Industrial hydrogen production is achieved by the water-gas shift (WGS) reaction (H2O + CO &lt;--&gt; H2 + CO2). Traditionally, the Cr2O3-Fe2O3 catalyst was used to perform the high temperature WGS and Cu-ZnO-Al2O3 catalyst to perform the low temperature WGS to enhance hydrogen production. The structure and promotion mechanism of Cr in the Cr2O3-Fe2O3 catalyst, however, are still not well understood due to the lack of surface characterization studies under reaction conditions.<br/>The unsupported Fe2O3 and Cr2O3-Fe2O3 catalysts were synthesized by co-precipitation from the corresponding aqueous nitrates. All the catalysts were investigated with modern cutting edge in situ, operando (Raman, XANES/EXAFS and near ambient pressure XPS) spectroscopy during the high temperature WGS reaction. HS-LEIS was used to determine the outermost surface information of the catalyst. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) and DFT calculation were applied to both experimentally and theoretically understand the reaction mechanism.<br/>The fresh, fully oxidized Cr2O3-Fe2O3 catalyst consists of surface Cr6+ species and bulk Fe2-xCrxO3. During the WGS reaction, the surface Cr6+ extensively reduces to Cr3+ and the bulk Fe2-xCrxO3 transforms to bulk Fe3-xCrxO4. A surface FeO (Fe+2) layer, however, is present during WGS. The HT WGS catalyst operates via a redox reaction mechanism and the catalytic active site is the surface FeOx species on Fe3-xCrxO4 that undergo the Fe+2 &lt;--&gt; Fe+3 redox cycle. The Cr promoter does not exhibit redox characteristics in the reaction environment and its only role is to increase the surface area of the dominant Fe3-xCrxO4 phase (textural promoter). Steady-State Isotopic Transient Kinetic Analysis (SSITKA) with CO218-CO216 isotopes further confirmed the WGS reaction follows a redox mechanism only involving surface oxygen species that are the most abundant reactive intermediates (MARI). These new insights have led to a new fundamental understanding of the HT WGS catalysts and the development of a kinetic model that follows the surface Mars-van Krevelen mechanism.

Promotion mechanisms of iron oxide-based high temperature water−gas shift catalysts by chromium and copper

253rd National Meeting (2017)

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