Noble metals supported on carbon black composites as catalysts for the wet-air oxidation of phenol

Pt, Pd and Ru catalysts supported on carbon black composites (CBC) were characterized in the wet air oxidation of phenol solution using a fixed-bed reactor working in a trickle-flow regime under relatively mild conditions: temperature, 393-433 K; pressure, 50-80 bar; liquid hourly space velocity (LHSV), 0.5-6 h⁻¹. The activity of the catalysts decreases in the following order: Pt/CBC>Pd/CBC≈Ru/CBC?CBC. The physicochemical properties of the CBC are affected by its reaction with oxygen during the oxidation process. Combustion of the CBC material in the aqueous phase proceeds at a lower temperature than that in the gas phase; its surface properties change according the same rules as during low temperature oxidation by gaseous air.     

Preparation of low-density carbon aerogels by ambient pressure drying

With the addition of hexamethylenetetramine (HMTA) and alcohol as solvent, an ambient pressure drying technique was developed for the fabrication of low-density organic aerogels and related carbon aerogels. When a suitable ratio of resorcinol to HMTA (R/H ratio) and ratio of resorcinol to solvents (R/S ratio) are selected, the low-density alco-gels obtained can be dried under ambient pressure conditions without observable shrinkage. The addition of HMTA increases the size of carbon nano-particles and the pore size of the aerogels that are produced. The carbon aerogels prepared in this work have similar nano-particle structures typical of the aerogels prepared with CO₂ or by the isopropanol supercritical drying technique.     

Carbon aerogels derived from cresol–resorcinol–formaldehyde for supercapacitors

The objective of the present paper is to demonstrate the possibility to synthesize mixed carbon aerogels (denoted C_mRF) from cresol (C_m), resorcinol (R) and formaldehyde (F), as an alternative economic route to the classical RF synthesis. These porous carbon aerogels can be used as electrode materials for supercapacitors with a high volume-specific capacitance. Organic precursor gels were synthesized via polycondensation of a mixture of resorcinol and cresol with formaldehyde in an aqueous alkaline (NaOH) solution. After gelation and aging the solvent was removed via drying at ambient pressure to produce organic aerogels. Upon pyrolysis of the organic aerogels at 1173 K, monolithic carbon aerogels can be obtained. By controlling the catalyst (Cat) molar ratio (C_m+R/Cat) in the range 200–500, up to 70% of the resorcinol can be replaced with the cheap cresol. The resulting homogeneous organic aerogels exhibit a drying shrinkage below 15% (linear). The shrinkage and mass loss upon pyrolysis of the mixed aerogels increase with increasing cresol content. Nitrogen adsorption at 77 K was employed to characterize the microstructure of the carbon aerogels. The data show that the porous structure of mixed carbon aerogels is similar to that of RF carbon aerogels. Cyclic voltammetry measurements show that the as-prepared C_mRF carbon aerogels exhibit a high volume-specific capacitance of up to 77 F/cm³.     

Morphology of heat-treated tunsgten doped monolithic carbon aerogels

The morphology of a tungsten-doped monolithic organic aerogel, prepared by the sol-gel method from the polymerisation of a resorcinol, formaldehyde and ammonium tungstate mixture, and of its carbonized derivatives at 500 and 1000 °C was studied by scanning and high-resolution electron microscopy. Tungsten influenced the surface morphology of the carbon aerogels. The tungsten-containing phase was homogeneously distributed in the organic aerogel and its heat treatment produced changes in the metal phase distribution throughout the pellets. Tungsten oxide particles of needle-like structure similar to hollow tubules were detected after heat treatments at different temperatures. In addition, particles of dendritic appearance, formed by tungsten carbide and an intermediate Magnelli phase, appeared when the heat treatment was carried out at 1000 °C.     

Organic and carbon aerogels derived from poly(vinyl chloride)

Organic aerogels were derived from dimethylformamide solution of poly(vinyl chloride) (PVC) via dehydrochlorination using a strong base, 1,8-diazabicyclo[5,4,0]undec-7-ene, and supercritical drying using carbon dioxide. From these organic aerogels, carbon aerogels were yielded via stabilization and carbonization. Changes in the porous structure of the aerogels during the preparation process and influences of the preparation conditions on the porous structure were investigated. The framework of the aerogels composed the walls of the meso- and macropores. The volume and the size of these pores were reduced during stabilization and carbonization due to the shrinkage of the framework caused by the release of decomposition gases and densification of the material. Simultaneously, the release of decomposition gases produced additional micropores. The extent of dehydrochlorination, the concentration of PVC in the starting solution and the molecular weight of PVC were the factors with which the porous structure of the aerogels could be controlled over a wide range. In addition, the stabilization conditions notably influenced the carbonization behavior of the organic aerogels and the porous structure of the carbon aerogels. The optimum stabilization conditions that minimized the loss of mass and maximized the pore volume of the carbon aerogels were determined.     

Structural changes in carbon aerogels with high temperature treatment

The structural change of carbon aerogels at high temperatures up to 2800°C has been investigated. Change in microtexture of fine particles, which constitute carbon aerogels derived from phenolic resin, was of a typical non-graphitized carbon. The microporosity decreased with an increase of heat-treatment temperature, and disappeared at 2000°C. The mesoporosity still remained even after heat-treatment up to 2800°C, though 50% of mesopore volume was lost because of the fusion of the particles with the change of carbon microtexture.     

A study of carbon nanotube formation by C2H2 decomposition on an iron based catalyst using a pulsed method

The synthesis of multiwalled carbon nanotubes has been studied using a pulsed catalytic method. Acetylene was used as carbon generation reactant and silica or alumina supported iron as catalysts. Different metal loadings and various reaction temperatures were evaluated. The carbon growth over the catalysts has been followed gravimetrically ‘in situ’. The reaction was stopped after different pulse numbers in an attempt to control both the diameters and the lengths of the carbon nanotubes, which were characterised by transmission electron microscopy (TEM). A consecutive step growing mechanism is used to describe our experiments.     

Methanol electro-oxidation on mesocarbon microbead supported Pt catalysts

Mesocarbon microbeads (MCMB) as Pt catalyst supports were characterized by X-ray electron diffraction, thermal field emission scanning electron microscope and electrochemical analysis. MCMB with different pretreatment were used as the catalyst supports. The XRD patterns show the existence of Pt and the micrograph of SEM shows Pt is absorbed uniformly on the surface of MCMB particles and the platinum grain size is ca. 3-5 nm. The effect of the pretreatment of the support on the catalyst performance of methanol electrooxidation was studied. Electrochemical analysis shows that MCMB are excellent candidates to be used as the support of catalyst for methanol electrochemical oxidation. The catalyst with MCMB boiled in KOH for 1 h as support exhibits a high catalytic activity during the electrooxidation of methanol.     

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.     

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.     

Catalytic activity of carbon nanotubes in the oxidative dehydrogenation of ethylbenzene

Multi-walled carbon nanotubes (MWNT), before and after different oxidative treatments and hence possessing different oxygen surface groups, were used as catalysts for the oxidative dehydrogenation of ethylbenzene (ODE), and their performances compared to those of activated carbon and graphite samples. MWNT are active catalysts for ODE and show the highest specific activity per initial surface area amongst all the materials tested in this study. Moreover, the main advantage of using MWNT over activated carbons is their higher stability under oxidative conditions. It was shown that pre-oxidised MWNT are more active for ODE during the initial stages of the reaction, highlighting the importance of oxygenated surface groups.     

Carbon nanotube synthesis upon stainless steel meshes

In this paper we report and interpret the effectiveness of different bulk metal catalyst preparations and of various components within reactive gas mixtures for carbon nanotube (CNT) synthesis. The combined catalyst precursor and supporting material is type 304 stainless steel mesh. The steel mesh keenly illustrates the net effect of different pretreatments upon the catalyst because of its resistance to oxidation. These preparative treatments include oxidation, reduction, and their combinations. Finally the utility of the different components within the reactive gas mixture are illustrated by synthesis tests in their individual absence. The effect of catalyst preparation and gas mixture on CNT synthesis is judged on the basis of the relative surface density and morphology of the CNTs (as observed via SEM) and their graphitic structure (as observed via TEM).     

On the determination of platinum particle size in carbon-supported platinum electrocatalysts for fuel cell applications

Using a conventional laboratory diffractometer, small angle X-ray scattering (SAXS) was used to determine the average platinum particle size in samples of carbon with a range of platinum loadings. The results obtained were compared with those obtained from wide-angle X-ray diffraction (WAXS) studies. SAXS was more effective than WAXS for determining the average platinum particle size in samples where the grains were so small that the resultant diffraction peaks in the WAXS profiles were too broad to accurately determine the peak width for use in the Scherrer equation.