According to Environmental Protection Agency, over 55% of NOx production in the US is caused by automobile exhaust, also resulting in the formation of CO2 and hydrocarbon residues. Transition metal doped CeO2-ZrO2 (CZO) nanoparticles (NPs) are extensively employed in three-way catalysis due to their ability to adsorb and release oxygen helping to prevent coking. These CZO NPs have shown a 5.3 mW/mg catalytic activity comparable to state of the art catalysts utilized in dry reforming of methane. However, the activity of these structures under reaction conditions, which is function of the position of the active site (Ni), has been shown to reduce the oxygen storage capacity and increase coking over time. To address this, the current work aims to understand the structural and electronic properties of the active site of CZO NPs, allowing for an in-depth study of the formation of oxygen vacancies in the lattice due to the position of the active site. In this work, a two-step process, co-precipitation/molten salt synthesis, has been explored for CZO:Ni catalyst, comparing its catalytic activity and oxygen vacancy concentration to a standard prepared by sol-gel technique. TEM and XRD characterizations identify that the as-synthesized NPs are monodisperse spheres, with 10 nm diameter, exhibiting fluorite phase. The concentration of oxygen vacancies in these NPs was studied using Raman spectroscopy, which shows a higher intensity defect-induced (D) band at 600 cm-1 for the as-synthesized catalyst than the reported standard. Furthermore, the information regarding the site occupancy of the active site (Ni) in these catalysts was obtained by calculating the change in FWHM of the F2g band. To further correlate the concentration of oxygen vacancies to the position of Ni in the lattice, surface sensitive electronic characterizations such as X-ray Photoelectron (XPS) and X-ray absorption spectroscopy (XAS) were performed. The oxidation state of Ni was identified as 2+, indicating that the metallic Ni clusters are not present. Furthermore, the local bond structure and geometry of the active site was accurately probed via Ni L edge XAS measurements in addition to the site occupancy measurements from Raman. These results on CZO:Ni catalysts demonstrate that controlling the oxygen vacancies in the lattice by varying the position of the active site (Ni) is the first step to understand the diffusion of the active site during a reaction and deactivation pattern of the catalyst.

Controlling the concentration of oxygen vacancies in CeO2-ZrO2 nanoparticles via spatial tailoring of the active site

2019 Spring National Meeting

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