The preparation of CeO2-Co2O3/BiVO4 by electrodeposition and its applied study in photocatalytic degradation of wastewater

Technical Summary: Preparation of CeO2-Co2O3/BiVO4 by Electrodeposition and Its Application in Photocatalytic Degradation of Wastewater

Problem Statement
The study addresses the limitations of existing semiconductor photocatalysts, specifically Bismuth Vanadate (BiVO4), in the treatment of complex organic wastewater. While BiVO4 is an environmentally friendly catalyst with a suitable bandgap (2.4 eV) for visible light response, its practical application is hindered by a high rate of photogenerated electron-hole recombination and weak adsorption capacity for organic pollutants. Furthermore, the biochemical effluents from coking wastewater contain refractory organic compounds that are difficult to degrade to discharge standards (e.g., reducing Chemical Oxygen Demand (COD) from >250 mg/L to <50 mg/L) using conventional deep oxidation methods. The authors aim to develop a composite catalyst that enhances charge separation, improves pollutant adsorption, and maximizes visible light utilization to overcome these barriers.

Methodology
The research employed a multi-step synthesis and characterization approach:

• Synthesis:
  • BiVO4: Synthesized via a hydrothermal method using bismuth nitrate and ammonium metavanadate.
  • Co2O3/BiVO4: Prepared using a wet impregnation method followed by calcination at 850°C.
  • CeO2-Co2O3/BiVO4 (Ternary Composite): Prepared via electrodeposition. The Co2O3/BiVO4 coated on a foam nickel working electrode was immersed in a phosphate buffer solution (pH ~7.0) containing Ce(NO3)2. A bias voltage of 0.5 V was applied for 6–8 hours, followed by calcination at 600°C to remove organic binders. The use of a phosphate buffer was critical to prevent Ce³⁺ hydrolysis and ensure uniform nucleation.
• Characterization: The catalysts were analyzed using X-ray Diffraction (XRD) for crystalline structure, Scanning Electron Microscopy (SEM) for morphology, and X-ray Photoelectron Spectroscopy (XPS) for elemental valence states. Electrochemical workstations were used to measure oxygen evolution overpotential and impedance.
• Photocatalytic Testing: Experiments were conducted using coking wastewater (initial COD ~252 mg/L) under a 500 W xenon lamp. The study screened optimal light wavelengths (320, 345, 435, 495 nm), evaluated degradation over time (up to 6 hours), tested catalyst dosages (0.25–1.25 g/L), and assessed the effect of pH (6.0, 7.0, 8.0).
• Mechanism Analysis: The generation of hydroxyl radicals (·OH) was quantified using terephthalic acid fluorescence, and 3D fluorescence spectroscopy was used to track the degradation of dissolved organic compounds.

Key Contributions

1. Novel Synthesis Technique: The paper demonstrates the efficacy of using electrodeposition in a phosphate buffer solution to load CeO2 onto Co2O3/BiVO4. This method allows for precise control over the loading and dispersion of active components, preventing pore blockage and ensuring uniform particle growth compared to traditional impregnation.
2. Ternary Composite Design: The study introduces a ternary system (CeO2-Co2O3/BiVO4) that leverages the p-type nature of both CeO2 and Co2O3 to form heterojunctions with the n-type BiVO4. This configuration is designed to create potential differences that facilitate electron-hole separation.
3. Optimization of Operational Parameters: The research identifies specific optimal conditions for the degradation of coking tailwater: a visible light wavelength of 435 nm, a catalyst dosage of 0.5 g/L, and a neutral pH of 7.0.

Results

• Structural Confirmation: XRD and XPS confirmed the successful formation of monoclinic scheelite BiVO4, the loading of Co2O3, and the crystalline deposition of CeO2 (Ce⁴⁺ state) on the composite surface. SEM images revealed that the ternary catalyst possessed a rougher surface with increased fine particles, suggesting a higher specific surface area and more active sites compared to the binary catalyst.
• Degradation Efficiency:
  • Under optimal conditions (435 nm, 0.5 g/L dosage, pH 7.0), the CeO2-Co2O3/BiVO4 catalyst achieved a 98% COD degradation of coking wastewater after 6 hours of irradiation.
  • This performance was significantly superior to the binary Co2O3/BiVO4 (85% degradation) and pure BiVO4 (58% degradation). The ternary catalyst showed a 13% improvement over the binary catalyst.
  • 3D fluorescence spectroscopy confirmed the complete removal of refractory aromatic and soluble organic compounds after 6 hours.
• Mechanism Insights: The ternary catalyst generated the highest concentration of hydroxyl radicals (65 µmol/L at 10 minutes), which was 20% higher than the binary catalyst. The proposed mechanism suggests that photogenerated electrons transfer from CeO2 and Co2O3 to BiVO4, while holes transfer in the opposite direction, effectively delaying recombination and promoting the generation of ·OH radicals for pollutant oxidation.
• Parameter Sensitivity: Increasing the catalyst dosage beyond 0.5 g/L reduced efficiency due to light scattering and reduced transmittance. The system performed best at neutral pH, indicating no need for pH adjustment in the target wastewater.

Significance
The paper claims that the CeO2-Co2O3/BiVO4 composite, prepared via the described electrodeposition method, offers a significant advancement in the photocatalytic treatment of refractory coking wastewater. The primary significance lies in the demonstrated ability of CeO2 loading to effectively reduce electron-hole recombination and enhance the generation of hydroxyl radicals, thereby pushing COD degradation close to the theoretical limit (98%). The study validates that the phosphate buffer-assisted electrodeposition is a viable, controllable method for creating stable, high-performance ternary photocatalysts without the need for complex post-synthesis treatments. The findings suggest that this specific composite catalyst is a promising candidate for practical wastewater treatment applications under visible light.