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Structural Investigation of Pd(II) in Concentrated Nitric and Perchloric Acid Solutions by XAFS

Abstract : XAFS spectra of palladium(II) in concentrated HNO3/HClO4 acid mixtures have been recorded and analyzed. Structural parameters of the Pd(H2O)42+ complex and the mixed nitric Pd(NO3)2(H2O)2 complex, for the first time, were determined by the XAFS method. For pure 5 M HClO4 and for mixtures (0-0.3 M HNO3), the XAFS spectra of the 0.02 M Pd solutions are indeed very similar and originated from four Pd-Ow equivalent distances. For the Pd(H2O)42+ square-planar aqua ion in strong perchloric acid, the use of an FEFF6 theoretical approach led to a first-shell Pd-Ow distance of 2.00 (1) Å and a Debye-Waller (DW) factor of 2 = 0.0030 (3) Å2. Four water molecules are tightly bound to the Pd2+ ion in the equatorial plane, while two (or one) axial water molecules are weakly bound to the metal ion at 2.5 Å with a DW factor of 0.015 (5) Å2. For highly concentrated mixtures (4-6 M HNO3) and for pure concentrated (4-6 M) nitric acid as well as for crystalline powder Pd(NO3)2(H2O)2, the XAFS spectra are very similar and are determined by the mixed nitric complex Pd(NO3)2(H2O)2: four Pd-O near-equivalent distances of 2.01 (1) Å from two H2O and two NO3 molecules with a total DW factor of 2 = 0.0037 (3) Å2. Moreover, two Pd---N distances of 2.8-2.9 Å were determined in the second coordination shell. Finally, for intermediate mixtures (1-3 M HNO3 in 5 M HClO4), the XAFS spectra are a superposition of the XAFS of Pd(H2O)42+ and Pd(NO3)2(H2O)2 complexes. The mean ligand number NO3- around Pd2+ has been calculated, and the XAFS results at pH close to zero confirm the spectrophotometric results previously published.
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Submitted on : Monday, February 20, 2006 - 3:05:38 PM
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J. Purans, B. Fourest, C. Cannes, V. Sladkov, F. David, et al.. Structural Investigation of Pd(II) in Concentrated Nitric and Perchloric Acid Solutions by XAFS. Journal of Physical Chemistry B, American Chemical Society, 2005, 109, pp.11074-11082. ⟨10.1021/jp045489n⟩. ⟨in2p3-00025649⟩



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