Recyclable Platinum Nanocatalyst for Nitroarene Hydrogenation: Gum Acacia Polymer-Stabilized Pt Nanoparticles with TiO₂ Support

Abstract

Platinum has emerged as an optimal catalyst for the selective hydrogenation of nitroarenes owing to its high hydrogenation activity, selectivity, and stability. In this study, we report the fabrication of platinum nanoparticles stabilized on a composite support consisting of gum acacia polymer (GAP) and TiO₂. It was engineered for the targeted reduction of nitroarenes to arylamines via selective hydrogenation in methanol at ambient temperature. The non-toxic and biocompatible properties of GAP enable it to act as a reducing and stabilizing agent during synthesis. The synthesized nanocatalyst was characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Morphological and structural analyses revealed that the fabricated catalyst consisted of minuscule Pt nanoparticles integrated within the GAP framework, accompanied by the corresponding TiO₂ nanoparticles. Inductively coupled plasma optical emission spectrometry (ICP-OES) was employed to ascertain the Pt content. The mild reaction conditions, decent yields, trouble-free workup, and facile separation of the catalyst make this method a clean and practical alternative to nitroreduction. Selective hydrogenation yielded an average arylamine production of 97.6% over five consecutive cycles, demonstrating the stability of the nanocatalyst without detectable leaching.

1. Introduction

Organic amines and their derivatives are essential components in the production of a wide range of products, including organic substrates, dyes, agrochemicals, pharmaceuticals, polymers, pesticides, and surfactants [1]. Among the various techniques available for their synthesis, the selective hydrogenation of nitroarenes has gained paramount importance [2]. Nevertheless, this method appears to be difficult because of the harsh conditions required for the traditional methods. Achieving high selectivity during the reduction process without side reactions remains elusive. Furthermore, conventional catalytic systems are not only costly but also display poor functional group selectivity coupled with the generation of toxic effluents that cause environmental issues [3].

The major characteristic of homogeneous catalysis is that the catalysts are in the same phase as the reactants and products. The extraction of the catalysts from the reaction mixture to avoid metal carryover is such a problem that it poses a major challenge. In many instances, this prevents industrial applications to a considerable extent [4]. Therefore, catalytic nanoparticles are fixed on a solid support to obviate this difficulty so that recovering the catalysts from the reaction mixture is rather effortless [5]. Immobilized catalysts have several advantages in terms of their catalytic processes. This enhances the efficiency of reagent trapping, provides a large accessible surface area, is conducive to favorable interactions between the support and metal nanoparticles, and prevents the agglomeration of metal nanoparticles during reactions [6]. Nevertheless, heterogeneous ligand-free catalysis has gained popularity because of its environmental benefits in a sustainable context [7].

Metal catalysts are employed in the hydrogenation of nitrobenzene, with copper being preferred for vapor/gas phase applications [8] or in combination with other metals [9]. Nickel is commonly utilized for both gas- [10] and liquid-phase hydrogenation [11]. Precious metals, such as palladium, platinum, ruthenium, and their combinations, are predominantly used in the liquid phase [8]. Platinum-based catalysts exhibit superior activity, selectivity, and stability for the hydrogenation of nitroarenes. Despite their high costs, their effectiveness under mild conditions justifies their investment. Platinum catalysts supported on TiO₂, MgO, and CeO₂ exhibit enhanced performance owing to their synergistic effects. An important challenge in the field of catalysis is the development of safe, environmentally benign, and cost-effective approaches to synthesize nanostructured catalytic materials. Biomolecules and biopolymers have emerged as promising candidates due to their non-toxic, biocompatible, and renewable nature [12]. Previous studies have demonstrated the use of various biomass-derived materials, such as chitosan [13], cellulose [14], starch [15], and lignin [16], as reducing and stabilizing agents for the fabrication of metal nanoparticles [17]. Among these biopolymers, gum acacia, a naturally occurring, water-soluble, and non-toxic polysaccharide, has garnered significant interest because of its exceptional properties [18].

In addition to its role as a reducing and stabilizing agent for metal nanoparticles, gum acacia has been investigated as an effective support for depositing metal nanoparticles [19]. This is attributed to gum acacia’s high surface area, distinctive structural properties, and metal-binding capacity [20]. These characteristics facilitate a synergistic environment between the metal nanoparticles and support, resulting in enhanced catalytic performance [21].

Previous research conducted by Sreedhar et al. [22] illustrated the application of gum acacia polymer (GAP) to stabilize Pt nanoparticles in a water-based medium, resulting in an efficient and reusable colloidal catalyst for the selective hydrogenation of nitroarenes. Nonetheless, colloidal systems often lack the mechanical durability and structural firmness required for prolonged heterogeneous application. To address these challenges, we integrated gum acacia with nanocrystalline TiO₂ and prepared a hybrid support that combined the metal-binding and stabilizing characteristics of GAP with the structural strength and electron transport capabilities of TiO₂. This strategy afforded a Pt nanocatalyst that exhibited exceptional activity and long-term recyclability for nitroarene hydrogenation under mild conditions.

2. Materials and Methods

2.1. Materials

Gum acacia and chloroplatinic acid were procured from Sigma-Aldrich (St. Louis, MO, USA). Titanium isopropoxide and sodium hydroxide were obtained from S. D. Fine Chemicals Pvt. Ltd. (Hyderabad, India) and were used without further purification. All solvents were of analytical grade and were distilled before use.

2.2. Preparation of Pt@TiO₂–GAP

To synthesize Pt@TiO₂–GAP, 1 mmol of titanium isopropoxide (0.284 g) was initially dispersed in 20 mL of a 1.0% (1 g dissolved in 100 mL) aqueous solution of gum acacia under continuous stirring. The precipitation of TiO₂ was subsequently induced by the controlled dropwise addition of a 0.1 M NaOH solution. The pH of the mixture was measured to be between 8.5 and 9.5 using pH paper. A stock solution of chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O; 0.517 g in 100 mL of deionized water) was prepared, yielding a Pt concentration of 0.02 mmol/mL. Thereafter, 10 mL of this solution (0.2 mmol Pt) was added dropwise to the TiO₂–GAP precursor mixture under continuous stirring. After Pt addition, the final pH decreased slightly and stabilized between 8.0 and 8.5. The solution was then transferred to a Teflon-lined stainless-steel autoclave and subjected to hydrothermal treatment at 150 °C for 6 h. The solution was left undisturbed for 24 to 48 h to facilitate the nanoparticle settling. The resulting Pt@TiO₂–GAP nanoparticles were subsequently separated by ultracentrifugation, washed with distilled water and ethanol, and dried in an oven at 100 °C for 6 h.

2.3. Characterization Studies

X-ray diffraction (XRD) analysis was performed using a Rigaku diffractometer (Rigaku Corporation, Tokyo, Japan) with CuKα radiation (λ = 0.1546 nm) operating at 40 kV and 40 mA. Field-emission scanning electron microscopy (FE-SEM) images were obtained using a JEOL instrument (Tokyo, Japan). TEM (transmission electron microscopy) images were captured using a PHILIPS TECNAI FE12 microscope (PHILIPS, Eindhoven, The Netherlands) operated at 120 kV. Selected area diffraction (SAED) patterns were recorded at a camera length of 660 mm in order to examine the crystal structure and phase composition. Fourier-transform infrared (FT-IR) spectra were recorded using a Thermo Nicolet Nexus 670 spectrophotometer (Thermo Fisher Scientific, Madison, WI, USA). Thermal stability and compositional changes were analyzed using thermogravimetric analysis (TGA) coupled with differential thermal analysis (DTA). These measurements were performed with an SDT Q600 V20 thermal analyzer (TA Instruments, New Castle, DE, USA) integrated with a Discovery MS, TA instrument under a N₂ atmosphere at a heating rate of 10 °C/min from room temperature to 600 °C. For surface composition analysis, X-ray photoelectron spectroscopy measurements were carried out using a KRATOS AXIS 165 spectrometer (Kratos Analytical Ltd., Manchester, UK) with Mg Kα radiation (hν = 1253.6 eV). To minimize contamination, the samples were degassed in an XPS chamber for several hours prior to analysis. A charge neutralizer (2 eV) was used, and the binding energy of the C 1s core level (284.63 eV) of adventitious hydrocarbons was used as an internal reference.

For structural characterization, ¹H and ¹³C NMR spectra were recorded using CDCl₃ as the solvent and TMS as an internal reference. Chemical shifts (δ) are reported in parts per million (ppm), and coupling constants (J) are given in hertz (Hz). The Pt content in the Pt@TiO₂–GAP nanocomposite was quantified using inductively coupled plasma optical emission spectroscopy (ICP-OES). For this purpose, 1 mg of the compound was initially digested with 0.5 mL of HNO₃ in a quartz tube and further diluted to 100 mL with double-distilled water. This solution was then analyzed using an ICP-OES instrument provided by Themofischer Scientifics, Oxford, UK, the iCAP-6500-DUO with a 40.68 MHz RF generator. The measurements were performed at a wavelength of 324.270 nm, and a three-point calibration method was adopted by calibrating the instrument with standard platinum solutions of 0.5, 1.0 mg/L, and 1.5 mg/L. The structures of the prepared compounds were confirmed by comparing the spectroscopic data with the literature data (details in SI) [23,24,25,26].

2.4. Catalytic Hydrogenation Studies

A 100 mg quantity of Pt@TiO₂–GAP (0.536 mol%) was added to a reaction flask containing 1 mmol of nitroarene in 5 mL of methanol for a standard catalytic reaction. A hydrogen balloon was fitted at the top of the flask to continuously provide the hydrogen for the reaction. The reaction mixture was stirred at room temperature and periodically monitored using thin-layer chromatography (TLC). Upon completion of the reaction, 5 mL of ethyl acetate was used to dilute the mixture, followed by centrifugation to recover the catalyst. The supernatant liquid was carefully separated, and the catalyst was washed with ethyl acetate and dried in an oven for use in subsequent cycles. The organic phases were then combined, placed in a separating funnel, and washed thoroughly with water. The supernatant liquid was collected and concentrated under reduced pressure to yield a crude product, which was subsequently purified via column chromatography using hexane and ethyl acetate as eluents.

3. Results and Discussion

3.1. Characterization of Pt@TiO₂–GAP

A comprehensive analysis of the recoverable hybrid catalyst was performed using various analytical techniques to gain insights into its structural characteristics.

3.1.1. FT-IR and ICP-OES

Fourier-transform infrared (FT-IR) spectroscopy was used to investigate the incorporation of platinum nanoparticles (Pt NPs) into the gum acacia polymer (GAP) matrix. Furthermore, this study confirmed the synergistic interaction between nanoparticles and their respective supports. The FTIR spectrum of the biopolymer GAP exhibited several characteristic absorption bands (Figure 1).

The GAP spectrum displays a peak at 3465 cm⁻¹, which is associated with the stretching of hydroxyl (–OH) groups found in water, polysaccharides, and carboxyl groups. In Pt@TiO₂–GAP, this band experienced a shift and reduction in intensity, indicating hydrogen bonding interactions between gum acacia and titanium dioxide (TiO₂). The surface hydroxyl groups on TiO₂ form hydrogen bonds with the functional groups of gum acacia, leading to changes in its spectral properties [27]. The peak observed at 2939 cm⁻¹ is indicative of C–H stretching vibrations, which are characteristic of aliphatic groups in GAP. The appearance of this peak appears at 2927 cm⁻¹ in the Pt@TiO₂–GAP composite, suggesting that the aliphatic structures of gum acacia are largely preserved following the integration of TiO₂ [28]. The peak near 1659 cm⁻¹ corresponds to C=O stretching and N–H bending vibrations in GAP. The retention of this peak at 1686 cm⁻¹ in the Pt@TiO₂–GAP composite implies that the carbonyl functionalities remained intact, indicating minimal disturbance to these groups during the incorporation of TiO₂. The peaks at 1426 and 1112 cm⁻¹ are associated with C–O stretching vibrations, which are indicative of the polysaccharide structures in GAP. The presence of these peaks at 1444 and 1181 cm⁻¹ confirms that the polysaccharide backbone of gum acacia was maintained after TiO₂ incorporation. Additionally, a broad and intense band below 504 cm⁻¹ in Pt@TiO₂–GAP is attributed to Ti–O–Ti vibrations, confirming the presence of TiO₂ in the composite. Subtle changes in the absorption frequencies of Pt@TiO₂–GAP indicate interactions between the biopolymer and embedded metal nanoparticles. Observations have shown that proteins bound to TiO₂ nanoparticles are free amine groups or carboxylate ions of amino acid residues, enabling nanoparticle stabilization by surface-bound proteins [29]. The interaction between gum acacia and TiO₂ nanoparticles via the hydroxyl groups of arabinoses, rhamnose, galactose, and the carboxylic groups of glucuronic acid facilitates Pt encapsulation with TiO₂ nanoparticles, providing steric stabilization. Gum acacia proved to be an effective capping agent that enabled surface passivation and suppressed nanoparticle growth via the –COO and –OH functional groups. The Pt loading in the final composite was determined using inductively coupled plasma optical emission spectrometry, which revealed a Pt content of 4.48% in the Pt@TiO₂–GAP nanocomposite.

3.1.2. XPS Analysis

An in-depth investigation of the elemental composition, surface properties, and oxidation states of the platinum species within the Pt@TiO₂–GAP nanocomposite was performed using X-ray photoelectron spectroscopy (Figure 2a). The survey scan spectra confirmed the presence of core elements. Peaks corresponding to O 1s (~530 eV), Ti 2p (~460 eV), N 1s (~400 eV), C 1s (~285 eV), and Pt 4f (~71–75 eV) were detected, verifying the integration of C, N, O, Ti, and Pt into the GAP matrix. The signals for C, N, and O were linked to the functional groups, such as –NH₂ and –COOH, found in the GA biopolymer.

Figure 2b shows deconvoluted narrow scans of C 1s, highlighting the chemical diversity of carbon in the Pt@TiO₂–GAP nanocomposite. The peaks associated with C–H/C–C (~285 eV), C–O (~286 eV), and O–C=O (~290 eV) functionalities reflect the varied carbon environments within the gum acacia. The presence of oxygen-containing functional groups indicates that gum acacia is abundant in hydroxyl and carboxyl groups, which assist in stabilizing the Pt and TiO₂ nanoparticles. These functional groups are instrumental for binding Pt and TiO₂, thereby enhancing the stability of the composite [30]. Figure 2c shows the core-level Pt 4f spectra of the nano, revealing two spin–orbit doublets. The primary peaks at lower binding energies, Pt 4f7/2 (~71 eV) and Pt 4f5/2 (~74 eV), indicate metallic platinum (Pt⁰), while the peaks at higher binding energies (~72 eV and ~75.5 eV) correspond to oxidized platinum (Pt²⁺) from surface oxidation during nanoparticle formation. During the hydrothermal process, Pt⁰ species were generated in situ as the hydroxyl and carboxyl groups in gum acacia served as gentle biogenic reducing agents, transforming Pt⁴⁺ from chloroplatinic acid into Pt⁰ nanoparticles. This eco-friendly synthetic approach eliminates the need for additional reducing agents and is consistent with existing studies on the reduction of noble metals using polysaccharides [31]. The coexistence of Pt⁰ and Pt²⁺ indicates the presence of a thin surface oxide layer, which contributes to catalytic stability during hydrogenation reactions [32]. Although both oxidation states are typical for Pt-based nanocatalysts, spectral deconvolution is limited without a GAP–TiO₂ control sample for background subtraction. Nevertheless, the distinct Pt 4f peaks confirmed Pt incorporation, and the dual oxidation states aligned with the literature on redox activity in heterogeneous catalysts [33]. These findings were supported by ICP-OES and FTIR analyses, confirming the chemical stability of the Pt@TiO₂–GAP system.

3.1.3. XRD Analysis

The X-ray diffraction pattern of the Pt@TiO₂–GAP catalyst, shown in Figure 3, indicates the presence of both anatase and rutile phases of TiO₂. A significant peak at 2θ = 25.3° is associated with the (101) plane of anatase, whereas the other reflections at 2θ = 48.0°, 54.5°, 62.5°, 68.9°, and 75.0° correspond to the (200), (211), (204), (116), and (215) planes of anatase, respectively. The characteristic peaks of the rutile phase are also evident at 2θ = 27.4°, 36.1°, 39.2°, 44.0°, and 54.3°, which align with the (110), (101), (200), (210), and (211) planes (JCPDS No. 21-1276), confirming the mixed-phase nature of the TiO₂ support [34]. No distinct diffraction peaks for metallic Pt were detected, likely due to the low Pt loading and high dispersion of Pt nanoparticles within the TiO₂–GAP framework, as corroborated by the TEM and XPS data. The coexistence of anatase and rutile phases in TiO₂ is known to influence its catalytic behaviour. Anatase provides higher surface activity and promotes electron transfer, whereas rutile enhances the structural robustness and thermal stability of the support [35]. This combination improves the efficiency and durability of the catalyst during nitroarene hydrogenation. The XRD pattern of the Pt/TiO₂ catalyst (without GAP) is presented in the Supplementary Information (Figure S1) for structural comparison. It shows similar TiO₂ phase reflections but lacks discernible Pt peaks. However, initial tests indicated that this catalyst experienced significant Pt leaching and poor recyclability. These issues underscore the importance of gum acacia in the Pt@TiO₂–GAP catalyst, which aids Pt immobilization, enhances dispersion, and ensures long-term catalytic stability.

The average crystallite sizes were calculated using the Debye–Scherrer [36] Equation (1), and the average size of the dispersed TiO₂ was found to be 12.55 nm.

$$D={\displaystyle \frac{K\lambda }{\beta cos\theta }}$$

(1)

where ‘D’ is the average crystallite size, ‘λ’ is the wavelength of the incoming radiation (Cu Kα with a wavelength of 1.540 Å), ‘K’ is a constant (0.89), often known as the shape factor, ‘β’ is the FWHM, and ‘θ’ is the diffraction angle.

3.1.4. FE-SEM Study

Field-emission scanning electron microscopy (FE-SEM) imaging of Pt@TiO₂–GAP revealed the morphology and dispersion of nanoparticles within the gum acacia matrix. At a lower magnification (Figure 4a), agglomerates of particles were discernible, resulting from the interaction between gum acacia (GAP), TiO₂, and Pt nanoparticles. A rough, porous texture is advantageous for catalyst supports because it provides an increased surface area for active sites [37]. The non-uniform particle distribution suggests partial agglomeration of TiO₂ or Pt nanoparticles, a phenomenon commonly observed in biopolymers such as gum acacia [38]. The web-like features represent the organic residues of gum acacia, which encapsulate TiO₂ and Pt. At a higher magnification (Figure 4b), smaller particles and detailed textures became apparent. Pt nanoparticles manifested as bright dots across the TiO₂–gum acacia surface. The entangled matrix illustrates the polymeric nature of gum acacia, which stabilizes the TiO₂ and Pt particles. The contact between the components indicates efficient electron/charge transfer pathways, which are beneficial for applications in photocatalysis, sensing, and fuel cell supports.

3.1.5. TEM Study

Transmission electron microscopy (TEM) images of Pt@TiO₂–GAP, shown in Figure 5a, reveal an irregularly shaped, agglomerated particle network, suggesting strong interactions between TiO₂, Pt, and gum acacia. The dark regions likely correspond to dense Pt and TiO₂ particles, whereas the lighter regions are associated with the biopolymer matrix.

Figure 5b shows TiO₂ and Pt nanoparticles embedded within the gum acacia matrix, with nanosheets supporting TiO₂ and Pt stabilization. The well-dispersed Pt nanoparticles are crucial for catalytic applications. Figure 5c reveals small, distributed nanoparticles, confirming successful Pt deposition onto the TiO₂–GAP support, with visible lattice patterns indicating a crystalline nature. The selected area electron diffraction (SAED) pattern in Figure 5d shows bright, concentric rings corresponding to TiO₂ and Pt crystal planes, with distinct diffraction spots confirming their crystalline structure, essential for catalytic efficiency.

3.1.6. TGA-DTG Analysis

The thermal stability and decomposition characteristics of the nanocomposite were evaluated by thermogravimetric analysis (TGA). A detailed examination of the TGA profiles of Pt@TiO₂–GAP composite, conducted under a nitrogen atmosphere at a heating rate of 20 °C/min over a temperature range of 25–1000 °C, revealed distinct thermal behaviours characterized by significant mass loss in successive stages (Figure 6). The thermogram of the Pt@TiO₂–GAP composite shows three distinct decomposition steps. The initial weight loss of 12.45% observed between 30 and 115 °C indicates the removal of loosely bound water molecules from the biopolymer framework. The subsequent decomposition steps, accounting for 4.0% between 115 and 220 °C, are attributed to the decomposition of low-molecular-weight organic compounds or weakly bound volatile components in gum acacia, while a 5.3% loss between 230 and 470 °C is associated with the decomposition of the organic matrix (gum acacia) and potential oxidation of the remaining carbonaceous content. No significant weight loss was observed above 500 °C because the remaining material primarily consisted of TiO₂ and Pt. The results underscore the hybrid nature of the composite, with contributions from the inorganic metal oxide phase and the organic biopolymer scaffold. The Pt@TiO₂–GAP composite demonstrated good thermal stability, with the main decomposition of the organic content occurring below 500 °C. The presence of TiO₂ and Pt enhances thermal resistance, rendering it suitable for high-temperature applications such as catalysis.

3.2. Catalytic Activity of Pt@TiO₂–GAP

The well-known Pt/TiO₂ catalyst was initially subjected to hydrogenation under 1 atm of H₂ pressure at room temperature in methanol, without the addition of gum acacia (GAP), for comparative analysis. This catalyst exhibits significant metal leaching during these reactions, leading to poor recyclability and diminished catalytic efficiency. This behaviour, similar to homogeneous catalysis owing to the weak attachment of metals, highlights the importance of adding GAP to improve the stability of nanoparticles and prevent the loss of Pt during the process. Then, 4-methoxy nitrobenzene was subjected to hydrogenation under the same reaction conditions with Pt@TiO₂–GAP, followed by 4-nitrobenzylalcohol, 4-butyl nitrobenzene, 4-tert-butyl nitrobenzene, and 4-(trifluoromethyl) nitrobenzene (Scheme 1). Additional substrates were tested under optimized reaction conditions.

As shown in

Table 1. Hydrogenation of nitroarene by Pt@TiO₂–GAP nanocatalyst ^a.

Entry	Substrate	Product	Time (h)	Yield (%)
1			2	99
2			3	97
3			2	99
4			2	98
5			3	94

^a Reaction conditions: nitroarene (1 mmol) in methanol, H₂ (1 atm), and 100 mg of nanocatalyst at room temperature.

, Pt@TiO₂–GAP exhibited exceptional catalytic activity and selectivity, assisting the selective hydrogenation of nitroarenes to the desired aniline products. Exhibiting exceptional catalytic performance, Pt@TiO₂–GAP facilitated the transformation of five distinct nitroarene substrates, each characterized by unique functional groups and steric properties, with all reactions successfully completed within 2 h. The halogen substituent remained unaltered throughout the reaction, indicating the potential for the additional functionalization of the amine products. Steric hindrance at different positions is well tolerated. Overall, these findings reflect the excellent catalytic ability of Pt@TiO₂–GAP and its enhanced activity. The structures of the products were determined from their ¹H and ¹³C NMR spectra in the literature (S2–S7).

3.3. Proposed Mechanism

The hydrogenation of nitro compounds to amines is a well-established transformation, with Pt@TiO₂–GAP nanocomposites serving as efficient heterogeneous catalysts (Scheme 2). Upon exposure to H₂, platinum nanoparticles within the TiO₂–GAP matrix facilitate molecular hydrogen dissociation into reactive atomic hydrogen species. The process is initiated by Pt⁰ centers, as verified by XPS analysis (Pt 4f₇/₂ at approximately 71 eV), which initiate hydrogenation.

The TiO₂ support, consisting of anatase and rutile phases, improves electron mobility and stabilizes Pt nanoparticles, whereas the gum acacia polymer assists in dispersing and stabilizing catalytic sites and enhances substrate binding through hydrogen bonding and π–π interactions. The adsorbed nitroarene undergoes stepwise reduction on the hydrogen-rich Pt surface, transforming –NO₂ into a nitroso intermediate (R–NO), then into hydroxylamine (R–NHOH), and, finally, into a primary amine (R–NH₂). The presence of surface-bound hydrogen and the stabilization of intermediates allows this sequence to proceed efficiently. The catalytic cycle is completed by desorption of the product and regeneration of the active Pt sites. This mechanistic pathway is supported by the selectivity of the catalyst, high yields across various substrates, and its ability to be recycled over multiple cycles. These findings, along with detailed structural (XPS, FTIR, TEM, and ICP-OES) and spectroscopic analyses, confirm the active role of the Pt species within the TiO₂–GAP matrix. The synergy between metallic Pt, the mixed-phase TiO₂ support, and the functional biopolymer scaffold enables efficient hydrogen activation and substrate transformation, which is consistent with the established mechanistic frameworks for Pt-catalyzed nitroarene hydrogenation [2,3,36].

3.4. Heterogeneity and Recyclability Studies

A heterogeneity test was conducted to verify whether nitro reduction occurs through purely heterogeneous catalysis or involves any partial contribution of leached metal species. This method is crucial for differentiating real heterogeneous catalysis, where the activity is strictly confined to the solid catalyst surface, from homogeneous contributions originating from dissolved active species within the reaction medium. In a typical reaction, the catalyst was filtered off after partial conversion, and the reaction was continued with the filtrate. The results revealed no further substrate conversion, confirming that the nanocatalyst was truly heterogeneous.

To study the recycling efficacy of 4-methoxy aniline, multiple consecutive runs were conducted under identical reaction conditions (

Table 2. Reusability of Pt@TiO₂–GAP applied for catalytic reduction of 4-methoxy nitrobenzene over five consecutive cycles.

Cycle (% Yield)
Entry	Catalyst	1st	2nd	3rd	4th	5th	Avg. % Yield
1	Pt@TiO2–GAP	99	99	98	97	95	97.6

). After each cycle, the catalyst was recovered by simple filtration, washed thoroughly with methanol, and reused in the subsequent cycles. The Pt@TiO₂–GAP catalyst maintained over 95% of its initial catalytic activity, demonstrating exceptional stability and recyclability even after five successive cycles (Figure 7).

The findings of this study were compared with the performance of various catalysts documented in the literature for the reduction of nitroarenes under mild conditions (

Table 3. Comparison of Pt@TiO₂–GAP’s ability with other reported catalysts.

Catalyst	Substrate	% Yield	Time (min)	Reaction Conditions	TOF (min−1)	Ref.
Au/TiO2@PVA	1-methyl-4-nitrobenzene	97	300	MeOH, RT, H2 (1 atm)	0.166	[39]
Pd/TiO2	p-nitrophenol	81	360	MeOH, RT, H2 (1 atm)	0.507	[40]
Pt/TiO2@glass film	Nitrobenzene	87	30	RT, H2 (1 atm)	0.631	[41]
Fe3O4@TiO2	p-nitroanisole	90	150	EtOH, RT, H2 (1 atm)	0.143	[42]
Pt/TiO2@RGO	Nitrobenzene	95	1200	MeOH, RT, H2 (1 atm)	0.309	[43]
Ru/TiO2	p-chloro-nitrobenzene	76	120	MeOH, RT, H2 (1 atm)	28.45	[44]
Pd/RGO	p-methoxy nitrobenzene	99	300	EtOH\H2O, RT, H2 (1 atm)	0.943	[45]
Pt/TiO2@GAP	p-methoxy nitrobenzene	99	120	MeOH, RT, H2 (1 atm)	0.362	This work

). Although many studies have focused on yield and reaction time, Turnover Frequency (TOF) provides deeper insight into catalytic efficiency by reflecting the number of product molecules formed per active site per unit time. The Au/TiO₂@PVA catalyst achieved a 97% yield in 240 min under ambient conditions (MeOH, RT, 1 atm H₂) with a moderate TOF of 0.207 min⁻¹ [39].

Pd/TiO₂ showed an 81% yield in 360 min, corresponding to a higher TOF of 0.507 min⁻¹ [40], whereas Pt/TiO₂@glass film delivered a similar yield of 87% in 30 min but with a notable TOF of 0.631 min⁻¹ [41]. Fe₃O₄@TiO₂ offered a 90% yield in only 150 min with a TOF of 0.143 min⁻¹ [42], and Pt/TiO₂@RGO gave a lower 95% yield in 1200 min with a TOF of 0.305 min⁻¹ [43]. Notably, the Ru/TiO₂ catalyst demonstrated the highest intrinsic activity, with a TOF of 28.45 min⁻¹, reaching a 76% yield in 120 min, and the Pd/RGO catalyst exhibited a 99% yield over 300 min, with a respectable TOF of 0.943 min⁻¹ [44,45]. A higher TOF indicates a more kinetically active catalyst, which is crucial for comparing systems with varying metal concentrations and reaction times. In this study, the Pt@TiO₂–GAP catalyst achieved a 99% yield in just 120 min under mild conditions (MeOH, RT, and 1 atm H₂), with a TOF of 0.362 min⁻¹. Although it does not have the highest TOF overall, it is notable for its combination of rapid reaction kinetics, high yield, and a straightforward, scalable synthetic process. Compared with many other Pt- and Pd-based systems, it offers a better balance between performance and practicality. Therefore, our Pt@TiO₂–GAP catalyst is a promising alternative for efficient nitroarene reduction, combining excellent reactivity with favourable green chemistry metrics.

4. Conclusions

An innovative and eco-friendly approach was established for the synthesis of a highly efficient and reusable Pt@TiO₂–GAP nanocatalyst tailored for the selective hydrogenation of nitroarene compounds. The results reveal that these heterogeneous catalysts are remarkably effective in reducing a variety of nitroarenes without the necessity of protecting the functional groups and achieving rapid reaction times and conversion yields ranging up to 94–99% for different nitroarenes using non-toxic agents. Moreover, the straightforward recovery of nanocatalysts effectively addresses the issue of catalyst separation after the reaction, thereby enhancing the sustainability of this method. Analytical techniques, including FT-IR and XPS, along with structural (XRD), morphological (SEM and TEM), and thermal (TGA–DTG) analyses, indicated that gum acacia plays a crucial role in the reduction and stabilization of active platinum species. This is achieved through electrostatic interactions with the carboxylate ion groups of amino acid residues in proteins. Additionally, the presence of nano TiO₂ significantly boosts the catalytic performance and aids the efficient recovery of the synthesized Pt@TiO₂–GAP nanocatalyst.