Porous anodic aluminium oxide (AAO) membranes have various practical applications in separation and purification technologies. Numerous approaches have been utilized to tailor the transport ...properties of porous AAO films, but all of them assumed the isotropic nature of anodized aluminium. Here, the impact of aluminium crystallography on the permeability of AAO membranes is disclosed. A comparative study of AAO membranes formed on low-index aluminium surfaces by anodizing in sulphuric acid electrolyte is presented. The small-angle x-ray scattering technique is used to quantify the out-of-plane arrangement of pores. The AAO grown on the Al(100) substrate possesses a porous structure with a minimum number of point defects and minimum tortuosity of the pores, resulting in the highest permeability of individual gases in the series of AAO membranes obtained on low-index aluminium surfaces. Our results demonstrate how to fabricate AAO membranes with high permeability on the polycrystalline Al foils as well.
We report on the specific properties of O- and OH-terminated Ti3C2Tx nanosheets contributing the permeance of MXene membranes with sorption type selectivity. Thin MXene selective layers on porous ...support demonstrated separation factor for NH3/H2 pair over 50 with ammonia permeance up to 5.0 m3(STP)∙m−2∙bar−1∙h−1 facilitated for humid medium and separation factor for H2O/N2 over 1000 with water permeance over 30 m3(STP)∙m−2∙bar−1∙h−1 close to P0(H2O). According to QCM and GIWAXS studies, high permeance and selectivity of membranes towards ammonia and water vapors are attributed both to ultimate sorption capacity for “basic” vapors and condensation-induced expansion of the interlayer space. Sorption capacity of MXene over 0.045 g∙g(MXene)−1 at 0.3P0 for both NH3 and H2O, significantly exceeds the sorption capacity for permanent gases. Calculations of the diffusion coefficient suggest labyrinthine transport of vapors. For permanent gases no variation of diffusion coefficients was observed with feed pressure, indicating Knudsen diffusion mechanism. Diffusion coefficient of strongly absorbed gases significantly increases with pressure. According to GIWAXS the effect was attributed to the increase of the interlayer distance with saturation of membrane with vapors and reduction of activation barriers for hopping diffusion. Demonstrated characteristics of Ti3C2Tx nanosheets makes it a promising candidate for developing of ammonia selective membranes for Haber-Bosch process membrane extractors.
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•Acidic properties of OH groups on MXene surface determine affinity to “basic” gases.•NH3 sorption coefficient attains 52 mmol/(m3·Pa), drastically exceed sorption for other gases.•Membranes with Ti3C2Tx selective layer were obtained by spin-coating technique.•Selectivity for a NH3/H2 exceeds 50 with ammonia permeance up to 3.91 m3 m−2 bar−1∙h−1.•The increase of interlayer distance with MXene saturation with vapors was founded.
An experimental study on the permeability of anodic alumina (20-120 nm) and track-etched (30 nm) nanoporous membranes for different gases in the transitional flow regime is reported in the range of ...Knudsen numbers from 0.1 to 10. A significant variation (up to 30%) of the membrane permeance for different gases at the same Knudsen numbers is reported with certainty. It is established that this discrepancy relates to a molecule's effective collision area, which is poorly described in the frameworks of conventional gas permeation models. Two models are proposed for the description of the effect: self-diffusion of penetrate gases due to intermolecular collisions and enhancement of the slip flow contribution due to tangential momentum accommodation growth with the decrease of a molecule's effective collision area. The best fit parameters for the simultaneous fit of the experimental data with different models for 30 membrane-gas pairs are given.
Here we report on the spontaneous assembly of Ti
3
C
2
T
x
MXene flakes into monolayer films at the liquid-air interface. According to X-ray reflectivity and grazing incidence X-ray fluorescence both ...the structure of the layers and assembly kinetics depends on the pH value of the solution. At pH > 4 MXene flakes form a single ∼1.5 nm thick layer carrying a negative charge, while in the acidic medium the layer contains coordinated anions with the formation of the Br
aq
−
/Ti
3
C
2
T
x
/subphase interface. The surface layer compression allows the assembling of MXene flakes into a dense monolayer films with the surface coverage of up to 96% and surface pressure exceeding 40 mN m
−1
in the case of the acidic subphase. The films can readily be transferred onto solid substrates by the conventional Langmuir-Blodgett approach or modified by surfactants to form MXene/surfactant composite films.
Here we report on the spontaneous assembly of Ti
3
C
2
T
x
MXene flakes into monolayer films at the liquid-air interface.
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•Membrane contactor performance with alkali absorbent was studied.•Removing of CO2 and H2S from gaseous stream with NaOH solution was studied.•In-liquid diffusion and gas/membrane ...contact time are key factor influence on performance.•Utilization of membrane contactor allows to reduce H2S content down to 7 ppm.•Conditions for selective H2S removing in presence of CO2 was found, allowing to attain H2S/CO2 selectivity >1500.
The paper reports a comprehensive review of the performance of nanoporous gas-liquid polypropylene membrane contactor with NaOH absorbents for removal of H2S and CO2 components from the gas streams. The experiments at different operation conditions, including variation of acid gas content in a feed stream, alkali concentration in absorbent, gas and liquid absorbent volume flow rates, absolute and gas-liquid differential pressures are discussed as a function of the absorbent saturation levels. In-liquid diffusion of components and gas/membrane contact times were determined as main governing factors limiting contactor efficiency. Mass transfer rates of acid gas removal on the membrane contactor over 3 × 10−3 mol/(m2 × s) for CO2 and 7.5 × 10−3 mol/(m2 × s) for H2S were attained. Ultimate processed gas quality with H2S content below 5 ppm and CO2 content below 0.01% was achieved at the contactor performance over 7 m3/(m2 × h) and initial acidic gas content of 2%, while achieving a membrane packing density in the contactor over 3000 m2/m3. The paper also provides an experimentally-proven theoretical model for calculating removal efficiency and residual acid gas partial pressures depending on the membrane parameters and operation conditions. It is shown, the mass transfer coefficient and removal efficiency differ significantly for H2S and CO2 due to the difference in dissolution mechanism involving kinetically limited deprotonation reaction of solvated water in case of CO2(aq) while engaging direct deprotonation of H2S. This allows to attain residual partial pressure of H2S in retentate stream equal to the equilibrium pressure above the absorbent solution, while CO2 residual pressure exceeds an equilibrium value few orders of magnitude. The effect has been successfully utilized for the selective removal of H2S from both CO2 and H2S-containing mixtures with H2S/CO2 selectivity exceeding 1500.
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•Anodic alumina membranes modification performed using octadecylphosphonic acid.•Surface modification was proved by IR- and Raman spectroscopy and thermal analysis.•Modification ...allows to control molecule residence time in adsorbed stage.•For hydrocarbons TMAC coefficients depend on surface saturation of grafted layer.•Modification allows to achieve n-C4H10/CH4 separation factors up to 32.3.
Here we report evidence for substantial changes in the separation efficiency of nanoporous anodic alumina membranes with nanochannel diameters ranging from 10–100 nm modified with octadecylphosphonic acid in the transitional flow regime. Softening of surface by alkyl groups with a surface density ∼2 groups/nm2 leads to a general permeance decrease in 3–500 times, depending strongly on the penetrant gas nature and the channels diameter. The divergence of the permeance, for different gases, increases with the decreasing diameter of the pores. For a surface-functionalized membrane, with 10-nm channel diameters, it results in n-C4H10/CH4 ideal and mixed gas separation factors up to 32.3 and 9.0 respectively at a n-C4H10 permeance up to 3.5 m3/(m2·bar·h). The effect is related to the changes of the ratio of molecule travelling time to residence time in the adsorbed state, as well as a strong influence of surface saturation by the absorbed molecules on the tangential momentum accommodation coefficient, which is supported by the derived model. Synergetic contribution of these two factors allows to enhance the separation factor of permanent and condensable gases strongly beyond the Knudsen limit, while maintaining a high permeance of porous membranes.
We propose a one-step procedure to stabilize mesoporous silicon nanoparticles in aqueous solutions by polyethylene glycol (PEG) coating during grinding in a planetary ball mill. The milling is done ...in aqueous medium that allows us to directly obtain the aqueous suspension of PEGylated nanoparticles. The prepared nanoparticles are investigated by means of the scanning electron microscopy, energy dispersive X-ray spectroscopy, low-temperature nitrogen sorption, dynamic light scattering, Fourier transform infrared spectroscopy, Raman and photoluminescence spectroscopy, which reveal the nanoparticle size of 50–100 nm, preservation of the nanocrystallinity and mesopores. The PEGylated nanoparticles are found to be stable in aqueous solution for at least 24 h. The proposed PEGylation method can be used to control the physical properties and stability of mesoporous silicon nanoparticles for biomedical applications.
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•PEG-coated mesoporous silicon nanoparticles were formed by grinding in water.•Raman spectroscopy reveals nanocristalline structure of PEG-coating mesoporous silicon nanoparticles.•PEG-coated mesoporous silicon nanoparticles exhibit enhanced photoluminescence.•PEGylation is a one-step way of regulating the dissolution of silicon nanoparticles.•PEGylation of silicon nanoparticles is promising for use of latter as nanocontainers.
Abstract
Here we report a successful one-step synthesis of vanadium-doped anatase mesocrystals by reactive annealing of NH
4
TiOF
3
/PEG2000 mesocrystal precursors with NH
4
VO
3
. The formation ...solid solution Ti
1−
x
V
x
O
2
with vanadium content up to
x
= 25 at% inheriting the structure of mesocrystals is observed for the first time. The doping mechanism via vapor phase transport of vanadium is proposed. The Ti
1−
x
V
x
O
2
mesocrystals exhibit improved specific capacity of 175 mAh g
−1
(compared to 150 mAh g
−1
for pure anatase phase) and decreased potential gap between charge and discharge processes.
Mass-transport properties of anodic alumina membranes exploited in a number of technological areas are strongly affected by the real pore structure and arrangement of channels that can split or ...terminate during the anodization process. This paper focuses on the investigation of pore branching and rearrangement caused by voltage variation in the course of the anodic oxidation of aluminum. Gas-transport measurements were utilized for the quantitative determination of an effective through porosity of multilayer anodic alumina membranes with branched channels obtained by variation of anodization voltage. It was shown that on decrease of anodization voltage a branching of pores occurs, while an increase of anodization voltage leads to the termination of some of the pores with an increase in the diameter of others. Gas permeance measurements combined with electron microscopy unambiguously prove dead-end pore formation on voltage increase, while no pore merging appears. This generally affects any mass-transport properties and applications of anodic alumina membranes as the delivery of any species (e.g. ions, gas molecules, etc) through the blocked channels is impossible.
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