Metal–organic frameworks (MOFs)
are often synthesized using
various additives to modulate the crystallization. Here, we report
the direct imaging of the crystal surface of MOF MIL-101 synthesized
with different additives, using low-dose high-resolution transmission
electron microscopy (HRTEM), and identify three distinct surface structures,
at subunit cell resolution. We find that the mesoporous cages at the
outermost surface of MIL-101 can be opened up by vacuum heating treatment
at different temperatures, depending on the MIL-101 samples. We monitor
the structural evolution of MIL-101 upon vacuum heating, using in
situ X-ray diffraction, and find the results to be in good agreement
with HRTEM observations, which leads us to speculate that additives
have an influence not only on the surface structure but also on the
stability of framework. In addition, we observe solid–solid
phase transformation from MIL-101 to MIL-53 taking place in the sample
synthesized with hydrofluoric acid.
is currently based on structures floating on water. [11,13,17,[19][20][21][22] By using graphite membranes, the highest solar thermal efficiency (see Equation (4)) of 85% has been reported under an equivalent solar intensity of 10 suns. [13] One of the main drawback of this approach, besides the large light intensity requirement, lies in the fact that carbon-based systems (both graphite and graphene) are vulnerable to the contamination of pollutants or salt in the water, thus limiting the recyclability of the membrane after prolonged use.In this article, we developed a different approach, which makes use of a flat-optics metasurface [23][24][25][26][27][28] composed of suitably engineered plasmonic nanoparticles. [29,30] Plasmonics structures, thanks to the possibility of localizing light energy at the nanoscale, have demonstrated a great potential in converting light energy into heat for a variety of photothermal applications. [31][32][33][34] In plasmonic steam generation, the best results have been obtained with porous films, which reported efficiencies of ≈60% under 1 sun. [19,20] The efficiency limits of these structures originate from the resonant nature of classical plasmonic materials, which usually harvest energy only at characteristic frequencies of the solar spectrum. In our metasurface, we overcome this problem by using a completely new mechanism of heat generation supported by biomimetic nanoparticles that behave as an almost ideal blackbody. These nanoparticles mimic the shell of an Asian specie of beetle that possesses an exceptional ability in controlling light reflection. [29] Even when these nanoparticles are used in extremely small volumes, they completely absorb input photons at all frequencies and polarizations. By suitably dispersing these nanoparticles in a paper film, we created a metasurface with a remarkable ability in transforming light energy into heat, leading to a dramatic increase of solar thermal generation efficiency if compared to classical structures. Our nanoparticles are fully recyclable from the paper substrate and invulnerable to saline water corrosion, representing an ideal system for solar steam generation. [35] 2. Results
Sample Design and Fabrication
The production of 1-butene by ethylene dimerization is an important chemical industrial process currently implemented using homogeneous catalysts. Here, we describe a highly active heterogeneous catalyst (Ni-ZIF-8) for ethylene dimerization, which consists of isolating Ni-active sites selectively located on the crystal surface of a zeolitic imidazolate framework. Ni-ZIF-8 can be easily prepared by a simple one-pot synthesis method in which site-specific anchoring of Ni is achieved spontaneously because of the incompatibility between the d 8 electronic configuration of Ni 2+ and the three-dimensional framework of ZIF-8. The full exposure and square-planar coordination of the Ni sites accounts for the high catalytic activity of Ni-ZIF-8. It exhibits an average ethylene turnover frequency greater than 1 000 000 h −1 (1-butene selectivity >85%) at 35 °C and 50 bar, far exceeding the activities of previously reported heterogeneous catalysts and many homogeneous catalysts under similar conditions. Moreover, compared to molecular Ni complexes used as homogeneous catalysts for ethylene dimerization, Ni-ZIF-8 has significantly higher stability and shows constant activity during 4 h of continuous reaction. Isotopic labeling experiments indicate that ethylene dimerization over Ni-ZIF-8 follows the Cossee-Arlman mechanism, and detailed characterizations combined with density functional theory calculations rationalize this observed high activity.
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