Reversible Redox Property of Co(III) in Amorphous Co-Doped SiO(2)/γ-Al(2)O(3) Layered Composites

This paper reports on a unique reversible reducing and oxidizing (redox) property of Co(III) in Co-doped amorphous SiO(2)/γ-Al(2)O(3) composites. The Fenton reaction during the H(2)O(2)-catalyzed sol–gel synthesis utilized in this study lead to the partial formation of Co(III) in addition to Co(II) within the composites. High-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analyses for the composite powder sample with a composition of Al:Si:Co = 85:10:5 showed the amorphous state of the Co-doped SiO(2) that modified γ-Al(2)O(3) nanocrystalline surfaces. In situ X-ray absorption fine structure (XAFS) spectroscopic analysis suggested reversible redox reactions of Co species in the composite powder sample during heat-treatment under H(2) at 500 °C followed by subsequent cooling to RT under Ar. Further analyses by in situ IR spectroscopy combined with cyclic temperature programmed reduction/desorption (TPR/TPD) measurements and X-ray photoelectron spectroscopic (XPS) analysis revealed that the alternating Co(III)/(II) redox reactions were associated with OH formation (hydrogenation)-deformation (dehydrogenation) of the amorphous aluminosilicate matrix formed in situ at the SiO(2)/γ-Al(2)O(3) hetero interface, and the redox reactions were governed by the H(2) partial pressure at 250–500 °C. As a result, a supported mesoporous γ-Al(2)O(3)/Co-doped amorphous SiO(2)/mesoporous γ-Al(2)O(3) three-layered composite membrane exhibited an H(2)-triggered chemical valve property: mesopores under H(2) flow (open) and micropores under He flow (closure) at 300–500 °C.