The basis of recently developed quantitative TG-MS analysis was used for the evaluation of TPD-MS measurements in order to characterize the changes in the surface chemistry of carbon cryogels after ...the oxidation/activation treatment. This methodology was applied to the analysis of partially pyrolysed carbon cryogels (400 °C), which were oxidized/activated in four different ways (phosphoric acid, Fenton like reaction, ammonium persulfate with sulfuric acid and nitric acid with hydrogen peroxide). The quantitative TPD-MS evaluation enables analysis of all the evolved gases during the sample heating (without the calibration of all gases) and distinguishes the carbonization process from the decomposition of the more thermally stable oxygen surface groups (analysis of the effect of oxidation/activation reagents). The activation by H3PO4 results in the hydrolysis and esterification of anhydrides, lactones and phenols and creation of phosphoric acid esters (C-O-P), which decompose into C3-P=O linkages with the CO evolution. All the oxidation treatments had a similar effect on the surface chemistry. The amounts of carboxylic acids and anhydrides increased significantly, while the amount of lactones and phenol groups decreased (oxidized into carbonyls). Application of TG-MS fundamentals to the evaluation of TPD-MS profiles from a wide range of samples allows determining all the gaseous products.
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This work shows the potential application of carbon materials prepared by three different ionic liquid-based methods, using 1-butyl-3-methylimidazolium methanesulfonate bmimMeSO3, for electrochemical ...supercapacitors. The effects of bmimMeSO3 on morphology, texture and surface chemistry of prepared materials has been explored by SEM/TEM, N2/CO2 adsorption measurements and XPS. The results indicate the possibility of synthesis of carbon materials with tunable physicochemical properties using ionic liquid based methods. The charge storage behavior of all materials was studied in three different pH aqueous electrolytes. The pseudocapacitive and double layer contributions were estimated and discussed from the aspect of the textural changes and the changes of the chemical composition of surface functional groups containing heteroatoms. CO type functional groups, with the contribution of COOH groups, were found to be responsible for a different amount of charge, which could be stored in alkaline and acidic electrolytic solution. The material prepared by direct carbonization of bmimMeSO3, showed the best electrochemical performance in alkaline electrolyte with a capacitance of 187 F g−1 at 5 mV s−1 (or 148 F g−1 at 1 A g−1), due to the contribution of both electric-double layer capacitance and pseudocapacitance which arises from oxygen, nitrogen and sulfur functional groups.
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•Carbon materials were synthesized using new ionic liquid (IL) based methods.•Charge storage behavior in three pH-different electrolytes was compared.•Double layer and pseudocapacitance were separated and compared.•CO and COOH were found to be responsible for different charge storage.•Carbonized IL showed the highest capacitance due to contribution of N and S groups.
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•ZnO materials were loaded with gold nanoparticles by a double impregnation method.•Bare ZnO efficiency depends on both textural and surface properties.•Depending on the ZnO support ...Au nanoparticles act as electron sinks or sensitizers.•Photogenerated holes play a major role in phenol degradation but free radicals are also important.
Gold nanoparticles were loaded on ZnO materials by a double impregnation method. Materials were characterized by spectroscopic, microscopic and N2 adsorption techniques, and tested on the photocatalytic oxidation of phenol in aqueous solutions, under simulated solar light. Compared with bare ZnO, the Au-loaded catalysts presented increased activity, which has been evaluated in terms of the pseudo-first-order kinetic constant (kapp) and phenol mineralization. The best activities were obtained with the Au-loaded ZnO samples produced by chemical vapor deposition, and by thermal decomposition of zinc acetate (kapp=5.6min−1mgAu−1 for both materials), achieving mineralizations of 92% and 82%, respectively. Depending on the ZnO material, on the gold nanoparticle dimensions and on the irradiation wavelength used, gold particles may act as electron sinks or light harvesters. Selective trapping of photogenerated holes and radicals by selective scavengers showed that holes are crucial, but free radicals do also participate on phenol’s photodegradation pathway.
Design of graphene-based TiO2 photocatalysts—a review Morales-Torres, Sergio; Pastrana-Martínez, Luisa M.; Figueiredo, José L. ...
Environmental science and pollution research international,
11/2012, Volume:
19, Issue:
9
Journal Article
Peer reviewed
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–TiO
2
, graphene ...oxide–TiO
2
, and reduced graphene oxide–TiO
2
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.
Characterization of Active Sites on Carbon Catalysts Figueiredo, José L; Pereira, Manuel F. R; Freitas, Maria M. A ...
Industrial & engineering chemistry research,
06/2007, Volume:
46, Issue:
12
Journal Article
Peer reviewed
A method based on the deconvolution of TPD spectra is proposed for the characterization of surface oxygen groups, which can act as the active sites on carbon catalysts. The method, which was ...previously used to characterize activated carbons oxidized in the gas phase, has been extended and applied to other materials, carbons oxidized in the liquid phase. It is shown that this method fits quite well the TPD experimental data of the original activated carbon as well as the gas-phase and liquid-phase oxidized materials and is suitable to estimate the amounts of each type of oxygen surface groups.
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•N-doped graphene oxide (GO) samples were prepared with different N precursors.•A metal-free N-doped GO membrane was also developed.•The treatment of organic pollutants by persulfate ...(PS) activation was investigated.•Using the membrane, instead of powders, avoids the catalyst separation step.•Loss of N-pyridinic groups decreases the membrane catalytic activity.
The persulfate (PS) activation can be successfully applied in water treatment and the use of carbon materials to activate PS has been thoroughly described in the literature. Nevertheless, the prospect of employing a metal-free catalytic membrane as the single activator of PS has to be better studied. In the present work, graphite oxide was prepared by the modified Hummers’ method and different nitrogen precursors (melamine, urea and gaseous ammonia) were investigated to produce N-doped graphene oxide catalysts. The material prepared with melamine presented the highest nitrogen content (23.78 wt%) and the highest catalytic activity for the degradation of phenol in aqueous solution. The membrane fabricated with this catalyst was applied in filtration experiments for the degradation of phenol and oxalic acid. The catalytic mechanism is governed by both the radical and the non-radical degradation of the pollutants, but a more notorious contribution of singlet oxygen species was observed. It was demonstrated that the membrane lost the N-pyridinic groups during the reaction and, as consequence, part of its catalytic activity, whereas N-quaternary species remained in the used membrane. An important advantage associated with the application of this metal-free catalytic membrane, instead of a catalyst in powder form, is to avoid the final catalyst separation step.
► Acidic character of GOP composites varied with the GO content and heat-treatments. ► The mesoporosity of the composites increases with GO wt. but not the microporosity. ► GO aggregates coated with ...P25 nanoparticles are stabilized with TiOC bonds. ► Optimization of the GO content is fundamental in the photocatalytic efficiency. ► The formation of reduced graphene oxide improves the photocatalytic performance.
Graphene oxide (GO) and the benchmark TiO2 photocatalyst (P25) were used to prepare different composites (GOP), by a simple method of mixing and sonication, varying the GO content and the heat-treatment temperature under nitrogen. The composites were characterized by thermogravimetric (TG) and differential thermogravimetric (DTG) analyses, scanning electron microscopy (SEM), physical adsorption of nitrogen, UV–Vis and IR diffuse reflectance spectroscopies (DRUV and DRIFT), and point of zero charge (pHPZC) measurements. The morphology, microporosity and SBET of the composites did not vary significantly in comparison to P25, while an increase of their mesoporosity and mesopore diameter were observed due to the formation of GO aggregates coated with P25 nanoparticles. The aggregates were stabilized by the formation of TiOC bonds, which in turn produced a narrowing of the band gap relative to P25. The surface chemistry of GOP composites varied with the GO content, being more acidic when higher GO content was used. The photocatalytic performance was evaluated for the degradation of diphenhydramine (DP) pharmaceutical and methyl orange (MO) dye under near-UV/Vis irradiation. The first order rate constant of MO photodegradation increased four times for some GOP composites with relation to P25 (i.e., from k=52×10−3 to 207×10−3min−1). Comparable efficiencies were observed when DP was used as model pollutant (i.e., around k=54×10−3min−1). The best performing photocatalyst was that containing 1.4wt.% GO and treated at 200–300°C. The improved performance was attributed to the reduction of GO during the thermal treatment and to the good contact between the TiO2 and the carbon phases.
Multi-walled carbon nanotubes (MWCNTs) with different surface chemical properties were prepared by oxidative treatments with HNO
3, H
2O
2 and O
2 to introduce oxygen-containing surface groups and by ...thermal treatments for their selective removal. The texture and surface chemistry of the MWCNTs were characterized by nitrogen adsorption, temperature programmed desorption (TPD) and pH at the point of zero charge. A deconvolution procedure of the TPD spectra was used to quantify the oxygenated surface groups. These materials were used as catalysts for ozone decomposition, and for the ozonation of oxalic and oxamic acids. Generally, all these catalytic processes are favoured by carbon nanotubes with low acidic character. MWCNTs were shown to exhibit higher activity for the ozonation of oxalic and oxamic acids, compared to activated carbon. Successive experimental runs of oxalic acid removal carried out with a selected MWCNT sample show that the catalyst suffers some deactivation as a result of the introduction of oxygenated groups on the surface. Therefore, the effect of the surface chemistry is mainly observed for the fresh catalysts.
We investigate a photonic regenerative memory based upon a neuromorphic oscillator with a delayed self-feedback (autaptic) connection. We disclose the existence of a unique temporal response ...characteristic of localized structures enabling an ideal support for bits in an optical buffer memory for storage and reshaping of data information. We link our experimental implementation, based upon a nanoscale nonlinear resonant tunneling diode driving a laser, to the paradigm of neuronal activity, the FitzHugh-Nagumo model with delayed feedback. This proof-of-concept photonic regenerative memory might constitute a building block for a new class of neuron-inspired photonic memories that can handle high bit-rate optical signals.
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