Abstract
We investigated redox mechanisms for the high temperature (HT) water–gas shift reaction (WGSR) on the Fe3O4 (1 1 1) surface using density functional theory (DFT) calculations and mean-field microkinetic modeling (MKM). The redox pathways branched into three Langmuir-Hinshelwood processes (LH1, LH2 and LH3) and one Mars-van-Krevelen (MVK) process (in the presence of oxygen vacancy) from multidentate binding of CO and CO2 over four available reactive sites (Feoct2, Fetet1, Fe bridge, and O1). We found that the LH1 and LH2 processes have CO adsorption at a single iron site (Feoct2 or Fetet1), while the LH3 and MVK processes have stronger chemisorption of CO or CO2 by both the Feoct2 site and the O1 site. From the mechanistic study of these reaction path, we recognized that availability of O1 sites was key to proceed to either CO oxidation by the single site (LH1, LH2) or by the dual site (LH3, MVK). We observed that the reaction energetics were significantly different in the CO oxidation steps, where the single or dual site results in markedly different apparent activation energy (73–281 kJ/mol) and reaction rates (10−3–10−10 mol⋅m−2⋅s−1) among the four reaction mechanisms. The utilization of mean field MKM with DFT reaction energetics helps explain the experimental debates for the catalytic reaction details as well as providing a possible direction to engineer the catalyst with higher activity.
