Abstract
U.v.-vis-n.i.r. spectroscopy was used to identify the consecutive formation of hydrated octahedral, tetrahedral monoaquo, and trigonal bare Fe(II) species during the dehydration of Fe(II)-A zeolite. The amount of residual water was determined simultaneously using near-infrared water vibration bands. The adsorption of NO probe molecules proved the accessibility of the surface trigonal iron sites. The resulting optical spectrum indicated the formation of a mononitrosyl [Fe(I)-NO +] complex. Upon contact with O 2 at 700 K, the ferrous ions of Fe(II)-A were oxidized to ferric species, consuming about one oxygen atom per two iron cations. Upon subsequent reduction with H 2, CH 4, or CO at 600–700 K, most of the Fe cations were reduced back to surface trigonal Fe(II) species, while a smaller fraction was converted to amorphous ferrous species. These amorphous oxidic species exhibited a higher redox activity than that of the surface trigonal Fe cations.
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