Inorganic Chemistry
Article
CO sorption at 195 K (Figures 3 and S15). The gas sorption
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Figure 4. IR spectra for samples of (a) activated ZrL1, (b) ZrL1-
Co (CO) , and (c) ZrL1-CoO.
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ZrL1-CoO to be 1:1.49. With the above ZrL1 formula, a L1/
Co ratio of 1:7.2 can be deduced, corresponding to about half
Figure 3. CO (195 K) adsorption and desorption isotherms for an
activated ZrL1 sample (red graphs), ZrL1-CoO sample (blue graphs),
and ZrL1-CoO-600 sample (green graphs).
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(
7.2/16 = 45%) of the eight alkyne side arms of each L1 linker
being engaged as the [Co (CO) (RCCR)] complex. The
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efficient uptake of Co (CO) as enabled by the alkyne side
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which allows the selective penetration by CO (and not by N )
arms is also highlighted by the poor Co (CO) uptake (e.g.,
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because of its smaller kinetic diameter, quadruple moment, and
mostly limited to the external surfaces) of a MOF wherein the
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stronger thermal motion at 195 K (relative to 77 K for N2).
alkyne units were built into the linker backbone instead.
Notably, activated ZrL1 is quite persistent (e.g., even after
exposure to the moist air of Hong Kong for 40 days, its
diffraction profile remains unchanged (Figure 2d)). The air
stability may be partially ascribed to the bulky hydrophobic
side arms that prevent water from eroding the metal node.
Interestingly, upon contact with the DMF solvent, the
contracted phase (i.e., activated ZrL1) reverts to the open
phase of the as-made sample, as revealed by PXRD (pattern e,
Figure 2; cf. pattern b). The swift recovery of the pristine phase
is likely associated with the linker deficiency (i.e., 37.5%),
Comparison with other related MOFs (e.g., the isoreticular
PCN-606 and NU-901 ) with regard to metalation and
catalysis, however, still warrants attention.
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The ZrL1-CoO solid continues to feature a type-I isotherm
in CO sorption (195 K, Figures 3 and S16), with a smaller
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−1
Langmuir surface (340 m g ) than for activated ZrL1 (486
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−1
m g ), which is consistent with partial pore occupation by
the CoO guests. ZrL1-CoO also exhibits greater hysteresis
indicative of ultramicropore blocking by the CoO guests.
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leading to lower-connected Zr nodes (i.e., five-connected) and
host frameworks remain upstanding (Figure S13), although an
unusually strong background (likely due to the CoO
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1
consequently more flexibility for framework dynamics.
Framework dynamics of Zr-MOF solids often arise from the
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fluorescence ) largely obscures the diffraction signals.
Notably, no diffraction peaks from CoO or other phases
were observed, indicating the finely dispersed (and thus
nondiffracting) nature of the CoO species within the MOF
ZrL1-CoO, which reveal no large aggregated particles (e.g., of
ZrL1, ZrL1-Co (CO) , and ZrL1-CoO solids to be about 2.19,
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21,39,63,64
supple topology,
linker flexibility,
and partial
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hydrolysis of the carboxylate donors (e.g., switching from
the bidentate bridging mode to the monodentate or
uncoordinated mode). Further study is needed on how these
factors contribute to the dynamic, breathing properties of
ZrL1.
Unlike the strenuous (e.g., by vacuum deposition) grafting
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of metal carbonyl guests into other MOF matrixes,
the
alkyne-functionalized ZrL1 solid (activated, orange-red) read-
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ily binds Co (CO) from a THF solution to form the dark-
1.72, and 1.25 eV, respectively.
CoO is known to be a good electrocatalyst for the oxygen
evolution reaction (OER) when strongly coupled with various
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brown ZrL1-Co (CO) solid. The characteristic, strong IR
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−
1
peaks at 2089, 2054, and 2028 cm (Figure 4b) of the CO
stretches point to the known μ -alkyne dicobalt hexacarbonyl
carbon substrates including graphene and amorphous
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complexes [Co (CO) (RCCR)].
Compared with the
carbon.
To enhance charge transport for electrocatalysis
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yellow-fluorescing ZrL1 (emission peak at 585 nm for the
applications, the ZrL1-CoO sample was heated to 600 °C for 3
h to carbonize the organic linkers (e.g., by cyclizing the alkyne
units). The resulting black sample, ZrL1-CoO-600, remains
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2
−1
The ZrL1-Co (CO) solid (brown) can be heated in air
e.g., at 75 °C for 3 h) to remove the CO ligands (IR evidence;
lack of distinct peaks in its PXRD pattern (Figure S23)
indicates an amorphous structure with the Co species
remaining highly dispersed (i.e., they do not aggregate into
crystalline particles in the thermal treatment). In a 0.1 M KOH
and a small TS of 80 mV/dec (Figure S24), which are
significantly enhanced over the control sample (i.e., without
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(
Figure 4c). The resultant solid was denoted ZrL1-CoO
because XPS reveals the Co 2p peak at 779.7 eV with distinct
content, with Zr/Co atomic ratios of 1:1.5 and 1:1.4 for ZrL1-
Co (CO) and ZrL1-CoO, respectively. Further elemental
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Co components) of ZrL1-600 (η > 577 mV at 1.0 mV/cm , TS
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analysis by ICP-OES quantifies the Zr/Co atomic ratio in
= 284 mV/dec). ZrL1-CoO-600 as an OER catalyst was thus
C
Inorg. Chem. XXXX, XXX, XXX−XXX