John W. Geus

Carbon nanofibers: catalytic synthesis and applications

Carbon nanofibers (diameter range, 3–100 nm; length range, 0.1–1000 µm) have been known for a long time as a nuisance that often emerges during catalytic conversion of carbon-containing gases. The recent outburst of interest in these graphitic materials originates from their potential for unique applications as well as their chemical similarity to fullerenes and carbon nanotubes. In this review, we focus on the growth of nanofibers using metallic particles as a catalyst to precipitate the graphitic carbon. First, we summarize some of the earlier literature that has contributed greatly to understand the nucleation and growth of carbon nanofibers and nanotubes. Thereafter, we describe in detail recent progress to control the fiber surface structure, texture, and growth into mechanically strong agglomerates. It is argued that carbon nanofibers are unique high-surface-area materials (˜200 m2/g) that can expose exclusively either basal graphite planes or edge planes. Subsequently, we will present the recently explored applications of carbon nanofibers: polymer additives, gas storage materials, and catalyst supports. The latter application is described in detail. It is shown that the graphite surface structure and the lyophilicity play a crucial role during metal emplacement and catalytic use in liquid-phase catalysis. A case in point is fiber-supported Pd catalysts for nitrobenzene hydrogenation. Finally, we summarize issues with respect to the large-scale production of carbon nanofibers, including production cost estimates and research items to be dealt with in future work.

Synthesis of supported catalysts by impregnation and drying using aqueous chelated metal complexes

Abstract Work is reviewed on the synthesis of supported metal and metal oxide catalysts using impregnation of an aqueous solution of chelated metal ions followed by drying. The nature of the aqueous solutions of chelated complexes is discussed first. Upon solvent evaporation a steep increase in viscosity is apparent, which inhibits redistribution of impregnated solution upon drying of the support bodies. Furthermore, a gel-like phase is formed that favors high dispersions of the active phase after full drying. Second, several examples are dealt with in some detail, in particular supported iron, nickel, and cobalt–molybdenum catalysts. Finally an overview is presented for metal and metal oxide precursors that can be suitably deposited upon support materials using chelated aqueous metal complex solutions.

The formation of filamentous carbon on iron and nickel catalysts : II. Mechanism

The mechanism of filamentous carbon growth on iron and nickel catalysts has been studied using a combination of magnetic techniques and temperature-programmed hydrogenation. CO and CH4 were used as carburizing agents. It is concluded that high carbide contents are a prerequisite for the nucleation of filamentous carbon. The presence of a substoichiometric nickel carbide during steady-state growth is established. In the case of iron the hexagonal a-Fe2C or .a’-Fe& is proposed to be the intermediate in filamentous growth. The driving force for carbon transport can be appreciated considering the presence of a gradient in the carbon content of nonstoichiometric carbides.

The formation of filamentous carbon on iron and nickel catalysts. I: Thermodynamics

The thermodynamic properties of filamentous carbon formed from CO and CH4 on iron and nickel catalysts have been determined in the temperature range 650–1000 K. Catalyst samples quenched from a steady-state growth of carbon have been studied using temperature-programmed hydrogenation, thermomagnetic analysis, and electron microscopic techniques. It is concluded that carbon is deposited as a metastable carbide intermediate, leading to filamentous carbon on decomposition. The presence of carbide intermediates has been demonstrated both by magnetic measurements and by temperature-programmed hydrogenation.

The formation of filamentous carbon on iron and nickel catalysts: III. Morphology

The microstructure of primary carbon filaments formed on supported iron and nickel catalysts has been investigated using transmission electron microscopy, dark-field imaging and (selected-area) electron diffraction. It has been established that the filaments consist of cone-shaped graphite layers, stacked with their c-axes in a direction normal to the metal-carbon interface. A growth mechanism is proposed involving the excretion of cone-shaped graphite layers. To explain the constancy of the filament diameter, slippage of these layers over one another is invoked. Edge dislocations are brought about by differences in the rate of carbon excretion.

Monoliths in catalytic oxidation

Abstract Catalytic combustion is useful to avoid emission of nitrogen oxides, to combust fuel gas of different calorific levels, and to combust low contents of badly smelling or hazardous gaseous compounds. After dealing with some characteristics of catalytic combustion it is argued that catalytic combustion to a final temperature lower than about 800°C calls for a rapid transport of thermal energy out of the reactor. A fluidized bed in which combustion has been successfully performed is dealt with as well as a reactor filled with metal bodies sintered to each other and to the wall of the reactor. To achieve a sufficiently high catalytically active surface area a thin layer of silicone rubber is applied to the surface of the metal bodies and subsequently pyrolyzed to a highly porous layer of silica. To raise the thermostability alumina can be added to the silica layer. To establish a final temperature above 900°C the homogeneous gas-phase combustion can be ignited by a solid catalyst or the reaction can be performed completely catalytically. Since the combustion reaction proceeds very rapidly at elevated temperatures, a large gas flow can be utilized, which calls for a reactor exhibiting a low-pressure drop. For catalytic combustion monoliths and gauzes are appropriate. The chemical composition of ceramic and metallic monoliths is dealt with as well as the cell densities and wall thicknesses of commercial monoliths. The application of active components to the surface of the walls of monoliths is subsequently discussed. Since monoliths do not allow radial mixing, a homogeneous gas mixture has to be fed to the monolith to prevent very high temperature levels moving randomly over the channels of the monolith and deactivating the catalyst. With monoliths in gas turbines often catalytic ignition is used. To limit the temperature a fraction of the fuel feed is injected into the homogeneous combustion chamber. A number of alternatives of transporting the fresh fuel to the homogeneous combustion zone is mentioned. The cause of the catalyst temperature being higher than that of the gas flow is dealt with as well as the low volatility at elevated temperatures required for the catalytic components. Selection of the catalytically active materials is discussed and the procedure to bring the gas flow at the light-off temperature of the catalyst. Monolithic combustors used in radiant heaters display often an oscillatory behavior. After dealing with the cause of the oscillations, prevention by means of a flame arrestor is mentioned.

Impact of agglomeration state of nano- and submicron sized gold particles on pulmonary inflammation

BackgroundNanoparticle (NP) toxicity testing comes with many challenges. Characterization of the test substance is of crucial importance and in the case of NPs, agglomeration/aggregation state in physiological media needs to be considered. In this study, we have addressed the effect of agglomerated versus single particle suspensions of nano- and submicron sized gold on the inflammatory response in the lung. Rats were exposed to a single dose of 1.6 mg/kg body weight (bw) of spherical gold particles with geometric diameters of 50 nm or 250 nm diluted either by ultrapure water or by adding phosphate buffered saline (PBS). A single dose of 1.6 mg/kg bw DQ12 quartz was used as a positive control for pulmonary inflammation. Extensive characterization of the particle suspensions has been performed by determining the zetapotential, pH, gold concentration and particle size distribution. Primary particle size and particle purity has been verified using transmission electron microscopy (TEM) techniques. Pulmonary inflammation (total cell number, differential cell count and pro-inflammatory cytokines), cell damage (total protein and albumin) and cytotoxicity (alkaline phosphatase and lactate dehydrogenase) were determined in bronchoalveolar lavage fluid (BALF) and acute systemic effects in blood (total cell number, differential cell counts, fibrinogen and C-reactive protein) 3 and 24 hours post exposure. Uptake of gold particles in alveolar macrophages has been determined by TEM.ResultsParticles diluted in ultrapure water are well dispersed, while agglomerates are formed when diluting in PBS. The particle size of the 50 nm particles was confirmed, while the 250 nm particles appear to be 200 nm using tracking analysis and 210 nm using TEM. No major differences in pulmonary and systemic toxicity markers were observed after instillation of agglomerated versus single gold particles of different sizes. Both agglomerated as well as single nanoparticles were taken up by macrophages.ConclusionPrimary particle size, gold concentration and particle purity are important features to check, since these characteristics may deviate from the manufacturers description. Suspensions of well dispersed 50 nm and 250 nm particles as well as their agglomerates produced very mild pulmonary inflammation at the same mass based dose. We conclude that single 50 nm gold particles do not pose a greater acute hazard than their agglomerates or slightly larger gold particles when using pulmonary inflammation as a marker for toxicity.

Remarkable spreading behavior of molybdena on silica catalysts: an in situ EXAFS-Raman study

In contrast to the frequently reported lack of interaction between hexavalent molybdenum and SiO2 and the tendency of silica-supported MoO3 to coalescence, it has been found that on dehydration small molybdenum oxide clusters spread over a silica support. A combined Raman spectroscopy-X-ray absorption study shows a significantly altered structure of the molybdenum oxide phase after dehydration. In EXAFS the total Mo-Mo coordination number drops from 3.27 to 0.20 after anin situ thermal treatment at 673 K. The increase of the peak in the XANES region (Is -→ 4d) indicates that the coordination sphere of the molybdenum atoms strongly alters after dehydration. The Raman spectra reflect the change of the structure through a shift of the position of the terminal Mo=O bond from 944 to 986 cm−1 and the disappearance of the bridged Mo-O-Mo vibration at 880 cm−1. It is concluded that dehydration produces almost isolated molybdenum sites in this highly dispersed sample. Water ligands stabilize the oligomeric clusters under ambient conditions; the removal of water causes spreading of these clusters.

Hydrolysis-precipitation studies of aluminum (III) solutions. I. Titration of acidified aluminum nitrate solutions

Abstract Acidified aluminum nitrate solutions were titrated with alkali (NaOH or KOH) over a temperature range of 24°C to 90°C. A homogeneous distribution of added base was achieved by: (i) in situ decomposition of urea (90°C); and (ii) a novel method involving injection through a capillary submerged in the agitated salt solution. The experimental pH curves are characterized by two plateaus (or platforms) separated by a steep jump in pH. These characteristic features were not previously observed in continuous titration experiments. This is especially true of the second platform, which begins at an OH Al ratio larger than 2.5. Stable colloidal solutions or gels are formed on traversing this plateau. The hydrolysis-precipitation behavior was also followed by light scattering and x-ray diffraction measurements. A qualitative picture of the neutralization process is developed.

Preparation and properties of iron oxide and metallic iron catalysts.

Abstract After an introduction dealing with the applications of iron oxide and metallic iron catalysts, the preparation of iron (oxide) catalysts is discussed. The aqueous chemistry of iron(III) and iron (II) is surveyed as well as two reactions where the extent of hydration of the iron oxide is important, viz. the removal of hydrogen sulfide and the oxidation of carbon monoxide. The activity of iron oxide in the carbon monoxide shift conversion is subsequently considered. It is argued that the reduction of (supported) iron oxide is governed by the transport of water vapor out of the pores of the system being reduced. Especially with iron oxide supported on highly porous carriers the transport of water out of the system and thus the reduction is difficult. Interaction with the support causes the reduction to proceed through an intermediate iron(II) oxide phase. Experiments on single crystals have shown that a monolayer of oxygen on the Fe(100) surface is very stable and almost impossible to remove by reduction with hydrogen. Oxygen present on the closely packed Fe(110) surface, on the other hand, is easily removed by reaction with hydrogen. The implications for the determination of iron surface areas by hydrogen adsorption and the ammonia synthesis are discussed.