(578e) Water-Gas Shift Reaction on Iron Oxide Catalysts – a DFT Study

The water-gas shift (WGS) reaction is used for hydrogen enrichment and decreasing the carbon monoxide concentration in synthesis gas (H₂/CO). The copper supported chromium-iron oxide catalyst is used as the primary industrial catalyst for the high temperature (320-450 ^oC) WGS (HT-WGS) reaction since the Fe/Cr-based oxide catalysts can reduce the amount of CO from 10-15% to 2-3% [1]. Chromium is used as textural promoter to prevent thermal sintering and copper provides higher catalytic activity. [2]

The Feoct2 termination of the Fe₃O₄ (111) plane and Cr atoms substituted Fe₃O₄ (111) slab surfaces as the HT-WGS catalysts are investigated by means of spin-polarized periodic DFT approach using the Vienna Ab-initio Simulation Package (VASP) with the projector-augmented wave (PAW) method. Ferrimagnetic Fe₃O₄ is successfully optimized with nearly the same magnetic moment for octahedral and tetrahedral sites and the results are in line with the experimental values. The Feoct2 termination reveals two Fe atom layers located at the octahedral site (topmost) and tetrahedral site over four oxygen atom layer is studied among the six Fe₃O₄ (111) terminations. The DFT calculations indicate that the Cr atoms prefer being below the FeO₆ sites, which agrees with the LEIS surface analysis of Cr-Fe oxide catalyst activated under HT-WGS reaction conditions. The oxygen atom located at the topmost layer is energetically favorable for oxygen vacancy formation and the presence of Cr in the structure increases the energy for oxygen vacancy formation, which is in line with experimental kinetic measurements. Water adsorption on both Feoct2 and Cr-Feoct2 surfaces is dissociative with the adsorption energy values of -159 and -154 kJ/mol, respectively. There is only a minor effect of Cr on adsorption of water. The adsorption energy for CO on both surfaces are nearly same (the activation barrier for CO₂ formation on the Feoct2 site is 48 kJ/mol). Copper promotion of iron oxide based catalysts has also been investigated.