Electrocatalytic activity of calcined manganese ferrite solid nanospheres in the oxygen reduction reaction.

In this study, we synthesized MnFe2O4 solid nanospheres (MSN) calcined at different temperatures (200-500 °C) and MSN-based materials mixed with carbon black, for their use as electrocatalysts in the oxygen reduction reaction (ORR) in alkaline medium (0.1 M KOH). It was demonstrated that the calcination temperature of MSN material determined its chemical surface composition and microstructure and it had an important effect on the electrocatalytic properties for ORR, which in turn was reflected in the performance of MSN/CB-based electrocatalysts. The study revealed that the presence of Mn species plays a key role in the ORR activity. Among tested, MSN200/CB and MSN350/CB exhibited the best electrochemical performances together with outstanding stability.

Adsorption mechanisms of metal cations from water on an oxidized carbon surface.

Adsorption of Cr(III), Mn(II), Cu(II) and Zn(II) on an oxidized activated carbon cloth was studied. Its surface chemistry was characterized by potentiometric titration. This technique revealed the amount of surface oxygen functionalities and their acidity constant distribution. The acidity constant range involved in the metal cation adsorption was obtained from this distribution. Metal cation adsorption increased with higher adsorption temperature due to the increase in the negative surface charge of the oxidized activated carbon. Adsorption was by proton exchange and the number, amount and strength of the surface acid groups involved could be obtained. The proton exchange was by an inner-sphere or outer-sphere surface metal complex formation mechanism. In the case of divalent cation adsorption, the increase in temperature changed the adsorption mechanism from outer-sphere to inner-sphere. However, the adsorption mechanism of Cr(III) was outer-sphere and independent of temperature. Adsorption capacity augmented with the increase in the charge-to-size ratio of the hexa-aquo cations. In addition, the adsorption capacity of divalent cations increased with the rise in stability of the surface metal complex formed.

Batch and column adsorption of herbicide fluroxypyr on different types of activated carbons from water with varied degrees of hardness and alkalinity.

There has been little research into the effects of the water hardness and alkalinity of surface waters on the adsorption of herbicides on activated carbons. The aim of this study was to determine the influence of these water characteristics on fluroxypyr adsorption on different activated carbons. At low fluroxypyr surface concentrations, the amount adsorbed from distilled water was related to the surface hydrophobicity. Surface area of carbons covered by fluroxypyr molecules ranged from 60 to 65%. Variations in fluroxypyr solubility with water hardness and alkalinity showed a salting-in effect. Calcium, magnesium and bicarbonate ions were adsorbed to a varied extent on the activated carbons. The presence of fluroxypyr in solution decreased their adsorption due to a competition effect. K(F) from the Freundlich equation linearly increased with water hardness due to salt-screened electrostatic repulsions between charged fluroxypyr molecules. The amount adsorbed from distilled water was largest at high fluroxypyr solution concentrations, because there was no competition between inorganic ions and fluroxypyr molecules. The column breakthrough volume and the amount adsorbed at breakthrough were smaller in tap versus distilled water. Carbon consumption was lower with activated carbon cloth than with the use of granular activated carbon.

Adsorption and thermal desorption of the herbicide fluroxypyr on activated carbon fibers and cloth at different pH values.

Adsorption of fluroxypyr was studied at pH values between 2 and 10 and at temperatures of 298 and 313 K. Adsorption capacity decreased when the solution pH increased. This was explained by changes in fluroxypyr solubility and in dispersive and electrostatic adsorbent-adsorbate interactions with the increase in pH. The highest adsorption was found at pH 2, when the solubility was the lowest and only dispersive interactions operated. An increase in temperature produced a decrease in adsorption capacity. Thermal desorption of fluroxypyr up to 1073 K left a residue on the carbon surface, which increased with higher adsorption pH. Differential thermogravimetry (DTG) profiles showed two desorption peaks at pH values of 2 and 4 and only one peak at pH values of 7 and 10. The appearance of one or two peaks may be related to the type of adsorbate-adsorbent interactions established during adsorption. The predominance of electrostatic interactions favours the strongly bound or chemisorbed fluroxypyr. One important conclusion is that the highest amounts of fluroxypyr are adsorbed and thermally desorbed when there are only non-electrostatic interactions between fluroxypyr molecules and carbon surface. Activation desorption energy and pre-exponential factor were obtained from the shift in temperature of desorption peaks with higher heating rate.

Inter- and intra-primary-particle structure of monolithic carbon aerogels obtained with varying solvents.

Different carbon aerogels were obtained by carbonization of organic aerogels prepared from the polymerization of resorcinol and formaldehyde using potassium carbonate as catalyst. Various solvents were added to the initial mixture to study their effects on the inter- and intra-primary-particle structure of the carbon aerogels. To carry out this study, various techniques were used, including high-resolution transmission and scanning electron microscopy, mercury porosimetry, mechanical tests, N2 and CO2 adsorption at -196 and 0 degrees C, respectively, and immersion calorimetry into benzene. Variation of the solvent used produced changes in the gelation time of the organic aerogels, which gave rise to variations in the inter- and intra-primary-particle structure of the carbon aerogels obtained. The monolith density of the carbon aerogels ranged from 0.37 to 0.87 g/cm3. Samples with a density higher than 0.61 g/cm3 had micropores and mesopores but no macropores. Macro- and mesoporosity had a monomodal pore size distribution. The elastic modulus showed a scaling relationship with density. In all samples studied, which had a mean micropore width of 0.62-1.06 nm, the surface area obtained by enthalpy of immersion into benzene yielded a realistic value of their total surface area.

Kinetics of diuron and amitrole adsorption from aqueous solution on activated carbons.

A study was conducted on the adsorption kinetics of diuron and amitrole from aqueous solutions on activated carbons of different particle sizes and on an activated carbon fiber. Different kinetic models were applied to the experimental results obtained. A pseudo-second-order rate equation fitted the adsorption kinetics data better than a pseudo-first-order rate equation. Amitrole showed faster adsorption kinetics compared with diuron because of the smaller size of the former herbicide, despite its lower driving force for adsorption. Both reaction rate constants increased when the particle size decreased. The activated carbon fiber and the activated carbon of smallest particle size (0.03 mm) showed similar adsorption kinetics. The intraparticle diffusion rate constant increased with higher initial concentration of herbicides in solution and with lower particle size of the adsorbent. This is because the rise in initial concentration increased the amount adsorbed at equilibrium, and the reduction in particle size increased the number of collisions between adsorbate and adsorbent particles. Demineralization of the activated carbon with particle size of 0.5mm had practically no effect on the adsorption kinetics.

Adsorption of benzene, toluene, and xylenes on monolithic carbon aerogels from dry air flows.

Monolithic carbon aerogels were obtained by carbonization of organic aerogels prepared by polymerization of resorcinol and formaldehyde under different conditions. Some carbon aerogels obtained were further CO2-activated. Samples were characterized by gas adsorption, scanning electron microscopy, high-resolution transmission electron microscopy, and mechanical tests. Benzene, toluene and xylenes were adsorbed from dry air by using carbon bed columns, obtaining the breakthrough curves. There was no correlation between the amount adsorbed at the breakthrough point and the volume of micropores narrower than 0.7 nm. Conversely, a good linear relationship between the amount adsorbed at the breakthrough point and the total micropore volume up to a mean micropore width of around 1.05 nm was found. In addition, the height of the mass transfer zone decreased with the mean width of the total micropores up to a value of around 1.05-1.10 nm. One of the best adsorbents obtained showed the lowest height of the mass transfer zone and one of the highest amounts adsorbed at the breakthrough point, either per mass or volume unit. However, it had a lower elastic modulus and compressive strength than other monolithic carbon aerogels, although its compressive strength (3 MPa) was still high enough to use it in carbon bed columns. The sample with the best mechanical properties was a poorer adsorbent. Regeneration of the exhausted adsorbents allowed the recovery of the hydrocarbons adsorbed without any appreciable loss of adsorption capacity of the carbon bed.

Removal of diuron and amitrole from water under static and dynamic conditions using activated carbons in form of fibers, cloth, and grains.

This study investigated the removal of the herbicides diuron and amitrole from water under static and dynamic conditions using different activated carbons in the form of fibers, cloth, and grains. In all cases, there was much greater adsorption of diuron than of amitrole due to the lower solubility, greater hydrophobicity, and larger dipolar moment of the former. The activated carbon cloth was the best adsorbent for diuron under dynamic conditions because it had the largest mesopore volume, water-accessible pore volume, and surface area. However, the best adsorbent for amitrole under dynamic conditions was the granular activated carbon due to its higher surface basicity. Comparisons using the best adsorbent for each herbicide showed that diuron was removed by the activated carbon more efficiently compared with amitrole under both dynamic and static conditions.

Effect of surface chemistry, solution pH, and ionic strength on the removal of herbicides diuron and amitrole from water by an activated carbon fiber.

A study was conducted on the effects of carbon surface chemistry, solution pH, and ionic strength on the removal of diuron and amitrole from aqueous solutions by adsorption on an as-received and oxidized activated carbon fiber. Results obtained were explained by the surface characteristics of the adsorbents and the characteristics of the herbicide molecules. Under the experimental conditions used, diuron uptake was much higher than that of amitrole, despite its larger molecular dimensions, due to the lesser water solubility, greater hydrophobicity, and larger dipolar moment of diuron compared with amitrole. Uptake variations associated with differences in carbon surface oxidation, solution pH, and ionic strength were explained by corresponding changes in electrostatic, hydrophobic, and van der Waals interactions.

Molybdenum carbide formation in molybdenum-doped organic and carbon aerogels.

A Mo-doped organic aerogel and its corresponding carbonized derivative at 1000 degrees C were obtained. Both samples were treated in a H(2)/Ar flow. After this treatment, a mixture of Mo(VI) and Mo(2)C was detected in both samples. The results obtained indicate that the presence of H(2) in the gas flow is necessary to obtain the carbide phase, due to the formation of CH(4) or even CH(x)() species that reduce and carburize the molybdenum oxide phase. Carburization of the Mo-doped organic aerogel yielded better results compared with carburization of the Mo-doped carbon aerogel.

Bisphenol A removal from water by activated carbon. Effects of carbon characteristics and solution chemistry.

The present study aimed to analyze the behavior of different activated carbons in the adsorption and removal of bisphenol A (2-2-bis-4-hydroxypheniyl propane) from aqueous solutions in order to identify the parameters that determine this process. Two commercial activated carbons and one prepared in our laboratory from almond shells were used; they were texturally and chemically characterized, obtaining the surface area, pore size distribution, mineral matter content, elemental analysis, oxygen surface groups, and pH of the point of zero charge (pH(PZC)), among other parameters. Adsorption isotherms of bisphenol A and adsorption capacities were obtained. The capacity of the carbons to remove bisphenol A was related to their characteristics. Thus, the adsorption of bisphenol A on activated carbon fundamentally depends on the chemical nature of the carbon surface and the pH of the solution. The most favorable experimental conditions for this process are those in which the net charge density of the carbon is zero and the bisphenol A is in molecular form. Under these conditions, the adsorbent-adsorbate interactions that govern the adsorption mechanism are enhanced. Influences of the mineral matter present in the carbon samples and the solution chemistry (pH and ionic strength) were also analyzed. The presence of mineral matter in carbons reduces their adsorption capacity because of the hydrophilic nature of the matter. The presence of electrolytes in the solution favor the adsorption process because of the screening effect produced between the positively charged carbon surface and the bisphenol A molecules, with a resulting increase in adsorbent-adsorbate interactions.

Cadmium ion adsorption on different carbon adsorbents from aqueous solutions. Effect of surface chemistry, pore texture, ionic strength, and dissolved natural organic matter.

Adsorption of Cd(II) species at pH = 5 was studied on three carbon adsorbents: granular activated carbon, activated carbon fiber, and activated carbon cloth. As-received and oxidized adsorbents were used. Cd(II) adsorption greatly increased after oxidation due to the introduction of carboxyl groups. The use of a buffer solution to control the pH introduced some changes in the surface chemistry of carbons through the adsorption of one of the compounds used, biphthalate anions. The increase in ionic strength reduced Cd(II) uptake on both as-received and oxidized carbons due to a screening of the electrostatic attractions between the Cd(II) positive species and the negative surface charge, which in the case of as-received carbons derived from the biphthalate anions adsorbed and in the oxidized ones from the carboxyl groups. Tannic acid was used as a model compound for natural organic matter. Its adsorption was greatly reduced after oxidation, and most of the carbon adsorbents preadsorbed with tannic acid showed an increase in Cd(II) uptake. In the case of competitive adsorption between Cd(II) species and tannic acid molecules, there was a decrease in Cd(II) uptake on the as-received carbon whereas the contrary occurred with the oxidized carbons. These results illustrate the great importance of carbon surface chemistry in this competitive adsorption process. Finally, under all experimental conditions used, when the adsorption capacity of carbons was compared under the same conditions it increased in the following order: granular activated carbon < activated carbon fiber < activated carbon cloth.

Application of ammonia intermittent temperature-programmed desorption to evaluate surface acidity of tungsten oxide supported on activated carbon.

Intermittent temperature-programmed desorption of ammonia was used to study the strength and population of surface acid sites of tungsten oxide supported on activated carbon pretreated at 350 and 700 degrees C. Catalysts pretreated at 350 degrees C showed two types of surface acid sites and desorption occurred with free readsorption until a temperature of around 300 degrees C was reached. Pretreatment at 700 degrees C produced three different states of ammonia adsorbed on the catalysts and desorption occurred with free readsorption.