Role of the Oxide Support on the Structural and Chemical Evolution of Fe Catalysts during the Hydrogenation of CO2

Mauricio Lopez Luna,See Wee Chee,Beatriz Roldan Cuenya,Clara Rettenmaier,Simon R Bare,Adam S Hoffman,David Kordus,Shamil Shaikhutdinov,Janis Timoshenko

doi:10.1021/acscatal.1c01549

Abstract

Iron-based catalysts are considered active for the hydrogenation of CO2 toward high-order hydrocarbons. Here, we address the structural and chemical evolution of oxide-supported iron nanoparticles (NPs) during the activation stages and during the CO2 hydrogenation reaction. Fe NPs were deposited onto planar SiO2 and Al2O3 substrates by dip coating with a colloidal NP precursor and by physical vapor deposition of Fe. These model catalysts were studied in situ by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in pure O2, then in H2, and finally in the CO2 + H2 (1:3) reaction mixture in the mbar pressure range and at elevated temperatures. The NAP-XPS results revealed the preferential formation of Fe(III)- and Fe(II)-containing surface oxides under reaction conditions, independently of the initial degree of iron reduction prior to the reaction, suggesting that CO2 behaves as an oxidizing agent even in excess of hydrogen. The formation of the iron carbide phase, often reported for unsupported Fe catalysts in this reaction, was never observed in our systems, even on the samples exposed to industrially relevant pressure and temperature (e.g., 10 bar of CO2 + H2, 300 °C). Moreover, the same behavior is observed for Fe NPs deposited on nanocrystalline silica and alumina powder supports, which were monitored in situ by X-ray absorption spectroscopy (XAS). Our findings are assigned to the nanometer size of the Fe particles, which undergo strong interaction with the oxide support. The combined XPS and XAS results suggest that a core (metal-rich)–shell (oxide-rich) structure is formed within the Fe NPs during the CO2 hydrogenation reaction. The results highlight the important role played by the oxide support in the final structure and composition of nanosized catalysts.

Highlights

IntroductionDue to ecological and global climate concerns, there is a growing interest in developing environmentally friendly technologies to utilize CO2, especially in applications where it is reacted with H2 (which can be obtained from renewable sources) to produce value-added chemical products such as high-order hydrocarbons and alcohols.[1,2] In this respect, one approach that has received considerable attention is the socalled “modified” Fischer−Tropsch (FT) synthesis (or CO2− FTS), which uses CO2 as a feedstock instead of CO
Due to ecological and global climate concerns, there is a growing interest in developing environmentally friendly technologies to utilize CO2, especially in applications where it is reacted with H2 to produce value-added chemical products such as high-order hydrocarbons and alcohols.[1,2] In this respect, one approach that has received considerable attention is the socalled “modified” Fischer−Tropsch (FT) synthesis, which uses CO2 as a feedstock instead of CO
A comparative study of CO2−FTS catalysts prepared from Fe2O3 and CuFeO2 and activated in H2 revealed a greater extent of the Fe(III) → Fe(0) reduction in CuFeO2, that favored the selective carburization toward the Hagg carbide and improved the selectivity toward higher (C5+) hydrocarbons considerably.[4]

Summary

Introduction

Due to ecological and global climate concerns, there is a growing interest in developing environmentally friendly technologies to utilize CO2, especially in applications where it is reacted with H2 (which can be obtained from renewable sources) to produce value-added chemical products such as high-order hydrocarbons and alcohols.[1,2] In this respect, one approach that has received considerable attention is the socalled “modified” Fischer−Tropsch (FT) synthesis (or CO2− FTS), which uses CO2 as a feedstock instead of CO. Since Fe-based catalysts were found active for both reactions, iron has become the most studied component of the CO2−FTS catalysts. In coexisting with tphaertFiceuolaxridHe apghgascea.8r−b1id2eIr(oχn-Fcea5rCbi2d)e, sparroebaablsloy frequently considered to be the active phase(s) for CO2 hydrogenation, with the reduced state of iron playing an essential role for the carbide formation.[13] For example, a comparative study of CO2−FTS catalysts prepared from Fe2O3 and CuFeO2 and activated in H2 revealed a greater extent of the Fe(III) → Fe(0) reduction in CuFeO2, that favored the selective carburization toward the Hagg carbide and improved the selectivity toward higher (C5+) hydrocarbons considerably.[4] Direct conversion of CO2 to gasoline-range (C5−C11) hydrocarbons was demonstrated on a multifunctional catalyst (Na−Fe3O4/HZSM-5), which cooperatively catalyzed a tandem reaction, presumably on three types of active sites (Fe3O4, Fe5C2, and acid sites in zeolite).[14] Recently, the Received: April 5, 2021 Revised: April 27, 2021 Published: May 7, 2021

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