGPs are crucial in modern catalysis, with platinum standing out for its multifunctional catalytic properties. The key to their effectiveness lies in the molecular structure and morphology of their nanoparticles—factors that determine their performance in various technological applications [34].

The catalyst developed in this research contains an active phase, ZnO-Pt, and is supported by a characteristic spherical alumina (Al₂O₃) shape. This support has been the subject of research and development for many years. Alumina exhibits a cubic crystal structure with a molecular weight of 103.235 g/mol and a molar volume of 25.575 cc/mol. The compound comprises 96% SiO₂, with the remainder comprising other elements such as Fe, B₂O₃, Al₂O₃, Na₂O, or K₂O [35]. Al₂O₃ is the most used support for industrial hydrogenation catalysts due to its large specific surface area, concentrated pore distribution, and affordable price [36]. Different coordination states of aluminum have been observed to affect the electronic and geometric configuration of supported active components. However, there have been few studies on this topic to date. Consequently, it remains unclear how different aluminum coordination affects the surface acidity [35,37].

The doping of ZnO with Pt represents a novel strategy in the field of advanced photocatalysis, since the properties of the semiconductor can be adjusted when doped with metal ions such as platinum, modifying the electronic structure of the material in a controlled manner; broadening its spectral response, catalytic activity, reusability, operational stability, and thermal stability; and generating more efficient active sites for the degradation of persistent organic pollutants [30,38,39].

For the above-mentioned reasons, the synthesis of a zinc oxide (ZnO) catalyst doped with platinum nanoparticles (Pt) and supported on alumina (Al₂O₃), which has been little explored, was carried out in this research. A fluidized bed reactor (FBR), commonly used in biological processes, was constructed to degrade pyridine. Unlike other conventional reactors (batch or tubular), it has a higher mass transfer, a high exposure to UV light, and an improved catalyst distribution. A response surface design (Box–Behnken) was used to study the interactions between variables (pH, pyridine concentration, amount of catalyst) in order to analyze the operating conditions and understand the complex non-linear interactions, positioning this work as a novel, scalable, and applicable system for the treatment of complex industrial effluents.

2. Results and Discussion

2.1. Photocatalyst Characterization

The results of the Fourier transform infrared spectroscopy (FTIR) analysis are presented in Figure 1, which shows the superposition of the synthesized materials: zinc oxide (ZnO), zinc oxide doped with platinum nanoparticles (Pt-ZnO), and zinc oxide doped with platinum nanoparticles supported on alumina (Pt-ZnO/Al₂O₃). A broad and intense band was observed from 3500 cm⁻¹ to 3000 cm⁻¹, characteristic of the water present in the sample and associated with O-H tensile and H-O-H bending motions, respectively [40,41]. Peaks were seen between 2900 and 3000, which corresponded to the C-H stretching vibrations of the methyl (-CH3) and methylene (-CH2-) groups, due to organic contaminants adsorbed on the surface of the samples, such as solvent residues or environmental impurities [42]. Peaks corresponding to the asymmetric vibration of adsorbed CO₂ were present at 2350. Around 1400 cm⁻¹, a band attributed to the tetrahedral ammonium molecule (NH4) was observed, indicating that the sample contained ammonium salts generated as a product during the synthesis.

In the platinum-doped and alumina-supported FTIR spectra, prominent bands were observed at 1200–1000 cm^−1, corresponding to Pt-ZnO bonds [43]. The approximate bands at 604 cm⁻¹ indicate the stretching of the Zn-O bond. The peak corresponding to 874 cm⁻¹ was due to the formation of a tetrahedral coordination of Zn. The FTIR spectrum of the catalyst supported on alumina (Al₂O₃) presented Al-O bonds of Al₂O₃, which were assigned prominent peaks between 554 cm⁻¹ and 456 cm⁻¹ [13,44]. The presence of characteristic Zn-O bands confirmed the ZnO structure in all the samples; however, changes in the ZnO bands were noted upon the incorporation of Pt, which may indicate a modification in the crystal lattice due to the addition of the metal.

Figure 2 shows the results obtained from an X-ray diffraction (XRD) analysis of the ZnO and Pt-ZnO nanoparticles. The more intense peaks were found at 31.75° (1 0 0), 34.41° (0 0 2), 36. 23° (1 0 1), 47.51° (0 1 2), 56.57° (1 1 0), 62.81° (0 1 3), 66.35° (2 0 0), 67.91° (1 1 2), 69.05° (2 0 1), 72.55° (0 0 4), and 76. 91° (2 0 2) crystal-faceted planes of the hexagonal ZnO wurtzite, according to the Crystallography Open Database (COD), thus demonstrating the formation of a hexagonal crystal structure. The value of the maximum peaks agreed with the COD-2300450 standard [45]. The average crystallite size obtained was 53.17822 nm. The crystallite size depended on several factors, including the intensity, peak broadening, sharpness, dislocation density, and deformation. The peaks were thin and well defined, indicating the high crystallinity of ZnO. There were no signs of amorphism or poorly crystallized phases.

The dislocation density is due to irregularities or cracks in the crystal structure. Increasing the dislocation density increases deformation, which in turn decreases the crystallite size. The results confirmed that size is directly related to deformation and the dislocation density. Peak displacement or peak broadening depends on the deformation present in the materials [46,47,48]. The results indicate that the crystallites decreased in size as the deformation increased up to 0.00064.

In the case of the Pt-doped catalyst, the peaks coincided with the crystalline planes of the 56.93° (1 0 −1) and 66.35° (2 0 0) facets of Pt according to the COD-1001824 standard [49] for Pt, where the intensity of the peaks increased with increasing Pt concentration. The average crystal size obtained was 57.21340 nm, which was larger than the crystal size of ZnO due to the addition of the metallic compound. The maximum achieved crystal deformation was 0.00037.

No peak shifts were observed when Pt was loaded onto the ZnO surface, indicating that the Pt particles did not modify the crystal structure of ZnO [39]. Figure 2B1,B2 shows the X-ray dispersive fluorescence spectra for ZnO and platinum-doped ZnO. In this case, the Pt content in the catalyst is 0.374% by mass, confirmed by the presence of platinum in the ZnO-Pt catalyst.

The scanning electron microscopy (SEM) results for the synthesized ZnO and Pt-ZnO NPs can be seen in Figure 3a and b, respectively. Figure 3a reveals the morphology and size of the ZnO particles, which did not present a homogeneous structure due to the synthesis method used (precipitation method), as observed in the distribution graph. The material presented a spherical shape with an average diameter of 114.10 nm.

Figure 3b shows the morphology obtained from the synthesis of Pt-ZnO, which had a rod shape with a homogeneous distribution and an average diameter of 99.70 nm. A structural change was observed when platinum was added to the semiconductor through the wet impregnation method.

The change in morphology from spherical (ZnO) to rod-like (Pt-ZnO) can be attributed to the synthesis method used for the semiconductor (precipitation method), which tends to produce spherical particles due to homogeneous nucleation and isotropic growth. However, when doped with Pt by wet impregnation followed by calcination, the noble metal acts as a heterogeneous nucleation center, favoring anisotropic growth and producing elongated morphologies such as nanorods. The rod shape of the nanoparticles depends on the synthesis temperature; at temperatures below 100 °C, rod structures are formed, and as the temperature increases, the shape of the particles changes [50,51]. Studies such as those by Hong et al. [39] show that the presence of Pt modifies the surface energy of the crystalline facets of ZnO, promoting preferential growth in a specific direction. Other studies confirm that Pt doping induces deformations in the ZnO crystal lattice, which can reorient growth toward one-dimensional morphologies [48].

The rod shape is one of the best structures for photocatalytic processes compared to other types of morphologies, such as spherical or hexagonal ones, because it exposes an enlarged active surface, which increases the sites available for the adsorption of reagents (e.g., organic pollutants or water molecules) and their interaction with light [52]. The elongated morphology facilitates the directional movement of electrons along the longitudinal axis of the rod, reducing electron–hole pair recombination. Since they are one-dimensional structures that provide more efficient carrier transport due to decreased surface defects, the exposed crystalline faces on nanowires are usually more reactive [53]. These faces generate oxidizing species (such as -OH) and facilitate charge transfer, thereby enhancing the photocatalytic activity. This one-dimensional structure also provides more efficient carrier transport due to the decrease in surface defects, disorder, and discontinuous interfaces [54].

2.2. Kinetic Analysis

The kinetic analysis indicated that the photocatalytic oxidation reactions followed Langmuir–Hinshelwood–Hougen–Watson (LH–HW)-type kinetics [48,52,53,54] as follows:

$${-r}_a=-{\displaystyle \frac{dC}{dt}}={\displaystyle \frac{K_{1}C}{1+K_{2}C+\sum K_i{C}_i}}$$

(1)

where K₁C represents the kinetic term of the rate equation, K₂C represents the adsorption term of the reactant, and ∑(K_i C_i) represents the adsorption term of all the intermediate products of the degradation reaction of organic compounds.

If the experimental data are analyzed for very short reaction times, the adsorption term of the intermediate products can be neglected.

Based on the above, it can be shown that the following equation represents the general kinetic form:

$$r_a=-{\displaystyle \frac{dC}{dt}}={\displaystyle \frac{K_1{C}^m}{{1+K}_2{C}^n}}$$

(2)

If the exponents m and n have a value of 1, the constants K₁ and K₂ can be determined directly from the graph of the reaction rate versus concentration.

Equation (2) can be linearized in the manner recommended by Fogler [55] and Moctezuma [56]; using the initial conditions of t = 0, C = C₀, and the reaction rate, the equations obtained are as follows:

$${\left.r_a\right|}_{t=0}={\displaystyle \frac{K_1{C}_0}{{1+K}_2{C}_0}}$$

(3)

$${\displaystyle \frac{1}{{\left.{-r}_a\right|}_{t=0}}}={\displaystyle \frac{{1+K}_2{C}_0}{K_1{C}_0}}={\displaystyle \frac{1}{K_1{C}_0}}={\displaystyle \frac{K_2{C}_0}{K_1{C}_0}}$$

(4)

$${\displaystyle \frac{1}{{\left.{-r}_a\right|}_{t=0}}}={\displaystyle \frac{1}{K_1{C}_0}}+{\displaystyle \frac{K_2}{K_1}}$$

(5)

Equation (5)’s behavior is represented in Figure 4, where the ordinate at the origin is K₂/K₁ and the slope is given by 1/K₁ [57,58].

$$-r_{AC}=-{\displaystyle \frac{d{C}_{AC}}{dt}}={\displaystyle \frac{K_1{C}_{AC}}{1+K_2{C}_{AC}}}$$

(6)

It is necessary to specify that five replicates were conducted for each of the concentrations handled (20, 160, and 300 ppm), that is, 15 series of data, and for each concentration, nine time points were recorded; for each concentration, a total of 45 data points were collected, and for the three concentrations combined, 135 data points were ued for the kinetic analysis. From the values obtained in Figure 4, we obtained the values of the constants, and therefore, the previous equation can be expressed as follows:

$$-r_{AC}={\displaystyle \frac{0.0061\times C_{PY}}{1+0.0039\times C_{PY}}}$$

(7)

The values of the reaction constants K₁ = 0.0061 min⁻¹ are low compared with the results presented by Leyva [59], but the system was a 300 mL reactor, and the catalyst was only zinc oxide where pyridine was degraded using UV lamps. The adsorption constant K₂ = 0.0039 L/mg was slightly lower than the results presented by this author, but it can be said that the adsorption of pyridine is not favorable in this system, since most of the reaction is due to the formation of hydroxyl radicals.

Figure 5 shows the behavior of the LH–HW model compared with the experimental data. The data were observed to have a behavior similar to the model, fitting its behavior.

2.3. Evaluation of Critical Variables in Pyridine Removal by DOE

The efficiency of the photocatalytic pyridine removal process critically depends on variables such as the pH of the medium, the initial pyridine concentration, and the amount of catalyst used. The pH can influence the surface charge of the catalyst and the ionization state of pyridine, thus affecting adsorption and radical generation. Furthermore, both high-pyridine and high-catalyst concentrations can limit light absorption or the availability of active sites. Therefore, systematically evaluating the effect of these variables is crucial for optimizing the photodegradation process and designing efficient and sustainable treatment systems. Figure 6a presents the experimental values for the degradation of pyridine. It can be observed that treatments 5 and 8 exhibited the highest rate of pyridine degradation, characterized by a low pH level (pH of 4) and intermediate to high levels of the pyridine catalyst. Figure 6b shows the influence of the pH, pyridine concentration, and amount of catalyst on the mean pyridine removal. It can be observed that, at an acidic pH (pH of 4), the removal percentage was high. This can be attributed to a greater generation of hydroxyl radicals (·OH) and the better adsorption of pyridine on the catalyst surface. As the pH increased (pH of 7–10), the removal percentage tended to decrease; this could be due to an electrostatic repulsion between the catalyst surface and the anionic species formed at a high pH. On the other hand, at moderate concentrations of pyridine (160 ppm), a moderate removal percentage was obtained. High concentrations of pyridine did not show an improvement in the removal percentage. This could be due to the saturation of the active sites and the formation of intermediate products that compete for the active sites. It was observed that an increase in the amount of catalyst increased the removal percentage due to a greater availability of active sites and the improved generation of reactive species. However, an excessive amount can cause the agglomeration of the catalyst particles, leading to light scattering and, therefore, reducing the process efficiency.

A Box–Behnken design for the experiments was implemented to optimize pyridine degradation in aqueous solutions, enabling the modeling and understanding of the interactions between the studied factors with a limited number of experiments. The results obtained are presented below. Figure 7 shows the Pareto chart, a statistical tool that identifies and prioritizes the factors most significantly influencing a pyridine removal process [60]. Factor A corresponds to the pH, factor B corresponds to the pyridine concentration, and factor C corresponds to the amount of catalyst. As can be seen, the factor with the most significant impact was the pH, along with its quadratic term. This effect was due to the pH of the medium being a determining factor in the removal of organic compounds such as pyridine. Previous work has shown that a pH close to neutral favors the generation of hydroxyl radicals (·OH), highly reactive species responsible for oxidation reactions.

Furthermore, the pH directly affects the catalyst’s surface charge and the organic compound’s ionization, influencing the compound’s adsorption. Shu et al. [59] achieved the efficient photocatalytic degradation of pyridine using TiO₂ modified with La and Fe at a pH of 8. They attributed this efficiency to improved pyridine adsorption on the catalyst and the more significant generation of reactive species under slightly alkaline conditions [61]. The Pareto diagram indicates that the factors do not act independently in the degradation of pyridine. There are important interactions between the factors, such as AC and AB.

Figure 8 shows how each factor individually influences the photocatalytic removal of pyridine. It has been confirmed that the pH is the most significant factor influencing the process. As mentioned above, an adequate pH level optimizes the generation of hydroxyl radicals (·OH) [62]. Likewise, the pH affects the surface charge of the catalyst and improves pyridine adsorption by ionizing it. The figure also shows that the pyridine and catalyst factors do not have a marked effect within the study range. However, it was observed that the initial pyridine concentration directly influences its removal. At low pyridine concentrations, active sites are available on the catalyst for pyridine adsorption, which facilitates its oxidation [63]. However, the active sites may become saturated at higher concentrations, and the accumulation of intermediates may inhibit the process. Meanwhile, a high amount of catalyst ensures a more significant number of active sites responsible for generating reactive species and a larger adsorption surface area [64].

Figure 9 illustrates the interactions corresponding to the different levels of the studied factors. Figure 9a shows that high levels of pyridine require an acidic pH to enhance pyridine degradation; this is because, at an acidic pH, pyridine is protonated, producing the pyridine ion (PyH⁺), which increases its solubility for the degradation and adsorption processes [65]. In Figure 9b, it can be observed that the highest percentage of pyridine degradation occurred at low pH levels. At high catalyst levels, this interaction was attributed to the fact that the pH influences the generation of reactive species, such as hydroxyl radicals (·OH), which are responsible for the oxidation of pyridine. As can be seen, at high pH levels, the degradation efficiency decreases; this could be due to the precipitation of the metallic species present in the catalyst [62]. A high catalyst level improves the availability of active sites for generating reactive species. Figure 9c shows an optimal concentration range for pyridine and the catalyst. Excessive amounts of pyridine can saturate the catalyst’s active sites, while increased amounts of the catalyst can cause particle agglomeration and light obstruction, reducing the process efficiency.

A polynomial regression model was developed to represent the percentage of pyridine removal as a function of the process input variables (pH, initial pyridine concentration, and catalyst). The calculated regression model is presented in Equation (8), which provides an adequate fit to the experimental data (R² = 0.943).

$$\begin{gathered} \mathrm{\%}\mathrm{R}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}=29.80-7.497\mathrm{p}\mathrm{H}-1.453\mathrm{P}+1.049\mathrm{C}+11.43{\mathrm{p}\mathrm{H}}^2+1.11{\mathrm{P}}^2 \\ +0.93{\mathrm{C}}^2-6.72\mathrm{p}\mathrm{H}·\mathrm{P}-6.85\mathrm{p}\mathrm{H}·\mathrm{C}-2.03\mathrm{P}·\mathrm{C} \end{gathered}$$

(8)

Figure 10, for the pH, shows the normal probability plot, which evaluates the assumption of a normal distribution in the fitted regression model. It can be seen that most of the points are close to the red line, indicating that the normality of the residuals was met. Although there are a few residual points that are slightly off the scale, they are not different enough to invalidate the model.

Below (

Table 1. Comparative data on pyridine removal.

Catalyst	Reactor	% Piridina Remotion	Time (h)	Reference
Pt-ZnO/Al2O3	Fluidized bed photocatalytic reactor with UV light lamps	57.7	4	Present study
ZnO-Al	Rotating photodisco reactor (RPR) with natural light lamps	62	72	[66]
TiO2	Photoreactor with UV lamps	100	5	[59]
Al-ZnO/Al2O3	Fluidized bed photocatalytic reactor with UV light lamps	70	5	[10]
Modified Shell Powder/La-Fe-TiO2	Not specified	80.23	3	[61]
Gamma radiation	Not applicable (direct irradiation)	71	1	[8]

) is a comparison between various synthesized materials with the percentages of pyridine removal to compare the viability of the catalyst and the use of the fluidized bed photocatalytic reactor.

The Pt-ZnO/Al₂O₃ catalyst used in this study stands out for its efficiency, achieving a 57.7% pyridine removal in 4 h under UV light in a fluidized bed reactor. This performance surpasses that of ZnO-Al under natural light (62% in 72 h), which, although it takes advantage of an energy-efficient source, is slower for practical applications. In addition, it approaches the performance of Al-ZnO/Al₂O₃ (70% in 5 h), but with a shorter treatment time, suggesting that including Pt in the catalyst could improve the photocatalytic activity.

Although conventional TiO₂ (100% in 5 h) and Modified Shell Powder/La-Fe-TiO₂ (80.23% in 3 h) achieve higher efficiencies, they require longer times or complex modifications to the material. The system in the present study offers a faster alternative than traditional photocatalysts and is more scalable than gamma radiation (71% in 1 h), which depends on specialized equipment. Therefore, the Pt-ZnO/Al₂O₃ catalyst is positioned as a competitive and promising catalyst, with the potential to further optimize the photocatalysis process through adjustments in catalyst composition or operating conditions.

3. Methodology

3.1. Chemical Products and Reagents

The reagents used to carry out the synthesis of the catalyst were zinc nitrate hexahydrate (Zn (NO₃)₂∙6H₂O) with a 99% purity, purchased from Meyer (St. Louis, MO, USA); potassium hydroxide (KOH) with an 85% purity, purchased from Meyer; platinum acetylacetonate (C₁₀H₁₄O₄Pt) with a 97% purity, obtained from Sigma-Aldrich (St. Louis, MO, USA); and acetone ((CH₃)₂CO) with a 99.6% purity, purchased from JT Baker (Phillipsburg, NJ, USA). The following reagents for the photoactivity evaluation of the catalyst were used: pyridine (C₅H₅N), which was obtained from Sigma-Aldrich, and distilled water, which was obtained from Wöhler (Hayward, CA, USA) (reagent grade).

3.2. Photocatalyst Preparation

The synthesis of the semiconductor zinc oxide (ZnO) was carried out through the direct precipitation method [48]. Two solutions were prepared: zinc nitrate (0.2 M) and potassium hydroxide (0.4 M), both in distilled water as the solvent. The potassium hydroxide solution was added to the zinc nitrate solution under magnetic stirring. The mixture was left to stand for 20 min and then filtered under a vacuum, and the obtained precipitate was washed with distilled water and ethanol to remove impurities. Finally, it was dried at 100 °C for 24 h and calcined at 500 °C for three hours.

The doping of ZnO with platinum nanoparticles (Pt-ZnO) was performed using the incipient impregnation technique [44] in the following steps: A 0.5 wt.% platinum acetylacetonate solution diluted in acetone was prepared and added to ZnO over a heating grid at 90 °C for subsequent calcination in a muffle at 400 °C for two hours.

The Pt-ZnO was supported on alumina (Al₂O₃) using the wet impregnation technique [44]. A solution of 3 g of Pt-ZnO was prepared in 250 mL of distilled water, which was added to the Al₂O₃ beads while stirring for 30 min. The mixture of the Pt-ZnO solution and the alumina beads was left under UV irradiation for 24 h to achieve maximum adsorption of the semiconductor onto the support. Finally, the calcination of the support was carried out in a flask at 550 °C for three hours.

3.3. Photocatalyst Characterization

A Fourier transform infrared spectroscopy (FTIR) analysis was performed using an Agilent Cary 660 spectrometer (Santa Clara, CA, USA) with a scanning range of 4000 to 400 cm⁻¹ and attenuated total reflectance (ATR) to verify the presence of ZnO and Pt. To evaluate the crystal structure of the synthesized photocatalyst, a Rigaku Ultima IV diffractometer (Tokyo, Japan) with a thin-film module (Cu radiation Kα λ = 0.15418 nm, 40 kV, 44 mA, and angle step of 0.02°) in the Bragg–Brentano configuration was used. Subsequently, the results obtained were analyzed with the Match! 3.3 software [67,68]. The morphology and size distribution of the Pt-ZnO/Al₂O₃ NPs were analyzed using scanning electron microscopy (SEM) with an INSTRUMENT JSM-6490 scanning electron microscope (Tokyo, Japan) at a voltage of 20 kV and a magnification of 20,000 magnification. The micrographs were interpreted in ImageJ software version 1.53k to determine the approximate size of the NPs.

3.4. Fluidized Bed Photocatalytic Reactor Used for Photocatalytic Experiments

A fluidized bed photocatalytic reactor was used for the pyridine degradation runs. This reactor consists of a borosilicate glass cell 5 cm in diameter and 50 cm in length, with an effective volume of 500 mL. The external body of the reactor is made of stainless steel and is resistant to corrosive conditions; its diameter is 31 cm, and its length is 56 cm; and Pt-ZnO/Al₂O₃ photocatalyst beads constitute the reactor bed. A Mayware fan was installed to lower the temperature within the reaction, thereby minimizing the impact on the photocatalytic process. Two UV lamps with a wavelength (λ) of 365 nm and 15 watts were placed inside the reactor housing; a recirculation pump was also placed to circulate the solution countercurrent through the system. Figure 11 shows a diagram of the fluidized bed reactor. In Figure 12, we can appreciate the image of the photoreactor with the catalyst specifications described in Figure 11.

For the construction of the reactor cell, it was considered that the material to be used should be transparent to UV radiation. Quartz is an ideal material due to its good UV light transmittance and chemical and thermal resistance; however, its high price limits its application in photocatalysis for this project. Therefore, it was decided to use borosilicate glass, as it has excellent UV light transmittance.

One of the critical parameters to take into account is the diameter of the reactor, which must be considered in order to have a balance between the illumination, the amount of catalyst, and the solution. In tubular photoreactors, practical diameters generally range from 25 to 50 mm. Smaller diameters can result in a high pressure drop, while larger diameters will have less volume illumination, reducing process efficiency. Therefore, the reactor was built with a diameter of 5 cm and a length of 50 cm, which allowed considerable amounts of charge to be handled with the maximum possible number of useful photons reaching the cell, and the volume would guarantee the monitoring of the dissolution, thus enabling an evaluation of the phenomenological behavior of the pyridine in contact with the catalyst. In Figure 12, we can see an actual image of the photoreactor with the specifications described in Figure 11.

3.5. Photocatalytic Experiments

The pyridine degradation runs were carried out following a Box–Behnken-type experimental design (

Table 2. Experimental Box–Behnken design.

Experiments	Coded Variables			Natural Variables
	pH (−)	Concentration (ppm)	Time (min)	pH (−)	Concentration (ppm)	Catalyst (g)
1	−1	0	−1	4	160	20
2	−1	−1	0	4	20	60
3	0	0	0	7	160	60
4	0	−1	−1	7	20	20
5	−1	0	1	4	160	100
6	0	0	0	7	160	60
7	1	0	1	10	160	100
8	−1	1	0	4	300	60
9	1	0	−1	10	160	20
10	0	−1	1	7	20	100
11	1	−1	0	10	20	60
12	0	1	1	7	300	100
13	0	0	0	7	160	60
14	0	1	−1	7	300	20
15	1	1	0	10	300	60

), where the independent variables were the following: the pyridine concentration (20 ppm, 160 ppm, or 300 ppm), the amount of catalyst (20, 60, or 100 g), and the pH (4, 7, or 10), under normal pressure and temperature conditions (1 atm, 27 °C). For the kinetic analysis, concentrations of 40, 60, and 80 ppm were added, and the same reaction times were used. The percentage of pyridine degradation was measured as the response variable. These conditions simulate wastewater discharges from industries that manufacture pyridine and its derivatives [9]. The pH and catalyst amount conditions were used to analyze their effect during the photocatalytic process.

Samples were taken every 30 min for 4 h to measure the concentration of pyridine inside the photoreactor. Each sample was analyzed using a UV-VIS VE-5100UV VELAB UV-VIS spectrometer (Pharr, TX, USA).

4. Conclusions

The experimental results obtained from the degradation runs show that the pH was the most influential factor in the degradation process, i.e., acidic conditions maximized the efficiency, along with a moderate pyridine concentration and high catalyst loading. However, high concentrations over time saturate the active sites, leading to agglomeration and reduced light penetration.

The fluidized bed reactor demonstrated improved mass transfer, enhanced catalytic exposure to UV light, and catalyst stability, making it ideal for treating wastewater with recalcitrant contaminants. The Pt-ZnO/Al₂O₃ catalyst in an FBR achieved high efficiency and stability, and demonstrated good potential to degrade pyridine in industrial effluents. Its nano-barrier morphology and synergy with the FBR offer a promising solution for N-heterocycles, which may lay the foundation for industrial-scale applications.