(621t) Quantitative Determination of Number of Active Sites and Tofs for Cr2O3-Fe2O3 Water-Gas Shift Catalysts

Introduction

        Industrial hydrogen production is achieved by the water-gas shift (WGS) reaction (H₂O + CO ↔ H₂ + CO₂) [1]. Traditionally, the CuO-Cr₂O₃-Fe₂O₃ catalyst was used to perform the high temperature WGS reaction. Many researchers have been tried to develop chromium-free catalyst due to the toxic nature of hexavalent chromium [2,3]. However, the reaction mechanism have not yet been fully understood, and there is no standard for direct comparison of catalysts’ performance.

Materials and Methods

        The unsupported Fe₂O₃ and Cr₂O₃-Fe₂O₃ catalysts were synthesized by co-precipitation from the corresponding aqueous nitrates. All the catalysts were investigated with in situ, operando Raman and XANES/EXAFS spectroscopy during the high temperature WGS reaction. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) were performed to study its reaction mechanism.

Results and discussion

        The HT WGS catalyst operates via a redox reaction mechanism and the catalytic active site is the surface Fe species on Fe_3-xCr_xO₄ that undergo the Fe⁺² ↔ 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 Fe_3-xCr_xO₄ phase (textural promoter). 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). The number of catalytic active sites were able to be calculated based on the exchanged surface oxygen. The turn-over frequency (TOF) was then derived for Fe₂O₃ and Cr₂O₃-Fe₂O₃. These new insights have led to a new fundamental understanding of the HT WGS catalysts and the development of a method to calculate TOF of Iron-based high temperature WGS catalsysts.

References

1) C. Rhodes, G.J. Hutchings, A.M. Ward, Catal Today 23 (1995) 43.

2) D. W. Lee, M.S. Lee, J.Y. Lee, S. Kim, H.-J. Eom, D.J. Moon, K.-Y. Lee, Catal Today 210 (2013) 2

3) G.C. de Araujo, M.D. Rangel, Catal Today 62 (2000) 201