How magnetic field affects catalytic CO2 hydrogenation over Fe-Cu/MCM-41: In situ active metal phase—reactivity observation during activation and reaction

Abstract

In this work, a systematic investigation of external magnetic field affecting metal phase transformation of Fe-Cu/MCM-41 catalysts both during activation and reaction, and their performances by means of CO2 conversion and C2 + C3 hydrocarbons selectivity were for the first time examined in-situ using time-resolved X-ray absorption spectroscopy (TRXAS) and an online quadrupole mass spectrometer. The in-situ XANES result confirmed that FeO and Fe3O4 are major active forms of the catalyst. With magnetic flux intensities of 28.91–58.59 mT, iron oxide phases, specifically Fe3O4 and FeO, within the catalysts were stabilized and maintained. The CO2 conversion was increased for 1.03 – 1.09 times, while the concentrations of C2 + C3 hydrocarbons were considerably improved by 24.03 – 35.37 times compared to those without magnetic field. The reaction steps of CO2 hydrogenation were theoretically confirmed on the isolated catalysts using M06-L with the 6-31G(d,p) + LANL2DZ mixed basis set based on the results from LCF analysis. Fe4, Fe4O4, and Fe2Cu2 clusters were used to represent the active species of pristine Fe and iron oxide (FeO) and bimetallic Fe-Cu catalyst, respectively. It was found that CO2 hydrogenation could take place via the bimolecular route in which adsorbed CO2 on the Fe center was hydrogenated by dissociative adsorption of H2 on the Cu center—promoting chain growth probability of CO2 to form C2 + C3 hydrocarbons due to the active iron oxides (Fe3O4 and FeO) were maintained by magnetic field during the reaction.