Crucial Role of Surface FeO_x Components on Supported Fe-Based Nanocatalysts for CO₂ Hydrogenation to Light Olefins

Abstract

Currently, Fe-based catalysts show excellent catalytic performance in the hydrogenation of CO 2 to produce light olefins. Despite numerous studies, the identification of the role of catalytically active components over Fe-based catalysts for CO 2 hydrogenation is not quite clear. In this work, a series of ZrO 2supported cobalt-doped Fe-based catalysts were fabricated by a microliquid film reactor-assisted coprecipitation method. It was demonstrated that appropriately strong interactions between Fecontaining species and the ZrO 2 support in catalyst precursors could facilitate the generation of a defective FeO x component upon reduction treatment and inhibit the carburization of metallic iron during the reaction. Among all Fe-based catalysts, the catalyst bearing a (Co + Fe)/ZrO 2 mass ratio of 3:7 and a Co/Fe molar ratio of 1:9 achieved a highest light olefins selectivity of about 38% at 49% CO 2 conversion under reaction conditions (i.e., 320 °C, 2.0 MPa, and 4800 mL•g cat −1 •h −1 ), along with a high Fe time yield to light olefins of 9.2 μmol COd 2 •g Fe −1•s −1 and a low CO selectivity of 4.7%. Comprehensive structural characterization and catalytic CO 2 hydrogenation experiments clarified that the surface-dominant defect-rich FeO x component as catalytically active species substantially played a crucial role in governing the hydrogenation of CO 2 , mainly owing to their enhanced adsorption and binding ability for CO 2 and the CO intermediate. This study offers a new strategy to design Fe-based catalysts and provides deep insight into the role of iron oxide species for CO 2 hydrogenation to produce light olefins.

Key Role of Metal Oxide Support in Tuning Active Surface Components of Fe-Based Catalysts for CO2 Hydrogenation

“…In previous studies on the CO 2 hydrogenation to olefins, supported iron catalysts have been widely investigated. − For example, compared to the tetragonal ZrO 2 support, the high-surface-area monoclinic ZrO 2 support improved the Fe dispersion, as well as the Fe–ZrO 2 charge transfer induced by oxygen vacancies (O v ), thus favoring the generation of the χ-Fe 5 C 2 phase, contributing to the production of light olefins . Recently, through investigating the transformation of iron-containing phases in Fe-based catalysts during activation and reaction, our group revealed the crucial role of Fe 3 O 4 species having numerous oxygen vacancies on the activity and selectivity toward light olefins during CO 2 hydrogenation . Additionally, Shaikhutdinov et al reported that nanometer-sized Fe particles underwent strong interaction with planar oxide supports (i.e., SiO 2 , Al 2 O 3 ) to form a core (metal-rich)–shell (oxide-rich) structure during CO 2 hydrogenation, catalyzing the formation of CO and CH 4 as main products, as well as a small amount of C 2+ products with the low selectivity of 6–7%.…”