Agustín F. Pérez-Cadenas

On the nature of surface acid sites of chlorinated activated carbons

Abstract Two activated carbons containing different amounts of chlorine were obtained by chlorination of an activated carbon prepared from olive stones. Variations in surface physics and chemistry of the samples were studied by N2 and CO2 adsorption, mercury porosimetry, TPD, XPS, pHPZC measurements, and by testing their behaviour as catalysts in the decomposition reaction of isopropanol. Our results indicate that chlorination of activated carbon increases its Lewis acidity but decreases its Bronsted acidity, which can be explained by the resonance effect introduced into the aromatic rings of graphene layers by the chlorine atoms covalently bound to their edges. This resonance effect could also explain the changes observed in the thermal stability of C–Cl and C–O bonds.

Surface Chemistry, Porous Texture, and Morphology of N-Doped Carbon Xerogels

N-doped carbon xerogels were obtained from organic xerogels prepared using different N-containing organic compounds, including 3-hydroxy aniline, melamine, and 3-hydroxy pyridine. Carbonization was carried out between 500 and 900 degreeC. The surface chemistry of samples was determined by elemental analysis and X-ray photoelectron spectroscopy, their porous texture was determined by N2 and CO2 adsorption at (-)196 and 0degreeC, respectively, and their morphology was determined by scanning electron microscopy. N-doped carbon xerogels with a wide variety of N contents and functionalities were obtained according to the ingredients and carbonization temperature used. Carbon xerogels contained, in different proportions, three/four N functionalities: pyridinic, pyrrolic and/or pyridonic, and quaternary N functionalities. They were microporous carbons with narrow micropores that had constrictions at their entrances, producing higher CO2(-) than N2-determined micropore surface areas. Morphology studies showed samples to be constituted by isolated microspheres or microsphere clusters. Microsphere diameters depended on the recipe and carbonization temperature used.

Design of low-temperature Pt-carbon combustion catalysts for VOC's treatments.

Two series of Pt/C-catalysts were prepared using pure carbon aerogels as supports. The influence of porosity, surface chemistry and Pt dispersion on the activity of Pt/C combustion catalysts was analyzed. The synthesis of the supports was fitted to have a monomodal pore size distribution in the meso and macropore range respectively. Both supports were functionalized by oxidation treatment with H(2)O(2) or (NH(4))(2)S(2)O(8). These treatments did not modify the porosity significantly, but the surface chemistry changed from basic to acid as oxygen content increased. In this way, Pt-dispersion decreased as a result of the low thermal stability of surface carboxylic acid groups. Benzene was selected as target VOCs and the catalytic combustion performance depended mainly on the porous texture and Pt-dispersion, while the variations in the surface chemistry of carbon supports due to oxidation treatments seemed to have a weak influence on this kind of reaction.

Tungsten oxide catalysts supported on activated carbons: effect of tungsten precursor and pretreatment on dispersion, distribution, and surface acidity of catalysts

Tungsten catalysts supported on activated carbon were prepared using tungsten hexacarbonyl, ammonium tungstate, and tungsten pentaethoxide. The catalysts were pretreated in He, dry air, or wet air at 623 K for 6 h before being characterized by N2 adsorption at 77 K, temperature-programmed desorption, X-ray diffraction, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy, and by testing their behavior in the decomposition reaction of isopropanol. The dispersion of the supported tungsten oxide phase and their surface acidity depended on the metal precursor and atmosphere of pretreatments. Thus, the highest dispersion and surface acidity were found for the catalyst prepared from W(CO)6 and the lowest dispersion for that prepared from (NH4)2WO4. Dry air gave the highest dispersion, whereas wet air yielded the highest surface acidity. Air pretreatments of the catalyst prepared from W(CO)6 seem to create metal oxide–support interactions, because of the very low particle size of the supported tungsten oxide phase and its homogeneous distribution.

Carbon-Based Honeycomb Monoliths for Environmental Gas-Phase Applications

Honeycomb monoliths consist of a large number of parallel channels that provide high contact efficiencies between the monolith and gas flow streams. These structures are used as adsorbents or supports for catalysts when large gas volumes are treated, because they offer very low pressure drop, short diffusion lengths and no obstruction by particulate matter. Carbon-based honeycomb monoliths can be integral or carbon-coated ceramic monoliths, and they take advantage of the versatility of the surface area, pore texture and surface chemistry of carbon materials. Here, we review the preparation methods of these monoliths, their characteristics and environmental applications.

Influence of carbon–oxygen surface complexes on the surface acidity of tungsten oxide catalysts supported on activated carbons

Tungsten oxide catalysts supported on activated carbons were prepared by using tungsten hexacarbonyl, ammonium tungstate, and tungsten pentaethoxide as precursors. An activated carbon was obtained from olive stone by physical activation. A portion of this activated carbon was oxidized with ammonium peroxydisulfate in order to introduce different oxygen surface complexes. Subsequently, different portions of this oxidized activated carbon were heat treated in nitrogen flow at various temperatures to partially remove the oxygen surface complexes. In this way, activated carbons with different amounts of oxygen surface complexes were obtained, which were then used as supports for the tungsten oxide catalysts. Both the supports and the supported catalysts were pre-treated either in He, dry air or wet air flow at 623 K for 6 h. They were then characterized by X-ray photoelectron spectroscopy, X-ray diffraction, measurements of the pH of the point of zero charge, and activity in the decomposition reaction of isopropanol. Turnover frequencies for the formation of propene were obtained. According to these results, the oxygen surface complexes of the support have a major influence on the total acidity of the tungsten oxide supported catalysts. In some supported catalysts, W(VI) was reduced to W(V) during the decomposition reaction of isopropanol as a consequence of the hydrogen evolution. The results indicate that oxygen surface complexes of the support may play an important role in this reduction process, which was inhibited when the support had high surface oxygen content.

Synthesis and Properties of Phloroglucinol−Phenol−Formaldehyde Carbon Aerogels and Xerogels

Carbon aerogels and xerogels were successfully prepared from phloroglucinol-phenol mixtures and characterized by different techniques to determine their potential. We examined the influence of the phloroglucinol/phenol ratio, reactant concentration, cure conditions, and drying method on the morphology and porosity of the samples. The gelation time was found to be independent of the phloroglucinol/phenol ratio in spite of the different reactivities of both monomers. In general, carbon aerogels have a high volume of mesopores and of micropores without diffusion restrictions. Carbon xerogels are denser materials without mesopores but with a well-developed microporosity that shows a strong molecular sieve effect. Therefore, while micro-/mesoporous carbon aerogels can be used as catalyst supports or VOC adsorbents, the microporous carbon xerogel could offer high selectivity in the separation of small molecules from gaseous mixtures.

Development of Carbon Coatings for Cordierite Foams : An Alternative to Cordierite Honeycombs

Cordierite foams were prepared by replication of polyurethane foams and were coated with three types of carbon xerogels. The dip coating and synthesis conditions were optimized, and the coated foams were characterized exhaustively. The composition of the starting solution, coat loading, and carbonization temperature are the most important parameters determining both textural and mechanical properties. Carbon xerogel coatings obtained from aqueous solutions of resorcinol (R) and formaldehyde (F) are macro-, meso-, and microporous but present the greatest shrinkage, which causes a loss of adhesion between ceramic foams and carbon coatings. The coatings from polyfurfuryl alcohol (PFA) and RF-poly(vinyl butyral) (Butvar) resin are highly microporous and present very good adhesion even after carbonization. In all cases, coatings induce the improvement of the mechanical properties, which is related to the fact that the coating fills the defects present in the cordierite foams, thereby affecting both the rigidity and the way cracks propagate through the coated samples. These materials, due to the synergetic role of the highly porous coatings and the tortuous channels of the ceramic foams, are suitable materials for adsorption or catalytic treatments of fluids.

Solution study and 2-D layered structures of zinc(II) and cadmium(II) complexes with N-2-(6-amino-3,4-dihydro-3-methyl-5-nitroso-4-oxopyrimidinyl)-l-methionine as ligand

Abstract The solution study of the N-2-(6-amino-3,4-dihydro-3-methyl-5-nitroso-4-oxopyrimidinyl)- l -methionine has revealed that it acts as a monoprotic acid in the pH range 2.5–9.5. In solution the complexes detected with Zn(II) and Cd(II) at 1:1 and 1:4 metal-to-ligand molar ratios were the mononuclear species [ZnL2], [CdL2] and [Cd(HL)]2+, with the ligand coordinating through the carboxylate group in the two former and through the pyrimidine ring in the later. From these, two layered complexes of Zn(II) and Cd(II) with HL were obtained. {[Zn(μ-L)2]·2H2O}n, monoclinic, space group P21, a=10.882(3), b=7.473(2), c=18.534(4) A, β=90.92(2)°, Z=2; {[Cd(μ-L)2]·3H2O}n, triclinic, space group P1, a=7.615(2), b=10.427(2), c=11.129(2) A, α=86.29(3), β=78.87(3), γ=71.13(3)°, Z=1. The structures of both complexes are very similar except for the coordination numbers of five and seven for Zn(II) and Cd(II), respectively. These facts point out the versatility of this family of N-pyrimidine amino acids as ligands, as well as the importance of the crystallization processes in obtaining supramolecular compounds.

Tailoring activated carbons for the development of specific adsorbents of gasoline vapors

The specific adsorption of oxygenated and aliphatic gasoline components onto activated carbons (ACs) was studied under static and dynamic conditions. Ethanol and n-octane were selected as target molecules. A highly porous activated carbon (CA) was prepared by means of two processes: carbonization and chemical activation of olive stone residues. Different types of oxygenated groups, identified and quantified by TPD and XPS, were generated on the CA surface using an oxidation treatment with ammonium peroxydisulfate and then selectively removed by thermal treatments, as confirmed by TPD results. Chemical and porous transformations were carefully analyzed throughout these processes and related to their VOC removal performance. The analysis of the adsorption process under static conditions and the thermal desorption of VOCs enabled us to determine the total adsorption capacity and regeneration possibilities. Breakthrough curves obtained for the adsorption process carried out under dynamic conditions provided information about the mass transfer zone in each adsorption bed. While n-octane adsorption is mainly determined by the porosity of activated carbons, ethanol adsorption is related to their surface chemistry, and in particular is enhanced by the presence of carboxylic acid groups.