A simple method for determining the neutralization point in Boehm titration regardless of the CO2 effect

We report a simple method by which the neutralization point in the Boehm titration can be easily determined without going through a pre-screening process to remove the effect of atmospheric carbon dioxide (CO₂). The proposed method is based on the principle that the equivalence and the corresponding neutralization point of the reaction bases remains unchanged regardless of the dissolution of CO₂ in the reaction bases. This method was used to measure the surface functionality of acid-treated multi-walled carbon nanotubes with high precision.     

High-crystallization polyoxymethylene modification on carbon nanotubes with assistance of supercritical carbon dioxide: Molecular interactions and their thermal stability

As a typical engineering plastic and high-crystallization polymer, polyoxymethylene (POM) has been successfully wrapped on single-walled carbon nanotubes (SWCNTs) using a simple supercritical carbon dioxide (SC CO₂) antisolvent-induced polymer epitaxy method. The characterization results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reveal that the SWCNTs are coated by laminar POM with the thicknesses of a few nanometers. The polymer adsorption on CNTs via multiple weak molecular interactions of CH groups with CNTs has been identified with FTIR and Raman spectroscopy. The experimental results indicate that the decorating degree of POM on the surface of CNTs increases significantly with the increase of SC CO₂ pressure, and accordingly the dispersion of SWCNT modified by POM at higher pressure are more excellent than that of obtained at lower pressure. Further the processing stability of POM/CNTs composites are investigated by differential scanning calorimetry and thermogravimetric analysis. The experimental results obtained show that their thermal stability behavior is closely related to surface properties of CNTs. Apparently, the composites with POM-decorating SWCNTs as the filler shows higher melting points compared to the POM composites with pristine SWCNTs as the filler. Therefore, we anticipate this work may lead to a controllable method making use of peculiar properties of SC CO₂ to help to fabricate the functional CNTs-based nanocomposites containing highly crystalline thermoplastic materials such as POM.     

Carbon nanotube/carbon nanofiber growth from industrial by-product gases on low- and high-alloy steels

The growth of carbon nanotubes (CNTs) on sheet metal surfaces (including low- and high-alloyed steel and Ni-plated steel) has been explored using a mixture of CO, CO₂, and H₂ as the precursor feedstock in a thermal chemical vapor deposition process. The influence of various experimental parameters such as the reactor temperature, reaction time, and precursor composition on the yield, purity, and dimensions of the CNTs has been elucidated. Addition of CO₂ during CNT growth leads to higher carbon deposition rates, especially for low- and high-alloyed steel. The diameters of the obtained CNTs range from 12 to 300 nm at carbon deposition rates of ～0.3 mg cm^?2 min^?1. The CNTs are observed to be uniformly distributed and adhered firmly to the substrates. The experimental conditions for CNT growth on sheet metal surfaces are very similar to concentrations and temperatures of a typical effluent stream of the steel industry. This process thus holds potential to harness waste gases to fabricate CNT-based coatings that impart added functionality to sheet metals, while further reducing the carbon footprint of steel plants.     

Adsorption of binary CO2/CH4 mixtures using carbon nanotubes: Effects of confinement and surface functionalization

Effect of confinement and surface functionalization in carbon nanotubes (CNTs) on the competitive adsorption of a binary CO₂/CH₄ mixture has been investigated by grand canonical Monte Carlo simulations. Adsorption using CNTs with different functionalization arrangements, different diameters, different functionalization degrees, and different functional groups, such as –COOH, –CO, –OH, –CH₃, is investigated. Effects of (a) the pore textural properties, such as pore size and accessible surface area, and (b) the gas–adsorbent interaction, especially the electrostatic interaction, are discussed. From these results, we discuss the impact that variables such as confinement and surface functionalization have on the performance for CO₂ separation.     

Effects of the catalyst pretreatment on CO2 sensors made by carbon nanotubes

A novel CO₂ sensor was made by carbon nanotubes (CNTs). The CNTs were synthesized by catalystic thermal chemical vapour deposition at 700 °C. Prior to the synthesis, the Fe catalysts were pretreated by H₂ plasma for different times. Two terminal resistance of the as-grown CNTs mat was measured under different CO₂ concentrations. It was found that without the catalyst pretreatment, the sensitivity was about 4% when the CNTs mat was exposed to 800 mTorr CO₂ concentration. However, with various catalyst pretreatment times of 5, 10, 15 and 20 min, the sensitivity was 3.69%, 6.27%, 9.54%, and 12.1%, respectively. The Raman spectroscopy showed the I_D/I_G decreased from 0.668 to 0.539 as the catalyst pretreatment time increased. The XPS also showed the correlation of surface chemical components with the Raman spectroscopy. The Fe catalyst H₂ plasma pretreatment affected both the graphitization and surface binding sites of CNTs.     

Quest for high performance carbon capture membranes: Fabrication of SAPO-34 and CNTs-doped polyethersulfone-based mixed-matrix membranes

Gas separation process is an effective method for capturing and removing CO₂ from post-combustion flue gases. Due to their various essential properties such as ability to improve process efficiency, polymeric membranes are known to dominate the market. Trade-off between gas permeability and selectivity through membranes limits their separation performance. In this study, solution casting cum phase separation method was utilized to create polyethersulfone-based composite membranes doped with carbon nanotubes (CNTs) and silico aluminophosphate (SAPO-34) as nanofiller materials. Membrane properties were then examined by performing gas permeation test, chemical structural analysis and optical microscopy. While enhancing membranes CO₂ permeance, SAPO-34 and CNTs mixture improved their CO₂/N₂ selectivity. By carefully adjusting membrane casting factors such as filler loadings. Using Taguchi statistical analysis, their carbon capture efficiency was improved. The improved mixed-matrix membrane with loading of 5 wt% CNTs and 10 wt% SAPO-34 in PES showed highly promising separation performance with a CO₂ permeability of 319 Barrer and an ideal CO₂/N₂ selectivity of 12, both of which are within the 2008 Robeson upper bound. A better mixed-matrix membrane with outstanding CO₂/N₂ selectivity and CO₂ permeability was produced as a result of the synergistic effect of adding two types of fillers in optimized loading.     

Toughening effect of carbon nanotubes and carbon nanofibres in epoxy adhesives for joining carbon fibre laminates

The effect of carbon nanotubes (CNTs) and carbon nanofibres (CNFs) on mode I adhesive fracture energy (G_IC) of double cantilever beam (DCB) joints of carbon fibre-reinforced laminates bonded with an epoxy adhesive has been studied. It was observed that the presence of carbon nanofillers in the epoxy adhesive results in a significant increase in the propagation value of mode I adhesive fracture energy with CNTs producing the largest increase. The toughening mechanisms, analysed using scanning electron microscopy (SEM), for the two nanofiller systems differed: pull-out with CNFs, and pull-out and crack bridging with CNTs. At the macroscopic level there was also a change in the failure mode, with an increased proportion of delamination occurring in the nanoreinforced joints in comparison with the unreinforced. Two different surface treatments were also applied to the laminates: grit blasting and atmospheric plasma. The highest fracture energy was obtained in the grit blasted joints.     

Aligned carbon nanotubes fabricated by thermal CVD at atmospheric pressure using Co as catalyst with NH3 as reactive gas

Co is used as a catalyst for chemical vapor deposition (CVD) of vertically aligned multi-walled carbon nanotubes (CNTs) in a tube furnace at atmospheric pressure. C₂H₂ and NH₃ were used for the carbon feedstock and reaction control, respectively. The CVD process parameters determine the chemical properties of the Co particles and subsequently the morphologies and field emission behavior of CNTs as they strongly depend upon the catalyst condition. The flow rate ratio of NH₃ to C₂H₂ is shown to be central to the synthesis of vertically aligned CNTs. Repeatable synthesis of vertically aligned CNTs at atmospheric pressure in a tube furnace is cost effective for large area deposition of such structures which may be used, for example, in vacuum field emission devices.     

Coating carbon nanotubes with metal oxides in a supercritical carbon dioxide-ethanol solution

Metal oxide films, including Ce₂O₃ and/or CeO₂, Al₂O₃, La₂O₃, were deposited on the outer surfaces of carbon nanotubes (CNTs) through the decomposition of metal nitrate precursors in supercritical CO₂ modified with ethanol. Transmission electron microscopy showed that CNTs could be coated with metal oxide layers that were nominally complete and uniform. The thickness of the coating could be readily tailored by tuning the ratio of the initial mass of precursors to CNTs. The as-prepared CeO₂-CNT composites showed high sensitivity and selectivity to acetone on the basis of chemiluminescence detection.     

A study of the electrical properties of carbon nanotube-NiFe2O4 composites: Effect of the surface treatment of the carbon nanotubes

Novel carbon nanotube-NiFe₂O₄ composite materials have been prepared for the first time by in situ chemical precipitation of metal hydroxides in ethanol in the presence of carbon nanotubes (CNTs) and followed by hydrothermal processing. The obtained composite powders were characterized using XRD, TEM and EDS. The effect of surface oxidation treatment of CNTs on their properties was investigated by FTIR, zeta potential and hydrodynamic radius distribution characterization. Electrical conductivity measurements show that surface oxidation treatment of CNTs can improve the electrical conductivity of the composites more pronouncedly than pristine CNTs do. With 10 wt.% addition of surface treated CNTs, the electrical conductivity is increased by 5 orders of magnitude. The surface oxidized CNTs are crucial for this significant increase in electrical conductivity, which provides strong adhesion between the nanotubes and the matrix to give a homogeneous carbon nanotube-NiFe₂O₄ composite.     

Adsorption of single-ringed N- and S-heterocyclic aromatics on carbon nanotubes

Adsorption of single-ringed N- and S-heterocyclic aromatics on single-walled carbon nanotubes (SWCNTs) was examined to explore the potential of using carbon nanotubes (CNTs) as drug carriers and environmental adsorbents. Adsorbates included pyrimidine, 2-aminopyrimidine, 4,6-diaminopyrimidine, thiophene, benzene and aniline. Adsorbents included pristine SWCNTs, oxidized SWCNTs, and nonporous graphite. Adsorption of N- and S-heterocyclic aromatics was significantly enhanced by non-hydrophobic interactions. Particularly, the -NH₂-substituted compounds exhibited much stronger (up to 2 orders of magnitude) adsorption affinities to oxidized SWCNTs than benzene, even though they are much less hydrophobic. The π-π coupling or electron donor-acceptor (EDA) interactions are likely adsorption-enhancement mechanisms for all six compounds. The lone-pair electrons of the N heteroatoms and the -NH₂ group can enable n-π EDA interactions with SWCNT surfaces. Lewis acid-base interactions are another significant adsorption-enhancement mechanism for the -NH₂-substituted compounds (and possibly for pyrimidine) on SWCNTs. For the N-heterocyclic aromatics, adsorption affinity is highly dependent on the O-functionality of the SWCNT surface and on solution pH, due to the speciation reactions of both adsorbates and SWCNT surface O-functional groups, indicating that selective adsorption of N-heterocyclic aromatics is possible by combining the surface functionality of CNTs and solution chemistry.     

Improvements in interfacial and heat-resistant properties of carbon fiber/methylphenylsilicone resins composites by incorporating silica-coated multi-walled carbon nanotubes

The multi-scale reinforcement and interfacial strengthening on carbon fiber (CF)-reinforced methylphenylsilicone resin (MPSR) composites by adding silica-coated multi-walled carbon nanotubes (SiO₂-CNTs) were investigated. SiO₂-CNT has been successfully prepared via the hydrolysis of tetraethoxysilane in the presence of acid-oxidized multi-walled carbon nanotubes. Transmission electron microscopy, X-ray diffraction, and Fourier Transform infrared spectroscopy were carried out to examine the functional groups and structures of CNTs. Then, SiO₂-CNT was incorporated into MPSR matrix to prepare CF/MPSR-based composites by the compression molding method. The effects of the introduced SiO₂-CNT on the interfacial, impact, and heat-resistant properties of CF/MPSR composites were evaluated by short-beam bend method, impact test, and thermal oxygen aging experiments, respectively. Experimental results revealed that the CF/MPSR composites reinforced with 0.5 wt% SiO₂-CNT showed a significant increase 34.53% in the interlaminar shear strength (ILSS) and 20.10% in impact properties. Moreover, the heat-resistant properties of composites were enhanced significantly by adding SiO₂-CNT hybrid nanoparticles. These enhancements are mainly attributed to the improved matrix performance resulted from the molecular-level dispersion of SiO₂-CNT in MPSR matrix and the strong interfacial adhesion between SiO₂-CNT and matrix resin, which are beneficial to improve the mechanical stress transfer from MPSR matrix to CFs reinforcement and alleviate stress concentrations.     

Carbon dioxide-assisted fabrication of self-organized tubular carbon micropatterns on silicon substrates

Hierarchical three-dimensional (3D) tubular micropatterns made of carbon nanotubes (CNTs) are fabricated on silicon substrates by catalytic decomposition of a ferrocene-cyclohexane mixture at 850 °C in the presence of CO₂. It is found that the catalyst concentration, temperature and the presence of CO₂ are key factors that govern the assembly and growth of CNTs. The self-assembled patterns of catalysts in the initial stage are responsible for the formation of CNT patterns in which a multi-level self-assembly is involved. The potential use of the tubular CNT micropatterns as electrode in the electroanalysis of biomolecules (dopamine) has been demonstrated.     

The release of free standing vertically-aligned carbon nanotube arrays from a substrate using CO2 oxidation

Free standing vertically-aligned carbon nanotube (CNT) arrays were released from a quartz substrate on a large scale by using CO₂ as an oxidative reagent to weaken the array-substrate interaction and facilitate their harvest. During the oxidation process, amorphous carbon and carbon in contact with metallic particles were preferentially etched, leading to the easy release of high quality free standing CNT arrays. O₂ was also used as an oxidant for comparison. The O₂ showed a stronger oxidizability and caused the formation of CNTs with a turbostratic-graphitic carbon heterojunction structure. The mechanisms of CO₂ and O₂ oxidation of CNT arrays were investigated, and based on these results, CO₂ was considered a more suitable oxidant for the purpose.     

Superhydrophobicity of a three-tier roughened texture of microscale carbon fabrics decorated with silica spheres and carbon nanotubes

A stable superhydrophobic surface with low contact angle hysteresis using microscale carbon fabrics decorated with submicroscale silica (SiO₂) spheres and carbon nanotubes (CNTs) is created. Without any surface treatment, superhydrophobicity is achieved, and a microsized water drop can be suspended on the three-tier roughened surface, leaving an air film underneath the droplet. A modified Cassie–Baxter model analyzes that the combined effect of SiO₂ spheres and CNTs contributes a high area fraction of a water droplet in contact with air, leading to superhydrophobicity. Such a three-tier surface texture has robust superhydrophobic properties.     

Using supercritical carbon dioxide in preparing carbon nanotube nanocomposite: Improved dispersion and mechanical properties

Improvements in carbon nanotube (CNT) dispersion and subsequent mechanical properties of CNT/poly(phenylsulfone) (PPSF) composites were obtained by applying the supercritical CO₂ (scCO₂)‐aided melt‐blending technique that has been used in our laboratory for nanoclay/polymer composite preparation. The preparation process relied on rapid expansion of the CNTs followed by melt blending using a single‐screw extruder. Scanning electronic microscopy results revealed that the CNTs exposed to scCO₂ at certain pressures, temperatures, exposure time, and depressurization rates have a more dispersed structure. Microscopy results showed improved CNT dispersion in the polymer matrix and more uniform networks formed with the use of scCO₂, which indicated that CO₂‐expanded CNTs are easier to disperse into the polymer matrix during the blending procedure. The CNT/PPSF composites prepared with scCO₂‐aided melt blending and conventional melt blending showed similar tensile strength and elongation at break. The Young's modulus of the composite prepared by means of conventional direct melt blending failed to increase beyond the addition of 1 wt% CNT, but the scCO₂‐aided melt‐blending method provided continuous improvements in Young's modulus up to the addition of 7 wt% CNT. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Electrospun melamine-blended activated carbon nanofibers for enhanced control of indoor CO2

Electrospun carbon nanofibers were activated with melamine–polyacrylonitrile [melamine-blended carbon nanofibers (MACNFs)] for use as a fibrous adsorbent for indoor CO₂ removal. Although, melamine doping was intended solely to incorporate basic nitrogen functionalities on the nanofibers, it also shortened fabrication time, conserving time, and energy cost. The specific surface area and microporosity of the fibers were enhanced from 412 m² g⁻¹ and 0.1646 cm³ g⁻¹ to 547 m² g⁻¹ and 0.220 cm³ g⁻¹, respectively, upon final CO₂ activation of the nanofibers. With the chemical properties, we observed significant tethering of pyridine functionality. The sample, MACNF-7 (10 mL of polymer solution doped with 0.7 g of melamine), provided the optimum melamine doping condition to achieve the highest CO₂ adsorption capacity of 3.15 mmol g⁻¹. The adsorption performance was based on simultaneous improvement in microporosity (physical) and surface basicity (chemical) properties of the adsorbent. However, in a binary mixture with nitrogen, the selective adsorption of CO₂ showed the predominance of the improved surface basicity over microporosity. The highest CO₂ selective capture (1.22 mmol g⁻¹) was occurred for a CO₂:N₂ ratio of 0.15:0.85, with a selectivity of 58.19 at 273 K. In a regeneration test, stable and robust performance was achieved more than five cycles. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47747.     

Effect of H2S on the CO2 corrosion of carbon steel in acidic solutions

The objective of this study is to evaluate the effect of low-level hydrogen sulfide (H₂S) on carbon dioxide (CO₂) corrosion of carbon steel in acidic solutions, and to investigate the mechanism of iron sulfide scale formation in CO₂/H₂S environments. Corrosion tests were conducted using 1018 carbon steel in 1 wt.% NaCl solution (25 °C) at pH of 3 and 4, and under atmospheric pressure. The test solution was saturated with flowing gases that change with increasing time from CO₂ (stage 1) to CO₂/100 ppm H₂S (stage 2) and back to CO₂ (stage 3). Corrosion rate and behavior were investigated using linear polarization resistance (LPR) technique. Electrochemical impedance spectroscopy (EIS) and potentiodynamic tests were performed at the end of each stage. The morphology and compositions of surface corrosion products were analyzed using scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results showed that the addition of 100 ppm H₂S to CO₂ induced rapid reduction in the corrosion rate at both pHs 3 and 4. This H₂S inhibition effect is attributed to the formation of thin FeS film (tarnish) on the steel surface that suppressed the anodic dissolution reaction. The study results suggested that the precipitation of iron sulfide as well as iron carbonate film is possible in the acidic solutions due to the local supersaturation in regions immediately above the steel surface, and these films provide corrosion protection in the acidic solutions.     

Growing carbon nanotubes on patterned submicron-size SiO2 spheres

Growing carbon nanotubes (CNTs) perpendicularly to the surface of submicron-size SiO₂ spheres by pyrolyzing iron(II) phthalocyanine (FePc) is reported for the first time in this paper. The large curvature isolates the nanotubes and forms unique structures. The density, lengths and morphology of CNTs on SiO₂ spheres can be controlled by varying the experimental conditions. A method of growing CNTs on patterned SiO₂ spheres on conducting surface by photolithography is further developed based on the selective growth of CNTs. This may offer an effective way to control the density of patterned, aligned CNTs on conducting substrates for various applications, particularly for field emission.     

Alkaline carbon nanotubes as effective catalysts for H2S oxidation

Carbon nanotubes (CNTs) were made alkaline by impregnation with Na₂CO₃ and used for the direct oxidation of H₂S into sulfur at 30 °C. The alkaline CNTs before and after H₂S oxidation were characterized by N₂ adsorption, scanning and transmission electron microscopy, X-ray diffraction, and thermogravimetry analysis. The effects of Na₂CO₃ loading and the structure of the CNTs on the catalytic activity of alkaline CNTs were investigated. Results indicated that the saturation sulfur capacity of alkaline CNTs was up to 1.86 g H₂S/g catalyst, which was about 3.9 times higher than that of a common commercial H₂S oxidation catalyst. The introduction of Na₂CO₃, which provided alkalinity needed for H₂S dissociation, significantly enhanced the catalytic performance. The optimum content of Na₂CO₃ loading was determined to be 20 wt.% in the CNTs. The catalytic performance was also dependent on the structure of the CNTs, and the single-walled CNTs with the smallest tube diameter exhibited the highest sulfur capacity. Tangled CNTs provided their external voids to store the sulfur produced, which was a key feature of this high-performance CNT catalyst.