CO2 adsorption on branched polyethyleneimine-impregnated mesoporous silica SBA-15

Adsorption of pure CO₂ on SBA-15 impregnated with branched polyethyleneimine (PEI) has been studied. Materials were prepared by impregnating the pore surface of SBA-15 mesoporous silica with different amounts of branched PEI (10, 30, 50 and 70 wt%). Textural properties, elemental analysis and low angle XRD measurements of the prepared samples showed a progressive pore filling of SBA-15 as PEI loading was increased. Pure CO₂ adsorption isotherms on these modified SBA-15 materials were obtained at 45 °C, showing high adsorption efficiency for CO₂ removal at 1 bar. Chemisorption of CO₂ on amino sites of the modified SBA-15 seems to be the main adsorption mechanism. PEI content of impregnated SBA-15 influences the adsorption capacity of the material, being a relevant variable for CO₂ removal by adsorption. Temperature effect on adsorption was also studied in the range 25-75 °C, showing that temperature strongly influences CO₂ adsorption capacity. Adsorption capacity was also tested after regeneration of the PEI-impregnated SBA-15 materials. Our results show that these branched PEI-impregnated materials are very efficient even at low pressure and after several adsorption-regeneration cycles.     

Adsorption of CO2 on PbO at ambient temperature

The adsorption of CO₂ on metal oxides at ambient temperature received less study largely due to the small adsorption amount. However, the adsorption is of interest in refreshing the atmosphere of isolated spaces. It was shown in the present work that P_bO was sensitive to low concentration CO₂ in the presence of water. An XPS examination indicated that P_bO changed to P_bCO₃ after the adsorption of CO₂; therefore, the adsorption is chemical in nature. In order to enlarge the CO₂ capacity, P_bO was dispersed on the surface of a silica gel with large surface area (710 m²/g). Both CO₂ capacity and adsorption rate indicated that the optimal dispersion manner of P_bO is the mono-molecular layer surface coverage. Breakthrough experiments showed that the prepared adsorbent could effectively capture low-concentration CO₂ at ambient temperature and pressure yielding a CO₂ capacity of 59.1 mg g⁻¹. The saturated adsorbent was regenerated on heating at 380 °C and the CO₂ capability was recovered.     

Regeneration of thiol-functionalized mesostructured silica adsorbents of mercury

The regeneration of thiol-functionalized SBA-15 adsorbents of mercury is presented in this article. The influence of temperature and pH on the adsorption process was studied. The effect due to the presence of complexing agents in aqueous solution on the desorption step was also evaluated. Hg(II) maximum adsorption capacities at different temperatures ranging from 20 °C to 60 °C were obtained and it was found that temperature does not affect the adsorption process. Mercury adsorption capacity was also determined in the presence of HNO₃ and HCl up to 3 M concentration. The comparison of the results showed that whereas hydrochloric acid exhibits an appreciable capacity to regenerate the thiol-functionalized SBA-15 adsorbent, the nitric acid results inefficient. The difference was attributed to the mercury complexing ability of chloride anion. Four complexing compounds, KBr, KSCN, (NH₂)₂CS, and HBr were tested for desorbing mercury in regeneration experiments. All agents were able to remove significant amounts of adsorbed mercury, being hydrobromic acid the complexing compound that yields the best results.     

Ammonia modification of activated carbon to enhance carbon dioxide adsorption: Effect of pre-oxidation

A commercial granular activated carbon (GAC) was subjected to thermal treatment with ammonia for obtaining an efficient carbon dioxide (CO₂) adsorbent. In general, CO₂ adsorption capacity of activated carbon can be increased by introduction of basic nitrogen functionalities onto the carbon surface. In this work, the effect of oxygen surface groups before introduction of basic nitrogen functionalities to the carbon surface on CO₂ adsorption capacity was investigated. For this purpose two different approaches of ammonia treatment without preliminary oxidation and amination of oxidized samples were studied. Modified carbons were characterized by elemental analysis and Fourier Transform Infrared spectroscopy (FT-IR) to study the impact of changes in surface chemistry and formation of specific surface groups on adsorption properties. The texture of the samples was characterized by conducting N₂ adsorption/desorption at −196 °C. CO₂ capture performance of the samples was investigated using a thermogravimetric analysis (TGA). It was found that in both modification techniques, the presence of nitrogen functionalities on carbon surface generally increased the CO₂ adsorption capacity. The results indicated that oxidation followed by high temperature ammonia treatment (800 °C) considerably enhanced the CO₂ uptake at higher temperatures.     

Functionalized SBA-15 materials for bilirubin adsorption

To investigate the driving force for bilirubin adsorption on mesoporous materials, a comparative study was carried out between pure siliceous SBA-15 and three functionalized SBA-15 mesoporous materials: CH₃-SBA-15 (MS), NH₂-SBA-15 (AS), and CH₃/NH₂-SBA-15 (AMS) that were synthesized by one-pot method. The obtained materials exhibited large surface areas (553-810 m²/g) and pore size (6.6-7.1 nm) demonstrated by XRD and N₂-ad/desorption analysis. The SEM images showed that the materials had similar fiberlike morphology. The functionalization extent was calculated according to ²⁹Si MAS NMR spectra and it was close to the designed value (10%). The synthesized mesoporous materials were used as bilirubin adsorbents and showed higher bilirubin adsorption capacities than the commercial active carbon. The adsorption capacities of amine functionalized samples AMS and AS were larger than those of pure siliceous SBA-15 and MS, indicating that electrostatic interaction was the dominant driving force for bilirubin adsorption on mesoporous materials. Increasing the ionic strength of bilirubin solution by adding NaCl would decrease the bilirubin adsorption capacity of mesoporous material, which further demonstrated that the electrostatic interaction was the dominant driving force for bilirubin adsorption. In addition, the hydrophobic interaction provided by methyl groups could promote the bilirubin adsorption.     

High surface area thermally stabilized porous iron oxide/silica nanocomposites via a formamide modified sol-gel process

Iron oxide/silica (Fe:Si as 1:10 atomic ratio) composite materials have been prepared by calcination for 3 h at different temperatures (400-900 °C) of xerogel precursor obtained via a formamide modified sol-gel process. The process involved TEOS and iron(III) nitrate, nitric acid and formamide. Genesis of the composite materials from the xerogel precursor has been investigated by TGA, DSC, FTIR, XRD, SEM and EDX. Results indicated that all the calcined composites are mainly composed of amorphous iron oxide dispersed as finely divided particles in amorphous silica matrixes. Nitrogen adsorption/desorption isotherms revealed a reversible type I of isotherms indicative of microporosity. However, high S_BET surface area and microsporosity were observed for the calcined composite materials (e.g. S_BET = 625 m² g⁻¹, and S_αs = 556 m² g⁻¹ for the composite calcined at 400 °C). Formation of the porous texture was discussed in terms of the action of formamide, which enhanced strengthening of the silica gel network during evaporation of the more volatile components within the composite body during the drying process.     

Studies on ordered mesoporous materials for potential environmental and clean energy applications

Two series of ordered mesoporous materials, SBA-15 silica and CMK-3 carbon were synthesized. The ordered nanostructure of these materials was confirmed by TEM and XRD analysis. Structural parameters including the specific surface area, pore volume and pore size distribution were determined on the basis of nitrogen adsorption data at 77 K. Potential applications of these materials were explored in relation to the CO₂ sequestering, methane storage and fuel desulfurization. Initial studies of both materials showed their usefulness for environmental and clean energy applications. SBA-15 modified with triethanolamine showed a very good adsorption selectivity for CO₂ while its adsorption reversibility was retained. Also, this material after CuCl deposition was useful for removal of fuel thiophenes. However, CMK-3 was shown to be promising material for storage of natural gas. As high as 41 wt.% of methane was stored in this material in the presence of appropriate amount of water.     

Adsorption of octadecyltrichlorosilane on mesoporous SBA-15

Adsorption of octadecyltrichlorosilane (OTS) on mesoporous SBA-15 has been studied by using Brunauer-Emmett-Teller (BET) surface area analysis, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermo-gravimetric analysis (TGA) techniques. BET surface area analysis shows decrease of surface area from 930 to 416 m²/g after OTS adsorption. SEM pictures show close attachment of SBA-15 particles. EDAX measurements show increase of carbon weight percentage and decrease of oxygen and silicon weight percentage. XPS results closely support EDAX analysis. FTIR spectra shows presence of methyl (-CH₃) and methylene (-CH₂) bands and oriented OTS monolayer on SBA-15. Thermo-gravimetric analysis shows that the OTS adsorbed on SBA-15 are stable up to a temperature of 230 °C and that the OTS monolayers decompose between 230 and 400 °C.     

Reactivity screening of silica

As a screening of the chemical activity of silica [SiO₂/Si(1 0 0)], which is one of the most often used supports for nanostructures, thermal desorption spectroscopy data have been gathered for a variety of gases such as n-nonane, n-hexane, n-butane, iso-butane, ethane, CO₂, CO, O₂, and H/H₂. Whereas, the alkanes with chain lengths larger than three adsorb with large binding energies (E_d = 50-70 kJ/mol), the activity towards the other probe molecules is negligible (<24 kJ/mol) down to adsorption temperatures of 95 K. The adsorption of n- and iso-butane has additionally been studied by molecular beam scattering and follows standard precursor mediated adsorption dynamics.     

CO2 adsorption on the bimetallic Zn-on-Cu(1 1 0) system

Adsorption probability measurements (molecular beam scattering) have been conducted to examine the adsorption dynamics (i.e. the gas-surface energy transfer processes) of CO₂ adsorption on the Zn-on-Cu(1 1 0) bimetallic system. The results indicate surface alloy formation, which is in agreement with prior studies. Depositing Zn at 300 K on Cu(1 1 0), above the condensation temperature of CO₂, leads to a “blocking” of CO₂ adsorption sites by Zn which is incorporated in the Cu(1 1 0) surface. This apparent site blocking effect indicates a lowering of the CO₂ binding energy on the alloyed surface as compared with the clean Cu(1 1 0) support. The Zn coverage has been calibrated by Auger electron spectroscopy and thermal desorption spectroscopy.     

Adsorption of CO, CO2 and NO molecules on a BaTiO3 (0 0 1) surface

Since the development of Scanning Tunnelling Microscopy (STM) technique, considerable attention has been devoted to various molecules adsorbed on various surfaces. Also, a new concept emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale are thus particularly invaluable. The present work describes the first Density Functional Theory (DFT) study of adsorption of CO, CO₂ and NO molecules on a BaTiO₃ surface following a first preliminary calculation of O and O₂ adsorption on the same surface. In the previously considered work, we found that a (0 0 1) surface with BaO termination is more stable than the one with TiO₂-termination. Consequently, we extended our study to CO, CO₂ and NO molecules adsorbed on a (0 0 1) surface with BaO termination. The present calculation was performed on a (1 × 1) cell with one monolayer of adsorbed molecules. Especially, a series of cases implying CO molecules adsorbed in various geometrical configurations has been examined. The corresponding adsorption energy varies in the range of −0.17 to −0.10 eV. The adsorption energy of a CO₂ molecule directly located above an O surface atom (called O_s) is of the order of −0.18 eV. The O-C distance length is then 1.24 Å and the O-C-O and O-C-O_s angles are 134.0° and 113.0°, respectively. For NO adsorption, the most important induced structural changes are the followings: (i) the N-O bond is broken when a NO molecule is absorbed on a Ba-O_s bridge site. In that case, N and O atoms are located above an O and a Ba surface atom, respectively, whereas the O-Ba-O_s and N-O_s-Ba angles are 106.5° and 63.0°, respectively. The N-O distance is as large as 2.58 Å and the adsorption energy is as much as −2.28 eV. (ii) In the second stable position, the NO molecule has its N atom adsorbed above an O_s atom, the N-O axis being tilted toward the Ba atom. The N-O_s-Ba angle is then 41.1° while the adsorption energy is only −0.10 eV. At last, the local densities of states around C, O as well as N atoms of the considered adsorbed molecules have also been discussed.     

Characterization of the synthesis and reactivity behavior of nanostructured vanadia model catalysts using XPS and vibrational spectroscopy

Nanostructured vanadia model catalysts, i.e., highly dispersed vanadium oxide supported on mesoporous silica SBA-15 (VO_x/SBA-15), were prepared. The mechanism for the synthesis of VO_x/SBA-15 was elucidated by detailed characterization of the individual synthesis steps using XPS and vibrational spectroscopy. The resulting surface vanadium oxide species (0-2.3 V/nm²), grafted on the inner pores of the SBA-15 silica matrix, consists of tetrahedrally coordinated vanadia as inferred from UV-VIS- and Raman spectroscopy. The prepared vanadia model catalysts were tested in the partial oxidation of methanol to formaldehyde yielding high formaldehyde selectivities of 94% at 350 °C. XPS and Raman analysis of the catalyst after reaction reveal the presence of methoxy as well as a significant amount of carbonaceous species on the surface. Our results demonstrate that a detailed understanding of partial oxidation reactions requires the combination of complementary spectroscopic techniques ultimately within one experimental set-up.     

Facile post-synthesis and photocatalytic activity of N-doped ZnO–SBA-15

N-doped ZnO–SBA-15 materials (denoted as nN–xZnO–SBA-15, where n is number of urea treatments and x is the weight ratio of ZnO/(ZnO+SBA-15)) were successfully synthesized by a two-step procedure. First, xZnO–SBA-15 was prepared by impregnating SBA-15 with Zn(NO₃)₂, followed by calcinating at 550 °C. In the second step, xZnO–SBA-15 was modified n times by doping nitrogen with the assistance of urea. The resulting nN–xZnO–SBA-15 materials prepared with various numbers of urea treatments were characterized by XRD, TEM, SEM, EDS, N₂ adsorption/desorption at 77 K, diffuse reflectance UV–vis, and XPS. The results show that the nN–xZnO–SBA-15 maintains its ordered hexagonal mesostructure and exhibits light absorbance in the visible region. The nN–xZnO–SBA-15 samples were investigated with the photodegradation of methylene blue under visible light, and exhibited significant photocatalytic activity. The kinetics of the reaction obeyed the Langmuir–Hinshelwood model.     

Structure and acid-base properties of surface-modified mesoporous silica

A series of surface-modified mesoporous silica endowing with acid-base properties have been successfully synthesized in one pot by in situ introduction of zirconium and magnesium salts into the initial mixture of synthesizing mesoporous silica (SBA-15) and this method combines into a single step to form a novel material with a periodically ordered mesoporous backbone and specific chemical reactivity of the acid-basic sites. X-ray diffraction, high-resolution transmission electron microscopes (HRTEM), N₂ adsorption, FT-IR transmission spectra, ²⁹Si MAS NMR spectra, NH₃- and CO₂-temperature programmed desorption (TPD) are employed to characterize the titled mesoporous materials. The results indicate that the product possesses excellent acid-basic properties with well mesoporous structure, which make it promising for their application in heterogeneous catalysis and adsorption-separation processes.     

Facile synthesis and characterization of acid-base bifunctionalized mesoporous silica

A simple acid-base bifunctionalized approach has been developed through grinding the precursors, magnesium and aluminium nitrates, with the as-prepared SBA-15, and then the generation of acid-base active sites and removal of host template were combined into a single step in the subsequent calcination procedure. A series of acid-base bifunctional mesoporous materials MgO-Al₂O₃-SBA-15 (MA-SBA-15) have been successfully synthesized by means of this approach. X-ray diffraction (XRD), high-resolution transmission electron microscopes (HRTEMs), N₂ adsorption, FT-IR spectra, ²⁷Al and ²⁹Si magic-angle-spinning (MAS) NMR, NH₃- and CO₂-temperature programmed desorption (TPD), pyridine adsorption were employed to characterize the resultant mesoporous materials. The results indicate that the guests can be well dispersed in the channel of SBA-15, and the resultant materials exhibit excellent acid-basic properties with well mesoporous backbone, which make it possessing high activity for the synthesis of ethyl methyl carbonate, an important asymmetric carbonate ester compound.     

Naproxen drug delivery using periodic mesoporous silica SBA-15

In this paper, we present the release of naproxen from hexagonal periodic mesoporous silica SBA-15, which serves as a drug delivery system. Naproxen, the well-known nonsteroidal anti-inflammatory drug (NSAID), was loaded into the pores of SBA-15 silica modified with aminopropyl groups. The physicochemical properties of the modified sample (A-SBA-15/napro) were compared with the unmodified SBA-15 mesoporous silica loaded with the drug (SBA-15/napro). The kinetic of the naproxen release into the physiological solution was studied. The released amount of naproxen represented 90.7% from the unmodified SBA-15 in 72 h, while from the sample A-SBA-15/napro the released amount represented about 80.9%. The prepared materials were characterized by nitrogen adsorption/desorption, Small angle X-ray scattering (SAXS), Fourier-transform infrared spectroscopy (FT-IR) and the thermoanalytical methods (TG/DTA). Thin layer chromatography (TLC) was used for quantitative determination of the released naproxen.     

CO2 adsorption on Cr(1 1 0) and Cr2O3(0 0 0 1)/Cr(1 1 0)

We attempt to correlate qualitatively the surface structure with the chemical activity for a metal surface, Cr(1 1 0), and one of its surface oxides, Cr₂O₃(0 0 0 1)/Cr(1 1 0). The kinetics and dynamics of CO₂ adsorption have been studied by low energy electron diffraction (LEED), Aug er electron spectroscopy (AES), and thermal desorption spectroscopy (TDS), as well as adsorption probability measurements conducted for impact energies of E_i = 0.1-1.1 eV and adsorption temperatures of T_s = 92-135 K. The Cr(1 1 0) surface is characterized by a square shaped LEED pattern, contamination free Cr AES, and a single dominant TDS peak (binding energy E_d = 33.3 kJ/mol, first order pre-exponential 1 × 10¹³ s⁻¹). The oxide exhibits a hexagonal shaped LEED pattern, Cr AES with an additional O-line, and two TDS peaks (E_d = 39.5 and 30.5 kJ/mol). The initial adsorption probability, S₀, is independent of T_s for both systems and decreases exponentially from 0.69 to 0.22 for Cr(1 1 0) with increasing E_i, with S₀ smaller by ∼0.15 for the surface oxide. The coverage dependence of the adsorption probability, S(Θ), at low E_i is approx. independent of coverage (Kisliuk-shape) and increases initially at large E_i with coverage (adsorbate-assisted adsorption). CO₂ physisorbs on both systems and the adsorption is non-activated and precursor mediated. Monte Carlo simulations (MCS) have been used to parameterize the beam scattering data. The coverage dependence of E_d has been obtained by means of a Redhead analysis of the TDS curves.     

One-, two- and three-photon spectroscopy of π-conjugated dendrimers: cooperative enhancement and coherent domains

We use wavelength tunable femtosecond pulses to measure intrinsic (simultaneous) two-photon absorption (2PA) and three-photon absorption (3PA) molecular cross section in two series of π-conjugated dendrimers built of identical 4,4′-bis(diphenylamino) stilbene (BDPAS) and 4,4′-bis(diphenylamino) distyrylbenzene (BDPADSB) repeat units. Record large 2PA cross sections, σ₂=10⁻⁴⁶ cm⁴ s are obtained for the largest second-generation BDPAS-based dendrimer, as well as zeroth-generation 4-arm BDPADSB-based dendrimer. In both series, maximum 2PA cross section increases nonlinearly with the number of π-electrons, whereas for higher generations this dependence turns to linear one. 3PA cross section also increases nonlinearly with the size of the system in the series of BDPAS-based molecules, amounting a record large value, σ₃=10⁻⁷⁹ cm⁶ s², for the largest, second-generation dendrimer. We interpret these results in terms of direct inter-branch conjugation, which facilitates cooperative enhancement of the nonlinear-optical response. We propose a simple model which allows us to determine the effective size of coherent domains (extent of conjugation), which, in turn, determines the optimum dendrimer size for most efficient nonlinear response.     

Theoretical study of CO adsorption on the illuminated TiO2 (0 0 1) surface

A quantum modeling of the CO adsorption on illuminated anatase TiO₂ (0 0 1) is presented. The calculated adsorption energy and geometries of illuminated case are compared with the ground state case. The calculations were achieved by using DFT formalism and the BH and HLYP. Upon photoexcitation, an electron-hole pair is generated. Comparing of natural population in the ground state and the exited state, shows that an electron is trapped in a Ti⁴⁺ ion and a hole is localized in an oxygen ion. The photoelectron helps generation of a CO₂ molecule on the TiO₂ surface. As shown by optimization of these systems, the CO molecule adsorbed vertically on the TiO₂ (0 0 1) surface in the ground state case while the CO molecule made an angle of 134.3° to this surface at the excited state case. Based on the here used model the obtained adsorption energy was 0.36 eV which is in excellent agreement with the reported experimental value. In the present work the C-O stretch IR frequencies are calculated which are 1366.53 and 1423.16 cm⁻¹. These results are in good agreement with the earlier reported works for the surface carbonaceous compounds, and oxygenated carbon species.     

Adsorption CO2 on the perfect and oxygen vacancy defect surfaces of anatase TiO2 and its photocatalytic mechanism of conversion to CO

The adsorption energies for physisorption and the most stable chemisorption of CO₂ on the neutral charge of perfect anatase [TiO₂] (0 0 1) are −9.03 and −24.66 kcal/mol on the spin-unpolarized and −12.98 and −26.19 kcal/mol on the spin-polarized surface. The small activation barriers of 1.67 kcal/mol on the spin-unpolarized surface and of 6.66 kcal/mol on the spin-unpolarized surface were obtained. The adsorption mechanism of CO₂ on the oxygen vacancy defect [TiO₂ + V_O] surface of anatase TiO₂ using density functional theory calculations was investigated. The energetically preferred conversion of CO₂ to CO was found either on the spin-unpolarized or spin-polarized surfaces of oxygen vacancy defect surface [TiO₂ + V_O] as photocatalyst.