Holey Graphitic Carbon Derived from Covalent Organic Polymers Impregnated with Nonprecious Metals for CO2 Capture from Natural Gas

Natural gas (NG), as a renewable and clean energy gas, is considered to be one of the most attractive energy carriers owing to its high calorific value, low price, and less pollution. Efficiently capturing CO₂ from NG is a very important issue since CO₂ reduces energy density of natural gas and corrodes equipment in the presence of water. In this study, the authors use holey graphene‐like carbon derived from covalent organic polymers (COP) impregnated with nonprecious metals, i.e., COP graphene, as highly efficient separation materials. The dual‐site Langmuir–Freundlich adsorption model based ideal absorbed solution theory is applied to explore the adsorption selectivity. The experimental results along with first principles calculations show Mn‐impregnated COP graphene exhibits greater CO₂/CH₄ selectivity than Fe and Co impregnated materials. Particularly, the selectivity of C–COP–P–Mn reaches 11.4 at 298 K and 12 bars, which are much higher than those in many reported conventional porous materials and can be compared to the highest separation performance under similar condition. Importantly, all the three COP graphene show remarkably high regenerability (R > 77%), which are much better than many reported promising zeolites, active carbon, and metal organic frameworks. Accordingly, COP graphene are promising cyclic adsorbents with high selectivity for separation and purification of CO₂ from natural gas.     

Density functional study of the reaction of O2 with a single site on the zigzag edge of graphene

The reaction between molecular oxygen and an isolated zigzag graphene edge has been studied using density functional theory at the B3LYP/6-31G(d) level of theory. The initial reaction forms a peroxide, ΔH = −135 kJ mol⁻¹. If the graphene edge is pre-oxidised, the dangling peroxy atom can (E_a = 91 kJ mol⁻¹) migrate across contiguous ketone groups until finding another vacant site and stabilizing as a ketone. However, if no further vacant sites are available, the peroxy oxygen has a number of other possibilities open to it, including desorption of an oxygen atom (E_a = 140 kJ mol⁻¹), migration via the basal plane to form a lactone (E_a = 147 kJ mol⁻¹), and direct interaction with an adjacent oxide to form the lactone or a carbonate (E_a = 146 kJ mol⁻¹). The combination of thermal energy and the heat released in the initial formation of the peroxy adduct is likely to be sufficient to overcome these secondary barriers at modest temperatures.Transfer of the dangling peroxy O to the basal plane produces an epoxide that is mobile on the basal surface (E_a = 40–80 kJ mol⁻¹) but that is transferred back to the edge upon coming into proximity of either a vacant edge site or ketone. The instability of the edge epoxide structure implies that it cannot play a significant role in carbon gasification through promoting the reactivity of ketones, contrary to earlier suggestions.The desorption of an oxygen atom creates a very active species capable of reacting with basal or edge sites as well as with oxygen complexes. The reaction of ketone + O has been reported to yield a five-membered ring + CO₂, leading to an overall stoichiometry which is consistent with the observed oxyreactivity of carbon surface oxides identified in isotopic labelling studies in which one O atom is gasified and the other forms a new surface oxide.     

“Rotating” steps in Si(0 0 1) homoepitaxy

Steps on Si(0 0 1) surfaces which are initially not aligned along the high symmetry directions of the dimer reconstruction are observed, by scanning tunneling microscopy, to “rotate” toward [1 1 0] directions during Si growth. This step “rotation” occurs due to a faceting of the step edges. A theoretical analysis of adatom incorporation into the steps shows that this kinetic instability may be caused by a suppressed mobility of the growing species along the S_A step edge.     

Direct FTIR high resolution probe of small and medium size Arn(CO2)m van der Waals complexes formed in a slit supersonic expansion

A slit nozzle continuous expansion of argon seeded with carbon dioxide is probed using a Bruker IFS 120 HR FTIR spectrometer operating at 0.05 cm⁻¹ or 0.01 cm⁻¹ spectral resolution. About 250 individual rovibrational transitions are assigned which belong to the asymmetric stretch of the CO₂ moiety in Ar-CO₂ and (CO₂)₂ complexes. This made it possible to refine the set of spectroscopic constants characterizing these van der Waals species. Analysis of the observed spectral features allowed for evaluation of the number densities of complexes formed in a jet. The manifold of rovibrational lines of van der Waals complexes (along with the monomer lines) sits on an unresolved pedestal, the shape of which varies dramatically as a function of reservoir pressure and initial CO₂-Ar mixing ratios. Thorough examination of these variations allows for the observed features to be assigned to Ar_n(CO₂)_m clusters formed in CO₂ seeded Ar expansion.     

Synthesis of basic magnesium carbonate microrods with a “house of cards” surface structure using rod-like particle template

Basic magnesium carbonate (Mg₅(CO₃)₄(OH)₂·4H₂O) microrods with a surface structure of “house of cards” have been synthesized without any alkaline reagent, using rod-like particles, magnesium carbonate trihydrate, as templates. The product was characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). The transformation process from rod-like MgCO₃·3H₂O particles to Mg₅(CO₃)₄(OH)₂·4H₂O microrods with a surface structure of “house of cards” was recorded. Preliminary discussions on possible growth mechanisms of Mg₅(CO₃)₄(OH)₂·4H₂O microrods are also proposed in this paper.     

Modeling of the adsorption deformation of microporous adsorbents interacting with gases and vapors

A problem of modeling the adsorption deformation is examined. A model describing the elastic adsorption deformation of microporous adsorbents is suggested. Results of modeling of the CO ₂ — AUK microporous carbon adsorbent system and their comparison with the available experimental data are presented for pressures and temperatures changing in wide ranges. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 26–32, November, 2005.     

CO2 adsorption on the bimetallic Zn-on-Cu(1 1 0) system

Adsorption probability measurements (molecular beam scattering) have been conducted to examine the adsorption dynamics (i.e. the gas-surface energy transfer processes) of CO₂ adsorption on the Zn-on-Cu(1 1 0) bimetallic system. The results indicate surface alloy formation, which is in agreement with prior studies. Depositing Zn at 300 K on Cu(1 1 0), above the condensation temperature of CO₂, leads to a “blocking” of CO₂ adsorption sites by Zn which is incorporated in the Cu(1 1 0) surface. This apparent site blocking effect indicates a lowering of the CO₂ binding energy on the alloyed surface as compared with the clean Cu(1 1 0) support. The Zn coverage has been calibrated by Auger electron spectroscopy and thermal desorption spectroscopy.     

NOISE GENERATED BY A COANDA WALL JET CIRCULATION CONTROL DEVICE

An analysis is made of the production of sound by a hydrofoil with a Coanda wall jet circulation control (CC-) device. Three principal sources are identified in the vicinity of the trailing edge of the hydrofoil. The radiation at very low frequencies is dominated by “curvature noise” generated by the interaction of boundary layer turbulence with the rounded trailing edge of the CC-hydrofoil; this is similar in character and magnitude to the low-frequency component of the conventional trailing edge noise produced by a hydrofoil of the same chord, but with a sharp trailing edge. Higher frequency sound is produced principally at the Coanda jet slot. “Passive slot noise” is caused by the “scattering” by the slot lip of nearfield pressure fluctuations in the turbulent boundary layer of the exterior mean flow past the slot. This is of comparable intensity to high frequency, sharp-edged trailing edge noise. However, the acoustic spectrum is greatly extended to much higher frequencies if the Coanda jet is turbulent; the sound produced by the interaction of this turbulence with the lip tends to dominate the spectrum at frequencies f (Hz) greater than about U_j/h, where h is the slot width and U_jthe Coanda jet speed. Sample numerical results are presented for a typical underwater application that indicate that at this and higher frequencies the slot noise can be 20 dB or more greater than conventional trailing edge noise, although the differences become smaller as the thickness of the slot lip increases.     

Passivation of the surface of aluminum nanopowders by protective coatings of the different chemical origin

The results of investigation and analysis of electro-exploded aluminum nanopowders, whose surface were passivated with the following substances: liquids - nitrocellulose (NC), oleic acid (C₁₇H₃₃COOH) and stearic acid (C₁₇H₃₅COOH), suspended in kerosene and ethanol, fluoropolymer; solids - boron and nickel; gases - N₂, CO₂ and air (for a comparison) are discussed. The surface protection for the aluminum nanopowders by coatings of different chemical origins leads to the some advantages of the powders properties for an application in energetic systems, e.g. solid propellants and “green” propellants (Al-H₂O). Aluminum nanopowders with a protected surface showed the increased stability to oxidation in air during the storage period and higher reactivity by heating. The TEM-visual diagram of the formation and stabilization of the coatings on the particles has been proposed on the basis of experimental results. The kinetics of the interaction of aluminum nanopowders with air has been discussed. The recommendations concerning an efficiency of the protective “non-Al₂O₃” layers on aluminum nanoparticles were proposed.     

Influence of crystallization on the spectral features of nano-sized ferroelectric barium strontium titanate (Ba0.7Sr0.3Tio3) thin films

Ferroelectric barium strontium titanate (Ba_0.7Sr_0.3TiO₃)(BST) thin films have been prepared from barium 2-ethylhexanoate [Ba[CH₃(CH₂)₃CH(C₂H₅)CO₂]₂]_, strontium 2-ethylhexanoate [Sr[CH₃(CH₂)₃CH(C₂H₅)CO₂]₂] and titanium(IV) isopropoxide [TiOCH(CH₃)₂]₄ precursors using a modified sol-gel technique. The precursor except [TiOCH(CH₃)₂]₄ were synthesized in the laboratory. Transparent and crack-free films were fabricated on pre-cleaned quartz substrates by spin coating. The structural and optical properties of films annealed at different temperatures have been investigated. The as-fired films were found to be amorphous that crystallized to the tetragonal phase after annealing at 550 °C for 1 h in air. The lattice constants “a” and “c” were found to be 3.974 A and 3.990 A, respectively. The grain sizes of the films annealed at 450, 500 and 550 °C were found to be 30.8, 36.0 and 39.8 nm respectively. The amorphous film showed very high transparency (∼95%), which decreases slightly after crystallization (∼90%). The band gap and refractive index of the amorphous and crystalline films were estimated. The optical dispersion data are also analyzed in the light of the single oscillator model and are discussed.     

Effects of Cl, NO3 and SO4 anions on the anodic behavior of carbon steel in deaerated 0.50 M NaHCO3 solutions

The effects of Cl⁻, NO₃⁻ and SO₄²⁻ aggressive anions on the corrosion and passivation behavior of carbon steel electrode in deaerated 0.50 M NaHCO₃ solutions were studied using potentiodynamic anodic polarization and SEM techniques. It was found that the presence of Cl⁻, NO₃⁻ and SO₄²⁻ anions stimulates the anodic dissolution rate in both the active and the pre-passive potential regions. Moreover, significantly great effects were observed in both the passive and the trans-passive potential regions. Pitting corrosion was observed only in the presence of Cl⁻ anions, while the presence of NO₃⁻ and SO₄²⁻ anions facilitate only passivation by oxygen of water without themselves participating in the cathodic process. Also, it was observed that the effect of NO₃⁻ anion, which is a strong oxidizing agent acting “primarily” as stimulator of the cathodic process and then its reaction product acts “indirectly” retarding the anodic process. On the other hand, the effect of SO₄²⁻ anion, which is a non-oxidizing agent, exerts an “indirect” effect on the cathodic reaction increasing its rate and then “directly” influence on the anodic reaction, retarding it.     

Carbon nanosheets by microwave plasma enhanced chemical vapor deposition in CH4-Ar system

We employ a new gas mixture of CH₄-Ar to fabricate carbon nanosheets by microwave plasma enhanced chemical vapor deposition at the growth temperature of less than 500 °C. The catalyst-free nanosheets possess flower-like structures with a large amount of sharp edges, which consist of a few layers of graphene sheets according to the observation by transmission electron microscopy. These high-quality carbon nanosheets demonstrated a faster electron transfer between the electrolyte and the nanosheet surface, due to their edge defects and graphene structures.     

The stoichiometry and kinetics of carbon combustion at low temperature: A surface complex approach

The stoichiometry and rate of carbon combustion at low temperature (673 K) were investigated. Oxidation and TPD experimental data provide quantification of gaseous products and stable surface complexes over a broad range of conversion. Our analysis distinguishes between surface complexes forming CO and CO₂ and has assumed a certain fraction of each complex type decomposes instantaneously upon formation, leaving the remainder on the surface as stable complexes, C(O) and C(O₂). This analysis suggests that a maximum of 25% of CO-complexes and 89% of CO₂-complexes are unstable upon formation. At low conversion, unstable complex formation is the dominant pathway for the CO product. As conversion increases, decomposition of stable CO-complexes eventually becomes the main source of CO. Formation of unstable CO₂-complexes is the dominant pathway for the CO₂ product at all times. The combustion rate is initially high due to a high availability of vacant active sites, decreases sharply as these sites are filled with stable complexes, and gradually increases as the stable complexes promote CO₂-complex formation, in turn, driving their decomposition. The dynamics of formation and decomposition of C(O) and C(O₂) dictates their ratio on the carbon surface at any moment, which may be measured by TPD. This work may help in developing new kinetic models of carbon combustion which can predict the stoichiometry as well as the rate.     

An optical study of the correlation between growth kinetics and microstructure of μc-Si grown by SiH4-H2 PECVD

Fully microcrystalline silicon, μc-Si, thin films have been deposited on corning glass by plasma enhanced chemical vapor deposition (PECVD) using SiH₄-H₂. The effects of the surface treatment and of the deposition temperature on microstructure of μc-Si films are investigated by “in situ” laser reflectance interferometry (LRI), “ex situ” spectroscopic ellipsometry (SE) and Raman spectroscopy. LRI indicated the existence of a “crystalline seeding time”, which is indicative of the crystallite nucleation, and depends on substrate treatments. Longer “crystalline seeding time” results in a lower density of crystalline nuclei, which grow laterally, yielding to complete suppression of the amorphous incubation layer and to growth of very dense, fully crystalline layer at a growth temperature as low as 120 °C.     

The nano-structural properties of hydrogenated a-Si and Si-C thin films alloys by GISAXS and vibrational spectroscopy

Amorphous hydrogenated silicon (a-Si:H) with high hydrogen content (10-40 at.%), and non-stehiometric silicon-carbon (Si_1−xC_x) thin films with a variation of the carbon to silicon ratio up to 0.3, were deposited by using a magnetron sputtering source. The Si_1−xC_x thin films were partially crystallised after deposition by thermal annealing up to 1050 °C.The GISAXS (Grazing Incidence Small Angle X-ray Scattering) spectra of all of the prepared specimens indicate the presence of “particles” in the “bulk” of the films. For the a-Si:H samples, “particles” are most probably voids agglomerates with a variation in size between 3 and 6 nm. The mean value of the size distribution of the “particles” increases while its width slightly decreases with the hydrogen content in the film. This indicates a better structural ordering which is consistent with the results of Raman spectroscopy that show a decrease of the ratio between intensities of transversal acoustic (TA) and transversal optic (TO) phonon peaks, I_TA/I_TO, and a narrowing of the TO peak with increasing hydrogen content. These results are discussed as a consequence of the beneficial influence of hydrogen bombardment during the film growth.For Si_1−xC_x thin films, the “particles” are assumed to be SiC nano-crystals with a size between 2 and 14 nm and they are larger in films with a higher carbon concentration. Inside each of the films, the crystals are larger closer to surface and they grow faster in the direction parallel to the surface than in that which is perpendicular to it.     

Using of chelating resin to study the kinetic desorption of Eu(III) from humic acid-Al2O3 colloid surfaces

The association of organic-inorganic colloid-borne trace elements was investigated. Radionuclide ^152+154Eu(III) was chosen as a representative and chemical homologue for trivalent lanthanide and actinide ions present in radioactive nuclear waste. Effect of pH and contact time of organic-inorganic/Eu(III) on the kinetic dissociation of Eu(III) from HA-Al₂O₃ colloids was studied. The kinetic desorption behavior of sorbed ^152+154Eu(III) from humic acid-γ-Al₂O₃ colloids was studied at pH values of 4.5 ± 0.2, 5.3 ± 0.2 and 6.5 ± 0.2, respectively, by the addition of the chelating resin. The experimental results suggest that the fractions of irreversible sorption of radionuclide ^152+154Eu(III) to HA-Al₂O₃ colloids increase with increasing pH values, and are independent of aging time. At least two different species, “weak” and “strong” dissociation fractions, are required to simulate the kinetic desorption of ^152+154Eu(III) from HA-Al₂O₃ colloids. The species of Eu(III) sorbed on HA-Al₂O₃ colloids move from “weak” sites to “strong” sites with increasing aging time, whereas the fractions of irreversible sorption are independent of aging time. The results are important for the evaluation of radionuclides’ behavior in the environment.     

The effects of respiratory CO2 fluctuations in the resting-state BOLD signal differ between eyes open and eyes closed

Resting fluctuations in arterial CO₂ (a cerebral vasodilator) are believed to be an important source of low-frequency blood oxygenation level dependent (BOLD) signal fluctuations. In this study we focus on the two commonly used resting-states in functional magnetic resonance imaging experiments, eyes open and eyes closed, and quantify the degree to which measured spontaneous fluctuations in the partial pressure of end-tidal CO₂ (Pet_co2) relate to BOLD signal time series. A significantly longer latency of BOLD signal changes following Pet_co2 fluctuations was found in the eyes closed condition compared to with eyes open, which may reveal different intrinsic vascular response delays in CO₂ reactivity or an alteration in the net BOLD signal arising from Pet_co2 fluctuations and altered neural activity with eyes closed. By allowing a spatially varying time delay for the compensation of this temporal difference, a more spatially consistent CO₂ correlation map can be obtained. Finally, Granger-causality analysis demonstrated a “causal” relationship between Pet_co2 and BOLD. The identified dominant Pet_co2→BOLD directional coupling supports the notion that Pet_co2 fluctuations are indeed a cause of resting BOLD variance in the majority of subjects.     

“Quantization” of the q-vector in microcrystals of K0.3MoO3 and NbSe3

We report steps of conductivity between discrete conducting states in microsamples of quasi-one-dimensional conductors K_0.3MoO₃ and NbSe₃. The steps reveal single phase-slip events, and the discrete states reveal “quantization” of the wave vector q of the charge-density wave. The “quantization” is observed due to the coherence of the CDW within the sample volume and tight boundary conditions at the contacts. The distribution of steps in temperature gives the temperature dependences of the q-vectors, while the steps' height provides the carriers' mobilities. For the case of NbSe₃ we give the 1st direct confirmation of the extremely high mobility of the “pocket” holes at low temperatures.