Reverse water gas shift reaction over a Cu/ZnO catalyst supported on regenerated spent bleaching earth (RSBE) in a slurry reactor: the effect of the Cu/Zn ratio on the catalytic activity

Abstract

The catalytic conversion of CO₂ via the Reverse Water Gas Shift (RWGS) reaction for CO production is a promising environment-friendly approach. The greenhouse gas emissions from burning fossil fuels can be used to produce valuable fuels or chemicals through CO₂ hydrogenation. Therefore, this project was to study the CO₂ conversion via RWGS over various Cu/ZnO catalysts supported by regenerated spent bleaching earth (RSBE) prepared by wet impregnation technique with different Cu : Zn ratios (0.5, 1.0, 1.5, 2.0, 3.0). The causes of environmental pollution from the disposal of spent bleaching earth (SBE) from an edible oil refinery can be eliminated by using it as catalyst support after the regeneration process. The synthesized catalysts were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), temperature-programmed reduction of hydrogen (TPR-H₂), pyridine-adsorbed Fourier transform infrared (FTIR-pyridine), temperature programmed desorption of carbon dioxide (TPD-CO₂), N₂ physisorption, and Fourier transform infrared (FTIR) analysis. The RWGS reaction was carried out in a slurry reactor at 200 °C, with a pressure of 3 MPa, a residence time of 4 h, and catalyst loading of 1.0 g with an H₂/CO₂ ratio of 3. According to experimental data, the Cu/Zn ratio significantly impacts the catalytic structure and performance. The catalytic activity increased until the Cu : Zn ratio reached the maximum value of 1.5, while a further increase in Cu/Zn ratio inhibited the catalytic performance. The CZR3 catalyst (Cu/Zn ratio of 1.5) with a higher catalytic reducibility, high copper dispersion with small crystalline size, lower total pore volume as well as higher basicity showed superior catalytic performance in terms of CO₂ conversion (40.67%) and CO yield (39.91%). Findings on the effect of reaction conditions revealed that higher temperature (>240 °C), higher pressure (>3 MPa), higher reaction time (>4 h) and higher catalyst loading (>1.25 g) could improve CO₂ conversion to CO yield. A maximum CO₂ conversion of 45.8% and multiple recycling stability of the catalyst were achieved, showing no significant decrease in CO₂ conversion.