Palladium is a fantastic catalyst for many organic transformations, for example selective oxidation, and as such understanding the surface chemistry is vital and has been extensively studied.
Pd2+ species have the capability to reduce under an X-ray beam and as such should ideally be the first region analysed during a measurement.(1)
Typically, the 3d region is the most useful and widely studied. It has a doublet separation of 5.26 eV and a standard 3d5/2 : 3d3/2 area ratio of 3:2. There is a significant energy separation between Pd metal and PdO (1.7 eV)(2) which facilitates trivial deconvolution of the two species.
Figure . Fitted Pd(3d)/Au(4d) region for a AuPd/TiO2 catalysts for hydrogen peroxide formation, where: Blue = Pd(0), Lilac = Pd(2+) and Green = Au(0)
The most significant overlapping spectral features are the 3p peaks from zirconium, the 4d region of gold and some secondary auger features from the Mg KLL auger when using an aluminium X-ray source (1486.7 eV), so take care when analysing mixtures of these, you may potentially need to look at another Pd orbital (although this is complicated due to an overlap between Pd 3p3/2 and O 1s, leaving only the relatively weak Pd 3p1/2 and Pd 3s available for the majority of samples).
|Species||3d5/2 Binding energy / eV||Ref|
|Pd metal||334.8-335.5 (Typically taken as 335.2)(4)||5,6,7,8|
|PdCl2||338.4 (Pd0 = 335.2)||10|
|PdS2||336.6 (Pd0 = 335.5)||11|
|Na2PdCl4||337.9 (Pd0 = 335.5)||11|
|K2PdCl4||338.1 (Pd0 = 335.5)||11|
If it is possible to accurately obtain, the modified auger parameter (M4,5N1N2,3 & Pd 3d5/2) may provide a more reliable measure of the electronic state of the palladium, although the M4,5N1N2,3 auger is surprisingly weak from Pd metal.(12)
The modified auger parameter (M4,5N1N2,3) for Pd metal is 242.8±0.2 eV, and for PdO is 240.5±0.2.(12) If considering the M4,5VV auger, the parameter for Pd metal becomes 327.5±0.2 eV, while difficulties over the determination of a precise peak maxima render this choice of auger less desirable for PdO.(12)
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- Kim, K. S., et al. (1974). “X-ray photoelectron spectroscopic studies of palladium oxides and the palladium-oxygen electrode.” Analytical Chemistry 46(2): 197-200. Read it online here.
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- Militello, M. C. and S. J. Simko (1994). “Palladium chloride (PdCl2) by XPS.” Surface Science Spectra 3(4): 402-409. Read it online here.
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- Brun, M., et al. (1999). “XPS, AES and Auger parameter of Pd and PdO.” Journal of Electron Spectroscopy and Related Phenomena 104(1-3): 55-60. Read it online here.