Layered Pr-dodecyl sulfate mesophases as precursors of Pr2O2SO4 having a large oxygen-storage capacity

Surfactant-assisted synthesis of the oxysulfate Pr2O2SO4 with a large oxygen-storage capacity was studied with the aim of increasing the rate of redox cycles at lower temperatures. From an aqueous solution of nitrate, Pr-based surfactant mesophases templated by dodecyl sulfate anion (DS = C12H25OSO3-) were synthesized using ammonia or urea as precipitants. The precipitated mesophases, Pr-DS-NH3 and Pr-DS-N2H4CO, exhibited ordered layered structures with interlayer spacings of 2.63 and 3.68 nm, respectively, suggesting alternative stacking of a Pr hydroxide layer and a DS layer. The different interlayer spacing can be explained by the fact that Pr- DS- NH3 with the composition Pr-2(OH)(5)(DS) center dot 2.5H(2)O contains monolayers of DS, whereas Pr- DS- N2H4CO contains bilayers of DS. As was revealed by EXAFS and FT-IR results, the sulfate head group of each DS is strongly bound to uncoordinated Pr3+, so that heating of Pr-DS-NH3 could directly yield single-phase Pr2O2SO4 at a low temperature ( 500 degrees C). In contrast to the macropores (>= 12 nm in size, 8 m(2) g(-1)) of Pr2O2SO4 prepared by calcining Pr-2(SO4)(3), Pr- DS- NH3 after calcination showed pores of 2 - 3 nm in size and a larger surface area (28 m(2) g(-1)). The microstructure is very effective in accelerating the solid - gas reaction in oxygen release as well as storage processes, which enables the present system to work at temperatures <= 600 degrees C, compared to >= 700 degrees C required for Pr2O2SO4 prepared from Pr-2(SO4)(3).