The diffusion of methane in inorganic membrane pores under Knudsen conditions: the kinetics and depth of pyrocarbon deposition and its influence on transport properties

The kinetics of the topochemical reaction of methane dehydrogenation was studied using TRUMEM ultrafiltration membranes (TiO₂ + Cr₂O₃ on porous steel, the size of transport pores 50 nm). The depth of the deposition of pyrocarbon nanocrystallites (PNC) into pores was determined. The depth of PNC deposition was estimated using scanning electron microscopy and high-resolution energy-dispersion spectrometry. The deposition of PNC at a 4.9 kPa methane pressure in the reaction zone created Knudsen conditions for methane diffusion in pores. The deposition of PNC therefore occurred over the whole area of pore surfaces. Studies of the kinetics of PNC formation on the surface of pores showed that reaction rate V and its constant k substantially depended on reaction duration. The influence of PNC deposition on the electrosurface properties and permeability of the membranes to ethanol and dodecane was studied. After the deposition of PNC with L _c = 1.0–1.1 nm on the surface of pores, the ζ-potential and surface charge density (σ) decreased; simultaneously, the efficiency with respect to ethanol increased.     

A new method of modification of inorganic membranes with pyrocarbon nano-sized crystallites

Gas-separating membranes were obtained according to an original method of directed modification of ultra filtration membranes having 50 nm pores by means of pyrocarbon nano-sized crystallites (PNCs). For the first time, the size of the pores was reduced not along the entire depth of the selective layer but only at the pore orifice, increasing their productivity by several orders of magnitude. PNC deposition was done at 800 and 650°C using methane and propane as a pyrolyzed gas at 90–100 kPa, respectively. The depth of PNC deposition was determined by scanning electron microscopy and energy dispersive spectrometry. It was demonstrated that at the used pressure, the depth of deposition was 1.5 μm, i.e., PNCs were deposited in the pore orifices. Gas permeability analysis for a membrane with r _pore ～ 0.8 nm allowed us to estimate the Knudsen ratio and surface diffusion. The Knudsen and surface diffusion fluxes were estimated. The advantages and challenges of the proposed approach are discussed; these are mainly related to the thermal stability of the porous structure and the compatibility of the thermal expansion coefficients of the porous substrate and deposited PNCs. An analysis of the obtained results is presented, along with an outline for further studies.     

Hydrogen adsorption in the series of carbon nanostructures: Graphenes–graphene nanotubes–nanocrystallites

A comparative analysis of hydrogen absorption capability is performed for the first time for three types of carbon nanostructures: graphenes, oriented carbon nanotubes with graphene walls (OCNTGs), and pyrocarbon nanocrystallites (PCNs) synthesized in the pores of TRUMEM ultrafiltration membranes with mean diameters (D_m) of 50 and 90 nm, using methane as the pyrolized gas. The morphology of the carbon nanostructures is studied by means of powder X-ray diffraction, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM). Hydrogen adsorption is investigated via thermogravimetric analysis (TGA) in combination with mass-spectrometry. It is shown that only OCNTGs can adsorb and store hydrogen, the desorption of which under atmospheric pressure occurs at a temperature of around 175°C. Hydrogen adsorption by OCNTGs is quantitatively determined and found to be about 1.5% of their mass. Applying certain assumptions, the relationship between the mass of carbon required for the formation of single-wall OCNTGs in membrane pores and the surface area of pores is established. Numerical factor Ψ = m_dep/m_calc, where m_dep is the actual mass of carbon deposited upon the formation of OCNTGs and mcalc is the calculated mass of carbon necessary for the formation of OCNTGs is introduced. It is found that the dependence of specific hydrogen adsorption on the magnitude of the factor has a maximum at Ψ = 1.2, and OCNTGs can adsorb and store hydrogen in the interval 0.4 to 0.6 < Ψ < 1.5 to 1.7. Possible mechanisms of hydrogen adsorption and its relationship to the structure of carbon nanoformations are examined.     

Preparation and characterization of activated carbon fibers from liquefied wood

Wooden activated carbon fibers (WACFs) were prepared from phenolated Chinese fir (Cunninghamia lanceolata) using CO₂ activation; microstructure characterization, the adsorption capacity, BET-specific surface area, and pore distribution of WACFs were investigated by SEM and X-ray analysis. Results showed that WACFs have a smooth surface and round or elliptical cross-section. The (002) crystal plane diffraction peak of the WACFs was obviously heightened, also showing an apparent (100) diffraction peak. With increased activation temperature, the value of d ₍₀₀₂₎ gradually decreased, whereas the values of the crystallite sizes L _a and L _c initially decreased and then increased. The L _c/d ₍₀₀₂₎ and g values corresponding to the degree of change in the graphitization structure increased. WACFs mainly have micropores as well as a few macro- and mesopores. The micropore diameter of WACFs has a narrow range (0.3–0.5 nm). With increased activation temperature, the single-point surface area, Brunauer-Emmett-Teller surface area, micropore area, single-point total pore volume, and micropore volume of WACFs increased, while the pore diameter decreased. At 900 °C, the iodine adsorption and yield rate of WACFs were 779.22 mg/g and 51.48 %, respectively.     

X-ray spectroscopy study of lithiated graphite obtained by thermal deposition of lithium

X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy are used for in situ studies of the electronic structure of lithiated natural graphite produced by thermal deposition of lithium upon graphite in a vacuum. By XPS and NEXAFS spectroscopy it is found that lithium vapor thermal deposition results in the formation of a lithiated graphite surface layer and a change in its electronic structure. Based on the quantum chemical simulation of the experimental СK_α XES spectrum of lithiated graphite, it is found that lithium atoms are located mostly on the edges of graphite crystallites. Atomic force microscopy reveals that the size of natural graphite flakes varies from 50 nm to 200 nm.     

Noncatalytic hydrogenation of naphthalene in nanosized membrane reactors with accumulated hydrogen and controlled adjustment of their reaction zone volumes

As part of ongoing studies aimed at designing the next generation of nanosized membrane reactors (NMRs) with accumulated hydrogen, the noncatalytic hydrogenation of naphthalene in pores of ceramic membranes (TRUMEM ultrafiltration membranes with D _av = 50 and 90 nm) is performed for the first time, using hydrogen preadsorbed in a hybrid carbon nanostructure: mono- and multilayered oriented carbon nanotubes with graphene walls (OCNTGs) that form on inner pore surfaces. In this technique, the reaction proceeds in the temperature range of 330–390°C at contact times of 10–16 h. The feedstock is an 8% naphthalene solution in decane. The products are analyzed via chromatography on a quartz capillary column coated with polydimethylsiloxane (SE-30). It is established for the first time that in NMRs, the noncatalytic hydrogenation of naphthalene occurs at 370–390°C, forming 1,2,3,4-tetrahydronaphthalene in amounts of up to 0.61%. The rate constants and activation energy (123.5 kJ/mol) of the noncatalytic hydrogenation reaction are determined for the first time. The possibility of designing an NMR with an adjustable reaction zone volume is explored. Changes in the pore structure of the membranes after their modification with pyrocarbon nanosized crystallites (PNCs) are therefore studied as well. It is shown that lengthening the process time reduces pore size: within 23 h after the deposition of PNCs, the average pore radius (r _av) falls from 25 to 3.1 nm. The proposed approach would allow us to design nanoreactors of molecular size and conduct hydrogenation reactions within certain guidelines to synthesize new chemical compounds.

     

Chemical bath deposition of thin nanocrystalline tin(II) sulfide films with thioacetamide

Nanocrystalline tin(II) sulfide layers 100–650 nm thick were prepared by hydrochemical deposition on glass–ceramic supports from a citrate system at 323?343 K using thioacetamide as chalcogenizing agent. These films are of interest for the development of thin-film solar radiation converters based on the multicomponent compound Cu₂ZnSnS₄ of kesterite structure, prepared using a cost-saving process. Examination by scanning electron microscopy shows that tin(II) sulfide layers are formed from spherical nanocrystallites of 20–40 nm size. X-ray diffraction analysis shows that they crystallize in the orthorhombic system with the unit cell parameters a = 4.276, b = 11.243, and c = 3.986 Å. Analysis by X-ray photoelectron spectroscopy revealed the presence of up to 44.86 at. % oxygen in the surface layer of the film. This oxygen is mainly present in surface contaminants and is also incorporated in SnO present on the surface.     

The depth of nanocrystallites of pyrocarbon deposition in pores of ultrafiltration membranes and its influence on their efficiency

Scanning electron microscopy and energy dispersion spectrometry were used to determine the depth of the deposition of pyrocarbon nanocrystals (PNCs) on the pore surface of TRUMEM inorganic ultrafiltration membranes (D _pore = 50 nm) depending on the pressure of the gas subjected to pyrolysis (methane). It was shown that, while the deposition of PNCs occurred over the whole pore length equal to the selective layer thickness (20 μm) at 4.9 kPa methane pressure, an increase in pressure to 40.0 kPa decreased the deposition depth to 5.0 μm. Simultaneously, the λ/D ratio decreased from 94 to 11. At a pressure increased to 100.0 kPa, the depth of PNC deposition was ～1.5 μm (λ/D = 5). Changes in the depth of PNC deposition and the λ/D ratio depending on pressure were similar in character and could be described by exponential dependences. With PNCs deposited ～1.5 nm deep (100 kPa), membranes with pore size 8–17 nm were obtained. Their efficiency, for instance, for ethanol, was 50–75% of that of the initial membranes. The diameter of pores decreased by 3–6 times.     

Surface properties of the Ni-silica gel catalyst precursors for the vegetable oil hydrogenation process: N2 sorption and XPS studies

The effect of the type of the silica gel pore structure on the surface properties of the Ni-silica gel catalyst precursors for the vegetable oil hydrogenation process has been examined applying N₂ sorption and X-ray photoelectron spectroscopy techniques. The nickel catalyst precursors with identical composition (SiO₂/Ni = 1.0) has been synthesized by precipitation of Ni(NO₃)₂ · 6H₂O solution with Na₂CO₃ solution on the three types of silica gel with different pore structures. It is shown that the usage of the silica gel supports with different texture as source of SiO₂ causes different location of Ni-species into the support pores and on the external surface area. The XPS data confirm the formation of surface species with different strength of interaction and different dispersion. These surface characteristics of the precursors will predetermine the formation of the active nickel metallic phase as well as the mass transfer of the reactants and products to and from the catalytic sites.     

Electrochemical polymerization of polyaniline doped with Zn2+ as the electrode material for electrochemical supercapacitors

Polyaniline doped with Zn²⁺ (PANI/Zn²⁺) films was synthesized by cyclic voltammetric method on stainless steel mesh substrates in 0.2 mol L^?1 aniline and 0.5 mol L^?1 sulfuric acid electrolyte with various concentrations of zinc sulfate (ZnSO₄·7H₂O). The structure and morphology of PANI and PANI/Zn²⁺ films were characterized by Fourier transform infrared, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy techniques, respectively. The electrochemical properties of PANI and PANI/Zn²⁺ films were investigated by cyclic voltammetry, galvanostatic charge–discharge test, and electrochemical impedance spectroscopy in 0.5 mol L^?1 H₂SO₄ electrolyte in a three-electrode system. The results show that the surface morphology of PANI/Zn²⁺ is more rough than that of pure PANI. The specific capacitance of the PANI/Zn²⁺ film displays a larger specific capacitance of 738 F g^?1, lower resistance, and better stability as compared with the pure PANI film. Thus, good capacitive performance demonstrates its potential superiority for supercapacitors.     

Preparation of porous TiO2/silica composites without any surfactants

TiO₂-SiO₂ composites, with high specific surface area (up to 308 m²/g), large pore volume, and narrow distribution with average pore sizes of 3.2 nm, have been synthesized from wollastonite and titanium sulfate in the absence of any surfactants. Calcium sulfate, a microsolubility salt, plays an important role in the formation of pores in this porous TiO₂/silica composite. The microstructure and chemical composition of composite were characterized by X-ray diffractometry (XRD), transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectrometer (XPS) and N₂ adsorption and desorption analysis. The as-prepared porous titanium dioxide-silicon dioxide composites with high specific surface area and well-crystallized anatase contents were used as an efficient photocatalyst.     

酸法纳米纤维素模板剂合成介孔TiO2及光催化活性

以生物可再生资源的酸法纳米纤维素为模板剂,四氯化钛为钛源,采用液相水解-沉淀法制备了具有介孔结构的TiO2光催化剂。采用低温N2物理吸附-脱附、透射电镜(TEM)、X射线衍射(XRD)、热重-量热扫描(TG/DSC)、傅里叶变换红外(FTIR)、X射线光电子能谱(XPS)等对介孔TiO2进行了表征,并以甲基橙为模型物,考察了介孔TiO2光催化活性。结果表明,以酸法纳米纤维素为模板剂合成的TiO2光催化活性显著提高,且具有良好的孔隙结构,平均孔径5.03 nm、总孔容积0.35 cm3.g-1、比表面积192m2.g-1;纤维素模板剂合成的TiO2表面羟基数量降低;纤维素长链分子结构之间的羟基与TiO2表面羟基的键合,可有效限制TiO2前驱体的生长和团聚,并抑制锐钛矿相TiO2向金红石相转变。     

Improvement of pore geometry and performances of poly(ethylene terephthalate) track membranes by a protective layer method using plasma-induced graft polymerization of 1H,1H,2H-perfluoro-1-octene monomer

The process of geometrical modification of pores in poly(ethylene terephthalate) track-etched membranes (TM) by use of plasma deposition of a fluorine-containing polymer protective layer on one membrane surface and alkali etching of the other surface has been studied in order to produce membranes with improved performance characteristics. Samples of membranes with conical pores have been obtained which have better filtration efficiency compared with initial TM with cylindric pores. Plasma polymerization of 1H,1H,2H-perfluoro-1-octene at the membrane surface was used to produce the protective layer resistant to alkali solutions. The occurrence of plasma modification and changing of pore geometry have been verified by X-ray photoelectron spectroscopy and scanning electron microscopy studies. The filtration efficiency and selectivity of the modified membranes have been studied.     

An XPS and STM Study of Oxidized Platinum Particles Formed by the Interaction between Pt/HOPG with NO<Subscript>2</Subscript>

X-ray photoelectron spectroscopy (XPS) (with AlK_α and AgL_α radiations) and scanning tunneling microscopy (STM) were used to study the interaction of two model samples prepared by vacuum evaporation of platinum on highly oriented pyrolytic graphite (HOPG) with NO₂ at room temperature. According to STM data, platinum evaporation on the graphite surface produced particles of a flattened shape. In the Pt/HOPGS1 sample with a lower concentration of platinum, the average diameter of particles d and the height-to-diameter ratio h/d were 2.8 nm and 0.29, respectively. In the Pt/HOPG-S2 sample with a higher concentration of platinum, the average values of d and h/d were 5.1 nm and 0.32. When the samples interacted with NO₂ (P ≈ 3 × 10^–6 mbar), the particles of metallic platinum completely converted to the particles of PtO_2· Upon oxidation, the shape of larger platinum particles in the Pt/HOPG-S2 sample did not change, although for the dispersed particles in the Pt/HOPG-S1 samples under these conditions, the h/d ratio increases. The reduction of oxide to metal particles on heating the Pt/HOPG-S1 sample in vacuum at 460°С is accompanied by an increase in the size of particles. Their shape became more round compared to the initial one. It was found that X-ray radiation affects the state of platinum in the oxidized sample by reducing the surface layer of PtO₂ to PtO.     

A spiral-like chain from a hydrogen-bonded cyclic dichloride containing a water dimer in a quaternary diammonium dichloride trihydrate

The reaction of benzyl chloride with tetramethylethylenediamine (tmen) results in the formation of the quaternary diammonium dichloride trihydrate (dbtmen)Cl₂·3H₂O 1 (dbtmen is N,N′-dibenzyl-N,N,N′,N′-tetramethylethylenediammonium) in good yields. 1 crystallises in the monoclinic P2₁/c space group and its structure consists of N,N′-dibenzyl-N,N,N′,N′-tetramethylethylenediammonium dication, two chloride anions and three crystal water molecules all of which are located in general positions. The organic dication is H-bonded to the chloride anions and the crystal waters with the help of intra-and intermolecular C-H···Cl and C-H···O interactions, while the chloride anions are linked to the crystal waters via O-H···Cl interactions. One of the crystal waters is linked through an intermolecular O-H···O bond with another water resulting in the formation of a water dimer. The O-H···Cl and O-H···O interactions between the chloride anions and water molecules lead to the formation of a five-membered {O₃Cl₂} cyclic dichloride containing a water dimer. The five-membered rings are linked into a chain with the aid of a O-H···Cl interaction. The organic cations are organised in zigzag fashion on either side of the chain and are further linked to the anionic water chain via weak C-H···O and C-H···Cl interactions, leading to the supramolecular organisation of the rings into a spiral-like of chain. Dedicated to Prof. Sabyasachi Sarkar on the occasion of his 60^th birthday     

Porous α-Fe2O3 hollow microspheres and their application for acetone sensor

Porous α-Fe₂O₃ hollow microspheres were synthesized through a simple and efficient carbon sphere template method. The samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy and N₂ adsorption-desorption. Structural characterization indicated that as-prepared α-Fe₂O₃ hollow microspheres had porous structure with around 200 nm in diameter and thin shell about 10 nm thick. The average pore size and Brunauer-Emmett-Teller specific surface area of α-Fe₂O₃ hollow microspheres were 6.5 nm and 111.6 m²/g, respectively. The gas sensing behavior investigation showed that as-synthesized α-Fe₂O₃ hollow microspheres exhibited very good gas sensing property to acetone vapor.     

Remarkable shifts of Csp2-H and O-H stretching frequencies and stability of complexes of formic acid with formaldehydes and thioformaldehydes

Thirty-six stable complexes of formic acid with formaldehydes and thioformaldehydes were determined on the potential energy surface, in which the XCHO···HCOOH complexes are found to be more stable than the XCHS···HCOOH counterparts, with X = H, F, Cl, Br, CH₃, NH₂. All complexes are stabilized by hydrogen bonds, and their contribution to the total stabilization energy of the complexes increases in going from C-H···S to C-H···O to O-H···S and finally to O-H···O. Remarkably, a significant blueshift of C_sp2-H bond by 81–96 cm⁻¹ in the C_sp2-H···O hydrogen bond has hardly ever been reported, and a considerable redshift of O-H stretching frequency by 206–544 cm⁻¹ in the O-H···O/S hydrogen bonds is also predicted. The obtained results in our present work and previous literatures support that a distance contraction and a stretching frequency blueshift of C-H bond involving hydrogen bond depend mainly on its polarity and gas phase basicity of proton acceptor, besides the rearrangement of electron density due to complex formation. Markedly, we suggest the ratio of deprotonation enthalpy to proton affinity (R _c) as an indicator to prospect for classification of hydrogen bonds. The symmetry adapted perturbation theory results show a larger role of attractive electrostatic term in XO-n as compared to that in XS-n and the electrostatic interaction is overwhelming dispersion or induction counterparts in stabilizing XO-n and XS-n , with n = 1, 2, 3. © 2019 Wiley Periodicals, Inc.     

Characterization and evaluation of the photocatalytic properties of wormhole-like mesoporous silica incorporating TiO2, prepared using different hydrothermal and calcination temperatures

TiO₂?CSiO₂ mesoporous materials were synthesised by deposition of TiO₂ nanoparticles prepared by the sol?Cgel method on to the internal pore surface of wormhole-like mesoporous silica. In this work we synthesised wormhole-like mesoporous silica of different surface area by changing the hydrothermal temperature (70, 100, or 130?°C). Subsequent to this, titania solution was deposited on to the inner surface of the pores and this was followed by calcination at different temperatures (400, 600, or 800?°C). The effect of different hydrothermal and calcination temperature on the photocatalytic properties was evaluated. The samples were characterized by N₂-sorption, X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy. The effect of different hydrothermal and calcination temperatures on the photocatalytic properties was evaluated by measuring the degradation of methylene blue in aqueous solution under UV light irradiation (mercury lamp, 125?W). The results indicated that appropriate surface area and degree of crystallinity are two important factors for obtaining high photocatalytic efficiency. Samples prepared at a hydrothermal temperature of 100?°C and calcined at 800?°C had the best photocatalytic performance, because of the highest surface area and high crystallinity.     

Stepwise synthesis, characterization, and electrochemical properties of ordered mesoporous carbons containing well-dispersed Pt nanoparticles using a functionalized template route

A stepwise method is described for the accurately controlled growth of Pt nanoparticles supported on ordered mesoporous carbons (Pt-OMC) by the nanocasting of carbon and metal precursors in the pore channels of mesoporous silicas functionalized with Si-H groups. Results obtained from N₂ adsorption/desorption isotherms and transmission electron microscopy showed well-dispersed Pt nanoparticles (2-3 nm) on Pt-OMC with high surface area (837 m² g⁻¹) and regular pore channels (2.9 nm), which facilitate reactant/product diffusion. X-ray diffraction and X-ray photoelectron spectroscopy indicated that Pt nanoparticles in the Pt-OMC sample were mostly present in the metallic form of a face-centered cubic (fcc) crystalline structure. The Pt-OMC catalyst was found to have superior electrocatalytic properties during oxygen reduction reaction as compared to typical commercial electrocatalysts.     

Modification of polysulfone membranes 4. Ammonia plasma treatment

The effect of NH₃ and NH₃/Ar plasma on ultrafiltration polysulfone membranes have been studied. Results of contact angle, FTIR-ATR and X-ray photoelectron spectroscopy experiments clearly showed that both plasmas introduced hydrophilic, nitrogen- and oxygen-containing moieties on the polymer surface and that NH₃/Ar plasma was more efficient. That plasma was also more aggressive--signs of strong etching could be seen on the SEM pictures. Redeposition of etched material seemed to take place inside the pores. On the contrary, ammonia plasma was soft and caused cleaning the surface and pores enlargement. Performance of ammonia plasma modified membranes was greatly improved and independent on solution pH. The last observation proved amphoteric character of the surface. NH₃/Ar plasma treatment gave membranes of acidic surface and filtration indices not so good as for ammonia plasma.