3268 wileyonlinelibrary.com inorganic nodes in MOFs can be intrinsic active sites for catalysis, e.g., copperbased MOFs have been demonstrated as an excellent solid Lewis acid catalyst for a few reactions; [ 4,7 ] (ii) extrinsic functional species (e.g., nanoparticles) can be introduced to MOFs structure, either on the external surfaces or inside the bulks or shells of MOFs. Various catalytic nanoparticles have been successfully incorporated into different MOFs matrices, including Au, Ag, Pt, Cu, or their combination etc.; [8][9][10] and (iii) MOFs also serve as solid precursors for metal or metaloxide catalysts due to their high metal density within the frameworks and easy transformation. Many porous transition metal oxides, such as CuO, Cu 2 O, Mn 2 O 3 , Mn 3 O 4 , Co 3 O 4 , and ZnO among others, have been derived through thermolysis under controlled atmospheres. [11][12][13][14][15][16] For example, porous CuO hollow architectures with perfect octahedral shape were prepared by annealing HKUST-1 precursor in fl owing air. [ 12 ] Moreover, zeolitic imidazolate framework-67 (ZIF-67) was used to design and generate Co 3 O 4 or structurally more complex Co 3 O 4 /NiCo 2 O 4 . [ 13,14 ] Although increasing research attention has been paid to the catalytic applications of MOFs, to our knowledge, shapes of the studied MOFs are mostly followed in 3D isotropic polyhedrons (i.e., high symmetric MOF crystals with sizes greater than 100 nm), and the catalytic systems derived from MOFs with anisotropic shapes such as nanofi bers or nanosheets with a dimension(s) less than 10 nm have seldom been explored.Similar to many inorganic nanomaterials, tailoring of size and shape is essential to customize MOFs for specifi c applications according to the principle of "structure dictates function." [ 17 ] Although a fi ne control over the size and size-distribution of MOFs has been realized, e.g., in the range of 100 nm to several micrometers, [ 18 ] fashioning MOFs into desired shapes for specifi c applications remain to be challenging. Nevertheless, several attempts to the architectural control of MOFs shapes have recently appeared in the literature, ranging from 1D, 2D to 3D nanostructures. [ 19,20 ] For example, nanosheets of MOFs as molecular sieving membranes for gas separation have been made in two recent reports, [ 21,22 ] which achieved much higher separation selectivity than MOFs in other
Biosynthesis of Ag nanoparticles (AgNPs) by Cacumen Platycladi extract was investigated. The AgNPs were characterized by ultravioletÀvisible absorption spectroscopy (UVÀvis), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and X-ray diffraction (XRD). The results showed that increasing the initial AgNO 3 concentration at 30 or 60 °C increased the mean size and widened the size distribution of the AgNPs leading to red shift and broadening of the Surface Plasmon Resonance absorption. The conversion of silver ions was determined by atomic absorption spectroscopy (AAS) and to discuss the bioreductive mechanism, the reducing sugar, flavonoid, saccharide, protein contents in the extract, and the antioxidant activity were measured using 3,5-dinitrosalicylic acid colorimetric; Coomassie brilliant blue; phenol-sulfuric acid; rutin-based spectrophotometry method; and 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical-scavenging assay methods. The results showed that the reducing sugars and flavonoids were mainly responsible for the bioreduction of the silver ions and their reductive capability promoted at 90 °C, leading to the formation of AgNPs (18.4 ( 4.6 nm) with narrow size distribution. Finally, the antibacterial activity of the AgNPs against E. coli and S. aureus was assessed to determine their potential applications in silver-loaded antibacterial materials. This work provides useful technical parameters for industrialization of the biosynthetic technique and further antibacterial application of the AgNPs. Furthermore, the elucidation of bioreductive mechanism of silver ions by measuring the change of the biomolecular concentrations in plant extract exemplifies understanding the formation mechanism of such biogenic AgNPs.
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.
CO2 hydrogenation to produce useful C1 chemicals (such as CO, CH4, and CH3OH) plays a pivotal role in future energy conversion and storage, in which catalysts lie at the heart. However, our fundamental understanding of the correlation between catalyst structures and product selectivity is still limited because in most cases the catalyst structures in nanoscale are not well-defined. Herein, we report the design and synthesis of nanoreactors by phase transformations of sandwich-structured ZIF-67@Pt@mSiO2 nanocubes via a simple water-soaking method where ZIF-67 serves not only as a morphological template but also as a sacrificial cobalt source. The resultant porous mazelike nanoreactors are highly active in gas-phase CO2 hydrogenation, in which the reaction pathway involves (i) dissociation of CO2 to form CO over Pt site via reverse water–gas shift reaction and then (ii) methanation of CO catalyzed by the nearby cobalt site. It was found that the overall “long retention time” for feed gases on catalysts significantly affected the product distribution. Thus, the specific activity (in the form of turnover frequency) of the nanoreactor having prolonged diffusion paths was around six times as much as that of other comparative catalysts with shorter diffusion paths. This work contributes insights to the CO2 hydrogenation to methane over bifunctional nanoreactors with designed structures.
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