The continuous and dramatic increase of global demand for energy resources makes it urgent to develop affordable nanostructured materials to act as efficient electrocatalysts (ECs) in the ...energy-related reactions. Metal–organic framework (MOF) template pyrolysis for the production of nanostructured carbon-based materials is a very promising methodology to produce carbon-based ECs. Herein, we report the preparation, characterization (XPS, CHNS analysis, ICP-OES, XRD, TEM, and SEM/EDS), and application as oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) ECs of a plethora of nanostructured carbon materials derived from RT-synthesized (RT, room temperature) MOF-74 with different metal compositions, Co, Ni and Co/Ni, and dopant heteroatoms, N–, S– and N/S-dual-doping. This has allowed the study of the aforementioned parameters’ influence on the OER and ORR electrocatalytic activity in alkaline medium. Highly synergetic effects have been detected in two cases: (1) when N/S-dual-doped carbon is produced from a monometallic Co-MOF-74 template, N,S–Co@C, and (2) when an undoped carbon is derived from bimetallic Co/Ni-MOF-74, Co/Ni@C. These two samples achieve OER performances with η10 = 0.41 and 0.44 V, respectively, along with Tafel slopes of 101 and 93 mV dec–1, being close to the state-of-art OER catalyst performance. In addition, ORR tests showed that the effect of heteroatom doping on ORR activity is always positive, regardless of metal composition.
The novel porous {M(F‐pymo)2}n⋅2.5n H2O coordination networks (M=Co, Zn; F‐pymo=5‐fluoropyrimidin‐2‐olate), possessing sodalitic topology, have been synthesised and structurally characterised by ...means of powder diffraction methods. Thermodiffractometry demonstrated their plasticity: when heated up to 363 K, they reversibly transform into three‐dimensional dehydrated {M(F‐pymo)2}n species, with significantly different lattice parameters. Further heating induces irreversible polymorphic transformations into layered phases, in which the original MN4 coordination sphere changes into an MN3O one. A mixed‐metal phase, {CoxZn1−x(F‐pymo)2}n⋅2.5n H2O, was also prepared, showing that zinc is preferentially inserted, when starting from a Co/Zn reagent ratio of 1:1. The solid–gas adsorption properties of the anhydrous 3D frameworks have been explored towards N2, H2 (77 K) and CH4, CO2 (273 K). These results show that these materials permit the diffusion of CO2 molecules only. Remarkably, the CO2 adsorption process for the {Co(F‐pymo)2}n network proceeds in two steps: the first step takes place at low pressures (<600 kPa) and the second one above a threshold pressure of 600 kPa. By contrast, the {Zn(F‐pymo)2}n network only permits CO2 diffusion by applying pressures above 900 kPa. This type of behaviour is typical of porous networks with gated channels. The high CO2 selectivity of these systems over the rest of the essayed probe gases is explained in terms of flexibility and polarity of the porous network. Finally, the magnetic studies on the CoII systems reveal that the as synthesised {Co(F‐pymo)2}n⋅2.5n H2O material behaves as an antiferromagnet with a TN of about 29 K. At variance, the {Co(F‐pymo)2}n layered phase shows an unusually weak ferromagnetic ordering below 17 K, arising from a spin‐canting phenomenon.
Flexible MOFs! Application of high‐pressure CO2 or exposure to moisture of a zeomimetic coordination network induces a reversible structural change from a non‐porous β‐phase to a porous α‐phase (see figure). An additional structural transformation into a layered γ‐phase is promoted by thermal treatment implying a concomitant modification of the physicochemical properties.
A novel approach toward the obtaining of a Pd (II) carbon-supported catalyst based on a statistical distributed polymer (hyperbranched polyethyleneimine) and its use as a highly efficient and ...sustainable catalyst for Sonogashira-type coupling reactions is presented. The heterogenous Pd (II)/HBPEI catalyst fabricated through a facile, cost-effective and scalable procedure provides an innovative catalyst structure based on statistical distributed singular catalytic centers with well-defined Pd-Polymer chemical interactions, highly resistant to suffer leaching processes and extensively and homogenously distributed across the whole carbon surface. The catalyst exhibited excellent catalytic and recycling activity for several cross-coupling reactions between different aryl halides and terminal alkynes, free of any undesired by-product and in excellent yields, under air atmosphere, using water as solvent, at moderate temperatures (65 °C) and avoiding the use of phosphine derivatives, Cu(I) as co-catalyst and excess of any of the reactant. Therefore, the reported Pd(II)-HBPEI carbon-supported material can be considered a truly competitive heterogeneous Pd(II) catalyst for large-scale under green chemistry conditions.
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•Hyperbranched polyethyleneimine Pd(II)-complexing properties•Heterogeneous polymer-based Sonogashira catalyst for large-scale applications•Pd (II) catalyst based on porous carbon supports•Phosphine-, and sulfur-free heterogeneous palladium catalyst•Cross-coupling reaction under mild, aerobic, green and co-catalyst free conditions
A novel and straightforward approach to accurately predict, through direct potentiometric measurements, the complexing properties and adsorption capacity of novel carbon-based hybrid scavengers ...toward particular metal ions is described. This innovative approach establishes a linear free-energy relationship (LFER) between the complexing properties of the free-complexing function in solution and the complexing retention capacity of the functionalized carbon-based hybrid material (complexing function supported). In particular, we report the first detailed study concerning the metal ion complexing properties, as well as the complexing mechanisms involved in covalently-bonded carbon-supported hyperbranched polyethyleneimines, towards a series of metal ions of environmental and technical interest, such as Mn
2+
, Cd
2+
, Hg
2+
, Pd
2+
and Cr
3+
. The study based on adsorption isotherm measurements and potentiometric studies of non-bonded hyperbranched polyethyleneimines (HBPEIs) (free in solution) and carbon-supported HBPEI (hybrid material, C-PEI) revealed that bonded HBPEI molecules fully define and control the metal ion complexing abilities of C-PEI, while the carbon structure acts only as a solid support. These results corroborate the prevalence of a reliable linear free-energy relationship (LFER) between the retention capacities of the hybrid material (C-PEI) and the complexing properties of free HBPEI in solution, which supposes a novel approach to easily predict the metal ion complexing properties of carbon-based hybrid materials, through a simple potentiometric titration analysis of the free complexing ligand in solution. This approach has the potential to predict retention capacities of solid materials (actual or proposed) exhibiting surfaces functionalized with HPBEI.
Assessment of complexing properties and mechanisms involved in covalently-bonded carbon-supported hyperbranched polyethyleneimines towards metal ions of environmental and technical interest.
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•H2 plasma removes oxygen groups but not the sulfonyl and sulfonic groups.•Water treatment + H2 plasma treatment removes 60% of oxygen and 100% of sulfur.•This methodology removes as ...much oxygen as NaBH4 but is simpler, faster and cleaner.•The oxygen remaining after H2 plasma reduction forms labile surface groups.•Three GOs were tested rendering similar results.
We report the reduction of GO by hydrogen cold plasma as an effective alternative to the usual chemical (with harsh reducing agents) or physical (through high-temperature treatments) methods. The hydrogen plasma is generated through microwave radiation and the reduction is performed barely above room temperature, avoiding structural degradation of the graphene oxide. Three commercial GOs, with several oxygen and sulfur contents, have been used in this study. As a consequence of the exposure to the hydrogen plasma, the oxygen content is decreased in large extent while sulfur is scarcely removed. Thus a two-stage methodology consisting of a simple water treatment under reflux conditions of GO (to remove sulfur) and further treatment with H2 plasma is proposed. The combination of both steps allows the total elimination of sulfur while achieving very high degrees of reduction, lowering the initial oxygen content by more than 60%. These degrees of reduction are comparable to those obtained when the reduction is carried out by using the standard procedure with NaBH4 on the same materials. The reduction with hydrogen plasma clearly shows advantages over the conventional reduction procedures due to its simplicity: no chemicals or high temperatures are needed, and the procedure is very fast.
A guest@host POM@ZIF nanocomposite—PW11Co@ZIF-67—has been synthesized using an in situ strategy. This new nanocomposite exhibits (i) individually ZIF-67-cage-confined POM units, (ii) structural ...defects in the ZIF-67 host induced by the POM, and (iii) charge transfer from the ZIF-67 to the confined POM. In addition, it has served as a template to produce a set of derived samples by applying thermal treatment at various temperatures (200, 400, 500, 600, and 950 °C) under a N2 flow. We have used multiple characterization techniques, ICP-OES, CHNS analysis, XPS, ATR-IR, PXRD, Raman spectroscopy, N2/CO2 adsorption analysis, CV, and TEM/EDS, to fully assess the thermally-induced variation tendencies. The first two derivatives—D200 and D400—show the same nanoarrangement as the PW11Co@ZIF-67 precursor, although with incipient signs of both POM and ZIF-67 structural decompositions. The following samples—D500, D600, and D950—exhibit a carbonaceous nature consisting of C-embedded compositionally complex nanoparticles that involve Co and W present as diverse species, metallic/oxide/phosphate/phosphide. D500 presents the best intrinsic electrochemistry, probably due to the high proportion of pyridinic N moieties doping its C matrix combined with small-sized and highly dispersed Co-enriched nanoparticles. This study focuses on the need for a thorough physicochemical characterization of this class of highly nanostructured materials with a view to exploring their application in electrocatalysis.
A stable, extensive amine functionalization was introduced on the surface of an activated carbon (AC) through very effective grafting of a polyethyleneimine layer onto the AC surface. The procedures ...used are simple, sustainable and easily scalable. Among the large variety of possible applications that the polyamine functionality confers to the AC, its outstanding properties as Pd(II) adsorbent are here presented.
The selective functionalization of carbon nanotube surfaces is crucial for many potential applications of these materials. For this purpose several oxidants, among other substances, are used. The aim ...is to reach a large degree of functionalization which depends on the oxidant character of the reagent. For this reason the functionalization of multiwall carbon nanotubes (MWCNTs) by treatment with ozone in basic solution is studied. At basic pHs, ozone results into hydroxyl radicals whose reduction potential is very high (E° = 3.06 V). The results have been compared to those obtained by ozone in gas phase and with cold oxygen plasma. The oxidation with ozone in basic solution seems to be kinetically restricted. As a consequence, the degree of oxidation in this medium is smaller than this of ozone gas, in spite of the larger oxidation capacity of the former. The oxygen-containing groups fixed by these two treatments are mainly attached to defects of the nanotubes. Moreover, no modification of the graphene layers and no porosity result from these treatments. The oxygen plasma treatment stands out in the content of oxygen groups fixed to the MWCNTs, as it is by far more effective, although some of these groups have relatively low thermal stability. Nevertheless, this treatment mainly fixes the oxygen groups on the walls of the nanotubes.
Photo-induced darkening of red cinnabar (HgS) has attracted the interest of many researchers as it drastically impacts the visual perception of artworks. Darkening has commonly been related to ...metallic mercury (Hg
) formation in the presence of chlorides. Based on the study of UV-aged cinnabar pigment and tempera paint we propose an alternative pathway for the blackening reaction of cinnabar, considering its semiconductor properties and pigment-binder interactions. We demonstrate that darkening is caused by the oxidation of cinnabar to mercury sulfates and subsequent reduction to Hg
via photo-induced electron transfer without the involvement of chlorides, and provide direct evidence for the presence of Hg
on UV-aged tempera paint. Photooxidation also affects the organic binder, causing a competing depletion of photo-generated holes and consequently limiting but not impeding mercury sulfate formation and subsequent reduction to Hg
. In addition, organics provide active sites for Hg
sorption, which is ultimately responsible for the darkening of cinnabar-based paint.
A new Y-based metal–organic framework (MOF) GR-MOF-6 with a chemical formula of {YL(DMF)2·(DMF)} n {H3L = 5-(4-carboxyphenyl)ethynyl isophthalic acid; DMF = N,N-dimethylformamide} has been prepared ...by a solvothermal route. Structural characterization reveals that this novel material is a three-dimensional MOF in which the coordination of the tritopic ligand to Y(III) metal ions leads to an intercrossing channel system extending over three dimensions. This material has proven to be a very efficient catalyst in the cyanosilylation of carbonyls, ranking second in catalytic activity among the reported rare earth metal-based MOFs described so far but with the lowest required catalyst loading. In addition, its electrophoretic behavior has been studied in depth, providing a zero-charge point between pH 4 and 5, a peak electrophoretic mobility of −1.553 μm cm V–1 s–1, and a ζ potential of −19.8 mV at pH 10.
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