Canonical Monte Carlo simulation of adsorption of O2 and N2 mixture on single walled carbon nanotube at different temperatures and pressures

Adsorption of pure and mixtures of O₂ and N₂ on isolated single‐walled carbon nanotube (SWCNT) have been investigated at the subcritical (77 K) and different supercritical (273, 293, and 313K) temperatures for the pressure range between 1 and 31 MPa using (N,V,T) Monte Carlo simulation. Both O₂ and N₂ gravimetric storage capacity exhibit similar behaviors, gas adsorption is higher on outer surface of tube, compared to the inner surface. Results are consistent with the experimental adsorption measurements. All adsorption isotherms for pure and mixture of O₂ and N₂ are characterized by type I (Langmuir shape), indicating enhanced solid‐fluid interactions. Comparative studies reveal that, under identical conditions, O₂ adsorption is higher than N₂ adsorption, due to the adsorbate structure. Excess amount of O₂ and N₂ adsorption reach to a maximum at each temperature and specified pressure which can be suggested an optimum pressure for O₂ and N₂ storage. In addition, adsorptions of O₂ and N₂ mixtures have been investigated in two different compositions: (i) an equimolar gas mixture and (ii) air composition. Also, selectivity of nanotube to adsorption of O₂ and N₂ gases has been calculated for air composition at ambient condition. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

CO Adsorption on N2‐Precovered NaY Faujasite: A FTIR Analysis of the Resulting Adsorbed Species

To productively complete the information regarding the reversible adsorption of a gas mixture on the micropores of cationic zeolites, the adsorption of the two gases N₂ and CO on NaY faujasite is taken as a model case study. We analyze herein CO adsorption (77 K) on two distinct N₂‐precovered NaY sets (low and medium). We outline the continuous desorption of N₂ adducts during CO admittance to full N₂ desorption for the highest CO loadings. These features contrast with preceding results obtained for N₂ loading on CO‐precovered NaY. By comparing these results with the sole CO admission and combining both studies regarding the co‐adsorption sets, we demonstrate the influence of the basic strength of the two gases regarding the nature of the surface‐adsorbed species formed. We also propose and discuss a hypothesis regarding the formation of adsorbed mixed species having both N₂ and CO as ligands. These new findings strengthen the statistical response of IR signatures as a helpful proposal for analyzing adsorbed species and their assignments. This survey completes the molecular understanding of gas‐mixture adsorption that lacks experimental data to date.     

New method for calculating the adsorption of noble gases on amorphous surfaces

A new approach to calculating the equilibrium characteristics of the adsorption of noble gases on the amorphous surfaces of adsorbents was developed and applied to the Ar−TiO₂(rutile) system. Intermolecular adsorbate-adsorbate interactions are taken into account for the nearest neighbors in the quasi-chemical approximation. The lattice energy parameters of all interactions of the model are determined from the Lennard-Jones potential (12-6). The formation of amorphous TiO₂(rutile) surface includes completion of the surface layers and partial removal of the surface oxygen ions. The quality of the amorphization procedure was confirmed by the experimentally measured heats and isotherms of adsorption of the system under study. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1109–1118, June, 1997.     

Multicomponent Equilibrium Adsorption on Heterogeneous Adsorbents

A generalized method for prediction of multicomponent adsorption is suggested based on representing that adsorbent volume as energetically inhomogeneous. The method depends on extending the Polanyi potential theory to mixture adsorption. The main feature of the method is that, at constant partial pressure and temperature the composition of an adsorbed phase is not uniform over its volume. Results of applying this theory to non-porous adsorbents have been considered. The prediction ability of the theory is confirmed for the strongly non-ideal system acetone–chloroform–graphitized carbon black. It was shown that the departure from ideal behavior of adsorbed phase is quite close to that for the liquid mixture. Another system considered was oxygen–nitrogen–anatase at 78 K. Although this mixture is ideal, it has been found that there is significant variation in composition over the adsorbed layer due to the difference in the interactions of the quadrupolar N₂ molecule and nonpolar O₂ molecule with the anatase surface.     

Selective dissolution of TiO2 crystalline phases: Physicochemical characterization and photocatalytic activity

In this study, different commercially available TiO₂ powders (Degussa P25, pure anatase, and rutile) were submitted to selective dissolution treatments, with H₂O₂/NH₄OH and 10% HF, known to remove rutile and anatase from physical mixtures. The aim was to check whether a particular separation method designed to remove a specific crystalline phase influences the properties of the other phase from the mixture or not. More precisely, we have studied how the HF dissolution method designed to selectively remove the anatase affected the physicochemical and photocatalytic properties of rutile. In a similar way, the changes in the anatase properties were studied, after the H₂O₂/NH₄OH treatment, initially used to remove rutile from the mixture. All the samples were characterized by X-ray diffraction, nitrogen adsorption–desorption, transmission electron microscopy, diffuse reflectance (DR) ultraviolet–visible, and Raman spectroscopy. The photocatalytic activity of these powders was tested in the oxidation of p-chlorophenol from water. The selective treatment methods not only dissolved the target phase but also changed some physicochemical and the photocatalytic performances of the other TiO₂ crystalline phase in a considerable manner. These aspects should be taken into account in the studies regarding the synergistic effects of anatase and rutile, especially in reconstructed TiO₂ photocatalysts.     

First‐Principles Calculations on Electronic Structures of N/V‐Doped and N‐V‐Dodoped Anatase TiO2 (101) Surfaces

The energetic and electronic properties of N/V‐doped and N‐V‐codoped anatase TiO₂ (101) surfaces are investigated by first‐principles calculations, with the aim to elucidate the relationship between the electronic structure and the photocatalytic performance of N‐V‐codoped TiO₂. Several substitutional and interstitial configurations for the N and/or V impurities in the bulk phase and on the surface are studied, and the relative stability of different doping configurations is compared by the impurity formation energy. Systematic calculations reveal that N and V impurities can be encapsulated by TiO₂ to form stable structures as a result of strong N‐V interactions both in the bulk and the surface model. Through analyzing and comparing the electronic structures of different doping systems, the synergistic doping effects are discussed in detail. Based on these discussions, we suggest that N_OV_Ti codoping cannot only narrow the band gap of anatase TiO₂, but also forms impurity states, which are propitious for the separation of photoexcited electron–hole pairs. In the case of N_OV_Ti‐codoped TiO₂ (101) surfaces, this phenomenon is especially prominent. Finally, a feasible synthesis route for N_OV_Ti codoping into anatase TiO₂ is proposed.     

Effect of quenching on the methane coupling reaction

The effect of reaction mixture quenching on C₂ product formation in the methane coupling reaction on La₂O₃/CaO is disclosed. For reaction with the mixture (vol. %): 54.5 CH₄, 9.1 O₂ and 36.4 N₂ at 973 K, quenching of products results in a two-fold decrease in C₂ product yield. The results give evidence that in methane oxidative coupling methyl radical formation can occur in the gas phase to an extent comparable with that on the catalyst surface. The effect described must be taken into account in the case of an industrial application of methane oxidative coupling, too, because quenching is a regular procedure in the high temperature oxidative processes.     

Adsorption of a four-component gas mixture on zeolite NaX

Adsorption of N₂, CH₄, C₂H₆, C₃H₈, and their mixture on zeolite NaX was studied by the volumetric method under static conditions at 278 K in the pressure range from 0.1 to 0.8 MPa. Compressibility factors were calculated in order to take into account the nonideal character of the gas phase. Adsorption isotherms of individual gases and partial isotherms were obtained. The adsorption properties of gases in the adsorption of a mixture and its components were compared. The selectivity coefficient of adsorption of propane in the N₂-CH₄-C₂H₆-C₃H₈-NaX system was calculated, and its dependence on the total pressure was determined.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 839–841, April, 1996.     

TPD study of NH3 adsorption/desorption on the surface of V/Ti, V/Al based catalysts for selective catalytic reduction of NOx by ammonia 1. TPD test of γ-Al2O3, TiO2 (anatase) and alumina-supported vanadia catalysts

The effect of temperature on the adsorption/desorption of ammonia from the air mixture on the surface of γ-Al₂O₃, TiO₂ (anatase) and alumina-supported vanadia catalyst samples has been investigated using temperature-programmed desorption (TPD). When the vanadia loading was increased, the fraction of the acid sites providing the NH₃ adsorption in the high-temperature state decreased. At the same time, the fraction of the medium temperature state significantly increased.     

Active Sites for Adsorption and Reaction of Molecules on Rutile TiO2(110) and Anatase TiO2(001) Surfaces (cited: 1)

The reactivity of specific sites on rutile TiO₂(110)-(1×1) surface and anatase TiO₂(001)-(1×4) surface has been comparably studied by means of high resolution scanning tunneling microscopy. At the rutile TiO₂(110)-(1×1) surface, we find the defects of oxygen vacancy provide distinct reactivity for O₂ and CO₂ adsorption, while the terminal fivefold-coordinated Ti sites dominate the photocatalytic reactivity for H₂O and CH₃OH dissociation. At the anatase TiO₂(001)-(1×4) surface, the sixfold-coordinated terminal Ti sites at the oxidized surface seem to be inert in both O₂ and H₂O reactions, but the Ti-rich defects which introduce the Ti³⁺ state into the reduced surface are found to provide high reactivity for the reactions of O₂ and H₂O. By comparing the reactions on both rutile and anatase surfaces under similar experimental conditions, we find the reactivity of anatase TiO₂(001) is actually lower than rutile TiO₂(110), which challenges the conventional knowledge that the anatase (001) is the most reactive TiO₂ surface. Our findings could provide atomic level insights into the mechanisms of TiO₂ based catalytic and photocatalytic chemical reactions.     

EPR and kinetic investigation of free cyanide oxidation by photocatalysis and ozonation

Oxidation of free cyanide in aqueous suspensions of three commercial TiO₂ specimens, with different anatase crystal size, has been carried out in a batch photoreactor by simultaneously applying ozonation and photocatalysis. Dissolved ozone participates both in homogeneous and catalytic reactions with cyanide; the extents of these two processes are comparable to that of the photodegradation with oxygen. The reactivity results are well described by the Langmuir-Hinshelwood kinetic model, providing the values of the kinetic and equilibrium adsorption constants for the catalytic and photocatalytic reactions contributing to cyanide oxidation. The cyanide concentration decreases faster with time for catalysts with increasing anatase crystal size, being more marked under UV irradiation. EPR studies on gaseous ozone adsorption on the three samples in the dark have shown stronger ozone interactions with Ti⁴⁺ and O^2? ions of the samples with largest anatase crystal size, leading to the formation of significant signals of Ti³⁺ and _s O^??O₂ radicals than with the anatase with the lowest crystal size, where ozone was mainly adsorbed on water molecular arrangements covering its surface. The hampering of the ozone and/or cyanide adsorption by the water molecular arrangements covering the surface of the catalyst with the lowest crystal size would justify the low cyanide degradation rate observed for this sample.     

Impact of surface hydroxylation in MgO-/SnO-nanocluster modified TiO2 anatase (101) composites on visible light absorption,charge separation and reducibility

Anatase TiO₂ surfaces, whether oxidised or hydroxylated, can be modified by nanoclusters of SnO and MgO to give a red shift in light absorption, enhanced charge separation and high reducibility.     

Predicting Low-Pressure O2 Adsorption in Nanoporous Framework Materials for Sensing Applications

A set of 98 nanoporous framework material (NFM) structures was investigated by classical Grand canonical Monte Carlo simulations for low-pressure O₂ adsorption properties (Henry’s constant and isosteric heat of adsorption). The set of materials includes those that have shown high O₂ uptake experimentally as well as a subset of more than 2000 structures previously screened for noble-gas uptake. While use of the general force field UFF is fruitful for noble-gas adsorption studies, its use is shown to be limited for the case of O₂ adsorption—one distinct limitation is a lack of sufficient O₂–metal interactions to be able to describe O₂ interaction with open metal sites. Nonetheless, those structures without open metal sites that have very small pores (<2.5 Å) show increased O₂/N₂ selectivity. Additionally, O₂/N₂ mixture simulations show that in some cases, H₂O or N₂ can hinder O₂ uptake for NFMs with small pores due to competitive adsorption.     

Synergistic Nitrogen Binding Sites in a Metal-Organic Framework for Efficient N2/O2 Separation

Porous materials with d₃ electronic configuration open metal sites have been proved to be effective adsorbents for N₂ capture and N₂/O₂ separation. However, the reported materials remain challenging to address the trade-off between adsorption capacity and selectivity. Herein, we report a robust MOF, MIL-102Cr, that features two binding sites, can synergistically afford strong interactions for N₂ capture. The synergistic adsorption site exhibits a benchmark Q_st of 45.0 kJ mol⁻¹ for N₂ among the Cr-based MOFs, a record-high volumetric N₂ uptake (31.38 cm³ cm⁻³), and highest N₂/O₂ selectivity (13.11) at 298 K and 1.0 bar. Breakthrough experiments reveal that MIL-102Cr can efficiently capture N₂ from a 79/21 N₂/O₂ mixture, providing a record 99.99 % pure O₂ productivity of 0.75 mmol g⁻¹. In situ infrared spectroscopy and computational modelling studies revealed that a synergistic adsorption effect by open Cr(III) and fluorine sites was accountable for the strong interactions between the MOF and N₂.     

Density dependence of electron mobility in dense gases

The density (N) dependence of electron mobility (μ) in various dense gases (H₂, N₂, O₂, CO₂ and rare gases) has been calculated using a multiple-scattering approach. Deviation of the high density gas from its perfect gas behaviour has been taken into account through the temperature-dependent second virial coefficient. Multiple scattering of electrons leads to shifts in their kinetic energy and it also changes their distribution functions. This unified approach predicts both positive and negative effects. The positive (negative) effect entails on increase (decrease) of μ withN. We have assessed the available data on momentum transfer cross-sections by comparing the mobility at very low densities (Nμ)₀ with experimental results. The density dependence is studied by comparing the calculated ratioNμ)/Nμ)₀ with the observed values and other theoretical work. The Legler model which assumes a constant cross-section is inadequate for predicting the observed density dependence. We obtain good agreement with available experimental work for all the atomic and molecular species studied here.     

Dependence of the effective coefficient of separating a mixture of NH3-O2 and NH3-N2 on the rate of distillation

The effective coefficient for separating a mixture of NH₃-O₂ and NH₃-N₂ at different rates of distillation is determined experimentally and theoretically. The effect of the temperature of a surface layer of liquid ammonia on the purification process from permanent gases (O₂ and N₂) is studied. The results indicate that upon an increase in the rate of distillation, the efficiency of separating a mixture of ammonia and impurities rises as heat transfer in the surface layer becomes difficult.     

Role of Adsorption of Reagents in Reduction of NO by Propene on ZrO2-Based Complex Oxide Systems

We have used IR spectroscopy to study adsorption of NO, propene, and their mixture on Rh-Cr₂O₃/ZrO₂ and Rh-CeO₂/ZrO₂ catalysts at temperatures of 293-623 K. We have established that adsorption and coadsorption of the reagents (NO and C₃H₆) have important differences, depending on the nature of the surface. Weak adsorption interactions of the reaction mixture on Rh-CeO₂/ZrO₂ lead to significantly lower activity of this catalyst in selective catalytic reduction (SCR) of NO by propene.     

Controlled gas adsorption properties of various pillared clays

Microporous pillared clays (PILC) were prepared by the intercalation of montmorillonite with particles of titania (Ti-PILC), zirconia (Zr-PILC), alumina (Al-PILC), iron oxide (Fe-PILC) and mixed lanthania/alumina (LaAl-PILC). Nitrogen adsorption isotherms (77 K) and XRD data provided information on the porosity, surface area, micropore volume and interlayer distance of these samples. The surface area varied between 198 and 266 m²/g for Ti- and Fe-PILC, respectively. The titania pillared clay had also the highest micropore volume (0.142 cc/g) and interlayer spacing (16–20 Å), compared to the Zr-PILC, which had the smallest spacing between the layers (max, 4 Å). Despite this fact, Zr-PILC always showed a high adsorption capacity for gases such as N₂, O₂, Ar or CO₂, due to its high adsorption field in the very small micropores.From gas adsorption experiments on these various PILCs, it became clear that their adsorption properties depend on the pillars in three ways: (i) the pillar height, (ii) the distribution of the pillars between the clay layers and (iii) the nature of the pillaring species.The incorporation of other elements in the pillars leads to specific adsorption sites in the pores. This was demonstrated by the preparation of mixed Fe/Cr and Fe/Zr pillared clays. Compared to the parent Fe-PILC, the incorporation of chromium and zirconium in the iron oxide pillars had a positive influence on the adsorption capacity. Also the modification of a PILC with cations increases both capacity and selectivity for gases. This was confirmed by the increased adsorption of N₂, O₂ and CO₂ at 273 K on a Sr²⁺ exchanged Al-PILC.     

Effect of Hydrogen on O2 Adsorption and Dissociation on a TiO2 Anatase (001) Surface

The effect of hydrogen on the adsorption and dissociation of the oxygen molecule on a TiO₂ anatase (001) surface is studied by first‐principles calculations coupled with the nudged elastic band (NEB) method. Hydrogen adatoms on the surface can increase the absolute value of the adsorption energy of the oxygen molecule. A single H adatom on an anatase (001) surface can lower dramatically the dissociation barrier of the oxygen molecule. The adsorption energy of an O₂ molecule is high enough to break the O?O bond. The system energy is lowered after dissociation. If two H adatoms are together on the surface, an oxygen molecule can be also strongly adsorbed, and the adsorption energy is high enough to break the O?O bond. However, the system energy increases after dissociation. Because dissociation of the oxygen molecule on a hydrogenated anatase (001) surface is more efficient, and the oxygen adatoms on the anatase surface can be used to oxidize other adsorbed toxic small gas molecules, hydrogenated anatase is a promising catalyst candidate.     

Unique adsorption behaviors of NO and O2 at hydrogenated anatase TiO2(101)

The extra electron on the hydrogenated anatase TiO₂(101) is localized at the nearest Ti_5c only, and the chargetransfer promoted NO and O₂ adsorptions are also site-selective. These results are totally different from those at hydrogenated rutile TiO₂(110).