Francisco J. Maldonado-Hódar

Effects of non-oxidant and oxidant acid treatments on the surface properties of an activated carbon with very low ash content

Abstract An activated carbon obtained from olive stones and with very low ash content (0.10%) was treated with either HCl, HF or HNO3. The changes in surface area and porosity resulting from the acid treatments were studied by N2 and CO2 adsorption at 77 and 273 K, respectively and by mercury porosimetry. The changes in surface chemistry were studied by temperature-programmed desorption and Fourier transformed infrared spectroscopy. The treatments with HCl yielded activated carbons on which some chlorine remained chemisorbed, whereas the HF treatment did not fix any fluorine. Due to this, the HCl treatment had a slight effect on the microporosity of the samples. Moreover, the HF treatment increased the amount of CO-evolving surface groups. The treatment with HNO3 destroyed the pore walls to a large extent, fixing a large amount of oxygen surface groups. The nature and structure of the CO- and CO2-evolving groups will be discussed in detail.

Optimization of conditions for the preparation of activated carbons from olive-waste cakes

Abstract An experimental design (Doehlert matrix) has been drawn up to optimize the experimental conditions of the preparation of activated carbon from olive-waste cakes. A series of activated carbons have been prepared by physical activation with steam. Adsorption of N2 (77 K), CO2 (273 K) and mercury porosimetry experiments have been carried out to determine the characteristics of all carbons prepared. Adsorption of iodine and methylene blue was used as a primary indicator of the adsorption capacity of these carbons. The experimental response varied between: 13–27% for the total yield (Y1), 115–490 mg/g for the adsorption of methylene blue (Y2), 741–1495 mg/g for the adsorption of iodine (Y3), 514–1271 m2/g for the BET surface area (Y4), 0.225–0.377 cm3/g for the micropore volume (Y5), 0.217–0.557 cm3/g for the volume of pores with a diameter greater than 3.7 nm (Y6) and 31.3–132 m2/g for the external surface area (Y7). The results obtained were exploited using response surface methodology. These responses have been represented and studied in all experimental regions of activation time and activation temperature, the most influential factors in activated carbon preparation. Optimization to obtain activated carbons with textural characteristics suitable to use in water treatment has been carried out. The optimal activated carbon is obtained when using 68 min as activation time and 1095 K as activation temperature.

Synthesis and textural characteristics of organic aerogels, transition-metal-containing organic aerogels and their carbonized derivatives

Abstract Four organic aerogels were prepared by the sol-gel method from polymerization of resorcinol with formaldehyde catalyzed by Na 2 CO 3 . These aerogels were further pyrolyzed in N 2 in order to obtain their corresponding carbon aerogels, and after that activated with either steam or CO 2 to obtain activated carbon aerogels. Another series of organic aerogels and their pyrolyzed and activated derivatives were prepared without a catalyst or in the presence of Ag, Pd or Pt. All these samples were texturally characterized, which has shown the porous modifications undergone by the aerogels during the pyrolysis and activation processes. The textural characteristics of the transition-metal-containing activated carbon aerogels depend on the nature of the metal. Thus, whereas the sample containing Pt, in a very small amount, had the largest meso and macropore volume (0.822 and 2.982 cm 3 g −1 ) found, those containing either Pd or Ag were essentially microporous.

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.

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.

Treatment of azo dye-containing wastewater by a Fenton-like process in a continuous packed-bed reactor filled with activated carbon

In this work, oxidation with a Fenton-like process of a dye solution was carried out in a packed-bed reactor. Activated carbon Norit RX 3 Extra was impregnated with ferrous sulfate and used as catalyst (7 wt.% of iron). The effect of the main operating conditions in the Chicago Sky Blue (CSB) degradation was analyzed. It was found that the increase in temperature leads to a higher removal of the dye and an increased mineralization. However, it also increases the iron leaching, but the values observed were below 0.4 ppm (thus, far below European Union limits). It was possible to reach, at steady-state, a dye conversion of 88%, with a total organic carbon (TOC) removal of ca. 47%, being the reactor operated at 50°C, pH 3, W(cat)/Q=4.1 g min mL(-1) (W(cat) is the mass of catalyst and Q the total feed flow rate) and a H(2)O(2) feed concentration of 2.25 mM (for a CSB feed concentration of 0.012 mM). The same performance was reached in three consecutive cycles.

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.

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.

Metal-carbon aerogels as catalysts and catalyst supports

Abstract Several transition-metal-containing organic aerogels (Ti, Cr, Mo, W, Fe, Co and Ni) were obtained by polymerization of resorcinol and formaldehyde. The metal-carbon aerogels were obtained by pyrolysis of these aerogels in an inert atmosphere. The porous texture of the samples was characterized by different techniques (mercury porosimetry, He and Hg density and gas adsorption) as well as their surface chemistry and morphology (XPS, XRD, DTG, TEM, SEM). Some of the samples were tested as catalysts in reactions such as the conversion of alcohols and the skeletal isomerization of 1-butene.