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.     

Treatment of chlorophenols-bearing wastewaters through hydrodechlorination using Pd/activated carbon catalysts

The catalytic hydrodechlorination of 2-chlorophenol, 4-chlorophenol and 2,4-dichlorophenol in aqueous solution over Pd/activated carbon catalysts (0.5% w/w Pd) was studied in a fixed bed reactor. The reactor was fed with a 100 mg/l solution of chlorophenol and a H₂/N₂ (1:1) gas stream. The ranges studied for temperature, pressure and space-time were 25-100 °C, 1.8-6.0 bar and 14-55 kg h/mol, respectively. A commercial and some home-made catalysts were tested. The carbon supports were subjected to oxidation with nitric acid and sodium persulfate. Chlorophenols conversion was found to peak for pressure values ≈2.4 bar. In these conditions, an increase of reaction temperature increases conversion. In the runs carried out at high space-time both the reactants conversion and the selectivity towards end chain reaction products was enhanced. The oxidation of the carbon support with nitric acid prior to impregnation improves conversion. At the optimum conditions (2.4 bar, 75 °C and nitric acid oxidation of carbon support) conversion values over 95% were reached for all chlorophenols. As a result of this treatment the toxicity of the initial solutions was reduced by more than 90%.     

Transport and sorption of water vapour in activated carbons

The adsorption of water vapour on microporous carbons derived from the carbonization of coconut shell has been studied. The adsorption and desorption characteristics of water vapour on the activated carbons were investigated over the pressure range p/p₀ 0–0.95 in a static water vapour system. In these experiments the process of water adsorption/desorption was investigated by both kinetic and equilibrium experimental data. Activated carbons differing by the degree of burn-off have shown the importance of the microstructure. A carbon enriched with nitrogen functions underlined the influence of the surface chemistry.     

Effect of thermal and oxidative treatments of activated carbon on its surface structure and suitability as a support for barium-promoted ruthenium in ammonia synthesis catalysts

The effect of heat-treating a typical activated carbon at 1600-2500 °C on its structural and textural properties was investigated by gas sorption and X-ray diffraction. Particular attention was paid to the recovery of the surface area and porosity of thermally treated carbons by means of oxidation treatments. It was found that the thermal modification of activated carbon could eliminate carbon impurities, bring about different degrees of graphitization, and improve the resistance of the carbon support to undergo methanation under ammonia synthesis conditions, but the surface area and porosity decreased dramatically. Oxidative treatment partially recovered the surface area and porosity. The higher the thermal treatment temperature, the greater the stability of the carbon support, and the more difficult the recovery of the surface area and porosity becomes. A series of unpromoted ruthenium/carbon catalysts showed that the highly developed texture of the carbon support resulted in a significant increase in Ru dispersion. The ammonia synthesis activity of a Ba-promoted Ru catalyst was greatly improved by using activated carbon C1900ox as a support, which was heated at 1900 °C in an inert atmosphere and then subjected to oxidation treatment.     

Modelling of water adsorption by activated carbons: effects of microporous structure and oxygen content

The present paper examines the adsorption of water by microporous carbons containing various amounts of surface oxygen and a smaller proportion of basic centres. The modelling of water adsorption for 293 and 310 K, using variable pore size distributions (PSD), confirms that the overall type IV isotherm is the sum of a type I isotherm associated with the specific interactions, and a type V isotherm reflecting the non-specific interactions. The principle of temperature invariance is followed by these isotherms, which indicates that modelling leads to the Dubinin-Astakhov equation.The present approach allows the prediction of water adsorption near room temperature, on the basis of the PSD and the density of oxygen present on the surface area of the micropores. It is assumed, to a first and good approximation, that the pores are slit-shaped and the oxygen distribution is random.     

Ultrasound-promoted N-propargylation of imidazole by alkaline-doped carbons

The synthesis of N-propargyl imidazoles via alkylation of imidazole with propargyl bromide by sonochemical and thermally activated reactions over two alkaline promoted carbons (Na and Cs-Norit) as catalysts is reported. In this green, solvent free procedure, we produce exclusively N-propargyl imidazoles in very high yields (>80%) when the Cs⁺-Norit carbon is employed under ultrasound activation. The basicity of the alkaline cation increases the conversion and ultrasound activation affords a remarkable increase in the yields. The catalysts were characterized by thermal analyses, X-ray photoelectron spectroscopy and nitrogen adsorption isotherms.     

Influence of chemical modification of anthracite on the porosity of the resulting activated carbons

Activated carbons in a wide range of porosity (from essentially microporous to essentially mesoporous) have been prepared from anthracite by the combination of a chemical treatment with HClO₄ or Mg(ClO₄)₂ and a physical activation with CO₂ at 850°C. The main effects of the chemical treatment are a modification of anthracite microstructure by insertion and oxidation. The extent of these two reactions was found to be related to the nature of the chemical agents, the treatment time and temperature, and the textural properties of anthracite. As a consequence, the pretreatment of anthracite causes a noticeable reduction of the activation time and a change of the final pore size distribution. The influence of chemical treatment parameters has been analysed by means of XPS, FTIR, TGA, mass spectrometry, elemental analysis and gas adsorption techniques. The optimal conditions for producing activated carbons from chemically modified anthracites were identified. Step by step increasing of the temperature of anthracite treatment by HClO₄ up to 160°C seems to be the best way to obtain a precursor of highly activated carbon with well-balanced micro and meso porosity.     

Influence of surface properties and iron addition on the SO2 adsorption capacity of activated carbons

When iron derivatives are added to low ash activated carbons having basic surface characteristics (obtained by suitable oxidation at 800°C with 2% of O₂ in N₂), certain materials are obtained showing high SO₂ sorbent properties from gaseous mixtures having a composition close to that of the flue gases. This behaviour seems to be related to the simultaneous presence of both basic surface sites promoting the initial adsorption of SO₂ and iron promoting the transformation of the adsorbed SO₂ into other, more stable forms. The sorbent properties of these activated carbons are more stable, following consecutive cycles, in the processes of adsorption and desorption of SO₂, than those shown by similar carbons with different surface characteristics.     

Adsorption of dyes on activated carbons: influence of surface chemical groups

The surface chemistry of a commercial activated carbon has been selectively modified, without changing significantly its textural properties, by means of chemical treatments, using HNO₃, H₂O₂, NH₃, and thermal treatments under a flow of H₂ or N₂. The resultant samples were characterized in terms of their surface chemistry and textural properties, and subsequently tested in the removal of different classes of dyes. It was shown that the surface chemistry of the activated carbon plays a key role in dye adsorption performance. The basic sample obtained by thermal treatment under H₂ flow at 700 °C is the best material for the adsorption of most of the dyes tested. For anionic dyes (reactive, direct and acid) a close relationship between the surface basicity of the adsorbents and dye adsorption was shown, the interaction between the oxygen-free Lewis basic sites and the free electrons of the dye molecule being the main adsorption mechanism. For cationic dyes (basic) the acid oxygen-containing surface groups show a positive effect but thermally treated samples still present good performances, showing the existence of two parallel adsorption mechanisms involving electrostatic and dispersive interactions. The conclusions obtained for each dye individually were confirmed in the colour removal from a real textile process effluent.     

The characterization of activated carbons with oxygen and nitrogen surface groups

The physicochemical properties and the surface chemical structure of the carbon materials obtained by the modification of the commercial activated carbon D43/1 (Carbo-Tech, Essen, Germany) were studied. The previously de-ashed activated carbon was subjected to the following modification procedures: high-temperature treatment (1000 K) under vacuum; oxidation with conc, nitric acid; and ammonia-treatment of annealed and oxidised carbons at high temperatures. The porous structure and the surface area of the five different carbon samples obtained were estimated by means of mercury porosimetry and from low-temperature nitrogen adsorption data. The thermogravimetric analysis and the quantitative determination of surface functional groups by selective neutralisation of bases and pH-metric titration were carried out. FTIR spectra (transmission) and X-ray photoelectron spectra (Cls, Ols and Nls) were obtained for all the carbon samples and compared with one another. The changes in the porosity and the chemical properties of the carbon surface caused by the modification were analysed. Some possible surface functional species, their structure and surface state are discussed.     

Adsorption of phenanthrene on activated carbons: Breakthrough curve modeling

The modeling of breakthrough curves obtained in the adsorption of phenanthrene as model compound on different activated carbons is described. All the runs were performed in a fixed bed reactor at atmospheric pressure, with a process temperature of 150 °C and low contaminant concentrations. Therefore, experimental conditions were similar to the observed in the flue gases of energy generation systems. This work is mainly focused on the study of how adsorbent characteristics (surface area and micropore size distribution) influence the kinetics of the adsorption process. First, equilibrium values are found from the breakthrough curves and they are satisfactory fitted to the Freundlich isotherm. Using the obtained equilibrium values together with the linear driving force model as kinetic expression, the breakthrough curve modeling is achieved. It was found that this model fits all the breakthrough curves and it is a useful tool for modeling purposes. Values for the phenanthrene surface and effective diffusion coefficients are calculated and reported, and a relationship with the microporosity is found. As it was expected, it is observed that the phenanthrene molecule finds kinetic restrictions for the diffusion in those carbons with narrow microporosity, especially in those with a mean pore diameter close to the molecular size.     

On the nature of surface acid sites of chlorinated activated carbons

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, pH_PZC 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 Brönsted 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.     

Influence of activated carbon pore structure on oxygen reduction at catalyst layers supported on rotating disk electrodes

Carbon materials are often used as catalyst supports, and for catalysts in electrodes of a polymer electrolyte fuel cell, carbon black has been used. Recently, it was found, however, that activated carbon could replace carbon black and besides, significantly improve the activity of the electrode catalyst layer for oxygen reduction. In the present study, to optimize the pore structure of activated carbon for further activity improvement, the influence of the pore structure on the activity was investigated using activated carbon of various specific surface areas and mean pore diameters. A catalyst layer was formed from activated carbon loaded with platinum and a polymer electrolyte. The activity of the layer was measured in an oxygen-saturated perchloric acid solution, supporting the layer on a rotating glassy carbon disk electrode. We found that increases in the specific surface area and mean pore diameter increased the activity and that the latter was more effective than the former mainly due to the enhanced mass-transfer in the pores; the catalyst layer formed from activated carbon with the largest mean pore diameter was the most active. Unless pores excessively develop and lose connections between particles, a large pore diameter is therefore desired for the fuel cell electrodes.