Selective Reverse Water–Gas Shift Reaction through MnOx Modification of Supported Pd Catalysts

Abstract

Controlling the selectivity for CO₂ hydrogenation is crucial in catalysis. While the reverse water–gas shift (rWGS) reaction offers a promising strategy for recycling atmospheric CO₂, achieving selective rWGS with conventional supported metal catalysts is challenging owing to the successive hydrogenation of CO to CH₄ (methanation). In this study, we show that selective rWGS can be achieved by modifying an Al₂O₃-supported Pd catalyst with an amorphous manganese oxide (MnO_x). We fabricated MnO_x-modified Pd catalysts supported on Al₂O₃ using a coimpregnation method. Electron microscopy coupled with energy-dispersive spectroscopy revealed that Pd was loaded as nanoparticles with an average size of 7.7 nm, while MnO_x was distributed across the catalyst surface. An in-depth analysis using X-ray photoelectron spectroscopy indicated that the Pd particles were covered with MnO_x overlayers. A Pd catalyst afforded both CO and CH₄, whereas the MnO_x-modified Pd catalyst, which was optimized with 2 mol % Pd and 22 mol % Mn, selectively afforded CO at 673 K with a 28.9% yield. This performance was 8.3 times higher than that of the Pd catalyst and 1.5–1.8 times than those of the reference catalysts (Cu/ZnO/Al₂O₃ and FeCrCuO_x). In situ Fourier transform infrared spectroscopy and temperature-programmed desorption measurements proved that MnO_x inhibited the adsorption of bridged CO species to suppress the successive hydrogenation of CO, while also providing adsorption sites for CO₂ to enhance the conversion of CO₂. Our findings demonstrate the effectiveness of MnO_x modification in enhancing the performance of supported metal catalysts for the rWGS reaction, offering insights into efficient catalytic design for industrial CO₂ recycling.