Water Adsorption Desorption Research Articles

Porosity and microstructure of plasma deposited TiO 2 thin films

The microstructure of TiO 2 thin films prepared by plasma enhanced chemical vapour deposition has been assessed by using water adsorption–desorption isotherms measured by means of a quartz crystal monitor (QCM). Thin films have been deposited by using titanium tetraisopropoxide as a precursor and by changing different experimental parameters of the deposition procedure such as temperature of the substrate, pressure, and gas composition in the plasma. The films were characteristic of different microstructures that, according to their scanning electron micrographs, have been categorized as columnar, homogeneous and crystalline. They also have different refraction indices with values between 1.95 and 2.41. Water and toluene adsorption isotherms have been measured by means of a QCM monitor for the films heated in vacuum to remove the water previously adsorbed in their pores. The analysis of the adsorption-desorption isotherms by means of the so called “ t-plots” and the determination of the pore size distribution curves rendered that the three kinds of microstructures presented different kinds of isotherms and water adsorption behaviours. Columnar films consisted of micro- and meso-pores had a very high adsorption hystheresis at low pressures. Homogeneous films only had micropores and presented no adsorption hystheresis. Crystalline films consisted of both micro- and meso-pores but had no adsorption hystheresis at low pressures. A zone scheme has been proposed to account for the microstructure of the films depending on the plasma conditions utilized. The implications of the different water adsorption behaviours of the films for the determination of their refraction indices are discussed.

Highly ordered self-assembled mesostructured membranes: Porous structure and pore surface coverage

Highly ordered silica and hybrid organic–inorganic membranes have been obtained through a slow and controlled solvent evaporation in the presence of a triblock copolymer as the structure-directing agent. Crack-free mesostructured membranes of large dimensions have been prepared. The mesophase is self-organized into a periodic cubic structure, Im 3 ¯ m , and consists of macroscopic mesostructured domains that maintain the out-of-plane order throughout the material. After calcination at 350 °C the membranes remain transparent even though they break in fragments of some centimeters. Different techniques have been employed to characterize the mesoporous materials: small angle X-ray scattering (SAXS) using synchrotron radiation and transmission electron microscopy to study the porous structure, Fourier transform infrared spectroscopy (FTIR) in vacuum at different partial pressures to investigate the nature of the pore surface. SAXS analysis has been used to map the whole membrane, revealing some differences in the pore organization that can be attributed to a gradient in the evaporation rate. The infrared measurements at different partial pressures have elucidated the mechanism of water adsorption–desorption on the pore surface. The changes of the FTIR spectra with different partial pressure have demonstrated that the pore surface is covered by a layer of molecular water that is hydrogen bonded to silanol species.

Water adsorption behavior of ordered mesoporous silicas modified with an organosilane composed of hydrophobic alkyl chain and hydrophilic polyethylene oxide groups

FSM-type mesoporous silicas with different pore diameters were synthesized and modified with an organosilane composed of hydrophobic alkyl chain and hydrophilic polyethylene oxide (PEO) groups. In the slightly silylated material, both alkyl chains and PEO groups are located on the surfaces of ordered mesopores. In the fully silylated material, it is considered that alkyl chains are present near the silicate frameworks and the residual spaces are filled with PEO groups. Accordingly, inner spaces of ordered mesopores can be designed by the modification. On the basis of the water adsorption–desorption measurements, surface properties of the modified materials was investigated in detail. Although surfaces of ordered mesoporous silicas must be extremely hydrophobic by the presence of surface alkyl chains, the property was largely reduced by the presence of hydrophilic PEO groups and then water molecules can be adsorbed on the modified surfaces and penetrated into PEO moieties even after the organic modification.

Modeling extreme ultraviolet∕H2O oxidation of ruthenium optic coatings

A mathematical model of extreme ultraviolet (EUV)-induced oxidation of a ruthenium (Ru)-coated EUV optic is presented. The model describes the key processes that contribute to the growth of the oxide within the optic when exposed to EUV radiation in the presence of water vapor. These processes include the adsorption and thermal desorption of water to and from the Ru surface, molecular diffusion of water across the optic surface, and the dissociation of the water by both direct EUV ionization and secondary electron excitation. Oxygen produced by dissociation may associatively desorb from the surface or may diffuse into the Ru subsurface where it can react to form an oxide. The presence of oxygen in the Ru coating, whether as oxide or atomic oxygen, reduces the reflectivity of the optic and the overall throughput of the EUV lithographic system. The model predicts oxide thickness over time, which may later be used to estimate the reflectively loss attributable to the oxide in any given EUV environment. Model predictions for EUV-induced oxide growth provide a good description of the oxide growth observed in available electron-beam experiments. The model is also used to estimate oxygen penetration into the Ru coating under various conditions of water partial pressure, EUV power, and temperature. The model predicts reduced oxidation with higher temperatures and for substrates that bind water less tightly than ruthenium.

Morphology and surface properties of fumed silicas

Several series of fumed silicas and mixed fumed oxides produced and treated under different conditions were studied in gaseous and liquid media using nitrogen and water adsorption–desorption, mass spectrometry, FTIR, NMR, thermally stimulated depolarization current (TSDC), photon correlation spectroscopy (PCS), zeta potential, potentiometric titration, and Auger electron spectroscopy methods. Aggregation of primary particles and adsorption capacity ( V p) decrease and hysteresis loops of nitrogen adsorption–desorption isotherms becomes shorter with decreasing specific surface area ( S BET). However, the shape of nitrogen adsorption–desorption isotherms can be assigned to the same type independent of S BET value. The main maximum of pore size distribution (gaps between primary nonporous particles in aggregates and agglomerates) shifts toward larger pore size and its intensity decreases with decreasing S BET value. The water adsorption increases with increasing S BET value; however, the opposite effect is observed for the content of surface hydroxyls (in mmol/m 2). Associative desorption of water (2( Si OH) → Si O Si + H 2O) depends on both the morphology and synthesis conditions of fumed silica. The silica dissolution rate increases with increasing S BET and pH values. However, surface charge density and the modulus of zeta-potential increase with decreasing S BET value. The PCS, 1H NMR, and TSDC spectra demonstrate rearrangement of the fumed silica dispersion depending on the S BET value and the silica concentration ( C SiO 2 ) in the aqueous suspensions. A specific state of the dispersion is observed at the C SiO 2 values corresponding to the bulk density of the initial silica powder.

Water adsorption–desorption on aluminum surface

Water adsorption–desorption on aluminum surface studied at the Synchrotron Radiation Research Center (SRRC) is described. Both the thermal desorption and the photon stimulated desorption were studied. It was observed that the outgassing behavior of water is quite different from that of the other gases. Experimental results showed that the processes of venting, evacuation and vacuum treatment are sensitive to the water desorption behavior.

Contribution to the study of framework modification of SAPO-34 and SAPO-37 upon water adsorption by thermogravimetry

The adsorption–desorption of water vapor in SAPO-34 and SAPO-37 is followed in isobar conditions ( P H 2O =17.6 mbar ) by thermogravimetry at temperatures from 20°C to 400°C. The phenomenon is reversible in SAPO-34 but not in SAPO-37. The rise in water uptake is the highest at around 60–70°C, i.e. in range where an attack of the frameworks was previously reported. The reversibility (or not) of water adsorption–desorption uptake is linked to this reversible (or not) framework modification. The kinetics of desorption in SAPO-34 is strongly slowed down between 40°C and 60°C. This suggests that the full restructuration of the framework requires some time (700 min in the experimental conditions used). The morphology of the crystals of the two SAPOs is unchanged upon hydration but template-free SAPO-34 crystals become progressively destroyed in the electron beam of an electron microscope.

Quantitative ESR study of the CuH-ZSM-5 system: influence of preparation and pretreatment techniques on the valence state of copper

Abstract A quantitative ESR study of copper-deficient CuH-ZSM-5 demonstrates that the valence state of copper in this system is determined by the techniques of sample preparation and pretreatment. Oxidative calcination of the system at 500–550°C results, however, in a stabilization of the majority of copper by the H-ZSM-5 matrix in the form of isolated Cu 2+ cations, irrespective of the method of copper introduction. No measurable auto-reduction of Cu 2+ occurs upon calcination of the samples which are free of organic impurities. Peculiarities in the change of the ESR signal intensity upon water adsorption–desorption are related to the change in the coordination of Cu 2+ and the spin–lattice interaction between Cu 2+ ions and the zeolitic framework.

Temperature-programmed desorption and high-resolution electron energy loss spectroscopy studies of the interaction of water with the GaAs (001)-(4×2) surface

The adsorption, desorption, and dissociation of water on the GaAs(001)-(4×2) surface have been studied using Auger electron spectroscopy (AES), temperature-programmed desorption, and high-resolution electron energy loss spectroscopy. We have found that water first adsorbs molecularly at 100 K and dissociates readily upon annealing by virtue of overlapping desorption and dissociation temperatures between 150 and 200 K. The dissociation probability of water on the GaAs(001)-(4×2) surface is approximately 0.8 at low coverages (exposures below 0.5 L). However, the decomposition products of water exhibit a high recombination probability, making the oxidation of GaAs difficult. A large fraction of surface hydroxyls are rehydrogenated to produce desorbing water at temperatures between 300 and 700 K. Hence, we have applied a cycling treatment (repeated adsorption of water at 100 K followed by annealing to 750 K) in order to effectively oxidize the GaAs surface. During cycling, we have monitored GaAs–oxide growth using AES. In addition, thermal desorption spectra recorded after exposure of the cycling-treated GaAs surface to water at 100 K point to molecular adsorption and intact desorption of water with little evidence of dissociation, which suggests that the surface has been significantly oxidized by the cycling treatment of water.

Interaction of water with clean and gold-precovered tungsten field emitters: adsorption and desorption

The results of field emission microscopy investigation of the adsorption and thermal desorption of water deposited on a thermally clean and gold-covered tungsten emitter are presented. The desorption experiments indicate that H 2O molecules dissociate in contact with surfaces of the clean tungsten emitter, but no traces of dissociation of water adsorbed on Au W were found, if only the gold coverage was sufficiently high ( Θ ≥ 2). H 2O is probably adsorbed in two adsorption states on Au W . Water desorption from the first state was carried out in the temperature range 145–155 K with an activation energy of 51.5 ± 1.5 kJ/mol, whereas the desorption from the second state was observed at 220–240 K. Generally, the adsorption of water causes the reduction of the emission current at a constant high voltage, both for W and Au W , but for the second adsorption state in the system H 2O/Au/W a slight increase of the emission current was noticed. The behaviour of water adlayer in the presence of the high electric field is discussed.

The preparation and properties of some fluorinated activated carbons

Granular activated carbons were modified by treatment with fluorine (5% vol in N 2) at temperatures below 200°C. The nature and amounts of any gaseous products formed were dependent upon the type of carbon and the reaction temperature, but in each case the extent of carbon gasification was small. Nitrogen adsorption indicated that fluorination resulted in a progressive reduction in micropore volume and external surface area. Water adsorption—desorption isotherms suggested that the polarity of the carbon surface was not reduced as a consequence of fluorination and also that the modified carbons were unstable. Qualitative differences in the thermal stability of the modified carbons were noticeable, and it was apparent that the bulk of the fluorine was weakly adsorbed. Carbons fluorinated or outgassed below 350°C were acidic in nature, and liberated iodine from potassium iodide solution. The adsorption of chloropicrin from a humid airstream by all the modified carbons was poor in comparison to control samples.

Adsorption of surrounding gas molecules on pure metal surfaces during wear processes

A direct measurement of gas consumption was made during the sliding of twenty-seven elemental metals on each other when they were rubbed together in a vacuum chamber with a restricted volume. The gas pressure in the chamber decreased with time through the adsorption of gas molecules on to freshly generated metal surfaces. Using a mass spectrometer, the nature of the specific gases being adsorbed could be determined through measurement of the pressure drop during the wear process. Through a series of experimental studies, the following facts were observed: 1. (1) Generally speaking, oxygen and water molecules were adsorbed on metal surfaces, and hydrogen molecules were desorbed from metal surfaces during sliding. Far more adsorption of oxygen was observed during mild wear than during severe wear. 2. (2) Oxygen adsorption on elemental metal surfaces during wear was governed by the position of the element in the periodic table. The rare earth metals (f-character elements, such as lanthanum, praseodymium, samarium etc.) adsorbed oxygen most actively; transition metals (d-character elements, such as molybdenum, iron, nickel etc.) were next, and the other metals (s, p-elements, such as silver, zinc, tin etc.) adsorbed the least gas during wear processes. 3. (3) When a magnetic field was applied during iron/iron rubbing, oxygen and water adsorption and hydrogen desorption were greatly accelerated compared with the rates under ordinary conditions. From these findings the authors propose that the action of gas molecules on the wearing surfaces of metals is chemisorption, and they also discuss the role of gas chemisorption in the wear of metals.

Solar energy storage through water adsorption—desorption cycles in zeolitic tuffs

The process of thermal activation of an Italian natural zeolite, namely chabazitic tuff, is studied in the view of its possible use for solar energy storage. An evaluation of the amount of heat storable through the dehydration of the zeolite is also given.

The Effects of the Adsorption-Desorption Cycles on the Adsorption Characteristics of Synthetic Zeolites

Relationship between the adsorption characteristics of synthetic zeolites and a number of water adsorption-desorption cycles was investigated with a specially designed test apparatus illustrated in the text.The Na-A, (Na, K)-A and faujasite type synthetic zeolites were stable in their crystal structure at a temperature of 700°C for more than several days under the atmospheric condition. However, water adsorption rate of (Na, K)-A type zeolite after repeated several tens of water adsorption (at the room temperature)-thermal desorption cycles (below 300°C) in the apparatus was considerably decreased with increasing the number of cycles, scarcely adsorbing methanol. In the case of Na-A type zeolite after the same treatment, the propylene adsorption rate was decreased, but the decrease of water adsorption rate was not observed. The crystal structures of both resultant zeolites were preserved, as confirmed by X-ray diffraction.From these results, it is believed that the pore size of zeolite shrinked, these phenomena being attributable to the influence of high temperature water vapor that was generated from the zeolite during the thermal desorption process.Faujasite type synthetic zeolite, treated in the same apparatus under the same conditions, was investigated for adsorption of 1, 3, 5-trimethylbenzen, but no such phenomenon as in the case with the A-type zeolite was observed.