Engineering Dopant Position in Structure-Controlled CeO2-ZrO2 Catalysts

Abstract

CeO2-ZrO2 (CZO) nanoparticles (NPs) have application in many catalytic reactions, such as methane reformation, due to their oxygen cycling ability. Ni doping has been shown to improve the catalytic activity and acts as an active site for the decomposition of methane. In this work, Ni:CZO NPs were synthesized via a two-step co-precipitation/molten salt synthesis to compare Ni distribution, oxygen vacancy concentration, and catalytic activity relative to a reference state-of-the-art catalyst. To better understand the effects of Ni position and dispersion, and oxygen vacancy formation in these materials, the Ni concentration, reaction time, and deposition methods were varied. X-ray diffraction (XRD) measurements show a cubic phase with little to no segregation of Ni/NiO. Catalytic activity measurements displayed similar activity per surface area with an order of magnitude decrease in the coking rate for the particles synthesized by the molten salt method compared to a traditional insipient wetness impregnation synthesis. Additionally, this new approach resulted in an order of magnitude increase in oxygen vacancies which is attributed to the high dispersion of Ni2+ ions in the NP core. Tailoring active sites position and concentration on the catalyst surface has been shown to effect activity and stability of a catalyst. After an active Ni:CZO core has been finalized, a shell layer was subsequently deposited to active site concentration and dispersion. The robust structure of the core of the catalyst that is synthesized helps achieve better dispersion of active sites on the surface. Better dispersion of active sites along with availability of oxygen vacancies from the core resulted in a five-fold increase in catalytic activity per surface area and an order of magnitude decrease in coking. 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.