Enhancing CO₂ Capture and Conversion to Formic Acid via a Membrane-Photocatalytic Hybrid System with ZnO–ZnS Heterojunction Catalyst

A combined approach for CO₂ capture and photoreduction provides a comprehensive solution to address exhaust emissions. This study aims to develop a hybrid system integrating membrane contactor and photocatalytic technology for CO₂ conversion to formic acid by optimizing the synthesis of ZnO-ZnS heterojunction photocatalysts through controlled variations in precursor concentrations and calcination temperatures. The catalysts are characterized to assess their structural and optical properties, photocatalytic activity, stability and reaction kinetics. Additionally, the photocatalytic performance is also tested using a model gas composition that simulates power plant emission with UV or visible light serving as the energy source. The synthesized ZnO-ZnS catalysts exhibit diffraction patterns consistent with standard references, with a measured band gap interval of 3.06-3.13 eV. Among the three most effective catalysts, labeled as Z1 (ZnO:ZnS ratio of 1:2 at 400 °C), Z2 (ZnO:ZnS ratio of 1:1 at 400 °C), and Z4 (ZnO:ZnS ratio of 1:2 at 500 °C), the formic acid yields were 0.643, 0.554, and 0.626 mmol/(L g_cat h), respectively. The highest yield, 0.936 mmol/(L g_cat), was achieved under a low CO₂ feed gas concentration (15 vol%). Furthermore, under LED irradiation, the Z1 catalyst produced a formic acid yield of 0.394 mmol/(L g_cat) after 4 h, demonstrating higher selectivity for formic acid production. Electrochemical impedance spectroscopy (EIS) analysis shows that Z1 exhibits lower resistance, enhancing charge transfer efficiency. Scanning electron microscopy (SEM) analysis reveals nanorod-like ZnO and globular ZnS structures ranging from 50 to 100 nm, while high-resolution transmission electron microscopy (HRTEM) confirms the presence of ZnO-ZnS diffraction patterns. After 4 h of photocatalytic test, the XRD analysis confirmed that most of the ZnO-ZnS catalyst peaks remained intact, indicating structural stability. Ultimately, the optimized ZnO-ZnS catalysts demonstrate promising efficiency for selective CO₂ conversion to formic acid under visible light, offering a viable approach for emission reduction through advanced hybrid membrane-photocatalytic technology.