Substituting Chromium in Iron-Based Catalysts for the High-Temperature Water–Gas Shift Reaction

Abstract

A set of doped iron oxides (chromium, aluminum, gallium, indium, manganese, zinc, niobium) were prepared by a one-step coprecipitation/calcination approach evaluated for their WGS activity under industrially relevant conditions and characterized in detail. The WGS activity after ageing the doped catalyst for 4 days at 25 bar follows the order chromium ≈ aluminum > gallium > indium > manganese > zinc > niobium for copper-codoped catalysts. The activated catalysts predominantly consist of magnetite, irrespective of the dopant. Mössbauer spectra of aged catalysts showed that aluminum and zinc occupy both tetrahedral and octahedral sites of magnetite, while chromium, gallium, indium, manganese, and niobium preferentially substitute octahedral iron. The incorporation of trivalent metal ions of similar size to octahedral Fe3+ (i.e., chromium, aluminum, gallium) results in moderate to high CO conversion, irrespective of incorporation in tetrahedral or octahedral sites. The substitution of Fe2+ with Mn2+ results in an increased Fe3+/Fe2+ ratio. Incorporation of Zn2+ in tetrahedral sites (replacing Fe3+ ions) leads to a complex structure where the charge balance is compensated from the octahedral sites. Separate dopant metal oxide phases were observed in indium- and niobium-doped catalysts. XPS shows that copper is present as a separate phase in activated copper-codoped catalysts. Aluminum is identified as the most promising promoter for substituting chromium in commercial high-temperature WGS catalysts on the basis of their similar high CO conversion although incorporation of these dopants into the magnetite structure differed substantially.

Volcano-Shaped Correlation Dictated Superior Activity for Ultralow Al-Doped Iron Oxide toward High-Temperature Water–Gas Shift Reaction

“…Among all the dopants (Cr, Al, Ga, In, Mn, Zn, and Nb) tested, Al is also identified as the most promising one for substituting Cr in commercial applications. 12 Although plenty of knowledge has been attained, most of the previous studies focused on the establishment of a structure−activity relationship based on the fresh (simplified as f-) precursor, which is hematite (α-Fe 2 O 3 ), whereas the dynamic transformation of Al and its resultant spatial distribution within magnetite (the active site) is yet to be understood. In principle, a prereduction of hematite in a reducing environment (e.g., H 2 ) could induce the formation of oxygen vacancy on the surface, which in turn promotes the inward diffusion of the reduced Fe 2+ and the outward diffusion of bulk oxygen and cations (e.g., Al 3+ ) with a strong oxophilicity toward the surface, thereby stabilizing and reconstructing the surface structure as well.…”

“…More recently, by comparing a series of doped Fe 2 O 3 with 5.3 wt % of Al 2 O 3 (2.81 wt % of Al), it was found that the Al substitute occupies both tetrahedral and octahedral sites in the bulk Fe 3 O 4 , resulting in a comparable activity and stability as Cr-doped iron oxide catalyst. Among all the dopants (Cr, Al, Ga, In, Mn, Zn, and Nb) tested, Al is also identified as the most promising one for substituting Cr in commercial applications . Although plenty of knowledge has been attained, most of the previous studies focused on the establishment of a structure–activity relationship based on the fresh (simplified as f-) precursor, which is hematite (α-Fe 2 O 3 ), whereas the dynamic transformation of Al and its resultant spatial distribution within magnetite (the active site) is yet to be understood.…”