ELUCIDATION OF MECHANISMS OF PALLADIUM INDUCED MODIFICATION OF SELENIUM IN ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY BY INVESTIGATION OF CONDENSED SPECIES USING SYNCHROTRON X-RAY-ABSORPTION SPECTROSCOPY

Condensed species, associated with Se/Pd on the surface of powdered gr aphite granules at elevated temperatures, were investigated in situ by synchrotron X-ray absorption spectroscopy to elucidate the mechanisms that control Pd induced modification of Se. Negative 4.4-eV shifts we re observed in the X-ray absorption near-edge structure (XANES) of the Se K-edge when Se was in the presence of Pd, regardless of the initia l oxidation state of Pd. This corresponds to an average change in the oxidation state of + 4, whereas in the absence of Pd, Se underwent an average change in the oxidation state of about + 2.5. The XANES result s also suggest that the mechanism of conversion of selenous acid into different Se species depends on the initial oxidation state of Pd. Fou rier transformations of the extended X-ray absorption fine structure ( EXAFS) show different coordinations of Pd with Se, depending on the ox idation state of the Pd. Formation of a [SexPdy] or [SexPdyOz] compoun d is indicated. This is consistent with earlier gas phase investigatio ns by mass spectroscopy. The exact stoichiometry of these condensed ph ase selenium species could not be determined. (C) 1998 Elsevier Scienc e B.V. All rights reserved.