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)

We report here the synthesis of CexZr1-xO2 and (Ce0.7Zr0.3O2)(x)(Al2O3)(1-x) core-shell nanopowders in a single step by liquid-feed flame spray pyrolysis (LF-FSP) of the metalloorganic precursors, Ce(O2CCH2CH3)(3)(OH), alumatrane [N(CH2CH2O)(3)Al], and Zr(O2CCH2CH3)(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 similar to 1000 degrees C/ms to form CexZr1-xO2 and (Ce0.7Zr0.3O2)(x)(Al2O3)(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 CexZr1-xO2 nanopowders and ternary (Ce0.7Zr0.3O2)(x)(Al2O3)(1-x) nanopowders in one step. The obtained Ce0.7Zr0.3O2 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 (Ce0.7Zr0.3O2)(x)@(Al2O3)(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 CexZr1-xO2 solid solutions and those that condense to delta-Al2O3 during processing. These as-produced materials are without microporosity at surface areas of >= 30 m(2)/g. Evidence is presented suggesting the formation of (Ce/Zr)(3+) species in the as-processed (Ce0.7Zr0.3O2)(x)(Al2O3)(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.