Local structure of a Pd-doped polymer investigated using a linear combination of XANES spectra

Methods of principal components analysis and linear combinations of XANES (X-ray absorption near edge structure) spectra were applied to determine the composition of Pd species in Pd-doped poly-o-methoxyaniline (POM). A set of reference spectra reproducing the absorption spectrum of Pd/POM polymer was easy to find because of the square-planar structure common for the d(8) metal ion Pd(H). Absorption spectra of reference samples such as PdO, Pd(C5H7O2)(2), Pd(CH3COO)(2), [Pd(NH3)(4)]Cl-2, [Pd(NH3)(4)](NO3)(2), (C3H5PdCl)(2), PdCl2(CH3CN)(2), PdCl2, (NH4)(2) [PdCl4], K2PdCl4 and Pd metallic foil were taken into account. Although the XANES fits for the best two sets of reference compounds, (1) Pd(CH3COO)(2), (NH4)(2) [PdCl4], Pd foil and (2) [Pd(NH3)(4)]Cl-2, (NH4)(2) [PdCl4], Pd foil, were in very good agreement with the experimental XANES spectrum of Pd/POM, only the first model was established as being correct. This was achieved by examining the Fourier transforms of reconstructed EXAFS spectra in comparison to the Fourier transform for the Pd/POM polymer. A quantitative XANES analysis of the composition of Pd/POM led to a final result of 46.0% Pd metal and 54% Pd2+ ions surrounded by four neighbours, including Cl and O ions in an average ratio of 2.6:1.4. The coordination numbers for the Pd-O, Pd-Cl and Pd-Pd near-neighbour shells in the Pd/POM polymer were simply evaluated from the XANES fit analysis. (C) 2003 Elsevier B.V. All rights reserved.