Topochemical Speciation of Intercalated Palladium in Graphite by Valence Band X-ray Spectrometry in the Electron Microprobe

Abstract

The binding state of palladium was studied within the frame of an investigation on the mechanism of analyte fixation during the pyrolysis step in graphite furnace atomic spectrometry. One approach was the determination of the palladium intercalated in the pyrolytic coating of the graphite tube. Due to the low concentrations of intercalated palladium in the pyrolytic coating, precise determination of the shift of certain X-ray lines was chosen. From several investigated valence state sensitive X-ray transitions, the Pd Lbeta2/15 (L3-N4,5) line shift was the one best determinable. The measured line shifts are in the range of -0.14 to 0.71 eV at line widths of 13 eV (fwhm) and a line energy of 3.1729 keV. These very small line shifts were determined by electron probe microanalysis. The detection of the small line shifts was performed with a new method-by evaluation of the change of the intensity in the flanks of the X-ray line. The measurements yielded the following results: inside the pyrolytic graphite, the palladium is distributed inhomogeneously in the form of clusters or islands and in the form of particles on the surface of the pyrolytic graphite. The differentiation between particles and clusters is a very practical one: as long as a particle can be seen in the SEM we talk of particles. Often, however, Pd is detected in an area on the tube or platform surface without detection of a particle. Hence, it can be assumed that the Pd is present in the form of clusters which might even be intercalated in the uppermost graphite layers. The valence state inside these clusters does not appear to be uniform. It can be interpreted as a mixture of Pd with PdO in the center of the clusters or particles (positive peak shift) and of Pd bound to the graphite (strong negative peak shift). On the basis of these observations, a way is proposed to determine how activated Pd atoms in intercalated Pd domains are forming strong covalent bonds to analytes. These bonds are responsible for the analyte fixation of even very volatile analytes.

Possibilities of space-resolved solid-state speciation

“…25 Recently it was successfully attempted to apply this method to palladium. 26 Due to the relative reduction of the line shift with rising atomic number it was necessary to devise a new method for the expected very small shifts of Pd. This method is based on the fact that a peak shift leads to a pronounced change of intensity in the flanks of the peak.…”

“…25 It was then consequently developed further by us. 26 This method is based on the concomitant changes of both, the intensity ratio and the wavelength shifts of the Fe L a and Fe L b X-ray emission lines with variations of the Fe 2+ to Fe 3+ -ratio. 25 If such measurements are carried out by the use of synchrotron radiation, a lateral resolution level of a few micrometers can be reached together with a high detection sensitivity.…”

Atomic Spectroscopy

“…Here both Pd, W, Rh, Ir, Nb, Ni, Zr, and Mg salts or binary mixtures of them can be used to retain the analytes while evaporating the matrix elements. In efforts to clarify the mechanisms of these modifiers, Rohr et al (22) investigated whether intercalation of Pd in the graphite occurs with the aid of valence bond X-ray spectrometry. Maia et al (23) investigated the use of a permanently modified graphite tube surface to eliminate interferences in coal analysis, whereas Fischer (24) studied the electrothermal atomization of Pdstabilized Se in the presence of phosphate.…”