Water Adsorption Properties Research Articles

The effect of water on the structural, electronic and photocatalytic properties of graphitic carbon nitride

g-C3N4, as a typical metal-free catalyst for water splitting, has attracted special attention. The structural and electronic properties of water adsorption on g-C3N4 play a key role in understanding the water splitting mechanism at the atomic level. The properties of a single g-C3N4 sheet and the water adsorption on a single g-C3N4 sheet were thoroughly explored based on density functional theory (DFT) calculations. The results show that water adsorption on one side of the single g-C3N4 sheet will lead the initial flat structure to change to a buckle one, while water molecule adsorption on both sides of g-C3N4 will not disturb the flat structure. The flat g-C3N4 is an indirect semiconductor, and interestingly the band structure of g-C3N4 changes from an indirect to a direct one during the flat structure transformation from flat to buckle because of the water adsorption. Water molecules prefer to adsorb around the intrinsic vacancy of the single g-C3N4 sheet at low coverage, and further adsorbed water molecules stay around the intrinsic vacancy. Water adsorption also affects the band edge position of g-C3N4 for water splitting. These results provide a deep insight into the structure and adsorption properties of g-C3N4 in the water environment, which will greatly help to design a new type of metal-free catalyst for water-splitting.

Effect of hydrazine post-treatment on natural fibre reinforced polymer composites

Surface modified natural fibres have received great attention due to their several advantages such as non-toxic, low cost and environmentally friendly nature. The development of composite materials, surface modified natural fibre used as reinforcing filler and thermoplastic polymers as matrixes is currently at the centre of attention. In the present work, coir fibre reinforced polypropylene composites were manufactured using a single screw extruder and an injection moulding machine. Raw coir fibre was oxidised, and manufactured composites were post-treated with phenylhydrazine in order to increase the interfacial adhesion between the coir fibre and the polypropylene matrix. Both raw and oxidised were utilised during manufacturing. Coir fibre at 15, 20, 25, 30 and 35 wt-% was utilised. Microstructural analysis (scanning electron microscopy and Fourier transform infrared spectroscopy), mechanical tests (tensile, flexural and impact, hardness) and Water adsorption property were conducted. Post-treated specimens yielded better mechanical properties compared to the oxidised and raw ones. Based on fibre loading, 30% fibre reinforced composites had the optimum set of mechanical properties. Authors propose that the bonding between the polypropylene matrix and the phenylhydrazine treated coir fibre must be improved in order to have better mechanical properties at higher fibre content.

Shaping adsorption properties of nano-porous molecular sieves for solar thermal energy storage and heat pump applications

The water adsorption properties of modified porous sorbents for solar thermal energy storage and heat transformation have been investigated by measurements of water adsorption isotherms, microcalorimetry and storage tests. A chabazite type SAPO, a dealuminated faujasite type zeolite, and a mesostructured aluminosilicate, have been synthesized and compared with common zeolites NaX, NaY and silica gel. It has been found that optimized lattice composition and pore architecture contribute to well adapt hydrophilic properties and a beneficial steep isotherm.

Acoustic chemometrics for on-line monitoring and end-point determination of fluidised bed drying

An emerging process analytical technology (PAT) technique, acoustic chemometrics, was applied for monitoring of heated fluidised bed drying process. The feasibility of quantitative on-line monitoring of the drying progress and end-point determination of silica gel dried in a heated fluidised bed was investigated. Silica gel was used owing to its high water adsorption property and stability to heat. Acoustic signals were acquired using four accelerometers mounted at different locations on the wall of the fluidised bed. The accelerometer located close to the base was the best based results from this study. Prediction models validated with independent acoustic data (test set validation) were developed using partial least square regression, PLS-R. Some data pre-preprocessing techniques were applied to improve the developed prediction model. The final prediction results were satisfactory for monitoring of the drying progress and end point determination. The prediction results based on the independent data indicated a slope=0.97, correlation coefficient, R2=0.99 and the root mean square error of prediction was 1.71 water %, within the range 0–35.69 water % of sampled reference. It was concluded that the results from this feasibility study show that acoustic chemometrics is a viable on-line technique for monitoring the drying progress and for determining the end-point during drying of particulate matter. This on-line monitoring technique for the drying process developed in this study can be applied in many relevant industries in order to improve the overall economics of material drying by optimising fluidised bed drying technology using this decisive end-point determination approach.

Free Energy and Electronic Properties of Water Adsorption on the SnO2(110) Surface

A molecular understanding of the adsorption of water on SnO2 surfaces is crucial for several applications of this metal oxide, including catalysis and gas sensing. We have investigated water adsorption on the SnO2(110) surface using a combination of dynamic and static calculations to gain fundamental insight into the reaction mechanism at room temperature. The reaction dynamics are studied by following water adsorption and dissociation on the SnO2 surface with metadynamics calculations at low and high coverage. The electronic structure in the relevant isolated minima is investigated through Mulliken charge analysis and projected density of states analysis. Surface bridging oxygen (Obr) is found to play a decisive role in water adsorption forming rooted hydroxyl groups with the water H atoms. Bond formation with H significantly changes the electronic configuration of Obr and presumably leads to reduced band bending at the SnO2 surface. The free-energy estimation indicates that on a clean SnO2(110) surface at room temperature both associative and dissociative adsorption occur, with the latter being thermodynamically favored. Oxygen coverage strongly affects the ratio between associatively and dissociatively adsorbed H2O, favoring associative adsorption at high oxygen coverage (oxidized surface) and dissociative adsorption at low oxygen coverage (reduced surface). Electronic analyses of isolated surface minima show the existence of two different electron-transfer phenomena occurring at the surface, depending on the water adsorption mechanism. The relevance of these findings in explaining the changes in electric conductivity occurring in SnO2-based gas sensors upon water adsorption is discussed. Whereas associative adsorption leads to electron enrichment of the metal oxide surface, dissociative adsorption induces surface electron depletion. Both mechanisms are consistent with the electrical conductivity changes occurring upon interaction of SnO2 with water, causing cross sensitivity to the latter. The theoretical results form the basis for correlating the existing atomistic models with the experimental data and offer a coherent description of the reaction events on the surface at room temperature.

DFT+U Study of Molecular and Dissociative Water Adsorptions on the Fe3O4(110) Surface

Spin-polarized density functional theory method (GGA+U) and periodic supercell model have been used to study water adsorption properties on the Fe3O4(110) surface, which has A and B terminations in close surface energy. The adsorption of one and two water molecules is molecular on the A termination, while dissociative on the B termination. For the adsorption of three and four water molecules, mixed dissociative and molecular coadsorption is preferred on the A termination, and fully dissociative adsorption as well as mixed molecular and dissociative coadsorptions are preferred on the B terminations. The stepwise adsorption energies show that the full monolayer water adsorption on both terminations is thermodynamically possible. Further analysis shows that surface iron atoms and hydrogen bonding contribute to the adsorption energies. The adsorption mechanism has been analyzed on the basis of projected density of states (PDOS).

A New Family of Nonsiliceous Porous Hybrids from Bisphosphonates

This paper reviews the development of nonsiliceous porous hybrids, espetially a new family of surfactant assisted mesoporous metal organophosphonates (MOPs) prepared by the reactions between metal sources and bisphosphonates. This synthetic concept was proved by successful construction of aluminium organophosphonate (AOP) frameworks around supramolecular assembly mediated mesopores. In AOP-type materials, a series of ordered mesoporous materials can be obtained after optimizing the synthetic conditions including elimination of oligomeric surfactants and triblock copolymers. Water adsorption properties over the AOP-type materials were exceptional and promising as high-performance devise materials in gas heat-pomp and desiccant systems. A variety of ordered mesoporous titanium organophosphonates (TOPs) have also been prepared with investigation on multi-functions in liquid phases (removal of toxic heavy metal ions, photodegradation of organic pollutants, CO2 capture, chromatographic separation, etc.). Possibility to combine both functions due to inorganic units and organic groups has so far been explored. Through further development of processing and engineering methodology with the structural (mesoscale), compositional (inorganic and organic parts), and morphological (macroscale) designs, nonsiliceous MOP-type porous materials will widely be developed for potential and practical applications.

Molecular dynamics simulations of supercritical water at the iron hydroxide surface

The adsorption properties of supercritical water confined between electrostatically neutral but hydrophilic surfaces of iron (II) hydroxide were determined through molecular dynamics simulations. Simulations were conducted at temperatures of 715, 814 and 913K, and at water densities typically found in the heat transport system of the supercritical water cooled nuclear reactor (SCWR). Surface water layer densities were obtained and compared to those of the bulk water. Adsorption coverage was calculated as a function of the number of waters per OH group on the surface. Images of the water molecules configurations are provided along with the density profile of the adsorption layer. The localized adsorption and surface clustering of supercritical water, as seen in this study, would likely produce more localized corrosion phenomena in the water bearing components of the SCWR.

Molecular Simulations of Water Adsorbed on Mesoporous Silica Thin Films

Grand canonical Monte Carlo and canonical ensemble molecular dynamics simulations were used to investigate water adsorption properties in mesoporous silica thin films. The effect of pore radius on the adsorption properties was assessed using two models of mesoporous silica thin films having different pore radius and film thickness (1.38 and 5.66 nm in Model 1, respectively, and 1.81 and 7.30 nm in Model 2, respectively). In the simulations, a water adsorption layer or water menisci were formed in a mesopore accompanying the growth or shrinkage of stable adsorption layers on the upper and lower surfaces. The thickness of a water adsorption layer immediately before the onset of capillary condensation and the curvature radius of a water meniscus prior to capillary evaporation were identified. In the initial stage of layer adsorption, the surface state and radius of the pore affected the coordination structure of water around silanol groups. In the pore-filling state, the hydrogen bond network of water in the pore was similar but not the same as that in the bulk liquid; the difference was because the pore wall affected the position of water molecules. This difference may confer unique dynamic properties upon the water in mesoporous silica.

Water adsorption and dissociation on Ni surface: Effects of steps, dopants, coverage and self-aggregation

Water-metal interaction has been receiving extensive attention due to its interdisciplinary application. In this paper, on the basis of first-principle calculations and slab models, the behavior of water adsorption and dissociation on planar, stepped and blocked Ni surfaces is investigated, the effects of steps, Au and Ag dopants, coverage and self-aggregation of water are also disclosed. The results show that: step not only strengthens water-substrate interaction but also facilitates water dissociation. With dopants modification, the adsorption and dissociation properties remain relatively unchanged at lower coverage (1/9 on facets and 1/12 ML on steps) while at higher coverage (1/4 on facets and 1/6 ML on steps) water adsorption is weakened and dissociation activity decreases dramatically. Water adsorption and dissociation properties on Ni surfaces are essentially unaffected with the increase of coverage. On doped surfaces, adsorption properties and dissociation activities are closely associated with the ligand effect, which is dependent on the dopant, dopant concentration and surface morphology. Water self-aggregation enhances water-surface interaction on all studied surfaces due to hydrogen bond (network) formation. Furthermore, investigation shows that it does not assist water dissociation.

Postsynthetic tuning of hydrophilicity in pyrazolate MOFs to modulate water adsorption properties

Metal–organic frameworks (MOFs) have attracted interest as adsorbents in water-based adsorption heat pumps owing to their potential for increased water loading capacities and structural and functional tunability versus traditionally used materials such as zeolites and silica. Although pyrazolate-based MOFs exhibit exceptional hydrolytic stability, the water adsorption characteristics of this class of frameworks have remained unexplored in this context. In this report, we describe the modular synthesis of novel dipyrazole ligands containing naphthalenediimide cores functionalized with –H (H2NDI–H), –NHEt (H2NDI–NHEt), or –SEt (H2NDI–SEt) groups. Reaction of these ligands with Zn(NO3)2 afforded an isostructural series of MOFs, Zn(NDI–X), featuring infinite chains of tetrahedral Zn2+ ions bridged by pyrazolate groups and ∼16 A-wide channels with functionalized naphthalenediimide linkers lining the channel surface. The Type V water adsorption isotherms measured for these materials show water uptake in the 40–50% relative humidity range, suggesting hydrophobic channel interiors. Postsynthetic oxidation of Zn(NDI–SEt) with dimethyldioxirane was used to generate ethyl sulfoxide and ethyl sulfone groups, thereby rendering the channels more hydrophilic, as evidenced by shifts in water uptake to the 30–40% relative humidity range. Such tunability in water adsorption characteristics may find utility in the design of new adsorbents for adsorption-based heat transfer processes. An original MATLAB script, MOF-FIT, which allows for visual modeling of breathing and other structural deformations in MOFs is also presented.

Water adsorption properties controlled by coating/filling ordered mesoporous silica inside cellulose membranes

Porous organic membranes have been utilized as hard templates not only for replication of porous macrostructures but also for fabrication of hierarchical porous solids through infiltration of precursor solutions in ordered mesoporous materials. However, such organic membranes are usually burned out as sacrificial skeletons by calcination. In addition, replicated macropores are too big to enhance properties due to inorganic oxide frameworks. In this study, when cellulose membranes were used as organic membranes, a coating/filling technology of ordered mesoporous silicas was proposed and the water adsorption-desorption properties were directly investigated by using the composite membranes after extraction of nonionic surfactants used. The composite membranes possessed enough adsorption capacity for water, which will be potentially useful for improving total energy efficiency in heat-pump and desiccant air conditioning systems.

Adjusting the Stability of Metal–Organic Frameworks under Humid Conditions by Ligand Functionalization

The practical use of metal-organic frameworks (MOFs) in applications ranging from adsorption separations to controlled storage and release hinges on their stability in humid or aqueous environments. The sensitivity of certain MOFs under humid conditions is well-known, but systematic studies of water adsorption properties of MOFs are lacking. This information is critical for developing design criteria for directing future synthesis efforts. The goal of this work is to understand the influence of the extent of Zn-O bond shielding on the relative stabilities of MOFs belonging to same family of isostructural, noncatenated pillared MOFs [Zn(L)(DABCO)(0.5)], where L is the functionalized BDC (1,4-benzenedicarboxylic acid) linker. The different extent of Zn-O bond shielding is provided by incorporating a broad range of functional groups on the BDC ligand. The resulting MOFs have varying surface areas, pore sizes, and pore volumes. Stability is assessed through water vapor adsorption isotherms combined with powder X-ray diffraction (PXRD) experiments and surface area analyses. Our study demonstrates that integration of polar functional groups (e.g., nitro, bromo, chloro, hydroxy, etc.) on the dicarboxylate linker renders these MOFs water unstable compared to the parent MOF as these polar functional groups have a negative shielding effect; i.e., they facilitate hydrolysis of the Zn-O bond. On the other hand, placing nonpolar groups (e.g., methyl) on the BDC ligand results in structurally robust MOFs because the Zn-O bond is effectively shielded from attack by water molecules. Therefore, the anthracene- and tetramethyl-BDC MOFs do not lose crystallinity or surface area after water exposure, in spite of the large amount of water adsorption due to capillary condensation at ∼20% relative humidity (RH). This has been observed rarely in the MOF literature. The results of this work show that by ligand functionalization it is possible to adjust the water stability of a pillared MOF in both the positive and negative directions and, thus, provide an important step toward understanding the water adsorption behavior of MOFs.

Ethanol and Water Adsorption on Close-Packed 3d, 4d, and 5d Transition-Metal Surfaces: A Density Functional Theory Investigation with van der Waals Correction

Nowadays, there is a great interest in the economic success of direct-ethanol fuel cells; however, our atomistic understanding of the designing of stable and low-cost catalysts for the steam reforming of ethanol is still far from satisfactory, in particular due to the large number of undesirable intermediates. In this study, we will report a first-principles investigation of the adsorption properties of ethanol and water at low coverage on close-packed transition-metal (TM) surfaces, namely, Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111), employing density functional theory (DFT) calculations. We employed the generalized gradient approximation with the formulation proposed by Perdew, Burke, and Erzenholf (PBE) to the exchange-correlation functional and the empirical correction proposed by S. Grimme (DFT+D3) for the van der Waals correction. We found that both adsorbates binds preferentially near or on the on-top sites of the TM surfaces through the O atoms. The PBE adsorption energies of ethanol and water decreases almost linearly with the increased occupation of the 4d and 5dd-band, while there is a deviation for the 3d systems. The van der Waals correction affects the linear behavior and increases the adsorption energy for both adsorbates, which is expected as the van der Waals energy due to the correlation effects is strongly underestimated by DFT-PBE for weak interacting systems. The geometric parameters for water/TM are not affected by the van der Waals correction, i.e., both DFT and DFT+D3 yield an almost parallel orientation for water on the TM surfaces; however, DFT+D3 changes drastically the ethanol orientation. For example, DFT yields an almost perpendicular orientation of the C–C bond to the TM surface, while the C–C bond is almost parallel to the surface using DFT+D3 for all systems, except for ethanol/Fe(110). Thus, the van der Waals correction decreases the distance of the C atoms to the TM surfaces, which might contribute to break the C–C bond. The work function decreases upon the adsorption of ethanol and water, and both follow the same trends, however, with different magnitude (larger for ethanol/TM) due to the weak binding of water to the surface. The electron density increases mainly in the region between the topmost layer and the adsorbates, which explains the reduction of the substrate work function.

Comparative study on the properties of ZnO nanowires and nanocrystalline thin films

The microstructural, morphological, optical and water-adsorption properties of nanocrystalline ZnO thin films and ZnO nanowires were studied and compared. The ZnO thin films were obtained by a sol–gel process, while the ZnO nanowires were electrochemically grown onto a ZnO sol–gel spin-coated seed layer. Thin films and nanowire samples were deposited onto crystalline quartz substrates covered by an Au electrode, able to be used in a quartz crystal microbalance. X-ray diffraction measurements reveal in both cases a typical diffraction pattern of ZnO wurtzite structure. Scanning electron microscopic images of nanowire samples show the presence of nanowires with hexagonal sections, with diameters ranging from 30 to 90nm. Optical characterization reveals a bandgap energy of 3.29eV for the nanowires and 3.35eV for the thin films. A quartz crystal microbalance placed in a vacuum chamber was used to quantify the amount and kinetics of water adsorption onto the samples. Nanowire samples, which have higher surface areas than the thin films, adsorb significantly more water.

Adsorption of ammonia and water on functionalized edge-rich carbon nanofibers

The preparation, characterization and ammonia and water adsorption properties of edge-rich carbon nanofibers (CNFs) were studied, including platelet CNFs (PCNFs) and cup-stacked CNFs (CSCNFs). Since PCNFs and CSCNFs have many chemically active exposed edges, functionalization by oxidizing the edges was carried out by ozone stream and by nitric acid. Transmission electron microscopy, N2 adsorption isotherms and temperature-programmed desorption analysis showed that the nitric acid treatment partly destroyed the graphite structure of the PCNFs and created acid functional groups and micropores, whereas the ozone treatment created functional groups without damaging the structure. Ammonia adsorption isotherms clarified that NH3 adsorption on PCNFs and CSCNFs occurred mainly on oxygen-containing groups, whereas the adsorption on activated carbon fibers (ACFs) occurred on both oxygen-containing groups and the carbon surface without the functional groups, and the CSCNFs showed larger amounts of adsorbed ammonia compared to the PCNFs. Especially at a relatively low pressure range (<0.2 atm), the PCNFs/CSCNFs/ACFs showed the same ammonia adsorption mechanism; that is, the one-to-one interaction between oxygen atoms in the functional groups and hydrogen atoms in ammonia molecules. In addition, the adsorption on the ACFs appeared to occur mainly by interaction with the carbon surface at relatively high pressure (0.3–1.0 atm). Our experimental results and previous findings suggest that NH3 adsorption on PCNFs is due mainly to NH…O hydrogen bonding between oxygen-containing groups and ammonia rather than to chemical bonding.

Thermal, mechanical and water adsorption properties of corn starch–carboxymethylcellulose/methylcellulose biodegradable films

The objective of this study was to investigate the effect of the addition of methylcellulose and carboxymethylcellulose on the thermal, mechanical and water adsorption properties of starch-based films plasticized with glycerol or polyethylene glycol (PEG). Mechanical tests showed that as the methylcellulose and carboxymethylcellulose proportion increased, starch films became more resistant to break, resulting in higher TS values. Besides there has been a positive effect on the elasticity of starch films realized by a considerable increase in E% values. Depending on the plasticizer type, either single or dual glass transitions were seen in DSC thermograms. One glass transition temperature was observed for films plasticized with glycerol, on the contrary, dual glass transitions were detected for PEG plasticized films. This behavior was attributed to the phase separation of the PEG. In addition, the presence of an endothermic peak in the thermograms of PEG plasticized films was taken as another indicator of the phase separation. As a result, it was suggested that PEG was not as compatible as glycerol with the composite polysaccharide matrix and plasticizer type was the main factor that shaped the thermal profiles of the film samples. Water adsorption isotherm data showed that samples displayed nonlinear sorption profile which is typical for hydrophilic films. In all films tested, equilibrium moisture contents, increased almost linearly up to a aw of 0.65–0.85, beyond where a sharp increase was noted. Adsorption data was adequately fitted by BET and GAB models. Eventually, it can be concluded that film forming properties of starch can be improved by incorporation of methylcellulose and carboxymethylcellulose to the polymer matrix.

Effect and mechanism of controlled permeable formwork on concrete water adsorption

Controlled permeability formwork (CPF) is a kind of non-woven formwork, which is a widely used product in the spinning industry. In this paper, the effect of CPF on the water adsorption property of concrete was studied. Surface appearance, meso-surface morphology and microstructures of concrete blocks were characterized to analyze the affecting mechanism of CPF on concrete adsorption. It was found that, by applying CPF, concrete permeability was markedly reduced due to the improved surface and microstructures. In addition, the surface structure of the CPF liner was studied by SEM. It was indicated that the CPF was a beneficial and promising material for the improvement of concrete performances.

Cellular and soft tissue compatibility to high interconnectivity between pores of chitosan scaffold

In the field of tissue engineering and regenerative medicine, porosity, pore size, and pore interconnectivity of three-dimensional scaffolds affect cellular attachment, proliferation and, migration, and degree of tissue infiltration. In this study, porous chitosan scaffolds with macropores interconnected by micropores were prepared using a thermally induced phase-separation process. Water adsorption properties of scaffolds were investigated by soaking in phosphate-buffered saline (PBS) at room temperature. The chitosan scaffold with macropores interconnected by micropores (CS-10B) adsorbed PBS more rapidly than a chitosan scaffold containing only macropores (CS). An investigation of cell attachment and distribution showed that human dermal fibroblasts (HDFs) culture on chitosan scaffolds in vitro were present on both the surface and cross-sections of the CS-10B scaffold, but only on the surface of the CS scaffold. Studies of tissue responses to the prepared scaffolds, evaluated by subcutaneous implantation in rats, showed that the organization of tissues in the CS-10B scaffold was superior to that in the CS scaffold, and the CS-10B implant degraded faster than the CS implant in vivo. The high interconnectivity of porous scaffolds with macropores interconnected by micropores enhanced biocompatibility and biodegradability, suggesting the excellent potential applications of such chitosan scaffolds in the field of tissue engineering and regenerative medicine. Open image in new window

Application of zeolitic materials prepared from fly ash to water vapor adsorption for solar cooling

The water vapor adsorption properties of raw and hydrothermally treated fly ashes with NaOH and their application prospect as evaporative coolers of roof surfaces were studied. Initially, samples were characterized through techniques like elemental analysis, X-ray diffraction, thermogravimetry, reflectance measurements and water vapor adsorption isotherms. Moreover, the water adsorption properties and the associated temperature variations were determined in a specific wind tunnel with controllable environmental conditions. The adsorption isotherms for fly ash were of type III indicating hydrophobic material with low water vapor adsorption. The hydrothermal treatment in an alkaline solution transformed the fly ash in hydrophilic material of type IV. Moreover, the treated samples were capable of lowering their surface temperatures due to water evaporation and the release of the latent heat. The maximum difference of temperature increase under simulated solar irradiation was observed between the treated fly ash and the concrete with values of 5.0, 5.4 and 7.5°C for the surface, middle and bottom position, respectively. The results indicate that the zeolitic materials prepared from the fly ash samples have a significant potential for solar cooling applications.