Reduced Metal-Codoped TiO₂/LSX Zeolite Catalysis: A Sustainable Solution for Crystal Violet Dye Degradation

Abstract

Water pollution due to synthetic dyes such as crystal violet (CV) poses significant environmental and health risks due to its toxicity and persistence. This study investigates the synthesis of titanium dioxide (TiO₂) and TiO₂/low silica Type X (LSX) zeolite composites using different TiO₂ precursor concentrations (0.1, 0.25, 0.5, and 1.0 M) and metal-doped variants (Ag⁰/TiO₂, Ag⁰–TiO₂/LSX, Fe⁰/TiO₂, and Fe⁰–TiO₂/LSX) via sol–gel and borohydride reduction methods. Characterization techniques used are XRD, FTIR, SEM, EDX, BET, TG-DTG, UV–vis spectroscopy, and TEM and HRTEM analysis. Photodegradation has been carried out in the removal of CV dye from aqueous solutions. LSX due to its high surface area (687.408 m²/g) and TiO₂ with a high bandgap of 3.2 eV are found most suitable in UV photocatalysis. Metal doping reduced the band gaps, e.g., from 3.2 eV of TiO₂ to 2.64 eV of Ag⁰/TiO₂ and 1.91 eV of Fe⁰/TiO₂, enhancing UV-light responsiveness. Batch adsorption and photocatalytic degradation under dark and UV-assisted radiation revealed that Ag⁰/TiO₂ and Fe⁰/TiO₂ photocatalysts showed the highest removal efficiencies of 97.45% and 84.81%, along with adsorption capacities up to 20.98 and 18.26 mg/g, respectively. The comparatively lower performance of TiO₂/LSX composites than that of pure TiO₂ and pure Na-LSX is attributed to surface coverage by TiO₂, which limited the accessibility of active sites. Challenges such as pore blockage, surface coverage, dealumination, and Ag⁰ and Fe⁰ agglomeration necessitate further optimization. These findings highlight the potential of Ag⁰-and Fe⁰-doped zeolite composites for sustainable wastewater treatment along with future research needed to enhance scalability and real-world applicability.

1. Introduction

Industrial activities, particularly textile dyeing, contribute significantly to water pollution through the release of synthetic dyes like crystal violet (CV), a toxic triphenylmethane cationic dye known for its environmental persistence and health risks. Conventional treatment methods, such as adsorption and chemical oxidation, often face limitations due to high costs, incomplete pollutant removal, or secondary pollution. Heterogeneous photocatalysis using TiO₂-based materials offers a promising, eco-friendly alternative due to TiO₂’s chemical stability, low cost, and high photocatalytic efficiency. However, TiO₂’s wide bandgap (3.2 eV for anatase) restricts its activity to UV light, limiting its practical application under solar irradiation. By combining TiO₂ with some adsorbent such as low-silica Type X (Na-LSX), their high surface area can be leveraged for enhanced adsorption and synergistic photocatalysis. The synthetic X-type zeolites are members of the Faujasite group, which has the following structure: α-cages/large interior super cages are formed by connecting β-cages with hexagonal prisms. The 12-membered ring (12MR) allows molecules, including CO₂, to pass through and into the α-cages. The diameter of the windows is 10.8 Å. These zeolites can be broadly categorized into two groups: X-type zeolites, which have low silicon content, and Y-type zeolites, which have high silicon content. The chemical composition and recognized chemical formula of X-type zeolites demonstrate their significant diversity. The zeolite-supported TiO₂ has greater photocatalytic activity as compared to that of unsupported TiO₂ when it comes to breaking down different types of contaminants. When the zeolite has a lower Si:Al ratio, the photoactivity of a TiO₂-based photocatalyst supported on it is higher.

Zeolite is typically used in conjunction with titanium dioxide as a photocatalyst, acting as a supportive medium for the doping of TiO₂. This is mostly due to the catalyst’s capacity to increase photocatalytic activity through high zeolite adsorption. However, several studies showed that these advantages depend heavily on the synthesis and treatment conditions. Faujasite zeolites like LSX undergo dealumination or structural coverage, which collapses their crystalline framework and decreases porosity. Such degradation lowers the adsorption capacity and hinders TiO₂-zeolite synergy. Preserving the zeolite crystallinity is critical for optimal photocatalytic efficiency in TiO₂/zeolite composites. While TiO₂/zeolite composites have gained significant interest, an unresolved challenge is preserving zeolite’s natural adsorption ability when high amounts of TiO₂ are loaded.

Zeolites are highly porous materials with a strong adsorption capacity. They can also enrich pollutants around the catalyst’s surface, increasing the rate of photocatalytic reactions via the ‘preconcentration effect’. However, several studies state that higher TiO₂ loading can clog the zeolite’s micropores, decrease the surface area, and ultimately hinder the internal diffusion of dye molecules within the structure. Furthermore, LSX and other FAU-type zeolites have been reported to be sensitive to the highly acidic conditions that are formed during the sol–gel process, which can cause dealumination and a partial loss of crystallinity and therefore a reduced adsorption capacity within the framework. Doping with metals like Fe⁰ and Ag⁰ enhances visible-light responsiveness by reducing the bandgap and improving charge separation. A lot of research has been conducted on zerovalent metals, such as zerovalent iron (Fe⁰) to reduce certain pollutants. The composite’s catalytic efficiency is increased by the mutual remediation of Fe⁰ and TiO₂. Iron is most frequently employed for doping due to its nontoxic nature, abundance in the earth crust, and ability to absorb TiO₂ with a red shift. While doping is an essential strategy for enhancing the optoelectronic characteristics of semiconducting metal oxides, the preparation technique has a significant impact on the extent and efficiency of doping. Another efficient way to increase TiO₂ photoactivity is to deposit a noble metal, like silver, on its surface. Due to the Schottky barrier on the surface of TiO₂, Ag⁰ is an important acceptor of photogenerated electrons, which slows down the rate at which charge carriers recombine. There are also three interrelated considerations that supported the choice to use zerovalent metals such as Ag⁰ and Fe⁰. They create more and highly reactive surface sites that destroy target pollutants, they create enhanced charge separation and electron transfer in photocatalytic systems, and they enhance stability and dispersion of the metals when supported on zeolite as compared to unsupported metal particles. Several studies are found in the literature that provide a description of the improvement as a result of the addition of zerovalent metals to photocatalytic systems. Li et al., (2018) and Li et al., (2020) reported the increased elimination of organics and inorganics from polluted water due to the use of the adsorption and reductive transformation processes with the introduction of nanoscale zerovalent iron immobilized on zeolite. , In a similar way, it is possible to achieve the enhanced photocatalytic activity of titania and silica under UV light because of the electron-accepting capacity and plasmonic enhancement of silver metal. , The reactions and interactions between the metals and zeolite may occur in two ways: First is by ion exchange of metal cations (Ag⁺ and Fe²⁺/Fe³⁺) with the zeolite, which is reduced either thermally or chemically to the zerovalent form. The second method is by using a metal salt impregnation, followed by an in situ reducing treatment, which will result in the creation of metal nanoparticles within readily accessible pores and at the exterior. Again, in each of the two structures a scaffold is applied where the metal particles are supposed to be located and dispersed in order to make them nonaggregate and to stabilize smaller nanoparticles or clusters.

The three main ways through which the synergistic effect occurs are as follows. Zero-valent metals may (i) include redox-active sites that can reduce or deprotonate a variety of different contaminants (this effect has been noted especially in the case of Fe⁰), (ii) transport photogenerated electrons, thereby preventing electron–hole recombination and increasing photocatalysis (this effect has been associated with Ag⁰ in TiO₂), and (iii) alter the surface adsorption or surface chemistry of the substance in such a way that a greater number of the target species can bind about the active sites and bind more strongly. These active roles have been proven by a number of studies: in particular, nZVI/zeolite composites exhibit both adsorption and reductive removal, , and Ag⁰-decorated TiO₂/SiO₂ showed better activity in the visible part of the light spectrum because of the simultaneous action of plasmon activity and electron sink. ,

The present study addresses these issues by focusing on the effect of different amounts of TiO₂ on LSX adsorption and dye removal photocatalytic activity. Rather than solely concentrating on maximizing the TiO₂ content, moderate levels of its loading (0.1 to 1.0 M) are designed to attempt to strike a compromise between preserving the zeolite’s surface area and achieving sufficient TiO₂ surface coverage to enable photocatalysis. Moreover, the introduction of Ag⁰ and Fe⁰ is meant to enhance charge separation and retard electron hole recombination, thereby featuring substantial photocatalytic activity without a high degree of TiO₂ deposition. With this developed method, there is much more scope to give practical insight on the relationship between TiO₂ loading, zeolite integrity, and metal doping, which is of a profound nature to the structure of composite adsorbents to enhance their efficiency in hybrid adsorbent photocatalytic systems. This study aims to synthesize TiO₂, TiO₂/LSX composites with varying molarities (0.1, 0.25, 0.5, and 1.0 M), and metal-doped variants (Ag⁰/TiO₂, Ag⁰–TiO₂/LSX, Fe⁰/TiO₂, and Fe⁰–TiO₂/LSX) using sol–gel and borohydride reduction methods. The synthesized zeolite composites were characterized by X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray (EDX), Brunauer–Emmett–Teller (BET) analysis, Thermogravimetric Analysis (TG-DTG), TEM, HRTEM, and Ultraviolet and Visible (UV–vis) spectroscopy. The adsorption efficiency was evaluated by TiO₂ (Titanium dioxide), TiO₂/LSX, Ag⁰/TiO₂, Ag⁰–TiO₂/LSX, Fe⁰/TiO₂ and Fe⁰–TiO₂/LSX photocatalyst for the removal of CV dye under dark and light-assisted irradiations. The research addresses the need for efficient, cost-effective solutions to mitigate dye pollution and contribute to environmental remediation efforts.

2. Materials and Methods

2.1. Materials

Na-LSX zeolite was supplied from LuoYang Jian Long Co., Ltd. CV dye of high purity, deionized water (DI), and ethanol (99.9%) were supplied by Merck KGaA, Germany; FeSO₄·7H₂O and sodium borohydride (98%) were purchased from Sigma-Aldrich. TiCl₄ was purchased from Fluka, and ammonia (32%) and HCl (37%) were purchased from Merck Schuchardt, Germany, and AgNO₃ (99%) was purchased from Sigma-Aldrich.

2.2. Methods

2.2.1. Synthesis of TiO₂

The TiCl₄ solution (1.5 mL) was added dropwise into 25 mL of ice-cold water, and the solution was kept stirring for about 10 min. As 3–4 mL ammonia solution (32%) was added dropwise to the solution, gel formation started and this mixture was stirred for 1 h at 250 rpm. The mixture was filtered and washed with deionized water, to maintain the pH of the solution up to 6. After filtration the gel was dried in an oven for 24 h at 100 °C. The obtained mass was crushed to fine powder and then finally calcined at 600 °C. Synthesis of TiO₂-LSX.

The TiCl₄ solution (1.5 mL) was added dropwise into 25 mL of ice-cold water, and 1 g of Na-LSX zeolite added to the above solution and stirred for about 10 min. Then 3–4 mL of ammonia solution (32%) was added dropwise into the solution, and gel formation started. This mixture was kept on stirring for 1 h at 250 rpm. The mixture was filtered and washed 2 to 3 times with deionized water to maintain the pH of the solution up to 6. After filtration the gel was dried in an oven for 24 h at 100 °C. The dried mass was crushed to fine powder and then calcined at 600 °C. TiO₂-LSX composites were synthesized by treating TiCl₄ precursor solutions of varying molar concentrations (0.1, 0.25, 0.5, and 1.0 M) with LSX to achieve composites with differing TiO₂ loadings. A TiO₂ precursor (TiCl₄ via sol–gel) to LSX mass ratio of 0.200:1 was used, corresponding to moderate loading. Ag⁰ and Fe⁰ were doped on TiO₂-LSX composites separately.

2.2.2. Synthesis of Fe⁰/TiO₂

The Fe⁰/TiO₂ composite was synthesized by a simple reduction method applying sodium borohydride (NaBH₄) as a reducing agent. The presynthesized TiO₂ (800 mg) was added to a 30 mL ethanol and water (5:25 v/v) mixture and sonicated for 5 min. Then, 1200 mg of FeSO₄·7H₂O was added to the solution of TiO₂ and stirred for about 30 min at 250 rpm under nitrogen purging. 1200 mg of NaBH₄ () was added in 40 mL of deionized water; this solution was then added dropwise to the TiO₂ mixture under the inert atmosphere (N₂ purging). Black precipitate of Fe⁰/TiO₂ composite was formed at the addition of the first drop of NaBH₄. To ensure maximum reduction, NaBH₄ (40 mL) was added continuously to the solution while stirring. Following that, the Fe⁰/TiO₂ black precipitate was separated and washed with ethanol, vacuum-dried at 60 °C for 2 h, and then stored at room temperature after grinding.

2.2.3. Post Synthesis of the Fe⁰–TiO₂/LSX Zeolite Composite

The Fe⁰–TiO₂/LSX composite was synthesized by a simple reduction method applying sodium borohydride (NaBH₄) as a reducing agent. The presynthesized 0.1 M TiO₂/LSX zeolite composite (800 mg) was added to a 30 mL ethanol and water (5:25 v/v) mixture and sonicated for about 5 min. Then, 1200 mg of FeSO₄·7H₂O was added to the solution of TiO₂ and stirred for about 30 min at 250 rpm under nitrogen purging. Afterward, a solution containing NaBH₄ (1200 mg) in 40 mL of deionized water was added dropwise to the TiO₂ mixture under an inert atmosphere (N₂ purging). A black precipitate of Fe⁰–TiO₂/LSX zeolite composite was successfully formed upon addition of the first drop of NaBH₄. To ensure maximum reduction, a solution of NaBH₄ was continuously added to the TiO₂ mixture during continuous stirring. Afterward, the Fe⁰–TiO₂/LSX black precipitates were separated, washed with ethanol, vacuum-dried at 60 °C for 2 h, and then stored at room temperature after grinding.

2.2.4. Synthesis of Ag⁰–TiO₂

The Ag⁰–TiO₂ composite was synthesized by a simple reduction method applying sodium borohydride (NaBH₄) as a reducing agent. Presynthesized TiO₂ (800 mg) was added to a 30 mL ethanol and water (5:25 v/v) mixture and sonicated for about 5 min at 250 rpm. Under nitrogen purging, 1200 mg of silver nitrate was added to the solution of TiO₂ and stirred for about 30 min. Afterward, a solution of NaBH₄ (1200 mg) prepared in 40 mL of deionized water was added dropwise into the TiO₂ mixture under N₂ purging. Grayish-brown precipitates of Ag⁰/TiO₂ composite were formed upon addition of the first drop of NaBH₄. To ensure maximum reduction, NaBH₄ was added continuously to the solution with continuous stirring. Following that, the Ag⁰/TiO₂ grayish-brown precipitates were separated, washed with ethanol, vacuum-dried at 60 °C for 2 h, and then stored at room temperature after grinding.

2.2.5. Post Synthesis of the Ag⁰–TiO₂/LSX Zeolite Composite

The Ag⁰–TiO₂/LSX composite was synthesized by a simple reduction method applying sodium borohydride (NaBH₄) as reducing agent. 800 mg of presynthesized 0.1 M TiO₂/LSX zeolite was taken in 30 mL ethanol and water (5:25 v/v) mixture and sonicated for about 5 min, then 1200 mg of silver nitrate was added to TiO₂ solution and stirred for about 30 min at 250 rpm under nitrogen purging. Then the same procedure is repeated for the preparation of the Ag⁰–TiO₂/LSX zeolite composite as used for Ag⁰–TiO₂. Grayish brown precipitates of Ag⁰–TiO₂/LSX zeolite composite were separated, washed with ethanol, vacuum-dried at 60 °C for 2 h, and then stored at room temperature after grinding.

2.3. Physicochemical Characterizations

The composites were characterized by using multiple techniques to assess their structural, morphological, and optical properties. XRD was performed using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 0.1542 nm) to identify crystalline phases, with scans from 2θ = 5° to 80°. SEM and EDX were conducted using a JEOL SEM IT200 instrument to analyze surface morphology and elemental composition. FTIR spectra were recorded on Bruker ALPHA-T ranges from 4000 to 400 cm^–1 to identify vibrational modes. BET surface area analysis was performed using a Beijing JWGB Sci& Tech Co., Ltd. with N₂ adsorption–desorption isotherms. UV–vis spectroscopy was conducted on a Shimadzu UV-2600 instrument to determine absorption edges and bandgaps, with measurements from 200 to 800 nm. TG-DTG was performed using a PerkinElmer STA 8000, heating from 0 to 900 °C at the rate of 10 °C/min.

2.4. Photocatalytic and Adsorption Experiments

Batch experiments were conducted to evaluate CV dye (20 mg/L, pH 6.2) removal under dark and UV-assisted irradiation (254 nm, 15.7 W/m²) using a solution prepared by adding 0.1 g of catalyst in 50 mL of deionized water. In all experiments, the lamp was placed at a constant distance of 0.39 m above the surface of the solution in which the reaction occurred to provide uniform illumination. To maintain thermal control of the reactor, continuous airflow was allowed around the reactor, and temperature was controlled to ensure that there was no excessive accumulation of heat. During the irradiation process, the reaction mixture was kept on constant stirring using a magnetic stirrer that would keep the catalyst suspended in the mixture and prevent sedimentation. Adsorption was conducted in the dark for 30 min, followed by UV irradiation for 120 min. Dye concentration was measured using a UV–vis spectrophotometer at λ_max = 590 nm. Removal efficiency (%) was calculated using eq .

$$\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}(\%)=100\times C_0-C/C_0$$ 1

where C ₀ and C are initial and final dye concentrations, respectively. Adsorption capacity (qt, mg/g) of zeolite composites is determined via eq .

$$q_t=(C_0-C)\times V/m$$ 2

where V is solution volume (L) and m is catalyst mass (g).

2.5. Adsorption and Degradation Kinetics

To determine the adsorption rate of TiO₂/LSX composites and propose their adsorption mechanism requires both PFO and PSO models, which were used to interpret experimental data for adsorption studies. The Legergren’s rate equation is one of themost versatile models to be used as an allowed model to explain adsorption of an adsorbate in the liquid state. The linear forms of the pseudo-first and pseudo-second order kinetic models are represented by eqs and , respectively

$$\ln (q_e-q_t)=\ln q_e-k_1·\mathrm{t}$$ 3

$$\frac{t}{q_t}=\frac{1}{{{k_2·q}_e}^2}+\frac{t}{q_e}$$ 4

where q _e and q _t represent the amount of CV dye (mg/g) adsorbed at equilibrium and at time t respectively, k ₁ and k ₂ are equilibrium pseudo-first and pseudo-second order rate constants, respectively.

The CV dye degradation kinetics was used to study the pseudo-first-order kinetics model for degradation, as represented by eq

$$\ln \frac{C_0}{C}=k_{\mathrm{a}\mathrm{p}\mathrm{p}}·\mathrm{t}$$ 5

where C ₀ and C are the concentrations (mg/L) of the CV dyes at time = 0 and time = t (min), respectively, k _app is the apparent rate constant, and t is the time interval.

3. Results and Discussion

3.1. XRD Analysis

XRD analysis confirmed Na-LSX zeolite with characteristic peaks at 2θ = 6.2°, 10.2°, 15.6°, 23.4°, and 31°, as shown in Figure , confirming the Faujasite-type framework’s high crystallinity. Pure TiO₂, synthesized via the sol–gel method and calcined at 600 °C, exhibited sharp peaks at 2θ = 25.3°, 37.8°, 48.0°, 54.0°, and 62.7°, corresponding to the 101, 004, 200, 105, and 204 planes of the anatase phase, which indicated phase purity and high crystallinity aligned with Diebold. The TiO₂/LSX composites showed prominent anatase TiO₂ peaks at the same 2θ values across all molarities, but LSX peaks were unexpectedly absent, likely due to TiO₂ dominance (Figure A­(c–f)). TiO₂ particles deposited on the Na-LSX are capable of covering crystalline domains and interrupting the crystallinity of LSX; thereby, fewer LSX peaks were visible. In general, the LSX diffraction peak attenuation, or loss, in these composites can be best explained by the combined action of pore blocking and partial surface coverage with TiO₂ particles, instead of complete zeolite structure breakdown. TiO₂ (anatase) peak intensities increased from 0.1 to 1.0 M, with slight broadening at 1.0 M, suggesting smaller crystallite sizes or lattice strain, possibly linked to the heavy TiO₂ coverage observed in SEM, TEM, and HR-TEM, as shown in Figure K–N.

1.

(A) XRD spectra of (a) Na-LSX, (b) TiO₂, (c) 0.1, (d) 0.25, (e) 0.5, and (f) 1.0 M TiO₂/LSX. (B) XRD spectra of (a) Na-LSX, (b) TiO₂, (c) Ag⁰/TiO₂, (d) Ag⁰–TiO₂/LSX, (e) Fe⁰/TiO₂, and (f) Fe⁰–TiO₂/LSX.

3.

SEM images of (A) Na-LSX, (B) TiO₂, (C) 0.1, (D) 0.25, (E) 0.5, and (F) 1.0 M TiO₂/LSX, (G) Fe⁰/TiO₂, (H) Fe⁰–TiO₂/LSX, (I) Ag⁰/TiO₂, and (J) Ag⁰–TiO₂/LSX; TEM images of (K) TiO₂, (L) Ag⁰–TiO₂/LSX; and HRTEM of (M) TiO₂, (N) Ag⁰–TiO₂/LSX.

In the doped composites, Ag⁰/TiO₂ exhibited anatase peaks and additional peaks at 2θ = 38.1°, 44.3°, 64.4°, and 77.5° corresponding to the 111, 200, and 220 planes of metallic silver (Ag⁰), confirming reduction, which is found in close agreement with the literature. Ag⁰–TiO₂/LSX displayed peaks of the anatase phase of TiO₂ and Ag⁰, verifying retention of all crystalline phases with uniform Ag⁰ dispersion. The weakening of Faujasite’s diffraction lines is not caused by a decrease in crystallinity but by the significant X-ray absorption of Ag⁰. Fe⁰/TiO₂ retained anatase peaks at 2θ = 25.3°, 37.8°, 48.0°, 54.0°, and 62.7°, with slight broadening and reduced intensity, indicating Fe⁰ incorporation into the TiO₂ lattice without forming iron oxide phases, as shown in Figure B­(e). Fe⁰ peaks present at 2θ = 44.1° and 64.8° are found consistent with that reported by Abdelfatah. Figure B­(f) revealed Fe⁰ peaks present at 2θ = 44° and 64.86° in Fe⁰–TiO₂/LSX that confirmed the presence of metallic iron by the JCPDS card number 01–1252. These peaks appear together with the characteristic anatase peaks aligning with reported literature. The small peaks of Fe⁰ may result from the strong oxidation of Fe⁰ on the zeolite surface. Furthermore, the presence of the peaks of Fe₃O₄ or Fe₂O₃ at 2θ = 35.6° and 43.3° suggests partial surface oxidation that is characteristic of Fe⁰ kept in the air. These findings are also supported by EDX analysis, which confirms the homogeneous distribution of Fe⁰ in the composites. The XRD patterns of the Fe⁰–TiO₂/LSX and Ag⁰–TiO₂/LSX composites illustrate the disappearance of LSX zeolite peaks, which could be attributed to pore blocking and partial surface coverage. The Na-LSX structure comprises aluminosilicate cages that contain Na⁺ ions that counterbalance a negative charge of AlO₄ ^– tetrahedra, and these Na⁺ ions are loosely held and can be easily replaced by other cations with the same charge and size. The decrease in crystallinity is thoroughly explained by the fact that zerovalent metal (Fe⁰ and Ag⁰) and TiO₂ clusters deposited on the zeolite surface partly block the pores and cover the crystalline framework. During synthesis, the starting metal precursors Ag⁺ (1.26 Å) or Fe³⁺ (0.64 Å) may undergo limited exchange with Na⁺ (0.97 Å) sites; because Ag⁺ has a larger ionic radius and greater polarizability, it can substitute Na⁺ in the supercages and sodalite sites where the coordination environment is similar. However, this replacement slightly stretches the local lattice, as Ag⁺ occupies more space and forces distortion of nearby Si–O–Al bonds. In comparison, Fe³⁺ being smaller and carrying a higher charge density, tends to interact more strongly with framework oxygen and could be linked by Fe–O–Si or Fe–O–Al bonds. These new bonds locally disturb the Faujasite lattice, disrupting the long-range symmetry of the lattice and reducing the overall crystallinity. However, further reduction to Fe⁰ and Ag⁰ gives rise to metallic particles, which are attached physically or by surface oxygen links (Ag–O and Fe–O), instead of being substituted by the framework. These Fe⁰ and Ag⁰ particles, along with the TiO₂ coating, lead to partial amorphization and masking of the zeolite diffraction signals. − Pore-blocking and surface coverage are commonly observed in metal-loaded TiO₂/zeolite composites.

Beyond the surface coverage, it is definite that the acidic sol–gel environment (particularly the use of TiCl₄, which hydrolyzes to produce HCl) will lead to partial dealumination or alteration of the structure of the LSX zeolites (e.g., dealumination by strong acid or acid leaching). ,

The loss of the order and, at the same time, the attenuation of the XRD peaks are the results of the leaching of the aluminum through the exposure of the structure to the acid, although the external shape remains the same. The near disappearance of the typical LSX peaks that can be observed is most probably the consequence of the combination of the exposure to strong acids and the reorganization of the structure that simultaneously took place and is not only the coverage by TiO₂ on the surface. The synthesis pathway (i.e., acidic sol–gel and deposition) in the present study probably led to partial framework Al extraction, formation of structural defects or extra-framework aluminum species, and loss of long-range crystallinity. ,

3.2. FTIR Analysis

FTIR analysis confirmed the vibrational modes and functional groups of zeolite composites, as shown in Figure . In the Na-LSX structure, the asymmetric stretching vibration of tetrahedral Si (Al)–O framework bonds is detected by the absorption peak at 958 cm^–1 (Figure A­(a)). Symmetric stretching vibrations of tetrahedral bonds located within and outside the framework are represented by the weaker absorption peaks at 746, 672, and 454 cm^–1. Na-LSX zeolite exhibited strong absorption bands at 3517 cm^–1 (O–H stretching of surface hydroxyls and water), 1644 cm^–1 (H–O–H bending of adsorbed water), and 746 and 454 cm^–1 (Si–O/Al–O framework vibrations), consistent with the reported literature for Faujasite-type zeolites. TiO₂ showed a characteristic peak at 535 cm^–1 by the presence of a Ti–O bond in the TiO₂ anatase phase, with no significant O–H or framework bands. Ti species influence on the zeolite framework causes a minor change in the stretching vibrations of T-O-T or O-T-O (T = Si or Al) in the 1000 cm^–1 region. As can be seen in Figure A­(c-f), the TiO₂/LSX composites displayed combined features, with Ti–O stretching at 958 cm^–1 and LSX framework vibrations at 746 and 454 cm^–1, while the peaks of the O–H (3517 cm^–1) and H–O–H (1644 cm^–1) decreased slightly with increasing TiO₂ concentration, indicating surface coverage or reduced water adsorption.

2.

(A) FTIR spectra of (a) Na-LSX, (b) TiO₂, (c) 0.1, (d) 0.25, (e) 0.5, and (f) 1.0 M TiO₂/LSX. (B) FTIR spectra of (a) Na-LSX, (b) TiO₂, (c) Ag⁰/TiO₂, (d) Ag⁰–TiO₂/LSX, (e) Fe⁰/TiO₂, and (f) Fe⁰–TiO₂/LSX.

The metal-modified composites further revealed additional vibrational characteristics. Ag⁰/TiO₂ and Ag⁰–TiO₂/LSX exhibited a new peak at 1385 cm^–1 attributed to NO₃ ^– (likely from AgNO₃ precursor), alongside Ti–O and Na-LSX framework bands on metal doping effects. The band identification indicates that the band at 1644 cm^–1 in Fe⁰/TiO₂ is produced by the Ti–OH bending vibrations. In Fe⁰/TiO₂, the Ti–OH bending vibrations are shown by the spectral peak at 1640 cm^–1. The C–H bending is responsible for the peak at 1335 cm^–1, whereas the peak at 1644 cm^–1 appears in both Fe⁰/TiO₂ and Fe⁰ alone. The persistence of zeolite framework vibrations across composites confirms structural integrity postmodification, while the reduction in water-related peaks with TiO₂ or metal addition indicates surface alterations.

3.3. Morphological Analysis

SEM analysis reveals that Na-LSX has well-defined octahedral morphology with smooth surfaces, ranging from 3 to 10 μm in size, indicating a suitable support for TiO₂ (Figure ). TiO₂ appeared as spherical microparticles (2–10 μm) with significant clustering, suggesting reduced active surface area due to agglomeration. TiO₂/LSX composites (0.1M) show sparse TiO₂ (2–10 μm) distributed on the zeolite, preserving its octahedral structure. As the TiO₂ concentration increased (0.25, 0.5, and 1.0 M), the coverage increased, forming a uniform layer on the zeolite, with heavy coating and agglomeration at 1.0 M, potentially limiting pore accessibility. As can be seen in Figure (G and H) Fe⁰/TiO₂ revealed TiO₂ microparticles decorated with Fe⁰ particles (2–10 μm), appearing as dark spots, which reduced TiO₂ agglomeration. Fe⁰–TiO₂/LSX showed TiO₂ nanoparticles decorated with Fe⁰ particles (2–10 μm), appearing as dark spots, imparting a rough, sponge-like morphology to the composite, and enhancing surface area. Ag⁰/TiO₂ showed Ag⁰ nanoparticles (2–10 μm) as spheres (bright spots) on TiO₂, indicating uniform dispersion and improved charge separation. Ag⁰–TiO₂/LSX displayed Ag⁰ (2–10 μm) as bright spots on TiO₂, with the LSX zeolite retaining partial visibility and TiO₂ and Ag⁰ evenly distributed as shown in Figure I,J, suggesting a synergistic effect for adsorption and photocatalysis. ,

In Figure K,L, the TEM micrographs indicate that pure TiO₂ is composed of nanosized crystallites, which form irregular and porous agglomerates. The TiO₂ nanoparticles are mostly 10–20 nm, whereas the agglomerates are up to 50–150 nm, which is characteristic of high-surface-energy metal oxides. The differences in contrast between the particles correlated to the difference in thickness and overlapping nanocrystals and there are no secondary phases or impure particles found. The TEM images of the Ag⁰–TiO₂/LSX composite reveal that TiO₂ nanoparticles are successfully deposited on the LSX zeolite framework, and darker contrast spherical spots represent metallic Ag⁰ nanoparticles because of the high density of electrons. Both TiO₂ and Ag⁰ have good spatial distribution on the LSX surface, and no major aggregation to large domains is observed. This reaffirms the development of the Ag⁰–TiO₂/LSX hybrid structure of close interfacial contact.

Figure M,N shows high-resolution TEM images of TiO₂ and Ag⁰–TiO₂/LSX that revealed additional information on the crystallinity and structure of the nanoparticles. The HRTEM image shows a different crystalline region with a clear lattice pattern inside the material. The interplanar distancing measured at 0.32 nm corresponds to the plane (101) of anatase TiO₂ indicating its crystalline phase, as shown in Figure M . The presence of a variety of fringe orientations in single clusters points out that the agglomerates are made of a variety of nanocrystals, with each having its own lattice orientation. This proves that the particles are nonoriented, softly aggregated, but not epitaxially fused. The inset confirms the lattice by showing a more magnified and filtered view. This spacing is consistent with the reported literature that identify anatase TiO₂using HRTEM from its (101) plane around 0.32 nm. The HR-TEM image shows a small crystalline region inside the zeolite particle. Clear and straight lattice fringes can be seen in Figure M,N, which means that the material is well-crystallized. The measured spacing between these fringes is 0.224 nm, as marked in the image of Ag⁰–TiO₂/LSX. The interplanar distancing measured at 0.224 nm corresponds to the plane (111) of Ag⁰, as shown in Figure N. This confirms that silver nanoparticles were successfully formed inside the zeolite structure. The inset image shows a magnified lattice region, which further highlights the periodic arrangement of the atomic planes. The surrounding area appears more amorphous, which is typical for zeolite frameworks, but the silver region is clearly crystalline. These findings are in close agreement with the reported HR-TEM studies, where Ag nanoparticles inside zeolites show d-spacing around 0.224 nm (111). These values confirm that the particle in the image is metallic silver. The EDXA spectrum of the Na-LSX zeolite sample revealed strong peaks for oxygen (0.5 keV), along with clear signals for sodium (1.04 keV), aluminum (1.49 keV), and silicon (1.74 keV), as shown in Figure S1. The elemental composition showed 76.16% O, 3.82% Na, 7.79% Al, and 12.22% Si, confirming the typical aluminosilicate structure of Faujasite-type LSX.

In the pure TiO₂ sample, strong peaks for titanium at 4.5 keV and oxygen at 0.5 keV were observed, confirming the formation of TiO₂ with no foreign elements. For the TiO₂/LSX composites prepared with 0.1, 0.25, 0.5, and 1.0 M precursor concentrations, all samples exhibited peaks for Ti (4.5 keV), O (0.5 keV), silicon (Si) at 1.7 keV, and aluminum (Al) at 1.5 keV, verifying the presence of both TiO₂ and the LSX zeolite framework. The intensity of the Ti peak increased progressively from 0.1 to 1.0 M, indicating an increasing TiO₂ loading. The weight and atomic percentages of the material were summarized in Table , where O (76.16 wt and 84.2 at·%), Si (12.22 wt and 7.7 at·%), Al (7.79 wt and 5.1 at·%), and Na (3.82 wt and 2.9 at·%) are the key elements in the Na-LSX structure. Pure TiO₂, on the other hand, comprised only Ti and O elements (Ti: 30.55 wt and 21.2 at·%; O: 69.45 wt and 78.8 at·%), which is indicative of its stoichiometric composition TiO₂.

1. EDXA Analysis of Na-LSX, TiO₂, 0.1, 0.25, 0.5, 1.0 M TiO₂/LSX, Ag⁰/TiO₂, Ag⁰–TiO₂/LSX, Fe⁰/TiO_2, and Fe⁰–TiO₂/LSX.

sample	elemental composition (%)
	Al		Si		O		Na		Ti		Ag		Fe
	wt·%	at·%	wt·%	at·%	wt·%	at·%	wt·%	at·%	wt·%	at·%	wt·%	at·%	wt·%	at·%
Na-LSX	7.79	5.1	12.22	7.7	76.16	84.2	3.82	2.9	-	-	-	-	-	-
TiO2	-	-	-	-	30.55	78.7	-	-	24.65	21.2	-	-	-	-
0.1 M TiO2/LSX	10.40	8.2	11.01	8.3	53.29	70.5	6.28	5.8	16.28	7.2	-	-	-	-
0.25 TiO2/LSX	8.15	6.3	7.88	5.9	56.82	74.3	5.12	4.7	20.13	8.8	-	-	-	-
0.5 TiO2/LSX	5.44	4.1	5.52	4.0	61.42	78.9	3.32	2.9	22.96	9.9	-	-	-	-
1.0 TiO2/LSX	4.42	3.2	3.84	2.6	69.52	84.2	2.90	2.4	18.60	7.5	-	-	-	-
Ag0/TiO2	-	-	-	-	78.44	93.9	-	-	10.21	4.1	11.35	2.01	-	-
Ag0–TiO2/LSX	4.34	3.8	4.31	3.6	56.01	82.1	1.69	1.7	5.63	2.7	28.02	6.1	-	-
Fe0/TiO2	-	-	-	-	49.29	76.4	-	-	15.12	7.8	-	-	35.60	15.8
Fe0–TiO2/LSX	7.29	6.7	7.59	6.7	42.0	64.9	3.64	3.9	12.00	6.2	-	-	25.95	11.5

When TiO₂ was loaded, the TiO₂/LSX composites showed a progressive rise in the Ti content with the increase in molar ratio, i.e., 16.28 wt·% (7.2 at %) of TiO₂ at 0.1 M TiO₂/LSX, and 22.96 wt·% (9.9 at %) at 0.5 M, which confirmed a successful incorporation of TiO₂. Accordingly, the relative fractions of Si, Al and Na decreased, suggesting gradual coverage of the surface of the LSX framework and partial masking of the pore by TiO₂ particles. This change is further confirmed by the steady increase in oxygen content and is consistent with XRD results of weak LSX peak intensity.

The Ag⁰/TiO₂ composite showed prominent peaks for oxygen (0.5 keV), silver (3.0–3.2 keV), and titanium (4.5 keV). The elemental composition consisted of 78.44% O, 10.21% of the Ti, and 11.35% of the Ag⁰ confirming the successful incorporation of both TiO₂ and metallic silver. The Ag⁰–TiO₂/LSX composite exhibited characteristic peaks for Ti, O, Si, and Al, along with a strong silver (Ag⁰) peak at 3.0 keV, as shown in Figure S1, indicating successful reduction and dispersion of metallic silver within the composite. The Fe⁰/TiO₂ sample showed peaks for Ti and O, along with a distinct iron (Fe) peak at 6.4 keV, confirming successful iron doping. The Fe⁰–TiO₂/LSX composite showed elemental peaks for Ti, O, Fe, Si, and Al, with the Fe⁰ peak again visible at 6.4 keV, confirming its inclusion along with the zeolite support. The Ag⁰–TiO₂/LSX composite also contained clear Ag peaks (28.02 wt and 6.1 at %), and thus Ag⁰ incorporation was successful. Simultaneously, a reduction in Na at % (1.7%) and Si/Al indicates that some of the ion exchange and surface deposition of Ag⁰ particles is taking place, which is consistent with literature accounts of Ag being found in the external pore mouths and supercages. Similarly, the Fe⁰–TiO₂/LSX sample showed a strong Fe signal (25.95 wt·% and 11.5 at·%), which confirmed the existence of metallic iron. The moderate reduction in percentages of framework elements and the higher percentage of oxygen suggest partial oxidation of Fe⁰ to Fe-oxo/hydroxo compounds during synthesis, which proves the possibility of Fe⁰ oxidation to Fe-oxo/hydroxo species during the formation of the element, which was also reported earlier. In all LSX-containing samples, the consistent presence of Si and Al further confirmed the retention of the zeolite framework. The EDX data, supported by these energy-specific peaks, validate the chemical composition and successful synthesis of all zeolite composites. Overall, the EDXA data confirms the successful incorporation of TiO₂, Ag⁰ and Fe⁰ in the LSX matrix, and compositional changes are aligned with the desired molar ratios and structural and textural changes in the XRD and BET analyses.

3.4. BET Analysis

BET analysis, as listed in Table , revealed that the Na-LSX possesses a highly accessible pore system, characterized by high surface area (687.408 m²/g), micropore volume (0.2730 cm³/g), and median micropore size (0.7742 nm), which are found in good agreement with the literature. This open network allows dye molecules to diffuse rapidly and occupy internal adsorption sites, which explains its high removal efficiency. TiO₂ showed a significantly lower surface area of 38.113 m²/g, with a mesopore volume of 0.313 cm³/g, an average mesopore size of 31.937 nm, a micropore volume of 0.0148 cm³/g, and a median micropore size of 0.7518 nm, as shown in Table _. The TiO₂/LSX zeolite composites indicated a decreasing surface area trend from 90.83 m²/g (0.1 M) to 66.983 m²/g (1.0 M) with mesopore volumes ranging from 0.316 to 0.245 cm³/g, average mesopore sizes from 13.369 to 13.887 nm. Micropore volumes of TiO₂/LSX composites range from 0.0351 to 0.0259 cm³/g, and median micropore sizes range from 0.8446 to 0.8332 nm, reflecting increased TiO₂ coverage and reduced porosity with higher molarity. In the TiO₂/LSX composite samples, however, both the micropore volume and BET surface area decrease sharply with the micropore volume dropping from 0.2730 to 0.025–0.035 cm³/g and the BET surface area decreasing from around 687 to 67–91 m²/g. At the same time, the average mesopore diameter narrows significantly (from 45.7 to about 12–14 nm). These changes suggest that TiO₂ particles and the incorporated metal species either fill or coat the large mesopore channels, blocking many micropore entrances. As a result, most internal adsorption sites become inaccessible, and mass transfer into the remaining pores slows down considerably. As a result, the system will no longer rely on high-capacity adsorption like in pure LSX, but rather it will primarily utilize limited surface photocatalysis that results in the observed decline in total removal and adsorption efficiency, and these pore-blocking and surface-coating effects have already been reported earlier for TiO₂/zeolite composites. ,

2. BET Surface Area, Pore Volume, and Size of Different Photocatalysts.

Sample	Surface area (m2/g)	Micropore volume (cm3/g)	Median micropore size (nm)	Mesopore volume (cm3/g)	Average mesopore size (nm)
Na-LSX	687.41	0.27	0.77	0.35	45.70
TiO2	38.11	0.02	0.75	0.31	31.94
0.1 M TiO2/LSX	90.83	0.04	0.84	0.32	13.37
0.25 TiO2/LSX	75.24	0.03	0.85	0.25	12.71
0.5 TiO2/LSX	77.53	0.03	0.84	0.25	12.19
1.0 TiO2/LSX	66.98	0.03	0.83	0.25	13.89
Ag0/TiO2	8.01	0.002	1.09	0.03	13.20
Ag0–TiO2/LSX	46.91	0.02	0.83	0.21	17.18
Fe0/TiO2	55.79	0.02	0.85	0.21	14.60
Fe0–TiO2/LSX	58.23	0.02	0.84	0.18	11.96

The metal-modified composites exhibited varied textural properties. Fe⁰/TiO₂ has a surface area of 55.789 m²/g, a mesopore volume of 0.214 cm³/g, an average mesopore size of 14.602 nm, a micropore volume of 0.0212 cm³/g, and a median micropore size of 0.8514 nm, indicating Fe⁰ incorporation reduced surface area compared to TiO₂. Moreover, Fe⁰–TiO₂/LSX showed a slight increase to 58.234 m²/g, with a mesopore volume of 0.184 cm³/g, an average mesopore size of 11.961 nm, a micropore volume of 0.0224 cm³/g, and a median micropore size of 0.8444 nm, suggesting partial retention of zeolite porosity (Table ). Furthermore, a significant reduction in surface area and pore volume was noted upon loading Fe⁰ onto TiO₂/LSX. The Fe⁰ particles block the pores of TiO₂/LSX. The presence of Fe⁰ in the extremely porous TiO₂/LSX matrix has been determined to reduce its specific surface area by blocking its pores. ^, Despite the fact that Fe⁰ is initially used in its metallic state, its surface is oxidized and hydrolyzed easily during synthesis and in aqueous surroundings, giving rise to the presence of Fe-oxo and Fe-hydroxo clusters. These clusters may build up in the zeolite channels, causing structural disorder and deformation of the pore structure.

Ag⁰/TiO₂ has the lowest surface area of 8.007 m²/g with a mesopore volume of 0.033 cm³/g, an average mesopore size of 13.208 nm, a micropore volume of 0.0017 cm³/g, and a median micropore size of 0.802 nm, likely due to Ag⁰ agglomeration. While Ag⁰–TiO₂/LSX improved to 46.91 m²/g, with a mesopore volume of 0.212 cm³/g, an average mesopore size of 17.182 nm, a micropore volume of 0.0180 cm³/g, and a median micropore size of 0.8319 nm, indicating better dispersion with LSX support. It is quite obvious that the BET surface area of Ag⁰–TiO₂/LSX is larger than Ag⁰/TiO₂ because of the zeolite’s role as a support in the preparation of the Ag⁰–TiO₂/LSX photocatalyst. It is reported in the literature that zeolite has the ability to increase the specific surface area of TiO_2. , Silver is found in the form of Ag⁰ clusters and tends to cluster around the pore openings and agglomerate, which essentially decreases the amount of the micropore and blocks the diffusion routes. This explains the low surface area and low adsorption capacity especially in Ag-containing composites. ,

Not only do these clusters occupy adsorption sites, but they also reduce the effective microporosity. In addition, high surface loading of TiO₂ and metal particles on the reduced size mesopores enhances turbidity in suspension and brings about partial light shielding or aggregation of particles. This decreases the proportion of active photocatalysts when exposed to light, which further decreases photocatalytic performances. Combining the effects of the layer of micropore blockage, mesopores constriction, pore-mouth sealing with metal cluster expanses, and optical shielding, it is possible to explain the reduced efficiency of TiO₂, Ag⁰/TiO₂ and Fe⁰/TiO₂ when incorporated into the LSX structure despite single component performance.

Agglomeration occurs due to the increased level of precursors that stimulates the growth of particles and deposition on pore openings, which restricts the accessibility of micropores and reduces adsorption efficiency, as shown in Figure K,L. The same trend in the BET and adsorption data can be attributed to the correlation between TiO₂ loading, particle dispersion, and pore accessibility. The LSX surface at low concentrations of TiO₂ offers sufficient anchoring sites to allow TiO₂ to cover the surface uniformly, retaining the majority of the microporous structure and high surface area. Nevertheless, with a higher concentration of TiO₂, the nucleation of particles starts to become faster and hits other particles, creating bigger agglomerates. These agglomerates tend to form at the pore openings and block the micropores and internal surface area, eventually decreasing the adsorption capacity. Such observation is common in TiO₂-zeolite composites, in which excess TiO₂ causes severe pore occlusivity and less effective adsorption-photocatalysis synergy.

To mitigate these limitations, there are a number of approaches that may be employed to check particle growth and dispersion. To prevent the agglomeration of TiO₂ nuclei to larger agglomerates, sol–gel synthesis (i.e., with the aid of CTAB or PEG) can be utilized. Anchoring interactions are also enhanced by functionalizing the zeolite surface, which decreases the rate at which the particles move and grow during the calcination process. Moreover, slow rates of hydrolysis, dilute precursor solution, and mild thermal treatment may help to preserve the porous structure and minimize the complexity of pore blockage. The literature has extensively reported these techniques as useful means to reduce agglomeration in TiO₂-based composite photocatalysts.

3.5. Thermal Stability Analysis

TG-DTG spectra are shown in Figure A–J. The Na-LSX zeolite suffered thermal degradation in three distinct stages. In order to measure the water weight loss (%) regarding water types using TG profiles, three temperature scales were chosen. The desorption of physisorbed water was responsible for the initial major weight loss, which was approximately 15.7% measured in the temperature range of 27–197 °C. The second weight loss of 3.2% within the temperature range of 197–297 °C resulted from the dehydration of chemisorbed water. The final weight loss of traces in the range of 297–427 °C decomposition of crystal water in the form of hydroxyl groups of water molecules is consistent with the findings of Panezai et al., on the thermal dehydration kinetics of LSX zeolites. , TiO₂ showed a minor weight loss of 2.1% below 726 °C because moisture on the surface evaporates, and it loses even less weight of 0.1% in the range of 726–900 °C because it loses hydroxyl groups. No significant decomposition takes place, suggesting that TiO₂ is stable up to 900 °C. TiO₂/LSX composites (0.1, 0.25, 0.5, and 1.0 M) displayed combined profiles, with total weight losses of 6.4, 4.6, 4.5, and 3.3% from 30 to 550 °C due to loss of physisorbed water. The second weight loss in the zeolite composites shows dehydration of water from internal cavities of zeolite at 550–900 °C. Ag⁰/TiO₂ and Ag⁰–TiO₂/LSX showed a major weight loss of 9.7% and 3.6% from 31 to 377 °C due to dehydration of physisorbed water and further minor weight loss of 0.4% and 1.5% up to 900 °C due to dehydroxylation, indicating stable Ag⁰ nanoparticles. Fe⁰/TiO₂ and Fe⁰–TiO₂/LSX exhibit an initial weight loss of 6.9 and 5.4% around 255 °C, attributed largely to loss of adsorbed water. Above 600 °C, there are relatively small changes in weight, such as an increase in weight by 1.9% in Fe⁰/TiO₂ and 0.3% in Fe⁰–TiO₂/LSX due to oxidation of the Fe⁰, and minor weight loss of 0.2% from 500 to 600 °C due to dehydroxylation. Lastly, a weight gain of 3.2% was observed in Fe⁰–TiO₂/LSX caused by additional oxidation. The increase in mass of the Fe-containing samples with heating, as observed in our TG-DTG results, is a known indication of oxidation of metallic iron (Fe⁰) to iron oxides (Figure I,J). As Fe⁰ combines with oxygen at higher temperature or when the sample is exposed to air, iron oxides (e.g., FeO, Fe₃O₄, and Fe₂O₃) are formed, and the oxygen atoms that are added to the sample raise the mass of the sample unlike reactions where volatile products are lost to the system. This effect has been observed in thermogravimetric experimental studies of pure iron powders: the first stage of rapid mass gain and the second stage of slower increase in mass when the oxide layer builds up. , Therefore, the increase in weight in our Fe⁰–TiO₂/LSX samples is good circumstantial evidence that there was initially metallic Fe⁰, which was oxidized during the TG run.

4.

TG-DTG spectra of (A) Na-LSX, (B) TiO₂, (C) 0.1, (D) 0.25, (E) 0.5, and (F) 1.0 M TiO₂/LSX, (G) Ag⁰/TiO₂, (H) Ag⁰–TiO₂/LSX, (I) Fe⁰/TiO₂, and (J) Fe⁰–TiO₂/LSX.

3.6. Bandgap Analysis

The optical bandgaps of the TiO₂/LSX zeolite composites were determined through UV–vis absorption spectra analysis using the Tauc relation presented in eq

$${(Ahv)}^n=a(hv-E_g)$$ 6

where A is denoted as absorbance, h is Planck’s constant, v is light frequency, E _g is the material’s bandgap energy, and a is a proportionality constant that works with the value of exponent n, which is 2 for allowed direct transitions and $\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ for allowed indirect transitions.

The following energy in eq is from quantum mechanics

$$E=\frac{hv}{\lambda }$$ 7

In this equation, energy is denoted as the bandgap, represented by E, h denotes Planck’s constant (6.626 × 10^–34 J·s), ν signifies the speed of light (2.99 × 10⁸ m/s), and λ represents the absorption peak wavelength. The bandgap energies of LSX, TiO₂, TiO₂/LSX (0.1, 0.25, 0.5, and 1.0 M), Fe⁰/TiO₂, Fe⁰–TiO₂/LSX, Ag⁰/TiO₂ and Ag⁰–TiO₂/LSX zeolite composites are determined by plotting (Ahv)^1/2 against energy (eV) for indirect gaps, while (Ahv)² was plotted against energy (eV) for direct bandgaps. The linear portion of the (Ahv) ⁿ vs energy (eV) curve are plotted and shown in Figure S2, while the bandgap values are listed in Table .

3. Calculated Bandgaps of Different Photocatalysts.

photocatalyst	bandgap energy V
	direct bandgap	indirect bandgap
Na-LSX	4.82	2.73
TiO2 (Anatase)	3.27	3.14
0.1 M TiO2/LSX	3.26	2.85
0.25 TiO2/LSX	3.16	3.04
0.5 TiO2/LSX	2.97	2.96
1.0 TiO2/LSX	2.90	2.91
Ag0/TiO2	2.64	2.22
Ag0–TiO2/LSX	3.12	2.90
Fe0/TiO2	1.91	1.39
Fe0–TiO2/LSX	2.13	1.75

The bandgap of a semiconductor like TiO₂ is the energy difference between the conduction band, occupied by free electrons, and the valence band where the electrons are localized. This is the minimum energy that the photons must possess in order to excite electrons, which is a fundamental component of the photocatalytic degradation of CV dye under UV irradiation. When only TiO₂ was used, the band gap was about 3.2 eV, as shown in Figure S2, which is characteristic of its anatase phase and well suited for UV-driven photocatalysis due to its efficient electron–hole pair generation under ultraviolet light. Even though both direct and indirect Tauc plots are plotted, and it is known that anatase TiO₂ is primarily an indirect bandgap semiconductor. , Any linear region that is seen in the direct Tauc plot will probably not be a true direct transition. Rather, they can be a result of defect states, oxygen vacancies, surface states, or metal-related energy levels that are added in the process of doping or forming a composite and can distort the absorption edge. , This is why the indirect bandgap value of anatase-based materials is the significant value, and this is the value used to describe the optical properties of the samples. In TiO₂/LSX composites, there is a slight decrease in band gap (3.26 to 2.90 eV, direct) with increasing concentrations of TiO₂ precursors (0.1 to 1.0 M), but this is expected to be caused by interactions between the TiO₂ and the LSX zeolite structure, leading to the formation of lattice defects that reduce the excitation energy necessary. Fe⁰ incorporation of TiO₂ reduced the band gap, 1.91 eV (direct) and 1.39 eV (indirect), as shown in Figure S2. This decrease may be explained by some of the Fe⁰ connections, through the addition of intermediate energy levels in the TiO₂ band gap, which act as electron traps and inhibit the recombination of electron–hole pairs during UV irradiation. Therefore, increased generation of reactive species, including hydroxyl radicals, increases CV dye degradation, reaching a removal efficiency of 84.81%. Likewise, Ag⁰/TiO₂ exhibited a band gap of 2.64 eV (direct) and 2.22 eV (indirect), as shown in Figure S2, which was due to a Schottky junction at the Ag⁰/TiO₂ interface. This crossroad collects photogenerated electrons, reducing recombination and enhancing the availability of oxidative holes, which explains the high 97.45% efficiency of the CV removal. In the case of Fe⁰–TiO₂/LSX and Ag⁰–TiO₂/LSX, the band gaps were somewhat greater (2.13 and 3.12 eV, direct), and it is the moderating effect of the LSX framework, but not the high surface area, that promotes dye adsorption, which facilitates photocatalysis. The reduced bandgaps of doped composites enable the effective excitation of electrons by UV light, which enhances the continuing production of reactive species to effectively degrade CV.

3.7. Photocatalytic and Adsorption Performance

Wide ranges of CV dye removal efficiencies and adsorption capacities are provided via synthesized composites (listed in Table ), offering insights into their practical potential. Pure Na-LSX exhibits a relatively high dye removal and adsorption efficiency, which is attributed to the open and accessible system of pores. Na-LSX zeolite achieved a removal efficiency of 76.39% with an adsorption capacity of 16.45 mg/g, driven by its high surface area (687.408 m²/g), which supports strong physical adsorption of CV, consistent with Faujasite zeolite behavior. The material has an average mesopore size of 45.7 nm that enables dyes to pass through the material and get into adsorption sites inside the material. Pure TiO₂ reached 89.98% efficiency with 19.37 mg/g capacity, reflecting its effective photocatalytic activity under UV light due to anatase’s electron–hole generation. TiO₂/LSX composites showed removal efficiencies (60.31% at 0.1 M to 77.34% at 1.0 M) with adsorption capacities (12.98 to 17.65 mg/g). The 1.0 M TiO₂/LSX composite showed the highest removal efficiency (77.34%) compared to other molarities, likely due to the higher TiO₂ content enhancing its efficiency. Upon the inclusion of TiO₂ into the LSX structure, however, a noticeable change occurs. The mean mesopore size declines drastically to approximately 12–14 nm, and the micropore size slightly increases to approximately 0.83–0.85 nm. Meanwhile, the total surface area and pore volume decrease significantly, indicating the fact that TiO₂ particles partially clog the pores or cover the surface of zeolites. These particles tend to accumulate on the openings of the pores or even lie inside the mesopores, thereby blocking access to the inner channels and reducing the number of active sites for adsorption. Consequently, dye adsorption and removal efficiency of TiO₂/LSX composites is significantly lower as compared to that of Na-LSX and TiO₂.

4. Photocatalytic Removal and Adsorption Performance of CV Dye.

sample	removal efficiency (%)	adsorption capacity (q t , mg/g)
Na-LSX	76.39	16.45
TiO2	89.98	19.37
0.1 M TiO2/LSX	60.31	12.98
0.25 TiO2/LSX	62.01	13.35
0.5 TiO2/LSX	63.91	13.61
1.0 TiO2/LSX	77.34	17.65
Ag0/TiO2	97.45	20.98
Ag0–TiO2/LSX	79.92	17.21
Fe0/TiO2	84.81	18.26
Fe0–TiO2/LSX	88.29	19.02

The lower adsorption efficiency of the TiO₂/LSX composites compared with that of pure Na-LSX can be attributed to the lower textural properties of these composites. The analysis of BET reveals that the surface area of the 0.1–1.0 M TiO₂/LSX composites (75.2–90.8 m²/g) was lower than that of Na-LSX (687.4 m²/g), and the micropore volume was reduced from 0.27 cm³/g of Na-LSX to 0.03–0.04 cm³/g of composites. This significant loss in accessible micropores and surface area is associated with the reduced crystal violet dye removal (60–77% for composites and 76.39% for pure LSX), which also indicates that pore blockage by TiO₂ plays a crucial role in limiting adsorption capacity. , These results quantitatively confirm that the inclusion of TiO₂ provides photocatalytic activity but also disrupts the preconcentration properties of the zeolite because of structural coverage.

The 1.0 M TiO₂/LSX composites have a lower surface area (66.98 m²/g), although it still has a higher dye removal (77.34%), which indicates that the process of photocatalysis would take over at high concentrations of TiO₂. The greater the concentration of TiO₂ the more photocatalytically active sites there are and the better the photon absorption rate, enabling the production of more electron–hole pairs, hydroxyl radicals, and superoxide radicals. These ROS species accelerate the degradation of dyes in the case when adsorption is restricted. This tendency is thoroughly described: even when TiO₂ loading goes beyond the range of optimal adsorption, photodegradation can be enhanced due to the high dependence of catalytic activity on the amount of semiconductor, but not only on surface area.

In addition, the LSX support remains effective even at high TiO₂ loading to enhance charge separation and decrease electron–hole recombination that is essential in preserving photocatalytic efficiency. Interfaces of TiO₂/zeolite facilitate electron transfer between TiO₂ and the aluminosilicate framework, increasing the number of radicals and reducing losses of recombination. Even though the agglomeration decreases the pore accessibility and adsorption capacity, the contribution of photocatalysis is still high enough to generate greater total dye removal in the 1.0 M TiO₂/LSX sample. That is why this concentration of TiO₂ remains the best despite the anticipated surface area constraints.

Fe⁰/TiO₂ achieved 84.81% efficiency with 18.26 mg/g capacity, benefiting from its reduced 1.91 eV bandgap, while Fe⁰–TiO₂/LSX reached with 88.29% with 19.02 mg/g, moderated by LSX’s influence. Ag⁰/TiO₂ showed the highest efficiency at with 97.45% with 20.98 mg/g capacity, driven by Ag⁰’s plasmonic enhancement, though Ag⁰–TiO₂/LSX dropped to 79.92% with17.21 mg/g, likely due to Ag⁰ agglomeration in porous LSX covering the active sites for adsorption, which leads to reduction in surface area.76 The same trend has been observed with Ag⁰–TiO₂/LSX and Fe⁰–TiO₂/LSX composites. Upon the addition of the Ag⁰ (0.05 μm) or Fe⁰ (0.90 μm), they are likely to disrupt the structure internal pores by creating small aggregates of Ag or Fe oxides. These clusters are able to fill in the mesopores to some extent and even block some of the micropores, as is also reported by Sun and Tasharrofi, , which leads to a reduction in the average mesopore size by a significant margin from 45.7 nm in Na-LSX to 17.18 and 11.96 nm in the Ag-modified and Fe-modified samples, respectively. The smaller pore size makes it harder for the diffusion of dye molecules into the inner channels of the zeolite, which reduces the overall rate of adsorption. Moreover, the introduction of metal ions may alter the local crystal structure and reduce the quantity of hydroxyl groups on the surface to bind the dye. Thus, although TiO₂, Ag⁰/TiO₂, and Fe⁰/TiO₂ are quite active individually, they do not exhibit good adsorption behavior or photocatalytic activity with LSX. This reduction mainly arises from pore blockage, limited dye accessibility, and partial damage to the zeolite framework.

The large particle size and reduced surface area observed in SEM images and BET analysis are indeed key factors limiting the performance of metal-loaded TiO₂/LSX composites. Although the bandgap decreases upon metal modification, this advantage cannot be fully utilized because photocatalysis under UV irradiation depends strongly on the presence of active sites on the surface and effective adsorption. The mechanical processing and deposition of TiO₂ reduces the BET surface area of the TiO₂/LSX composites to 66–90 m²/g when compared to Na-LSX (687 m²/g), as shown in Table . Clusters of TiO₂ can be seen by using SEM as well as TEM and HR-TEM images. Such loss of surface area and microporosity is expected in TiO₂/zeolite composites, where it is noted that the TiO₂ forms clusters that partially block zeolite pores. , This decreases the number of active sites in the surface area, and despite a decrease in bandgap, the photocatalytic activity also does not enhance in direct proportionality. Literature supports the idea that bigger particles and agglomeration have a strong effect in decreasing the surface area, and this directly decreases the density of the surface hydroxyl groups and adsorption sites required to produce radicals. , In addition, it has been demonstrated that a smaller bandgap does not necessarily enhance photocatalysis when the material has poor textural properties or particle aggregation. However, in UV-driven photocatalysis, surface area is not the only determining factor of activity. It has been noted that high crystallinity, good effective dopant dispersion, and efficient separation of the electron–hole pairs affect the photocatalytic degradation rates. The Ag⁰ and Fe⁰ dopants improve charge separation and electron trapping, which can balance the impact of lower active surface sites. In addition, the LSX framework still provides microporous domains that pack the dye molecules around the active photocatalytic sites of TiO₂ enhancing its effective degradation, which explains the low external surface area. Thus, our findings are found to be consistent with the literature: the possible advantage of reducing the bandgap is compensated by the active sites lost due to particle agglomeration.

Figure shows the UV–vis absorbance spectra of CV dye removal via Na-LSX, TiO₂, TiO₂/LSX, Ag⁰/TiO₂, Ag⁰–TiO₂/LSX, Fe⁰/TiO₂, and Fe⁰–TiO₂/LSX composites. The photocatalytic activity of the prepared samples was investigated for the removal of CV dye from an aqueous solution in the presence of UV light. In the case of Na-LSX zeolite, the absorbance has slightly decreased because the CV dye molecules were adsorbed on the surface of the zeolite. In addition, pure TiO₂ shows a more pronounced decrease in absorbance due to its photocatalytic activity under UV light, as shown in Figure A,B. CV dye removal is also improved by the TiO₂/LSX zeolite composites of various molarities (0.1, 0.25, 0.5, and 1.0 M) because LSX offers adsorption sites and TiO₂ degrades the dye, thus creating a synergistic effect. At higher TiO₂ loadings, the absorption peaks drop more gradually, with increased degradation in a shorter period. As can be seen in Figure G–J, the spectra indicate that further doping of Ag⁰ and Fe⁰ on TiO₂/LSX zeolite composites results in an even stronger reduction of the intensity of the absorbance, which implies that the efficiency of the photocatalyst is greater. This is improved by enhanced charge separation and light absorption because of the dopants, which results in enhanced degradation of CV dye.

5.

UV–vis absorbance spectra of CV dye: (A) Na-LSX, (B) TiO₂, (C) 0.1, (D) 0.25, (E) 0.5, and (F) 1.0 M TiO₂/LSX, (G) Ag⁰/TiO₂, (H) Ag⁰–TiO₂/LSX, (I) Fe⁰/TiO₂, and (J) Fe⁰–TiO₂/LSX.

3.8. Adsorption and Photocatalytic Degradation Kinetics

The kinetics study of CV dye adsorption and photocatalytic degradation on TiO₂-supported LSX composites revealed distinct mechanisms governing each process, as shown in Table . Analysis of adsorption kinetics showed that across all samples the pseudo-second-order (PSO) model provided a significantly better fit than the pseudo-first-order (PFO) model, indicated by higher R ² values listed in Table . These findings indicate that the pseudo-second-order model is a significantly better fit to all of the catalysts, most of them giving R ² values over 0.97, which means that adsorption is primarily influenced by surface interaction and availability of active sites and not just by simple diffusion.

5. Kinetic Analysis of the Adsorption and Degradation Processes of CV Dye.

		Adsorption						Degradation
		Pseudo-1st order model			Pseudo-2nd order model			Pseudo-1st order model
Sample	q e (exp)	q e (calc)	K 1	R 2	q e (calc)	K 2	R 2	K app	R 2
Na-LSX	4.1407	1.3730	0.0619	0.1947	4.0833	0.4537	0.9981	0.0069	0.9463
TiO2	4.8711	1.6767	0.0347	0.0981	5.6402	0.7686	0.9994	0.0823	0.9895
0.1 M TiO2/LSX	2.27	1.25	0.0478	0.1892	2.68	0.29	0.9894	0.0186	0.9031
0.25 TiO2/LSX	4.1407	1.9919	0.0525	0.3205	4.4964	0.3206	0.9964	0.0161	0.9825
0.5 TiO2/LSX	4.4123	1.3932	0.0705	0.1892	4.1964	0.4312	0.998	0.0137	0.9673
1.0 TiO2/LSX	4.3681	1.6572	0.0606	0.2367	4.7037	0.4036	0.9978	0.0486	0.9152
Fe0–TiO2/LSX	3.9818	1.8923	0.069	0.2581	4.9164	0.3283	0.9974	0.0041	0.8722
Ag0–TiO2/LSX	2.2373	1.2898	0.0018	0.0052	3.2755	0.1556	0.9715	0.0005	0.981

To illustrate this, Na-LSX has an R ² of 0.9981 with the pseudo-second-order model and 0.1947 with the pseudo-first-order model, which proves the second-order model is more suitable. The similar trend is followed by pure TiO₂ (R ² = 0.9994), 0.25 M TiO₂/LSX (R ² = 0.9964), and 0.5 M TiO₂/LSX (R ² = 0.998). The higher R ² values show that adsorption capacity increases rapidly at the start, followed by equilibrium caused by filling of available sites. The experimental qe values also correlate with the calculated qe values of the pseudo second-order model. This agreement implies that adsorption primarily takes place by surface interaction between dye molecules and the catalyst on the surface. This type of behavior is usually observed in dye adsorption of zeolite-based and TiO₂-based materials in which electron exchange, electrostatic attraction, or surface complexation is the major adsorption mechanism as opposed to simple physical diffusion.

The rate laws of the photocatalytic degradation of crystal violet dye under UV light were pseudo-first-order for all samples. The rate constants (K _app) of degradation appear to be clearly different between the catalysts. Pure TiO₂ has a high rate constant (0.0823 min^–1), which indicates the high intrinsic photocatalytic activity of this material. The 1.0 M TiO₂/LSX sample has a higher rate of degradation (0.0486 min^–1) as compared to the lower-loading samples, indicating that an increase in the TiO₂ loading causes the reaction to proceed primarily by surface photocatalysis and no longer by adsorption (Table ). The trends of the metal-modified samples are different. Ag⁰–TiO₂/LSX has a very low K _app value (0.0005 min^–1), which is in line with its low surface area and pore blockage due to deposition of heavy metals. Nevertheless, Fe⁰–TiO₂/LSX exhibits moderate kinetics (0.0041 min^–1), which implies that even in the case of pore structure blockage, Fe⁰ is capable of supporting electron transfer. These trends can further be seen in Figure A,C where adsorption data overlap PSO plots (linear correlation) and the degradation data overlap PFO plots (linear correlation) for 0.1 M TiO₂/LSX composites supporting the model assignments. These findings are related to reports that photocatalytic degradation of TiO₂ typically follows pseudo-first-order kinetics and that metal particles have the ability to change charge separation efficiency and degradation rates.

6.

Pseudo second-order model vs adsorption time (A); pseudo first-order model vs adsorption (B); peudo first-order model vs degradation (C).

4. Conclusions

The present study reveals that Ag⁰ and Fe⁰-doped TiO₂ composites are effective photocatalysts in the degradation of CV dye under UV light, which can be used as a sustainable method for treating wastewater. The composites were synthesized by a sol–gel and borohydride reduction procedure; TiO₂/LSX zeolite composites were synthesized with 0.1–1.0 M and metals (Ag⁰ or Fe⁰) doped. X-ray diffraction patterns identified the presence of the anatase phase of TiO₂ and partial retention of the Na-LSX structure, although there was a minor interference at increased TiO₂ loadings. An octahedral Na-LSX, spherical TiO₂ with a size of 5–10 μm, and evenly dispersed metals are exhibited by SEM and supported by EDXA spectroscopy. BET analysis revealed the large surface area of 687.408 m²/g of Na-LSX, which was decreased by the addition of TiO₂ and thermogravimetric analysis confirmed the thermal stability of zeolite composites up to 600 °C. Photocatalytic analysis revealed that Ag⁰/TiO₂ and Fe⁰/TiO₂ proved to be the most efficient photocatalysts with a removal efficiency of 93.84 and 84.81% and an adsorption capacity of 8.36 and 18.26 mg/g, respectively, due to their lower bandgaps of 2.64 and 1.9 eV, respectively. This increases the electron–hole pair generation in UV light. However, TiO₂/LSX (0.1, 0.25, 0.5, and 1.0 M) showed comparatively lower removal efficiencies in the range from 34.81 to 55.84 and Ag⁰–TiO₂/LSX showed 79.92% removal efficiency. Structural studies indicated that deposition of TiO₂ blocked zeolite pores and deformed the LSX structure, whereas Ag⁰and Fe⁰ particles were likely to agglomerate on the surface, thus decreasing the available active sites. These observations highlight the fact that preserving the structural integrity and the textural properties of the zeolite is necessary to realize the actual synergistic behavior in such hybrid systems. The major problems are the disruption of zeolite frameworks, blocking of pores, and Ag⁰ and Fe⁰ agglomeration, which call upon the need for optimized synthesis conditions. Future efforts should be directed to better optimization of synthesis and post-treatment conditions, the use of milder processing parameters, and alternative methods of metal loading or deposition to improve metal dispersion and maintain zeolite crystallinity to enhance composite performance. By addressing these areas, this research lays a solid foundation for developing sustainable, cost-effective, and environmentally friendly photocatalysts, contributing significantly to the global effort to protect and preserve water resources.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c11853.

• Elemental data of zeolite, titania, and their various composites (Figure S1); Direct and Indirect bandgap of zeolite, titania, and their various composite (Figure S2) (PDF)

The authors declare no competing financial interest.