Microwave Synthesis of SnO2 Nanospheres Using Polyethylene Terephthalate (PET) Waste for the CO2 Electrochemical Reduction to Formate

Terephthalic acid (TPA) has been extracted from waste polyethylene terephthalate (PET) bottles, which has been used to prepare the Sn-based catalyst precursors. The precursors were calcined to obtain SnO₂ catalysts. SnO₂ catalysts have been synthesized using microwave synthesizer [SnO₂ (M)] and oil bath reflux setup [SnO₂ (O)]. For comparison, a catalyst has been prepared without using TPA extracted from PET waste, [SnO₂ (C)]. The catalysts were characterized by various techniques such as X-ray diffraction (XRD), N₂ adsorption–desorption analysis, Fourier transform-infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), field emission transmission electron microscopy (FETEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) spectroscopy, and CO₂-temperature programmed desorption (CO₂ TPD) analysis. The catalysts were found to be crystalline in nature and in tetragonal rutile phase. SnO₂ (M) was found to have the highest Brunauer–Emmett–Teller (BET) surface area of 179.05 m²/g. Further, SnO₂ (M) displayed stronger adsorption property towards CO₂. Electrochemical characterization of the catalysts was done using linear sweep voltammetry (LSV), cyclic voltammetry (CV), and chronoamperometry techniques. The synthesized catalysts were explored for their performance towards CO₂ electrochemical reduction to formate using a H-shaped electrochemical cell. Among the catalysts, SnO₂ (M) showed the lowest onset potential −1.41 V (versus Ag/AgCl) and highest double layer capacitance (0.0076 mF/cm²). SnO₂ (M) also exhibited the highest Faradaic efficiency (FE_Formate) of 78.2% in 1 h at −1.7 V (versus Ag/AgCl) with the current density of 15 mA/cm². The catalysts were found to be electrochemically stable for 1 h.