Pd and Pt catalysts supported on carbon-coated monoliths for low-temperature combustion of xylenes

Carbon-coated monoliths were prepared from polyfurfuryl alcohol coated cordierite structures obtained by dip-coating. In this way, thin, homogeneous, consistent and good adhered carbon layers were obtained. The different steps followed in the preparation of these catalyst supports were studied by scanning electron microscopy. Pd and Pt catalysts were prepared by equilibrium impregnation of the monolithic supports with an aqueous solution of the corresponding tetraammine metal (II) nitrates. The catalysts were pretreated in H₂ at 300 °C before their characterization by chemisorption or before testing their catalytic activity. This pretreatment was monitored by temperature programme reduction. Catalytic activities in xylene combustion were evaluated as a function of the reaction temperature as well as against time on stream. The monolithic catalysts were thermally stable during the reaction. The Pt catalysts were more active than the Pd ones. The Pd catalysts with smaller Pd-particle sizes were more active. In the case of Pt catalysts however, the opposite was observed, which might be due to a structure sensitivity effect. Complete xylene combustion was reached in the range between 150 and 180 °C with a total selectivity to CO₂ and H₂O. Combustion of m-xylene was easier than p-xylene over Pt.     

Controlling the diameter distribution of carbon nanotubes grown from camphor on a zeolite support

Single-wall and multi-wall carbon nanotubes (SWNTs and MWNTs, respectively) of controlled diameter distribution were selectively grown by thermal decomposition of a botanical hydrocarbon, camphor, on a high-silica zeolite support impregnated with Fe-Co catalyst. Effects of catalyst concentration, growth temperature and camphor vapor pressure were investigated in wide ranges, and diameter distribution statistics of as-grown nanotubes was analyzed. High yields of metal-free MWNTs of fairly uniform diameter (∼10 nm) were grown at 600-700 °C, whereas significant amounts (∼30%) of SWNTs were formed at 850-900 °C within a narrow diameter range of 0.86-1.23 nm. Transmission electron microscopy and micro-Raman spectroscopy reveal that camphor-grown nanotubes are highly graphitized as compared to those grown from conventional CNT precursors used in chemical vapor deposition.     

Carbon nanocage: a large-pore cage-type mesoporous carbon material as an adsorbent for biomolecules

Novel large pore cage type mesoporous carbons, carbon nanocages, abbreviated as CKT (carbon from KIT-5) using three dimensional large cage type face centered cubic Fm3m mesoporous silica materials (KIT-5) as inorganic templates prepared at different temperature were synthesized and characterized. The specific pore volume and the specific surface area of the carbon nanocage materials are much higher as compared to CMK-3 mesoporous carbon. Adsorption of amino acid (histidine), flavonoid (catechin), vitamin (vitamin E, α-tocopherol), endocrine disrupter (nonylphenol), and enzyme (lysozyme) over the carbon nanocage materials was also studied. Interestingly, carbon nanocage materials showed an unusual adsorption capacity of catechin in aqueous solution. The CKT carbon nanocage materials also exhibited higher adsorption capabilities of other biomolecules used in this study as compared with the CMK-3 mesoporous carbon.     

Surface oxygen complexes as governors of neopentane sorption in multiwalled carbon nanotubes

Multiwalled carbon nanotube (MWCNT) samples were obtained by purification and ball-milling of a MWCNT sample synthesized by catalytic chemical vapor decomposition. These samples were oxidized with KMnO₄-H₂SO₄ solution. A heat treatment was employed to decompose the oxygenated groups created by the oxidation treatment. The samples were characterized by transmission electron microscopy (TEM), adsorption of nitrogen, pore size distribution, and by thermogravimetric analysis combined with mass spectroscopy (TG-MS). The dynamics of 2,2-dimethyl-propane (neopentane) sorption was studied by frequency-response (FR) spectroscopy. The FR spectra obtained clearly demonstrate the effect of (a) the ball-milling, (b) the oxidative treatment and (c) the heat treatment on the sorption uptake processes involved in these treated samples.     

Adsorption of chromium by activated carbon from aqueous solution

Adsorption isotherms of Cr(III) and Cr(VI) ions on two samples of activated carbon fibres and two samples of granulated activated carbons from aqueous solutions in the concentration range 20–1000 mg/l have been studied. The adsorption isotherms have been determined after modifying the activated carbon surfaces by oxidation with nitric acid, ammonium persulphate, hydrogen peroxide and oxygen gas at 350°C and after degassing at different temperatures. The adsorption of Cr(III) ions increases on oxidation and decreases on degassing. On the other hand, the adsorption of Cr(VI) ions decreases on oxidation and increases on degassing. The increase of Cr(III) and the decrease of Cr(VI) on oxidation and the decrease of Cr(III) and the increase of Cr(VI) on degassing have been attributed to the fact that the oxidation of the carbon surface enhances the amount of acidic carbon–oxygen surface groups while degassing eliminates these surface groups. Thus while the presence of acidic surface groups enhances the adsorption of Cr(III) cations, it suppresses the adsorption of Cr(VI) anions.     

Characterization and kinetic study of carbon-based briquettes for the reduction of NO

Carbon-based catalytic briquettes have been prepared for the reduction of NO in the presence of NH₃ at low temperatures (100-250 °C). In general terms, the reduction efficiency is promoted by an increase in the support porosity, and especially, by an oxidizing treatment prior to the impregnation. The apparent rate constants for the catalytic briquettes doped with the ashes of a petroleum coke are within the same order of magnitude as other catalysts for selective catalytic reduction (SCR) of NO at low temperature, demonstrating the feasibility of using them for this purpose.     

Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution

The objective of this research was to develop activated carbon selection criteria that assure the effective removal of trace organic contaminants from aqueous solution and to base the selection criteria on physical and chemical adsorbent characteristics. To systematically evaluate pore structure and surface chemistry effects, a matrix of activated carbon fibers (ACFs) with three activation levels and four surface chemistry levels was prepared and characterized. In addition, three granular activated carbons (GACs) were studied. Two common drinking water contaminants, relatively polar methyl tertiary-butyl ether (MTBE) and relatively nonpolar trichloroethene (TCE), served as adsorbate probes. TCE adsorbed primarily in micropores in the 7-10 Å width range while MTBE adsorbed primarily in micropores in the 8-11 Å width range. These results suggest that effective adsorbents should exhibit a large volume of micropores with widths that are about 1.3 to 1.8 times larger than the kinetic diameter of the target adsorbate. Hydrophobic adsorbents more effectively removed both TCE and MTBE from aqueous solution than hydrophilic adsorbents, a result that was explained by enhanced water adsorption on hydrophilic surfaces. To assure sufficient adsorbent hydrophobicity, the oxygen and nitrogen contents of an activated carbon should therefore sum to no more than about 2 to 3 mmol/g.     

Surface modification and characterisation of a coal-based activated carbon

Surface modification of a coal-based activated carbon (F400) was performed using thermal and chemical methods. Nitric acid oxidation of the conventional sample produced samples with weakly acidic functional groups and the presence of such groups was confirmed by Fourier transform infra red (FTIR) spectroscopy, pH titration, zeta potential measurements and sodium uptake capacity results. There was a significant loss in microporosity of the oxidised samples which was caused by humic substances that were formed as a by-product during the oxidation process. Thermal treatment produced a carbon with some basic character while amination of the thermally treated carbon gave a sample containing some amino (–NH₂) groups and these groups were detected by X-ray photoelectron spectroscopy (XPS) analysis.     

Preparation and characterization of multi-walled carbon nanotubes supported PtRu catalysts for proton exchange membrane fuel cells

A series of PtRu nanocomposites supported on H₂O₂-oxidized multi-walled carbon nanotubes (MWCNTs) were synthesized via two chemical reduction methods—one used aqueous formaldehyde (HCHO method) and the other used ethylene glycol (EG method) as the reducing agents. The effects of the solvents (water and ethylene glycol) and the surface composition of the MWCNTs on the deposition and the dispersion of the metal particles were investigated using N₂ adsorption, TEM, ICP-AES, FTIR and TPD. The wetting heats of the MWCNTs in corresponding solvents were also measured. The characterizations suggest that combination of the surface chemistry of the MWCNTs with the solvents decides the deposition and the dispersion of the metal nanoparticles. These nanocomposites were evaluated as proton exchange membrane fuel cell anode catalysts for oxidation of 50 ppm CO contaminated hydrogen and compared with a commercial PtRu/C catalyst. The data reveal superior performances for the nanocomposites prepared by the EG method to those by the HCHO method and even to that for the commercial analogue. Structure–performance relationship of the nanocomposites was also studied.     

Catalytic oxidization of SO2 from incineration flue gas over bimetallic Cu–Ce catalysts supported on pre-oxidized activated carbon

To improve the deep sulfation of alumina support, the inertness material activated carbon was used as an alternative support for copper/cerium catalysts to remove SO₂ from incineration flue gas which contained other air pollutants such as NO_X, CO, CO₂, HCl, carbon particulates, and heavy metal vapor. During the 473–820 K, the AC support showed no retention of SO₂. However, the metal Pb composed in the flue gas exhibited the toxic characterization to M/AC catalysts, which was due to the outer orbitals of d subshell all paired.     

Functionalization of carbon support and its influence on the electrocatalytic behaviour of Pt/C in H2 and CO electrooxidation

Chemical modification of Carbon Vulcan XC-72R for fuel cell applications has been undertaken. Treated carbons were used as carriers for the deposition of Pt nanoparticles and used as electrocatalysts. The influence of the carbon treatment, as well as that of the Pt nanoparticles generation and their deposition route has been studied. The behaviour of the electrocatalysts in the CO and hydrogen oxidation reaction (HOR) has been studied. It was observed that carbon pre-treatment lead to difference behaviour in the CO oxidation reaction compared with the performance over non treated supports. In this way, CO oxidation was controlled by the nature of the support rather than by the nature of the Pt particles alone.     

Effectiveness of carbon microcoils as a reinforcing material for a polymer matrix

Spring-shaped carbon microcoils (CMCs) were embedded in epoxy resins to form CMC/epoxy resin composites. The mechanical properties of the composites were examined and compared with those of conventional straight carbon fiber (CF) /epoxy resin composites. CMCs were found to be more effective than CFs as a reinforcing material for the epoxy resin having a low Young’s modulus (0.54 MPa). SEM images of the fractured cross-sections of the composite revealed that CMCs were not pulled out from the resin matrix but fractured with the matrix. This could be ascribed to the unique conformations of CMCs.     

Grafted and crosslinked carbon black as an electrode material for double layer capacitors

Isocyanate prepolymers can readily react with oxidic functional groups on carbon black. Different prepolymers were examined for their grafting behaviour. In carbon black grafted with di-isocyanates, reactive isocyanate groups are available for cross-linking to a polyurethane system. This crosslinked carbon black was designed as a new active material for electrochemical electrodes. The successful crosslinking of carbon black was demonstrated by determinations of the grafted groups via titration and via thermogravimetric analysis. Products of this type were tested as an active material for electric double layer capacitor application. Active material for electric double layer capacitor electrodes was produced which had a specific capacitance of up to 200 F/g. Crosslinking efficiencies of up to 58% of the utilized polymers were achieved.     

Monodispersed hard carbon spherules as a catalyst support for the electrooxidation of methanol

Hard carbon spherules (HCS) were used as support of Pt nanoparticles as electrocatalyst for direct methanol fuel cells (DMFCs). Scanning electron microscopy (SEM) images show that the size of the Pt particles on HCS by reduction of K₂PtCl₆ with ethylene glycol is 4-5 nm. High-resolution transmission electron microscopy (HRTEM) study reveals that the Pt particles on the HCS surface have faceted crystalline structures. The size and aggregation of the Pt particles depend on the surface properties of the carbon support and the medium of the reduction reaction. Cyclic voltammetry and galvanostatic polarization experiments show that the Pt/HCS catalyst exhibits a higher catalytic activity in the electrooxidation of methanol than either the Pt/MCMB or the commercial Pt/Vulcan XC-72 catalyst does.     

Polyaniline/porous carbon electrodes by chemical polymerisation: Effect of carbon surface chemistry

Polyaniline/porous carbon composite electrodes were prepared by chemical polymerisation and characterized in terms of porosity and performance as electrochemical capacitors.To obtain the composite electrodes two methods were used. The first method consisted of mixing, directly, the activated carbon with chemically polymerised polyaniline. The second one consisted of mixing the activated carbon with aniline and subsequent chemical polymerisation. Additionally, the second process was carried out with the porous carbon previously thermally treated in N₂ up to 900 °C in order to remove surface oxygen groups.Changes in porosity with the polyaniline addition were analysed. It has been proved that the method used strongly affects the porous structure. Dealing with the electrochemical performance, polyaniline and carbon mechanically mixed seem to work independently, being the composite behaviour a combination of the corresponding performance of both materials separately. The composites prepared by the second method (polymerisation over carbon) reveal the key role of surface chemistry in polyaniline coating. Aniline reacts with the oxygen complexes and their positive effect in capacitance is not observed.The second method (polymerisation over carbon) using a thermally treated carbon seems to be the best one since a more porous (or thinner) polyaniline film is produced.