The nature of the active sites for palladium supported on nitrogen doped carbon nanofibers (Pd/N-CNFs) catalysts for selective hydrogenation of acetylene to ethylene was examined. Palladium ...concentrations of 0.05–0.6 wt% were deposited on carbon nanofibers doped by nitrogen atoms (N-CNFs) and characterized by transmission electron microscopy (TEM), CO chemisorption, X-ray photoelectron spectroscopy (XPS), Extended X-ray Absorption Fine Structure (EXAFS) and quantum-chemical calculations. The data indicates that the Pd initially forms highly-dispersed particles. However, decreasing of the Pd concentration on the carbon nanofibers doped by nitrogen atoms below 0.15 wt% led to stabilization of the metal in the atomic state. Porphyrin-like defects with four nitrogen atoms are formed on the surface of the CNFs and strongly interact with palladium atoms, and thus can be sites for stabilization of atomic Pd. The catalytic activity and selectivity of Pd/N-CNFs catalysts depend on the ratio of these two palladium states.
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•Palladium was deposited on carbon nanofibers (CNFs) doped by nitrogen atoms.•Pd initially forms highly-dispersed particles.•Porphyrin-like defects with four nitrogen atoms are formed on the surface of the CNFs.•Porphyrin-like defects can be sites for stabilization of atomic Pd.
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•Comparative study of peroxide delignification of hardwood and softwood over catalyst TiO2.•Characterization of products of wood catalytic delignification.•Characterization of ...ethanol-lignin and acetone-lignin from aspen wood.•Catalytic depolymerization of lignins in supercritical ethanol and butanol.•Wood biorefinery based on the use of solid catalysts and green reagents.
Two ways of catalytic depolymerization of native and isolated wood lignins are described: the peroxide delignification of hardwood (aspen, birch) and softwood (abies) in the medium of acetic acid – water over TiO2 catalyst and the thermal dissolution of organosolv lignins (ethanol-lignin and acetone-lignin) in supercritical alcohols (ethanol and butanol) over solid Ni-containing catalysts.
The catalyst TiO2 in rutile modification has the higher activity in wood peroxide delignification at 100 °C as compared to TiO2 in anatase modification. The results of kinetic studies and optimization of the processes of peroxide depolymerization of hardwood (aspen, birch) and softwood (abies) lignins in the medium of acetic acid – water over catalyst TiO2 (rutile) at mild conditions (≤100 °C, atmospheric pressure) are compared. The catalyst TiO2 initiates the formation of OH and OOH radicals from H2O2 which promote the oxidative fragmentation of wood lignin. In this case, the peroxide depolymerization of softwood lignin, constructed from phenylpropane units of guaiacyl-type proceeds more difficult than the hardwood lignins, mainly containing syringyl-type units.
The solid and soluble products of peroxide catalytic delignification of wood under the optimized conditions were studied by FTIR, XRD, GC–MS and chemical methods. Regardless of the nature of wood the cellulosic products have a structure similar to microcrystalline cellulose. The soluble products mainly consist of monosaccharides and organic acids. Aromatic compounds are present only in a low amount which indicates the oxidative degradation of aromatic rings of lignin phenylpropane units under the used conditions of wood catalytic delignification.
The processes of thermal dissolution of acetone-lignin and ethanol-lignin from aspen-wood in supercritical ethanol and butanol over Ni-containing catalyst (NiCu/SiO2, NiCuMo/SiO2) are compared. The composition, structure and thermal properties of organosolv lignins were studied with the use of FTIR, GPC, 1H – 13C HSQC NMR, DTA and elemental analysis.
The influence of a composition of Ni-containing catalyst on the thermal conversion in supercritical butanol and ethanol of ethanol-lignin and acetone-lignin was established. The highest conversion of lignins (to 93% wt.) in supercritical alcohols and the highest yield of liquid products (to 90% wt.) were achieved at 300 °C in the presence of catalyst NiCuMo/SiO2.
Scheme of green biorefinery of wood based on the use of non-toxic and low-toxic reagents (H2O2, H2O, acetic acid, ethanol, butanol) and solid catalysts (TiO2, NiCuMo/SiO2) is suggested.
Experimental studies were focused on decomposition of formic acid over carbon nanofibers (CNF) and carbon nanotubes (CNT) to produce pure hydrogen. These carbon materials were shown to provide ...decomposition of formic acid to form preferably hydrogen and carbon dioxide. It was established that structural features of carbon nanomaterials affect their activity in the decomposition of formic acid. The specific catalytic activity of CNF (ini) is at least twice as high as that of CNT (ini). The treatment of CNF (NaOH) with an alkali solution of NaOH is shown to improve noticeably both activity and selectivity of the catalytic decomposition of formic acid. The synthesized catalysts were characterized using X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and adsorption. It was shown by XPS that the alkali treatment of CNF results in the appearance of C–O–Na fragments on the carbon surface.
•CNT and CNF with essentially different structures were synthesized.•The specific catalytic activity of CNF (ini) is at least twice as high as that of CNT (ini).•Alkali-treated CNF have significantly higher catalytic activity in the FA dehydrogenation.•Carbon nanofibers catalyzed FA decomposition to hydrogen and carbon dioxide.•The CNF (NaOH) catalyst is comparable in activity to the 0.2–0.4%Pt/N-graphene catalysts.
We studied the dechanneling length of 150 GeV/cπ− interacting with a short bent silicon crystal. Dechanneling length measures the rate and the strength of incoherent interactions of channeled ...particles in a crystal. The mechanism of dechanneling of negatively charged particles has been elucidated through simulation and experiment. It was found that the dechanneling length for negative particles is comparable to the nuclear dechanneling length for positive charges. Indeed, dechanneling of negative particles occurs as a result of incoherent interactions with the nuclei because the trajectories of such particles always intersect atomic planes, explaining the lower channeling efficiency for such particles. Obtained results can be useful for the design of crystals for manipulating high-energy negative particle beams through channeling.
Polymer monoliths containing open interconnected pores were synthesized by a visible-light induced polymerization of oligocarbonatedimethacrylate (OCM-2) in the presence of dialkylphthalates ...(C6H4C(O)OR2, R = C2H5, n-C4H9, n-C8H17 and n-C9H19) as porogenic agent. A closeness of the refractive indices of the forming polymer and dialkylphthalates provides for the formation of transparent porous monoliths, thus making manufacture of thick porous polymeric materials using visible light possible. The porous polymers derived from OCM-2 can sorb water and benzene. The filling of pores by water increases with an increase of a number of carbon atoms in the ester groups from 0.65 for diethyl phthalate to 1.0 for dinonyl phthalate. Experimentally defined value of Hildebrand solubility parameter δ for poly-OCM-2 is 24.9 MPa1/2. A closeness of the solubility parameters of the polymer (δp) and porogenic agent (δs) guarantees the porous architecture of the polymer formed. The porous polymeric materials have been studied in vitro using a culture of human dermal fibroblasts (HDF) of four passages. The MTT test (MTT is 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) has shown that the materials are not cytotoxic. During long-term cultivation of the cells on the material surface a pronounced cell adhesion as well as cell viability and proliferative activity have been detected.
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•Polymers containing open pores were synthesized by a visible light polymerization.•These materials are not cytotoxic and were studied in vitro using dermal fibroblasts.•The obtained cell scaffolds have a thickness of up to 4 mm.
The study involved a series of experiments on the coking of vacuum residue (VR) and of a mixture of carbon nanotubes (CNTs) and VR in an autoclave at 400–550°C. Using XRD, TEM, and electrical ...resistivity measurements, the properties of petroleum coke (petcoke) and CNT/petcoke composite were investigated and compared with those after calcination in an inert atmosphere at 1000–1100°C. It was found that the coking of a 2.5% CNT/VR mixture reinforces the petcoke with the CNTs. The CNT/petcoke composite exhibits a slightly higher crystallinity than the petcoke. However, the resistivity of the CNT/petcoke composite was found to be more than an order of magnitude lower than that of the petcoke. After high-temperature (1000–1100°C) calcination, the difference in resistivity between the petcoke and the 2.5% CNT/petcoke composite becomes substantially lower. This is associated with a resistivity drop in the petcoke itself, which approximates the resistivity of the carbon nanotubes. It was also identified that the high-temperature calcination reduces the sulfur content in the CNT/petcoke composite, and this reduction is more dramatic than that for the petroleum coke.
The study investigated thermal carbonization of oil tar in comparison with tar carbonization over a Ni/Sibunit catalyst at 450–600°C. The addition of Ni/Sibunit to the tar slightly enhanced the yield ...of coke. However, various sulfur compounds released during tar carbonization interact with metallic nickel to form nickel sulfide, thus rapidly deactivating the catalyst in carbonization that occurs by the carbide-cycle mechanism. To suppress the deactivation of 8%Ni/Sibunit under the impact of sulfur compounds, we hydrocracked the tar sample prior to feeding it to the carbonization. The tar hydrocracking was carried out in a flow-type reactor with a fixed bed of sulfided Ni–Mo/Al
2
O
3
catalyst in a hydrogen atmosphere (the other reaction conditions included 16.0 MPa, 420°C, LHSV 0.5 h
–1
, and a hydrogen/feed ratio of 2000 v/v). The hydrocracking appreciably reduced the content of sulfur and nitrogen in the liquid product. In the presence of 8%Ni/Sibunit, the carbonization of the liquid tar hydrocracking product enhanced the yield of coke compared to that in the thermal carbonization. The catalytic carbonization was found to change the carbon morphology. Specifically, in the 8%Ni/Sibunit addition case, carbon nanofibers 15–40 nm in diameter were detected. In addition to the carbon nanofibers, the formation of amorphous carbon sheets was observed.
The anthracene coking process has been investigated in the temperature range of 400–600°C. It has been shown that intermolecular interaction of two anthracene molecules resulting in the elimination ...of hydrogen and the formation of a C–C bond between the middle rings begins at a temperature of 450°C. Increasing the coking temperature to 500–600°C leads to the formation of poorly crystallized graphite. In the case of pure anthracene, the formation of micron-sized spherical carbon particles occurs. The addition of carbon nanotubes to anthracene leads to the formation of the carbon “coat” covering their surface. The thickness of the carbon “coat” depends on the temperature of coking. An amorphous carbon layer observed on the surface of carbon nanotubes has a thickness of 1–2 nm in the case of coking temperature of 450°C or 10–15 nm in the case of coking at 600°C.
Samples of Pd/C and Pd–Ag/C, where C represents carbon nanofibers (CNFs), are synthesized by methane decomposition on a Ni–Cu–Fe/Al
2
O
3
catalyst. The properties of Pd/CNF are studied in the ...reaction of selective hydrogenation of acetylene into ethylene. It is found that the activity of the catalyst in hydrogenation reaction increases, while selectivity decreases considerably when the palladium content rises. The obtained dependences are caused by the features of palladium’s interaction with the carbon support. At a low Pd content (up to 0.04 wt %) in the catalyst, the metal is inserted into the interlayer space of graphite and the catalytic activity is zero. It is established by EXAFS that the main share of palladium in catalysts of 0.05–0.1 wt % Pd/CNF constitutes the metal in the atomically dispersed state. The coordination environment of palladium atoms consists of carbon atoms. An increase in the palladium content in a Pd/CNF catalyst up to 0.3 wt % leads to the formation of highly dispersed (0.8–1 nm) Pd particles. The Pd/CNF samples where palladium is mainly in the atomically dispersed state exhibit the highest selectivity in the acetylene hydrogenation reaction. The addition of silver to a 0.1 wt % Pd/CNF catalyst initially probably leads to the formation of Pd–Ag clusters and then to alloyed Pd–Ag particles. An increase in the silver content in the catalyst above 0.3% causes the enlargement of the alloyed particles and the palladium atoms are blocked by a silver layer, which considerably decreases the catalytic activity in the selective hydrogenation of acetylene.
Free-radical photopolymerization has been widely used in additive technologies, in particular, stereolithography using single- and two-photon initiated polymerization. The single-photon ...stereolithography affords the objects with about 100 μm resolution. The two-photon stereolithography initiated with a femtosecond near-infrared laser can afford arbitrary 3D microstructures with ultrahigh resolution at micro- and nanoscale level (~100 nm). Herein each of the mentioned method and the mechanisms of single- and two-photon excitation are reviewed. The recent results on the components of the photopolymerizable resin as well as the approaches to decrease the size of the elements of objects and accelerate their formation have been generalizated and systematized.
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