Unravelling the New Roles of Na and Mn Promoter in CO₂ Hydrogenation over Fe₃O₄‐Based Catalysts for Enhanced Selectivity to Light α‐Olefins

Abstract

Abstract The direct production of light α‐olefins (C 2 = ‐C 4 = ) from CO 2 is of great importance as this process can convert the greenhouse gas into the desired chemicals. In this study, the crucial roles of Na and Mn promoter in CO 2 hydrogenation to produce light α‐olefins via the Fischer‐Tropsch synthesis (FTS) over Fe 3 O 4 ‐based catalysts are investigated. The results indicate that both Na and Mn promoter can enhance the reducibility of Fe 3 O 4 . In situ XPS and DFT calculations show that Na facilitates the reduction by electron donation from Na to Fe as the oxygen vacancy formation energy is reduced by Na. In contrast, Mn promotes the reduction by the presence of oxygen vacancy in MnO as the oxygen in Fe oxide can spillover to the vacancy in MnO spontaneously. For un‐promoted Fe 3 O 4 catalysts, CO 2 hydrogenation dominantly produces light n ‐paraffins. The addition of Na remarkably shifts the selectivity to light α‐olefins with a sharp decline in the selectivity to light n ‐paraffins, which is attributed to the electron donation from Na to Fe resulting in the promoted CO dissociation and the favorable β‐H abstraction of surface short alkyl‐metal intermediates. The addition of Mn into Na‐containing Fe 3 O 4 catalysts can obviously further enhance the selectivity to light α‐olefins as the spatial hindrance of Mn suppresses the chain growth to increase the amount of surface short alkyl‐metal intermediates.