Elucidation of the Reaction Mechanism for Higherature Water Gas Shift over an Industrial-Type Copper-Chromium-Iron Oxide Catalyst

Article abstract of DOI:10.1021/jacs.9b03516

The water gas shift (WGS) reaction is of paramount importance for the chemical industry, as it constitutes, coupled with methane reforming, the main industrial route to produce hydrogen. Copper-chromium-iron oxide-based catalysts have been widely used for the higherature WGS reaction industrially. The WGS reaction mechanism by the CuCrFeO_x catalyst has been debated for years, mainly between a "redox" mechanism involving the participation of atomic oxygen from the catalyst and an "associative" mechanism proceeding via a surface formate-like intermediate. In the present work, advanced in situ characterization techniques (infrared spectroscopy, temperature-programmed surface reaction (TPSR), near-ambient pressure XPS (NAP-XPS), and inelastic neutron scattering (INS)) were applied to determine the nature of the catalyst surface and identify surface intermediate species under WGS reaction conditions. The surface of the CuCrFeO_x catalyst is found to be dynamic and becomes partially reduced under WGS reaction conditions, forming metallic Cu nanoparticles on Fe₃O₄. Neither in situ IR not INS spectroscopy detect the presence of surface formate species during WGS. TPSR experiments demonstrate that the evolution of CO₂ and H₂ from the CO/H₂O reactants follows different kinetics than the evolution of CO₂ and H₂ from HCOOH decomposition (molecule mimicking the associative mechanism). Steady-state isotopic transient kinetic analysis (SSITKA) (CO + H₂¹⁶O → CO + H₂¹⁸O) exhibited significant ¹⁶O/¹⁸O scrambling, characteristic of a redox mechanism. Computed activation energies for elementary steps for the redox and associative mechanism by density functional theory (DFT) simulations indicate that the redox mechanism is favored over the associative mechanism. The combined spectroscopic, computational, and kinetic evidence in the present study finally resolves the WGS reaction mechanism on the industrial-type higherature CuCrFeO_x catalyst that is shown to proceed via the redox mechanism.