Reaction mechanism and kinetics of CO oxidation over a CuO/Ce0.75Zr0.25O2-δ catalyst

Abstract

The CuO/Ce0.75Zr0.25O2-δ and Ce0.75Zr0.25Oδ catalysts were prepared by the sol-gel method, and Cu0.07Ce0.75Zr0.25O2-δ was obtained by treating CuO/Ce0.75Zr0.25O2-δ with nitric acid to remove the well-dispersed CuO on the surface. Various characterizations were used to reveal the different active sites, such as surface-dispersed CuO and Cu-Ce-Zr-Oδ solid solutions in CuO/Ce0.75Zr0.25O2-δ, Cu-Ce-Zr-Oδ solid solutions in Cu0.07Ce0.75Zr0.25O2-δ and Ce-Zr-Oδ solid solutions in Ce0.75Zr0.25Oδ. The Raman and O2-TPD results showed that the concentration of oxygen vacancies in Cu-Ce-Zr-Oδ solid solutions was higher than that in Ce-Zr-Oδ solid solutions. CO oxidation testing suggested that the catalytic activity decreases in the order of CuO/Ce0.75Zr0.25O2-δ > Cu0.07Ce0.75Zr0.25O2-δ > Ce0.75Zr0.25Oδ. Combined with the in situ diffuse-reflectance Fourier transform (in situ DRIFT) results, the reaction sensitivity followed the order of CO linear chemisorption onto dispersed CuOx species (Mars-van Krevelen mechanism) > carbonate species onto a Cu-Ce-Zr-Oδ solid solution (Langmuir-Hinshelwood mechanism) > carbonate species onto a Ce-Zr-Oδ solid solution (Langmuir-Hinshelwood mechanism). Kinetic studies suggested that the power-law rate expressions and apparent activation energies were r = 6.02 × 10−7×PCO0.68PO20.03 (53 ± 3 kJ/mol) for CuO/Ce0.75Zr0.25O2-δ, r = 5.86 × 10−7×PCO0.8PO20.07 (105 ± 5 kJ/mol) for Cu0.07Ce0.75Zr0.25O2-δ and r = 5.7 × 10−7×PCO0.75PO20.12 (115 ± 6 kJ/mol) for Ce0.75Zr0.25Oδ. The Mars-van Krevelen mechanism should be the crucial reaction pathway over CuO/Ce0.75Zr0.25O2-δ in CO interfacial reactions, although the Langmuir-Hinshelwood mechanism cannot be ignored, and the Langmuir-Hinshelwood mechanism mainly occurred over the Cu0.07Ce0.75Zr0.25O2-δ and Ce0.75Zr0.25Oδ catalysts, where the contribution from the Mars-van Krevelen mechanism was negligible due to the absence of surface CuOx species.