Imre Kiricsi

Large scale production of short functionalized carbon nanotubes

A simple mechano-chemical modification of multiwall carbon nanotubes is described. The use of ball-milling in specific atmosphere allows us to introduce functional groups like thiol, amine, amide, carbonyl, chlorine, etc. onto carbon nanotubes. The resulted functional groups are characterized using infrared spectroscopy and X-ray photoelectron spectroscopy.

Production of nanotubes by the catalytic decomposition of different carbon-containing compounds

Carbon nanotubes were prepared in the catalytic decomposition of different carbon containing compounds over supported transition metal catalysts. Besides acetylene, ethylene, propylene, acetone, n-pentane, methanol, toluene, and methane were tested and each resulted in carbon nanotube formation. The quality of as-made nanotubes was investigated by TEM and was found to be at least as good as obtained in acetylene decomposition. Ethylene and propylene showed somewhat lower reactivity in the buckytube formation with respect to acetylene, simultaneously suppressed formation of amorphous carbon on the outer surface was found.

Reactivity of different kinds of carbon during oxidative purification of catalytically prepared carbon nanotubes

In the purification procedure of catalytically prepared carbon nanotube samples, the oxidative removal of amorphous carbon was investigated. Oxidation was carried out by KMnO4, H2O2, O3 or HClO4. The selectivity of the amorphous carbon burning reaction was studied as a function of the reaction temperature by transmission electron microscopy. The advantages of the different methods are as follows: although oxidation by KMnO4 in an acidic suspension provides nanotubes free of amorphous carbon, and can easily be controlled, it must be followed by filtration and cc HCl treatment in order to dissolve the MnO2 formed during the reaction. Oxidation with hydrogen peroxide does not give undissolved residue on the surface of the tubes. Ozone treatment produces gaseous by-products, but requires higher reaction temperature. This method is more difficult to control. During oxidation, competitive oxidation always takes place: carbon nanotubes and amorphous carbon react simultaneously. While amorphous carbon can be attacked from any direction, carbon nanotubes can be oxidized only from the ends.

Zeolite supported mono- and bimetallic systems : structure and performance as CO hydrogenation catalysts

Preparation, characterization and the CO hydrogenation over mono- and bimetallic catalysts supported on zeolite have been highlighted. The metal/zeolite system was found to contain highly dispersed metal particles which influence the mode of CO chemisorption, its hydrogenation leading to oxygenates and hydrocarbons. Various factors, like metal support interaction, effect of the second metal, etc. are considered in explaining the CO hydrogenation.

Catalytic synthesis of carbon nanotubes over Co, Fe and Ni containing conventional and sol–gel silica–aluminas

An attempt has been made to synthesise multiwalled carbon nanotubes using cobalt, iron and nickel supported on different types of silica–aluminas to investigate the rules governing their nanotube producing activity. Acetylene was used as the source of carbon. Decomposition of acetylene has been carried out at atmospheric pressure. The effect of reaction temperature in the 770–970 K range and the flow rate of the hydrocarbon has been investigated. The catalysts were analysed by XRD, UV–VIS, surface area and porosity measurements. Formation of carbon nanotubes was followed by electron microscopy. The amount of deposited carbon increased with increasing reaction temperature and the flow rate of acetylene, but decreased with increasing concentration of alumina in the catalyst support. Each catalyst showed high production of carbon nanotubes at 970 K; however, they were inactive at 770 K. The yield of tube formation was very low at 870 K. The high-resolution transmission electron microscopic (HREM) analysis showed that the outer diameter of the tubes generated varied from 8 to 40 nm, the tubes were multiwalled, and the number of the layers was between 8 and 30. Sol–gel derived samples were also found to be working catalysts, indicating the existence of an optimal metal particle size.

Three-Dimensional Carbon Nanotube Scaffolds as Particulate Filters and Catalyst Support Membranes

Three-dimensional carbon nanotube scaffolds created using micromachined Si/SiO2 templates are used as nanoparticulate filters and support membranes for gas-phase heterogeneous catalysis. The filtering efficiency of better than 99% is shown for the scaffolds in filtering submicrometer particles from air. In the hydrogenation of propene to propane reaction low activation energy of E(a) approximately 27.8 +/- 0.6 kJ x mol(-1), a considerably high turnover rate of approximately 1.1 molecules x Pd site(-1) x s(-1) and durable activity for the reaction are observed with Pd decorated membranes. It is demonstrated that appropriate engineering of macroscopic-ordered nanotube architectures can lead to multifunctional applications.

Metal Substitution in Keggin-Type Tridecameric Aluminum-Oxo-Hydroxy Clusters

The species resulting from a typical preparation for metal-substituted hybrids of the Keggin tridecamer, Al13 or [AlO4Al12(OH)24(OH2)12]7+, were examined by performing 27Al NMR on the solutions during aging and by studying the precipitated sulfate salts via solid state 27Al NMR and powder X-ray diffraction (XRD). Aqueous mixtures (0.25 mol L-1) of AlCl3 and another metal ion (M), in a 12:1 mole ratio (Al:M), where M = Fe3+, Zn2+, Ga3+, In3+, Sn2+, La3+, and Bi3+, were subjected to forced hydrolysis by addition of NaOH (1.0 mol L-1) until OH/(Al + M) = 2.25, and the kinetics of Al13 formation and disappearance with aging at 80 °C was monitored by 27Al NMR spectroscopy. Al13 units polymerize on aging with an apparent rate constant (k) of 4.8(8) × 10-2 h-1 to form a species referred to as AlP2. Only the solutions containing Ga3+ and Sn2+ exhibited faster Al13 conversion rates. GaAl12 forms quickly at 80 °C (k = 0.54 h-1) and is more stable than AlP2. Sn2+ apparently promotes AlP2 formation (k = 0.38 h-1). XRD and solid state NMR reveal that only the Ga hybrid can be prepared by this method. No hybrid formation was evidenced using M = Mg2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, In3+, La3+, or Ce3+ at 25 °C or M = Co2+ or La3+ under reflux conditions. Isostructural (cubic symmetry) single crystals were obtained for the sulfate salts of Al13 and GaAl12. Single-crystal XRD analysis of these two polyoxocations provides the first rigorous comparison between them and shows they have very similar structures. The main crystallographic data for Al13 and GaAl12 are as follows:  Na[AlO4Al12(OH)24(H2O)12](SO4)4·10H2O, cubic, F4̄3m, a = 17.856(2) Å, Z = 4; Na[GaO4Al12(OH)24(H2O)12](SO4)4·10H2O, cubic, F4̄3m, a = 17.869(3) Å, Z = 4. Thus, the greater thermal stability of GaAl12 cannot be rationalized in terms of the overall geometric considerations, as suggested by others. Solid state NMR also shows the coordination symmetries of the outer 12 Al nuclei in both clusters to be similar.

ON THE ROLE OF CATALYST, CATALYST SUPPORT AND THEIR INTERACTION IN SYNTHESIS OF CARBON NANOTUBES BY CCVD

The catalytic growth of carbon nanotubes was investigated from the point of view of reaction mechanism. A great variety of catalyst supports (silica gel, zeotype materials, alumina, etc.) with different pore diameter was tested in acetylene decomposition at 1000 K. Quality and existence of carbon deposit was followed by transmission electron microscopy and the state of catalyst was investigated by in situ X-ray photoelectron spectroscopy measurements. It was proved that only catalyst particles deposited on the external surfaces of porous support could take part in the catalytic carbon nanotube formation.

Novel two-step synthesis of controlled size and shape platinum nanoparticles encapsulated in mesoporous silica

Uniform shape and size platinum nanoparticles encapsulated in mesoporous silica (SBA-15) were prepared in the same solution by a novel two-step method. Platinum nanoparticles were prepared in aqueous solution of K2PtCl4, the reduction was carried out by bubbling hydrogen, the capping material was tri-block poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymer. The mesoporous silica was synthesized using the same copolymer as template from tetraethyl orthosilicate by hydrolysis in acidic conditions. The “Pt-nanoparticles-in-mesoporous-silica” system was characterized by a combination of low-angle powder X-ray diffraction, transmission electron microscopy and N2 porosimetry. The platinum nanoparticles are encapsulated in the mesopores and retained their size and morphology. It appears that this hybrid material should be a superior three-dimensional high-surface-area catalyst for selective platinum-catalyzed reactions.

Layered double hydroxides and their pillared derivatives – materials for solid base catalysis; synthesis and characterization

Abstract Mg–Al and Zn–Al L(ayered) D(ouble) H(ydroxide)s have been prepared and characterized. Thermal and dehydration/rehydration behaviour was studied by thermoanalytical methods (TG, DTG, DTA) and 27 Al M(agic) A(ngle) S(pinning) NMR spectroscopy, X-ray diffractometry (XRD) and S(canning) E(lectron) M(icroscopy). Acid–base properties of the calcined samples were investigated by T(emperature) P(rogrammed) D(esorption) of preadsorbed CO 2 and the double-bond isomerization reaction of 1-butene. Heat treatment destroyed the layered structure, which could be only partially reconstituted by rehydration. On calcination mixed oxide with the predominance of basic sites were formed. Pillaring of the LDHs with organic (terephtalate, dodecyl sulphate) as well as inorganic (oxometallate, ferrocyanide) anions were also performed. The materials were characterized by infrared (IR) spectroscopy, XRD and BET measurements. Heat stabilities of the pillared substances were investigated too. Pillaring by various methods proved to be successful, however, decreased heat resistance was found in the intercalated materials relative to the host LDHs.