From Organic Waste to Clean Fuel and Water: Plant-Extract-Assisted TiO₂ Nanoparticles for Simultaneous 2-Naphthol Degradation and H₂ Production

Abstract

The development of sustainable technologies capable of simultaneously addressing environmental pollution and renewable energy production remains a major scientific challenge. In this work, titanium dioxide nanoparticles (GTiO₂) were synthesized through a plant-extract-assisted route using Punica granatum (pomegranate) peel extract and subsequently modified with platinum nanoparticles (Pt NPs) to obtain an efficient photocatalyst for the photoreforming of organic pollutants. The resulting Pt-GTiO₂ material exhibited an anatase crystal structure with an average crystallite size of approximately 12 nm and a specific surface area of about 140 m² g⁻¹. Comprehensive characterization using XRD, BET, TEM, FTIR, Raman, and photoluminescence spectroscopy (PL) revealed favorable structural and optoelectronic properties that promote efficient charge separation. The photocatalytic performance of Pt-GTiO₂ was evaluated through the simultaneous degradation of 2-naphthol, a priority aromatic pollutant, and hydrogen evolution under simulated solar irradiation in anaerobic conditions. Under the investigated conditions, Pt-GTiO₂ effectively promoted 2-naphthol degradation, with substantial but incomplete mineralization, as confirmed by TOC removal. The synthesized catalyst showed degradation efficiency higher than Pt-UV100 and comparable to Pt-P25, while exhibiting superior hydrogen evolution when compared with Pt-P25. Mechanistic investigations combining scavenger experiments, electron paramagnetic resonance (EPR) spectroscopy, and the identification of reaction intermediates suggest that photogenerated holes play a major role in the initial oxidation step under the mechanistic test conditions. The detected intermediates indicate that photoreforming proceeds via multiple pathways, including hydroxylation, ring-opening, reduction, and fragmentation. These findings highlight the potential of biogenic TiO₂-based photocatalysts for converting hazardous organic pollutants into clean hydrogen fuel while simultaneously achieving wastewater purification, offering a promising route toward sustainable environmental and energy technologies.

1. Introduction

The growing demand for sustainable energy and the rapid increase in environmental pollution represent two major challenges facing modern society [1]. The heavy reliance on fossil fuels to meet the global energy demand has significantly contributed to climate change [2,3]. At the same time, conventional wastewater treatment methods are often insufficient to remove the large volumes of complex organic pollutants released daily from industrial and human activities [1,4]. Therefore, the development of integrated technologies capable of producing clean energy while simultaneously treating contaminated water is urgently needed.

As a promising alternative to fossil fuels due to its clean combustion, which produces only pure water, and its efficiency due to its exceptionally high heat-to-weight density, molecular hydrogen has become an attractive renewable energy carrier [5,6]. However, despite these advantageous properties, its traditional production methods, like water electrolysis and steam methane reforming, often rely on the consumption of large amounts of water and energy, exacerbating the problems of increasing water scarcity and increasing energy demand [5]. Therefore, there is an urgent need for renewable and sustainable hydrogen production pathways that do not rely primarily on the consumption of traditional energy resources and drinking water sources [7,8,9].

Semiconductor photocatalysis is a light-driven process that accelerates redox reactions at a catalyst surface which has been widely investigated for environmental remediation, fuel generation, and the synthesis of valuable organic compounds. The photoexcited catalysts can drive selective redox transformations under mild conditions [4,10]. In this context, photocatalytic reforming of organic pollutants could be one of the most attractive and sustainable methods as an alternative for H₂ production [2,4,5,6]. In fact, this novel approach offers a dual benefit through simultaneous wastewater treatment and transformation of the chemical energy stored inside the organic pollutants into sustainable H₂ [11,12]. Because this process relies primarily on solar energy, it aligns with sustainability principles by repurposing wastewater streams into valuable products, including a sustainable energy carrier and purified water, rather than releasing them into the environment [13].

2-Naphthol serves as a necessary raw and intermediate material for multiple chemical syntheses and industries, such as pigments, dyes, insecticides, fungicides, pharmaceuticals, and other chemicals. In addition, it is a widespread by-product contaminant in effluents from various industries, such as those originating from the petroleum, textile, and coal sectors, mainly due to its relatively higher water solubility [14]. Due to its derivation from polycyclic aromatic hydrocarbons (PAHs) and its human toxicity, 2-naphthol is classified as a priority pollutant [14,15]. Although some literature studies suggest that 2-naphthol is “readily biodegradable” under controlled and specific laboratory conditions, other sources highlight its resistance and slow degradation rates in the environment [16,17]. The main concern regarding this compound is its accumulation in aquatic ecosystems, enabling it to eventually enter the food chain [17,18,19]. As traditional wastewater purification methods are insufficient to remove this compound and its persistent derivatives, new technologies are urgently needed to eliminate these pollutants without producing equally or more toxic by-products. Recently, several studies have focused on the implementation of advanced oxidation processes (AOPs), such as photocatalysis, to eliminate these persistent pollutants from the environment [4,11].

Recently, bifunctional heterogeneous photocatalysis based on semiconductors has received increasing attention as a sustainable, cheap, and environmentally friendly strategy for both solar energy harvesting and environmental pollution remediation [4,20,21]. Among the broad number of candidate semiconductor materials, titanium dioxide nanoparticles (TiO₂) and their derivatives stand out as the most widely studied, engineered, and utilized due to their low toxicity, chemical stability, low cost, and high photoreactivity. These nanoparticles, with the aid of solar energy, can mineralize a broad spectrum of organic pollutants into harmless carbon dioxide (CO₂) and water [15,22], while converting the chemical energy stored in these pollutants into a sustainable, green carrier under specific conditions.

The conventional and industrial methods for synthesizing TiO₂ NPs involve the use of harsh and toxic chemicals such as Titanium(III) chloride, Titanium(IV) chloride, and titanyl sulfate. These chemicals pose a threat to the environment by producing hazardous waste [4,20,23]. Therefore, it is essential to transition toward more environmentally benign synthetic routes to achieve sustainability. Recently, the plant-extract-assisted synthesis of TiO₂ NPs based on plant or plant residue extracts, such as pomegranate peels, has been considered a useful alternative method for the synthesis of environmentally friendly, cost-effective, simple, and scalable materials [23,24]. This approach relies on naturally occurring reducing and capping agents extracted from plants using a suitable solvent, such as water, to facilitate the stabilization of the formed nanoparticles [25]. Several research groups have reported the successful synthesis of various metal oxide nanoparticles using a broad spectrum of plant or plant residue extracts [25,26]. Mohammadi et al. synthesized zinc oxide nanoparticles using Punica granatum fruit peel extract [27]. Moghni et al. prepared TiO₂ NPs via a plant-extract-assisted synthesis using Inula viscosa aqueous extract [28]. In this study, an aqueous extract of Punica granatum (pomegranate) peels was used for the synthesis of TiO₂ NPs due to its rich composition of bioactive compounds, including tannins, polyphenols, flavonoids, and carboxylic acids, which function as natural reducing and stabilizing agents. This approach is perfectly consistent with circular economy principles for the transformation of biowaste materials into higher-value products [23,25].

Despite extensive research on TiO₂-based photocatalysis, most previous studies have focused primarily on either pollutant degradation or hydrogen production as separate processes. Furthermore, many investigations employ simple sacrificial agents such as methanol or oxalic acid, which do not adequately represent the complexity of real organic contaminants present in industrial wastewater. In addition, conventional synthesis routes for TiO₂ often involve environmentally hazardous reagents, contradicting the sustainability principles that photocatalytic technologies aim to address.

This work presents a sustainable strategy that integrates plant-extract-assisted synthesis of TiO₂ nanoparticles using pomegranate peel extract with bifunctional photocatalytic reforming of the priority pollutant 2-naphthol for simultaneous wastewater remediation and hydrogen production. TiO₂ photocatalysis, Pt cocatalysts, and green nanoparticle synthesis are well established individually, and many previous studies have shown that TiO₂-based photocatalysts can couple pollutant oxidation with H₂ evolution [29,30,31,32]. However, their integration for the anaerobic photoreforming of priority aromatic pollutants remains insufficiently explored, as shown in the comparison of literature reports in Table S1. The novelty of the present work lies in coupling the photocatalytic reforming of 2-naphthol over pomegranate peel extract-assisted TiO₂ loaded with Pt NPs under oxygen-free and simulated solar light irradiation conditions, which was systematically investigated and accompanied by a detailed mechanistic analysis. The study combines comprehensive physicochemical characterization of the Pt-GTiO₂ catalyst with kinetic analysis, benchmarked directly against Pt-P25 and Pt-UV100 under identical reaction conditions, intermediate identification using GC-MS and ion chromatography, and reactive species investigation through scavenger experiments and EPR spectroscopy. This integrated approach provides new insights into the photoreforming pathways of aromatic pollutants, thereby contributing to the development of sustainable photocatalytic systems capable of converting organic pollutants into clean hydrogen fuel.

2. Materials and Methods

2.1. Raw Materials

The benchmark TiO₂ used in this study includes Aeroxide (P25, Evonik Corporation, Champaign, IL, USA) and Hombikat (UV100, Sachtleben Chemie GmbH, Duisburg, Germany). The following reagents were sourced from Sigma-Aldrich (St. Louis, MO, USA): Titanium Tetra Isopropoxide (TTIP, C₁₂H₂₈O₄Ti, 97%), naphthalen-2-ol (99%), chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O, 99.9%), potassium iodide (99.5%), ethyl acetate (GC grade), 5,5-Dimethyl-1-pyrroline N-oxide (DMPO, ≥98.0%), and barium sulfate (≥99%). Methanol (HPLC grade), hydrochloric acid (37 wt%), and tert-butanol (≥99%) were purchased from Carl Roth (Karlsruhe, Germany). OASIS HLB and Sep-Pak C18 Cartridges, 500 mg Sorbent (6 mL 500 mg⁻¹) were obtained from Waters^® (County Wexford, Ireland). All chemicals in this study were used in their received form without any additional purification. Milli-Q water (18.2 MΩ·cm at 25 °C) was prepared locally in the lab using the Millipore Milli-Q system.

2.2. Plant-Extract-Assisted Synthesis of TiO₂ Nanoparticles

A plant-extract-assisted method was used for the synthesis of titanium dioxide nanoparticles by incorporating a biowaste extract—namely, pomegranate peel extract—as a capping agent. The term plant-extract-assisted synthesis is used here to describe the use of aqueous pomegranate peel extract as a renewable source for capping and stabilizing agents that can assist particle formation and stabilization. Indeed, the sustainability advantages of this approach are relative. Although this route reduces the need for harsh and toxic organic compounds, it is not a completely impact-free synthesis. The process still uses a titanium precursor, washing solvents, and calcination at 500 °C, all of which carry environmental and energy costs [25].

Pomegranate fruit was purchased from a local market in Al Al-Karak province, Jordan. The collected fruit was cleaned twice with tap water and finally rinsed with Milli-Q water to ensure the removal of dust and potential contaminants. After drying, the peels were separated from the fruit, washed with Milli-Q water, chopped into small pieces, and left to dry at room temperature inside a clean room in the absence of direct sunlight contact for around 2 weeks. The resulting dried peels were then mixed very well and pulverized into a powder using an electric grinder apparatus. To prepare the extract solution, 50 g of the well-mixed peels’ fine powder was suspended in 100 mL of Milli-Q water, gently heated, and stirred at 50 °C/600 rpm for 5 h on a hot plate. After cooling, the resulting extracted solution was filtered twice using Whatman filter paper No. 1, stored in an amber glass bottle, and finally stored at 4 °C in a refrigerator for further analysis.

The TiO₂ nanoparticles sample (GTiO₂) was synthesized by adapting Aravind et al.’s method [33] with some modifications. First, 10.0 mL of titanium (IV) isopropoxide was placed inside a 250.0 mL Erlenmeyer flask and dissolved in 100.0 mL of Milli-Q water under vigorous stirring. To this solution, 50.0 mL of the pomegranate peel extract was added dropwise, controlling the addition rate at around 0.5 mL min⁻¹, under continued vigorous stirring at room temperature. Afterward, stirring of the resulting solution continued for an additional 3 h. During this stage, the solution color changed from pure white to yellowish, confirming the successful formation of the GTiO₂ nanoparticles. The obtained suspension was then filtered using Whatman filter paper No.1 to separate the formed NPs. The collected nanoparticles were washed twice with methanol and then repeatedly with Milli-Q water to remove organic and inorganic residues. Finally, the collected nanoparticles were dried overnight at 100 °C, then calcinated in a Muffle furnace at 500 °C for 4 h.

2.3. Platinization of the TiO₂ Materials

Platinization of the TiO₂ samples, GTiO₂, UV100, and P25, was performed using a refined photodeposition method, adapted from a protocol established by Al-Madanat et al. [34]. Initially, all TiO₂ nanoparticle samples were calcined at 400 °C under a continuous flow of oxygen for 2 h. Once cooled, a specified amount of bare TiO₂ nanoparticles was suspended in 100 mL of Milli-Q water inside a 150 mL borosilicate photoreactor. The suspension was sonicated for 60 min, after which the appropriate quantity of platinum precursor (H₂PtCl₆) was introduced to achieve the desired weight percentage. The resulting suspension was stirred overnight to promote the adsorption of Pt (IV) ions onto the surface of the TiO₂ nanoparticles. Following this, the system was purged with pure argon for 30 min to eliminate any oxygen. The mixture was then illuminated at room temperature for 30 min. After this initial period, 5 mL of methanol was added to the system, and the suspension was illuminated for an additional 5 h. Ultimately, the platinized material was centrifuged, washed three times with methanol, followed by several washes with Milli-Q water, and dried at 120 °C overnight. The obtained samples were labeled Pt-GTiO₂, Pt-UV100, and Pt-P25.

2.4. Photocatalytic Experiments

In each typical experiment for the photocatalytic H₂ evolution and 2-naphthol photooxidation, 1.0 mg mL⁻¹ of photocatalyst powder was suspended in a 20.0 ppm solution of 2-naphthol inside a 20.0 mL glass vial sealed with a silicone septa crimp cap, or a 50 mL borosilicate photoreactor. The suspension was then purged with high-purity argon gas for 30 min. The concentration of 2-naphthol was adjusted according to the specific requirements of each experiment, while its concentration before, during, and after the photocatalytic process was precisely determined (three times) through the High-Performance Liquid Chromatography with ultraviolet detection (HPLC-UV) technique. After preparing the suspension based on the reaction conditions, the vials were horizontally positioned within an orbital shaker water bath to maintain the reaction temperature at 25 °C ± 2 °C. Afterward, the samples were left in the dark for around 1 h to establish the adsorption–desorption equilibrium. The distance between the solar simulator and the shaker was adjusted to 30 cm, while the simulator consisted of a chamber designed to reflect the light from all directions and was fitted with a xenon lamp (1000 W, Hönle Technology, Munich, Germany). This light source was equipped with a specialized filter to reduce the UV irradiation. The photon flux density was measured from the spectral irradiance of the lamp in the wavelength range between 320 nm and 380 nm at the irradiated window of the photoreactor and was found to be I₀ = 3.31 × 10⁻⁴ mol m⁻² s⁻¹. All photocatalytic experiments concerning the photooxidation of 2-naphthol and H₂ production were conducted in triplicate, yielding a relative standard deviation (RSD) of less than 10%. Additionally, control experiments were performed using pristine GTiO₂ and pristine commercial TiO₂ materials (P25 and UV100), besides the corresponding Pt-loaded samples. Bare GTiO₂ was used as the plant-extract-assisted synthesized TiO₂ control without Pt, while P25 and UV100 represent commercial TiO₂ references without plant extract prepared in conventional methods. Pt-P25 and Pt-UV100 were included as Pt-loaded controls without the pomegranate peel extract. All samples were tested under identical reaction conditions to separate and distinguish the effects of Pt loading from the influence of the TiO₂ support.

For scavenger experiments, the initial 2-naphthol concentration was 20.0 ppm, the Pt-GTiO₂ loading was 1.0 mg mL⁻¹, and the suspension volume was 50.0 mL. The required amounts of the KI and tert-butyl alcohol (TBA) solutions were added to the reaction suspension to achieve a final concentration of 20.0 mM prior to the dark equilibration period. Afterward, the reactor was sealed, purged with argon for 30 min, kept in the dark for another 60 min, and finally illuminated under a Philips CLEO 15 W UV lamp at 25 °C [6].

2.5. Analytical Methods

To quantify the concentration of 2-naphthol and the total organic carbon in the reaction medium during or after each experiment of 2-naphthol photooxidation, a 1 mL sample of the reaction suspension was drawn using a disposable syringe pre-purged with argon. This sample was immediately filtered through a 0.2 µm PTFE online filter and then stored in a 1.5 mL amber glass GC vial. Finally, the samples were subsequently either injected immediately after each experiment or analyzed within 24 h in the chromatographic system or analytical equipment.

The extent of 2-naphthol mineralization during the photocatalytic process was determined employing a total organic carbon (TOC) analyzer (Shimadzu 5000A, Kyoto, Japan) to monitor the decrease in the TOC from the system medium.

The precise and accurate concentration of 2-naphthol was determined using the High-Performance Liquid Chromatography (HPLC) technique, equipped with UV-VIS detectors (Merck L 6200A, Hitachi, Chiyoda, Tokyo). The detector operated at 274 nm, and the temperature of the Nucleosil-C18 column (ID: 25 cm × 0.4.0 cm × 5.0 µm) was maintained during the analysis period at 35 °C. A gradient mobile phase consisting of methanol (A) and 1.0% acidified Milli-Q water (B) at a 1.0 mL/min flow rate during the analysis period was used to efficiently separate the present 2-naphthol in the 50 µL injected sample. Prior to the analysis, the solvents were degassed with argon gas for a period of 15 min. The eluent composition was initiated with 55% of methanol, which linearly increased the percentage level up to 85% over a period of 10 min and then maintained for another 2 min. In the final stage, the methanol percentage increased over a period of 2 min to 95%, where it was then maintained for another 4 min before it decreased to its initial stage to stabilize the system for the next analysis.

The photocatalytic H₂ evolution during the photoreforming of 2-naphthol was detected using gas chromatography (Shimadzu GC-8A, Kyoto, Japan) equipped with a thermal conductivity detector (GC-TCD) and quantified through building a standard calibration curve. At specific irradiation time intervals, the gas sample from the headspace of the reaction reactor was taken through a 50 µL Valco gas-tight sampling syringe, which was pre-purged with high-purity argon gas. Immediately, the collected sample was injected into the preheated injector of the system, whose temperature was fixed at 120 °C, as well as the TCD temperatures. Separation was performed inside a Molecular Sieve column (5 Å, Sigma-Aldrich, St. Louis, MO, USA), placed inside an oven to maintain the column temperature at 80 °C during the analysis time.

The electron paramagnetic resonance (EPR) experiments were conducted to investigate the formation of photogenerated hydroxyl radical (OH^•) using an X-band MiniScope MS400 spectrometer (Magnettech GmbH, Berlin, Germany) at a frequency of 9.42 GHz. A UV spot-light (LC8, Hamamatsu, Japan, 200 W mercury−xenon lamp) was used to illuminate the solution samples inside the instrument using a small quartz flat cell cuvette (FZK 160-7×0.3, Magnettech GmbH, Berlin, Germany). 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was used as the spin-trapping agent at a concentration of 100 mM. The Pt-GTiO₂ concentration was 1.0 mg mL⁻¹, and the 2-naphthol concentration was 20.0 ppm [6].

2.6. By-Products Identification

To identify the stable organic intermediates that are present in the photocatalytic system after photooxidation of 2-naphthol, the solid-phase extraction (SPE) technique has been used. For this purpose, the irradiated suspension was immediately filtered through a 0.2 µm PTFE filter, stored in an amber glass bottle at 4 °C, and the extraction was conducted within 24 h. In a typical experiment, a dual serial extraction cartridge mode was used to isolate a large spectrum of the formed organic intermediates. C18 SPE cartridges, followed by OASIS HLB cartridges, were linked serially together with the Pyrex glass SPE (Supelco, Bellefonte, PA, USA) attached to a vacuum pump. After preconditioning the SPE cartridges with the required solvent, the acidified sample (pH 2) was passed into the serial cartridges, one by one. The flow rate was maintained between 1 and 2 mL/min. Following this, the cartridges were washed with 2–3 mL of a 5% methanol solution and acidified Milli-Q water, respectively, and left for 10 min to dry under low air pressure vacuum. Finally, the captured organic compounds were eluted from the cartridges by passing (1 mL/min) ethyl acetate (2 × 3 mL) from the head of the first cartridge. The organic extract eluent was then dried using pre-heated anhydrous sodium sulfate, concentrated to about 0.5 mL, and finally transferred to an amber glass GC vial for gas chromatography–mass spectrometry (GC-MS) analysis.

The ethyl acetate organic extracts were analyzed using a Shimadzu GC-MS (QP5050, Kyoto, Japan) system equipped with an AOC-5000 Plus autosampler and an Agilent DB-5MS capillary column (30 m × 0.32 mm × 0.25 µm). High-purity helium was used as the carrier gas at a constant flow rate of 1.25 mL/min, while the detector interface and the injector temperatures were maintained at 280 °C and 250 °C, respectively. The GC oven temperature program was initialized at 70 °C (held for 3 min), ramped at 5 °C min⁻¹ to 180 °C (held for 2 min), then ramped at 5 °C min⁻¹ to a final temperature of 315 °C (held for 5 min). The mass spectrometer was operated in electron impact ionization (EI) mode at 70 eV, and data acquisition was performed in full scan mode. Constituent peaks were identified by comparing their mass spectra with those of reference compounds in the National Institute of Standards and Technology (NIST) mass spectral library. Only peaks with a NIST library match factor greater than 70% were considered in the pathway discussion. To exclude contamination during the extraction and analysis, a solvent blank was analyzed using the same extraction and GC-MS analysis methods.

The Dionex ICS-1000 High-Performance Ion Chromatography (HPIC) system (Sunnyvale, CA, USA) was employed to detect the simple organic acids generated as by-products in the illuminated 2-naphthol system. The chromatograph was fitted with an anion exchange resin column (Ion Pac AS9-HC 2 × 250 mm) connected to an electro-regenerative suppressor and a conductivity cell serving as detectors. The temperature was maintained at 35 °C throughout the 90 min analysis. A mixture of 1.5 × 10⁻³ mol/L NaHCO₃ and 8 × 10⁻³ mol/L Na₂CO₃ alkaline solutions, which flow inside the system at 0.3 mL/min, was used as an eluent. Each sample analyzed was processed prior to analysis according to the procedures outlined in the HPLC methodology.

2.7. Characterization of the Photocatalyst

The synthesized platinized and bare samples were characterized with different physicochemical techniques as follows. X-ray diffraction analysis (XRD) patterns were obtained using a Bruker D8 Advance diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with a Bragg–Brentano geometry and employing Cu Kα radiation (λ = 1.54060 Å). The patterns were recorded in the 2θ range from 10° to 80° in increments of 0.039°. The specific surface areas of the platinized TiO₂ samples were assessed using a FlowSorb II 2300 instrument (Micromeritics Instrument Corporation, Norcross, GA, USA), equipped with a Micromeritics AutoMate 23. Prior to the measurements, all synthesized samples were pre-degassed in a vacuum at 150 °C for one hour. The specific surface area was determined through single-point standard Brunauer–Emmett–Teller (BET) surface area measurements, with each measurement conducted in triplicate to ensure precision. UV-vis spectra in the wavelength range of 200 to 800 nm were recorded using a Varian Spectrophotometer (Cary 100 Bio, Agilent, Santa Clara, CA, USA) equipped with a diffuse reflectance accessory. The fluorimeter used to collect Photoluminescence (PL) data was a FluoroMax P HORIBA (Jobin Yvon fluorescence spectrophotometer, Edison, NJ, USA), which was equipped with a 150 W Xe lamp. The colloidal solutions for PL measurements were prepared with a mass concentration of 0.2 mg mL⁻¹ in Milli-Q water. The pH of the solution was adjusted to 3.4 using H₂SO₄ to reduce aggregation and maintain the dispersion of the NPs. The samples were excited at a wavelength of 290 nm, and the emission spectra were recorded in the range of 300 to 550 nm. The morphology of the samples was investigated using Transmission Electron Microscopy (TEM) measurements. The TEM images were obtained using a Tecnai G2 F20 TMP (FEI, Hillsboro, OR, USA) instrument with an accelerating voltage of 200 kV. The real (w/w) % of Pt NPs in the synthesized TiO₂ was evaluated using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Varian 715-ES, Santa Clara, CA, USA). About 20–50 mg of each Pt-TiO₂ material was digested (triplicated to ensure precision and accuracy) in 5.00 mL of aqua regia at 150 °C for 5 h until all the acid evaporated. After cooling, each sample was re-dissolved in 10.00 mL of 3% HNO₃, filtered, and finally stored at 4 °C until further analysis.

3. Results and Discussions

3.1. Characterization

As shown in Figure 1, the XRD patterns for 1 wt% Pt-GTiO₂, 1 wt% Pt-UV100, and 1 wt% Pt-P25 nanoparticles were recorded to identify the crystallographic structure and the phase composition. The recorded spectrum reveals that the diffraction patterns as well as the peak positions of 1 wt% Pt-GTiO₂ and 1 wt% Pt-UV100 nanoparticles are almost identical. Both materials exhibit distinct characteristic peaks attributed to the tetragonal crystal structure of the anatase phase (JCPDS card No. 21-1272) [15]. The XRD spectrum of 1 wt% Pt-GTiO₂ does not show any signal for phase impurity, confirming the purity of the anatase phase. While the recorded diffraction patterns for 1 wt% Pt-P25 material exhibit the existence of a mixture of anatase and rutile phases, which is consistent with the data of JCPDS card Nos. 21-1272 and 21-1276, respectively. Our results for the synthesized GTiO₂ NPs were in agreement with the results of several research groups that reported the formation of pure anatase phase after the green synthesis of TiO₂ NPs using different plant extracts, Ti precursors, and calcination temperatures [35,36].

Moreover, we observed that loading of the surface of all TiO₂ materials with Pt NPs does not result in any impact or change on the XRD pattern position or features, indicating that the photodeposition method for preparing the platinized materials did not significantly alter the crystallinity or the formed TiO₂ phase [15]. In addition, no diffraction peaks for the Pt NPs were observed in the recorded XRD diffraction for all TiO₂ materials (Figure 1), which can be attributed to the low loading amount of Pt and the uniform distribution of these Pt NPs on the surface of the prepared materials [15,34]. The sharper and thinner 101 plane peak at 25.0° observed for Pt-GTiO_2, compared to Pt-UV100, indicates a higher crystalline structure and larger crystallite size of Pt-GTiO₂ nanoparticles [36]. These observations are confirmed by estimating the crystallite size of these materials based on the Scherrer equation [15]. The calculated average crystallite sizes of 1 wt% Pt-GTiO₂, 1 wt% Pt-UV100, and 1 wt% Pt-P25 were found to be 12.4, 7.5, and 25.0 nm, respectively (

Table 1. Actual Pt NPs loading, crystallite size, and BET surface area for platinized TiO₂ materials.

Material	Actual Pt NPs Loading wt.%	Crystallite Size a nm	Measured Surface Area (BET) m2 g−1
1 wt% Pt-UV100	0.91 ± 0.05	7.5 ± 2.3	290 ± 5.3
1 wt% Pt-P25	0.92 ± 0.04	25.0 ± 3.4	51 ± 2.8
1 wt% Pt-GTiO2 (fresh sample)	0.89 ± 0.06	12.4 ± 1.7	137 ± 3.9
1 wt% Pt-GTiO2 (after 4 cycles)	0.76 ± 0.10	-	-

^a. The calculated sizes are obtained based on the Scherrer equation.

). Several research groups reported a similar crystallite size for the plant-extract-assisted TiO₂ using TTIP as precursor [35,36].

In order to verify the presence of Pt NPs on the synthesized materials and determine their actual loading percentage, ICP-OES analysis was performed. The results presented in Table 1 indicate that the actual loading percentage of Pt NPs is lower than the expected value in all samples analyzed. However, the reduction values fall in the accepted range of experimental error [19].

In a complementary manner, the textural characteristics of the photocatalyst, particularly the specific surface area, directly influence the density of active sites, adsorption capacity for reactants and, finally, the photocatalytic efficiency [4]. The BET surface area analysis of the synthesized and commercial Pt-TiO₂ materials reveals significant differences in their values, as summarized in Table 1. The 1 wt% Pt-UV100 sample exhibited the highest surface area of 290 ± 5.3 m² g⁻¹, which is consistent with the known properties of Hombikat UV100, a benchmark anatase TiO₂ specifically engineered for a very high surface area [6,15]. In contrast, the 1 wt% Pt-P25 exhibits a considerably lower surface area of 51 ± 2.8 m² g⁻¹. In fact, this value is one of P25’s well-reported characteristics, which has a low surface area to maintain higher crystallinity, leading to better charge carrier separation compared to the UV100 [15,37].

The plant-extract-assisted 1 wt% Pt-GTiO₂ shows an intermediate BET surface area between Pt-P25 and Pt-UV100, with a value of 137 ± 3.9 m² g⁻¹. This value is notably higher than that of Pt-P25 and lower than Pt-UV100, suggesting that the biogenic synthesis method successfully produced a TiO₂ material with favorable properties, potentially balancing between a high density of surface active sites (compared to its surface area with that for P25) and sufficient crystallinity with special surface chemistry to reduce the rapid recombination of photogenerated electron–hole pairs [38]. In addition, it has been observed that the surface area of plant-extract-assisted TiO₂ nanoparticles is proportionally affected by the concentration of the plant extract [38]. In our study, we adapted the synthesis method developed by Aravind et al. [33] to prepare the GTiO₂ nanoparticles, resulting in the production of a material with smaller particle sizes than those reported by the Aravind research group. This variation may be attributed to the higher concentration of pomegranate peel extract utilized in our research compared to that in the study by Aravind et al. Indeed, the crystallite size calculated using the Scherrer equation coincides with this textural analysis. The smaller crystallite size of Pt-GTiO₂ (12.4 nm) is directly linked to their surface areas, as reducing particle size increases the surface-to-volume ratio. From the BET specific surface area, it is possible to estimate the mean particle size of the GTiO₂ by assuming spherical and non-porous TiO₂ nanoparticles, employing Equation (1) [39] based on the theory of Brunauer, Emmett, and Teller. The mean particle size (d_BET) was found to be 11.26, which is close to the value estimated from the XRD result based on the Scherrer equation.

$$d_{BET\left(\mathrm{n}\mathrm{m}\right)}={\displaystyle \frac{6000}{{\rho }_{\left(\mathrm{g}{\mathrm{c}\mathrm{m}}^{-3}\right) \times S_{BET\left({\mathrm{m}}^2{\mathrm{g}}^{-1}\right)}}}}$$

(1)

where $S_{BET}$ is the measured BET surface area and ${\rho }_{\left(anatase\right)}$ is the apparent density of anatase TiO_2.

The optical absorption properties for all the platinized TiO₂ materials were characterized using DRS. The reflectance data were transformed via the Kubelka–Munk function, F(R∞), to determine absorption characteristics, and Tauc plots were constructed after baseline correction, assuming indirect transitions to estimate the bandgap energy of each material.

As shown in Figure 2, the estimated band gap for Pt-GTiO₂ and Pt-UV100 are 3.14 and 3.2 eV, respectively. Since both materials consist of the anatase phase, which is characterized by 3.2 eV, the lower band gap of Pt-GTiO₂ indicated a small redshift in its absorption edge compared to Pt-UV100. This bandgap reduction suggests that the plant-extract-assisted synthesis method in the case of Pt-GTiO₂ could introduce electronic structural modifications, such as defect-rich states and/or residual heteroatom-related states associated with the plant-derived synthesis environment, thereby altering crystallinity. This modification may arise from organic residues during plant-extract-assisted synthesis using Punica granatum peel extract, leading to differences in the optical properties [27,40]. Saini et al. [26] reported the formation of defect states such as oxygen vacancies in the plant-extract-assisted synthesized TiO₂ nanoparticles, which originated from the organic residues in the Tinospora cordifolia plant stem extract. Recently, Mohammadi et al. [27] synthesized ZnO nanoparticles using the Punica granatum fruit peel extract and observed a similar redshift in the plant-extract-assisted synthesized ZnO NPs sample compared to the chemically prepared one.

On the other hand, the Pt-P25, which consists of a mixed phase of anatase and rutile, exhibits a bandgap of 3.00 eV, which is lower than the value estimated for Pt-GTiO₂. This lower bandgap value for Pt-P25 can be attributed to the difference in phase composition between the materials. TiO₂-P25 consists of a mixed phase of anatase and rutile, and its bandgap is regularly reported to be 3.00 eV [15].

HR-TEM was employed to study the morphology of the synthesized platinized photocatalysts. The micrographs presented in Figure 3a–f clearly indicate the structural distinctions between Pt-GTiO₂ (Figure 3a,b), Pt-UV100 (Figure 3c,d), and Pt-P25 (Figure 3e,f). All TiO₂ nanocrystals are spherical with a clear difference in grain boundaries, indicating a difference in size and degree of crystallinity during the synthesis processes.

As clearly seen in Figure 3c, the Pt-UV100 sample exhibits high agglomeration and small particles, indicating a relatively larger surface area compared to the Pt-GTiO₂ nanoparticles in Figure 3a. The particles of Pt-GTiO₂ appear less agglomerated, more uniform, and have intermediate sizes, compared to both commercial TiO₂ materials. Based on the ImageJ software (version 1.52p) for analyzing micrographs in Figure 3a,c,e, the average crystallite sizes of Pt-UV100, Pt-GTiO₂, and Pt-P25, respectively, are 8.9 ± 1.6, 13.5 ± 2.0, and 21 ± 1.5 nm. These values are in agreement with those that were obtained from the XRD results based on the Scherrer equation. The small crystallite size of the GTiO₂ NPs can be attributed to the advantages of the biowaste-assisted synthesis method in material preparation. Several research groups have reported similar or comparable crystallite sizes for plant-extract-assisted TiO₂ NPs obtained using various plant extracts [26,41].

Furthermore, analysis of the micrographs shown in Figure 3 indicated the uniform distribution of Pt NPs on the surfaces of all the synthesized Pt-TiO₂ materials, with an average particle size of 2–4 nm. This suggests the success of the modified photodeposition method for achieving small and well-dispersed Pt NPs. We reported previously a similar particle size with uniform distribution for the Pt NPs on the surface of TiO₂, using a different photodeposition method [34].

The FFT analysis of specific regions in Figure 3b,d,f, obtained using the ImageJ program, revealed a well-defined d-spacing of 0.35 nm in both Pt-GTiO₂ (Figure 3b) and Pt-UV100 (Figure 3d). This value corresponds to the (101) plane of anatase, thus confirming the phase purity and its retention after the platinization process [15,34]. While similar analysis in Figure 3f of the Pt-P25 sample showed the appearance of diffraction fringes with a d-spacing of 0.32 nm associated with (110) planes of the rutile phase. This phase was observed alongside (101) planes of the anatase phase, confirming the biphasic nature of P25.

As shown in Figure 4, the pre-calcination ATR-FTIR spectrum reveals several transmittance bands indicative of organic functional groups derived from the plant extract, as well as TiO₂-related vibrations. The broad transmittance band between 3200 and 3600 cm⁻¹, associated with O–H stretching, indicates the presence of hydroxyl groups that could be related to the presence of alcohols, phenols, carboxylic acids, or adsorbed water molecules on the surface of freshly prepared GTiO₂. Indeed, Altarawneh et al. [42] reported the presence of such organic compounds in Punica granatum peel extracts that contain flavonoids, phenolic compounds, and other capping agents [27,42].

In addition, the band observed around 2850–3000 cm⁻¹ in the pre-calcinated GTiO₂ sample could be related to aliphatic C–H stretching, indicating the presence of organic residues like alkanes or alkyl groups in the plant extract, and O–H stretching from a carboxylic acid. The observed transmittance band at 2350 cm⁻¹ could be assigned to the adsorbed CO₂ on the surface of the pre-calcinated GTiO₂. Furthermore, on the surface of the pre-calcinated sample, the bands observed around 1240 and 1650 cm⁻¹ are related to the C–O stretching vibration of phenols and esters, and the C–H bending of aromatic compounds, respectively [23,25,43]. All of these bands were significantly quenched or disappeared after the calcination of GTiO₂ (Figure 4), indicating the removal of the organic functional groups from the plant extract used in the synthesis. Calcination at a temperature higher than 400 °C combusts organic capping agents, such as organic acids and polyphenols, that are present from the plant extract [35,44]. This aligns with several previous studies that reported the calcination process removes biogenic residues, enhancing TiO₂’s crystallinity, and improving photocatalytic properties [26]. Meanwhile, the observed broad peak below 1000 cm⁻¹ in both materials before and after calcination can be related to Ti–O–Ti stretching vibrations in anatase [44,45].

Raman spectroscopy analysis was employed to identify the purity and crystallinity of the plant-extract-assisted Pt-GTiO₂ NPs phase. As shown in Figure 5, the studied sample displays six active modes centered around 144, 197, 399, 513, 519, and 639 cm⁻¹. According to the literature, these vibrational bands are consistent with the anatase phase of TiO₂, which aligns perfectly with our results from the XRD analysis and HRTEM micrographs analysis. Anatase TiO₂ crystallizes in a tetragonal structure with the space group I41/amd and exhibits D₂d point group symmetry. According to group theory, the anatase phase possesses six Raman-active modes, designated as A₁g + 2B₁g + 3Eg [41]. The strong peak at 145 cm⁻¹ corresponds to the Eg symmetric stretching mode of O–Ti–O bonds, while the features at 198 cm⁻¹ and 397 cm⁻¹ are also attributed to Eg and B₁g modes, respectively. The doublet observed at 513 and 519 cm⁻¹ can be assigned to overlapping A₁g and B₁g modes, while the broad peak near 639 cm⁻¹ corresponds to the Eg mode. The obtained bands correlate well with the values in the literature for the anatase phase of TiO₂ that was greenly synthesized using gum Arabic and beta vulgaris extract [41,46].

PL emission spectroscopy was employed to study the dynamics and recombination process of photogenerated charge carriers. As shown in Figure 6, a sharp feature near 335 nm appears in the bare UV100, bare GTiO₂, and Pt-loaded GTiO₂ spectra. This band can be related to residual scattering of excitation light due to its closeness to the 290 nm excitation wavelength. Moreover, another broad emission bands across the UV-visible region centered around 390 nm, as well as minor peaks near 450, 465, and ~520 nm were recorded for the three studied materials. These emissions were previously reported to various radiative recombination pathways, including intrinsic band-to-band transitions and defect-related states [45,47]. Since the bandgap of anatase TiO₂ was documented between 387 and 390 nm, the observed strong band around 390 nm can be related to the direct recombination of the photogenerated electron/hole after excitation [4]. However, the observed emission bands in the visible region reflect the contributions from shallow and deep-level defect states that could be oxygen vacancies and surface hydroxyl groups [45].

Furthermore, as observed in Figure 6, the bare GTiO₂ sample exhibits a lower emission intensity compared to the bare UV100 at 390 nm wavelength. This decrease reflects the lower recombination process of the photogenerated charge carrier in the case of bare GTiO₂, which can be related to the benefits of the plant-extract-assisted method in forming a material with better crystallinity and fewer radiative recombination centers. This enhancement in charge-carrier separation will lead to improved photocatalytic efficiency. Sangeetha et al. [45] observed similar PL characteristics in extract-assisted TiO₂ nanoparticles and noted that both chemical and biological synthesis routes yielded comparable optical behaviors.

Moreover, the presence of the Pt NPs on the surface of GTiO₂ exhibits a significant reduction in the PL signal compared to the bare GTiO₂ material. This quenching can be related to better separation of the charge carrier in the presence of the Pt NPs on the surface of the GTiO₂ material, which serve as a sink for the photogenerated electrons, thus reducing the recombination of electron–hole pairs. This observation highlighted the main role of the Pt NPs in the charge carrier separation. It has been reported that the formation of the Schottky barrier between Pt NPs and GTiO₂ enhances the charge carrier separation and thus improves the photocatalytic efficiency of Pt-GTiO₂ [15].

3.2. Comparison Between Different Catalysts

The photocatalytic performance of the plant-extract-synthesized GTiO₂ was evaluated based on 2-naphthol photodegradation with simultaneous molecular H₂ evolution under anaerobic conditions using simulated solar light. To evaluate the photocatalytic performance of the synthesized GTiO₂ NPs before and after loading with Pt NPs, it was compared against commercial TiO₂ benchmarks, Aeroxide P25 and Hombikat UV100. This comparison was designed as a control set to clarify the role of each component. Bare GTiO₂ represents the pomegranate-peel-assisted TiO₂ support without Pt. Bare P25 and bare UV100 represent commercial TiO₂ supports without plant extract and without Pt. Pt-P25 and Pt-UV100 represent Pt-loaded TiO₂ controls without the pomegranate extract, while Pt-GTiO₂ contains both the plant-extract-assisted TiO₂ support and Pt cocatalyst. Therefore, the comparison between bare GTiO₂ and Pt-GTiO₂ reflects the effect of Pt loading on the green-synthesized material, whereas the comparison between Pt-GTiO₂ and Pt-P25/Pt-UV100 shows the effect of the TiO₂ support under the same Pt-loading approach.

In the dark, it was observed that there was no activity for molecular hydrogen formation in the absence or presence of 2-naphthol over all types of pristine TiO₂ materials. The dark adsorption experiment showed that the 2-naphthol concentration reached a near-stable and unchanged concentration after 50 min of contact with Pt-GTiO₂ in suspension, as shown in Figure S1. After 60 min in the dark, the adsorption removal of 2-naphthol on the surface of the Pt-GTiO₂ was around 13%. Therefore, the 60 min dark period was sufficient to approach adsorption equilibrium under the present experimental conditions. Similar behaviors and values were observed for the adsorption of 2-naphthol on the surface of TiO₂ [48] and ZnO nanoparticles [14]. Additionally, to distinguish the contribution of photolysis to the removal of 2-naphthol from that of photocatalytic degradation, a control experiment was performed. Therefore, dark adsorption, photolysis, and photocatalysis were compared directly. As shown in Figure S2, the removal of 2-naphthol under illumination in the absence of the Pt-TiO₂ photocatalyst is about 15% after 240 min of irradiation. This value was found to be comparable to the adsorption value of 2-naphthol on the surface of the same catalyst in the dark. In contrast, the Pt-GTiO₂ photocatalytic system exhibited ~80% removal under the same irradiation time. These results confirm that the observed conversion is mainly photocatalytic and not caused by adsorption or direct photolysis alone.

As presented in Figure 7, during the irradiation of the pristine and platinized TiO₂ materials studied, the H₂ evolution was not observed in the absence of 2-naphthol. This phenomenon was well documented previously, and it was attributed to the fast recombination of the charge carriers in the absence of a suitable hole scavenger, such as 2-naphthol, and the overpotential of molecular hydrogen formation on the surfaces of all types of studied TiO₂ photocatalysts [15,49]. Different types of pristine TiO₂ have been reported to be ineffective photocatalysts for molecular H₂ production. Hence, even in the presence of the organic pollutant, naphthalene, in the system as an electron donor, Al-Madanat et al. showed that bare Aeroxide P25 and Hombikat UV100 did not exhibit any efficiency for photocatalytic molecular H₂ formation [4,6,15]. Electron paramagnetic resonance (EPR) results have confirmed that while the photogenerated holes are consumed by the electron donor during illumination, the photoexcited electrons are trapped inside or on the surface of the TiO₂ as reduced Ti(III) ions instead of catalyzing the reduction of the proton [4].

Additionally, the bare GTiO₂ demonstrated higher and comparable degradation efficiency for 2-naphthol against commercial UV100 and P25 TiO₂ samples, respectively. However, pristine P25 showed higher photocatalytic activity in converting 2-naphthol than both UV100 and GTiO₂. This enhanced activity is attributed to the synergistic effect of the anatase–rutile heterojunction in P25 material, which promotes efficient charge separation. It has been reported that anatase TiO₂ has a greater electron affinity than rutile TiO₂, promoting the transfer of photogenerated electrons from rutile to anatase, while holes migrate to rutile, enhancing the availability of holes for oxidation reactions [4]. These results align with the work of Hurum et al. [37], who reported that the mixed-phase of TiO₂ in Degussa P25 improves charge separation, reduces recombination, and boosts photocatalytic performance. Qourzal et al. studied the photocatalytic degradation of 2-naphthol under different types of TiO₂ materials. The Degussa P25 was found to be the most efficient catalyst compared to the UV100 and PC-500 TiO₂ materials [48].

On the other hand, the improvement of the 2-naphthol photocatalytic removal efficiency on the surface of pristine GTiO₂ compared to UV100 is attributed to the lower rate of e⁻/h⁺ recombination in GTiO₂ compared to UV100. These results are consistent with agreements with the PL analysis results, where the GTiO₂ NPs exhibited lower recombination intensity than UV100. This serves as a positive indicator of enhanced charge separation and reduced carrier recombination, resulting in greater availability of photogenerated charge carriers for surface redox reactions. Consequently, this leads to an increase in photocatalytic efficiency [50].

As shown in Figure 7, a significant enhancement in molecular H₂ formation was observed after platinization of all the tested photocatalysts in the presence of 2-naphthol. Under the present anaerobic conditions, this observation confirms that 2-naphthol is not only a pollutant to be removed from the system, but also an effective hole scavenger or “organic electron donor” that enables molecular hydrogen formation. During illumination, the valence-band photogenerated holes and the photoformed oxidative species oxidize the adsorbed 2-naphthol due to its electron-rich aromatic system and highly favorable oxidation potential. Thereby, consuming the photogenerated holes at the photocatalyst surface, and significantly extends the lifetime of the conduction band electrons. This will suppress electron–hole recombination. As a result, the photogenerated electrons accumulated on the Pt NPs, thus facilitating the proton reduction to H₂. Several research groups have reported the role of a similar organic pollutant to 2-naphthol as a hole scavenger during the photocatalytic degradation of organic compounds, along with H₂ production [6,20].

The Pt-GTiO₂ NPs showed higher H₂ evolution than P25 but lower than UV100 at the same Pt weight percentage content. The evolution of molecular H₂ after platinization can be related to effective electron trapping on the Pt NPs and Schottky barrier formation at the Pt-TiO₂ interface, which promotes proton reduction [22]. The different activities in H₂ evolution between the tested materials align with the differences in the surface area, as shown in Table 1. The higher the surface area, the better the rate of H₂ formation. As the surface area of the photocatalyst increases, organic substrate adsorption is enhanced due to the greater availability of the active sites, which increases the electron transfer rate to Pt NPs, and thus promotes the formation of molecular hydrogen [6,15]. It has been reported that surface area is one factor affecting adsorption and reaction rates, but crystallinity, phase composition, Pt dispersion, and charge separation also contribute to the observed photocatalytic behavior [4,29,30]. It was observed that the platinization process leads to enhancing the photocatalytic degradation efficiency, indicating that Pt NPs not only catalyze the reduction of protons but also promote oxidation of the organic pollutants [51].

Furthermore, as shown in Figure 7, a different influence for the loading percentage of Pt NPs on the photocatalytic reforming of 2-naphthol was observed. By increasing the % weight of the Pt NPs from 0.5% to 1% on the surface of the tested materials, H₂ evolution and the degradation efficiency increase. The 1 wt% Pt-GTiO₂ NPs exhibited a degradation efficiency of about 70%, which was higher than that of 1 wt% Pt-UV100 and comparable to 1 wt% Pt-P25. Moreover, 1 wt% Pt-GTiO₂ showed a better H₂ evolution of about 300 µmol g⁻¹ than the 1 wt% Pt-P25. In addition, although Pt-P25 benefits from the anatase–rutile heterojunction, which is well known to promote charge separation and fast oxidation reactions, the superior H₂ evolution of Pt-GTiO₂ in the present system arises mainly from its textural and interfacial advantages, which balance between the higher BET surface area (137 ± 3.9 m² g⁻¹) and the smaller crystallite size. These features will lead to an increase in the number of accessible Pt-TiO₂ interfacial sites, improve the adsorption of 2-naphthol and its reforming intermediates, and facilitate electron transfer from TiO₂ to Pt for proton reduction. In addition, the biogenic synthesis route may generate beneficial surface states and hydroxyl-rich surface chemistry, further supporting interfacial charge transfer. Therefore, Pt-GTiO₂ provides a more favorable balance for hydrogen evolution, whereas Pt-P25 remains slightly more favorable for fast oxidative disappearance of the parent pollutant [28,40]. These presented results indicate that Pt loading is the main factor responsible for H₂ evolution, while the nature of the TiO₂ support controls the balance between pollutant oxidation and hydrogen production.

Based on the previous discussion, it was shown that Pt-GTiO₂ did not outperform all commercial Pt-TiO₂ references across all activity parameters; thus, their activity cannot be attributed only to the pomegranate peel extract. Instead, the extract-assisted synthesis appears to provide GTiO₂ with a useful combination of anatase phase, intermediate surface area, particle morphology, and lower charge recombination, which together contribute to its photocatalytic behavior. It should be noted, however, that the specific contribution of the pomegranate peel extract to the observed enhancement in the photocatalytic performance of GTiO₂ in the absence of a control TiO₂ sample synthesized under identical conditions but without the plant extract must be interpreted with caution. The absence of such baseline control limits the possibility of interpreting variation in photocatalytic activities based on the differences in crystallinity, surface chemistry, and charge-carrier dynamics solely to the biogenic extract, rather than to other synthesis parameters [23,25].

3.3. Comparison of Kinetic Profiles

Figure 8 shows the kinetic profiles of photocatalytic 2-naphthol degradation with simultaneous molecular H₂ production over 1 wt% Pt-GTiO₂, 1 wt% Pt-UV100, and 1 wt% Pt-P25 catalysts under the same experimental conditions. During the 60 min dark period before illumination, approximately 10% of the initial 2-naphthol concentration was adsorbed onto the surfaces of all the photocatalysts tested. In this stage, there is no activity for molecular hydrogen formation. Like other research groups’ reports, our FTIR analysis, which is presented in Figure 4, confirmed the hydroxylated nature of the TiO₂ surface [26,52]. As a result, a number of 2-naphthol molecules adsorb to the surface of TiO₂ through interactions with hydroxyl (~OH) and deprotonated oxyanion (~O~) groups.

Upon irradiation, a rapid decrease in the 2-naphthol concentration in the presence of all photocatalysts during the first 60 min of illumination (Figure 8a) was observed, with approximately 80–90% of the total converted 2-naphthol removed in this period. This fast conversion is found to be accompanied by rapid production of molecular H₂ (Figure 8a,b). After this period to the end of illumination, a slight decrease in 2-naphthol conversion, as well as evolved H₂, was noticed. This observation suggests that the photocatalyst surface is deactivated by accumulated intermediates occupying active sites, and that 2-naphthol and its by-products compete for the remaining available active sites and the photogenerated holes. This result aligns with previous reports indicating that the degradation of aromatic compounds photocatalytically leads to the formation of different types of harsh intermediates, such as polymeric nature and coupling compounds, which deactivate the catalyst surface [6,15,53].

Furthermore, the results in Figure 8b show that Pt-UV100 exhibits the highest H₂ production, followed by Pt-GTiO₂, and Pt-P25 with the lowest yield. The trends of both kinetic profiles for Pt-UV100 and Pt-GTiO₂ are similar. The observed behavior suggests that the Pt-UV100 is most effective for H₂ production, which can be attributed to its more negative conduction band (CB) edge and its higher surface area. The typical CB edge for the Pt-UV100 has been reported around −0.5 V vs. Normal Hydrogen Electrode (NHE), compared to 0.0 V vs. NHE to rutile’s CB [6,37]. This higher CB energy in UV100 provides a greater tendency for proton reduction to H₂.

On the other hand, although Pt-P25 showed the highest degradation efficiency (Figure 8a), its kinetic profile trend is very similar to that of Pt-UV100. Both materials exhibit a rapid photodegradation rate in the first 60 min of illumination, which then declines to near zero after 120 min. A different behavior was observed in the case of Pt-GTiO₂. Although its degradation rate decreases with time, it still exhibits better activity after the previously mentioned period to the end of the illumination compared to the other catalysts. This behavior can be attributed to its synthesis method, which leads to the formation of a material characterized by superior surface chemistry and lower charge carrier recombination, thus enhancing its photocatalytic activity by altering charge carrier dynamics.

To compare the photocatalysts more fairly, the initial rates for H₂ production ($r_{H_2}$), 2-naphthol degradation ($r_{deg}$), and normalized activities based on the surface area and the initial rates were calculated for the first 30 min of irradiation, which represent the early reaction region. The initial rates and the normalized values are listed in

Table 2. Initial and normalized photocatalytic H₂ production and 2-naphthol degradation rates over different photocatalysts calculated from the first 30 min of irradiation.

Catalyst	${\mathit{r}}_{{\mathit{H}}_2}$ (µmol g−1 h−1)	Normalized Activities H2 Rate/BET Area (µmol m−2 h−1)	R2 (H2)	${\mathit{r}}_{\mathit{d}\mathit{e}\mathit{g}}$ (µmol g−1 h−1)	Normalized Activities 2-Naphthol Rate/BET Area (µmol m−2 h−1)	R2 (deg)
Pt-GTiO2	225.668	1.647	0.953	82.998	0.606	0.949
Pt-UV100	254.928	0.879	0.896	108.386	0.374	0.999
Pt-P25	85.636	1.679	0.908	109.068	2.139	0.969

. Pt-UV100 showed the highest initial H₂ evolution rate per catalyst mass, while Pt-GTiO₂ gave a lower value but remained clearly higher than Pt-P25. After normalization to BET surface area, Pt-GTiO₂ and Pt-P25 showed much higher H₂ rates per surface area than Pt-UV100. This indicates that the higher accumulated H₂ over Pt-UV100 is partly related to its much larger surface area. For 2-naphthol degradation, Pt-P25 and Pt-UV100 showed similar initial degradation rates, higher than Pt-GTiO₂. However, after normalization to the BET surface area, Pt-GTiO₂ and Pt-P25 showed much higher degradation rates per surface area than Pt-UV100. Furthermore, Pt-GTiO₂ maintained activity for a longer irradiation period compared to the other catalysts.

3.4. Effect of TiO₂ Dosage

It is well known that the catalyst concentration in the photocatalytic system highly affects the rate of H₂ production and the degradation of organic pollutants, due to its direct correlation with the active sites available for the reaction. Increasing the catalyst dosage in the reaction system provides more active sites for the adsorption of organic molecules, generating active species (such as ROS) that participate in the degradation of the adsorbed molecules, and the reduction of the adsorbed proton to H₂ [26,48].

The results presented in Figure 9a exhibit clear dependence on catalyst concentration for both Pt-UV100 and Pt-GTiO₂ under illumination in the presence of 2-naphthol. By increasing the dose of the Pt-UV100 catalyst from 0.5 to 2.0 mg mL⁻¹, the H₂ evolution increases to reach approximately 400 µmol g⁻¹. However, increasing the concentration to 3.0 mg mL⁻¹ resulted in a slight decline, suggesting that the optimal dosage is around 2.0 mg mL⁻¹. Furthermore, the H₂ evolution over the Pt-UV100 catalyst is higher than that of the Pt-GTiO₂ at comparable dosages, which can be related to the difference in the surface area of the two materials. The surface area of the 1 wt% Pt-UV100 and the 1 wt% Pt-GTiO₂ are about 300 m² g⁻¹ and 140 m² g⁻¹, respectively. Thus, more active sites are available on the surface of the 1 wt% Pt-UV100 for the reduction of the H⁺ to molecular H₂. In contrast to 1 wt% Pt-UV100, the extract-assisted 1 wt% Pt-GTiO₂ catalyst demonstrates a continuous increase in H₂ evolution with increasing dosage from 0.5 to 3.0 mg mL⁻¹, with its maximum reaching around 375 µmol g⁻¹.

On the other hand, both tested catalysts exhibit a characteristic shape similar to a volcano during the photodegradation of 2-naphthol when the catalyst concentration is increased from 0.5 to 3.0 mg mL⁻¹, as shown in Figure 9b. The optimal concentration for both materials was found to be 1.5 mg mL⁻¹ for Pt-UV100 and 2.0 mg mL⁻¹ for Pt-GTiO₂. As observed in the results of the H₂ evolution, beyond the optimal values, the performance of both catalysts declined markedly during 2-naphthol conversion. In fact, this phenomenon is well known and has been previously reported in various TiO₂-based materials during the photocatalysis process [26,48]. Increasing the catalyst concentration results in more active sites for both the reduction and oxidation reactions on the catalyst’s surface, thereby enhancing H₂ production and pollutant conversion. However, exceeding the optimal concentration of the photocatalyst in the reaction medium leads to an increase in both light scattering during illumination and particle agglomeration. This will limit light penetration into the reaction medium, thereby reducing the number of excited particles and the photogenerated charge carriers. While the agglomeration reduces the available active sites, thus decreasing the overall photocatalytic efficiency [15,26].

Furthermore, as presented in Figure 9b, Pt-GTiO₂ showed better photodegradation efficiency for 2-naphthol compared to Pt-UV100. It nearly converted 2-napthol completely in the presence of 2.0 mg mL⁻¹ of Pt-TiO₂, and 85% conversion was achieved at a concentration of 3.0 mg mL⁻¹. In the case of Pt-UV100 at the same catalyst doses, the conversion efficiencies were found to be about 70% and 50%, respectively. The higher activity of Pt-GTiO₂ can be related to its plant-extract-assisted synthesis process. This method yields a higher crystallite size with distinct surface properties and morphology, which could exhibit less susceptibility to light scattering and agglomeration, thereby enhancing the degradation of 2-naphthol [54].

3.5. Effect of the pH

The effect of varying the pH in the range between 3.0 and 11.0 on the degradation of 2-naphthol and H₂ evolution during the photocatalytic reforming process over Pt-UV100 and Pt-GTiO₂ was evaluated. The results presented in Figure 10 reveal the high dependence of 2-naphthol degradation and molecular H₂ formation on the system’s initial pH. As shown in Figure 10a, increasing the initial pH of the reaction medium from 3 to 11 resulted in an enhancement in the degradation efficiency of 2-naphthol, following the same trend in both photocatalyst systems. The Pt-GTiO₂ system achieved higher degradation efficiencies than the Pt-UV100 system at all the studied pH levels. It exhibited almost complete conversion of 2-naphthol in an alkaline medium, and about 56% and 75% conversion efficiencies in acidic and neutral conditions, respectively. Meanwhile, the 2-naphthol degradation in the Pt-UV100 system was found to range from 50% to 86% when changing the initial pH of the system from acidic to basic, respectively.

The better performance of both materials in basic medium can be attributed to the interplay between the surface charge of the TiO₂ support and the acid–base properties of 2-naphthol. The point of zero charge (pzc) of anatase TiO₂ is typically around pH 6.2–6.8 [55,56]. At pH values below the pzc, the catalyst surface is positively charged (TiOH₂⁺) while, at pH values above the pzc, it becomes negatively charged (TiO~) [22,48]. Concurrently, 2-naphthol, with a pKa of approximately 9.5, exists in its molecular form at acidic and neutral pH and transitions to its anionic naphtholate form in strongly alkaline conditions (pH > 9.5). At this point, the adsorption of naphtholate ions is expected to be very small due to the repulsion with the negatively charged TiO₂ surface. However, this negative surface promotes the formation of hydroxyl radical (OH^•) species at the surface of the TiO₂ through the reaction of the high-concentration OH⁻ near the catalyst surface with photogenerated holes, which improves the efficiency of the photocatalysis process, as shown in Equations (2) and (3) [6,22,48,55].

$$\mathrm{Pt-Ti}{\mathrm{O}}_2+\mathrm{h}\nu ⟶\mathrm{Pt-Ti}{\mathrm{O}}_2[{\mathrm{h}}^{+}+{\mathrm{e}}^{-}]$$

(2)

$$\mathrm{Pt-Ti}{\mathrm{O}}_2\left[{\mathrm{h}}^{+}\right]+{\mathrm{O}\mathrm{H}}^{-}\left(\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{d}\right)⟶\mathrm{Pt-Ti}{\mathrm{O}}_2+\mathrm{O}{\mathrm{H}}^{•}$$

(3)

The superior performance of Pt-GTiO₂ in the photodegradation of 2-naphthol at all pH levels over Pt-UV100 is attributed to its favorable optical properties, which were formed during the plant extract synthesis method using Punica granatum peel extract. As previously discussed, the PL analysis confirms the lower rate of charge carrier combination during the excitation of Pt-GTiO₂. This lower recombination rate leads to more ROS generation and efficient charge separation, thereby enhancing the degradation efficiency.

On the other hand, although H₂ production is favored in alkaline conditions, a different relationship was observed under other pH media than those in 2-naphthol degradation. Pt-GTiO₂ reached a maximum H₂ rate of approximately 520, 285, and 190 µmol g⁻¹ in the basic, acidic, and neutral mediums, respectively. In the same manner, but with slightly higher values than Pt-GTiO₂, the Pt-UV100 photocatalyst exhibited H₂ production of nearly 780, 335, and 220 µmol g⁻¹ under alkaline, acidic, and neutral pH conditions. This trend reflects the strong relationship between H₂ production and 2-naphthyl degradation, as presented in Figure 10. The higher efficiency of hole consumption during 2-naphthol oxidation in alkaline media leads to the suppression of electron–hole recombination. This, in turn, improved charge separation, making a larger population of photogenerated electrons available at the Pt NPs to reduce protons to molecular H₂, according to Equations (4)–(6) [6,22,55].

$$\mathrm{Pt-Ti}{\mathrm{O}}_2\left[{\mathrm{e}}^{-}\right]+{\mathrm{H}}^{+}⟶\mathrm{Pt-Ti}{\mathrm{O}}_2+{\mathrm{H}}^{•}$$

(4)

$${2\mathrm{H}}^{•}⟶{\mathrm{H}}_2$$

(5)

$${2\mathrm{H}}_2\mathrm{O}+{2\mathrm{e}}^{-}⟶{\mathrm{H}}_2+2\mathrm{O}{\mathrm{H}}^{-}$$

(6)

The higher performance of Pt-UV100 compared to the Pt-GTiO₂ can be attributed to the differences in the specific surface area of the two materials, as previously discussed in Section 3.2. Higher values for the photocatalysis of molecular hydrogen were reported in a basic medium at a similar pH, as documented in the literature by several research groups [55,56].

The lower H₂ yield in neutral and acidic media, which is combined with moderate degradation, suggests that charge recombination or competing reductive pathways may be more prevalent in these media. In fact, the degradation of organic pollutants in acidic and neutral media often proceeds by attacking the surface-bound OH^• radicals, which are generated by the oxidation of H₂O/OH⁻ by the photogenerated holes. This pathway is indirect and often slow, and it is governed by the limit of the adsorption of the organic molecules on the surface of the photocatalyst, which is essentially a prerequisite for efficient degradation according to the Langmuir–Hinshelwood kinetics model [51].

3.6. Effect of the Initial Concentration of 2-Naphthol on the Degradation Efficiency

Unlike the previous experiments, which used a platinized catalyst with 1 wt% Pt NPs. In this study, Pt NPs were employed at a 0.5 wt% concentration on the surface of GTiO₂ and UV100 catalysts to facilitate monitoring and tracking the reaction rate at low substrate concentration. Figure 11a–c illustrates the photocatalytic performance of the synthesized 0.5 wt% Pt-GTiO₂ and 0.5 wt% Pt-UV100 catalysts for the degradation of 2-naphthol at varying initial substrate concentrations (20.0, 10.0, and 5.0 ppm). The presented results indicate clear concentration-dependent efficiency for both catalysts, with superior performance observed at lower substrate concentrations.

As shown in Figure 10a, at the highest substrate concentration (20.0 ppm), Pt-GTiO₂ exhibits significantly faster reduction in both pollutant concentration and TOC compared to Pt-UV100. Pt-GTiO₂ exhibits an outperforming approximately 65% conversion efficiency and 50% TOC removal within 240 min compared to Pt-UV100, which showed around 50% degradation and 30% TOC removal. The lower removal rate of the TOC in Pt-GTiO₂ and Pt-UV100 systems indicates that the oxidized amount of 2-naphthol is converted into stable, non-mineralized organic intermediates rather than being completely oxidized to CO₂ and H₂O [6]. In fact, this is a common phenomenon during the photocatalysis process, which can be attributed to the complex degradation pathways of aromatic pollutants [4].

Furthermore, overall improvements in 2-naphthol degradation and TOC removal efficiencies were observed in both systems after reducing the initial concentration to 10.0 ppm (Figure 11b), with better performance for Pt-GTiO₂. After 240 min of illumination, a nearly complete and around 75% conversion for 2-naphthol, combined with a partial mineralization of around 70% and 50% was observed in Pt-GTiO₂ and Pt-UV100 systems, respectively. With a further reduction in 2-naphthol concentration to 5 ppm (Figure 11c), although both catalysts demonstrated remarkable conversion efficiencies, Pt-GTiO₂ exhibited exceptional performance. It achieved a complete 2-naphthol degradation within 120 min and a substantial mineralization of about 90% after 240 min, overtaking the 90% degradation and the 70% TOC removal of its counterpart Pt-UV100, which was achieved after 240 min. This enhanced performance at lower substrate concentrations can be related to the increased availability of active sites on the photocatalyst surface relative to the number of substrate molecules. Indeed, the concentration-dependent behavior is explained by the Langmuir–Hinshelwood model [14,56]. At higher concentrations, the competition between the parent compound and the formed intermediates for these limited active sites becomes more intense, resulting in a reduced overall efficiency [14,48]. The most significant results at lower concentrations in the case of the Pt-GTiO₂ system suggest that once the parent pollutants are converted, the formed intermediate species are rapidly mineralized on the available active sites on the surface of the catalyst that have a powerful ability for ROS formation [15,48].

3.7. Reusability Test

It is well known that the stability and reusability of photocatalysts are crucial factors that directly influence the cost and the practical application of photocatalysis technology. Therefore, in this study, the photocatalytic activity of the 1 wt% Pt-GTiO₂ photocatalyst was evaluated regarding the photooxidation of 2-naphthol and its simultaneous H₂ formation after four successive cycles. Indeed, this test evaluates the reusability of Pt-GTiO₂ after a regeneration step, rather than continuous operation without treatment between cycles. To this end, after each cycle, the used photocatalyst was collected through centrifugation and subjected to irradiation in the presence of molecular oxygen for 60 min to eliminate any adsorbed organic compounds on its surface resulting from the previous reforming process. The photocatalyst was then washed twice with methanol and subsequently with Milli-Q water. Following this, it was centrifuged again and dried in the oven at 100 °C overnight.

As shown in Figure 12, the recycled 1 wt% Pt-GTiO₂ photocatalyst exhibits good photocatalytic performance and reasonable reusability after regeneration towards the photooxidation of 2-naphthol and molecular H₂ over four successive runs. It was observed that the photocatalytic degradation efficiency of 2-naphthol and the simultaneous H₂ production were slightly decreased compared to the fresh photocatalyst, where the gradual decrease between each cycle was found to be around 2–5%.

After completing four cycles, the percentage of remaining Pt nanoparticles on the GTiO₂ surface was analyzed using ICP-OES techniques, as shown in Table 1. The analysis revealed that the amount of Pt was about 15% lower than the original ratio, which can explain the limited drop in 2-naphthol photodegradation and H₂ production. In addition, post-reaction characterization, including XRD and ATR-FTIR, was performed after the fourth cycle, as shown in Figures S4 and S5. The XRD pattern of the used catalyst, shown in Figure S4, indicates that the anatase phase was retained and has the same features as the fresh Pt-TiO₂ samples before performing the reaction. Compared to the freshly synthesized catalyst, the post-reaction FTIR spectrum after four cycles showed a small band in the region between 1600 and 2400 cm⁻¹, similar to the fresh Pt-TiO₂ samples, suggesting no strong residual organic compounds or adsorbed species on the catalyst surface. These results support the practical robustness of the catalyst over repeated use, while also indicating that stronger Pt anchoring would be beneficial for longer-term operation. Furthermore, although the physical retention of Pt nanoparticles on the surface of GTiO₂ was confirmed by ICP-OES, the long-term chemical stability and potential changes in the oxidation states of the Pt species during repeated cycles were not directly evaluated by surface techniques such as X-ray photoelectron spectroscopy (XPS). Future investigations focusing on the surface chemical dynamics of the cocatalyst under prolonged irradiation will be valuable.

3.8. Mechanism of Photoreforming of 2-Naphthol

3.8.1. Identified By-Products

It is widely recognized that the identification of reaction intermediates provides essential insights into the mechanistic pathways underlying the photocatalytic degradation of organic pollutants [4,6]. In this study, we have successfully identified several by-products generated during the photocatalytic reforming of 2-naphthol on the surface of Pt-GTiO₂ in an inert atmosphere, employing a different chromatographic technique. Several of these intermediates were tentatively identified by GC-MS, which cannot be confirmed due to the lack of authentic standards for all compounds.

After analyzing the concentrated extract from the solid phase extraction using GC-MS chromatography (Figure S3), several oxidative and reductive aromatic and aliphatic stable intermediates were tentatively identified based on their retention times, diagnostic mass fragments, and comparison with the NIST mass spectral library, such as naphthalene-1,2-diol, 3-(2-Methylphenyl)propionic acid, Octanoic acid, and 3,4-Dihydro-2(1H) naphthalenone, as illustrated in Scheme 1 and Table S2.

The detected by-products, in fact, reflect their formation in the early stage of the degradation process and their maintenance in the reaction system during illumination, due to their continued formation through the conversion of the parent compound, 2-naphthol. These results, to the best of our knowledge, represent the first investigation into the photoreforming of 2-naphthol under inert conditions, which precludes comparison with previously reported intermediates. However, similar stable intermediates have been reported in the literature for similar compounds, such as naphthalene, under anaerobic conditions [6], and for 2-naphthol under aerobic conditions [48,57]. In this context, it is worth noting that strong adsorption of the photocatalyzed byproducts during the photocatalytic reforming of 2-naphthol limits full mass balance in the presented study. This observation is consistent with the previously discussed phenomenon of poisoning the surface of the catalyst, which leads to constraining the complete mineralization of the parent compound and the formed byproducts during the photocatalytic process under anaerobic conditions [6].

To extend the investigation of by-products, the reaction aqueous medium after illumination was analyzed using an HPIC chromatograph to identify the formation of small and short-chain organic acids. Trace amounts of formic acid and acetic acid were detected (Scheme 1), confirming the extension and continuation of 2-naphthol oxidation to the formation of CO₂ and H₂O.

3.8.2. Trapping and EPR Experiments

To determine the primary reactive species participating in the photocatalytic degradation of 2-naphthol, various trapping agents were added to the reaction medium. Potassium iodide (KI) and ter-butyl alcohol (TBA) were used as hole and hydroxyl radical scavengers, respectively (Figure 13) [6]. In this context, it is worth mentioning that a superoxide radical (${\mathrm{O}}_2^{•-}$) scavenger was not included in this investigation because the scavenger experiments were performed under oxygen-free conditions. After preparing the reaction suspension, the reactor or the vial was sealed and then purged with high-purity Ar gas. Therefore, molecular oxygen was not intentionally present in the system. Because ${\mathrm{O}}_2^{•-}$ is mainly produced by the reduction of dissolved or adsorbed O₂ by photogenerated electrons, and O₂ is removed from the reaction medium, ${\mathrm{O}}_2^{•-}$ was not expected to be a main reactive species in the present anaerobic photoreforming system. Nevertheless, it should be mentioned that the absence of a dedicated superoxide scavenger experiment partially limits a complete mapping of all potential minor reactive species pathways. This experiment could have provided complementary information and further confirmed the negligible involvement of ${\mathrm{O}}_2^{•-}$ in the present anaerobic system [51]. Unlike all previous experiments that used solar light as the irradiation source, UV light was employed in these trapping, EPR, and photolysis experiments in order to reduce the degradation rates of both the parent compound and the resulting intermediates. This modification of the light source guarantees the detection of EPR signals and clearer scavenger responses, thereby distinguishing the roles of the reactive species (holes and hydroxyl radicals) present in the reaction medium during the photodegradation of 2-naphthol. Indeed, the photolysis experiment was included in Figure 13 as an internal reference for the scavenger investigation, but not as a general control for the solar photocatalytic experiments. Since the scavenger experiments were performed under UV irradiation, the photolysis results show direct light-only removal of 2-naphthol under the same conditions. This comparison is important for the KI and TBA systems.

As shown in Figure 13, the photolysis experiment was conducted to monitor the degradation efficiency of 2-naphthol in the absence of the Pt-GTiO₂ photocatalyst under the same UV irradiation conditions used in the scavenger experiments. The photolysis of 2-naphthol after 120 min of illumination was found to be around 10% of the initial concentration of 2-naphthol. The addition of the hole scavenger, KI, to the system in the presence of Pt-GTiO₂ resulted in 15% degradation efficiency, which is very close to the photolysis value. In contrast, 50% conversion efficiency was achieved after addition of the TBA to the photocatalytic system. Compared to the control system (the photocatalytic system without any scavenger), which achieved about 60% removal efficiency, the addition of KI led to a 75% reduction in the removal of 2-naphthol, suggesting that the photogenerated holes (h⁺) played a significant role in the primary degradation pathway. The scavenging of photogenerated holes from the system leads to a removal efficiency comparable to the photolysis process, which is the least efficient degradation process. On the contrary, a slight reduction of about 15% in the removal efficiency of the control system was observed in the presence of the TBA in the reaction system, highlighting the limited contribution of the hydroxyl radicals in the primary degradation pathway [6].

This observation suggests that both photoformed reactive species (OH^•, h⁺) play synergistically but unevenly to enhance the photodegradation of 2-naphthol. Based on these results, it can be proposed that the degradation of 2-naphthol occurs mainly through a direct oxidation pathway. This pathway involves charge transfer between the substrate organic pollutant and the h⁺ on the material surface [6]. The other, very limited pathway is indirect, via the attack of 2-naphthol by OH^•. These OH^• species formed indirectly through the photooxidation of the adsorbed water molecules (Equation (7)) or the hydroxyl ions (Equation (3)) on the surface of the TiO₂ photocatalyst. Several research groups have reported that such suggested mechanisms are commonly involved in the photocatalytic degradation of organic pollutants on the surface of different photocatalysts [14,57]. This suggests a multi-step reaction mechanism in which OH^• and h⁺ play key roles, while H₂O acts as a facilitator in producing OH^• and other reactive species. In fact, the synergistic interaction of all active species is essential for achieving maximum photocatalytic activity.

$$\mathrm{Pt-Ti}{\mathrm{O}}_2\left[h^{+}\right]+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{d}\right)⟶\mathrm{Pt-Ti}{\mathrm{O}}_2+{\mathrm{H}}^{+}+\mathrm{O}{\mathrm{H}}^{•}$$

(7)

Furthermore, EPR spectroscopy, using DMPO as a spin-trapping agent, is regularly employed to confirm the production and dynamics of OH^• in the presence and absence of organic substrates, such as 2-naphthol, under illumination [58]. In fact, this technique was used to support the previously suggested mechanism for the reactive species involved in the system via monitoring the formation of OH^• [6,59]. In the dark, no signal for the DMPO–OH^• species was observed, indicating that no OH^• formed without illumination (not presented). As presented in Figure 14, upon starting the illumination of the Pt-GTiO₂/DMPO aqueous system in the absence of 2-naphthol, a weak DMPO–OH^• signal was detected, indicating the initiation of OH^• generation via water oxidation by the h⁺. However, after the addition of 2-naphthol to the system, the signal intensity decreased slightly, reflecting minor consumption of the OH^• in the system [6]. The stronger DMPO–OH^• signal in the absence of 2-naphthol compared to DMPO–OH^• in the 2-naphthol system confirmed the dominant role of the photogenerated holes in the degradation process. These findings align with previously reported proposed mechanisms, which suggest that h⁺ plays a significant role in the initial step of photooxidation of organic compounds such as naphthalene, benzene, oxalic acid, and methanol [6,21,22,34].

3.8.3. Proposed Mechanism

In order to understand the redox reaction pathways of the formed intermediates, such as diols, aldehydes, ketones, and carboxylic acids, that were identified in Scheme 1 during the photocatalytic reforming of 2-naphthol on the surface of Pt-GTiO₂, the expected mechanism is proposed based on the results presented in this study and the literature, and is shown in Scheme 2 and Scheme 3. In fact, the detected intermediates clearly indicate that the redox reactions during the photoreforming process involve several complex pathways, such as oxidation, hydroxylation, ring-opening, reduction, and fragmentation reactions [6,48]. To the best of our knowledge, this is the first study to document the photoreforming pathways for 2-naphthol on the surface of a TiO₂-based material.

As shown in Scheme 2 and Scheme 3, the photocatalytic reforming process is initiated upon starting the irradiation. The Pt-GTiO₂ photocatalyst absorbs photons that have an energy above or equal to its bandgap (≈3.18 eV). As a result, electron–hole pairs (e⁻/h⁺) are generated (Equation (2)), which migrate to the surface to participate in the redox reaction, unless they recombine [4]. The powerful oxidant holes may react directly with the bound water or hydroxide on the surface of the photocatalyst to form highly reactive OH^• (Equations (3) and (7)). The other strongly expected pathway for the holes is reaction with the adsorbed 2-naphthol, as revealed by the scavenger experiment shown in Figure 13. In this pathway, holes attack the naphtholic ring, leading to deprotonation and formation of a naphthoxide anion or radical intermediate via proton abstraction [4].

On the other hand, the surviving photogenerated electron from the recombination process takes advantage of the presence of co-catalyst Pt NPs on the surface of GTIO₂ and accumulates there. These electrons then participate in the reduction of the present protons to form hydrogen atoms (Equation (4)), which in turn dimerize to produce molecular hydrogen (Equation (5)) [6,21].

Subsequent oxidation of the formed naphthoxide intermediate through attachment of the formed surface-bound hydroxyl radicals yields naphthalene-1,2-diol (A). The formed diol is further oxidized by a similar mechanism to form 2-Hydroxy-1,4-naphthalenedione (B) [14], which is characterized by partial ring destabilization. This intermediate is a quinone derivative, often detected through the oxidation of phenolic and aromatic compounds [14,60]. A new hydroxylation step oxidizes the Dione intermediate (B), which successively undergoes a ring cleavage reaction to produce phthalic acid (K). This compound (K) may be involved in an alkylation reaction with alkyl radicals (R^•) present in the reaction medium, producing dialkylphthalates (E). Such compounds have been detected during the photocatalytic oxidation of naphthalene in aerobic and anaerobic conditions over different metal oxides [6,60,61].

Moreover, the further oxidation of phthalic acid (K) via the photo-Kolbe reaction leads to the formation of 2-Methylbenzaldehyde (D), which, with successive hydroxylation and partial ring destabilization, leads to ring cleavage and the generation of aliphatic species as octanoic acid (F) [48]. In the final stages of photooxidation of 2-naphthol, simple linear organic acids such as formic acid (I) and acetic acid (J) are produced as a result of the continuation of the photo-Kolbe reaction [57], which finally convert to H₂O and CO₂ [21,22]. It is well-documented in the literature that the attack of aromatic compounds by reactive species present in the photocatalytic system leads to hydroxylation, fragmentation, and eventual breaking of the aromatic ring, converting them into simpler, less-toxic compounds such as CO₂ and H₂O [48,61,62].

Another pathway for photocatalytic degradation of 2-naphthol is likely to occur via direct hole attack at the electron-rich positions of the naphthalene ring, forming a radical cation [6]. Simultaneously, as the formed by-products can compete with the parent compound for holes, they can also compete for the photogenerated electrons. Thus, this carbocation can involve several reductive reactions, as well as oxidation reactions leading to the formation of 5,6,7,8-tetrahydro-2-Naphthol (G). In a similar manner, the formation of 3,4-Dihydro-2(1H) naphthalenone (H) can be explained by the participation of the intermediate naphthalenone radical (L) in several reductive reactions, which, with further oxidation, can be converted to 3-(2-Methylphenyl)propionic acid (C) via ring-opened carboxylation. Several research groups have reported the involvement of aromatic intermediates in such reductive reaction pathways. For instance, Yoshida et al. [62] have reported the formation of cyclohexanol during the photocatalytic photoreforming of benzene. Meanwhile, Al-Madanat et al. [6] observed the formation of partially reduced compounds, such as 1,2,3,4-tetrahydro-1-naphthalenone and 1,2,3,4-tetrahydro-1-naphthalenol, during the photocatalytic reforming of naphthalene over Pt-UV100 in inert conditions.

4. Conclusions

The results of this study demonstrate that plant-extract-assisted TiO₂ NPs after loading with Pt NPs can efficiently catalyze decomposition of the organic pollutant 2-naphthol and molecular H₂ formation under simulated solar irradiation in the absence of molecular oxygen. The physicochemical characterization of Pt-GTiO₂ confirmed the formation of pure anatase phase with a crystallite size of about 12 nm and a surface area of 140 m² g⁻¹. Meanwhile, PL investigation revealed the formation of a photocatalyst characterized by a lower electron–hole recombination rate, which may be associated with the defect-rich surface chemistry generated during the plant extract synthesis route. The photocatalyst Pt-GTiO₂ has a higher efficiency for the degradation of 2-naphthol compared to Pt-UV100, and is similar to Pt-P25 commercial TiO₂ powders. In the case of molecular H₂, Pt-GTiO₂ exhibited a better and higher formation rate than that of Pt-P25. Optimization of the reaction conditions showed that higher 2-naphthol conversion and H₂ formation activities can be achieved in a basic medium, at an optimal catalyst dose of 2.0 mg mL⁻¹. It was found that Pt-GTiO₂ can efficiently degrade 2-naphthol and partially mineralize it to carbon dioxide, water, and molecular hydrogen. The detection of numerous stable aromatic, cyclic, and aliphatic compounds indicates that the photocatalytic reforming process involves several pathways, including hydroxylation, ring-opening, reduction, alkylation, and photo-Kolbe. The scavenging experiments, EPR investigation, and detected intermediates support a proposed photoreforming mechanism in which photogenerated holes appear to play a major role in initiating the oxidation process.