Oxidation of Ethylene Glycol into Valuable Chemical Using BiVO₄ Photoanode

Introduction Plastic photoreforming using photocatalysts is a solar-powered technology that facilitates the generation of hydrogen (H 2 ) from water while simultaneously breaking down plastic waste into valuable organic products. While the focus in the past few decades has predominantly centered on enhancing the H 2 production efficiency, recent attention has shifted towards selective plastic oxidation reactions (U. Nwosu, et al . Renew. Sust. Energ. Rev. , 2021, 148, 111266). However, the investigation of effective cocatalysts for selective oxidation of plastic compounds has not been well studied. The photoelectrochemical (PEC) system offers a promising approach by segregating the anodic and cathodic processes, thereby enabling a more concentrated focus on the oxidation reaction in the design of reactions. In this study, we evaluate the BiVO 4 photoanode modified with various cocatalysts, for example, cobalt phosphate (Co-Pi) for the oxidation of ethylene glycol (EG), a crucial component in the formation of polyethylene terephthalate (PET). Additionally, we introduce acetonitrile (MeCN) into the reactant mixture to suppress the oxygen evolution reaction (OER), which otherwise competes with the ethylene glycol oxidation reaction (EGOR), creating a more conducive reaction environment for enhanced efficiency. Materials and Methods The BiVO 4 photoanode on fluorine doped tin oxide (FTO) glass was fabricated as previously reported (V. Andrei, et al. Adv. Energy Mater , 2018, 8, 1801403). Briefly,BiOI precursor was electrochemically deposited on the FTO glass and then drop-casted a vanadyl acetylacetonate [VO(acac) 2 ] solution onto the surface and heated at 723 K for 60 min. The deposition of the Co-Pi cocatalyst onto BiVO 4 was achieved by photoassisted electrodeposition method. The obtained photoanodes were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). PEC measurements were carried out using a three-electrode configuration under simulated solar irradiation (100 mW cm − 2 , AM 1.5G). The experiments were performed in various solutions containing 1 M EG: 0.5 M Na 2 SO 4 in H 2 O, 0.1 M NaClO 4 in MeCN, and the mixed solution of MeCN and H 2 O. Product analysis was performed using proton nuclear magnetic resonance ( 1 H NMR). Results and discussion According to the XRD and SEM results, the formation of nanoporous BiVO 4 film on FTO glass was confirmed. To assess the PEC performance for EGOR using BiVO 4 electrode, we carried out the experiments under conditions with and without the EG substrate ( Figure (a) ). With the addition of EG, a notable shift of the onset potential from 0.6 V RHE to 0.5 V RHE was observed. In addition, the photocurrent density at 1.2 V RHE increased from 0.67 mA/cm 2 to 0.92 mA/cm 2 . This result indicates that EGOR is more favorable on the BiVO 4 surface compared to OER. The Co-Pi cocatalyst deposition on the BiVO 4 notably increased the current under both conditions, with and without the EG substrate. Although Co-Pi is commonly recognized for its catalytic role in OER, our experiment surprisingly revealed its positive impact on EG performance as well. The Co-Pi/BiVO 4 photoanode exhibited a negative shift of the onset potential to 0.3 V RHE and an enhancement of photocurrent density to 2.04 mA/cm 2 at 1.2 V RHE for EGOR compared to bare BiVO 4 . Additionally, the Co-Pi/BiVO 4 photoanode showed 16.9% and 2.7% faradaic efficiencies (FE) for formate and glyoxal production, respectively, while bare BiVO 4 exhibited 10.6% FE for formate and no glyoxal production. Under aqueous conditions, the FE for EGOR is currently low due to the competition between OER and EGOR. To overcome this challenge and enhance the EGOR efficiency, we introduced MeCN into the aqueous solution because MeCN has higher (more positive) anodic oxidation potential than the aqueous medium (L. M. Reid, et al . Sustainable Energy Fuels , 2018, 2, 1905-1927). In the MeCN-water mixed solution (MeCN:water = 9:1), OER was effectively suppressed to 0.33 mA/cm 2 , while EGOR was significantly enhanced to 2.50 mA/cm 2 at 1.2 V RHE potential ( Figure (b) ). This MeCN-water mixed solvent created a more favorable environment for EGOR on bare BiVO 4 , resulting in a notable increase in FE for formate to 19.8% and FE for glyoxal to 13.2%, which represents a 1.87 and 4.89 times improvement, respectively, compared to the pure aqueous condition. Conclusions The deposition of Co-Pi cocatalyst on the BiVO 4 film improved the photocurrent density and FE for formate production. Moreover, the introduction of MeCN in the solvent created a more favolable reaction condition for EGOR, suppressing OER and increasing the EGOR activity.