Carlos Moreno-Castilla

Changes in surface chemistry of activated carbons by wet oxidation

Abstract A series of activated carbons with different degrees of activation were oxidized with H2O2, (NH4)2S2O8 and HNO3 in order to introduce different oxygen surface complexes. Changes in the surface chemistry of the activated carbons after their oxidizing treatments were studied by different techniques including temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FTIR), titrations with HCl and NaOH, measurements of the pH of the point of zero charge and catalytic dehydration of methanol. Results showed that treatment with (NH4)2S2O8 fixed the lowest amount of both total oxygen and surface acid groups. However, this treatment yielded the acid groups with the highest acid strength. This could be because it favors fixation of carboxyl groups close to other groups, such as carbonyl and hydroxyl, which enhances their acidity.

On the characterization of acidic and basic surface sites on carbons by various techniques

Abstract Active carbons of different origins have been oxidized with H2O2 and (NH4)2S2O8 and their oxygen surface complexes have been characterized by TPD, classical titration following Boehms method and by neutralization calorimetry. The net enthalpies of neutralization, determined by immersion calorimetry into NaOH and HCl 2 N lead to −41.1±1.8 and −52.3±2.0 kJ eq−1 for the acidic and basic sites on the surface. Experiments with NaHCO3 lead to −39.7±1.7 kJ eq−1 for the carboxylic groups alone. These results suggest that the surface groups of active carbons can also be characterized by immersion calorimetry. Results are also given for the variation of the pH of the point of zero charge with the total oxygen content of the surface.

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.

Activated carbons from KOH-activation of argan (Argania spinosa) seed shells as supercapacitor electrodes.

Activated carbons were prepared by KOH-activation of argan seed shells (ASS). The activated carbon with the largest surface area and most developed porosity was superficially treated to introduce oxygen and nitrogen functionalities. Activated carbons with a surface area of around 2100 m(2)/g were obtained. Electrochemical measurements were carried out with a three-electrode cell using 1M H(2)SO(4) as electrolyte and Ag/AgCl as reference electrode. The O-rich activated carbon showed the lowest capacitance (259 F/g at 125 mA/g) and the lowest capacity retention (52% at 1A/g), due to surface carboxyl groups hindering electrolyte diffusion into the pores. Conversely, the N-rich activated carbon showed the highest capacitance (355 F/g at 125 mA/g) with the highest retention (93% at 1A/g), due to its well-developed micro-mesoporosity and the pseudocapacitance effects of N functionalities. This capacitance performance was among the highest reported for other activated carbons from a large variety of biomass precursors.

The creation of acid carbon surfaces by treatment with (NH4)2S2O8

Abstract Two activated carbons with different degree of activation, 20 and 46% burn-off, were obtained from olive stones by steam activation. These samples were oxidized with a concentrated solution of (NH4)2S2O8 for different periods of time, up to a maximum of 24 hours, studying the variation in surface area, pore texture and surface chemistry during the oxidation process. The surface area and porosity was studied by N2 and CO2 adsorption, mercury porosimetry, and helium and mercury densities. The surface complexes were characterized by temperature programmed desorption and FTIR techniques, selective neutralization, and mass titration. Detailed studies are made of the kinetics of the formation of CO and CO2-evolving groups and of their nature and structure.

Adsorption of some substituted phenols on activated carbons from a bituminous coal

Adsorption at 298 K of phenol, p-cresol, m-chlorophenol, m-aminophenol, and p-nitrophenol from aqueous solutions on activated carbons obtained from an original and a demineralized bituminous coal has been studied. The adsorption capacity of the activated carbons depended on the surface area and porosity of the carbon, the solubility of the phenolic compound, and the hydrophobicity of the substituent. The relative affinity of the phenolic compound toward the surface of the carbon was related to the electron donor-acceptor complexes formed between the basic sites on the surface of the carbon and the aromatic ring of the phenol. The adsorption capacity of the carbon depended on the solution pH. As a result, the adsorption capacity began to decrease at a pH value that depended on the difference between the external and internal surface charge density, as measured by electrophoresis and pH measurement of the slurry, respectively.

Regularities in the temperature-programmed desorption spectra of CO2 and CO from activated carbons

Temperature-programmed desorption (TPD) spectra of CO and CO2 have been obtained under vacuum or helium flow with activated carbons from different origins and oxidized in various ways. In particular, a special form of TPD, called Intermittent TPD, has been applied to one of the carbon samples (prepared from almond shells and oxidized with nitric acid) in order determine the variation of the apparent activation energy of desorption Ed as the coverage of the surface decreases. Regularities appeared in the set of results obtained: There are common features in TPD profiles of carbons from different origins and histories. This finding is confirmed by a re-examination of numerous CO and CO2 spectra found in the literature, including results obtained under ultra-vacuum. These regularities are interpreted by the existence of the same superficial entities (various oxygen groups with different environments).

Chemical and physical activation of olive-mill waste water to produce activated carbons

Abstract Olive-mill waste water is produced in large quantities during the manufacture process of the olive oil in mills. This by-product has been used as raw material to produce activated carbons by both chemical and physical activation methods. In the first case, KOH and H3PO4 were used as activating agent, and in the second case, CO2 at 840°C for different periods of time. Results obtained indicate that the chemical activation with KOH at 800°C, in an inert atmosphere, yielded activated carbons with the highest surface area and more developed micro, meso and macroporosity.

Platinum catalysts supported on activated carbons: I. Preparation and characterization

Several Pt catalysts supported on activated carbons (manufactured from olive stones and almond shells) have been prepared with both H2PtCl6 · 6H2O and [Pt(NH3)4]Cl2 as metal precursor and using different methods. Once reduced, the supported catalysts were characterized by H2 and CO chemisorption as well as X-ray diffraction and transmission electron microscopy. The effect of reduction conditions on metal dispersion has been studied and correlated with the surface properties of the supports. The results show that porosity with sizes ranging from 9 to 11 nm is decisive for obtaining a high Pt dispersion. To increase the dispersion of the catalysts prepared from [Pt(NH3)4]Cl2 a treatment with He prior to reduction of the catalysts in H2 seems to be essential in order to avoid the formation of an unstable hydride which leads to agglomeration of the Pt particles.