Showing papers by "Francisco Rodríguez-Reinoso published in 2011"

Ammonia removal using activated carbons: effect of the surface chemistry in dry and moist conditions.

Abstract: The effect of surface chemistry (nature and amount of oxygen groups) in the removal of ammonia was studied using a modified resin-based activated carbon. NH(3) breakthrough column experiments show that the modification of the original activated carbon with nitric acid, that is, the incorporation of oxygen surface groups, highly improves the adsorption behavior at room temperature. Apparently, there is a linear relationship between the total adsorption capacity and the amount of the more acidic and less stable oxygen surface groups. Similar experiments using moist air clearly show that the effect of humidity highly depends on the surface chemistry of the carbon used. Moisture highly improves the adsorption behavior for samples with a low concentration of oxygen functionalities, probably due to the preferential adsorption of ammonia via dissolution into water. On the contrary, moisture exhibits a small effect on samples with a rich surface chemistry due to the preferential adsorption pathway via Bronsted and Lewis acid centers from the carbon surface. FTIR analyses of the exhausted oxidized samples confirm both the formation of NH(4)(+) species interacting with the Bronsted acid sites, together with the presence of NH(3) species coordinated, through the lone pair electron, to Lewis acid sites on the graphene layers.

A site energy distribution function from Toth isotherm for adsorption of gases on heterogeneous surfaces

Abstract: A site energy distribution function based on a condensation approximation method is proposed for gas-phase adsorption systems following the Toth isotherm. The proposed model is successfully applied to estimate the site energy distribution of three pitch-based activated carbons (PA, PFeA and PBA) developed in our laboratory and also for other common adsorbent materials for different gas molecules. According to the proposed model the site energy distribution curves of the activated carbons are found to be exponential for hydrogen at 77 K. The site energy distribution of some of the activated carbon fibers, ambersorb, Dowex optipore, 13X Zeolite for different adsorbate molecules represents a quasi-Gaussian curve with a widened left hand side, indicating that most sites have adsorption energies lower than a statistical mean value.

Mercury removal from aqueous solution by adsorption on activated carbons prepared from olive stones

Abstract: The textural characterization of a series of activated carbons prepared from olive stones, by carbonization at different temperatures (400, 550, 700 and 850 °C) and thermal activation with CO2, has been investigated using N2 adsorption at −196 °C and CO2 adsorption at 0 °C. The effect of pre-oxidation of the carbonized precursor has also been studied, using temperature-programmed decomposition (TPD), to evaluate the effect of oxygen content of the chars in the performance of the obtained activated carbons for mercury removal. The adsorption of Hg(II) cations from aqueous solutions at room temperature by the prepared activated carbons was studied. Experimental results show that all samples exhibit a large microporosity (pore diameter below 0.56 nm). The amount of surface oxygen groups increased after pre-oxidation treatment, this enhancing the Hg(II) uptake (up to 72%). It can be concluded that these groups make the support more hydrophilic, thus providing a more efficient adsorption of Hg(II). The formation of a great amount of surface oxide groups such as carboxyl, phenol and lactone alters the surface charge properties of the carbon, this enhancing the surface-Hg(II) interaction.

Molecular Simulation of Hydrogen Physisorption and Chemisorption in Nanoporous Carbon Structures

Abstract: The effect of pore morphology on the hydrogen-storage capacity of carbon materials at room temperature (298 K) has been studied using Grand Canonical Monte Carlo (GCMC) molecular simulation. Prototypical pore geometries such as slit pores and nanotubes were considered along with carbon foams and a random disordered carbon structure. Both physisorption and chemisorption models were considered in order to take into account the most favourable adsorption scenarios. Fluid-fluid and fluid-solid interactions were assumed to follow a Lennard-Jones-type potential.It was seen that physisorption alone cannot account for the adsorption of more than a few weight percentages, regardless of the pore morphology. Under physisorption conditions, the simulation results show that carbons with a slitshaped pore geometry are more efficient than other geometries at storing hydrogen, particularly at pore distances which allow the formation of fluid layers commensurate with the pore geometry. Slit pores appear to store a maximum...

Anomaly of CH4 molecular assembly confined in single-wall carbon nanohorn spaces.

Abstract: Vibrational-rotational properties of CH(4) adsorbed on the nanopores of single-wall carbon nanohorns (SWCNHs) at 105-140 K were investigated using IR spectroscopy. The difference vibrational-rotational bands of the ν(3) and ν(4) modes below 130 K show suppression of the P and R branches, while the Q branches remain. The widths of the Q branches are much narrower than in the bulk gas phase due to suppression of the Doppler effect. These results indicate that the rotation of CH(4) confined in the nanospaces of SWCNHs is highly restricted, resulting in a rigid assembly structure, which is an anomaly in contrast to that in the bulk liquid phase.

Guest Diffusion in Interpenetrating Networks of Micro- and Mesopores

Abstract: Pulsed field gradient NMR is applied for monitoring the diffusion properties of guest molecules in hierarchical pore systems after pressure variation in the external atmosphere. Following previous studies with purely mesoporous solids, also in the material containing both micro- and mesopores (activated carbon MA2), the diffusivity of the guest molecules (cyclohexane) is found to be most decisively determined by the sample "history": at a given external pressure, diffusivities are always found to be larger if they are measured after pressure decrease (i.e., on the "desorption" branch) rather than after pressure increase (adsorption branch). Simple model consideration reproduces the order of magnitude of the measured diffusivities as well as the tendencies in their relation to each other and their concentration dependence.

Effect of the support, Al2O3 or SiO2, on the catalytic behaviour of Cr–ZnO promoted Pt catalysts in the selective hydrogenation of cinnamaldehyde

Abstract: The effect of the support, Al 2 O 3 or SiO 2 , on the catalytic behaviour of Cr–ZnO promoted Pt catalysts has been studied in the liquid phase hydrogenation of cinnamaldehyde. A model Pt/Cr–ZnO catalyst has been used as a reference. H 2 PtCl 6 in aqueous solution has been used as platinum precursor for the synthesis of catalysts. They have been calcined at 773 K and reduced at two different temperatures, 473 and 623 K, before the determination of their catalytic behaviour. It has been shown that the support greatly affects the properties and characteristic behaviour of the final catalysts. A higher promoter dispersion was achieved on the silica-supported sample, which favoured a high selectivity towards the unsaturated alcohol.

Immersion calorimetry as a tool to evaluate the catalytic performance of titanosilicate materials in the epoxidation of cyclohexene.

Abstract: Different types of titanosilicates are synthesized, structurally characterized, and subsequently catalytically tested in the liquid-phase epoxidation of cyclohexene. The performance of three types of combined zeolitic/mesoporous materials is compared with that of widely studied Ti-grafted-MCM-41 molecular sieve and the TS-1 microporous titanosilicate. The catalytic test results are correlated with the structural characteristics of the different catalysts. Moreover, for the first time, immersion calorimetry with the same substrate molecule as in the catalytic test reaction is applied as an extra means to interpret the catalytic results. A good correlation between catalytic performance and immersion calorimetry results is found. This work points out that the combination of catalytic testing and immersion calorimetry can lead to important insights into the influence of the materials structural characteristics on catalysis. Moreover, the potential of using immersion calorimetry as a screening tool for catalysts in epoxidation reactions is shown.

Influence of water/alkoxide ratio in the synthesis of nanosized sol-gel titania on the release of phenytoin.

Abstract: The sol−gel method was used to synthesize inorganic reservoirs with encapsulated antiepileptic drug phenytoin. The drug release profile was shown to depend on the morphology and surface properties ...

Effect of nanoscale curvature sign and bundle structure on supercritical H2 and CH4 adsorptivity of single wall carbon nanotube

Abstract: The adsorptivities of supercritical CH4 and H2 of the external and internal tube walls of single wall carbon nanotube (SWCNT) were determined. The internal tube wall of the negative curvature showed the higher adsorptivities for supercritical CH4 and H2 than the external tube wall of the positive curvature due to their interaction potential difference. Fine SWCNT bundles were prepared by the capillary force-aided drying treatment using toluene or methanol in order to produce the interstitial pore spaces having the strongest interaction potential for CH4 or H2; the bundled SWCNT showed the highest adsorptivity for supercritical CH4 and H2. It was clearly shown that these nanostructures of SWCNTs are crucial for supercritical gas adsorptivity.

Evaluation of a mixed geometry model for the characterization of activated carbons

Abstract: Fil: Toso, J. P.. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico San Luis. Instituto de Fisica Aplicada; Argentina. Universidad Nacional de San Luis; Argentina

Influence of the surface chemistry of activated carbons on the ATRP catalysis of methyl methacrylate polymerization

Abstract: A parent activated carbon (C-0) was subjected to four different treatments: (i) heat treatment at 1273 K in Ar (C-1); (ii) heat treatment at 473 K in air (C-2); (iii) oxidation with H 2 O 2 (C-3) and, (iv) oxidation with HNO 3 (C-4). These materials were evaluated as supports of CuBr–[1,1,4,7,10,10-hexamethyltriethylenetetramine] (Cu I Br–HMTETA), a catalyst for the Atom Transfer Radical Polymerization (ATRP) of methyl methacrylate (MMA) using methyl-α-bromophenylacetate (MBP) as radical initiator. The supported catalysts showed an adequate control of polymerization, evidenced by polydispersity indexes (PDI) falling in the range 1.13–1.55. The best performance was achieved when activated carbon was treated with nitric acid (C-4) and with air at 473 K (C-2). Some copper leaching was always detected. The catalysts were reused and an adequate polymerization control was obtained in subsequent runs.

Comparison of nanostructured titania matrices obtained by carbon template and sol-gel methods for controlled release of fluoxetine.

Abstract: Two types of titania matrix were investigated as a support for the prolonged drug delivery of the antidepressant fluoxetine. Sample MT was synthesized using carbon template and consisted of titania microtubes on which then fluoxetine was adsorbed. Sample SG was synthesized by the sol-gel method when the drug was added during the reaction. The morphology of the powder surfaces was found to be different: nanotubes versus almost spherical particles with much larger surface area of SG and smaller pores. The relative degrees of hydroxyl coverage of the surface were studied by FTIR-spectroscopy and were found to be much larger for the sol-gel complex. Theoretical modeling was applied to consider possible interactions between the drug and the matrices. The liberation of the drug was proved to be faster from complex MT and was attributed to weaker drug-matrix interactions in combination with larger pore size.

Molecular simulations to study the effect of pore geometry and structure on the adsorption of CH4/H2 mixtures in carbon pores

Abstract: We used molecular simulations to study the effect of pore structure and geometry on the selectivity of carbon materials for the adsorption of methane from a mixture of methane and hydrogen at 50 bar and 298 K. We modeled hydrogen as a more realistic dumb-bell shaped fluid and methane as spherical Lennard-Jones sphere. The pore geometry studied includes the slit, nanotube, random carbon structure and carbon pores that mimic a foam structure. Our results confirm that both the pore size and pore structure have a significant effect on the selectivity for methane molecules. An array of nanotubes that have a closed structure has the highest selectivity for methane molecules, followed by carbon foam, slit and random pore structures. Energetic and packing effects at low and high pressures, respectively, influence the selectivity for methane. Reduction in entropy of hydrogen in smaller pores increases the selectivity for methane molecules.