Abstract
Hydrogen spillover phenomenon is well-documented in hydrogenation catalysis but still highly disputed in hydrogen storage. Until now, the existence of hydrogen spillover through metal–organic frameworks (MOFs) remains a topic of ongoing debate and how far the split hydrogen atoms diffuse in such materials is unknown. Herein we provide experimental evidence of the occurrence of hydrogen spillover in microporous MOFs at elevated temperatures, and the penetration depths of atomic hydrogen were measured quantitatively. We have made Matryoshka-type (ZIFs@)n−1ZIFs (where ZIFs = ZIF-8 or ZIF-67) nanocubes, together with Pt nanoparticles loaded on their external surfaces to produce atomic hydrogen. Within the (ZIFs@)n−1ZIFs, the ZIF-8 shell served as a ruler to measure the travelling distance of H atoms while the ZIF-67 core as a terminator of H atoms. In addition to the hydrogenolysis at normal pressure, CO2 hydrogenation can also trace the migration of H atoms over the ZIF-8 at high pressure.
Highlights
Hydrogen spillover phenomenon is well-documented in hydrogenation catalysis but still highly disputed in hydrogen storage
Well-defined zeolitic imidazolate frameworks (ZIFs)-67@ZIF-8/Pt was prepared as an example to investigate hydrogen spillover over the ZIF-8 phase, in which decomposition of the encapsulated ZIF-67 happens only when the hydrogen atoms delivered from the Pt could migrate across the inert ZIF-8 shell and arrive at reactive ZIF-67 core
It was reported that interaction energies of cetyltrimethylammonium bromide (CTAB) with {100}, {110} and {111} facets in ZIF-8 were −775, −395 and −104 kcal/ mol, respectively.[23,24]
Summary
Hydrogen spillover phenomenon is well-documented in hydrogenation catalysis but still highly disputed in hydrogen storage. The physisorbed hydrogen atoms can diffuse freely on graphitic materials,[13] the theoretical studies suggest that spillover based on this manner would be plugged in an ideal MOF structure.[14] it has been found that diffusion of chemisorbed H in a carboxylate based MOF (e.g., IRMOF-1) is still not feasible due to a large energy barrier (up to 1.6 eV) for the migration.[14] an alternative holemediated (due to metal vacancies) hydrogen spillover mechanism in IRMOF-1 has been proposed, which can substantially lower the barriers to enable spillover at ambient condition.[15] In contrast to the view of spillover for enhanced hydrogen storage, some researchers believed that hydrogen spillover effect does not exist on MOF materials at ambient temperature, or the minor enhancement from or by spillover effect is below the detection limit.[10,16,17] A recent critical review has reported and analysed such an ongoing debate, in which erroneous or irreproducible data regarding spillover in H2 storage in the literature have been summarized in detail.[18] To date, it is still unclear whether hydrogen atoms diffuse through the MOF structure, and if they do, how far they could diffuse is another important question needed to be addressed. On identifying the extent of ZIF-67 decomposition via in-situ gravimetric measurement (under flowing H2) combined with ex-situ morphological/structural characterizations, we are able to qualitatively measure the concentration of atomic hydrogen arriving at the reactive core of ZIF-67 phase and thereby the penetrating depth of H atoms
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