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deriv. normalized E)<br />
Chapter 3 47<br />
In order to gather information on the valence of the Ru-sites in the Ru-MOFs, comparative<br />
X-ray absorption spectroscopy (XAS) measurements were subsequently carried out. The<br />
derivative of the edge-jump energy positions of Ru (22117 eV) and RuO2 (22131 eV) can<br />
be used as standards, representing the oxidation states +0 and +4, respectively. On the<br />
other hand, according to the literature, the oxidation state of Ru-center in SBU-a reveals<br />
+2.5. [203] Hence, the same position of derived edge-jump energy (ca. 22124 eV) for SBU-a<br />
- -d clearly indicates the oxidation state of Ru-centre to be +2.5 (Figure 3.16). To note, Ru-<br />
MOF sample 1_ex with the edge jump position at 22126 eV matches well with its mixed<br />
oxidation state documented in the literature by UHV-IR technique with CO probe. [82] All<br />
the other Ru-MOFs samples (2-4_ex) exhibit their edge-jump at the range of 22123-22128<br />
eV, suggesting the mixed oxidation state of the Ru-centers in their structure as well<br />
(Figure 3.17). The study on deviation of local coordination environments within these Ru-<br />
MOFs due to the change of the counter-ions (in particular by Extended X-Ray Absorption<br />
Fine Structure (EXAFS)) is one of our further direction.<br />
Ru<br />
Ru NP<br />
3_ex<br />
4_ex<br />
RuO 2<br />
2_ex<br />
1_ex<br />
22100 22120 22140 22160<br />
energy, eV<br />
Figure 3.17. XANES spectra (derivative normalized absorption vs. energy) of Ru-MOFs obtained<br />
from various Ru-SBUs (1-4_ex) in comparison with Ru-standard samples (Ru, Ru NP and RuO 2 ).
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