Adsorption Of Polar Molecules Research Articles

Study on the adsorption mechanism of polar and non-polar VOCs by the activated carbon with surface oxygen

In this study, CO2 was converted to activated carbon by the molten salt CO2 capture and electrochemical conversion (MSCC-EC) method, and the surface oxygen content of the activated carbon was further modified for the adsorption of polar molecules (chlorobenzene) and nonpolar molecules (toluene), the role of the pore structure and surface O-containing functional groups of the adsorbents in contributing to the adsorption process was systematically investigated. The results showed that MS-AC500-10, MS-AC500-15, and MS-AC550-15 have rich pore structures with specific surface areas of 811.56 m2/g, 835.35 m2/g, 1249.104 m2/g, and surface oxygen contents of about 10 %, 15 %, 15 %, respectively. The toluene adsorption capacity of MS-AC500-10, MS-AC500-15, and MS-AC550-15 was 203.82 mg/g, 229.72 mg/g, and 358.97 mg/g, respectively, and was dominated by pore adsorption, which was positively proportional to the specific surface areas and pore volumes, the adsorption process belonged to the pseudo-first-order kinetic model. The adsorption amounts of chlorobenzene by MS-AC500-10, MS-AC500-15, and MS-AC550-15 were 287.98 mg/g, 363.11 mg/g, and 495.41 mg/g, respectively. In addition to the pore adsorption, the oxygen content on the surface of the adsorbents had a non-negligible promotion effect on the adsorption of chlorobenzene, mainly because chlorobenzene is a polar molecule, and the asymmetric electrons on its surface have strong interactions with the O-containing functional groups on the surface of the adsorbents. Moreover, the prepared adsorbents were thermally stable, and the decay rate of adsorption performance was less than 5 % after five adsorption–desorption cycles.

Experimental and Theoretical Evidence for Distorted Tetrahedral ≡Ti–OH Sites Supported on Amorphous Silica and Their Effect on the Adsorption of Polar Molecules

Experimental and Theoretical Evidence for Distorted Tetrahedral ≡Ti–OH Sites Supported on Amorphous Silica and Their Effect on the Adsorption of Polar Molecules

Investigation of the Mechanisms of CO2/O2 Adsorption Selectivity on Carbon Materials Enhanced by Oxygen Functional Groups.

Power plant flue gas and industrial waste gas are produced in large quantities. Using these as feedstocks for CO2 electroreduction has important practical significance for the treatment of excessive CO2 emissions. However, O2 in such sources strongly inhibits the electrochemical conversion of CO2. The inhibitory effect of O2 can be mitigated by constructing CO2-enriched regions on the surface of the cathode. In this study, the reaction zone was controlled by the selective adsorption of CO2 on oxygen-functionalized carbon materials. The results of quantum chemical simulations showed that CO2 adsorption was mainly influenced by electrostatic interactions, whereas O2 adsorption was completely regulated by dispersion interactions. This distinction indicated that introducing polar oxygen functional groups at the edge of the carbon plane can significantly enhance the selectivity for CO2/O2 adsorption. The difference in the adsorption energy between CO2 and O2 increased most noticeably after the carboxyl groups were introduced. The results of the adsorption experiments showed that oxygen-functionalization increased the CO2/O2 selectivity of the carbon material under an atmosphere of multicomponent gases by more than 4.9 times. The carboxyl groups played a dominant role. Our findings might act as a reference for the selective adsorption of polar molecules over nonpolar molecules.

Investigation on Adsorption of Polar Molecules in Vegetable Insulating Oil by Functional Fossil Graphene.

As a new engineering dielectric, vegetable insulating oil is widely used in electrical equipment. Small polar molecules such as alcohol and acid will be produced during the oil-immersed electrical equipment operation, which seriously affects the safety of equipment. The polar molecule can be removed by using functional fossil graphene materials. However, the structural design and group modification of graphene materials lack a theoretical basis. Therefore, in this paper, molecular dynamics (MD) and quantum mechanics theory (Dmol3) were utilized to study the adsorption kinetics and mechanism of graphene (GE), porous graphene (PGE), porous hydroxy graphene (HPGE), and porous graphene modified by hydroxyl and carboxyl groups (COOH-HPGE) on polar small molecules in vegetable oil. The results show that graphene-based materials can effectively adsorb polar small molecules in vegetable oil, and that the modification of graphene materials with carboxyl and hydroxyl groups improves their adsorption ability for polar small molecules, which is attributed to the conversion of physical adsorption to chemical adsorption by the modification of oxygen-containing groups. This study provides a theoretical basis for the design and preparation of graphene materials with high adsorption properties.

Porous reduced graphene oxide for ultrasensitive detection of nitrogen dioxide

The defect engineering in graphene plays a significant role for the application of gas sensors. In this work, we proposed an efficient method to prepare ultrasensitive gas sensors based on the porous reduced graphene oxide (PRGO). Photo-Fenton etching was carried out on GO nanosheets in a controlled manner to enrich their vacancy defects. The resulting porous graphene oxide (PGO) was then drop-coated on interdigital electrodes and hydrothermal reduced at 180 °C. Controllable reduction was achieved by varying the water amount. The gas sensor based on PRGO-5 min-6 h exhibited superior sensing and selective performance toward nitrogen dioxide (NO2), with an exceptional high sensitivity up to 12 ppm−1. The theoretical limit of detection is down to 0.66 ppb. The excellent performance could be mainly attributed to the typical vacancy defects of PRGO. Some residue carboxylic groups on the edges could also facilitate the adsorption of polar molecules. The process has a great potential for scalable fabrication of high-performance NO2 gas sensors.

Surface thermodynamics and Lewis acid-base properties of metal-organic framework Crystals by Inverse gas chromatography at infinite dilution

In this study, the surface thermodynamic properties and more particularly, the dispersive component γsd of the surface energy of crystals of a Zr-based MOF, UiO-66 (Zr6O4(OH)4(BDC)6; BDC = benzene 1,4-dicarboxylic acid), the specific interactions, and their acid-base constants were determined by using different molecular models and inverse gas chromatography methods. The determination of γsd of the UiO-66 surface was obtained by using several models such as Dorris-Gray and those based on the Fowkes relation by applying the various molecular models giving the surface areas of n-alkanes and polar organic molecules. Six models were used: Kiselev, spherical, geometric, Van der Waals, Redlich-Kwong, and cylindrical models. The obtained results were corrected by using our model taking into account the thermal effect on the surface areas of molecules. A linear equation was obtained between γsd and the temperature.The specific free energy, enthalpy and entropy of adsorption of polar molecules, as well as the acid and base constants of UiO-66 particles were determined with an excellent precision.It was also proved that the UiO-66 surface exhibited an amphoteric acid-base character with stronger acidity. The linear variations of the specific free energy of interaction as a function of the temperature allowed to obtain the specific surface enthalpy and entropy of adsorption, as well as the acid and base constants of UiO-66 by using ten different models and methods. The best results were obtained by using our model that gave the more precise values of the acid constant KA=0.57, the base constant KD=0.18 of the MOF particles and the ratio KA/KD = 3.14 clearly proving a strong acid character of the UiO-66 surface.

Electrostatic Field Enhanced Photocatalytic CO2 Conversion on BiVO4 Nanowires

HighlightsBiVO4 nanowires with exposed (010) facets were deposited on the piezoelectric transducer piezo-substrate.CO2 conversion rate was significantly improved by 5.5-time in an electrostatic field generated by applying a stress on the piezo-substrate.The mole percentage of methane in products was increased from 29% to 64%.The improved performance was attributed to the facilitated photo-carriers separation/transfer, as well as the enhanced adsorption of polar molecules.

Novel Graphene Wool Gas Adsorbent for Volatile and Semivolatile Organic Compounds.

Volatile and semivolatile organic compounds in ambient air and occupational settings are of great concern due to their associated adverse human health and environmental impacts. Novel graphene wool samplers have been developed and tested to overcome limitations of commercially available sorbents that can only be used once and typically require solvent extraction. Graphene wool (GW) was synthesized by non-catalytic chemical vapor deposition with optimized conditions, resulting in a novel fibrous graphene wool that is very easy to manage and less rigid than other forms of graphene, lending itself to a wide range of potential applications. Here, the air pollutant sampling capabilities of the GW were of interest. The optimal packing weight of GW inside a glass tube (length 178 mm, i.d. 4 mm, o.d. 6 mm) was investigated by the adsorption of vaporized alkane standards on the GW, using a condensation aerosol generator in a temperature-controlled chamber and subsequent detection using a flame ionization detector. The optimized GW packing density was found to be 0.19 mg mm–3 at a flow rate of 500 mL min–1, which provided a gas collection efficiency of >90% for octane, decane, and hexadecane. The humidity uptake of the sampler is less than 1% (m/m) for ambient humidities <70%. Breakthrough studies showed the favorable adsorption of polar molecules, which is attributed to the defective nature of the graphene and the inhomogeneous coating of the graphene layers on the quartz wool, suggesting that the polar versus non-polar uptake potential of the GW can be tuned by varying the graphene layering on the quartz wool substrate during synthesis. Oxidized domains at the irregular edges of the graphene layers, due to a broken, non-pristine sp2 carbon network, allow for adsorption of polar molecules. The GW was applied and used in a combustion sampling campaign where the samplers proved to be comparable to frequently used polydimethylsiloxane sorbents in terms of sampling and thermal desorption of non-polar semivolatile organic compounds. The total alkane concentrations detected after thermal desorption of GW and PDMS samplers were found to be 17.96 ± 13.27 and 18.30 ± 16.42 μg m–3, respectively; thus, the difference in the alkane sampling concentration between the two sorbent systems was negligible. GW provides a new, exciting possibility for the monitoring of organic air pollutants with numerous advantages, including high sampling efficiencies, simple and cost-effective synthesis of the thermally stable GW, solvent-free and environmentally friendly analysis, and, importantly, the reusability of samplers.

New Methodology to Study the Dispersive Component of the Surface Energy and Acid-Base Properties of Silica Particles by Inverse Gas Chromatography at Infinite Dilution.

A new methodology was proposed to determine the dispersive component of the surface energy ${\gamma}_s^d$ of a solid taking into account the effect of the temperature on the surface area of n-alkanes, methylene group (${a}_{- CH2-}$) and polar molecules, thus defeating the method used by Dorris-Gray Schultz etal. We determined the correct ${\gamma}_s^d$ of the surface energy, the specific free energy, enthalpy and entropy of adsorption of polar molecules as well as the acid base constants of silica particles with an excellent accuracy. We confirmed the dependence of the dispersive component of the surface energy on the variations of the surface areas of organic molecules used in IGC technique at infinite dilution. The specific properties of interactions of silica particles were determined. The new proposed model took into account this thermal effect. Obtained results proved that the other used IGC methods gave inaccurate values of the specific parameters of silica surface, except for the vapor pressure method that led to excellent results of the specific free energy, enthalpy and entropy of adsorption, and the acid-base constants of the silica particles.

Ion-induced oil–water wettability alteration of rock surfaces. Part II: Base interactions between oil and solid

Following the first part of this study, herein, we focus on the oil-wet rock surfaces formed by the adsorption of polar molecules via base interactions. The effects of ions on the oil–water wettability of quartz surfaces and the mechanism behind the phenomenon are revealed through contact angle measurements and surface characterization. The results indicate that the ions can greatly alter the wettability of the surface to become more water-wet only when the salt solutions are alkaline, with a maximum decrease of approximately 20° in the water contact angle. NaCl and KCl solutions are more effective than CaCl2 and Na2SO4 solutions. In alkaline solutions, the hydroxyl ions can deprotonate the octadecylamine molecules to weaken their interactions with the surfaces, such that the cations can displace the adsorbed octadecylamine and lead to large desorption from the surfaces. Finally, a quantitative model is established to describe the ion-induced wettability alteration.

Energy of adsorption of polar molecules on NaLSX zeolite

The article presents precision data of isotherms and total thermodynamic characteristics, i.e. ΔH, ΔF and ΔS of H2O adsorption on NaLSX zeolite. The stepwise nature of the change in the heat of adsorption on the studied systems with surface filling is revealed, and the molecular mechanism of water adsorption on NaLSX zeolite in the entire filling area is revealed. It was found that the polar water molecule is adsorbed in the NaLSX zeolite in an amount of 31.8 H2O/1/8 e.u., of which 25.3 are in super cavities, and 6.5 are in β-cavities.

A Fouling-Resistant Voltammetric Sensing System for Wearable Electroactive Biomarker Monitoring

Voltammetry-based sensing systems are suitable for wearable monitoring of electroactive biomarkers in epidermally-retrievable biofluids (e.g., sweat), as they can directly transduce the redox features of the targets into electrical signals, delivering a sample-to-answer solution. To render reliable voltammetric readouts, the fundamental challenge of biofluid-induced fouling (causing rapid and irreversible signal degradation) must be addressed–an issue which has been overlooked in the previously reported wearable sensor development methods, but widely reported in conventional voltammetric platforms. To this end, here, we devise a fouling-resistant voltammetric sensing interface that exploits an inert electrode surface (specifically, boron-doped diamond, facilitating weak adsorption of polar molecules) as well as a hydrophilic membrane coating (chitosan) to mitigate the interaction of fouling agents with the surface. Utilizing this engineered sensing interface, sub-micromolar level quantification of uric acid was demonstrated, where the sensor presented minimal interference from non-target electroactive species (within their physiologically relevant concentration range). By interfacing the developed sensor with a wireless circuit board–augmented with a dedicated analytical framework–seamless uric acid readout acquisition in sweat was achieved. The presented biofouling characterization/mitigation methodology provides a basis for wearable electroactive sensing system development. [2020-0199]

On the capture of polar indoor air pollutants at sub-ppm level—A molecular simulation study

Adsorption of polar molecules, such as formaldehyde, ammonia, and water, is profoundly affected by both the pore structure and the functional groups (FGs) of an adsorbent. Investigating the effects of pore structures and FGs separately is important but intractable for experiments and the conventional modeling. Molecular simulation provides a powerful tool to probe the adsorption mechanism by extracting abundant microscopic details. In present study, we have conducted a Monte Carlo simulation of formaldehyde, ammonia, and water, aiming to offer new insights into the capture of polar indoor air pollutants by adsorption. Upon the adsorption isotherms of formaldehyde and ammonia in the carbon nanopores at sub-ppm level, the effects of pore size and the FGs were separately investigated using the isosteric heats at zero loading as a measure of guest–host interaction. Based on the analysis of the pairwise potential energies of formaldehyde, ammonia, and water as well as their cluster-mediated adsorption mechanisms, we proposed that water may act as strong anchors for capturing formaldehyde and ammonia, with the premise that water does not condense and block the pore. Further discussion on the development of effective water-resistant adsorbent for the polar indoor air pollutants was presented.

Impact of high adsorbent conductivity on adsorption of polar molecules: simulation of phenol adsorption on graphene sheets

The high conductivity of nanoprous carbons has a significant effect on adsorption of polar molecules; however, the mechanisms underlying this effect are not well-understood, and this effect is generally not considered in adsorption modeling. To investigate the impact of high host conductivity on the adsorption properties of vertical stacks of the graphene sheet, phenol vapor has been chosen as a simple polar adsorptive for our case studies. The influence of high surface conductivity of graphene is taken into account during Grand Canonical Monte Carlo (GCMC) simulations by considering the resulting image charges, and using the corresponding analytical solution to compute the electrostatic energy of the polar molecules confined between two infinite parallel conducting planes. We find that the consideration of the high conductivity affects the atomic configuration of the adsorbed molecules, based on our results for two different pore widths of 1 nm and 0.65 nm. Allowing for the high conductivity results in more stable energy levels, greater heat of adsorption and smaller distances between phenol molecules and the graphene sheet. There results are shown to be in accord with electronic density functional theory calculations, and literature experimental data. The contributions of different guest–guest (phenol–phenol) and guest-host (phenol and graphene) interactions between molecules are studied.

Effects of inorganic salt ions on the oil-water wettability of quartz surfaces

The oil-water wettability in reservoir rock plays an important role in the recovery of crude oil. However, the mechanism of wettability transition on reservoir rock surfaces after low salinity water injection is still controversial. In this paper, the influence of inorganic salt ions in water on the wettability of the rock surfaces with the adsorption of polar molecules and its mechanisms were studied. Amines commonly found in crude oil were selected to represent base polar components in oil, and a highly idealized experimental model was used to exclude the influence of various components and other effects in crude oil. By measuring the oil-water contact angle on the quartz surface, the influence of various salt ion concentration in water on the wettability of quartz in the presence of base polar molecule in oil was obtained, and the mechanism of wettability alteration was analyzed by Zeta potential and AFM measurement. The experiment result showed that, the cleaned surface of quartz was water-wet, when 0.1 mmol/L octadecylamine was added to decane, the contact angle rose to about 42°, that is, the surface of the quartz became more oil-wet. When salts with different kinds and concentrations were added to water, the water contact angles decreased sharply with the increase of the concentration of ions. The contact angles of NaCl solutions and KCl solutions were basically the same at the same concentrations. However, at low concentrations, the CaCl2 solutions reduced the contact angles more than the NaCl solutions. The contact angles of Na2SO4 solutions were smaller than those of NaCl solutions at low concentrations, while the contact angles of Na2SO4 solutions were similar to those of NaCl solutions at higher concentrations. When the ion concentrations reached a certain level, the contact angles of different solutions all tended to be about 15°, which indicated that the ions desorbed basically all of the adsorbed polar molecules, leading to the change of wettability of quartz surfaces to be water-wet. The clean quartz powder was highly electronegative when dispersed in pure water. After soaking in octadecylaminedecane, the surface of quartz powder was positively charged. As a contrast, the Zeta potential of the quartz powder after soaking in pure decane changed little, indicating that the potential change was caused by the adsorption of positive octadecylamine on the quartz surface. When the quartz powder soaked with octadecylaminedecane was reversely extracted into NaCl solution, the Zeta potential of the quartz powder would decrease, and the electropositivity would weaken. In high concentration NaCl solution, the Zeta potential of the quartz powder would quickly return to negative charge, indicating the desorption effect of ions in the solution on polar molecules. From the AFM roughness analysis, it can be seen that the surface roughness of clean quartz was small, after soaking in octadecylaminedecane, the roughness increased, indicating that octadecylamine was fully adsorbed on the surface of quartz. After the soaking of droplet, the surface roughness of quartz plate decreased, indicating that the partial desorption of octadecylamine under the effect of ions. Based on the Zeta potential measurement of quartz powder and AFM roughness analysis of quartz surfaces, two mechanisms were proposed: (1) The octadecylamine in decane adsorbed and protonated at oil-water interface, resulting in an electrostatic attraction with the anion, thus reducing the oil-water interfacial tension; (2) the cations in water could be adsorbed on the negatively-charged quartz surfaces, weakening the polarity of quartz surfaces and thus replacing part of adsorbed octadecylamine molecules on the quartz surfaces.

Multiscale study on ammonia adsorption by Li-Doped COF-10

Four types of Li-doped COF-10 have been designed and noted as COF-10-1Li, COF-10-2Li, COF-10-4Li, and COF-10-6Li. COF-10 and COF-10-1Li have been used to adsorb both ammonia and hydrogen, and results indicate that incorporation of lithium brings about more enhancement for the adsorption of polar molecules. Ammonia adsorption on all the Li-doped COF-10 has been investigated. More lithium means higher adsorption capacity; however, the promotion of adsorption amounts is not proportional to the number of lithium atoms. Difference in adsorption capacities between COF-10-1Li and COF-10-2Li is slight. While six lithium atoms are included, a considerable promotion can be witnessed. The possible reason is that the positive potential shield on the surface of COF-10-6Li protects ammonia from repulsion from negative charges on the inner skeleton. These results suggest that not only charge amounts but also charge distribution have significant influence on adsorption behaviors.

Adsorption of Polar Molecules on a Solid Substrate

Abstract—A simple scheme for estimating the adsorption energy of polar molecules on solid state substrates has been proposed within the Harrison bond orbitals method. Analytical expressions are obtained for the ionic and metallic components of the adsorption energy for diatomic and tetraatomic molecules. The adsorption of GaCl and AlCl3 on a semiconductor substrate is considered as an example.

The effect of atomic point charges on adsorption isotherms of CO2 and water in metal organic frameworks

The interactions between metal–organic frameworks (MOFs) and adsorbates have been increasingly predicted and studied by computer simulations, particularly by Grand-Canonical Monte Carlo (GCMC), as this method enables comparing the results with experimental data and also provides a degree of molecular level detail that is difficult to obtain in experiments. The assignment of atomic point charges to each atom of the framework is essential for modelling Coulombic interactions between the MOF and the adsorbate. Such interactions are important in adsorption of polar gases like water or carbon dioxide, both of which are central in carbon capture processes. The aim of this work is to systematically investigate the effect of varying atomic point charges on adsorption isotherm predictions, identify the underlying trends, and based on this knowledge to improve existing models in order to increase the accuracy of gas adsorption prediction in MOFs. Adsorption isotherms for CO2 and water in several MOFs were generated with GCMC, using the same computational parameters for each material except framework point charge sets that were obtained through a wide range of computational approaches. We carried out this work for 6 widely studied MOFs; IRMOF-1, MIL-47, UiO-66, CuBTC, Co-MOF-74 and SIFSIX-2-Cu-I. We included both MOFs with and without open metal sites (OMS), specifically to investigate whether this property affects the predicted adsorption behaviour. Our results show that point charges obtained from quantum mechanical calculations on fully periodic structures are generally more consistent and reliable than those obtained from either cluster-based QM calculations or semi-empirical approaches. Furthermore, adsorption in MOFs that contain OMS is much more sensitive to the point charge values, with particularly large variability being observed for water adsorption in such MOFs. This suggests that particular care must be taken when simulating adsorption of polar molecules in MOFs with open metal sites to ensure that accurate results are obtained.

Evaluating Charge Equilibration Methods To Generate Electrostatic Fields in Nanoporous Materials.

Charge equilibration (Qeq) methods can estimate the electrostatic potential of molecules and periodic frameworks by assigning point charges to each atom, using only a small fraction of the resources needed to compute density functional (DFT)-derived charges. This makes possible, for example, the computational screening of thousands of microporous structures to assess their performance for the adsorption of polar molecules. Recently, different variants of the original Qeq scheme were proposed to improve the quality of the computed point charges. One focus of this research was to improve the gas adsorption predictions in metal–organic frameworks (MOFs), for which many different structures are available. In this work, we review the evolution of the method from the original Qeq scheme, understanding the role of the different modifications on the final output. We evaluated the result of combining different protocols and set of parameters, by comparing the Qeq charges with high quality DFT-derived DDEC charges for 2338 MOF structures. We focused on the systematic errors that are attributable to specific atom types to quantify the final precision that one can expect from Qeq methods in the context of gas adsorption where the electrostatic potential plays a significant role, namely, CO2 and H2S adsorption. In conclusion, both the type of algorithm and the input parameters have a large impact on the resulting charges, and we draw some guidelines to help the user to choose the proper combination of the two for obtaining a meaningful set of charges. We show that, considering this set of MOFs, the accuracy of the original Qeq scheme is often still comparable with the most recent variants, even if it clearly fails in the presence of certain atom types, such as alkali metals.

Gate-Opening Mechanism of Hydrophilic–Hydrophobic Metal–Organic Frameworks: Molecular Simulations and Quasi-Equilibrated Desorption

Adsorption of polar and nonpolar molecules in hydrophilic–hydrophobic STAM-1 metal–organic framework was studied by means of quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) experimental technique and molecular simulation. The QE-TPDA measurements revealed that thermal stability of the studied material in the presence of water may be lower than determined from thermogravimetric analysis. Molecular dynamics showed evident impact of diffusion on the adsorption mechanism in STAM-1. The QE-TPDA profiles recorded for adsorption of n-alkanes, water, and alcohols indicate the gate-opening effect occurring only upon adsorption of polar molecules, which was confirmed by in situ IR spectroscopy. Monte Carlo molecular simulations agree with experimental data revealing preferable adsorption sites for the molecules of alcohols in STAM-1. Simulations also showed that the molecular mechanism of the gate-opening is dependent on the size of polar guest molecules.