Preparation of noble metal-free porous CuO–ceria and zirconia mixed oxide catalysts using the ammonia driven deposition precipitation method for toluene combustion

Abstract

Cu-containing Ce_xZr_1−xO₂ noble metal-free mixed metal oxide catalysts were obtained using a non-disclosed synthesis protocol that involves a soft template assisted hydrothermal approach for the synthesis of various Ce_xZr_1−xO₂ supports and the subsequent deposition of Cu with a water-based ammonia driven deposition precipitation (ADP) method. As shown using N₂-sorption, X-ray diffraction (XRD), Raman spectroscopy, UV-vis diffuse reflectance spectroscopy (UV-vis-DR) and scanning electron microscopy (SEM), varying the ratio between Ce and Zr resulted in different crystal structures, going from purely cubic CeO₂ to tetragonal Ce_0.25Zr_0.75O₂ and finally to ZrO₂ with co-existing tetragonal and monoclinic crystal phases. Additionally, the introduction of zirconium to the CeO₂ crystal lattice induced the formation of mesopores, yielding a larger available surface for ADP deposition of Cu. The addition of different Cu loadings had an impact on both structural and optical properties of the catalysts, as well as on their reducibility, which was examined via H₂-temperature programmed reduction (H₂-TPR). During testing of the catalytic performance in the combustion of toluene as a model reaction for aromatic VOC combustion, all catalysts showed extremely high selectivity towards H₂O and CO₂ and were more active than CuO-modified commercially available CeO₂ and ZrO₂ supports. The best performance was obtained over the Cu–Ce_0.75Zr_0.25O₂ catalysts whereby the different loadings of Cu yielded nearly identical results. The high activity of these catalysts can be attributed to the successful ADP deposition and the increased amount of oxygen vacancies due to structural defects induced by the incorporation of smaller Zr⁴⁺ cations into the cubic crystal structure of ceria. The presence of oxygen vacancies was analyzed using X-ray photoelectron spectroscopy (XPS).