Microwave-Assisted Synthesis of Pt/SnO2 for the Catalytic Reduction of 4-Nitrophenol to 4-Aminophenol

In this study, we present a new approach for the synthesis of Pt/SnO2 catalysts using microwave radiation. Pt(IV) and Sn(IV) inorganic precursors (H2PtCl6 and SnCl4) and ammonia were used, which allowed the controlled formation of platinum particles on the anisotropic SnO2 support. The synthesized Pt/SnO2 samples are mesoporous and exhibit a reversible physisorption isotherm of type IV. The XRD patterns confirmed the presence of platinum maxima in all Pt/SnO2 samples. The Williamson-Hall diagram showed SnO2 anisotropy with crystallite sizes of ~10 nm along the c-axis (< 00l >) and ~5 nm along the a-axis (< h00 >). SEM analysis revealed anisotropic, urchin-like SnO2 particles. XPS results indicated relatively low average oxidation states of platinum, close to Pt metal. 119Sn Mössbauer spectroscopy indicated electronic interactions between Pt and SnO2 particles. The synthesized samples were used for the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of excess NaBH4. The catalytic activity of the Pt/SnO2 samples for the reduction of 4-NP to 4-AP was optimized by varying the synthesis parameters and Pt loading. The optimal platinum loading for the reduction of 4-NP to 4-AP on the anisotropic SnO2 support is 5 mol% with an apparent rate constant k = 0.59 × 10–2 s–1. The Pt/SnO2 sample showed exceptional reusability and retained an efficiency of 81.4% after ten cycles.


Introduction
Platinum nanoparticles (PtNPs) have garnered extensive attention due to their diverse applications across fields like biomedicine, energy conversion, storage, environmental cleanup, sensing, and catalysis [1][2][3][4][5][6][7][8][9][10][11][12]. Via the customization of synthesis techniques and conditions, a wide array of platinum nanoparticle shapes can be achieved, each possessing distinct attributes and potential uses [2]. Platinum can be harnessed in the form of nanoparticles dispersed within aqueous or organic mediums, or it can be distributed onto different substrates [2][3][4][5][6][7][8][9][10][11]. While PtNPs in a medium can aggregate, proving challenging to stabilize, dispersing PtNPs on substrates offers versatility for multiple applications. For instance, Papandrew et al. [3] crafted solid acid fuel cells using vapor depositing platinum from Pt(acac) 2 onto solid acid CsH 2 PO 4 . Sized 2-4 nm, these PtNPs act as catalysts for oxygen reduction and electronic conductors in the electrode. In a separate study, Liu et al. [4] produced onion-like carbon nanospheres to anchor atomically dispersed platinum, serving as a catalyst for the hydrogen evolution reaction (HER). Calculations revealed a locally enhanced electric field at the curved support's platinum site, boosting hydrogen evolution kinetics. Haneda et al. [5] explored Pt dispersion's impact on oxidation reactions of carbon monoxide (CO) and propene (C 3 H 6 ), prominent vehicle exhaust pollutants. Intriguingly, while CO oxidation turnover frequency (TOF) remained consistent across Pt/Al 2 O 3 catalysts, C 3 H 6 oxidation TOF surged with increased Pt dispersion. In a distinct context, Ponjavic et al. [6] showcased bacterial nanocellulose (BNC) as an effective eco-friendly support for high-performance PtNPs, particularly in methanol oxidation reactions. This sustainable catalytic system, leveraging BNC instead of conventional carbon-based materials, holds promise for broader implementation. Addressing hydrogen storage, Kostoglou et al. [7] synthesized plasma-derived nanoporous graphene, subsequently decorating it with PtNPs without altering surface chemistry or pore structure. These insights offer a pathway for innovative graphene-based nanocomposites suited for hydrogen storage applications under ambient conditions. Rioux and colleagues [8] delved into PtNPs incorporated within mesoporous SBA-15 silica, unearthing remarkable thermal stability for Pt/SBA-15 catalysts in pertinent turnover scenarios. Their approach outlines a blueprint for constructing various metal/support arrangements to decipher structure-selectivity connections in catalysis. Bai et al. [9] introduced a fresh avenue detailing the creation of ultrasmall PtNPs (averaging 0.9-2.3 nm) stabilized on hollow polymer nanoshells. This composite exhibited prowess as a catalyst for organic oxidation reactions of alcohols under ambient conditions. Size-dependent catalytic trends suggested peak activity in PtNPs of ≈ 1.7 nm, notably evident in the oxidation of 1-phenylethanol to acetophenone. However, PtNPs struggled to stabilize emulsions, leading to the emergence of irregular and sizeable polymer aggregates (>7 nm). Elezovic et al. [10] tackled oxygen reduction reactions, employing PtNPs on two distinct tin oxide-based supports, Sb-SnO 2 and Ru-SnO 2 , in an acidic milieu. Both Pt/SnO 2 catalyst types showcased akin catalytic prowess as Pt supported on commercially accessible carbon supports. Moreover, stability assessments unveiled minimal erosion of the electrochemically active surface area of Pt/SnO 2 catalysts across repetitive cycles, highlighting their robustness for potential fuel cell applications. Smiechowicz et al. [11] investigated Pt/SnO 2 traits and catalytic aptitude in CO oxidation, revealing the substantial influence of treatment conditions-temperature and atmospheric setting during catalyst heating-on active phase dispersion, surface layer composition, and O 2 adsorption capacity of Pt/SnO 2 catalysts. XRD analyses unveiled that H 2 -based heat treatment spurred the formation of Pt-Sn bimetallic compounds and assorted species, profoundly influencing CO oxidation catalyst activity. Martyla et al. [12] adopted a sol-gel technique to craft Pt/SnO 2 systems, yielding high electrocatalytic activity post-low-temperature processing. The findings underscore the potential for facile generation of immensely active Pt/SnO 2 electrocatalysts devoid of the need for thermal reduction conditions. The choice of dispersion method depends on the particular requirements of the application and the nature of the medium into which the platinum nanoparticles are to be placed. Fine-tuning of the dispersion method is a critical factor in ensuring uniform distribution, stability, and desired properties of the dispersed nanoparticles. Ongoing research is therefore dedicated to the development of new synthesis techniques aimed at improving the catalytic properties of platinum and its effectiveness as a catalyst.
In this study, we use microwave radiation to decorate Pt/SnO 2 with different Pt content by modulating the molar ratios of platinum and reducible SnO 2 support. The platinum content was 1, 3, 5, 10, and 15 mol%, corresponding to a ratio [Pt IV /(Pt IV + Sn IV )] of up to 0.15. Our studies address platinum dispersions, physicochemical properties, and catalytic and reusable properties of Pt/SnO 2 in the context of the reduction of 4-NP to 4-AP.
Our study presents a new approach for the synthesis of Pt/SnO 2 catalysts using microwave radiation, which allows the controlled formation of platinum particles on an anisotropic SnO 2 support. The resulting samples possess mesoporous properties with uniformly dispersed PtNPs in the micrometer range. These catalysts exhibit exceptional efficiency in the catalytic reduction of 4-nitrophenol to 4-aminophenol, even after 10 cycles, with optimal performance achieved at a Pt loading of 5 mol%. The results suggest promising applications in catalysis, adsorption, and gas sensing, with the potential for advanced catalytic systems in various environmental and catalytic scenarios.

Stock Solution Preparation
The SnCl 4 solution was prepared by weighing 35.06 g of the solid SnCl 4 ·5 H 2 O and mixing it with 50 mL of deionized Milli-Q water. The H 2 PtCl 6 solution was prepared by mixing 1 g of the solid H 2 PtCl 6 with 986 µL of deionized Milli-Q water. The calculated concentrations of both the tin stock solution and the platinum stock solution were 2.0 mol dm -3 (2M SnCl 4 and 2M H 2 PtCl 6 ).

Synthesis of the Samples
From previously prepared stock solutions, appropriate amounts of the aliquots were taken so that the molar ratio of tin and platinum ions in the mixture was from 0.00 to 0.15 (Table 1). For example, for the synthesis of the SnO 2 sample doped with 15 mol% of Pt IV , i.e., at a ratio [Pt IV /(Pt IV + Sn IV )] = 0.15, the 450 µL of H 2 PtCl 6 stock solution was added to 2.55 mL of SnCl 4 stock solution. The aliquots were added to a 100 mL glass and diluted with 17 mL of deionized Milli-Q water. The solution was stirred with a magnetic stirrer for 15 min before 1 mL of 1 M ammonia was added to a stream and stirred for an additional 15 min. The pH of the solutions was measured using a pH meter before and after the addition of ammonia, and the initial pH of the solutions increased from 0.5-0.7 up to 0.9-1.1. The magnet was removed from the glass, and the newly formed suspension was quantitatively transferred to a glass cuvette, which was inserted in a microwave digestion platform. Microwave-assisted hydrothermal synthesis took place for 30 min at 230 • C. The suspension was then quantitatively transferred to a Petri dish and placed overnight in an oven at 90 • C in order to evaporate the residual water. The precipitate was scraped from the Petri dish and homogenized in a mortar and pestle. It was then heated in air at 400 • C for 2 h in a tube furnace before being used for analysis and characterization.
Scanning electron microscopy was performed on Jeol Ltd. (Tokyo, Japan). 700F fieldemission scanning electron microscope coupled with EDS/INCA 350 system for energy dispersive x-ray spectrometry manufactured by Oxford Instruments Ltd. (Abingdon, UK).
This study utilized an Atomic Resolution Scanning Transmission Electron Microscope (AR STEM), specifically the Jeol ARM 200 CF model, operating at 200 kV. This instrument was coupled with the Gatan Quantum ER system, incorporating capabilities for electron energy loss spectroscopy and energy dispersive X-ray spectrometry using the Jeol Centurio 100 module.
For the nitrogen adsorption analysis conducted at 77 K, the Quantachrome Autosorb iQ3 system was employed, employing the Brunauer-Emmett-Teller (BET) technique to assess the material properties. Prior to testing, a controlled heating process up to 250 • C was administered under vacuum conditions to eliminate residual gases and moisture. The evacuation process continued until the pressure variation ceased to rise rapidly, achieving a level below 50 millitorr per minute. Subsequent adsorption and desorption isotherm measurements at 77 K were performed across a relative pressure span encompassing approximately 10 -5 up to nearly 0.99.
To examine the oxidation state of Sn and Pt in the Pt/SnO 2 samples, X-ray photoelectron spectroscopy (XPS) was employed. The analysis was conducted under ultra-high vacuum (UHV) conditions, utilizing a SPECS instrument. The experimental setup utilized an excitation energy of 1486.74 eV derived from Al Kα X-ray emission and the Phoibos100 electron energy analyzer. In order to neutralize charge accumulation in nonconductive oxide samples, a 5-eV electron flooding method was applied during the XPS analysis. During the evaluation of Pt 4f core levels, a pass energy of 50 eV was selected, whereas for Sn 3d level spectra, a pass energy of 10 eV was employed. The fitting of experimental data curves was carried out using a combination of Gaussian and Lorentzian functions via Unifit software [13]. All photoemission spectra were calibrated utilizing the C 1s peak located at a binding energy (BE) of 284.5 eV. 119 Sn Mössbauer spectra were measured at room temperature in transmission geometry by using a standard WissEl Mössbauer spectrometer setup along with a 119m Sn(CaSnO 3 ) radioactive source (RITVERC JSC) with an activity of~2.2 mCi. The source movement followed a sinusoidal velocity signal with a velocity extrema of ±6.13 mm s -1 . The unfolded spectra were recorded into 2048 channels that were subsequently folded into 1024 channels for analysis and further processing. Isomer shift (δ) values are quoted with respect to a SnO 2 reference powder (Merck) whose isomer shift can be taken to be equal to that of the CaSnO 3 source matrix. The velocity axis was calibrated by measuring the reference SnO 2 (δ = 0 mm s -1 ) powder together with β-Sn (δ = 2.56 mm s -1 ). For the Mössbauer measurements, circular absorbers with a diameter of 15.5 mm were prepared by mixing 15 mg of the original powders evenly with~100 mg of cellulose used as filler material. The recorded spectra were analyzed with version 4.0i of the MossWinn program [14] by assuming the validity of the thin absorber approximation.
UV-vis reflectance spectra were collected with a Shimadzu UV/VIS/NIR spectrometer, model UV-3600. The used wavelength range was from 600 to 200 nm.

Catalytic Measurements
We examined the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using UV-visible spectrophotometry with the presence of our synthesized samples and NaBH 4 . Before catalytic measurements, the solutions containing 4-NP and NaBH 4 were not purged with nitrogen gas. The NaBH 4 solution in water was freshly prepared prior to each experiment. Typically, we mixed 0.3 µmol of 4-NP (20 µL from a 0.015 M solution) with 2.7 mL of ultrapure water in a quartz cuvette. Then, we added 79.3 µmol of NaBH 4 Nanomaterials 2023, 13, 2481 5 of 23 (20 µL from a 0.793 M solution). To this solution, we introduced 20 µL of the catalyst (at a concentration of 3 mg/mL in ultrapure water) using a micropipette, and we mixed it quickly. Right after adding the synthesized samples, we measured the UV-visible absorption spectra at specific time intervals until the nitrophenolate ions' maximum absorption at 400 nm disappeared. The transformation into 4-AP was tracked by observing an increase in the maximum absorption at 300 nm.
Recyclability (reusability) tests were performed using the settings described above. After each cycle, 0.3 µmol of 4-NP (20 µL of 0.015 M solution) was added to the suspension and rapidly mixed, after which UV-vis spectra were collected in the same manner. This was performed for a total of 10 cycles. Before the fourth and seventh cycles, 79.3 µmol of NaBH 4 (20 µL of 0.793 M solution) was also added to the 4-NP solution to ensure that NaBH 4 was in large excess. Figure 1 shows the XRD patterns of the synthesized samples. In the sample without platinum (SP0), only cassiterite was found in the XRD patterns. In addition to cassiterite, platinum was found in samples SP1, SP3, SP5, SP10, and SP15, while (NH 4 ) 2 [PtCl 6 ] was found in samples SP5, SP10, and SP15. The results of the phase analysis are summarized in Table 2. Description: SnO 2 = phase structurally closely related to cassiterite (ICDD Card no. 41-1445); Pt = phase structurally closely related to cubic platinum (ICDD Card no. 04-802); (NH 4 ) 2 PtCl 6 = phase structurally closely related to cubic ammonium platinum chloride (ICDD Card no. 7-218).

XRD Results
The Williamson-Hall plot [15] indicated the presence of size anisotropy with significantly narrower diffraction lines along the direction <00l> compared to the direction <h00> (Table 3 and Figure 2). Due to the pronounced size anisotropy, the values of crystallite sizes in several different hkl directions were estimated from the Scherrer equation: where D hkl is a volume average of the crystal thickness in the direction normal to the reflecting plane hkl, λ is the X-ray wavelength (CuKα), θ is the Bragg angle, and β hkl is the pure full width of the diffraction line (hkl) at half the maximum intensity. The values of β hkl were found from the observed full width at half the maximum intensity (B hkl ) of the diffraction lines after correction for instrumental broadening for which the corresponding width of the diffraction lines of well crystalline ZnO sample was used [16]. The B hkl values of the diffraction lines were determined using the individual profile fitting method (computer program XFIT [17]).    Figure 3 shows SEM images of samples SP1 to SP15 at low magnification, taken in backscatter mode. The much brighter irregular spots correspond to platinum. In Figure 3a, these spots are represented using arrows due to the relatively low platinum content in sample SP1 (1 mol%), while they are clearly visible in the other samples. It can be seen that these relatively large PtNPs are dispersed throughout the sample. The backscattered electron (BSE) signal intensity is approximately proportional to the atomic number (Z) of the elements, with an exponent of around 1.7. Consequently, heavier elements like platinum (Z = 78) exhibit significantly brighter BSE images compared to lighter elements like tin (Z = 50). This relationship contributes to the contrasting brightness observed in SEM-backscattered images of different elements.  Figure 3 shows SEM images of samples SP1 to SP15 at low magnification, taken in backscatter mode. The much brighter irregular spots correspond to platinum. In Figure  3a, these spots are represented using arrows due to the relatively low platinum content in sample SP1 (1 mol%), while they are clearly visible in the other samples. It can be seen that these relatively large PtNPs are dispersed throughout the sample. The backscattered electron (BSE) signal intensity is approximately proportional to the atomic number (Z) of the elements, with an exponent of around 1.7. Consequently, heavier elements like platinum (Z = 78) exhibit significantly brighter BSE images compared to lighter elements like tin (Z = 50). This relationship contributes to the contrasting brightness observed in SEMbackscattered images of different elements.    Figure 3 shows SEM images of samples SP1 to SP15 at low magnification, ta backscatter mode. The much brighter irregular spots correspond to platinum. In F 3a, these spots are represented using arrows due to the relatively low platinum con sample SP1 (1 mol%), while they are clearly visible in the other samples. It can b that these relatively large PtNPs are dispersed throughout the sample. The backsca electron (BSE) signal intensity is approximately proportional to the atomic number the elements, with an exponent of around 1.7. Consequently, heavier elements like num (Z = 78) exhibit significantly brighter BSE images compared to lighter elemen tin (Z = 50). This relationship contributes to the contrasting brightness observed in backscattered images of different elements.          PtNPs are marked with arrows as an example, while the bright spots in the other samples are not marked. Figure 4 shows an SEM image of sample SP5 that, along with big irregular particles (see Figures S4 and S7 in the Supplementary Materials), contains urchin-like anisotropic particles.      Figure 6 shows the STEM and SAED (selected area electron diffraction) results of sample SP3. Figure 6a shows the STEM dark field (DF) image at low magnification. Bright spots corresponding to heavy elements such as platinum can be seen in the DF image. However, in the present case, these platinum-rich areas appear as dark areas due to the large and thick platinum particles in the upper part of the image (see also inset), while the small SnO 2 nanoparticles appear as bright areas. Figure 6b shows a bright-field image (BF) at higher magnification with a SAED image in the inset. The powder patterns are indexed to SnO 2 (cassiterite). Figure 6c shows a high-resolution image of SnO 2 with an FFT (Fast Fourier Transform) image in the (-11-1) zone (inset). The patterns appear as bright dots in the FFT image, and the indexed pattern 011 with 2.64 angstroms, for example, corresponds to the (011) crystallographic planes of SnO 2 with a lattice spacing of 2.64 angstroms. Figure 6d shows a high-resolution image of several SnO 2 -NPs with clearly visible lattice fringes. Figure 6 shows the STEM and SAED (selected area electron diffraction) results of sample SP3. Figure 6a shows the STEM dark field (DF) image at low magnification. Bright spots corresponding to heavy elements such as platinum can be seen in the DF image. However, in the present case, these platinum-rich areas appear as dark areas due to the large and thick platinum particles in the upper part of the image (see also inset), while the small SnO2 nanoparticles appear as bright areas. Figure 6b shows a bright-field image (BF) at higher magnification with a SAED image in the inset. The powder patterns are indexed to SnO2 (cassiterite). Figure 6c shows a high-resolution image of SnO2 with an FFT (Fast Fourier Transform) image in the (-11-1) zone (inset). The patterns appear as bright dots in the FFT image, and the indexed pattern 011 with 2.64 angstroms, for example, corresponds to the (011) crystallographic planes of SnO2 with a lattice spacing of 2.64 angstroms. Figure  6d shows a high-resolution image of several SnO2-NPs with clearly visible lattice fringes.

BET Nitrogen Adsorption-Desorption Isotherm and Pore Volume
In Figure 8, the gas (N2) adsorption (depicted using the red line and squares) and desorption (illustrated with the blue line and triangles) isotherms for the SP0 to SP15 samples are showcased, alongside the corresponding analysis of pore volume distribution. These samples exhibit reversible physisorption isotherms classified as type IV, indicating their mesoporous nature. The evaluation of Brunauer, Emmett, and Teller (BET) specific surface areas reveals a declining trend in the BET surface area across the samples, ranging from 134.1 m 2 /g for the SP3 sample to 71.8 m 2 /g for the SP15 sample. The analysis of pore volume distribution across all samples consistently supports their mesoporous characteristics, with an average pore diameter of approximately 4 nm.

BET Nitrogen Adsorption-Desorption Isotherm and Pore Volume
In Figure 8, the gas (N 2 ) adsorption (depicted using the red line and squares) and desorption (illustrated with the blue line and triangles) isotherms for the SP0 to SP15 samples are showcased, alongside the corresponding analysis of pore volume distribution. These samples exhibit reversible physisorption isotherms classified as type IV, indicating their mesoporous nature. The evaluation of Brunauer, Emmett, and Teller (BET) specific surface areas reveals a declining trend in the BET surface area across the samples, ranging from 134.1 m 2 /g for the SP3 sample to 71.8 m 2 /g for the SP15 sample. The analysis of pore volume distribution across all samples consistently supports their mesoporous characteristics, with an average pore diameter of approximately 4 nm.

XPS Results
In the left part of Figure 9, the graphs depict photoemission spectra encompassing the Sn 3d core levels within the platinum-loaded SnO 2 samples. These spectra exhibit a dual spin-orbital doublet pattern, corresponding to distinct oxidation states of tin-Sn 2+ and Sn 4+ . The dominant signal stems from Sn atoms in the Sn 4+ state (SnO 2 ), evidenced using the Sn 3d 5/2 peak positioned around 486.7 eV binding energy (BE) and the 3d 3/2 peak shifted by 8.4 eV towards higher BE. Nanomaterials 2023, 13, x FOR PEER REVIEW 12 of 24   Figure 9. XPS spectra measured around Sn 3d and Pt 4f core levels of synthesized samples. Figure 9. XPS spectra measured around Sn 3d and Pt 4f core levels of synthesized samples.
Moving to the right panel in Figure 9, Pt 4f photoemission curves of the SnO 2 samples with platinum loading are displayed. These spectra are meticulously fitted with two or three doublets, ascribed to platinum oxidation states: Pt 0 , Pt 2+ , and Pt 4+ . The energy positions of Pt 4f 7/2 peaks emerge at approximately 70.8 eV (Pt 0 ), 72.5 eV (Pt 2+ ), and 74.3 eV (Pt 4+ ). Notably, the energy separation between Pt 4f 7/2 and Pt 4f 5/2 peaks is consistent at around 3.2 eV across all Pt oxidation states, aligning well with existing data on platinum oxides from previous literature.
The refined Pt 4f and Sn 3d spectra guide the determination of peak positions and relative contributions (%) of Pt 4+ , Pt 2+ , Pt 0 , Sn 4+ , and Sn 2+ within the synthesized samples. Comprehensive details of these outcomes, including numerical values, are provided in Supplementary Table S1. Meanwhile, Table 4 encapsulates the XPS-derived average oxidation states (AOS) of platinum (Pt) and tin (Sn) achieved in this investigation, juxtaposed with findings from reference [18]-a comparison elaborated upon in the ensuing discussion.

Mössbauer Spectroscopy
The room temperature 119 Sn Mössbauer spectra of the samples are displayed in Figure 10. The spectra were well fitted with a single symmetrical quadrupole doublet of Lorentzians, with the normalized chi-square of the fit being close to 1 in all the cases. The obtained Mössbauer parameters are listed in Table 5. The near-zero isomer shift values confirm that tin is present in the oxidation state of Sn 4+ . Absorption signals from stannous compounds were not detected. The obtained quadrupole splitting and line width values correspond well to those expected for crystalline SnO 2 (see, e.g., [19]) and thereby confirm the formation of cassiterite. There are small differences in the isomer shift values among the samples (Table 5), which nevertheless reveal a decreasing tendency with increasing Pt concentrations, as depicted in Figure 11. Table 5. Room temperature 119 Sn Mössbauer parameters obtained via the fitting of the spectra displayed in Figure 10 to a symmetrical quadrupole doublet of Lorentzians. δ and ∆ denote the 119 Sn isomer shift and quadrupole splitting, respectively, whereas W stands for the FWHM line width of the individual Lorentzians of the doublet. Numbers between parentheses denote the standard fit error (1σ) in the last digit(s). See also Figure 11 regarding the isomer shift values.     The untreated SP0 sample, void of added platinum, is entirely inactive for the catalytic reduction of 4-NP to 4-AP (refer to Supplementary Materials). Conversely, samples carrying loaded platinum display robust catalytic effectiveness. Among them, the 5-mol% platinum-supported SnO2 (sample SP5) emerges as the most catalytically potent for 4-NP to 4-AP reduction, marked by a kapp rate constant of 0.59 × 10 -2 s -1 . This catalytic proficiency mirrors the decline in absorbance at 400 nm due to 4-nitrophenolate ions and the concurrent rise in absorbance at 300 nm due to 4-aminophenol formation (see Supplementary  Materials).

Catalytic Measurements
The recyclability (reusability) test was performed for the sample SP5 with the highest catalytic activity. The test was performed in the same way as the other catalytic measurements for a total of 10 cycles. As can be seen in Figure 13, sample SP5 is very robust and exhibits very high efficiency in reducing 4-NP to 4-AP even after 10 cycles.  Figure 12 presents the progress of catalytic reduction from 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with excess NaBH 4 over time, utilizing platinum supported on SnO 2 (SP1 to SP15 samples). The insets showcase the ln(A t /A 0 ) plot against reaction time. The depicted values of the rate constant k app (s -1 ) stem from the linear segment slopes, founded on a pseudo-first-order kinetic equation:

Catalytic Measurements
The untreated SP0 sample, void of added platinum, is entirely inactive for the catalytic reduction of 4-NP to 4-AP (refer to Supplementary Materials). Conversely, samples carrying loaded platinum display robust catalytic effectiveness. Among them, the 5-mol% platinumsupported SnO 2 (sample SP5) emerges as the most catalytically potent for 4-NP to 4-AP reduction, marked by a k app rate constant of 0.59 × 10 -2 s -1 . This catalytic proficiency mirrors the decline in absorbance at 400 nm due to 4-nitrophenolate ions and the concurrent rise in absorbance at 300 nm due to 4-aminophenol formation (see Supplementary Materials).
The recyclability (reusability) test was performed for the sample SP5 with the highest catalytic activity. The test was performed in the same way as the other catalytic measurements for a total of 10 cycles. As can be seen in Figure 13, sample SP5 is very robust and exhibits very high efficiency in reducing 4-NP to 4-AP even after 10 cycles. Nanomaterials 2023, 13, x FOR PEER REVIEW 18 of 24

Discussion
In this study, we present a new approach for the synthesis of Pt/SnO2 catalysts using microwave radiation. In contrast to previous methods using organic precursors, we used Pt(IV) and Sn(IV) as inorganic precursors (namely H2PtCl6 and SnCl4) that allow the controlled formation of platinum particles dispersed on the anisotropic SnO2 support. In both syntheses, annealing at 400 °C in the air was performed as the final synthesis step. The XRD patterns (Figure 1) show the platinum maxima in all Pt/SnO2 samples. The Williamson-Hall diagram shows anisotropy of SnO2 (Table 3 and Figure 2), with SnO2 crystallite size of about 10 nm in the c-axis direction (< 00l >) and about 5 nm in the a-axis direction (< h00 >). SEM results confirm the anisotropic nature of SnO2 and show anisotropic, urchin-like SnO2 particles (Figures 4 and S7). SEM backscattering results show that the PtNPs in the Pt/SnO2 samples are quite homogeneously dispersed at the micrometer level ( Figure 3). However, at higher magnification ( Figure 5), it can be seen that the PtNPs are heterogeneously dispersed, with areas that are very rich in platinum and areas that contain almost no platinum (Figures 5 and 7). The results of the BET nitrogen adsorptiondesorption isotherms (Figure 8) show that the BET surface area decreases with platinum loading, consistent with the formation of large compact platinum nanoparticles that decrease the SnO2 surface area. The results of pore volume distributions show that all samples are mesoporous, and the pore diameter is about 4 nm. The presence of ammonia during the synthesis of the Pt/SnO2 powder samples and the subsequent heating at 400 °C act as a templating process, leading to the formation of mesoporous SnO2 samples. These pores provide a large surface area and an interconnected network of voids that can be beneficial for various applications such as catalysis, adsorption, and gas sensing. In addition, the good dispersion of relatively large platinum nanoparticles (PtNPs) may indicate a suitable surface area for catalytic reactions and potentially better accessibility for reactant molecules. Well-dispersed large PtNPs with high metal character could provide more active sites for reaction, contributing to the observed high catalytic efficiency.
The XPS results ( Figure 9 and Table 4) show that the average oxidation state (AOS) of platinum is relatively low, with a minimum of 0.4 for sample SP3 and a maximum of

Discussion
In this study, we present a new approach for the synthesis of Pt/SnO 2 catalysts using microwave radiation. In contrast to previous methods using organic precursors, we used Pt(IV) and Sn(IV) as inorganic precursors (namely H 2 PtCl 6 and SnCl 4 ) that allow the controlled formation of platinum particles dispersed on the anisotropic SnO 2 support. In both syntheses, annealing at 400 • C in the air was performed as the final synthesis step. The XRD patterns (Figure 1) show the platinum maxima in all Pt/SnO 2 samples. The Williamson-Hall diagram shows anisotropy of SnO 2 (Table 3 and Figure 2), with SnO 2 crystallite size of about 10 nm in the c-axis direction (< 00l >) and about 5 nm in the a-axis direction (< h00 >). SEM results confirm the anisotropic nature of SnO 2 and show anisotropic, urchin-like SnO 2 particles (Figure 4 and Figure S7). SEM backscattering results show that the PtNPs in the Pt/SnO 2 samples are quite homogeneously dispersed at the micrometer level ( Figure 3). However, at higher magnification ( Figure 5), it can be seen that the PtNPs are heterogeneously dispersed, with areas that are very rich in platinum and areas that contain almost no platinum (Figures 5 and 7). The results of the BET nitrogen adsorption-desorption isotherms (Figure 8) show that the BET surface area decreases with platinum loading, consistent with the formation of large compact platinum nanoparticles that decrease the SnO 2 surface area. The results of pore volume distributions show that all samples are mesoporous, and the pore diameter is about 4 nm. The presence of ammonia during the synthesis of the Pt/SnO 2 powder samples and the subsequent heating at 400 • C act as a templating process, leading to the formation of mesoporous SnO 2 samples. These pores provide a large surface area and an interconnected network of voids that can be beneficial for various applications such as catalysis, adsorption, and gas sensing. In addition, the good dispersion of relatively large platinum nanoparticles (PtNPs) may indicate a suitable surface area for catalytic reactions and potentially better accessibility for reactant molecules. Well-dispersed large PtNPs with high metal character could provide more active sites for reaction, contributing to the observed high catalytic efficiency.
The XPS results ( Figure 9 and Table 4) show that the average oxidation state (AOS) of platinum is relatively low, with a minimum of 0.4 for sample SP3 and a maximum of 1.25 for sample SP15. On the other hand, the AOS of PtNPs determined in previous work, with a size of about 1 nm (see Ref [18,20] and Table 4), is about two to four times higher than the values determined in this work. It is well known that non-stoichiometry dominates with decreasing particle size. For example, Fe 3 O 4 (stoichiometric magnetite) with a size of about 3 nm or less has a similar stoichiometry to γ-Fe 2 O 3 (maghemite) [21][22][23]. TiO 2 (titanium dioxide) nanoparticles with a size of about 4 nm can also deviate from the ideal stoichiometry [24,25]. PtNPs with a size of about 1 nm obtained in previous work (Table 4 and Refs. [18,20]) exhibit deviations from the ideal stoichiometry of the Pt metal. These small PtNPs are prone to oxidation [26][27][28][29], leading to a behavior that resembles that of sub-stoichiometric platinum oxide (PtO x ) or other surface-bound species. In contrast, the large PtNPs obtained in this work (Figures 3 and 6) have a stoichiometry much closer to the Pt metal ( Table 4).
The 119 Sn Mössbauer spectroscopy result shows that the 119 Sn isomer shift values of the samples are all slightly above zero, indicating the presence of a small excess of electron density at the 5s level of Sn 4+ compared to our SnO 2 reference. This excess could be due to lattice defects, such as oxygen vacancies in the present samples. The decrease in isomer shifts in response to an increase in Pt concentration ( Figure 11) can then be interpreted as a gradual decrease in electron density at the 5s level of Sn 4+ due to the involvement of Pt in the formation of the electronic structure of SnO 2 . Since the electronegativity of Pt is higher than that of tin, it is plausible to expect that the incorporation of Pt into the bulk crystal structure or its attachment to the surface layer of SnO 2 particles (e.g., as shown in [30]) exerts such an influence on the electronic structure of the host particles. Overall, the correlation between the 119 Sn isomeric shift of SnO 2 and the Pt concentration used suggests that (at least some of the) Pt atoms are in direct electronic contact with SnO 2 particles.
The catalytic activity of the synthesized Pt/SnO 2 samples was investigated using a model reaction of the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) [18,[31][32][33]. The reduction of 4-NP to 4-AP is an oxidation-reduction (redox) reaction in which the nitro group (-NO 2 ) of 4-NP is reduced to an amino group (-NH 2 ) of 4-AP. The catalytic reduction of 4-NP to 4-AP is a well-studied model reaction, but the actual mechanism is still controversial. For instance, Wunder et al. [34] proposed a Langmuir-Hinshelwood mechanism in which borohydride ions facilitate the transfer of surface hydrogen species to nanoparticles, allowing reversible adsorption of 4-NP molecules, their reduction by adsorbed hydrogen, and subsequent desorption of 4-AP products, driving the catalytic cycle. Gu et al. [35] emphasized that 4-nitrophenol is first reduced to 4-nitrosophenol and then rapidly converted to 4-hydroxylaminophenol (Hx), which is the only stable intermediate. In the second step, Hx is reduced to the final product, 4-aminophenol. All reactions take place on the surface of the metal particles. Iben Ayad et al. [36] studied the reduction of 4-nitrophenol using pseudo-first-order kinetics and fitting to a Langmuir-Hinshelwood model and discovered two effective catalytic mechanisms involving surface-mediated hydrogen and electron transfer on metallic nanoparticles. The reaction involves sequential adsorption of reactants, formation of hydrogen radicals, and generation of 4-aminophenol via intermediate species, which ends with separation of the product and allows initiation of a new catalytic cycle. Zhao et al. [37] challenge the conventional understanding of 4-nitrophenol reduction by proposing a new mechanism that emphasizes the central role of protic solvents, particularly water, over NaBH 4 as a hydrogen source. Their experiments with isotopically labeled solvents and reducing agents show that water is essential for the catalytic reduction of 4-aminophenol, with the hydrogen atoms originating from the solvent itself, challenging the established role of NaBH 4 . This proposed dynamic interaction between solvent, metal catalyst, and reactants shows that the reduction process relies on the protic solvent and opens a new perspective on this catalytic reaction.
The platinum-containing samples (samples SP1 to SP15) completely catalytically reduced 4-NP to 4-AP between 10 and 80 min ( Figure 12). The higher catalytic activity of samples SP5, SP10, and SP15, which contained (NH 4 ) 2 PtCl 6 , compared to samples SP1 and SP3, which did not contain (NH 4 ) 2 PtCl 6 , indicates the positive effect of this impurity on the catalytic reduction of 4-NP to 4-AP. The sample containing 5 mol% Pt (sample SP5) showed the highest catalytic activity for the reduction of 4-NP to 4-AP with an apparent rate constant k = 0.59 × 10 -2 s -1 , so 5 mol% is an optimal platinum loading on the SnO 2 support under the present experimental conditions. In general, the catalytic activity of Pt on metal oxide supports depends on several factors, such as platinum loading, dispersion of platinum, availability of platinum on the surface, and interaction of platinum with reducible metal oxide supports. Another very important property of the catalyst is its recyclability (reusability). The reusability test (Figure 13) showed that the best catalyst (sample SP5) is exceptionally robust and durable, with very high efficiency (81.4%) in reducing 4-NP to 4-AP after 10 cycles. The ability to reuse a catalyst multiple times not only offers economic advantages but is also consistent with the principles of sustainable and environmentally friendly chemical processes. Catalyst reusability is an essential criterion in catalyst development and selection because it directly affects the overall efficiency, cost, and environmental impact of chemical processes.
In the context of the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), several studies have reported remarkable catalysts and their respective results. For instance, Na et al. [38] presented a novel reusable catalyst consisting of platinum nanoparticles (PtNPs) on layered double hydroxide nanosheets (LDH). Their catalyst showed promising catalytic efficiency, achieving 92% conversion within 30 min. However, Na et al. performed reusability tests for only five cycles, making it difficult to determine the sustained reusability of the PtNP-LDH sample. Pandey et al. [39] synthesized PtNPs stabilized using guar gum and achieved over 90% conversion from 4-NP to 4-AP in 7 min. Although their catalyst exhibited high efficiency, the amount used was larger than that of our SP5 sample. It is noteworthy that no reusability tests were performed in their study. Ullah et al. [40] used phytochemicals for the biofabrication of PtNPs, which exhibited good catalytic efficiency. However, the conversion rate from 4-NP to 4-AP was less than 90%, with an apparent rate constant k app = 0.2 × 10 -2 s -1 . Unfortunately, their study did not include information on the reusability of the catalyst. Bogireddy et al. [41] synthesized 2D platinum superstructures that exhibited efficient catalytic activity in the reduction of 4-NP to 4-AP as well as methylene blue and their mixture. Their self-assembled PtNPs showed promising results, although the evaluation of their reusability was limited to only five cycles. In contrast, in the present work, a conversion rate of over 96% was maintained after five cycles and 81.4% after 10 cycles. Ko et al. [42] investigated PtNP-[C 60 ]fullerene nanowhisker composites for the reduction of 4-NP. While these composites showed catalytic activity at different temperatures, the catalyst exhibited higher performance at room temperature. Nevertheless, the apparent rate constants remained relatively low compared to our study. Liu et al. [43] described a method for embedding PtNPs in mesoporous carbon spheres that resulted in improved catalytic performance. Their Pt/C composites were found to be recyclable and achieved 80% conversion after six cycles. However, our SP5 sample surpassed their results by still achieving a conversion rate of over 80% after ten cycles. In summary, the catalytic reduction of 4-NP to 4-AP has been studied by several researchers, each presenting innovative catalysts with different efficiencies. Our current work with the SP5 catalyst showed exceptional results by achieving both high conversion rates and sustained efficiency over ten cycles. This exceeds the performance of catalysts presented in previous studies and underscores the potential impact of our approach.

Conclusions
In this study, a new approach for the synthesis of Pt/SnO 2 catalysts using microwave irradiation is presented. The use of inorganic precursors (H 2 PtCl 6 and SnCl 4 ) enables the controlled formation of platinum particles on the anisotropic SnO 2 support. The PtNPs are homogeneously dispersed in the micrometer range. The Pt/SnO 2 samples exhibit mesoporous properties that are advantageous for applications in catalysis, adsorption, and gas sensing. XPS analysis shows that the oxidation state of Pt is relatively low, and the stoichiometry of the large PtNPs obtained in this study is close to that of the Pt metal. The 119 Sn-Mössbauer spectroscopy indicates electronic interactions between Pt and SnO 2 particles.
The Pt/SnO 2 catalysts show excellent catalytic activity in the reduction of 4-nitrophenol to 4-aminophenol, with a 5-mol% Pt loading showing optimal performance.
The unique anisotropic structure of the SnO 2 support, combined with the controlled dispersion of PtNPs, resulted in a robust and durable Pt/SnO 2 catalyst with very high efficiency in reducing 4-NP to 4-AP even after 10 cycles. These findings open new avenues for the design and fabrication of advanced catalytic systems for future catalytic and environmental applications.