Visible-Light-Driven Photocatalytic Hydrogen Production from Polystyrene Nanoplastics Using Pd/TiO2 Nanoparticles

Abstract

The accumulation of microplastics and nanoplastics in aquatic environments has raised significant concerns in recent years, given the potential health risks to both aquatic ecosystems and humans; due to their nanometer size, they enter the food chain of aquatic species and consequently that of humans too. This study presents an efficient plasmonic photocatalyst for degrading polystyrene nanoplastics (PS NPs), while simultaneously generating green hydrogen in the process. Blank controls show that the presence of PS NPs is necessary for H₂ evolution, since under identical conditions, it does not occur in their absence. A series of visible light-responsive plasmonic photocatalysts consisting of TiO₂ nanoparticles (NPs) supporting Pd, Au, Pt, and Ag NPs were prepared via the impregnation method. Among the synthesized nanoparticle photocatalysts, the 3 wt % Pd/TiO₂ NP photocatalyst exhibited superior hydrogen generation, producing 1329.76 μmol_H2 g_cat ^-1 after 2 h of irradiation, while also achieving a reduction in the average PS NP diameter. This study illustrates the potential of solar NP photocatalysis for environmental remediation and simultaneous hydrogen evolution.

Keywords: hydrogen production; nanomaterials; nanoplastics; photocatalysis; photocatalytic degradation; polystyrene.

1. Schematic Representation of the Preparation of TiO₂ or CeO₂-Based Photocatalysts, (a) Pd/TiO₂, Pd/CeO₂, Ag/TiO₂, and Pt/TiO₂ Photocatalyst Nanoparticles and (b) Au/TiO₂ Photocatalyst Nanoparticles

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High-resolution TEM images of (a) Pd/TiO₂, (b) Au/TiO₂, (c) Pt/TiO₂, and (d) Ag/TiO₂. The insets of images (a) and (b) correspond to expansions of the regions marked by the white squares. The numbers in images (c) and (d) indicate the size of representative Pt and Ag NPs, respectively.

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X-ray diffractograms of (a) black line 1 wt % Pd/TiO₂, orange line 3 wt % Pd/TiO₂, and red line 5 wt % Pd/TiO₂. The most intense characteristic peak of Pd NPs is indicated by a black square. (b) Brown line TiO₂, black line Pd/TiO₂, blue line Au/TiO₂, bright-green line Pt/TiO₂, and dark-green line Ag/TiO₂. The position expected for the most intense characteristic peak of each metal has been indicated on the plots: (black square)­Pd, (black circle) Au, (black diamond) Pt, and (black triangle) Ag.

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UV–vis spectra of (a) cyan line TiO₂, black line 1 wt % Pd/TiO₂, violet line 1 wt % Au/TiO₂, bright-green line 1 wt % Pt/TiO₂, and dark-green line 1 wt % Ag/TiO₂. (b) Black line 1 wt % Pd/TiO₂, red line 3 wt % Pd/TiO₂, and gray line 5 wt % Pd/TiO₂.

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Experimental high-resolution XP spectra and the corresponding deconvolution to individual components for the best-performing photocatalyst 3 wt % Pd/TiO₂.

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(a) Hydrogen evolution (μmol_H2 g_cat ^–1) vs time (h) under visible light irradiation (λ > 450 nm) from a suspension of PS NPs in H₂O (10 mg L^–1) using as the photocatalyst: black solid square 1 wt % Pd/TiO₂; red solid circle 1 wt % Pd/C; or dark-green solid triangle 1 wt % Pd/CeO₂ (1.5 mg mL^–1). (b) Hydrogen evolution of photocatalytic tests carried out with 10 mg mL^–1 PS NPs and 1.5 mg mL^–1 1 wt.% Pd/TiO₂ as the photocatalyst, black solid square under visible light irradiation or dark-green open square without irradiation. (c) Photocatalytic tests performed with 1.5 mg mL^–1 of 1 wt % Pd/TiO₂ as the photocatalyst and black solid square PS NPs or blue checked box without PS NPs as sacrificial agent under visible light irradiation. (d) Size distribution measured by dynamic laser scattering of photocatalytic test at initial time red line and final time bright-green line carried out with PS NPs (10 mg L^–1) as the sacrificial agent and 1 wt % Pd/TiO₂ (1.5 mg mL^–1) as the photocatalyst.

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Temporal evolution of H₂ μmol_H2 g_cat ^–1 upon irradiation in the presence of 1 wt % Pd/TiO₂ as the photocatalyst with visible light irradiation (λ > 450 nm) in water containing PS NPs (10 mg L^–1) varying photocatalyst dose: 0.5 mg mL^–1 pink diamond; 1 mg mL^–1 brown square; 1.5 mg mL^–1 black square; and 2 mg mL^–1 blue inverted triangle.

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Comparison of temporal H₂ evolution (μmol_H2 mg_met ^–1) of the different metals loaded on TiO₂ nanoparticles (visible light irradiation (λ > 450 nm), PS NPs 10 mg L^–1 suspended in water (20 mL) and photocatalyst 1.5 mg mL^–1 (30 mg) 1 wt % Pd/TiO₂ black solid square, 1 wt % Au/TiO₂ violet open circle, 1 wt % Pt/TiO₂ green open diamond, and 1 wt % Ag/TiO₂ brown open square).

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(a) Diameter size distribution analyzed by dynamic laser scattering of PS NPs in water before (green line) and after 2 h irradiation with visible light using 3 wt.% Pd/TiO₂ (1.5 mg mL^–1) as the photocatalyst (red line). (b) Plot of H₂ evolution μmol_H2 g_cat ^–1 in the time under visible light (λ > 450 nm) irradiation of an aqueous suspension of PS NPs (10 mg L^–1) in the presence of 1 wt % Pd/TiO₂ black solid square, 3 wt % Pd/TiO₂ orange open circle, or 5 wt % Pd/TiO₂ gray open diamond (1.5 mg mL^–1) as the photocatalyst. (c) Photocatalytic H₂ evolution (μmol_H2 g_cat ^–1) for three consecutive uses (the photocatalyst is washed and dried at 200 °C overnight before use) of 3 wt % Pd/TiO₂ (first time black solid square, second time brown open triangle, and third time violet open circle). FE-SEM images of (d) PS NPs, (e) fresh 3 wt % Pd/TiO₂, (f) fresh 3 wt % Pd/TiO₂ (1.5 mg mL^–1) mixed with PS NPs (10 mg mL^–1) without irradiation and (g) sample after visible light irradiation.

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Experimental high-resolution XP spectra and the corresponding deconvolution to individual components for the best-performing photocatalyst 3 wt % Pd/TiO₂ after 2 h of after visible light irradiation.

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Proposed mechanism for the photocatalytic H₂ evolution from aqueous dispersion of PS NPs (detailed mechanism explained in the text). The energy levels (E) are in volts versus the normal hydrogen electrode (NHE).

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(a) Zeta potential analysis at neutral pH of the 3 wt % Pd/TiO₂ photocatalyst black line, PS NPs cyan line, and the combination of 3 wt % Pd/TiO₂ with PS NPs blue line. (b) FE-SEM image of 3 wt % Pd/TiO₂ (1.5 mg mL^–1) being exposed to PS NPs (10 mg mL^–1) at initial time of visible light irradiation.