(560hw) Engineering Dopant Position in Core-Shell CeO2-ZrO2 Nanoparticles to Control Catalytic Activity

Tailoring active sites position and concentration on the catalyst surface has been shown to effect activity and stability of a catalyst. Specifically, the surface coverage of transition metals on CeO2-ZrO2 (CZO) nanoparticles (NPs) is known to directly affect the redox property and ability to utilize lattice oxygens. Recently, Ni incorporation in the core of a CZO NPs has caused a two-fold increase in catalytic activity per surface area due to high oxygen storage capacity. However, the activity of the particle is dependent on the Ni surface concentration and is traditionally deposited via the insipid wetness impregnation technique, forming Ni on the surface of the particle. In this work, the role of Ni position on catalytic activity is probed to develop a two-step synthesis process which allows for spatially controlled dopant distribution for improved catalytic activity.
A two-step process, co-precipitation/molten salt synthesis (MSS), has been explored for the catalyst core, and urea deposition synthesis for the shell of CZO:Ni NPs. The concentration of oxygen vacancies in these NPs was studied using Raman spectroscopy, which shows an order of magnitude increase in oxygen vacancies based on the change in intensity of defect-induced (D) band at 600 cm-1 as the position of the active site is modified. High resolution transmission electron microscopy (HRTEM) and electron loss energy spectroscopy (EELS) identify that a core-shell structure is obtained, and incorporation of nickel changes the d-spacing of the CZO crystal structure. UV-Vis spectroscopy proved presence of Ni in the lattice whereas for the standard catalyst Ni is positioned on the surface. To further correlate the dopant position and oxygen vacancies, local bond structure and geometry of the active site (Ni) was studied by performing L edge X-ray Absorption Spectroscopy (XAS) measurements. Geometric distortions, eg and t2g orbitals, is dependent on the distribution of Ni within the NPs, therefore determines the Ni bonding and position. These preliminary results on CZO|Ni core-shell NPs demonstrate that controlling the position of the active site (Ni) modifies the oxygen vacancies of the NPs which is the first step to understand the diffusion of the active site during a reaction and deactivation pattern of the catalyst.