Quantitation of Redox States

In ceria-based catalysts, one main aspect studied with XPS is the cerium redox state (i.e. the proportion of Ce and Ce ) for different preparations or treatments. Peak u, located at Eb 916.5 eV, which appears to be typical of Ce and absent for Ce, and consequently decreases upon reduction, has been used frequently with this aim. The percentage %(u ) of the total integrated spectrum intensity (after baseline subtraction) lying within this peak in pure, nonreduced Ce02 has been computed to amount to 13.7% [108]. The %(u ) value of an analyzed material is frequently used in the literature to quantify its redox state, assuming e.g. that the the quotient betwen it and the value for Ce02 gives the fraction of Ce in the material. It has been however pointed out in early works [109], that this value is not proportional to the amount of Ce present specifically Romeo et al. [110] claimed that peaks v 

If only two components of fixed shape exist, corresponding to the two redox states (as indicated by the FA studies), then the analysis can be simplified as done by Appel et a. [119] the Ce component (obtained from a pure Ce02 spectrum) can be subtracted from a given spectrum in amounts appropriate to cancel the u peak the Ce fraction is thus obtained, which allows to quantify both. It should be noted that this conclusion of the FA contradicts the above mentioned claims of non-linear relationship between the u peak area and total reduction degree. One may note here that the FA results [111, 117] reproduce the small shake-up satellites described above as part of the Ce component the latter of these overlaps the u peak of Ce, and if not taken into account may contribute to the non-proportionality between %(u ) and the amount of Ce. The presence of a small component near the u peak position is visible in other spectra of nominally pure trivalent Ce [120]. Also, earlier works [121] suggest a nearly linear dependence between the v peak intensity and the reduction degree which is not a pure proportionality, as it includes some (u -V ) peaks intensity in the pure Ce state (Fig. 5.11) this might correspond to the mentioned satellite located near the u peak. 

In summary, there are still issues requiring further work if a well-stablished quantification method is desired. In any case, the accuracy of any method is affected 

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