José L. Figueiredo

Modification of the surface chemistry of activated carbons

Abstract A NORIT activated carbon was modified by different chemical and thermal treatments (including oxidation in the gas and liquid phases) in order to obtain materials with different surface properties. Several techniques were used to characterize these materials including nitrogen adsorption, chemical and thermal analyses, XPS, TPD and DRIFTS. The results obtained by TPD agree quantitatively with the elemental and proximate analyses of the oxidized materials, and qualitatively with the observations by DRIFTS. A simple deconvolution method is proposed to analyse the TPD spectra, allowing for the quantitative determination of the amount of each functional group on the surface. A multiple gaussian function has been shown to fit the data adequately, the parameters obtained for each fit matching very well the features observed in the experimentally determined TPD spectra. It is shown that gas phase oxidation of the carbon increases mainly the concentration of hydroxyl and carbonyl surface groups, while oxidations in the liquid phase increase especially the concentration of carboxylic acids.

Pyrolysis kinetics of lignocellulosic materials—three independent reactions model

Abstract The behaviour of biomass components (cellulose, xylan—representative of hemicelluloses—and lignin) was studied thermogravimetrically with linear temperature programming, under nitrogen and air. The results were compared and the pyrolysis kinetics of cellulose determined, assuming a first-order kinetic function. The thermal decomposition of xylan and lignin could not be modelled with acceptable errors by means of simple reactions. Thermograms were determined for pine and eucalyptus woods and pine bark, under inert (nitrogen) or oxidizing (air) atmosphere. The pyrolysis of these lignocellulosic materials was modelled with good approximation by three first-order independent reactions. One of these reactions is associated with the primary pyrolysis of cellulose, its parameters being previously determined and fixed in the model. The model parameters are the activation energies and pre-exponential factors for the pyrolysis of the remaining two pseudo-components and two additional parameters related to the biomass composition. A method to determine this composition was proposed. The results calculated in this way were compared with data from the literature and those determined thermogravimetrically in air, showing good agreement.

The effects of different activated carbon supports and support modifications on the properties of Pt/AC catalysts

Two commercial activated carbons were used as catalyst supports in Pt/AC catalysts. Activated carbons were used in their unoxidized form or after either liquid phase oxidation with HNO3 or gas phase oxidation with 5% O2. All six activated carbons were characterized by nitrogen adsorption, TPD, and SEM. Selected samples were tested by XPS. The types and abundance of the oxygen bearing surface groups on each of the activated carbon supports were determined from the deconvolution of the TPD profiles of the activated carbons. An evaluation of the results obtained from TPD and XPS together indicates that both oxidation treatments led to a nearly homogeneous increase of oxygen bearing groups on the outer surface as well as interior surfaces. XPS analysis revealed that the HNO3 oxidized samples have the highest concentration of oxygen bearing groups on the outermost surface. Pt/AC catalysts (1 wt.%) were prepared on these supports. The catalysts were characterized by TPR, H2 adsorption, SEM–EDS and tested in structure insensitive benzene hydrogenation as well. The results obtained indicate that the oxidation treatments affect drastically the properties of the activated carbons as well as the catalysts prepared on them. A comparison between benzene hydrogenation activities validates the dispersion values obtained from H2 adsorption at room temperature.

Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts. I. Influence of surface chemical groups

Activated carbons were used as catalysts for the oxidative dehydrogenation of ethylbenzene. The type and amount of functional groups on the surface of the carbon catalysts was modified by oxidative treatments in the gas or in the liquid phase, while heat treatments at different temperatures were used to selectively remove some of the functional groups. The performance of these catalysts was evaluated in terms of conversion, styrene yield and selectivity. The results show that the gas phase treatments lead to improved performance associated with an increase in the amount of carbonyl/quinone groups on the surface, which were identified as the active sites for the reaction. A good correlation between catalytic activity and the concentration of these surface groups was obtained.

Design of graphene-based TiO2 photocatalysts—a review

There is a recent increase in the interest of designing high-performance photocatalysts using graphene-based materials. This review gathers some important aspects of graphene–TiO2, graphene oxide–TiO2, and reduced graphene oxide–TiO2 composites, which are of especial relevance as next generation photocatalysts. The methods used for the preparation of these materials, the associated mechanistic fundamentals, and the application of graphene-based composites on the photocatalytic degradation of pollutants are reviewed. Some structural, textural, and chemical properties of these materials and other photo-assisted applications, such as hydrogen production from water splitting and dye-sensitized solar cells, are also briefly included.

Carbon fibers, filaments, and composites

I. Fibres.- Carbon fibers - present state and future expectations.- Pitch and mesophase fibers.- Vapor-grown carbon fibers.- The growth of vapor-deposited carbon fibres.- Structure and properties of carbon fibres.- Surface properties of carbon fibres.- Critical issues for carbon fibers.- II. Composites.- Carbon fibres and composites.- High performance carbon fibre composites with thermoplastic matrices.- The fundamentals of chemical interactions in composite interfaces.- Surface modification of carbon fibres by plasma polymerization.- Potential of carbon/carbon composites as structural materials.- Theoretical and practical aspects of liquid-phase pyrolysis as basis of the carbon matrix of CFRC.- Protective layers for special types of composites.- Carbon based materials in medical applications.- Fibre reinforced ceramic-matrix composites.- Critical issues concerning carbon fibre reinforced composites.- III. Filaments.- Electron microscopy studies of the catalytic growth of carbon filaments.- Filamentous carbon formation on metals and alloys.- Carbon deposition and filament initiation and growth mechanisms on iron particles and foils.- Structural characterization of filamentous carbons produced on metal catalysts.- Physical modeling of carbon filament growth.- Applications of carbon filaments.- Carbon filaments.- List of lectures and participants.

A model for pyrolysis of wet wood

Abstract A mathematical model of the pyrolysis of wet particles of wood is presented. The model integrates: (i) a conventional description of the physical and chemical phenomena involved in the pyrolysis of dry particles of wood, and (ii) a simplified drying model. The dry-pyrolysis model assumes a complex reaction scheme independently determined. The most important parameters were experimentally measured, including the thermal conductivities and all the kinetic parameters; other parameters were taken as average values from the literature. The drying model neglects bound-water diffusion, air/vapour diffusion and pressure gradients inside the solid. Free-water movement is not described. Drying is theredore controlled by heat supply. These assumptions restrict the model validity range to:(a) temperatures higher than 150°C, (b) initial moisture content below the free-water continuity point (∼45%), and (c) sample dimension in the wood longitudinal direction not much greater than the dimensions in the transversal directions. The combined wet pyrolysis model has been experimentally validated in the simulation of: (i) drying of pine wood cylinders of variable diameter above 150°C, and (ii) pyrolysis of dry and wet cylinders pine wood of variable diameter between 300 and 800°C. No parameters optimization was required.

Adsorption of ciprofloxacin on surface-modified carbon materials

The adsorption capacity of ciprofloxacin (CPX) was determined on three types of carbon-based materials: activated carbon (commercial sample), carbon nanotubes (commercial multi-walled carbon nanotubes) and carbon xerogel (prepared by the resorcinol/formaldehyde approach at pH 6.0). These materials were used as received/prepared and functionalised through oxidation with nitric acid. The oxidised materials were then heat treated under inert atmosphere (N2) at different temperatures (between 350 and 900°C). The obtained samples were characterised by adsorption of N2 at -196 °C, determination of the point of zero charge and by temperature programmed desorption. High adsorption capacities ranging from approximately 60 to 300 mgCPxgC(-1) were obtained (for oxidised carbon xerogel, and oxidised thermally treated activated carbon Norit ROX 8.0, respectively). In general, it was found that the nitric acid treatment of samples has a detrimental effect in adsorption capacity, whereas thermal treatments, especially at 900 °C after oxidation, enhance adsorption performance. This is due to the positive effect of the surface basicity. The kinetic curves obtained were fitted using 1st or 2nd order models, and the Langmuir and Freundlich models were used to describe the equilibrium isotherms obtained. The 2nd order and the Langmuir models, respectively, were shown to present the best fittings.

Catalytic properties of carbon materials for wet oxidation of aniline

A mesoporous carbon xerogel with a significant amount of oxygen functional groups and a commercial activated carbon, were tested in the catalytic wet air oxidation of aniline at 200 degrees C and 6.9 bar of oxygen partial pressure. Both carbon materials showed high activity in aniline and total organic carbon removal, a clear increase in the removal efficiency relatively to non-catalytic wet air oxidation being observed. The best results in terms of aniline removal were obtained with carbon xerogel, an almost complete aniline conversion after 1h oxidation with high selectivity to non-organic compounds being achieved. The materials were characterized by thermogravimetric analysis, temperature programmed desorption, N(2) adsorption and scanning electron microscopy, in order to relate their performances to the chemical and textural characteristics. It was concluded that the removal efficiency, attributed to both adsorption and catalytic activity, is related to the mesoporous character of the materials and to the presence of specific oxygen containing functional groups at their surface. The effect of catalytic activity was found to be more important in the removal of aniline than the effect of adsorption at the materials surface. The results obtained indicate that mesoporous carbon xerogels are promising catalysts for CWAO processes.

Oxidative dehydrogenation of ethylbenzene on activated carbon catalysts: 3. Catalyst deactivation

Abstract Extended catalytic tests show that coke deposition is responsible for the deactivation of activated carbon catalysts in the oxidative dehydrogenation of ethylbenzene (ODE). Temperature-programmed desorption (TPD), DRIFTS, textural and elemental analyses of used catalysts have shown that not only the great majority of the micropores become blocked, but also that the amounts of oxygen and hydrogen in the catalyst composition increase with time on stream, leading to a material increasingly more reactive towards oxidation. It was observed that after a few days on stream, the rate of carbon gasification became larger than the rate of coke deposition, leading to a decrease in catalyst weight. Working under milder conditions (lower temperature and oxygen partial pressure) delayed this effect. The increase in the concentration of surface groups with time does not result in a proportional increase in the activity of the catalysts, because the majority of the groups created are not active for the ODE reaction.