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

Abstract

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(II). Absorption spectra of reference samples such as PdO, Pd(C 5H 7O 2) 2, Pd(CH 3COO) 2, [Pd(NH 3) 4]Cl 2, [Pd(NH 3) 4](NO 3) 2, (C 3H 5PdCl) 2, PdCl 2(CH 3CN) 2, PdCl 2, (NH 4) 2[PdCl 4], K 2PdCl 4 and Pd metallic foil were taken into account. Although the XANES fits for the best two sets of reference compounds, (1) Pd(CH 3COO) 2, (NH 4) 2[PdCl 4], Pd foil and (2) [Pd(NH 3) 4]Cl 2, (NH 4) 2[PdCl 4], 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% Pd 2+ 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.