A Novel Adsorption Cycle for CO2 Recovery: Experimental and Theoretical Investigations of a Temperature Swing Compression Process

Abstract

A novel adsorption cycle is examined experimentally and theoretically for recovering carbon dioxide from a 50 mol% mixture with carbon monoxide. Several adsorbents are considered, and zeolite NaY is chosen for the process due to its high capacity and selectivity for CO₂ in the presence of CO. The process consists of three steps. The bed is fed the gas mixture at 273 K until CO₂ breakthrough occurs. The bed then undergoes countercurrent blowdown of CO₂ while heating at 391 K and is finally cooled to the initial feed temperature once the bed has been depleted of CO₂. Results are presented from laboratory scale experiments and are described using numerical simulations. This novel cycle provides a method for capturing and producing CO₂ without the need for a purge gas and has low energy requirements if waste heat is available.     

Effective Removal of Methylene Blue on EuVO4/g-C3N4 Mesoporous Nanosheets via Coupling Adsorption and Photocatalysis

In this study, we first manufactured ultrathin g-C₃N₄ (CN) nanosheets by thermal etching and ultrasonic techniques. Then, EuVO₄ (EV) nanoparticles were loaded onto CN nanosheets to form EuVO₄/g-C₃N₄ heterojunctions (EVCs). The ultrathin and porous structure of the EVCs increased the specific surface area and reaction active sites. The formation of the heterostructure extended visible light absorption and accelerated the separation of charge carriers. These two factors were advantageous to promote the synergistic effect of adsorption and photocatalysis, and ultimately enhanced the adsorption capability and photocatalytic removal efficiency of methylene blue (MB). EVC-2 (2 wt% of EV) exhibited the highest adsorption and photocatalytic performance. Almost 100% of MB was eliminated via the adsorption–photocatalysis synergistic process over EVC-2. The MB adsorption capability of EVC-2 was 6.2 times that of CN, and the zero-orderreaction rate constant was 5 times that of CN. The MB adsorption on EVC-2 followed the pseudo second-order kinetics model and the adsorption isotherm data complied with the Langmuir isotherm model. The photocatalytic degradation data of MB on EVC-2 obeyed the zero-order kinetics equation in 0–10 min and abided by the first-order kinetics equation for10–30 min. This study provided a promising EVC heterojunctions with superior synergetic effect of adsorption and photocatalysis for the potential application in wastewater treatment.     

Disclosing the Biocide Activity of α-Ag2−2xCuxWO4 (0 ≤ x ≤ 0.16) Solid Solutions

In this work, α-Ag_2−2xCu_xWO₄ (0 ≤ x ≤ 0.16) solid solutions with enhanced antibacterial (against methicillin-resistant Staphylococcus aureus) and antifungal (against Candida albicans) activities are reported. A plethora of techniques (X-ray diffraction with Rietveld refinements, inductively coupled plasma atomic emission spectrometry, micro-Raman spectroscopy, attenuated total reflectance–Fourier transform infrared spectroscopy, field emission scanning electron microscopy, ultraviolet–visible spectroscopy, photoluminescence emissions, and X-ray photoelectron spectroscopy) were employed to characterize the as-synthetized samples and determine the local coordination geometry of Cu²⁺ cations at the orthorhombic lattice. To find a correlation between morphology and biocide activity, the experimental results were sustained by first-principles calculations at the density functional theory level to decipher the cluster coordinations and electronic properties of the exposed surfaces. Based on the analysis of the under-coordinated Ag and Cu clusters at the (010) and (101) exposed surfaces, we propose a mechanism to explain the biocide activity of these solid solutions.     

Room temperature decomposition of N2O in the presence of gaseous oxygen on prereduced Rh supported catalysts

The room temperature decomposition of N₂O over prereduced Rh‐based catalysts (Rh supported on ceria, zirconia and titania–alumina) is studied as a function of the oxygen content in the feed. Results indicate that Rh supported on titania–alumina shows lower degree of inhibition by gaseous oxygen on this reaction, attributed to the role of the metal particle–support interface region in the reaction. The effect of Rh loading and of the reaction temperature are consistent with the hypothesis. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Characterization of the Micropore Structures of Some Activated Carbons of Plant Origin by N2 and CO2 Adsorptions

Abstract

In this study, active carbons prepared from almond and hazelnut shells under various experimental conditions were investigated. Merck-2514 and Merck-2184 active carbons were used for comparison. N₂ (77 K) gas and CO₂ (273 and 195 K) gas adsorptions were determined as comparison criteria. Regarding the specific surface area and micropore volume results obtained from these adsorption data, it is concluded that N₂ (77 K) adsorption by itself is inadequate in the characterization of active carbons which are low-sized microporous dominated. In addition, it is concluded that it would be useful to investigate CO₂ (195 and 273 K) adsorption. The iodine and methylene blue tests at 298 K were also applied for the characterization of the carbon adsorbents mentioned. From these data it was seen that the iodine test can be applied as a total porosity indicator and that the methylene blue test can be used as a developed microporosity indicator. These results indicate that the best adsorbents were those prepared from hazelnut shells. Depending on the preparation conditions, the physically activated carbon has an activation time up to 4 hours and has adsorption properties on the level of Merck commercial carbons.     

Separation of CO2 and N2 on Zeolite Silicalate-1 Membrane Synthesized on Novel Support

This paper reports on the properties of an MFI-type zeolite (silicalite-1) membrane synthesized on a novel tubular support with a 0.45 µm-pore size active layer consisting of zirconium and titanium oxides. Even though the membrane was synthesized by a pore plugging method, apart from penetrating into the support, the silicalite-1 crystals formed a 1.5 µm layer on top of the support. After the zeolite synthesis, the Si constituted more than 35% of the active layer of the support, which implies small size and close packing of the silicalite-1 crystals in the pores of the active layer.

Single gas permeation tests with N₂ and CO₂ revealed comparable N₂ and CO₂ permeances. On the other hand, CO₂/N₂ gas separation tests performed at different total feed pressures and feed compositions lead to CO₂/N₂ permselectivities as high as 26.0, with the corresponding CO₂ permeance of 6 × 10^?8 mol/m² Pa s. The effects of changing the partial pressure gradient of CO₂ across the membrane by means of varying the total feed pressure and the feed composition on the CO₂ permeance and CO₂/N₂ permselectivity are discussed.     

The temperature-programmed desorption of N2 from a Ru/MgO catalyst used for ammonia synthesis

The temperature-programmed desorption (TPD) of N₂ from a Ru/MgO catalyst used for ammonia synthesis was studied in a microreactor flow system operating at atmospheric pressure. Saturation with chemisorbed atomic nitrogen (N-*) was achieved by exposure to N₂ at 573 K for 14 h and subsequent cooling in N₂ to room temperature. With a heating rate of 5 K/min in He, a narrow and fairly symmetric N₂ TPD peak at about 640 K results. From experiments with varying heating rates a preexponential factor A_des = 1.5×10¹⁰ molecules/(site s) and an activation energy E_des = 158 kJ/mol was derived assuming secondorder desorption. This rate constant of desorption is in good agreement with results obtained with a Ru(0001) single crystal surface in ultra-high vacuum (UHV). The rate of dissociative chemisorption was determined by varying the N₂ exposure conditions. Determination of the coverage of N-^* was based on the integration of the subsequently recorded N₂ TPD traces yielding A_ads = 2×10^–6 (Pa s)^–1 and E_ads = 27 kJ/mol. The corresponding sticking coefficient of about 10^–14 at 300 K is in agreement with the inertness of Ru(0001) in UHV towards dissociative chemisorption of N₂. However, if the whole catalytic surface were in this state, then the resulting rate of N₂ dissociation would be several orders of magnitude lower than the observed rate of NH₃ formation. Hence only a small fraction of the total Rumetal surface area of Ru/MgO seems to be highly active dominating the rate of ammonia formation.     

Effect of TiO2 addition on the microstructures,mechanical and dielectric properties of porous Si3N4-based ceramics

Porous Si₃N₄-based ceramics with different TiO₂ contents were prepared by gas pressure sintering method. The effects of TiO₂ addition ranging from 0 to 25?wt-% on the phase compositions, microstructures, mechanical performance and dielectric properties were investigated. The addition of TiO₂ significantly promoted the density which increased from 1.64 to about 2.3?g?cm^?3. The mechanical properties of porous Si₃N₄-based ceramics with TiO₂ addition decreased first and then increased with the increase of TiO₂ content, and the flexural strength and elastic modulus are more than 167.4?MPa and 72.8?GPa, respectively, which were higher than that of the Si₃N₄ ceramic without TiO₂ addition. With the increase of TiO₂ content, both the dielectric constant and dielectric loss increased, and the dielectric constant enhanced obviously. These results suggested that the TiO₂ was beneficial for the improvement of mechanical properties and dielectric constant of porous Si₃N₄-based ceramics.     

Photocatalytic reduction of N2O on metal-supported TiO2 powder at room temperature in the presence of H2O and CH3OH vapor

Ag- and Cu-supported TiO₂ photocatalysts showed high activity for the reduction of N₂O to N₂ at room temperature in the presence of CH₃OH and H₂O vapor. The suppression by H₂O on the activity was not observed in the present photocatalyst system. The remarkable behavior of the Ag and Cu co-catalysts for TiO₂ photocatalysts agreed well with that of electro- and thermal catalyses. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pressure-Swing Adsorption Separation of H2S from CO2 with Molecular Sieves 4A, 5A,and 13X

The separation of bulk quantities of H₂S from CO₂ was investigated through a series of pressure-swing adsorption experiments utilizing 4A, 5A, and 13X molecular sieves. High selectivity of H₂S over CO₂ was encountered for all sieves, particularly for the 13X and 5A. Practically pure CO₂ was produced in the adsorption stage with fresh 5A and 13X sieves, at high product recovery rates. Efficient H₂S purification was obtained with fresh 5A and regenerated 4A zeolites. The experimental results were in line with theoretical predictions of the literature.     

The Synergistic Effect of Adsorption-Photocatalysis for Removal of Organic Pollutants on Mesoporous Cu2V2O7/Cu3V2O8/g-C3N4 Heterojunction

Cu₂V₂O₇/Cu₃V₂O₈/g-C₃N₄ heterojunctions (CVCs) were prepared successfully by the reheating synthesis method. The thermal etching process increased the specific surface area. The formation of heterojunctions enhanced the visible light absorption and improved the separation efficiency of photoinduced charge carriers. Therefore, CVCs exhibited superior adsorption capacity and photocatalytic performance in comparison with pristine g-C₃N₄ (CN). CVC-2 (containing 2 wt% of Cu₂V₂O₇/Cu₃V₂O₈) possessed the best synergistic removal efficiency for removal of dyes and antibiotics, in which 96.2% of methylene blue (MB), 97.3% of rhodamine B (RhB), 83.0% of ciprofloxacin (CIP), 86.0% of tetracycline (TC) and 80.5% of oxytetracycline (OTC) were eliminated by the adsorption and photocatalysis synergistic effect under visible light irradiation. The pseudo first order rate constants of MB and RhB photocatalytic degradation on CVC-2 were 3 times and 10 times that of pristine CN. For photocatalytic degradation of CIP, TC and OTC, it was 3.6, 1.8 and 6.1 times that of CN. DRS, XPS VB and ESR results suggested that CVCs had the characteristics of a Z-scheme photocatalytic system. This study provides a reliable reference for the treatment of real wastewater by the adsorption and photocatalysis synergistic process.     

Adsorption of CO2 and SO2 on activated carbons with a wide range of micropore size distribution

The adsorption isotherms of N₂ at 77K, CO₂ at 251, 273 and 298K, and SO₂ at 262 and 273K have been determined on a series of physically activated carbons with a wide range of micropore size distributions. Since the series includes carbons with very high burn-off, it shows the problems involved in the characterization of microporsity in superactivated carbons. On the other hand, the results show that the carbon surface-adsorbate interactions for SO₂ at low relative pressures are weaker than for N₂ and CO₂, as a result of the strong adsorptive-adsorptive interactions in the bulk gas phase.     

Adsorption of CH3 and its reactions with CO2 over TiO2

The adsorption, decomposition of CH₃ and its reactions with CO₂ were followed by means of Fourier transform infrared spectroscopy combined with mass spectrometry. Methyl radicals were produced by the pyrolysis of azomethane. Absorption bands, observed at room temperature adsorption, were attributed to adsorbed CH₃ and CH₃O species. The decomposition of adsorbed CH₃ in vacuum started above 400 K and was accelerated by CO₂. In the study of the interaction of methane with titania, activated in different ways, we found no convincing spectroscopic evidence for the activation of methane at 300 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of Na Addition, Pyridine Preadsorption, and Water Preadsorption on the Hydrogen Adsorption Property of Pt/SO4 2--ZrO2

Hydrogen adsorption was studied for Pt/SO₄ ^2--ZrO₂ samples modified with Na addition, pyridine preadsorption, and water preadsorption to elucidate the relation between the rate and capacity of hydrogen adsorption and the surface state. The surface states were monitored by XRD, IR, ammonia TPD, and measurement of catalytic activity for cyclohexane isomerization. All the modifications suppressed the hydrogen uptake. It was suggested that Lewis acid sites promote the hydrogen uptake by stabilizing spiltover hydrogen atoms.     

Enhancement of C2H6 Oxidation by O2 in the Presence of N2O over Fe Ion-Exchanged BEA Zeolite Catalyst

Selective catalytic reduction (SCR) of N₂O with C₂H₆ took place effectively over Fe ion-exchanged BEA zeolite catalyst (Fe-BEA) even in the presence of excess oxygen. The mechanism in the SCR of N₂O with C₂H₆ over Fe-BEA catalyst was studied by a transient response experiment and an in situ DRIFT spectroscopy. No oxidation of C₂H₆ by O₂ took place below 350 °C (in C₂H₆/O₂). In the N₂O/C₂H₆/O₂ system, however, it was found that the reaction of C₂H₆ with O₂ was drastically enhanced by the presence of N₂O even at low temperatures (200-300 °C). Therefore, it was concluded that N₂O played an important role in the oxidation of C₂H₆ (i.e., activation of C₂H₆ at an initial step). On the basis of these findings, the mechanism in the SCR of N₂O with C₂H₆ is discussed.     

CO2, N2 gas sorption and permeation behavior of chitosan membrane

The sorption equilibria for CO₂ and N₂ in dry chitosan membrane at 20 and 30 ‡C were measured by a pressure decay method. The steady-state permeation rates for CO₂ and N₂ in dry and wet (swollen with water vapor) chitosan membranes at 20 and 30 ‡C were measured by a variable volume method. The sorption equilibrium for N₂ obeyed Henry’s law, whereas that for CO₂ was described apparently by a dual-mode sorption model. This non-linear sorption equilibrium for CO₂ could be interpreted by the interaction of sorbed CO₂ with the chitosan matrix expressed as a reversible reaction. The logarithm of the mean permeability coefficient for CO₂ in dry chitosan membrane increased linearly with upstream gas pressure. A linear increase of the logarithmic mean permeability coefficient for CO₂ with the pressure could be interpreted in terms of a modified free-volume model. The mean per-meability coefficient for N₂ in dry chitosan membrane only slightly increased with upstream gas pressure. The per-meabilities for CO₂ and N₂ in wet chitosan membrane increased by 15 to 17 times and 11 to 15 times, respectively, as compared to those in the dry membrane.     

Adsorption and Reactions of CH2I2 on Clean and Oxygen-Modified Ag(111): a RAIRS and TPD Study

The surface chemistry of CH₂I₂ on Ag(111) in the presence and absence of pre-adsorbed O, produced by NO₂ adsorption at elevated temperature, has been examined using temperature-programmed desorption and reflection absorption infrared spectroscopy. There is good evidence for the formation of adsorbed methylene, CH₂(a), that reacts with another CH₂(a) to form and desorb ethylene, C₂H₄(g), in a reaction-limited process. Increasing the surface coverage of CH₂I₂ hinders both the dissociation and recombination processes indicated by the upward temperature shift in the formation of C₂H₄. Co-adsorbed O atoms strengthen the bonding of CH₂I₂ to the surface; the increased thermal stability is up to 60 K. The formation of C₂H₄ decreases with increasing amounts of pre-adsorbed O; the main reaction product is CH₂O produced in a reaction-limited process. CH₂O forms either on the chemisorbed or on the oxide phase with desorption peak temperatures of 225 and 270 K, respectively. The formation of gas-phase carbon dioxide suggests that a formate intermediate is involved in a secondary reaction pathway.     

Construction of Highly Active Zn3In2S6 (110)/g-C3N4 System by Low Temperature Solvothermal for Efficient Degradation of Tetracycline under Visible Light

Herein, Zn₃In₂S₆ photocatalyst with (110) exposed facet was prepared by low temperature solvothermal method. On this basis, a highly efficient binary Zn₃In₂S₆/g-C₃N₄ was obtained by low temperature solvothermal method and applied to the degradation of tetracycline (TC). The samples of the preparation were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope, UV–vis diffuse reflection spectroscopy, and photoluminescence spectroscopy. Furthermore, the degradation performance of photocatalysts on TC was investigated under different experimental conditions. Finally, the mechanism of Zn₃In₂S₆/g-C₃N₄ composite material degrading TC is discussed. The results show that Zn₃In₂S₆ and Zn₃In₂S₆/g-C₃N₄ photocatalysts with excellent performance could be successfully prepared at lower temperature. The Zn₃In₂S₆/g-C₃N₄ heterojunction photocatalyst could significantly improve the photocatalytic activity compared with g-C₃N₄. After 150 min of illumination, the efficiency of 80%Zn₃In₂S₆/g-C₃N₄ to degrade TC was 1.35 times that of g-C₃N₄. The improvement of photocatalytic activity was due to the formation of Zn₃In₂S₆/g-C₃N₄ heterojunction, which promoted the transfer of photogenerated electron–holes. The cycle experiment test confirmed that Zn₃In₂S₆/g-C₃N₄ composite material had excellent stability. The free radical capture experiment showed that ·O₂⁻ was the primary active material. This study provides a new strategy for the preparation of photocatalysts with excellent performance at low temperature.     

二维AuP2材料电催化固氮性能的理论研究

通过电化学反应将氮气（N₂）和水（H₂O）在常温常压的条件下转化为氨气（NH₃）是一种绿色环保的合成氨方法。但由于N₂具有非常高的化学惰性,必须借助电催化剂来加速反应的动力学过程。通过密度泛函理论计算揭示出新型二维无机材料AuP₂对N₂电化学还原制NH₃具有很好的催化活性。在二维AuP₂材料中,Au与P之间由于电负性差异发生显著的电荷转移,使带有正电荷的P可作为活性位点促进氮还原。计算表明整个反应的速控步是N₂生成^*NNH的过程,限制电压为1.2 V,催化活性可以跟部分金属催化剂相媲美。为设计高效氮还原电催化剂提供了新的思路。