One-Step Synthesis of Core−Shell (Ce0.7Zr0.3O2)x(Al2O3)1−x[(Ce0.7Zr0.3O2)@Al2O3] Nanopowders via Liquid-Feed Flame Spray Pyrolysis (LF-FSP)

Abstract

We report here the synthesis of Ce(x)Zr(1-x)O(2) and (Ce(0.7)Zr(0.3)O(2))(x)(Al(2)O(3))(1-x) core-shell nanopowders in a single step by liquid-feed flame spray pyrolysis (LF-FSP) of the metalloorganic precursors, Ce(O(2)CCH(2)CH(3))(3)(OH), alumatrane [N(CH(2)CH(2)O)(3)Al], and Zr(O(2)CCH(2)CH(3))(2)(OH)(2). Solutions of all three precursors in ethanol with ceramic yields of 2.5 wt% were aerosolized with O(2), combusted at temperatures above 1500 degrees C, and rapidly quenched at approximately 1000 degrees C/ms to form Ce(x)Zr(1-x)O(2) and (Ce(0.7)Zr(0.3)O(2))(x)(Al(2)O(3))(1-x) nanopowders of selected compositions, at rates of 50-100 g/h. The resulting, as-processed, materials are unaggregated nanopowders with average particle sizes (APSs) < 20 nm and corresponding specific surface areas of 30-50 m(2)/g. The as-processed powders were characterized in terms of phase, particle size, specific surface area, compositions, and morphology by XRD, BET, DLS, SEM, TEM, XPS, TGA-DTA, and FT-IR. LF-FSP provides access to binary Ce(x)Zr(1-x)O(2) nanopowders and ternary (Ce(0.7)Zr(0.3)O(2))(x)(Al(2)O(3))(1-x) nanopowders in one step. The obtained Ce(0.7)Zr(0.3)O(2) powders are solid solutions with a cubic phase. In contrast, LF-FSP of mixtures of the three precursors at specific compositions [x = 0.5, 0.7 for (Ce(0.7)Zr(0.3)O(2))(x)@(Al(2)O(3))(1-x)] provide core-shell nanopowders in a single step. The most reasonable explanation is that there are differences in the rates of condensation, nucleation and miscibility between the gas phase ions that form the Ce(x)Zr(1-x)O(2) solid solutions and those that condense to delta-Al(2)O(3) during processing. These as-produced materials are without microporosity at surface areas of > or = 30 m(2)/g. Evidence is presented suggesting the formation of (Ce/Zr)(3+) species in the as-processed (Ce(0.7)Zr(0.3)O(2))(x)(Al(2)O(3))(1-x) core-shell materials. An accompanying paper indicates that these materials offer significant and novel catalytic activities for hydrocarbon oxidation and deNO(x) processes without using platinum as a co-catalyst.