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

Abstract

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 Pr₂(OH)₅(DS)·2.5H₂O 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 °C). In contrast to the macropores (≥12 nm in size, 8 m² g⁻¹) of Pr₂O₂SO₄ prepared by calcining Pr₂(SO₄)₃, Pr-DS-NH₃ after calcination showed pores of 2–3 nm in size and a larger surface area (28 m² 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 °C, compared to ≥700 °C required for Pr₂O₂SO₄ prepared from Pr₂(SO₄)₃.