Rational Design of High Temperature Water-Gas Shift Catalysts with Non-Toxic Earth-Abundant Elements

Abstract The copper promoted chromium-iron oxide has for decades been used as the commercial catalyst for production of H2 via the High Temperature-Water Gas Shift reaction (HT-WGS). The wide operation temperature range, high activity and robust thermostability has made this catalyst the catalyst of choice for HT-WGS. The toxic nature of hexavalent chromium has motivated extensive research to develop non-chromium HT-WGS catalysts. The lack of fundamental understanding of this HT-WGS catalyst system (catalyst structure under working conditions, reaction mechanism and copper/chromium promotion mechanism), however, have hampered the developed of Cr-free catalysts. The objectives of the dissertation were (1) to resolve the fundamentals of copper and chromium promotion mechanisms for the HT-WGS reaction, and (2) then apply the new fundamental insights to guide the rational design of chromium-free iron oxide-based HT-WGS catalysts. Temperature programmed surface reaction (TPSR) was employed to resolve the decades long debate regarding the HT-WGS reaction mechanism on iron oxide-based catalysts. Isotope C16O2/C18O2 switch experiments provided insights on the nature of active sites and the participation of surface oxygen, which allowed for the first time to calculate the Turnover Frequency (TOF) of iron oxide-based HT-WGS catalysts. To understand the structure of copper promoted chromium-iron oxide catalyst under reaction condition, a series of modern characterization techniques were employed (XRD, in situ Raman spectroscopy, Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS), High-Sensitivity Low-Energy Ion Scattering (HS-LEIS) Spectroscopy, in situ X-ray Absorption Spectroscopy and TEM-EDX). The activated catalysts were chemically probed with CO-Temperature Programmed Reduction (TPR) to examine the effects of copper and chromium on the catalyst activity for removing oxygen by CO, which is the rate-determining-step. These findings provided critical insights into the promotion mechanisms of copper and chromium. Finally, based on the fundamental understanding of the existing commercial catalysis system, new chromium-free, environmentally friendly iron oxide based HT-WGS catalysts were rationally designed.