Nanoscale Heterogeneity in Ceria Zirconia with Low-Temperature Redox Properties

Abstract

We have investigated nanoscale compositional and structural variations in Ce(0.5)Zr(0.5)O(2) samples with different redox properties. Different samples were prepared using a spray freezing technique, and the synthesis conditions were varied to yield materials with reduction temperatures in the range of 400-750 degrees C. X-ray diffraction and thermal gravimetric analysis were used to characterize the average structures and redox properties of these materials. The nanoscale structural and compositional variations in individual nanoparticles of high activity were determined with atomic-scale electron imaging and nanometer-resolution electron energy loss spectroscopy. During the early stage of particle formation, the crystallization process is initiated via the nucleation of ceria-rich nanodomains. This results in the formation of high-surface-area materials that exhibit nanoscale compositional heterogeneity consisting of Ce-rich cores surrounded by Zr-rich shells. The effect of high-temperature redox cycling on the nanoscale structure, composition, and low-temperature redox properties was also determined. Our analysis suggests that our most active material exhibits significant compositional and structural heterogeneity at the nanometer level.

A spray drying system for synthesis of rare-earth doped cerium oxide nanoparticles

“…Various methods have been reported to synthesize high surface area (HSA) nanoparticles of these materials including flame synthesis [7,8], sol-gel processing [9], homogenous precipitation [10], hydrothermal synthesis [11,12], mechanochemical synthesis [13], microwave assisted [14], and sonochemical synthesis [15]. The use of a 'spray route' which includes spray pyrolysis, spray drying and spray freeze drying for nanosized particles has also been reported [16][17][18][19][20][21]. Nanopowders of mixed oxide ceramics with homogenous chemical composition and high surface area have been successfully prepared by spray drying method [22,23].…”

Inorganic Cryogels

“…A combination of these two different approaches, namely the Pechini method and freeze-drying of aqueous solutions, results in the "cryogenic polymer-gel synthesis" method [18][19][20][21][22][23][24][25][26]. This method consists of three independent steps: (1) gel formation, (2) freezing and freeze-drying, and (3) heat treatment, i.e., calcination.…”

“…Due to the small but different solubility of the precipitates formed by different cations, as well as due to the different rates of their precipitation from solution, the first and the last portions of the precipitate are enriched by different components. Although this nano-inhomogeneity was postulated a long time ago, modern experimental methods like high resolution electron microscopy have recently allowed the observation of this difference experimentally [26]. The products of co-precipitation are used mostly as precursors for the synthesis of complex oxides by means of thermal decomposition.…”

“…In the case of oxide solid solutions [28][29][30][31] or complex oxide formation [26,[32][33][34][35][36][37][38][39], a high chemical homogeneity of the precursor promotes fast and uniform formation of the target product during its thermal processing. However, even in the case of non-reacting components, it was shown that cryogel synthesis is useful for the synthesis of nanocomposites [27,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54].…”

Chemical Imaging at Atomic Resolution as a Technique To Refine the Local Structure of Nanocrystals

“…The structural transformation takes place during the reduction step of the cycle, in which the fully reduced mixed oxide with Ce/Zr molar ratio 1:1 adopts the Ce 2 Zr 2 O 7 stoichiometry. Nevertheless, HAADF studies have clearly shown that, in the case of ceria-zirconia mixed oxides, this cation-ordered arrangement is preserved even after full reoxidation, that is, in the oxide with Ce 2 Zr 2 O 8 stoichiometry, whenever the oxidation temperature does not exceed 823 K.[8]Electron-microscopy studies have also revealed [9][10][11] another remarkable feature of the ceria-zirconia aged oxides with the pyrochlore-type cation sublattice: the occurrence of compositional heterogeneities at the nanometer scale. Taking these observations into account and also considering that the disorder-order transition may not be completed in the time scale and under the temperature conditions used in the redox-cycling treatments, the important question arises whether these heterogeneities are in fact occurring on a finer scale, that is, at the atomic level.…”