Coadsorption Of Water Research Articles

CO2 separation and landfill biogas upgrading: A comparison of 4A and 13X zeolite adsorbents

Abstract CO2 separation from wet and dry synthetic biogas has been investigated using adsorption on 4A and 13X molecular sieves in dynamic conditions, flowing through fixed beds in an adsorption column connected with an IR detector. The adsorption of CO2 has also been investigated by IR spectroscopy over pressed disks of the zeolites in static dry and wet conditions. 13X molecular sieve has significant adsorption capacity of CO2 from dry biogas, which is further increased by the presence of moisture. This is due to the change of the adsorption mode of CO2 due to the copresence of water. Over wet 13X zeolite adsorption of CO2 mostly occurs in the form of bicarbonate ions interacting with coadsorbed water, while over the dry surface several different kinds of carbonate ions are formed together with molecular adsorbed species. The adsorption capacity of CO2 from biogas is definitely lower over 4A molecular sieve, where coadsorption of methane is also significant. However, regeneration of 4A by purging with nitrogen at r.t. is faster than that of 13X. Over 4A coadsorption of water do modifies the adsorption state of CO2 but has little effect on adsorption capacity.

ChemInform Abstract: Coadsorption of Water and Chlorine on Ag(110): Evidence for Adsorbate- Induced Hydrophilicity.

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Competitive adsorption study of CO 2 and SO 2 on Co II3[Co III(CN) 6] 2 using DRIFTS

Diffuse reflectance infrared Fourier transform spectroscopy was used to study the competitive adsorption of CO 2 and SO 2 on the cobalt Prussian blue analogue Co II 3[Co III(CN) 6] 2 at 298 K. Characteristic peaks for adsorbed CO 2 and SO 2 species were identified and their relative areas, measured simultaneously as a function of pressure at 298 K, varied in accordance with a Langmuir–Freundlich isotherm fitted to both gases in the low-coverage Henry's Law limit. Evidence for co-adsorption of trace water was also obtained, as well as the apparent formation of an analogous cobalt nitroprusside compound as a reaction product under certain conditions. The several aspects of the adsorption of CO 2 and SO 2 determined in this work point to an important role for real-time diffuse reflectance infrared measurements in adsorption studies, particularly in the case of competitive adsorption where the occurrence and fate of molecular-level markers arising from more than one adsorbed species can be monitored simultaneously. Depending on the application, this may more than offset certain quantitative limitations of the technique that confine measurements to a relatively narrow set of experimental conditions and demand careful consideration of the effects of sample preparation and treatment.

Adsorption of paracresol in silicalite-1 and pure silica faujasite. A comparison study using molecular simulation

This paper presents the results on the Grand-Canonical Monte Carlo simulations of the adsorption of the paracresol uremic toxin and water into the silicalite-1 and pure silica faujasite zeolites. The co-adsorption of water and paracresol seems to proceed along a cooperation effect between the toxin and the solvent. A model of adsorption that accounts for the effect of the solvent has been elaborated and verified using experimental isotherms. The model is based on the Langmuir isotherm in which an apparent adsorption enthalpy is used that changes with the concentration of the solute. The new expression for the isotherm reproduces the experimental isotherm with good accuracy and physical interpretation is given to justify the model.

Dissociation of water on oxygen-covered Rh{111}

The adsorption of water and coadsorption with oxygen on Rh{111} under ultrahigh vacuum conditions was studied using synchrotron-based photoemission and photoabsorption spectroscopy. Water adsorbs intact on the clean surface at temperatures below 154 K. Irradiation with x-rays, however, induces fast dissociation and the formation of a mixed OH+H(2)O layer indicating that the partially dissociated layer is thermodynamically more stable. Coadsorption of water and oxygen at a coverage below 0.3 monolayers has a similar effect, leading to the formation of a hydrogen-bonded network of water and hydroxyl molecules at a ratio of 3:2. The partially dissociated layers are more stable than chemisorbed intact water with the maximum desorption temperatures up to 30 K higher. For higher oxygen coverage, up to 0.5 monolayers, water does not dissociate and an intact water species is observed above 160 K, which is characterized by an O 1s binding energy 0.6 eV higher than that of chemisorbed water and a high desorption temperature similar to the partially dissociated layer. The extra stabilization is most likely due to hydrogen bonds with atomic oxygen.

Vanadia and Water Coadsorption on Tetragonal Zirconia Surfaces

The surface structure of vanadia supported on zirconia is examined by density functional theory with periodic boundary conditions. Adsorption of V2O5 and H2O on the (101) and (001) surfaces of tetragonal zirconia is studied for different loadings up to (V2O5)4 or V2O5·6H2O per eight Zr surface sites (monolayer coverage). The considered surface species include vanadia monomers, dimers, and polymers on water-free surfaces as well as different combinations of coadsorbed vanadia, hydroxyls, and water. Calculated vibrational spectra do not show unique features that could be used for identifying certain surface species. Statistical thermodynamics is used to evaluate the relative stability of different surface structures as function of vanadia loading, water partial pressure, and temperature (surface phase diagrams). The presence of water has a strong influence on the surface species. Under water free conditions and for Θ(V2O5) = 0.25 vanadia is present as a monomeric species on the (001) surface and as dimeric ...

Molecular modeling of the mechanism of ethyl fatty ester synthesis catalyzed by lipases. Effects of structural water and ethanol initial co-adsorption with the fatty acid

This manuscript presents an MM2 modeling of the esterification of model fatty acids (oleic acid and 9-decenoic acid) with ethanol using lipases from Candida antarctica B (CALB) and Candida rugosa (CRL). The role of water and ethanol as part of an H-bonding network acting at the adsorption of the fatty acid, the stabilization of intermediaries and the acyl enzyme and on the regeneration of the Serine OH are taken into account.The model includes the consideration of the tunnel and the catalytic triad/oxyanion hole – the large active site – for CRL and CALB-using structures of the open lipases from the PDB. Besides, an additional molecular modeling of the H-bonding distances found in the intermediaries and the acyl enzyme using CALB was carried out. The fatty acid esterification demonstrated to be activated for CRL and almost non-activated for CALB—in the context of the large active sites. When the results of the additional MM2 modeling (for a shorter model) were analyzed. H-bonding at the level of the strong and weak H-bonding between Histidine, water, ethanol and O from Serine were found. Molecular modeling results also revealed that the presence of a molecule of water at the catalytic triad of both lipases promotes a significant change in the calculated energy profiles to lower energies and a potential too strong adsorption of substrates and products. There is qualitative agreement between the theoretical modeling and the experimental work published elsewhere.

Adsorption into the MFI zeolite of aromatic molecule of biological relevance. Investigations by Monte Carlo simulations

Adsorption of paracresol and water into the silicalite-1 (MFI) zeolite has been investigated using canonical and grand-canonical Monte Carlo simulations. The most stable sites of adsorption of paracresol are found to be located at the channel intersections. Grand-canonical simulations have shown that at low loading, water molecules adsorb preferably at the vicinity of paracresol molecules, whereas they are also located in the sinusoidal channels as the loading increases. In order to explain the experimental adsorption isotherm observed for the coadsorption of water and paracresol in the MFI zeolite we propose a new concept of apparent adsorption enthalpy that varies with the concentration of the solution. The mathematical expression for the apparent enthalpy is introduced in an adsorption isotherm model. We shall refer to this theoretical isotherm as a non-langmuirian isotherm. The non-linear expression for the apparent adsorption enthalpy accounts for a variable accessibility of the sites of adsorption with respect to the concentration of the solution.

The Effect of Water on the CO Oxidation on Ag(111) and Au(111) Surfaces: A First-Principle Study

Density functional theory has been used to study the effect of water on the molecular (02, 0, and CO) adsorption, O-2 dissociation, and CO oxidation on the Ag(111) and Au(111) surfaces. It is found that, though CO oxidation with atomic O is facile on both surfaces with barriers less than 0.29 eV, considerable barriers for O-2 dissociation (1.03 eV for Ag(111) and 1.97 eV for Au(111)) and weak adsorption of reactants limit their overall reactivity. Our calculations show that the reactants can be stabilized by coadsorption of water via the formation of an H bond and/or the interaction mediated through the substrates. The stabilization induced by coadsorbed water affects not only adsorption of reactants but also the transition states and intermediates, which enhances overall reactivity for CO oxidation, correspondingly. H bonds facilitate the dissociation of O-2 with reduction of barrier by 0.36 eV on Ag(111). Moreover, a highly active reaction pathway for CO oxidation via molecular assistant by water with overall barrier of 0.15 eV is identified. Atomic oxygen formed may either react with adsorbed CO or react with adsorbed water to form hydroxyls, which oxidizes CO subsequently, to complete the catalytic cycle.

Adsorption of methane with the presence of water on oxide, polymer and carbon adsorbents studied using 1H NMR spectroscopy at low temperatures

Co-adsorption of water and methane onto polymeric adsorbent LiChrolut EN, activated carbon adsorbent, fumed alumina and Silochrome modified by 3-aminopropylmethyldimethoxy silane was studied using 1H NMR spectroscopy with layer-by-layer freezing out of bulk and bound adsorbates. At a certain amount of adsorbed water the adsorption of methane can increase. Bound water can be in strongly (changes in Gibbs free energy Δ G < −1 kJ/mol) and weakly (Δ G > −1 kJ/mol) bound and strongly (chemical shift of the proton resonance δ H = 3–6 ppm) and weakly ( δ H = 1–2 ppm) associated states. With the presence of adsorbed methane contribution of weakly associated water increases.

Modifying the adsorption characteristics of water on Ru{0001} with preadsorbed oxygen

The coadsorption of water and preadsorbed oxygen on Ru{0001) was studied by synchrotron-based high-resolution x-ray photoelectron spectroscopy. A dramatic change was observed in the interaction of water with oxygen between low and high oxygen precoverages. Low oxygen coverages below 0.18 ML induce partial dissociation, which leads to an adsorbed layer of H2O and OH. Around half the oxygen atoms take part in this reaction. All OH recombines upon heating to 200 K and desorbs together with H2O. Oxygen coverages between 0.20 and 0.50 ML inhibit dissociation, instead a highly stable intact water species is observed, which desorbs at 220 K. This species is significantly more stable than intact water on the clean surface. The stabilization is most likely due to the formation of hydrogen bonds with neighboring oxygen atoms. For intermediate oxygen coverages around 0.18 ML, the dissociation behavior depends on the preparation conditions, which points toward possible mechanisms and pathways for partial dissociation of water on Ru{0001}.

Hybrid QM/MM Investigations into the Structure and Properties of Oxygen-Donating Species in TS-1

The active oxidizing species in the H2O2/TS-1 catalytic system is investigated using a hybrid quantum mechanical/molecular mechanical approach. In this computational technique, the site of interest is described with the density functional theory using the BB1K exchange and correlation functional, and the remainder of the system is treated with a valence force field. We have examined the formation of dioxygen adsorbate structures with η1 and η2 Ti-peroxo configurations, including radical species, which arise from the attack of hydrogen peroxide on a tetrahedral TiIV site along with effects of water co-adsorption. Our results show that hydrogen peroxide physisorbed on tetra- and tripodal Ti sites, as well as coadsorbed H2O2/H2O, is energetically favorable and involves minor structural modifications in the framework, as Ti atoms change their coordination from four to six. Furthermore, our calculations of various Ti-peroxo complexes formed from tripodal sites and H2O2 strongly suggest that Ti-η2(OOH), Ti-η1(OOH), and Ti-η1(O(H)OH) complexes will all form in the pores of TS-1 under reactions with H2O2 with a preferential coordination of six in the presence of water. In particular, both six-coordinate Ti-η1(OOH) and Ti-η2(OOH) compare favorably with EXAFS data obtained from the mesoporous materials with the isomorphous Ti active sites. These results indicate that water is not just a medium for transporting reactants and products at the catalytic sites, but has an active role in stabilizing the peroxo species present on the working catalyst. The Ti−O bond distances are also close to those in the five-coordinated complexes reported for TS-1 (also based on EXAFS data). Although, we find that both anhydrous and hydrated η2(O2·) type species are unlikely to be the predominant oxygen-donating species, we have demonstrated that our calculated g-tensors for the short-lived radical species are in line with EPR data, which supports our assertion of the tripodal Ti site as the main active site in TS-1 for partial oxidation catalysis.

Structural features of polymer adsorbent LiChrolut EN and interfacial behavior of water and water/organic mixtures

The structural and adsorption characteristics of polymer adsorbent LiChrolut EN and the behavior of adsorbed water and water/organic mixtures were studied using adsorption, microcalorimetry, transmission and scanning electron microscopy, mass spectrometry, infrared spectroscopy, 1H NMR spectroscopy with layer-by-layer freezing-out of liquids (190–273 K), and thermally stimulated depolarization current method (90–265 K). This adsorbent is characterized by large specific surface area (∼1500 m 2/g) and pore volume (0.83 cm 3/g) with a major contribution of narrow pores ( R < 10 nm ) of a complicated shape (long hysteresis loop is in nitrogen adsorption–desorption isotherm). The adsorbent includes aromatic and aliphatic structures and oxygen-containing functionalities and can effectively adsorb organics and water/organic mixtures. On co-adsorption of water and organics (dimethyl sulfoxide, chloroform, methane), there is a weak influence of one on another adsorbate due to their poor mixing in pores. Weakly polar chloroform displaces a fraction of water from narrow pores. These effects can explain high efficiency of the adsorbent in solid-phase extraction of organics from aqueous solutions. The influence of structural features of several carbon and polymer adsorbents on adsorbed nitrogen, water and water/organics is compared on the basis of the adsorption and 1H NMR data.

Temperature dependence of co-adsorption of carbon monoxide and water on highly dispersed Pt/C and PtRu/C electrodes studied by in-situ ATR-FTIRAS

ATR-FTIRAS measurements were conducted to investigate nature of water molecules co-adsorbed with CO on highly dispersed PtRu alloy and Pt catalysts supported on carbon black in the temperature range between 23 °C and 60 °C. Each catalyst was uniformly dispersed and fixed by Nafion ® film of 0.0125 μm thickness on a chemically deposited gold film. Adsorption of CO was conducted and monitored by ATR-FTIRAS for 30 min in 1% CO saturated 0.1 M HClO 4 after stepping the potential from 1.2 V and 1.0 V to 0.05 V on Pt/C and PtRu/C, respectively. Similar atop and bridge bonded CO bands were observed on both PtRu/C and Pt/C, but a smaller relative band intensity, bridge bonded vs. atop CO, was observed on PtRu/C compared to Pt/C. A distinct O–H stretching band was found around 3643 cm −1 and 3630 cm −1 on PtRu/C and Pt/C, respectively, upon CO adsorption. They are assigned to non-hydrogen bonded water molecules co-adsorbed with CO on these catalysts. We found that the number of non-hydrogen bonded water molecules co-adsorbed with a given number of CO molecules decreases with increasing temperature and is higher on PtRu/C than Pt/C at each temperature. We interpret the higher ability of water co-adsorption at PtRu/C over Pt/C is due to stronger H 2O–metal interactions on the alloy surface. We present a model of the CO–H 2O co-adsorbed layer based on the bilayer model of water on metal surfaces.

Co-adsorption of water and hydrogen on Ni(111)

We have studied the surface coverage dependence of the co-adsorption of D and D(2)O on the Ni(111) surface under UHV conditions. We use detailed temperature-programmed desorption studies and high resolution electron energy loss spectroscopy to show how pre-covering the surface with various amounts of D affects adsorption and desorption of D(2)O. Our results show that the effects of co-adsorption are strongly dependent on D-coverage. In the deuterium pre-coverage range of 0-0.3 ML, adsorption of deuterium leaves a fraction of the available surface area bare for D(2)O adsorption, which shows no significant changes compared to adsorption on the bare surface. Our data indicate phase segregation of hydrogen and water into islands. At low post-coverages, D(2)O forms a two-phase system on the remaining bare surface that shows zero-order desorption kinetics. This two phase system likely consists of a 2-D solid phase of extended islands of hexamer rings and a 2-D water gas phase. Increasing the water post-dose leads at first to 'freezing' of the 2-D gas and is followed by formation of ordered, multilayered water islands in-between the deuterium islands. For deuterium pre-coverages between 0.3 and 0.5 ML, our data may be interpreted that the water hexamer ring structure, (D(2)O)(6), required for the formation of an ordered multilayer, does not form anymore. Instead, more disordered linear and branched chains of water molecules grow in-between the extended, hydrophobic deuterium islands. These deuterium islands have a D-atom density in agreement with a (2x2)-2D structure. The disordered water structures adsorbed in-between form nucleation sites for growth of 3-D water structures. Loss of regular lateral hydrogen bonding and weakened interaction with the substrate reduces the binding energy of water significantly in this regime and results in lowering of the desorption temperature. At deuterium pre-coverages greater than 0.5 ML, the saturated (2x2)-2D structure mixes with (1x1)-1D patches. The mixed structures are also hydrophobic. On such surfaces, submonolayer doses of water lead to formation of 3-D water structures well before wetting the entire hydrogen-covered surface.

DFT calculations, DRIFT spectroscopy and kinetic studies on the hydrolysis of isocyanic acid on the TiO 2-anatase (1 0 1) surface

The co-adsorption of isocyanic acid (HNCO) and water (H 2O) and their reaction to ammonia and carbon dioxide on the anatase phase of TiO 2 were studied with ab initio density functional theory (DFT) calculations using a cluster model as well as with in situ DRIFTS investigations and kinetic experiments. We found that isocyanic acid can in principle adsorb both molecularly and dissociatively on the TiO 2(1 0 1) surface, but the moment at which water gets involved in the process, is vital for determining the further course of the surface reaction. In the absence of water, it was found that HNCO can adsorb in molecular form on the TiO 2 surface. Assuming this case to be the first step of the HNCO hydrolysis, the surface HNCO rearranges into an intermediate complex with a modified N C O skeleton. After decarboxylation water attacks the complex from the gas phase and ammonia is finally formed. However, when water is present at the beginning of the hydrolysis reaction, it immediately attacks the NCO group present at the surface, yielding a carbamic acid complex, which is further transformed into a carbamate complex. After decarboxylation an NH 2 group remains at the surface. Finally, NH 3 is formed by hydrogen transfer from molecularly adsorbed water at a neighboring titanium center and the hydrolysis reaction is completed. Since water is always present in diesel exhaust gas, only the second mechanism is relevant under practical conditions. Moreover, the calculated energy barrier is lower for the second mechanism compared to the first reaction pathway. The comparison between the sum of the theoretical vibrational spectra of the reaction intermediates with the in situ DRIFT spectra also strongly supports the accuracy of the second reaction pathway. The experimental investigation of the kinetics of the HNCO hydrolysis on TiO 2-anatase revealed a second order reaction—first order with respect to HNCO and first order with respect to water, which can only be reconciled with the second mechanism.

Origin of Gate Hysteresis in Carbon Nanotube Field-Effect Transistors

We have studied gate hysteresis of carbon nanotube field-effect transistors (CNFETs) on silicon oxide substrates in an ultrahigh vacuum (UHV) at low temperatures. It is found that the hysteresis is neither reduced by thermal annealing at temperatures over 300 °C under UHV nor significantly affected by independent adsorption of ammonia or water at T = 56 K. However, the hysteresis decreases greatly upon coadsorption of water and ammonia below condensation temperatures and restores completely with desorption of the adsorbed water layer. On the basis of these results, it is concluded that the main cause of gate hysteresis in CNFETs on silicon oxide substrate is charge transfer between the carbon nanotube and charge traps at the silicon oxide/ambient interface. We propose a mechanism for gate hysteresis that involves surface silanol groups as the major sources of screening charges. This surface silanol model is supported by results from scanning surface potential microscopy (SSPM).

Complex formation and proton transfer between formic acid and water adsorbed on Au(111) surfaces under UHV conditions

The coadsorption of formic acid and water on Au(111) surfaces has been investigated by means of vibrational and photoelectron spectroscopy (HREELS, XPS). Formic acid adsorbs at 90 K molecularly with vibrational modes characteristic for flat lying zig-zag chains in the mono- and multilayer regime, like in solid formic acid. Annealing results in a complete desorption at 190 K. Sequential adsorption of formic acid and water at 90 K shows no significant chemical interaction. Upon annealing the coadsorbed layer to 140 K a hydrogen-bonded cyclic complex of formic acid with one water molecule could be identified using isotopically labelled adsorbates (D2O, H13COOD). Upon further annealing this complex decomposes leaving molecularly adsorbed formic acid on the surface at 160 K, accompanied by a proton exchange between formic acid and water.

Mechanisms of and Effect of Coadsorption on Water Dissociation on an Oxygen Vacancy of the MgO(100) Surface

The dissociation mechanism of a water molecule at an oxygen vacancy on the MgO(100) surface was studied by using the embedded cluster method at the DFT/B3 LYP level, while the energetic information was refined by using the IMOMO method at the CCSD level. We found that a water molecule initially adsorbs on one of the magnesium ions surrounding the vacancy site with a binding energy of 15.98 kcal mol(-1). It then can dissociate on the MgO(100) surface along two possible dissociation pathways. One pathway produces a hydroxyl group bonded to the original magnesium with a proton filling the vacancy via a transition state with a barrier of 4.67 kcal mol(-1) relative to the adsorbed water configuration. The other pathway yields two hydroxy groups; the hydroxy group originally belonging to the water molecule fills the vacancy, while the hydrogen atom binds with the surface oxygen to form the other hydroxy group. Hydrogen atoms of these hydroxy groups can recombine to form a hydrogen molecule and the surface is healed. Although the barrier (14.09 kcal mol(-1)) of the rate-controlling step of the latter pathway is higher than that of the former one, the energies of all of its stationary points are lower than that of the separated reactants (H(2)O+cluster). The effects of water coadsorption are modeled by placing an additional water molecule near the active center, which suggests that the more coadsorbed water molecules further stabilize the hydroxy species and prevent the hydrogen molecule formation through the latter pathway. The results support the photoemission spectral evidence of water dissociation on the defective MgO(100) surface at low water coverage.

Co-adsorption of water and hydroxyl on a Pt 2Ru surface

Gradient-corrected periodic density functional theory calculations are performed to explain the mobility of surface-bonded hydroxyl on a Pt 2Ru(1 1 1) surface in the presence of co-adsorbed water, as recently observed by Desai and Neurock [S.K. Desai, M. Neurock, Phys. Rev. B 68 (2003) 075420] in ab initio molecular dynamics calculations. The effect can to a large extent be explained by an energetic compensation effect: the loss in energy associated with transferring OH from a Ru site to a Pt site is compensated by a gain in energy with the associated transfer of H 2O from the Pt to the Ru site. The calculated property of the Pt 2Ru(1 1 1) surface would support the OH spillover mechanism suggested by Davies et al. [J.C. Davies, B.E. Hayden, D.J. Pegg, M.E. Rendall, Surf. Sci. 496 (2002) 110], at least in as far as it concerns OH spillover to Pt site neighboring a Ru site, and provides new insight into the classical bifunctional mechanism of CO electro-oxidation.