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-Q st<br />
, kJ/mol<br />
Chapter 4 95<br />
DEMOFs (1a and 1c): 1) strong binding affinity between Ru-CUSs and hydrogen, namely<br />
Ru 2+/3+ -sites in the regular paddlewheel units and the Ru δ+ -sites in the modified<br />
paddlewheels (in defects A); 2) the relatively weaker interactions (ie. physisorption and<br />
van der Waals induced interactions) between the hydrogen and ligands/pores walls in the<br />
framework. Obviously, due to the lack of Ru δ+ -sites in the parent Ru-MOF, the Ru-DEMOFs<br />
exhibit higher H2 uptake than the parent Ru-MOF. To quantitatively understand the H2<br />
binding affinity of these samples, the isosteric heats of H2 adsorption (–Qst) was calculated<br />
and reveal an order with 1c > 1a > parent Ru-MOF when comparing the respective values<br />
at 1 mmol (H2) / Ru2-paddlewheel (what might be considered as the saturation<br />
adsorption of H2 at the regular paddlewheel) (Figure 4.30). This suggests stronger binding<br />
affinity of H2 for Ru-DEMOF with 32% of incorporated 5-OH-ip (sample 1c), which again<br />
is attributed to the relative higher amount of Ru δ+ -sites in 1c.<br />
8 parent Ru-MOF<br />
1a<br />
1c<br />
7<br />
6<br />
5<br />
0.0<br />
0.0 0.5 1.0 1.5 2.0 2.5 3.0<br />
H 2<br />
adsorbed, mmol/Ru 2<br />
pw<br />
Figure 4.30. Isosteric heats of adsorption of 1a and 1c calculated from the H 2 adsorption<br />
isotherms at 77K and 87K. The vertical line stands for the position where one H 2 molecule is<br />
absorbed per Ru 2 -paddlewheel.
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