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The commercial high-temperature water-gas shift (HT-WGS) catalyst consists of CuO-Cr 2 O 3 -Fe 2 O 3 ,w here Cu functions as ac hemical promoter to increase the catalytic activity,but its promotion mechanism is poorly understood. In this work, as eries of iron-based model catalysts were investigated with in situ or pseudo in situ characterization, steadystate WGS reaction, and density function theory (DFT) calculations.F or the first time,astrong metal-support interaction (SMSI) between Cu and FeO x was directly observed. During the WGS reaction, athin FeO x overlayer migrates onto the metallic Cu particles,c reating ah ybrid surface structure with Cu-FeO x interfaces.T he synergistic interaction between Cu and FeO x not only stabilizes the Cu clusters,b ut also provides new catalytic active sites that facilitate CO adsorption, H 2 Odissociation, and WGS reaction. These new fundamental insights can potentially guide the rational design of improved iron-based HT-WGS catalysts.The strong metal-support interaction (SMSI) was originally reported decades ago by Tauster et al.,who found adramatic change in the chemisorption properties of Group 8n oble metals supported on TiO 2 after CO or H 2 reduction. [1] This has significantly changed the understanding of supported metal catalysts,w hose morphologies were realized to dynamically evolve under reaction conditions.U nder reducing environments,t he oxide support can decorate the surface of metal nanoparticles to generate special contacting zones with enhanced catalytic properties. [2] Thes trong interaction can also improve the dispersion of metals on certain oxides that results in increased metal surface coverage or decreased particle size. [2][3][4] Since then, the SMSI effect has been extensively studied and became essential for the rational design of metal-based catalysts for av ariety of reactions including,b ut not limited to,t he water-gas shift (WGS) reaction, [5,6] methanol synthesis, [7][8][9] CO oxidation, [10,11] CO 2 hydrogenation, [12] methane reforming, [13,14] oxygen reduction reaction (ORR), [15] and so on. Forexample,Fuetal. developed anew class of LT-WGS catalysts with Au or Pt supported on cerium oxide.T he catalytic active sites were proposed to be nonmetallic gold or platinum species strongly associated with surface cerium oxides. [16] Subsequently,R odriguez et al. demonstrated by investigating the inverse CeO 2 /Aua nd TiO 2 /Aum odel compounds that oxide-metal interface directly participated in the LT-WGS reaction. Theo xides were thought to assist water dissociation and the metals to be the catalytic active sites for CO adsorption. [5] Besides,t he SMSI effect has now also been extended to non-precious metals such as Cu. Behrens et al. studied the industrial Cu/ZnO/Al 2 O 3 catalyst for methanol synthesis and observed with transmission electron microscopy (TEM) that the catalytic active sites were Cu steps decorated with oxidized Zn atoms. [9] Metastable graphite-like ZnO layers on the top of Cu were found to be present during reductive activat...
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