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# Photoreforming of poly(ethylene-terephthalate) plastic into valuable chemicals and hydrogen over BiVO4/MoOx: Synergistic promotion of oxidation and reduction processes

Xinxin Liang, Ting Gao, Yongqian Cui, Qibing Dong, Ximing Li, Abdelkader

Labidi, Eric Lichtfouse, Fei Li, Feng Yu, Chuanyi Wang

► To cite this version:

Xinxin Liang, Ting Gao, Yongqian Cui, Qibing Dong, Ximing Li, et al.. Photoreforming of poly(ethylene-terephthalate) plastic into valuable chemicals and hydrogen over BiVO4/MoOx: Synergistic promotion of oxidation and reduction processes. Applied Catalysis B: Environmental, 2024, 357, pp.124326. 10.1016/j.apcatb.2024.124326. hal-04621136

HAL Id: hal-04621136

https://hal.science/hal-04621136v1

Submitted on 23 Jun 2024

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# Photoreforming of poly(ethylene-terephthalate) plastic into valuable chemicals and hydrogen over $BiVO_4/MoO_x$: Synergistic promotion of oxidation and reduction processes

Xinxin Liang<sup>a</sup>, Ting Gao<sup>a</sup>, Yongqian Cui<sup>a</sup>, Qibing Dong<sup>a</sup>, Ximing Li<sup>a</sup>, Abdelkader Labidi<sup>a</sup>, Eric Lichtfouse<sup>d</sup>, Fei Li<sup>a, *</sup>, Feng Yu<sup>c</sup>, Chuanyi Wang<sup>a, b, **</sup>

## ARTICLE INFO

### Keywords:

$H_2$ evolution

$MoO_x$ particle

Photoreforming plastic

Polarization effect

Valuable chemical

## ABSTRACT

Photoreforming plastic waste into value-added chemicals along with hydrogen evolution is crucial for the circular economy, but its efficiency is limited by mutual constraints between proton reduction and plastic oxidation. Herein, $MoO_x$ particles were decorated onto $BiVO_4$ attempting to promote photo-redox reactions, simultaneously. Experiments and theoretical calculations demonstrate that oxygen vacancy in $MoO_x$ promotes the adsorption of pretreated poly(ethylene-terephthalate), leading to enhanced oxidation of poly(ethylene-terephthalate). Meanwhile, $MoO_x$ boosts the transfer of photoelectrons from $BiVO_4$ to $MoO_x$ by $Mo^{5+}/6^+$ ions with variable valence states and lowers the free energy of $H^+ + e^- \rightarrow H^*$ from 0.39 to -0.12 eV, thereby promoting $H_2$ evolution. Consequently, the photoreforming efficiencies of converting poly(ethylene-terephthalate) into formate, acetate and $H_2$ by $BiVO_4/MoO_x$ are 0.29, 0.19 and 1.96 mmol g$^{-1}$ h$^{-1}$, which are respectively 6.2, 6.0 and 5.9 times higher than that by $BiVO_4$. This work presents avenues for designing high-performance catalysts to efficiently photo-reform plastic.

## 1. Introduction

Plastics have become a necessity in modern life and production of industrial products due to their affordability, stability, and processability, while more than 70 % of plastics are wasted in natural environment [1]. The spontaneous degradation of waste plastic typically takes hundreds of years because of the high chemical stability [2], resulting in serious environmental problems [3]. Moreover, the release of plastic waste has become a worldwide pollution emergency, with around 10 million metric tons of plastic entering into the Earth's oceans annually [4]. Traditional methods of plastic disposal are landfilling, incineration and secondary recycling, while these methods are incapable of sustainably and economically recycling the plastic waste. Taking landfill as an example, the plastic waste can be easily handled,

but it requires a large amount of land, changes the pH of soil, and affects the sustainable use of land [4,5]. Given this situation, it is tremendously necessary to explore sustainable and economically beneficial strategies for plastic conversion.

Plastic can be converted into fuel or valuable chemicals by reforming, in particular thermoreforming, electroreforming and photo-reforming [6]. Photoreforming of plastic waste appears very promising due to the unlimited availability of solar energy [7]. Polyethylene terephthalate (PET) is one of the most commonly used plastic materials, typically employed to produce water bottles and packaging boxes [8]. Therefore, the photoreforming of PET has attracted wide attention. In a typical photoreforming process, the photogenerated electrons and holes in a photocatalyst are transported to the surface, where they actively engage in redox reactions. However, the photoreforming performance is

<footnote><sup>a</sup> School of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, PR China</footnote>

<footnote><sup>b</sup> Jiangxi Key Lab of Flexible Electronics, Flexible Electronics Innovation Institute, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, PR China</footnote>

<footnote><sup>c</sup> Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, School of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832003, PR China</footnote>

<footnote><sup>d</sup> State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, PR China</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>** Corresponding author at: School of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, PR China.</footnote>

<footnote>E-mail addresses: fayeli@sust.edu.cn (F. Li), wangchuanyi@sust.edu.cn (C. Wang).</footnote>
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severely restricted by limited proton supply and slow oxidation process. Therefore, in the photoreforming of PET, it is highly desired to simultaneously enhance the hydrogen evolution reaction (HER) and facilitate the oxidation of plastic substrates.

Due to the insolubility of PET waste, it is generally hydrolyzed into terephthalate (TPA) and ethylene glycol (EG) in alkaline conditions, and only EG participates in the photoreforming process [9]. For example, Reisner et al. converted EG molecule into formate, glycolate and acetate, meanwhile, hydrogen ($H_2$) was generated from water ($H_2O$) [10]. Additionally, EG can be regarded as a sacrificial agent because it is preferentially oxidized compared to the oxidation of $H_2O$. Therefore, the photoreforming reaction of pretreated PET into $H_2$ and chemicals is nearly energy neutral [11]. As a result, the focus of PET photoreforming should be on the usage of narrow bandgap photocatalysts.

Monoclinic $BiVO_4$ is considered to be one of the most promising narrow-bandgap photocatalysts due to its high visible light response and good chemical stability [12]. However, its poor charge carrier transport properties and sluggish surface reactions constantly limit its photocatalytic efficiency [13]. Metal oxide particles are composed of a small number of atoms, which can expose massive active sites, making them highly beneficial for rapid redox reactions [14]. Moreover, when metal oxide particles are loaded onto photocatalysts, they can induce discrete energy bands to trap charge carriers, thereby facilitating the efficient separation of photogenerated electron-hole pairs [15]. Based on this, the use of particle catalysts to modify $BiVO_4$ holds the potential to enhance the activity of PET photoreforming.

The properties of metal oxide particle cocatalysts are dramatically influenced by the number and coordination environment of the active atoms [16]. Therefore, it is crucial to develop an approach that accurately controls the morphology of particles decorating the surface of photocatalysts. Herein, a simple hydrothermal strategy is developed to anchor molybdenum-oxygen ($MoO_x$) particles on the surface of $BiVO_4$ ($BiVO_4/MoO_x$). As shown in Scheme 1, benefiting from the low conduction band position and the variable valence $Mo^{5+}/6^+$ ions, $MoO_x$ can be regarded as an electron relay, enhancing the transfer of photo-generated electrons from $BiVO_4$ to $MoO_x$. This process reduces the free energy of $H^+ + e^- \rightarrow H^*$ from 0.39 to -0.12 eV, thus facilitating the evolution of $H_2$. Besides, the oxygen vacancy ($O_v$) in $MoO_x$ promotes the adsorption of pretreated PET, leading to enhanced oxidation of PET. As a result, cooperative photo-redox reactions in the process of PET photoreforming are achieved. The optimum activity of $BiVO_4/MoO_x$ for PET photoreforming into formate, acetate and $H_2$ reach 0.29, 0.19 and 1.96 mmol $g^{-1} h^{-1}$, which are 6.2, 6.0 and 5.9 times higher than that of $BiVO_4$, respectively. In short, the designed catalyst $BiVO_4/MoO_x$ can

break through the mutual restraints of redox reactions in the photoreforming process of PET, thus leading to excellent performance over most previously reported non-toxic photocatalysts.

## 2. Experimental section

### 2.1. Synthesis of catalysts

#### 2.1.1. Synthesis of pure $BiVO_4$

The synthesis of pure $BiVO_4$ was performed by adopting a protocol previously reported [17]. In a specific experiment, 1 mmol of $Bi(NO_3)_3$ and 0.5 mmol of citric acid were added into a 100 mL Teflon-lined autoclave containing 70 mL of deionized (DI) water. The mixture was then heated at 180 °C for 10 h. The white powder obtained was centrifuged at 10,000 revolutions per minute and then cleaned with DI water. Afterwards, the powder was dried at 60 °C under vacuum. Subsequently, light yellow $Bi_2O_3$ powder was obtained by calcination in air at 500 °C for 3 h. Finally, 2 mmol $Bi_2O_3$ powder and 2 mmol $V_2O_5$ powder were added to a $HNO_3$ solution (100 mL, 0.5 M) and stirred for 4 days at 25 °C. The obtained vivid yellow powder was then cleaned with DI water and dried at 60 °C for 1 day.

#### 2.1.2. Preparation of the $BiVO_4/MoO_x$

$BiVO_4/MoO_x$ photocatalysts were prepared using a hydrothermal method. In a typical procedure, 1 g of $BiVO_4$ and 0.1 g of $(NH_4)_6Mo_7O_{24} \cdot 4H_2O$ were mixed and transferred into a 100 mL Teflon-lined autoclave containing 70 mL of DI water, heated at 180 °C for 10 h. The resulting solid product was obtained by centrifugation at 10000 revolutions per minute, washed with DI water for five times, and dried overnight. Different amounts of $(NH_4)_6Mo_7O_{24} \cdot 4H_2O$ (0.05 g, 0.10 g, 0.15 g) were added, and designated as $BiVO_4/0.05 MoO_x$, $BiVO_4/0.10 MoO_x$, and $BiVO_4/0.15 MoO_x$.

### 2.2. Photocatalysts characterization

XRD patterns of all samples were confirmed using a Smart Lab 9 kW-diffractometer with Cu Kα radiation in the range from 10 to 70° (λ = 1.5406 Å, 40 kV and 100 mA). The Raman spectroscopy measurements were conducted using a Raman-microscope (Thermo-Fisher Scientific DXRxi incorporated with a 532 nm laser). The surface morphologies, elemental distribution, and lattice fringes of all samples were analyzed by scanning electron microscope (SEM-ZEISS Sigma 300) and high-resolution transmission electron microscope (HRTEM-JEOL JEM-F200). Micromeritics ASAP 2460 analyzer was used to assess the

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Schematic description of PET plastic photoreforming at $BiVO_4/MoO_x$.</figcaption></figure>
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concrete surface areas and pore size distributions for all the prepared materials. X-ray photoelectron spectroscopy (XPS) analyses were conducted on a Shimadzu/Kratos AXIS SUPRA system (hν = 1486.6 eV) at 12 kV and the binding energies of each element were corrected with C 1 s as the standard peak (284.6 eV). The element contents of materials were determined by an inductively coupled plasma optical emission spectrometry (ICP-OES). Ultraviolet photoelectron spectroscopy (UPS-Thermo ESCALAB 250XI) studies were conducted applying He I light source (21.22 eV). UV-visible diffuse reflectance spectra were recorded on an Agilent Cary 5000 spectrophotometer. The surface photovoltage spectroscopy (SPV) and field-induced surface photovoltage spectroscopy (FISPS) measurements were performed on a surface photovoltage spectrometer (CEL-SPS1000). The decay time spectra were recorded on a fluorescence spectrometer (FS5, Edinburgh Instruments Ltd.). The temperature-programmed desorption (TPD) of H₂O measurements were performed on a chemisorption apparatus (XIANQUAN TP-5080). The fluorescence lifetimes for pure BiVO₄ and BiVO₄/0.10 MoOₓ were measured with excitation and emission at 380 nm and 600 nm, respectively. ¹H NMR spectra were measured using a 500 MHz Bruker spectrometer. Electron spin resonance (ESR) analyses were conducted using a Bruker E-500. To capture the hydrogen radicals (•H) and hydroxyl radicals (•OH) during irradiation, a dispersion of catalyst containing 100 mM 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used, with a volume of 1.0 mL. The •O₂ was tested under the same condition as •H and •OH, with the only difference being that CH₃OH was used as the solvent. In situ FTIR analysis was conducted by Nicolet iS50 FT-IR spectrometer (Thermo-Fisher Scientific) with a mercury-cadmium telluride (MCT) detector. Photoelectrochemical measurements were executed in a three electrodes system with 0.5 M Na₂SO₄ as the electrolyte solution. A platinum net electrode, saturated calomel electrode and sample films were used as the counter electrode, reference electrode and working electrode (WE), respectively. The production processes of WE are described as follows: 5.0 mg of the photocatalyst was mixed in solvent including 100 µL of DI water, and 100 µL of ethanol and agitated under ultrasound for 3 minutes. Then, 100 µL of the suspension was applied to ITO with an area of 1 cm². The experiment was conducted with a 300 W xenon lamp, and the distance between WE and light source was 15 cm.

## 2.3. Plastic upcycling processes

### 2.3.1. Pretreatment of plastic

A cutting machine was used to cut 25 g of commercial PET granules into smaller particles. 1 g of as-obtained PET powder was mixed with 50 mL of 1 M KOH aqueous solution and stirred in the dark at 50 °C for 48 h. The supernatant was collected by centrifugation at 10000 revolutions per minute and used as raw material for photoreforming.

### 2.3.2. Photoreforming of PET

The photoreforming experiments of PET were conducted in a self-made photoreactor at temperature of 15 °C, under Xe lamp irradiation of 300 W. 50 mg of photocatalyst was added into 50 mL of supernatant collected from PET pretreatment. The mixture was agitated under ultrasound for 3 mins, and then Ar gas was used to exhaust the reactor and create an inert atmosphere for 30 mins.

### 2.3.3. Products analysis

The evolving H₂ was assessed through a meteorological analyzer (FULI GC9720 Plus) with Ar as the carrier gas. Isotopic labeling tests were conducted with substitute D₂O/NaOD for H₂O/KOH, and the gaseous products in the self-made photoreactor (100 µL) were quantified through a gas chromatograph-mass spectrometer (Shimadzu GCMS-QP2020NX). The composition and concentration of PET hydrolysate were analyzed using ¹H NMR spectra both before and after reaction. In a typical experiment, 0.3 mL of a liquid sample was mixed with 0.3 mL of D₂O and 0.1 mL 1 mmol of maleic acid standard. High-performance

liquid chromatography (HPLC, Shimadzu-LC-2030 C) carrying with an AMINEX®HPX-87 H column (300 mm × 7.8 mm) and UV detector were utilized to separate and analyze the liquid products from photo-reforming hydrolysate of PET. The elution phase and mobile phase were consisted of methanol/H₂O (1:1) and 10 mM H₂SO₄ solution, respectively and the flow rate of mobile phase was 10 µL/s.

## 2.4. Computational details

All DFT calculations were performed using a Vienna Ab initio Simulation Package (VASP) with a project-augmented wave (PAW) method [18,19]. The exchange-functional was conducted by the generalized gradient approximation (GGA) of Perdew-Burke-Emzerhof (PBE) functional [20], and the dispersion interactions were presented by Grimme's DFT-D3 methodology [21]. H 1 s, O 2s2p, V 3d4s, Mo 4d5s and Bi 6s6p electrons were treated as valence electrons, as well as the spin-polarized effects of the electrons considered during the calculations. The valence electronic states were expanded in plane-wave basis sets with an energy cutoff at 450 eV.

The BiVO₄ was modelled as a redefined two-Bi₄V₄O₁₆-layer BiVO₄(010) slab (7.3 × 14.8 × 14.5 Å) based on the TEM results (Fig. 2b), and the MoO₃ was modelled as a redefined p(1 × 2) MoO₃(010) slab (7.5 × 15.1 × 11 Å). The heterojunction (7.3 × 14.8 × 32.2 Å) was constructed by adding one MoO₃ layer on each side of BiVO₄, with a small lattice mismatch of ~2 %. The k-point mesh was employed as 2 × 1 × 1, and the vacuum thickness between slabs was ~15 Å. All the atoms except the bottom V-Bi-O in the BiVO₄ slab and the bottom Mo-O-V-Bi-O in the heterojunction were allowed to relaxed until the atomic force threshold of 0.05 eV/Å. The DOS of independent BiVO₄ and MoO₃ were leveled relative to the deep energy level of gas N₂.

# 3. Results and discussion

## 3.1. Photocatalysts characterization

The XRD patterns of BiVO₄ with different Mo contents at 15.13°, 19.0°, 28.9°, 30.7°, 35.3°, 42.5°, 47.3° and 53.3° correspond to the (020), (011), (121), (040) (002), (051), (042) and (222) planes of monoclinic BiVO₄ (JCPDS No. 14-0688) (Fig. 1a), while there are no characteristic peaks for MoOₓ due to its low content and high dispersion [22]. Besides, the characteristic peaks of BiVO₄/MoOₓ exhibit a slight shift towards higher angles as the MoOₓ loading increases in comparison with pure BiVO₄. Consequently, the presence of MoOₓ particles induce the crystal lattice distortion of BiVO₄. In addition, FT-IR spectral analysis (Fig. S1) was conducted to confirm the structure of BiVO₄ and BiVO₄/0.10 MoOₓ photocatalysts [23]. All the IR peaks are attributed to the characteristic peaks of BiVO₄, which is in accordance with the XRD results.

As shown in Fig. 1b, the characteristic peaks at around 824, 367, 330 and 211 cm⁻¹ appear, which are assigned to monoclinic BiVO₄ in all photocatalysts [24]. Additionally, the peak at around 824 cm⁻¹ is the most intense, which corresponds to the symmetric V–O stretching, the other peaks are in accordance with the symmetric V–O bending (367 cm⁻¹), antisymmetric V–O bending (330 cm⁻¹), and external mode (211 cm⁻¹), respectively [25]. Moreover, all the Raman peaks of BiVO₄/MoOₓ shift to lower frequencies with the increase of MoOₓ loading were compared with pure BiVO₄, further indicating the presence of MoOₓ particles induces the crystal lattice distortion of BiVO₄.

Fig. S2a shows the N₂ adsorption-desorption isotherms of BiVO₄ photocatalyst with various MoOₓ contents. Type-II adsorption isotherms and a loop ring of type-H3 were observed across all materials, with no saturation adsorption platforms appearing. This suggests irregular pore structures in the samples [26]. The BET surface areas were determined to be 2.36, 2.07, 1.78, and 1.58 m²/g for BiVO₄, BiVO₄/0.05 MoOₓ, BiVO₄/0.10 MoOₓ, and BiVO₄/0.15 MoOₓ, with mean pore sizes of 2.04, 1.94, 1.69 and 1.57 nm, respectively (Fig. S2b). The decrease in surface
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<figure><img src="image_3.png" /><figcaption>Fig. 1. XRD patterns (a) and Raman (b) spectra of BiVO₄, BiVO₄/0.05 MoOₓ, BiVO₄/0.10 MoOₓ and BiVO₄/0.15 MoOₓ.</figcaption></figure>

area and mean pore size is attributed to the deposition of MoOₓ. Fig. 2a-d show the transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) images of BiVO₄ and BiVO₄/0.10 MoOₓ. The pure BiVO₄ (Fig. 2a) exhibits a smooth surface, while the surface of BiVO₄/0.10 MoOₓ shows the presence of numerous nano-particles with a uniform size about 5 nm, as marked with red dotted circles (Fig. 2c). The lattice spacing of BiVO₄ 0.29 nm, which is corresponding to the (040) facet of BiVO₄. The lattice spacing of MoOₓ deposited on BiVO₄ (Fig. 2b and d) is 0.32 and 0.34 nm, corresponding to the (210) and (040) facet of MoO₃, respectively [22,27–29]. Furthermore, energy-dispersive spectroscopy (EDS) mapping analysis (Fig. 2e) verifies the homogeneous distribution of Bi, V, O, and Mo elements in BiVO₄/0.10 MoOₓ [30]. Additionally, the Mo element contents in BiVO₄/0.05 MoOₓ, BiVO₄/0.10 MoOₓ, and BiVO₄/0.15 MoOₓ were determined to be 0.06 wt%, 0.10 wt%, and 0.16 wt% respectively, using inductively coupled plasma optical emission spectrometry (ICP-OES).

X-ray photoelectron spectroscopy (XPS) was used to deeply analyze the electronic structures of as prepared photocatalysts. The overall XPS survey spectrum (Fig. 3a) indicates the existence of Bi, V, O and Mo elements on the surface of BiVO₄/0.10 MoOₓ, which is in good

agreement with the result of EDS mapping. As depicted in Fig. 3b, the O1s spectrum of pure BiVO₄ presents three peaks appearing at 529.37, 531.04, and 532.94 eV, which are attributed to the Bi–O, V–O, and C=O, respectively [31,32]. Meanwhile, BiVO₄/0.10 MoOₓ shows an additional peak at 531.75 eV, which belongs to the O element in the vicinity of the oxygen vacancy (V>O) [33]. As presented in Fig. 3c, peaks at binding energies of 233.01 and 234.77 eV are attributed to the 3d₃/₂ and 3d₅/₂ orbitals of Mo⁶⁺, respectively [34]. Besides, peaks at binding energies of 229.00 and 231.11 eV are attributed to the 3d₃/₂ and 3d₅/₂ orbitals of Mo⁵⁺, respectively. Meanwhile, the EPR signal (Fig. S4 a) of BiVO₄/0.10 MoOₓ can be attributed to Mo⁵⁺ and V>O induced by Mo⁵⁺ with the g factors of 1.899 and 2.003, respectively [35]. The V 2p spectrum is shown in Fig. 3d. It is noted that two new peaks appear at the binding energies of 518.19 and 524.96 eV, which are respectively attributed to the 2p₃/₂ and 2p₁/₂ orbitals of V element with V–O–Mo bonds [25,36]. Furthermore, the binding energy of Bi 4f in BiVO₄/MoOₓ is higher than that of pure BiVO₄, suggesting that electrons flow from BiVO₄ into MoOₓ. (Fig. S4 b). All the above results demonstrate that MoOₓ is well decorated on the surface of BiVO₄.

<figure><img src="image_4.png" /><figcaption>Fig. 2. TEM image of BiVO₄ (a)-(b) and MoOₓ-BiVO₄ (c)-(d); (e) Elemental mapping images of BiVO₄/0.10 MoOₓ.</figcaption></figure>
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<figure><img src="image_5.png" /><figcaption>Fig. 3. XPS spectra of BiVO₄ and BiVO₄/0.10 MoOₓ: (a) Survey, (b) O 1s, (c) Mo 3d and (d) V 2p.</figcaption></figure>

## 3.2. Evaluation of optical properties of BiVO₄/MoOₓ photocatalyst

The optical properties of BiVO₄ and BiVO₄/MoOₓ composites were evaluated using UV-vis diffuse reflectance spectroscopy (DRS). As shown in Fig. 4a, the optical absorption of BiVO₄/MoOₓ is slightly enhanced as the load of MoOₓ increases, expanding the absorption in the wavelength range from 350 to 800 nm. Accordingly, the indirect band gap of BiVO₄ is 2.43 eV, calculated by the Kubelka-Munk method (Fig. S5), which is consistent with what previously reported [37]. The valence band potential (VB) of pure BiVO₄ was determined using ultraviolet photoelectron spectroscopy (UPS) by subtracting the difference between fermi edge and cut-off edge from the initial energy (21.22 eV), resulting in a value of 6.76 eV [38]. Moreover, the conduction band position is estimated to be 4.33 eV by subtracting the band gap (Eg) from the valence band potential (Fig. S6). In order to convert these values from electron volts to electrochemical energy potentials in volts, a reference standard is used where 0 V versus a normal hydrogen electrode (NHE, pH = 7) is equivalent to -4.44 V versus the vacuum level. Hence, the E<sub>CB</sub> and E<sub>VB</sub> of BiVO₄ are -0.11 and 2.32 V, respectively (vs NHE pH = 7), with a suitable electronic band structure to oxidize PET hydrolysate and reduce H₂O into H₂, simultaneously.

As is known, the separation efficiency of photogenerated charge carriers in a catalyst is crucial to its photocatalytic performance [39,40]. Considering this, the steady-state photoluminescence (PL) spectra were collected to assess the separation efficiency of the photogenerated electron-hole pairs. As displayed in Fig. 4b, all the samples exhibit an emission peak around 580 nm, and the PL intensity significantly diminishes after the addition of MoOₓ onto BiVO₄. Among them, the BiVO₄/0.10 MoOₓ exhibits the weakest PL peak, indicating the highest efficiency in separating photogenerated charge carriers.

Meanwhile, transient photocurrent (PC) of all samples were measured under a 300 W Xe lamp (Fig. 4c), and the current density was enhanced after decorating BiVO₄ with MoOₓ. Among all the samples, BiVO₄/0.10 MoOₓ exhibits the highest photocurrent density, indicating a superior photoinduced charge-carrier separation efficiency. Besides,

electrochemical impedance spectroscopy (EIS) study was conducted to evaluate the behavior of charge carriers' migration. As presented in Fig. S7, it is evident that the capacitance arc for BiVO₄/0.10 MoOₓ is smaller compared to other samples. This observation indicates that the electron mobility of BiVO₄/0.10 MoOₓ has been enhanced, and the recombination of electron-hole pairs has been effectively suppressed following the decoration of MoOₓ on BiVO₄. Generally, efficient separation of charge carriers also results in an increase in carrier lifetime. To demonstrate this, transient fluorescence spectra for the BiVO₄/MoOₓ/0.05 MoOₓ, BiVO₄/0.10 MoOₓ and BiVO₄/0.15 MoOₓ were performed, and their average fluorescence lifetimes were derived to be 4.26, 5.67, 12.41, and 8.38 ns, respectively (Fig. 4d). The lifetimes of carriers were significantly enhanced, indicating that the MoOₓ nano-particles effectively promote the separation of photogenerated carriers in BiVO₄.

The separation of charge carriers over the photocatalyst is a pre-requisite for photocatalysis rection; however, sufficient active sites on the catalyst to manipulate electrons to result in efficient reduction is equally important. Based on linear sweep voltammetry (LSV) curves testing, the onset potential is positively shifted when decorating MoOₓ nano-particles onto BiVO₄, due to the co-catalytic effect of MoOₓ (Fig. 4e). Additionally, BiVO₄/0.10 MoOₓ exhibits the highest hydrogen evolution reaction (HER) activity among all BiVO₄/MoOₓ catalysts. Moreover, the promoted HER activity was understood by DFT calculations (Fig. 4f and S8). Compared to pure BiVO₄, the BiVO₄/0.10 MoOₓ exhibits more advantageous interactions with the H⁺ intermediates, primarily due to the change in coordination number of surface oxygen atoms from 2 to 1. Therefore, the decoration of MoOₓ is beneficial for boosting the H₂ evolution of the photocatalyst.

Surface photovoltage spectroscopy (SPV) was employed to investigate the dynamic behaviors of the photogenerated electron-hole pairs in both prepared BiVO₄ and BiVO₄/0.10 MoOₓ. As shown in Fig. 4g, both BiVO₄ and BiVO₄/0.10 MoOₓ present an intense SPV response region in the wavelength range from 300 to 520 nm, attributed to the band-to-band transition. The band-to-band transition of BiVO₄ is constructed by two processes: response in the UV region and visible light region,
<figure><img src="image_6.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>d</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>e</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>f</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>g</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>h</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>i</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 4. (a) UV-vis DRS spectra, (b) fluorescence spectra, (c) photocurrents, (d) PL decay traces, (e) Linear sweep voltammetry (LSV) curves of the BiVO₄/MoOₓ photocatalysts. (f) Computed free-energy diagram for HER on BiVO₄ and BiVO₄/MoOₓ. (g) Surface photovoltage spectra, (h) the field-induced surface photovoltage spectra under different external electric fields, and (i) H₂O-TPD for pure BiVO₄ and BiVO₄/0.10 MoOₓ.</figcaption></figure>

which are attributed to the band transition from O 2p and the hybrid orbital of Bi 6s and O 2p to V 3d orbital, respectively [41]. The stronger SPV intensity observed in BiVO₄/0.10 MoOₓ indicates that the decoloration of MoOₓ nano-particles enhances the separation efficiency of photogenerated electron-hole pairs.

Moreover, field induced surface photovoltage spectra (FISPS) to investigate the influence of polarization effect [42]. As shown in Fig. 4h, the SPV response intensities for both BiVO₄ and BiVO₄/0.10 MoOₓ increase with the increase of positive electric field, suggesting that the direction of external field and polarization electric field in BiVO₄ are consistent. More importantly, BiVO₄/0.10 MoOₓ exhibits higher SPV intensities in the response region (300–520 nm) than pure BiVO₄ at the same external voltage, suggesting that the V₀ in MoOₓ enhances the polarization effect of BiVO₄. Furthermore, the electric hysteresis loops for as-prepared samples were conducted to reveal the ferroelectric performance and the polarization electric field is mainly contributed by Pᵣ after the corona poling. As illustrated in Fig. S9, the values of Pᵣ were found to be 0.45 and 0.54 µC·cm⁻² for BiVO₄ and BiVO₄/0.10 MoOₓ, respectively. The improved ferroelectric polarization can be attributed to the V₀ in MoOₓ of BiVO₄/0.10 MoOₓ, which is consistent with FISPS results.

The interaction between reactant molecules and catalysts was

characterized through contact angle measurements of pretreated PET and temperature-programmed H₂O desorption (H₂O-TPD). The BiVO₄/0.10 MoOₓ exhibits stronger surface hydrophilicity compared to pure BiVO₄ (Fig. S10). Besides, the peaks of H₂O in the cases of BiVO₄ and BiVO₄/0.10 MoOₓ locate at 128 and 138 °C, respectively (Fig. 4i), indicating the favorable adsorption of H₂O with the introduction of MoOₓ, consistent with the contact angle measurements. In summary, on BiVO₄/0.10 MoOₓ exhibited a more profound polarization effect, leading to enhanced adsorption and activation of the pretreated PET, which is beneficial for its oxidation.

### 3.3. Photoreforming of pretreated PET

Due to the insolubility of the PET substrate, it was pretreated in a 1 M KOH solution to initiate the decomposition of the polymer (ethylene glycol and terephthalate) before photoreforming (Fig. S11). Quantitative 1 H nuclear magnetic resonance (NMR) spectroscopy shows that 37 % of ethylene glycol (EG) and 33 % of terephthalic acid (TPA) in PET were released with 2 mmol L⁻¹ of maleic acid as the standard (Table S1 and Fig. 5a). After 5 h photoreforming, three carboxylate products were detected from the pretreated PET, identified as formate, acetate, and a trace of glycolate. Besides, the concentration of EG monomers decreased
<figure><img src="image_15.png" /><figcaption>**Fig. 5.** ¹H NMR spectra of photoreforming the pretreated PET for 5 h; insets are the partial enlarged of the spectra marked by a blue color. A, B, C, D and E correspond to TPA, EG, formate, acetate and glycolate. (b) The evolution rate of formate, acetate and (c) H₂ during photoreforming PET process for 5 h. (d) H₂ yield in the process of photoreforming pretreated PET and PET bottle in 25 h. (e) Control experiments with different scavengers. (f) MS spectrum of PET photoreforming over BiVO₄/0.10 MoOₓ with D₂O and NaOD as hydrogen sources. Reaction conditions: 0.2 g PET powder was added into 10 mL D₂O with 1 M NaOD followed by the photoreforming tests (5 h) over BiVO₄/0.10 MoOₓ (50 mg).</figcaption></figure>

from 32.2 to 28.6 mmol L⁻¹ with photoreforming (Fig. 5a), whereas the concentration (from 28.8 to 28.7 mmol L⁻¹) of TPA monomers presented no significant change. This indicates that all the products were converted through the oxidation of the EG component of PET rather than TPA, and the conversion of EG was calculated to be 11.2 %.

Moreover, all liquid products were further quantified by HPLC. The formate generation rates of BiVO₄, BiVO₄/0.05 MoOₓ, BiVO₄/0.10 MoOₓ, and BiVO₄/0.15 MoOₓ were determined to be 0.047, 0.076, 0.29, and 0.18 mmol g⁻¹ h⁻¹ after 5 h of irradiation. Meanwhile, the acetate generation rates were calculated to be 0.032, 0.085, 0.19, and 0.13 mmol g⁻¹ h⁻¹, respectively (Fig. 5b and Fig. S12). Besides, the reduction half-reaction over BiVO₄, BiVO₄/0.05 MoOₓ, BiVO₄/0.10 MoOₓ, and BiVO₄/0.15 MoOₓ was quantified by gas chromatograph (GC). Typically, the H₂ evolution rates of all the samples were 0.33, 0.74, 1.96, and 1.10 mmol g⁻¹ h⁻¹ in the 5 h photoreforming (Fig. 5c), indicating efficient PET photoreforming performance in non-toxic photocatalysts (Table S2). Clearly, the BiVO₄/0.10 MoOₓ presents the highest activity in the reduction half-reaction, which is consistent with the results of the oxidation half-reaction. As displayed in Fig. 5d, BiVO₄/0.10 MoOₓ exhibits stable H₂ evolution over 25 h of PET photoreforming, with an average activity of 1.8 ± 0.08 mmol g⁻¹ h⁻¹. Moreover, the morphology and crystal structure of BiVO₄/0.10 MoOₓ are retained after photoreforming (Fig. S13), indicating its good durability. In order to identify the active species, various scavengers (i.e., Na₂S/Na₂SO₃, DMPO, tert-Butanol (TBA), p-Benzoquinone (p-BQ) and furfuryl alcohol (FFA) as hole, ●H radical, ●OH radical, ●O₂ radical and ¹O₂ scavengers, respectively) were added during the pretreated PET photoreforming process (Fig. 5e). Subsequently, the yield of formate and acetate significantly decrease with the addition of 0.05 M Na₂S/Na₂SO₃, while there is no clear change with the addition of other scavengers, suggesting that photogenerated holes play the major role in EG oxidation. Interestingly, the yield of H₂ decreases with the addition of 0.05 M DMPO, demonstrating that certain kind of free radicals, which are

generated through the reduction of electrons, participate in H₂ production. The origin of evolving H₂ was analyzed by the D₂O isotope-labeling experiment. The detected mass-spectrometry signals at *m/z* = 2, 3 and 4 are attributed to the H₂, HD and D₂, strongly indicating the H₂ product is derived from H₂O rather than PET (Fig. 5f). Additionally, CO₃²⁻ was formed as a side product through further oxidation of formate, and its concentration was quantified by a previously reported method with an accumulation of 0.062 mmol (Fig. S14) [43]. After 5 h of photoreforming of pretreated PET, 0.0725 mmol of formate, 0.0475 mmol of acetate, and 0.062 mmol of CO₃²⁻ were generated. The selectivity of the main products was evaluated to be 20.1 %, 26.4 % and 17.3 %, respectively (Table S3). The carbon balance of these products is approximately 63.8 %, which could be attributed to the fact that a few side products were not detected using ¹H NMR and HPLC. Moreover, to pave the way for the industrialization of photoreforming plastic, a typical real-world photoreforming experiment was conducted. As shown in Fig. 5d, when PET bottles were pretreated as feedstocks, the average H₂ generation rate is determined to be 1.6 ± 0.09 mmol g⁻¹ h⁻¹, which is close to that of PET powder photoreforming. Therefore, this technology displays positive versatility and scalability for utilizing photocatalysis process to promote the upcycling of PET waste.

### 3.4. Photoreforming mechanism

To deeply understand the mechanism of PET photoreforming, we utilized in situ characterization techniques to identify the possible reaction intermediates. Considering that various free radicals may be involved in the process as important active species, in situ EPR was conducted to investigate the radical species in PET photoreforming. Meanwhile, due to the short lifetimes of radicals, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was added as a spin trapping agent into the reaction solution. In Fig. 6a, it is shown that ●H and ●OH radicals can be trapped as a DMPO-H (hyperfine splitting constants, A<sub>N</sub> = 16.5 G, A<sub>H</sub> =
<figure><img src="image_16.png" /><figcaption>**Fig. 6.** (a) EPR spectra of ●H and ●OH radicals by using DMPO in 1.5 mL aqueous solution with dispersion of 5 mg BiVO₄/0.10 MoOₓ. (b) Solid EPR spectra of BiVO₄/0.10 MoOₓ in dark condition, under light irradiation and after 10 mins of light. (c) In situ infrared spectra of BiVO₄/0.10 MoOₓ. (d) In situ FTIR spectra of EG oxidation reaction on BiVO₄/0.10 MoOₓ in 1 M KOH. (e) The possible reaction pathways for the oxidation of EG to formic and glycolic acids. (f) The HOMO and LUMO electronic structures and gaps (ΔE<sub>H-L</sub>) of EG and its intermediates predicted by DFT calculations. (g) Electrostatic potential (ESP)-mapped molecular surface of EG and its intermediates.</figcaption></figure>

22.5 G) and DMPO-OH (A<sub>N</sub> = 15.0 G, A<sub>H</sub> = 14.8 G) adducts over BiVO<sub>4</sub>/0.10 MoO<sub>x</sub>, whereas O<sub>2</sub> radicals were not observed (Fig. S15), demonstrating the generation of ●H and ●OH radicals during PET photoreforming processes [44,45]. Since the signal of ●H radicals may interfere with the signal of ●OH radicals, Ag(NO<sub>3</sub>)<sub>3</sub> was employed as an electron sacrificial agent to prevent the generation of ●H radicals. As a result, only the distinctive peak of ●OH radicals emerged prominently (Fig. S16a). In order to explore the origin of ●H and ●OH radicals, EPR tests of pure BiVO<sub>4</sub> and MoO<sub>x</sub> were performed under the same conditions. As shown in Fig. S16b, no signals of ●H radicals were detected for pure MoO<sub>x</sub> and BiVO<sub>4</sub>, whereas pure BiVO<sub>4</sub> exhibited weak signals of ●OH radicals. This suggests that the MoO<sub>x</sub> particles could act as a

co-catalyst to enhance the production of ●H radicals, thereby facilitating the H₂ evolution. During the process of PET photoreforming, the ●OH radicals can be easily captured by TPA to generate TPA-OH. A characteristic PL peak of TPA-OH at 430 nm was observed (Fig. S17) [9], suggesting that ●OH radicals hardly participate in the PET photoreforming process, which is consistent with the results of the control experiments. Moreover, the in situ solid EPR measurements of BiVO₄/0.10 MoOₓ were performed under different conditions. As shown in Fig. 6b, the EPR signal can be attributed to Mo<sup>5+</sup> and V₀ with the set of g factors of 1.899 and 2.003 [46]. It is worth noting that the EPR signal was enhanced under light condition, while it remained relatively unchanged after illumination, compared to the signal observed under dark
conditions. This observation suggests that the presence of MoOₓ nano-particles serves as an intermediate medium for the capture and transfer of photogenerated electrons in BiVO₄. The electronic density of states of independent BiVO₄ and MoOₓ demonstrate that the conduction band minimum of BiVO₄ is higher than that of MoOₓ (Fig. S18). Therefore, it is thermodynamically feasible to transfer the photogenerated electrons from BiVO₄ to MoOₓ. Additionally, MoOₓ can effectively activate the adsorbed water molecules as proved by in situ FTIR of BiVO₄/0.10 MoOₓ [47]. As displayed in Fig. 6c, the water gradually adsorbs to the surface of BiVO₄/0.10 MoOₓ and forms -OH groups at about 3330 cm⁻¹. Subsequently, the -OH groups disappear under light irradiation, suggesting the consumption of water. To sum up, the MoOₓ nano-particles can trap and transfer the photogenerated electrons from BiVO₄ to water, thereby promoting the generation of ●H radicals and enhancing the performance of photocatalytic H₂ evolution.

Comparatively, the intermediates of the oxidation half-reaction were explored by in situ FTIR with an ATR method. Typically, to eliminate interference from TPA, pure EG was performed to reveal the mechanism of the PET photoreforming process. As shown in Fig. 6d, the FTIR peak at 1581 cm⁻¹ could be attributed to the C=O stretching vibration, while two peaks at 1382 and 1350 cm⁻¹ could be assigned to the δ(-C-H) and the symmetric stretching vibration of O-C-O (vs(OCO)), indicating the generation of HCOO⁻ [48,49]. Meanwhile, the peak at 1550 cm⁻¹ was because of the asymmetric stretching vibration of O-C-O, acetate. Based on the peak at 1581 cm⁻¹, glyoxal is one of the possible products, as the characteristic peak of glyoxal at 1071 cm⁻¹ also was observed. The simultaneous appearance of peaks at 1581 cm⁻¹, 1322 cm⁻¹, and 1072 cm⁻¹ suggests the potential production of glycolate species, which were also observed via ¹H NMR. Besides, the inverted peaks on both sides of 1072 cm⁻¹ are ascribed to the consumption of EG during photoreforming PET hydrolysate process, providing further evidence that the formate and acetate were transformed from EG. Therefore, the reaction pathway (Fig. 6e) for EG oxidation is proposed as follows: First, the EG molecules are adsorbed on the surface of BiVO₄ through bidentate configuration according to previous investigations; After that, the O-H bond cleavage occurs in EG to form the *OCH₂-CH₂O* intermediates; Finally, the *OCH₂-CH₂O* intermediates are converted into the *OCH-HCO* intermediates, and the *OCH-HCO* intermediates are oxidized to generate formic acid. Besides, glyoxal could be converted to glycolic acid by the Cannizzaro reaction in an alkaline solution.

To investigate the rationality of the reaction pathways, the impact of functional groups of EG and its intermediates were analyzed based on the orbital structures. From the HOMO and LUMO electronic structure diagrams in Fig. 6f, the functional groups of -OH, -O-, HC=O and -COOH have an obvious effect on electronic structures of HOMO and LUMO for EG and its intermediates. As displayed in Fig. 6f, the aldehyde groups in *OCH-HCO* intermediates impact HOMO-LUMO gap more than other intermediates with the lower ΔE<sub>H-L</sub> = 0.16 eV which indicates that this compound has more ability to react with water molecule to form other products in solution. Also, for *OCH₂-CH₂O* intermediates with ΔE<sub>H-L</sub> = 0.22 eV, the presence of peroxide substitution induces a clear impact on HOMO-LUMO gap higher than HCOOH with ΔE<sub>H-L</sub> = 0.28 eV close to the initial product EG, which indicates that *OCH₂-CH₂O* is more reactive than HCOOH. In summary, the effect of groups substitution in the process of PET photoreforming induces a large reduction in the HOMO-LUMO gap, thus the intermediate reacts easily to generate formic acid [50].

Furthermore, an analysis was conducted on the distribution of electrostatic potential (ESP) on the surface of the EG molecule and its intermediates (Fig. 6g). The electrostatic potential is found to be more negative around the region surrounding of oxygen groups atoms for EG and its intermediates, suggesting that the positions may be subject to electrophilic attack. From the electrostatic potential results, it is understandable that all the obtained intermediates show high reactivity compared to EG. However, the absolute ESP value of HCOOH is much greater than that of *OCH-HCO* intermediate, indicating that the

reaction from *OCH-HCO* intermediate to HCOOH is difficult to occur, due to the fact that the C-C bond cleavage of glyoxal intermediates is an endothermic process. Therefore, the cleavage of C-C bond is the rate-determining step during EG oxidation process [51].

According to the above results, the mechanism of the photoreforming pretreated PET into H₂ and formate over BiVO₄/MoOₓ can be divided into reduction and oxidation reactions, in which the EG (hydrolyzed monomer of PET) underwent the photogenerated holes direct oxidative dehydrogenation and C-C bond cleavage to form formate over BiVO₄/MoOₓ. Meanwhile, MoOₓ can trap and transfer the photogenerated electrons from BiVO₄ to promote H₂ evolution. Briefly, the overall photoreforming mechanism could be proposed as follows:

$$BiVO_4 \rightarrow e^- + h^+$$

Reduction reaction: H₂O was reduced to H₂

$$e^- + Mo^{6+} \rightarrow Mo^{5+}$$

$$Mo^{5+} + H^+ \rightarrow Mo^{6+} + \bullet H \ (\text{fast process})$$

$$2\bullet H \rightarrow H_2\uparrow$$

Oxidation reaction: EG was oxidized into formate

$$CH_2OHCH_2OH_{sol} \rightarrow CH_2OHCH_2OH_{ads}$$

$$CH_2OHCH_2OH_{ads} + 2 h^+ + 2OH^- \rightarrow *OCH_2-CH_2O* + 2 H_2O$$

$$*OCH_2-CH_2O* + 2 h^+ + 2OH^- \rightarrow *OCH-HCO* + 2 H_2O$$

$$*OCH-HCO* + 2 h^+ + 4OH^- \rightarrow 2HCOO^- + 2 H_2O$$

However, the acetate products do not appear in the proposed pathway as mentioned above. This may be due to the conversion of glycoaldehyde into acetaldehyde, followed by the oxidation of acetaldehyde into acetate [52]. Therefore, after 5 h of photoreforming of pretreated PET, 1.075 mmol of electrons (e⁻) and 0.72 mmol of holes (h⁺) were involved in the generation of various redox products (Table S2). The charge balance of these products was approximately 67 %, which could be attributed to that a few side products were not detected using ¹H NMR and HPLC [43].

## 4. Conclusion

In summary, MoOₓ nano-particles were employed as a non-noble cocatalyst and decorated onto BiVO₄, facilitating the photo-redox reactions of reforming PET. TEM and XPS measurements revealed the decoration of MoOₓ particles onto BiVO₄ through V-O-Mo bonds, resulting in the formation of nanoparticles sized about 5 nm. The oxygen vacancy (O_v) in MoOₓ particles promotes the adsorption of pretreated poly(ethylene-terephthalate) (PET), leading to enhanced oxidation of pretreated PET. Meanwhile, benefiting from the low conduction position and the variable valence of Mo⁵⁺/⁶⁺ ions, MoOₓ can be regarded as an electron relay, facilitating the transfer of photogenerated electrons from BiVO₄ to MoOₓ. As a result, the free energy of H⁺ + e⁻ → H⁺ is lowered down from 0.39 to -0.12 eV, thus promoting the reduction of protons to produce H₂. Meanwhile, in-situ FTIR spectral results confirm that the possible reaction pathway of formate is achieved though the oxidation of EG on the surface of BiVO₄/MoOₓ. In other words, a cooperative promotion of photo-redox was achieved in the process of PET photoreforming. The optimum activity of BiVO₄/MoOₓ for PET photoreforming into formate, acetate and H₂ are estimated to be 0.29, 0.19, and 1.96 mmol g⁻¹ h⁻¹, which are 6.2, 6.0, and 5.9 times higher than that of BiVO₄, respectively. The present work provides insights into designing highly efficient catalysts to achieve effective PET photoreforming into valuable chemicals accompanied by H₂ evolution under ambient condition.
CRediT authorship contribution statement

**Fei Li:** Software, Investigation. **Feng Yu:** Software. **Abdelkader Labidi:** Software. **Eric Lichtfouse:** Writing – review & editing. **Xinxin Liang:** Writing – review & editing, Writing – original draft, Investigation, Data curation. **Ting Gao:** Formal analysis. **Chuanyi Wang:** Writing – review & editing, Funding acquisition, Conceptualization. **Ximing Li:** Investigation. **Yongqian Cui:** Investigation. **Qibing Dong:** Data curation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data that has been used is confidential.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52161145409, 21976116), SAFEA of China ("Belt and Road" Innovative Talent Exchange Foreign Expert Project # 2023041004L) (High-end Foreign Expert Project # G2023041021L), and Alexander-von-Humboldt Foundation of Germany (Group-Linkage Program).

Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.apcatb.2024.124326.

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<figure><img src="image_17.png" /><figcaption>Figure S1 FTIR spectra of BiVO₄ and BiVO₄/0.10 MoOₓ photocatalysts.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Figure S2 (a) The N₂ adsorption–desorption isotherms and (b) pore size distribution of BiVO₄, BiVO₄/0.05 MoOₓ, BiVO₄/0.10 MoOₓ and BiVO₄/0.15 MoOₓ.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>b</figcaption></figure>

<caption>Figure S3 The SEM images of BiVO₄ and BiVO₄/0.10 MoOₓ.</caption>

<figure><img src="image_21.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>Figure S4 The EPR (a) and (b) XPS spectra of BiVO₄ and BiVO₄/0.10 MoOₓ.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>Figure S5. The $(\alpha hv)^2$ versus $hv$ curves of $BiVO_4$.</figcaption></figure>

<figure><img src="image_24.png" /><figcaption>Figure S6. UPS spectra of $BiVO_4$.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>Figure S7. EIS Nyquist plots of $BiVO_4$.</figcaption></figure>
<figure><img src="image_26.png" /></figure>

Figure S8. Atomic structure for (a) BiVO₄ crystal and (b) MoOₓ nano-clusters used for model construction. Grey-colour = V, purple = Bi, red = O and green = Mo. Computational model for interface structures of BiVO₄/MoOₓ. (c) Side (elevation) and (d) Top (plan), view.

<figure><img src="image_27.png" /></figure>

Figure S9. Electric hysteresis loop of BiVO₄ and BiVO₄/0.10 MoOₓ.

<figure><img src="image_28.png" /></figure>

Figure S10. The pretreated contact angles of BiVO₄ and BiVO₄/0.10 MoOₓ.
<figure><img src="image_29.png" /><figcaption>Figure S11. The processes of pretreated PET.</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>Figure S12. Calibration curves of formate (a) and acetic acid (b).</figcaption></figure>

<figure><img src="image_31.png" /><figcaption>Figure S13. XRD pattern of BiVO₄/0.10 MoOₓ before and after photoreforming.</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>Figure S14. A self-built methodology for CO₃²⁻ quantification.</figcaption></figure>
<figure><img src="image_33.png" /><figcaption>Figure S15. EPR spectra of •O₂⁻ radicals by using DMPO in an aqueous dispersion of BiVO₄/0.10 MoOₓ.</figcaption></figure>

<figure><img src="image_34.png" /><figcaption>Figure S16. In situ EPR spectra of (a) BiVO₄/0.10 MoOₓ with 10 mM Ag(NO₃)₃ as the electron scavengers. (b) In situ EPR spectra of •H and •OH radicals by using DMPO in an aqueous dispersion of pure BiVO₄ and MoOₓ.</figcaption></figure>

<figure><img src="image_35.png" /><figcaption>Figure S17. PL spectra of pure TPA-OH and the collected TPA after 5 h of photoreforming.</figcaption></figure>

<figure><img src="image_36.png" /><figcaption>Figure S18. The electronic density of states (DOS) of independent BiVO₄ and MoOₓ was leveled relative to the deep energy level of N₂ gas.</figcaption></figure>
<table><caption>Table S1. Quantification (by 1H-NMR spectroscopy) of polymer (PET and PLA) solubilization after pre-treatment (50 mL, 1M KOH, 48h).</caption><thead><tr><th>Sample</th><th>Addition</th><th>Components</th><th>Ideal quantity of component</th><th>Measured quantity of component</th><th>solubilization</th></tr></thead><tbody><tr><td rowspan="2">PET</td><td rowspan="2">1g</td><td>ethylene glycol</td><td>0.27</td><td>0.10</td><td>37%</td></tr><tr><td>terephthalate</td><td>0.73</td><td>0.24</td><td>33%</td></tr></tbody></table>

<table><caption>Table S2. Plastic photoreforming performance for selected state-of-the-art photocatalysts.</caption><thead><tr><th>Plastics</th><th>Catalyst</th><th>Solvent</th><th>Reduction product</th><th>Activity/ µmol g<sup>-1</sup>h<sup>-1</sup></th><th>Oxidation product</th><th>Activity/ µmol g<sup>-1</sup> h<sup>-1</sup></th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="2">Pre<sup>a)</sup>-PET<sup>b)</sup></td><td rowspan="2">BiVO<sub>4</sub>/MoO<sub>x</sub></td><td rowspan="2">1M NaOH</td><td rowspan="2">H<sub>2</sub></td><td rowspan="2">1.96×10<sup>3</sup></td><td>formate</td><td>0.29×10<sup>3</sup></td><td rowspan="2">This work</td></tr><tr><td>acetate</td><td>0.19×10<sup>3</sup></td></tr><tr><td>Pre-PET</td><td>CdS/CdO<sub>x</sub></td><td>10M NaOH</td><td>H<sub>2</sub></td><td>3.42×10<sup>3</sup></td><td>formate</td><td>N/A</td><td>[1]</td></tr><tr><td>PET</td><td>Co-Ga<sub>2</sub>O<sub>3</sub></td><td>Deionized water</td><td>H<sub>2</sub></td><td>0.384×10<sup>3</sup></td><td>CO</td><td>100.6</td><td>[2]</td></tr><tr><td>Pre-PET</td><td>CN<sub>x</sub>|Ni<sub>2</sub>P</td><td>1M KOH</td><td>H<sub>2</sub></td><td>25.7</td><td>formate</td><td>N/A</td><td>[3]</td></tr><tr><td>Pre-PET</td><td>Ag<sub>2</sub>O/Fe-MOF<sup>c)</sup></td><td>Deionized water</td><td>H<sub>2</sub></td><td>1.9×10<sup>3</sup></td><td>N/A</td><td>N/A</td><td>[4]</td></tr><tr><td>Pre-PET</td><td>MXene/<br/>Zn<sub>x</sub>Cd<sub>1-x</sub>S<sup>d)</sup></td><td>10M NaOH</td><td>H<sub>2</sub></td><td>14.17×10<sup>3</sup></td><td>formate</td><td>N/A</td><td>[5]</td></tr><tr><td>Pre-PET</td><td>MoS<sub>2</sub>/CdS</td><td>10 M KOH</td><td>H<sub>2</sub></td><td>3.90×10<sup>3</sup></td><td>formate</td><td>0.72×10<sup>3</sup></td><td>[6]</td></tr><tr><td rowspan="3">Pre-PET</td><td rowspan="3">d-NiPS<sub>3</sub>/CdS</td><td rowspan="3">2 M KOH</td><td rowspan="3">H<sub>2</sub></td><td rowspan="3">31.38×10<sup>3</sup></td><td>formate</td><td>2.6×10<sup>3</sup></td><td rowspan="3">[7]</td></tr><tr><td>acetate</td><td>1.5×10<sup>3</sup></td></tr><tr><td>glycolate</td><td>2.8×10<sup>3</sup></td></tr></tbody></table>

<footnote>a) Pre: Pretreated b) PET: polyethylene terephthalate; c) MOF: metal-organic-framework;</footnote>
<table><caption>Table S3. Product quantification after 5 h photoreforming of pretreated PET (50 mL) by GC, HPLC, and the calculation of charge transfer numbers.</caption><thead><tr><th>Products</th><th>Amount/mmol</th><th>Charge transfer numbers</th><th>Total /mmol</th><th>Charge balance</th><th>Selectivity</th></tr></thead><tbody><tr><td>H<sub>2</sub></td><td>0.49</td><td>e<sup>-</sup>: 0.49*2</td><td>e<sup>-</sup>: 0.98</td><td rowspan="4">67%<br>(h<sup>+</sup>/e<sup>-</sup>)</td><td>/</td></tr><tr><td>Formate</td><td>0.0725</td><td>h<sup>+</sup>: 0.0725*3</td><td>h<sup>+</sup>: 0.22</td><td>20.1%</td></tr><tr><td rowspan="2">Acetate</td><td rowspan="2">0.0475</td><td>e<sup>-</sup>: 0.0475*2</td><td>e<sup>-</sup>: 0.095</td><td rowspan="2">26.4%</td></tr><tr><td>h<sup>+</sup>: 0.0475*4</td><td>h<sup>+</sup>: 0.19</td></tr><tr><td>CO<sub>3</sub><sup>2-</sup></td><td>0.062</td><td>h<sup>+</sup>: 0.062*5</td><td>h<sup>+</sup>: 0.31</td><td></td><td>17.3%</td></tr></tbody></table>

Note: 1) The CO₃²⁻ obtained by further oxidizing of formate, the reaction pathway as follow:

$$HCOO^{-} + 2h^{+} + 3OH^{-} \rightarrow CO_{3}^{2-} + 2H_{2}O$$

Moreover, owing to being in the alkaline medium, CO₃²⁻ with an accumulative concentration of 0.062 mmol instead of gaseous CO₂ evolution was determined by a self-built methodology.

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