Simultaneous Photocatalytic Sugar Conversion and Hydrogen Production Using Pd Nanoparticles Decorated on Iron-Doped Hydroxyapatite

Abstract

Pd nanoparticles (PdNPs) were successfully deposited on the surface of Fe(III)-modified hydroxyapatite (HAp), which was subsequently used as a photocatalyst for simultaneous photocatalytic H₂ evolution and xylose conversion. The structural phase and morphology of the pristine HAp, FeHAp, and Pd@FeHAp were examined using XRD, SEM, and TEM instruments. At 20 °C, Pd@FeHAp provided a greater xylose conversion than pristine HAp and FeHAp, about 2.15 times and 1.41 times, respectively. In addition, lactic acid and formic acid production was increased by using Pd@FeHAp. The optimal condition was further investigated using Pd@FeHAp, which demonstrated around 70% xylose conversion within 60 min at 30 °C. Moreover, only Pd@FeHAp produced H₂ under light irradiation. To clarify the impact of Fe(III) doping in FeHAp and heterojunction between PdNPs and FeHAp in the composite relative to pure Hap, the optical and physicochemical properties of Pd@FeHAp samples were analyzed, which revealed the extraordinary ability of the material to separate and transport photogenerated electron-hole pairs, as demonstrated by a substantial reduction in photoluminescence intensity when compared to Hp and FeHAp. In addition, a decrease in electron trap density in the Pd@FeHAp composite using reversed double-beam photoacoustic spectroscopy was attributed to the higher photocatalytic activity rate. Furthermore, the development of new electronic levels by the addition of Fe(III) to the structure of HAp in FeHAp may improve the ability to absorb light by lessening the energy band gap. The photocatalytic performance of the Pd@FeHAp composite was improved by lowering charge recombination and narrowing the energy band gap. As a result, a newly developed Pd@FeHAp composite might be employed as a photocatalyst to generate both alternative H₂ energy and high-value chemicals.

1. Introduction

Hydrogen is regarded a clean fuel and a sustainable energy carrier, as it can be derived from natural sources and its combustion yields only water. However, photocatalytic water splitting is constrained by endergonic thermodynamics and the slow kinetics of the O₂ evolution half-reaction [1,2]. Consequently, non-renewable and overpriced sacrificial agents such as triethanolamine and methanol are widely used to assist the H₂ production [3,4,5,6,7,8,9,10]. Scalable and environmentally-friendly H₂ production may be possible via photo reformation of xylose, the second most abundant sugar in lignocellulosic biomass [11,12,13,14]. Photo reforming necessitates the utilization of a photocatalyst that can simultaneously undergo oxidation of xylose into value-added biochemicals and reduction of water to hydrogen in the presence of photogenerated charge carriers [15,16,17,18,19]. Among the various biochemicals produced, lactic acid is a key component of the biorefinery process used in numerous industries, including the formulation of cosmetics, the manufacture of pharmaceuticals, and biodegradable polylactide [20,21,22]. Additionally, formic acid can be produced and used in several applications such as medicine, food, and agriculture [23,24,25,26,27].

Calcium hydroxyapatite (HAp) usually has a high adsorption capacity and amphoteric characteristics. These properties set it apart as a potential adsorbent or catalytic support with tunable selectivity, making it appealing for both fundamental and practical research. The primary drawback of HAp is its wide bandgap energy larger than 4 eV and therefore, the photocatalytic reaction is less active when exposed to solar light. In addition, the photocatalytic efficiency of single phase of HAp is limited by the high rate of photogenerated charge carrier recombination. Hence, there has been extensive research into metal-doped HAp as a solution to overcome these problems of solar light harvesting and fast charge carrier recombination [28]. For instance, Fe(III) ions stand out among various metal ions due to their exceptional ability to absorb solar light, both UV and visible light [29,30,31]. Therefore, the optical and electrical properties of HAp can be altered by inserting Fe ions into its crystalline structure to produced iron-doped HAp (FeHAp), thus lowering the bandgap energy [32]. Due to these alterations, solar light absorption can be improved. On the other hand, incorporating palladium nanoparticles into the HAp structure has shown promising performance towards methane oxidation reaction [33], for green oxidation of alcohols [34] and glucose electrooxidation [35] as reported in previous literatures. The current conventional available system for sugar conversion and hydrogen production utilized HAp-based materials is summarized in

Table 1. Recent reports of using hydroxyapatite (HAp)-based materials for H₂ production and sugar conversion.

Materials	Applications	Processes	Ref.
HAp	Sugar conversion	Isomerization of glucose to fructose	[36]
Ru/HAp	H2 production	Hydrolysis of borane dimethyl amine	[37]
Au/HAp	Sugar conversion	Glucose oxidation to sodium gluconate	[38]
Cu/HAp	H2 production	Hydrolysis of NaBH4	[39]
Ba/HAp	Sugar conversion	Isomerization of glucose to fructose	[40]
Ni-Mo2C/HAp	H2 production	Biomass gasification	[41]
Au/HAp	H2 production	Hydrolytic oxidation of organosilanes	[42]
PaIn/HAp	Sugar conversion	Glucose electrooxidation for direct glucose fuel cells	[35]
Ti-HAp	H2 production	Photocatalytic water splitting	[43]
CdS quantum dots/Cd-HAp	H2 production	Photocatalytic H2 evolution	[44]

. The recent research works using HAp-based materials were mainly applied for H₂ production, glucose oxidation, and isomerization of glucose to fructose. However, the application of HAp-based materials for simultaneous photocatalytic H₂ evolution and sugar conversion to value-added chemicals is still limited.

Commonly, Pt nanoparticles (PtNPs) have been used as a co-catalyst for H₂ evolution reaction. However, due to the high cost of PtNPs, Pd nanoparticle (PdNPs) could be the alternative co-catalyst for H₂ evolution [45]. To reduce the experimental cost, sugar could be used as a sacrificial reagent instead of expensive chemicals such as triethanolamine [3,4]. To the best of our knowledge, no study has been reported on the decoration of Pd nanoparticle (PdNPs) on Fe-doped HAp (Pd@FeHAp) for application in photocatalysis. Herein, we have synthesized HAp derived from calcium oxide through a hydrothermal method, following an ion exchange reaction to produce FeHAp. After that, the deposition of Pd NPs on the surface of FeHAp was conducted using chemical reduction. Its photocatalytic performance was evaluated towards the simultaneous conversion of xylose into value-added biochemicals such as lactic acid and formic acid and evolution of hydrogen from water. The materials were analyzed using the following methods: X-ray powder diffraction (XRD), UV–Vis diffuse reflectance spectroscopy (UV–Vis/DRS), photoluminescence spectroscopy (PL), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy energy-dispersive X-ray spectroscopy (TEM-EDX). Using reversed double-beam photoacoustic spectroscopy (RDB-PAS), the surface electronic structure of the produced sample and electron transport in the composite were investigated.

2. Results and Discussion

2.1. Phase Structure and Chemical Composition

Figure 1a presents the results of the PXRD analysis of the crystal phase structures of HAp, FeHAp, and Pd@FeHAp composites. All the synthesized samples showed distinct diffraction peaks at 26.0°, 31.9°, 32.3°, and 33.1°, which correspond to the hydroxyapatite crystal phase (JCPDS No. 09-0432). Additionally, no peaks of new Fe species, such as iron oxide and metallic iron, were observed in the FeHAp sample during the Fe(III) modification. This result indicated that the Fe(III) might be able to replace the structural Ca in HAp. Furthermore, the loading of PdNPs using NaBH₄ as reducing reagent has no influence to the main structure of FeHAp because Pd(II) ions can adsorb on the surface of HAp via the electrostatic interaction between the positive charge of Pd(II) ions and negative hydroxyl group on the surface of HAp. Thus, the deposition of PdNPs on the surface of HAp will not distort the main structure of FeHAp. However, the characteristic peak of PdNPs was not observed in the diffraction pattern of Pd@FeHAp due to low concentration of PdNPs loading. Thus, to confirm the loading amount of PdNPs in the composite and Fe in the sample, XRF analysis was used. In

Table 2. Elemental composition (%wt) of HAp, FeHAp, and Pd@FeHAp.

	O	P	Ca	Fe	Pd
HAp	47.49	17.60	34.91	-	-
FeHAp	48.25	16.35	31.95	3.45	-
Pd@FeHAp	48.27	16.39	31.89	3.21	0.23

, the XRF result showed that the FeHAp and Pd@FeHAp showed smaller Ca content compared with pure HAp, while the appearance of Fe atom was found, confirming the substitution of Fe(III) ions with Ca(II) ions in the HAp structure during the modification step. Additionally, the ratio of Ca/Fe is about 9.9%, which is close to the added value of the Ca(II)/Fe(III) ratio, while the observed Pd content is about 0.23%. In addition, the chemical vibrational compositions of HAp, FeHAp and Pd@FeHAp were conducted by Raman spectroscopy as shown in Figure 1b. The strongest peak corresponding to the symmetric stretching of P−O bonds at 950 cm⁻¹ and the weak band positions at 1050 cm⁻¹ and 1080 cm⁻¹ of asymmetric ν3(P−O) stretching were observed in all samples. In addition, the other band vibrational modes originated from phosphate group at 576 cm⁻¹, 590 cm⁻¹, and 616 cm⁻¹ including the O-P-O bending modes at 430 cm⁻¹ and 450 cm⁻¹ were shown in all samples [46].

2.2. Optical and Surface Electronic Properties

The light absorption and energy band gaps of HAp, FeHAp, and Pd@FeHAp composites were determined using UV-DRS investigations. Figure 2a indicates that HAp has lowest absorbance, suggesting that the pure HAp has poor light absorption ability in both UV and visible light regions. However, after Fe(III) introduction in the HAp structure, FeHAp composite showed strong light absorption in the 300–500 nm wavelength range, making it an efficient photocatalyst in the UV and near-visible light region. Contrarily, compared to HAp and FeHAp, the Pd@FeHAp composites absorbed much more UV and visible light (approximate wavelength range of 300–600 nm). This result indicated that after being decorated with PdNPs, the optical properties of Pd@FeHAp had improved. Moreover, using Tauc’s equation [47], the calculated Eg values of HAp, FeHAp, and Pd@FeHAp were >4.0 eV, 3.27 eV, and 2.96 eV, respectively, as shown in Figure 2b. Pd@FeHAp composite had a lower Eg than pure HAp and FeHAp, indicating that the existence of the PdNPs on the surface of Pd@FeHAp could harvest more visible light region. Thus, the lowered Eg values of the composites indicate that the photocatalytic activity of the composites for the simultaneous reactions may be enhanced in consequence of the increased production of electron-hole pairs during light irradiation.

2.3. Morphology Results

SEM-EDX images of the surface morphologies of HAp, FeHAp, and Pd@FeHAp are shown in Figure 3a–c. Pure HAp surface morphology in Figure 3a showed aggregates of tiny HAp particles arranged in a spherical form with a diameter about 7.5 µm. Surprisingly, the addition of Fe(III) in the HAp structure caused the smaller particle size of FeHAp, indicating some distortion by substitution of Ca(II) by Fe(III) as shown in Figure 3b. Moreover, the difference of particle size of Pd@FeHAp and FeHAp could not be observe (Figure 3c). SEM-EDX mappings were used to clarify the distribution of each element in HAp, FeHAp, and Pd@FeHAp. It can be seen that HAp, FeHAp, and Pd@FeHAp showed homogenous distribution of the main three elements of O, P, and Ca which come from hydroxy appetite structure. Moreover, the Fe signal, which was well overlapping with O, P, and Ca, was detected in the FeHAp and Pd@FeHAp, suggesting that the Fe(III) might dope into the HAp structure to form Fe(III)-doped HAp. Additionally, Pd signal was detected in a similar location with the Ca, O, and P, suggesting good distribution of PdNPs on the surface of HAp in the composites.

The presence of PdNPs in the Pd@FeHAp composite was investigated using TEM and STEM analysis. The high angle annular dark field (HAADF-STEM) images of Pd@FeHAp in Figure 4a,b reveals the dark spot on the surface of FeHAp in the bright field image and the bright spot on the surface of FeHAp in the dark field image, both of which point to the presence of heavy metal nanoparticles on the surface of FeHAp. Additionally, the particle size of the PdNPs in the Pd@FeHAp was investigated by TEM observation in different magnifications (Figure 4c–e). The particle size of PdNPs could be observed around 2.2–3.5 nm. The larger particle size in the Pd@FeHAp might be due to the aggregation of the smaller PdNPs. STEM-EDX mapping results showed Ca, P, O, Fe, and Pd signals in the Pd@FeHAp composites. However, the signals of Pd were observed at the same location as the dark spot in the bright field STEM image, confirming the deposition of PdNPs in the composite (Figure 4f). Thus, the formation process of the PdNPs on the surface of Pd@FeHAp might be due to the adsorption of Pd²⁺ on the hydroxide functional group on the surface of HAp via an electrostatic interaction force, following by reduction of the Pd²⁺ on the surface of HAp by chemical reduction.

2.4. Chemical State and Band Positions

XPS was used to determine the chemical states of Ca, P, O, Fe, and Pd as well as the valence band top (VBT) locations in Hap, FeHAp, and Pd@FeHAp. In order to identify the main elements present, the survey XPS spectra of Hap, FeHAp, and Pd@FeHAp were examined as shown in Figure 5a. The signal of Ca 2p, P 2p, and O 1s were observed as major components of HAp and both modified samples. Moreover, two peaks of Fe 2p_3/2 and Fe 2p_1/2 signals were detected FeHAp and Pd@FeHAp, confirming the existence of Fe ions in the samples. In contrast, the Pd signal in Figure 5a was difficult to observe in survey scans because of low Pd concentration. A narrow scan of the Ca 2p spectra of HAp, FeHAp, and Pd@FeHAp are shown in Figure 5b. The strong and weak signals at 346.8 eV and 349.8 eV corresponded to Ca 2p_3/2 and Ca 2p_1/2, respectively [48]. In Figure 5c, the signals of the P 2p_3/2 and P 2p_1/2 in HAp, FeHAp, and Pd@FeHAp can be attributed to PO₄³⁻ in the structure of hydroxyapatite [49,50]. Additionally, the signals of Fe 2p in the FeHAp and Pd@FeHAp were observed at 712.3 eV, indicating the Fe (III) ions species in the structure [51]. The surface mole ratio of Fe/Ca of the FeHAp and Pd@FeHAp were investigated to compare with the XRF results to elucidate the location of the Fe in the composites. The XPS results showed the Fe/Ca of the FeHAp and Pd@FeHAp at about 0.203 and 0.201, respectively, while the XRF results suggests a lower Fe/Ca ratio of FeHAp (0.077) and Pd@FeHAp (0.072). These results indicated that the Fe ions might exist on the surface of HAp by substitution with the outer Ca surface of HAp. Additionally, the two peaks of Pd 3d_5/2 = 336.4 eV and Pd 3d_3/2 = 341.7 eV were found in the Pd@FeHAp sample as shown in Figure 5d, confirming the presence of decorated Pd(0) on the surface of FeHAp [35,52]. The valence band (VB) location of HAp, FeHAp, and Pd@FeHAp composite were also determined using XPS [53]. The VB position was estimated by the interception of a linear line from the slope of the XPS signal in the VB region at y = 0. In Figure 5f, the energy around 2.2 eV, 1.9 eV, and 0.46 eV, respectively, were VB positions for HAp, FeHAp, and Pd@FeHAp. The conduction band bottom (CBB) positions of FeHAp and Pd@FeHAp could be calculated based on the combination of VB and Eg results to be −1.37 and −2.5 eV, respectively.

2.5. Simultaneous Photocatalytic Xylose Conversion and Hydrogen Evolution

The simultaneous photocatalytic hydrogen evolution and xylose conversion using HAp, FeHAp, and Pd@FeHAp was conducted as shown in Figure 6a. HAp and FeHAp displayed less production of H₂ evolution than Pd@FeHAp at 20 °C and the rates of evolved H₂ of HAp, FeHAp, and Pd@FeHAp were 0.00, 0.00, and 0.0286 µmol/g/h, respectively. This result suggested that PdNPs can act as a co-catalyst for H₂ production to promote the reduction of H⁺ in the water to H₂. In addition, when the reaction temperature was increased to 30 °C, the rate of evolved H₂ for Pd@FeHAp was 0.0290 µmol/g/h. The results from the effect of reaction temperature indicated that the reaction temperature has no effect for the H₂ evolution. In addition, the photocatalytic H₂ production using Pd@HAp, which was produced in the same manner as the Pd@FeHAp, was performed to emphasize the role of Fe in the HAp. As shown in Figure S1, it can be seen that the Pd@HAp could not be generated under light irradiation, while the Pd@FeHAp could produce H₂ at about 0.0862 µmol/g at 30 °C. The poor production of H₂ over Pd@HAp might be due to the large Eg and lower light absorption ability of pristine HAp. This result indicated that the introduction of Fe into the structure of HAp could reduce the Eg of HAp from >4.0 eV to 3.27 eV, resulting in the change of its properties from insulator to semiconductor, which can work as photocatalysts. Simultaneously, xylose was converted to small molecules during the H₂ evolution reaction. The xylose conversions at 20 °C were around 16.5%, 25.2%, and 35.42% for HAp, FeHAp, and Pd@FeHAp, respectively, as shown in Figure 6c. After Fe(III) introduction into the structure of HAp, the conversion efficiency was improved due to reduction of Eg by new fermi level of Fe(III). When PdNPs were decorated on the surface of FeHAp in Pd@FeHAp sample, the photocatalytic xylose conversion efficiency increased due to less recombination of photogenerated charge carriers by the formation heterojunction between PdNPs and FeHAp. Interestingly, the photocatalytic activity of the Pd@FeHAp dramatically enhanced when the reaction temperature was raised to 30 °C because the conversion of xylose is an endothermic reaction, which is preferable at a high reaction temperature. Basically, xylose could be oxidized to several small organic compounds such as xylitol, erythritol, lactic acid, formic acid, and xylonic acid [5,15,54]. However, in this work, the product yields from xylose conversion were investigated based on two important high-value converted products such as lactic acid and formic acid. For this experiment, due to the high basic condition reaction, the final products are basically in their basic forms, such as lactate and formate. However, after the reaction, the solution was acidified using 1M H₂SO₄ (pH below 7). Thus, the final product forms are acids such as lactic acid and formic acid. At 20 °C, Pd@FeHAp showed higher production of lactic acid than the other samples, as shown in Figure 6d, while the higher formic acid yield was obtained by FeHAp (Figure 6e). In addition, based on the HPLC spectrum of the solution after the simultaneous photocatalytic sugar conversion and hydrogen production over Pd@HAp for 4 h at 30 °C (Figure S2), some other by-products except formic and lactic acid could be formed, such as succinic acid, citric acid, and some unidentified peaks that may be related to some small organic compounds, indicating the conversion of glucose xylose to small organic molecules. Additionally, the pH of the sugar solution after 4 h of reaction was measured and compared with the original pH of the sugar solution before reaction. The results show that the pH of the solution slightly changes after 4 h of the reaction from 12.96 to 12.83 due to the formation of lactate and formate. Catalyst stability and recyclability are significant aspects in their practical use. The photocatalytic H₂ evolution and xylose conversion to lactic acid and formic acid over Pd@FeHAp was retained over three cycles, as shown in Figure 6f,g, respectively. In addition, the structural stability of Pd@FeHAp after the photocatalytic reaction was investigated through XRD. In Figure 6h, the Pd@FeHAp had great stability after the simultaneous photocatalytic H₂ evolution and xylose conversion. Additionally, in this work, we transformed the non-active HAp which could not generate H₂ to possible photocatalyst for the production of H₂ by surface Fe modification. Thus, the production of hydrogen is quite lower than normal photocatalyst such as TiO₂. However, the results of this work can suggest the possible use of Fe-doped HAp as a photocatalyst for hydrogen production. In addition, Pd NPs was used as co-catalyst instead of Pt NPs, which is highly active and common as a co-catalyst for hydrogen production due to its lower price. Additionally, the xylose was used as a sacrificial agent instead of a triethanolamine, which is a common reagent in H₂ production, but has a higher cost compared with xylose. Thus, the combination of Fe-doped HAp and PdNPs can be considered as an alternative photocatalyst due to being cost effective.

2.6. Examination of the Transfer and Separation of Charges

The formation of heterojunction between FeHAp and PdNPs were investigated using PL measurements to examine the separation and transfer of e⁻ and h⁺ pairs. A low PL intensity often indicates a low rate of e^- and h⁺ pair recombination. As seen in Figure 7, reduced PL emission intensity of FeHAp compared to HAp at around 400 nm for a 350 nm excitation wavelength suggested that the new fermi level of Fe substitution in HAp structure could trap the excited electron from valence band and avoid e^--h⁺ pair recombination. Moreover, the PdNPs decoration on the surface of FeHAp improved the charge carrier separation.

The surface electronic characteristics of HAp, FeHAp, and Pd@FeHAp were also examined using reversed double-beam photoacoustic spectroscopy (RDB-PAS), which could have an influence on photocatalytic activity and electron transfer of the composite [55,56,57]. The energy-resolved distribution of electron trap (ERDT) pattern as a function of the energy from the valence band top (VBT) vs. the electron trap density was conducted for the HAp, FeHAp, and Pd@FeHAp samples as shown in Figure 8. It is evidence that HAp does not have an electron trap because of its large energy band gap, but after Fe modification for FeHAp, the modified sample displayed an electron accumulation density peak at about 3.1 eV, which was close to Eg of the FeHAp sample. This suggests that the Fe atom in the HAp structure may be able to generate a new electron trapped (ETs) state close to CBB. It is interesting that the ERDT patterns of the Pd@FeHAp composites could not be seen, suggesting that the PdNPs introduction on the surface of FeHAp promoted the migration of electrons from FeHAp to the surface of PdNPs. The decrease in electrons in the ETs of FeHAp might be due to the electron transfer from FeHAp to PdNPs through a heterojunction, which prevented charge carrier recombination and enhanced photocatalytic activity.

2.7. Photocatalytic Mechanism

A potential photocatalytic simultaneous photocatalytic H₂ evolution and xylose conversion process using Pd@FeHAp composite is shown in Scheme 1. The VB and CB of Pd@FeHAp composite were 0.46 and −2.5 eV, respectively. After light irradiation, the excited e^- moved from the VB of FeHAp to a new electronic state created by the Fe(III) Fermi level in the structure. Concurrently, h⁺ were created at the VB. The decoration of PdNPs on FeHAp prevented the recombination of e⁻ and h⁺ pairs owing to the migration of e⁻ to the surface of PdNPs through heterojunction. Through a photocatalytic process, the accumulated electrons on the surface of PdNPs can react with H⁺ and produce the H₂. Simultaneously, xylose can react with h⁺ in the VB of FeHAp and generate hydroxyl radicals to form formic acid and lactic acid. Thus, by forming a heterojunction using Pd@FeHAp, more active electrons and hydroxyl radicals were available for the simultaneous photocatalytic H₂ evolution and xylose conversion.

3. Materials and Methods

3.1. Chemicals

All pure analytical-grade chemicals used in this experiment, namely calcium oxide (CaO), iron (III) nitrate nonahydrate (Fe(NO₃)₃•9H₂O), diammonium hydrogen phosphate ((NH₄)₂HPO₄), xylose (C₅H₁₀O₅), sodium hydroxide (NaOH), and palladium(II) nitrate (Pd(NO₃)₂) were supplied by Fujifilm Wako Pure Chemical Co. (Osaka, Japan). None of the chemicals used underwent any further purification.

3.2. Preparation of HAp and FeHAp

A mixture of CaO (s) and (NH₄)₂HPO₄ (s) with a molar ratio of Ca/P = 1.67 in 50 mL of DI water was utilized to prepare hydroxyapatite under hydrothermal treatment. The initial pH was measured at 14.5. The Teflon autoclave was heated at 200 °C for 48 h. The obtained solid was then filtered and washed several times with ethanol and DI water. Additionally, the solid products were dried for 24 h at 75 °C. This product was denoted as HAp.

For Fe(III) modification on HAp, an ion exchange method was applied to introduce Fe(III) to as-prepared HAp by suspending HAp in Fe(NO₃)₃•9H₂O (aq) with a molar ratio of Ca/Fe = 10 under stirring at room temperature for 30 min. After that, the final solid product was filtered and washed with ethanol and DI water many times. The solid product was dried at 75 °C for 24 h. The resulting product was identified using FeHAp.

3.3. Preparation of Pd@HAp and Pd@FeHAp

Pd nanoparticles (PdNPs) were decorated on the surface of FeHAp by chemical reduction using NaBH₄. The synthetic method began with dissolving 3.2 mg of Pd(NO₃)₂ in 20 mL of DI water. Then, 32.25 mg of FeHAp was suspended in Pd solution under stirring at room temperature to achieve 0.5% Pd loading on the composite. After 30 min, 1 mL of 1 M NaBH₄ (aq) was then added to the mixture solution to form Pd nanoparticle under stirring for 30 min at room temperature. After that, the synthetic product was filtered and washed with ethanol and DI water several times before drying at 75 °C for 24 h. Pd@FeHAp was the designation given to the final product. In addition, the Pd@HAp was prepared in the same manner as the Pd@FeHAp for comparison of the photocatalytic H₂ production.

3.4. Characterization

X-ray diffraction (XRD) using the Ultima IV was used to identify the crystal phases of all materials (RIGAKU, Akishima, Japan). The chemical composition was figured out by Raman spectroscopy (Thermo, DXR Smart Raman, Waltham, MA, USA) using laser wavelength 532 nm. With high magnification, scanning electron microscopy (SEM, VE-9800, Keyence, Osaka, Japan) and STEM-EDX (JEM-2100HCKM, JEOL, Tokyo, Japan) were used to examine the morphology, particle sizes, and surface properties of all samples. The ability of the pure and composite materials to absorb light was measured using UV–Vis diffuse reflectance spectroscopy (UV-DRS, UV-2450 Shimadzu, Kyoto, Japan). All samples’ VB sites were identified by X-ray photoelectron spectroscopy (XPS, ESCA 5800; ULVAC-PHI, Inc., Kanagawa, Japan). In this work, the C-C of C 1s from contaminated carbon from the machine at 284.6 was used as a reference for calibration of the XPS spectra. The elemental compositions of all samples were determined by XRF spectroscopy using Rigaku ZSX Primus II in the wavelength dispersive mode (Akishima, Japan). Photoluminescence (PL) spectroscopic studies were carried out using an FP-6600 spectrofluorometer (FP-6600 spectrofluorometer, JASCO Corporation, Tokyo, Japan). An energy-resolved distribution of electron trap (ERDT) pattern of the generated materials was discovered using reversed double-beam photoacoustic spectroscopy (RDB-PAS) from 600 to 300 nm using an amplifier to double the photoacoustic signals.

3.5. Simultaneous Photocatalytic Sugar Conversion and Hydrogen Production

To elucidate the efficacy of photocatalytic activity of the obtained samples, the simultaneous photocatalytic sugar conversion and hydrogen production were conducted. Typically, 60 mg of the produced samples was added in 120 mL of 1000 ppm xylose in 1 M NaOH under N₂ purging for 30 min. Then, a 100 W Hg lamp was used to irradiate the suspensions. While the light was on, the produced H₂ gas was sampled to identify the concentration of H₂ using gas chromatography (GC, Nexis GC-2030, Shimadzu, Kyoto, Japan), with the Shincarbon ST column, TCD detector, and 50 mL/min flow rate of Ar gas as a mobile phase. In addition, the 0.5 mL from the photocatalytic solution was acidified using 0.5 mL of 1M H₂SO₄ for high-pressure liquid chromatography (HPLC) analysis. Thus, xylose, lactic acid, and formic acid concentration were determined by HPLC (JASCO, CO-2065 Plus, Tokyo, Japan) using cation-exchange column (Aminex HPX-87H, 300 × 7.8 mm, Bio-Rad) and eluted with the flow rate 0.6 mL/min of 5 mM H₂SO₄ at 50 °C. The detection of Xylose was calculated from RI detector, whereas the organic acid was determined by a UV detector at wavelength 210 nm. The xylose conversion and production yield were calculated following these equations as shown below [58];

$$\mathrm{Xylose} \mathrm{conversion} \left(\%\right)=\frac{\left(\mathrm{initial} \mathrm{concentration}-\mathrm{remained} \mathrm{concentration}\right)}{\mathrm{initial} \mathrm{concentration}}\times 100$$

(1)

$$\mathrm{Product} \mathrm{yield} \left(\%\right)=\frac{\mathrm{mole} \mathrm{of} \mathrm{product}}{\mathrm{mole} \mathrm{of} \mathrm{initial} \mathrm{xylose}}\times \frac{\mathrm{carbon} \mathrm{numbers} \mathrm{in} 1 \mathrm{mol} \mathrm{product} }{\mathrm{carbon} \mathrm{numbers} \mathrm{in} 1 \mathrm{mol} \mathrm{xylose}}\times 100$$

(2)

4. Conclusions

Pd@FeHAp composites were successfully prepared using a decoration of PdNPs on the surface of Fe(III) modification HAp. Pd@FeHAp provided a greater rate production of H₂ and xylose conversion than HAp and FeHAp, resulting from the lower recombination of charge carriers. According to PL studies, the formation of heterojunction between PdNPs and FeHAp in Pd@FeHAp composite reduced the recombination of electron-hole pairs. The surface electronic investigation using RDB-PAS analysis supported the PL result as the electron from VB of HAp was accumulated in the Fermi level of Fe(III). The heterojunction formation between PdNPs and FeHAp caused the lessened electron density in ERDT patterns due to electron migration from FeHAp to PdNPs. Thus, the heterojunction formation in Pd@FeHAp gave more active electrons and hydroxyl radicals for the simultaneous photocatalytic H₂ evolution and xylose conversion. Additionally, it is well known that hydroxyapatite can be prepared by natural wastes such as eggshells, animal bones, and cockle shells, and Fe is the fourth most abundant element in Earth’s crust. Moreover, the price of Pd is cheaper than that of Pt (a common cocatalyst), which is widely used in photocatalytic H₂ evolution. Thus, the concept of production of Pd@FeHAp as a low-cost photocatalyst for simultaneous H₂ evolution and lactic production can promote natural waste recycling in advanced purposes with a cost-effectiveness route. Thus, this work offers an alternative Pd@FeHAp photocatalyst for effective simultaneous photocatalytic H₂ evolution and sugar conversion to produce biochemicals and alternative energy.