Promotion Mechanisms of Iron Oxide-Based High Temperature Water–Gas Shift Catalysts by Chromium and Copper

Abstract

The Cr and Cu promotion mechanisms of high temperature water–gas shift (HT-WGS) iron oxide catalysts, synthesized by coprecipitation, were investigated as a function of reaction conditions. XRD and in situ Raman characterization showed that the initial calcined catalysts consisted of the Fe₂O₃ (hematite) bulk phase and transformed to the Fe₃O₄ (magnetite) phase during the HT-WGS reactions. In situ NAP-XPS and HS-LEIS surface analysis revealed that Cr was surface enriched as Cr⁶⁺ for the initial catalyst and reduced to Cr³⁺ during the HT-WGS reactions, with the Cr³⁺ dissolving into the bulk iron oxide lattice forming a solid solution. In situ NAP-XPS, XANES, and HS-LEIS characterization indicated that Cu was initially present as Cu²⁺ cations dissolved in the Fe₂O₃ bulk lattice and reduced to metallic Cu⁰ nanoparticles (∼3 nm) on the external surface of the iron oxide support during the HT-WGS reactions. In situ HS-LEIS surface analysis also suggests that ∼1/3 of the surface of the Cu nanoparticles was covered by a FeO_x overlayer. The CO-TPR probe demonstrated that Cr does not chemically promote the iron oxide catalyst and that only Cu is a chemical promoter for the iron oxide HT-WGS catalysts. The Cu promoter introduces new catalytic active sites that enhance the reaction rates of the WGS reactions

Operando Spectroscopic Study of Dynamic Structure of Iron Oxide Catalysts during CO2 Hydrogenation

“…[6][7][8] By using NAP-XPS and operando techniques, the mechanisms of Fe-based catalysts during hight emperature RWGS and WGS process have been revealed. [9,10] However, for the CTH process, both the structure of active phases and the structure-performance relationship of Fe-based catalysts are still ambiguous. [10,11] In particular, the origin of active sites of Fe-based catalysts and specific effects of different iron species on the product distributions are yet to be fully understood.…”

“…[9,10] However, for the CTH process, both the structure of active phases and the structure-performance relationship of Fe-based catalysts are still ambiguous. [10,11] In particular, the origin of active sites of Fe-based catalysts and specific effects of different iron species on the product distributions are yet to be fully understood. [12][13][14][15][16][17][18] In this study,o perando techniques, that is, operando Raman spectroscopy (ORS) and X-ray diffraction (OXRD) coupled with an online gas chromatography (GC), as well as ex situ characterization methods of (sub)-surface structures of iron-based catalysts before and after CO 2 hydrogenation, have been applied to explore the structure evolution of iron active phases.…”

Strong Metal–Support Interactions between Copper and Iron Oxide during the High‐Temperature Water‐Gas Shift Reaction

“…Cu also played an essential role in the industrial CuO‐Cr 2 O 3 ‐Fe 2 O 3 catalyst for HT‐WGS reaction. In contrast to Cr that only acts as a textural promoter, Cu was found to present in its metallic form and increase the turnover frequency (TOF) of iron‐based catalysts for HT‐WGS . In turn, FeO x was reported to retard the sintering of metallic Cu nanoparticles during the reaction .…”

“…Addition of Cu results in a new peak at 154 °C that is assigned to Cu 2+ →Cu 0 reduction and the reduction of Fe 2 O 3 slightly shifts to a lower temperature (249→241 °C) . After activation under WGS reaction conditions, the catalyst dramatically reconstructs with formation of bulk metallic Cu and Fe 3 O 4 . The CO‐TPR of the activated Fe 3 O 4 catalyst exhibits a peak at 188 °C that represents the reduction of surface oxide species (Figure d).…”

“…The CO‐TPR of the activated Fe 3 O 4 catalyst exhibits a peak at 188 °C that represents the reduction of surface oxide species (Figure d). For the Cu/Fe 2 O 3 catalyst, the reduction peak down‐shifts to 157 °C, reflecting the promotion of surface reduction by the presence of the metallic Cu 0 nanoparticles …”