Preferential Adsorption Of CO2 Research Articles

Preparation of carbon sphere from lactose by hydrothermal reaction and its performance in gas separation

Carbon spheres were produced from mild hydrothermal reaction of biomass lactose, followed by carbonization with low‐activation temperatures (400°C). The Brunauer‐Emmett‐Teller (BET)‐specific surface area of carbon at optimum condition was 403 m2/g. The resultant carbon shows good performance in preferential adsorption of CO2 to CH4 below 50 kPa at 293 K. The results of the isosteric enthalpies of adsorption for different gases on prepared carbon indicate mildly heterogeneous surfaces. The relatively good performance of the resultant carbon in kinetics indicates its potential for low‐pressure kinetic separation of carbon dioxide from carbon dioxide and methane mixture at room temperature.. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 581–587, 2014

Interfacial Tension and Wettability in Water–Carbon Dioxide Systems: Experiments and Self-consistent Field Modeling

This paper presents experimental and modeling results on water-CO2 interfacial tension (IFT) together with wettability studies of water on both hydrophilic and hydrophobic surfaces immersed in CO2. CO2-water interfacial tension (IFT) measurements showed that the IFT decreased with increasing pressure and the negative slopes of IFT-pressure isotherms decreased with increasing temperature. Water contact angle on a cellulose surface (hydrophilic) immersed in CO2 increased with pressure, whereas the water contact angle on a hydrophobic surface such as hexamethyl disilazane (HMDS) coated silicon surface was almost independent of pressure. These experimental findings were augmented by modeling using the self-consistent field theory. The theory applies the lattice discretization scheme of Scheutjens and Fleer, with a discretization length close to the size of the molecules. In line with this we have implemented a primitive molecular model, with just small variations in the molar volume. The theory makes use of the Bragg-Williams approximation and has binary Flory-Huggins interaction parameters (FH) between CO2, water, and free volume. Using this model, we generated the complete IFT-pressure isotherms at various temperatures, which coincided well with the trends reported in literature, that is, the water-CO2 interfacial tension decreased with increasing pressure for pressures ≤100 bar and became independent of pressure >100 bar. The transition point occurred at higher pressures with increasing temperature. At three-phase coexistence (water-CO2-free volume) and at the water-vapor interface (water-free volume), we always found the CO2 phase in between the water-rich and free volume-rich phases. This means that for the conditions studied, the water-vapor interface is always wet by CO2 and there are no signs of a nearby wetting transition. Calculation of the water contact angle on a solid surface was based on the computed adsorption isotherms of water from a vapor or from a pressurized CO2-rich phase and analysis of surface pressures at water-vapor or water-CO2 coexistence. The results matched reasonably well with the experimental contact angle data. Besides, we also computed the volume fraction profiles of the CO2, H2O, and the V phase, from which the preferential adsorption of CO2 near the hydrophilic surface was deduced.

Gas Adsorption Properties and Selectivity in CuII/Adeninato/Carboxylato Metal–Biomolecule Frameworks

AbstractHerein we report the analysis of the porosity of a family of MBioFs with formula [Cu2(μ3‐ade)2(μ‐OOCCH3)2]n (compound 1), [Cu2(μ3‐ade)2(μ‐OOCCH2CH3)2]n (compound 2) and [Cu2(μ3‐ade)2(μ‐OOC(CH2)2CH3)2]n (compound 3) [ade = adeninato], by means of the measurements of N2 (77 K), CO2 (273 K) and H2 (77 K) experimental adsorption isotherms. The experimental results for each adsorptive species show that the uptake capacity decreases on increasing the length of the aliphatic chain of the carboxylato ligand. Although, in general, all compounds show a moderate experimental uptake capacity, the amount of CO2 adsorbed in compound 1 exceeds the values reported for many other well‐known MOFs, which is related to the presence of the Watson–Crick face in the pore wall. The experimental data of all compounds is discussed on the basis of simulated isotherms computed from grand canonical Monte Carlo (GCMC) calculations. To assess the suitability of pores decorated by the Watson–Crick faces for the selective capture of CO2 and CO, the selectivity towards CO2/H2 and CO/H2 gas mixtures was analysed by computing the Henry constants and the binary adsorption isotherms. The results show that the selectivity towards CO2 and CO can be tuned by the carboxylato ligand, increasing its value with increasing length of the aliphatic chain. These selectivity values are relatively high in all compounds, especially for compound 3 in which the pore narrowing promoted by the lengthening of the aliphatic chain favours a preferential adsorption of CO2 and CO, leading to one of the highest values computed for MOFs to date. Moreover, the high affinity towards CO2 and CO, and the marked lowering of the selectivity with increasing pressure observed in compound 3 is explained on the basis of the analysis of the potential energy maps obtained by the simulations, which reveals two different adsorption sites.

CO2 adsorption on zeolite X/activated carbon composites

Two series of zeolite X/activated carbon composites with different ratios of zeolite and activated carbon were prepared through a combination process of CO2 activation of the mixtures of elutrilithe and pitch and subsequent hydrothermal crystallization in alkaline solution. An additional surface modification was achieved in diluted NH4Cl solution. CO2 and N2 uptakes on the composites before and after modification were determined for pressures up to 101 kPa at 273 and 298 K, respectively. Langmuir-Freundlich and Toth adsorption models were used to describe the adsorption isotherms of CO2 and the corresponding heats of adsorption were estimated with the Clausius-Clapeyron equation. Both before and after modification, all composites exhibited a remarkable preferential adsorption of CO2 compared to N2, with the modified composites showing a higher adsorption selectivity to CO2 over N2 than the unmodified composites. With an increasing ratio of zeolite in the composites, adsorption capacity and adsorption heat of CO2 on the composites increased simultaneously. Lower adsorption heat was observed both before and after modification especially at the low-loading region and when there was less energetic heterogeneity on the surface of the modified composites. The results may be attributed to the elimination of strong basic sites on the modified composites, which is favorable for desorption of CO2 on the adsorbents and application in pressure swing adsorption processes.

Separation of CO2 and N2 with a lithium-modified silicalite-1 zeolite membrane

Carbon dioxide capture and storage is gaining attention as a strategy to abate greenhouse gas emissions. However, currently available commercial processes to remove CO2 from flue gas streams, mainly from N2 are costly and energy intensive. A silicalite-1 membrane was prepared on a porous alumina support using hydrothermal synthesis. The resulting silicalite-1 membranes were modified using Li solutions in order to achieve better CO2 separation. SEM micrographs exhibited a thin, uniform and compact silicalite-1 film well adhered to the alumina support. Crystallinity of the membrane was confirmed by XRD. CO2 and N2 permeation experiments were carried out in a stainless steel module. CO2 single gas permeation was measured through a silicalite-1 membrane from room temperature to 400°C, having a 25psi pressure on the feed side. The resulting CO2 permeance measurements indicated molecular sieving as the principal transport mechanism for the unmodified silicalite-1 membrane, whereas surface diffusion is the principal transport mechanism for lithium-modified silicalite-1 membranes. The separation between CO2 and N2 takes place due to preferential CO2 adsorption and diffusion, which in turn hinders N2 permeation through the silicalite-1 pore network. A CO2/N2 separation factor up to 6 at 25psi and 400°C was obtained using the silicalite-1 modified membrane, whereas this was 1.46 using the unmodified silicalite-1 membrane.

Displacement of Methane by Coadsorbed Carbon Dioxide Is Facilitated In Narrow Carbon Nanopores

By using simulation methods, we studied the adsorption of binary CO2–CH4 mixtures on various CH4 preadsorbed carbonaceous materials (e.g., triply periodic carbon minimal surfaces, slit-shaped carbon micropores, and Harris’s virtual porous carbons) at 293 K. Regardless of the different micropore geometry, two-stage mechanism of CH4 displacement from carbon nanospaces by coadsorbed CO2 has been proposed. In the first stage, the coadsorbed CO2 molecules induced the enhancement of CH4 adsorbed amount. In the second stage, the stronger affinity of CO2 to flat/curved graphitic surfaces as well as CO2–CO2 interactions cause the displacement of CH4 molecules from carbonaceous materials. The operating conditions of CO2-induced cleaning of the adsorbed phase from CH4 mixture component strongly depend on the size of the carbon micropores, but, in general, the enhanced adsorption field in narrow carbon ultramicropores facilitates the nonreactive displacement of CH4 by coadsorbed CO2. This is because in narrow carbon ultramicropores the equilibrium CO2/CH4 selectivity (i.e., preferential adsorption toward CO2) increased significantly. The adsorption field in wider micropores (i.e., the overall surface energy) for both CO2 and CH4 is very similar, which decreases the preferential CO2 adsorption. This suppresses the displacement of CH4 by coadsorbed CO2 and assists further adsorption of CH4 from the bulk mixture (i.e., CO2/CH4 mixing in adsorbed phase).

Polarized Micropores in a Novel 3D Metal–Organic Framework for Selective Adsorption Properties

A novel 3D porous metal-organic framework with 1D polarized channels was synthesized, and its adsorption properties for gas separation and chemical sensing were studied. The framework shows a preferential adsorption of CO(2) over N(2) with a selectivity of 22:1. It also exhibits a very good sensitivity to water with respect to most of the organic solvents in view of chemical sensing applications.

Adsorption-Induced Deformation of Microporous Materials: Coal Swelling Induced by CO2–CH4 Competitive Adsorption

Carbon dioxide injection in coal seams is known to improve the methane production of the coal seam, while ensuring a safe and long-term carbon sequestration. This improvement is due to the preferential adsorption of CO(2) in coal with respect to CH(4): an injection of CO(2) thus results in a desorption of CH(4). However, this preferential adsorption is also known to cause a differential swelling of coal, which results in a significant decrease in the reservoir permeability during the injection process. Recent studies have shown that adsorption in coal micropores (few angströms in size) is the main cause of the swelling. In this work, we focus on the competitive adsorption behavior of CO(2) and CH(4) in micropores. We perform molecular simulations of adsorption with a realistic atomistic model for coal. The competitive adsorption is studied at various temperatures and pressures representative of those in geological reservoirs. With the help of a poromechanical model, we then quantify the subsequent differential swelling induced by the computed adsorption behaviors. The differential swelling is almost insensitive to the geological temperatures and pressures considered here and is proportional to the CO(2) mole fraction in the coal.

A lithium–organic framework with coordinatively unsaturated metal sites that reversibly binds water

The synthesis and characterisation of a three-dimensional lithium-organic framework MIL-145 is described, which upon thermal treatment yields a second open framework, MIL-146, that contains four and three-coordinate lithium centres: the coordinatively unsaturated trigonal planar lithium centres are able to reversibly bind water with crystallinity maintained, while the dehydrated phase shows preferential adsorption of CO(2) over N(2).

Use of carbon monoxide and cyanide to probe the active sites on nitrogen-doped carbon catalysts for oxygen reduction

Interaction of carbon monoxide with CNx catalysts was investigated using pulse chemisorption, DRIFTS, and cyclic voltammetry techniques. Pulsed chemisorption experiments showed no CO uptake over the CNx catalysts. Cyclic voltammetry and potential hold studies showed carbon monoxide not to have any electrocatalytic interaction with nitrogen-doped graphite surfaces and not to have any poisoning effect for the oxygen reduction reaction. This is in contrast to the preferential adsorption of CO in the presence of oxygen on ORR sites of Pt catalysts, inhibiting the oxygen reduction activity drastically. Cyanide poisoning experiments performed using KCN showed no activity loss for CNx, while Pt/VC catalysts showed significant deactivation. These studies suggest that it is unlikely for coordinated metal sites to have significant contribution to the oxygen reduction reaction in CNx catalysts.

Understanding the Preferential Adsorption of CO2 over N2 in a Flexible Metal–Organic Framework

The unusual uptake behavior and preferential adsorption of CO(2) over N(2) are investigated in a flexible metal-organic framework system, Zn(2)(bdc)(2)(bpee), where bpdc = 4,4'-biphenyl dicarboxylate and bpee = 1,2-bis(4-pyridyl)ethylene, using Raman and IR spectroscopy. The results indicate that the interaction of CO(2) with the framework induces a twisting of one of its ligands, which is possible because of the type of connectivity of the carboxylate end group of the ligand to the metal center and the specific interaction of CO(2) with the framework. The flexibility of the bpee pillars allows the structure to respond to the twisting, fostering the adsorption of more CO(2). DFT calculations support the qualitative picture derived from the experimental analysis. The adsorption sites at higher loading have been identified using a modified van der Waals-Density Functional Theory method, showing that the more energetically favorable positions for the CO(2) molecules are closer to the C═C bond of the bpee and the C-C bond of the bpdc ligands instead of the benzene and pyridine rings of these ligands. These findings are consistent with changes observed using Raman spectroscopy, which is useful for detecting both specific guest-host interactions and structural changes in the framework.

Adsorption and Separation of CO2/CH4 on Amorphous Silica Molecular Sieve

Adsorption and separation of CO2/CH4 were examined on two kinds of amorphous silica molecular sieves, ASMS-3A and ASMS-5A, that are composed of spherical cavities arranged in a face-centered-cubic array and connected through narrow necks of molecular dimensions. For ASMS-3A, the exclusion of CH4 molecules due to their dimensions into the narrow necks and preferential adsorption of CO2 with the smaller kinetic diameter is observed at 283 K. In addition, the amount of CO2 adsorbed at 283 K does not decrease at all with surface hydration. All these results suggest that ASMS-3A is a promising candidate for the separation and purification of CO2 from various CO2/CH4 mixtures such as natural gas and landfill gas by a PSA process.

Carbonate Selective Ca2Ru2O7−y Pyrochlore Enabling Room Temperature Carbonate Fuel Cells I. Synthesis and Physical Characterization: II. Verification of Carbonate Cycle and Electrochemical Performance

The electrochemical selectivity of a Ca2Ru2O7−y pyrochlore catalyst towards carbonate formation was investigated and compared to platinum. During constant current operation, an increase in CO2 content was observed at the anode effluent when Ca2Ru2O7−y was used as the cathode catalyst, compared to Pt/C. Carbonate selectivity for Pt/C was 1.78, while for Ca2Ru2O7−y it was 7.33, a 4.1X increase, which confirmed the preferential formation of carbonate on the pyrochlore. This elevated selectivity was attributed to the high surface basicity of the pyrochlore catalyst, which led to preferential CO2 adsorption. During constant voltage operation, a 134% CO2 concentration increase was measured at the anode effluent when 10% CO2 was supplied to the cathode inlet. Optimal fuel cell performance on the carbonate cycle was obtained with a 90% O2/ 10% CO2 cathode concentration, where maximum currents where 4X compared to operation with no CO2. Higher CO2 concentrations were matched with a gradual decrease in performance. This was attributed to a relatively low electrochemical activity, which combined with high surface basicity, led to O2 site blocking. Thin-film electrode experiments showed excellent electrochemical stability on the oxygen reduction region. An increase in the current and a positive shift in the half-wave potential were observed when CO2 was added to the electrolyte in ex-situ three electrode experiments.

Study of active sites and mechanism responsible for highly selective CO oxidation in H 2 rich atmospheres on a mixed Cu and Ce oxide catalyst

Preferential oxidation of CO in H 2 was studied over a series of CuO x –CeO 2 mixed oxide catalysts with differing Cu loadings and preparation techniques to elucidate structure and mechanism parameters required to achieve high selectivity. Using in situ diffuse reflectance infra-red Fourier transform spectroscopy (DRIFTS), CO oxidation activity is linked to Cu + carbonyl species (∼2110 cm −1), while catalyst deactivation is associated with catalyst hydration. Active sites for both reactions are found to be reducible oxygen from highly dispersed or surface incorporated CuO x species in proximity to Ce, and are quantified utilizing anaerobic titrations. Active site concentrations (2.1–3.5 × 10 −6 mol O ∗ m −2) are correlated with surface copper content, as determined by X-ray photoelectron spectroscopy (XPS). High reaction selectivity towards CO oxidation (100% selectivity above 200 ppm CO at 333 K) is attributed to preferential adsorption of CO as well as inhibited H 2 dissociation due to oxidized copper sites. A detailed Mars and van Krevelen mechanism for CO and H 2 oxidation is proposed and supported by comparing the model to the experimental data.

Impact of thickness on CO2 concentration profiles within polymer films swollen near the critical pressure

Abstract The isothermal swelling of polymer thin films by a supercritical fluid does not increase monotonically with increasing chemical potential (pressure), but rather a maximum in swelling is generally observed near the critical pressure. A reactive templating approach utilizing the condensation of silica within hydrophilic domains of a swollen amphiphilic polymer film enables visualization of the qualitative concentration profile of CO2 by the changes in the size of hydrophobic domains (pores) with cross sectional TEM microscopy; specifically, isothermal swelling of poly(ethylene oxide-propylene oxide-ethylene oxide) films by CO2 at 60 °C is examined. Films that contain thickness gradients are used to avoid any uncertainties in the impact of thickness due to variations in the temperature or pressure during the silica modification. A uniform pore size (local swelling) is observed for all film thicknesses when the pressure is outside of the anomalous maximum in the film swelling, except for a small increase at the buried interface due to preferential adsorption of CO2 to the native silicon oxide surface of the substrate. However at this swelling maximum, a gradient in the pore size is observed at both interfaces. These swelling gradients at interfaces appear to be responsible for the anomalous maximum in thin films. As the film thickness increases beyond 350 nm, there is a decrease in the maximum swelling at the free interface.

In situ ATR-FTIR study of bulk CO oxidation on a polycrystalline Pt electrode

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) measurements were conducted to analyze the electrochemical oxidation of CO dissolved in bulk electrolyte solution on a polycrystalline Pt electrode during linear sweep (cyclic) voltammetry (CV) in 1% and 100% CO-saturated 0.1 M HClO4. A CO adlayer was first formed on Pt at 0.05 V vs. the reversible hydrogen electrode (RHE) during CO bubbling in each electrolyte, followed by the CV measurement. In 1% CO-saturated 0.1 M HClO4, a well-developed prepeak was found, showing an onset and a peak at around 0.25–0.3 V and 0.42 V, respectively. A major contribution of bridge-bonded CO, CO(B), to the prepeak is concluded based on a decrease in the corresponding band intensity. In 100% CO-saturated 0.1 M HClO4, the onset of bulk CO oxidation takes place around 0.25–0.3 V, which is closely associated with a band intensity decrease of CO(B), whereas atop (linear) CO, CO(L), did not exhibit intensity change in this potential region. This suggests that vacant sites made available upon oxidation of CO(B) serve as active sites for bulk CO oxidation. The oxidation of CO(B) at such low potentials is interpreted in terms of an adsorption energy on Pt that is lower than that for CO(L) and also of the specific structure of an adlayer that consists of intermixed CO(L) and CO(B). The bulk CO oxidation becomes diffusion-limited by dissolved CO above ca. 0.72 V, where we observed hardly any infrared spectral features ascribed to reaction intermediates. During a negative-going scan back to 0.05 V from 1.2 V, a steep decrease of the bulk CO oxidation current takes place around 0.66 V, at which preferential adsorption of CO(L) is observed. A rigid CO(L) island formation is strongly suggested from its high CO stretching frequency vs. its very small initial coverage and from its subsequent dependence on potential, with a linear Stark shift characterized by a slope of −29 cm−1/V. Such an island formation is in marked contrast to the adlayer structure with intermixed CO(L) and CO(B) initially formed at 0.05 V during CO bubbling. It is concluded that the Pt surface saturated with the CO adlayer formed initially at 0.05 V exhibits a low overpotential for bulk CO oxidation owing to its adlayer structure with intermixed atop and bridge-bonded CO.

Adsorption Behavior of Carbon Dioxide and Methane on AlPO4-14: A Neutral Molecular Sieve

AlPO4-14 molecular sieve was prepared by hydrothermal crystallization of saturated gels. The synthesized molecular sieve was characterized by X-ray diffraction and scanning electron microscopy. For the first time, the adsorption behavior of CO2 and CH4 was investigated on AlPO4-14, a neutral molecular sieve with no extra-framework cations. Adsorption isotherms of CO2 and CH4 on AlPO4-14 were examined at 273 and 300 K. Henry’s law constants and isosteric heats of adsorption of all adsorbates were determined from adsorption isotherms. It was found that CO2 was strongly adsorbed on the molecular sieve, and the selectivity of CO2/CH4 can reach as high as 21.77 at 273 K. The adsorbate−adsorbent interaction and the steric effect should be responsible for the preferential adsorption of CO2 on AlPO4-14.

First-principles investigations of the Ni 3Sn alloy at steam reforming conditions

The structure and surface composition of a Ni 3Sn alloy at conditions relevant for the steam reforming reaction was investigated using density functional theory calculations. Both the flat Ni 3Sn(0 0 0 1) surface and a surface with steps in the closed packed direction [1 0 1 ¯ 0]were considered. The adsorption geometries and energies of the species CO, C, OH and H were calculated. Chemical potentials were used to map out which adsorbates are on the surface under varying conditions. It was found that adsorbates preferably bind to Ni as nearest neighbor with Sn as second-nearest neighbor. The binding energy is slightly stronger than on pure Ni. Adsorbate binding to Sn was found to be very unfavorable. Binding free energies indicate that at high temperature the alloy surface will be predominantly covered by CO and C, and at low temperatures one may find H and almost no OH. Even though the nominal composition of the investigated alloy is Ni 3Sn, the surface composition may differ significantly depending on temperature and pressure of the gas phase. This effect was investigated by calculating segregation energies both in the absence and in the presence of adsorbates. For the flat surface, it was found that only the bulk termination is present under relevant conditions. In contrast, it was found that for steps preferential adsorption of CO and C on Ni sites may lead to adsorption-induced segregation at temperatures below 400 °C. When taking segregation into account, the most stable Ni 3Sn surfaces will not bind CO or C at the same condition that Ni does. This is in excellent agreement with the previously proven ability of Ni–Sn alloys to inhibit graphite formation.

Microwave assisted phase transformation of silicoaluminophosphate zeolite crystals

SAPO-34 zeolite displaying ∼0.5 µm crystal size with a narrow particle size distribution and preferential adsorption of CO2 over that of CH4 was prepared via phase transformation of SAPO-5 under microwave heating.

An amine-functionalized metal organic framework for preferential CO2 adsorption at low pressures

A metal organic framework with amine-lined pores gives high values for surface area and heat of adsorption with CO(2) gas.