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

The CuO/Ce0.75Zr0.25O2-(delta) and Ce0.75Zr0.25O delta catalysts were prepared by the sol-gel method, and Cu0.07Ce0.75Zr0.25O2-delta was obtained by treatingCuO/Ce0.75Zr0.25O2-(delta) 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-O-delta solid solutions in CuO/Ce0.75Zr0.25O2-(delta) , Cu-Ce-Zr-O-delta solid solutions in Cu0.07Ce0.75Zr0.25O2-(delta) and Ce-Zr-O-delta solid solutions in CuO/Ce0.75Zr0.25O delta. The Raman and O-2-TPD results showed that the concentration of oxygen vacancies in Cu-Ce-Zr-O-delta solid solutions was higher than that in Ce-Zr-O-delta solid solutions. CO oxidation testing suggested that the catalytic activity decreases in the order of CuO/Ce0.75Zr0.25O2-(delta) > Cu0.07Ce0.75Zr0.25O2-delta > Ce0.75Zr0.25O delta. Combined with the in situ diffuse-reflectance Fourier transform (in situ DRIFT) results, the reaction senitivity followed the order of CO linear chemisorption onto dispersed CuOx species (Mars-van Krevelen mechanism) > carbonate species onto a Cu-Ce-Zr-O-delta solid solution (Langmuir-Hinshelwood mechanism) > carbonate species onto a Ce-Zr-O-delta solid solution (Langmuir-Hinshelwood mechanism). Kinetic studies suggested that the power-law rate expressions and apparent activation energies were r = 6.02 x 10(-)(7)x P(CO)(0.68)x P-O2(0.03) (53 +/- 3 kJ/mol) for CuO/Ce0.75Zr0.25O2-(delta), r = 5.86 x 10(-7) x P(CO)(0.8 )x P-O2(0.07) (105 +/- 5 kJ/mol) for Cu0.07Ce0.75Zr0.25O2-(delta) and r = 5.7 x 10(-7) x P(CO)(0.75 )x P-O2(0.12) (115 +/- 6 kJ/mol) for Ce0.75Zr0.25O delta. The Mars-van Krevelen mechanism should be the crucial reaction pathway over CuO/Ce0.75Zr0.25O2-(delta) in CO interfacial reactions, although the Langmuir-Hinshelwood mechanism cannot be ignored, and the Langmuir-Hinshelwood mechanism mainly occurred over theCu(0)(.)(07)Ce(0)(.)(75)Zr(0)(.)(25)O(2)-(delta) and Ce0.75Zr0.25O delta catalysts, where the contribution from the Mars-van Krevelen mechanism was negligible due to the absence of surface CuOx species.