Jean-Philippe Tessonnier

Recent Progress on the Growth Mechanism of Carbon Nanotubes: A Review

Tremendous progress has been achieved during the past 20 years on not only improving the yields of carbon nanotubes and move progressively towards their mass production, but also on gaining a profound fundamental understanding of the nucleation and the growth processes. Parameters that influence the yield but also the quality (e.g., microstructure, homogeneity within a batch) are better understood. The influence of the carbon precursor, the reaction conditions, the presence of a catalyst, the chemical and physical status of the latter, and other factors have been extensively studied. The purpose of the present Review is not to list all the experiments reported in the literature, but rather to identify trends and provide a comprehensive summary on the role of selected parameters. The role of the catalyst occupies a central place in this Review as a careful control of the metal particle size, particle dispersion on the support, the metastable phase formed under reaction conditions, its possible reconstruction, and faceting strongly influence the diameter of the carbon nanotubes, their structure (number of walls, graphene sheet orientation, chirality), their alignment, and the yield. The identified trends will be compared with recent observations on the growth of graphene. Recent results on metal-free catalysts will be analyzed from a different perspective.

Selective Deposition of Metal Nanoparticles Inside or Outside Multiwalled Carbon Nanotubes

A general method is described for the deposition of metal nanoparticles selectively either inside or outside of carbon nanotubes (CNTs). The method is based on the difference in the interface energies of organic and aqueous solutions with the CNT surface. Because of their lipophilic character, the organic solvent better wets the surface of the nanotubes compared to water and penetrates into the inner volume. The precise control of the volume of each phase allows filling the CNT with the organic phase and covering its outer surface with the aqueous one. Hence, metal nanoparticles can be put with high selectivity either inside or outside the CNT, just by choosing in which solvent the metal precursor is dissolved. SEM, TEM, and 3D-TEM investigations show that a selectivity in localization close to 75% can be reached by this technique. The nanoparticles are homogeneously dispersed and present a narrow size distribution, centered on 5 nm. In this way, one can decorate either the inner or the outer surface of open CNTs, without the need of discriminating the diameter of the opening and without any further step of functionalization than a treatment with nitric acid.

Synthesis and catalytic uses of carbon and silicon carbide nanostructures

Abstract Carbon nanofibers and nanotubes with controlled diameters were synthesized by catalytic decomposition of an ethane/hydrogen mixture over nickel and iron supported catalysts. The synthesis of the first silicon carbide (SiC) nanotubes was performed according to the shape memory synthesis (SMS) method. The benefit of using carbon and SiC nanotubes as catalyst supports was evidenced, respectively in the case of the selective Cue605C bond hydrogenation in the α,β-unsaturated cinnamaldehyde and the low temperature selective oxidation of H 2 S into elemental sulfur (60xa0°C). Carbon nanotubes as support allowed to increase both cinnamaldehyde conversion and selectivity toward Cue605C bond hydrogenation. Supporting a nickel-based catalyst on SiC nanotubes allowed to increase both desulfurization activity of the catalyst and its solid sulfur storage capacity. The inner partial pressure concept, or confinement effect, was developed to explain the high performances of this new SiC-based catalyst. The last section is devoted to further objectives for developing such highly performing new support materials.

Nanostructured Manganese Oxide Supported on Carbon Nanotubes for Electrocatalytic Water Splitting

Incipient wetness impregnation and a novel deposition symproportionation precipitation were used for the preparation of MnOx/CNT electrocatalysts for efficient water splitting. Nanostructured manganese oxides have been dispersed on commercial carbon nanotubes as a result of both preparation methods. A strong influence of the preparation history on the electrocatalytic performance was observed. The as‐prepared state of a 6.5u2005wt.u2009% MnOx/CNT sample could be comprehensively characterized by comparison to an unsupported MnOx reference sample. Various characterization techniques revealed distinct differences in the oxidation state of the Mn centers in the as‐prepared samples as a result of the two different preparation methods. As expected, the oxidation state is higher and near +4 for the symproportionated MnOx compared to the impregnated sample, where +2 was found. In both cases an easy adjustability of the oxidation state of Mn by post‐treatment of the catalysts was observed as a function of oxygen partial pressure and temperature. Similar adjustments of the oxidation state are also expected to happen under water splitting conditions. In particular, the 5u2005wt.u2009% MnO/CNT sample obtained by conventional impregnation was identified as a promising catalytic anode material for water electrolysis at neutral pH showing high activity and stability. Importantly, this catalytic material is comparable to state‐of‐art MnOx catalyst operating in strongly alkaline solutions and, therefore, offers advantages for hydrogen production from waste and sea water under neutral, hence, environmentally benign conditions.

Dissolved Carbon Controls the Initial Stages of Nanocarbon Growth

Carbon is a versatile material that, depending on its hybrid-ization and assembly in one-, two-, or three-dimensionalnetworks, exhibits important electronic and chemical proper-ties with countless practical applications. For example, it isfound in printer inks, pencils, water purification systems,thermal isolation, and antistatic materials.

Mesoporous carbon nanotubes for use as support in catalysis and as nanosized reactors for one-dimensional inorganic material synthesis

Abstract Mesoporous multi-walled carbon nanotubes (MWNTs) with an average inner diameter of about 50xa0nm were successfully used as a one-dimensional catalyst support either in gas-phase, i.e. selective oxidation of H 2 S into elemental sulfur in a thrickle-bed configuration, or in liquid-phase reactions, i.e. selective hydrogenation of nitrobenzene into aniline or Friedel-Crafts benzoylation reactions. On one hand, the combination of the small size, i.e. high external surface area, and of the tube structure, i.e. confinement effect, allowed a significant improvement of the catalytic performance when compared to that obtained on traditional grain shaped catalysts as observed in the selective oxidation of H 2 S and the selective hydrogenation of nitrobenzene reactions. On the other hand, the confinement effect induced by the high aspect ratio of the tubes could, in addition, be effectively used for the synthesis of one-dimensional nanowire zeolitic materials under non-hydrothermal macroscopic conditions. Carbon nanotubes template were removed by combustion leaving behind zeolite nanowires which were made up of zeolite particles of about 10–20xa0nm. The zeolitic catalysts exhibit a high benzoylation activity compared to that of the commercial one due to its high external surface area.

Spinel-Type Cobalt–Manganese-Based Mixed Oxide as Sacrificial Catalyst for the High-Yield Production of Homogeneous Carbon Nanotubes

Potential applications of carbon nanotubes (CNTs) in very different fields such as electronics, medicine, or catalysis have been widely demonstrated on a laboratory scale. The development of CNT-based commercial products now mainly relies on CNTs’ ability to also fulfill the expectations on an industrial scale and on our ability to produce them at a reasonable cost. In the past, several attractive materials, such as fullerenes or nanodiamonds, lost some of their appeal because their synthesis is difficult to scale up for technical and economical reasons. For multiwalled carbon nanotubes (MWCNTs), their integration in commercial products depends on the availability of highpurity nanotubes with high homogeneity in size and in structure at a relatively low price. To date, high-quality MWCNTs are produced by catalytic chemical vapor deposition (CCVD) using transition metal heterogeneous catalysts. Despite many efforts, the yields remain relatively low, at best a few tens of grams MWCNTs per gram of catalyst. Purification using strong acids is therefore necessary to remove the remaining catalyst, which significantly increases the complexity of the production process. Herein, we demonstrate that spinel-type cobalt– manganese-based mixed oxide is a unique catalyst to produce MWCNTs. The yield is typically 2–9 times higher than that obtained with conventional catalysts, so that the purification step is no longer needed. In addition, the MWCNTs are exceptionally homogeneous in diameter, the standard deviation being only 4 nm. The high quality and high yield obtained allow for, to our knowledge, the first time MWCNTs to become a commodity chemical. Co, Mn, Al, and Mg in their nitrate forms were coprecipitated at pH 10 by using sodium hydroxide (see the Supporting Information). The precipitate was then filtered and thoroughly washed with distilled water to eliminate sodium and nitrate ions until the conductivity of the washing solution matched that of the freshly distilled water. The filtrate was subsequently dried in air at 180 8C for 5 h and calcined at 400 8C for 4 h to decompose it into the corresponding spinel-type mixed oxide. Finally, its efficiency in growing MWCNTs was tested by reducing it in diluted hydrogen (50 %) and exposing it to an ethylene/hydrogen feed at 650 8C for 2 h (see the Supporting Information for additional details). The yield was 179 gCNT g 1 catalyst (17 900 wt %), which is 2–9 times higher than the best yields previously obtained under similar conditions (Figure 1). Furthermore, residual catalyst impurities in the synthesized material were scarce and the purity was close to 99.5 % carbon, which is sufficient for most applications. Therefore, the traditional purification step with strong mineral acids is no longer required, thus rendering the overall process safer, greener and cheaper. In addition, the diameter distribution calculated from high-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (HRTEM) images was centered at 14 nm with a standard deviation of 4 nm, which is remarkably narrow. The produced MWCNTs were fully character-

Transesterification of Triglycerides Using Nitrogen-Functionalized Carbon Nanotubes

Nitrogen-functionalized carbon nanotubes were synthesized by grafting amino groups to the surface of the nanotubes. The nanotubes exhibited promising results in the base-catalyzed liquid phase transesterification of glyceryl tributyrate with methanol, which is a model reaction for the production of biodiesel. The concentration of the active sites and the reaction parameters, such as temperature and glyceryl tributyrate to methanol ratio, were shown to significantly affect catalytic performance. The grafting technique employed allowed for design and control of the active sites. As a consequence, it was possible to design a nitrogen-functionalized carbon nanotube catalyst with a few strong, basic groups. This might be of interest for carbohydrate conversion reactions where strong basic sites are required but the pH of the solution should remain mild to avoid the degradation of the reactants and/or products.

New Insights from Microcalorimetry on the FeOx/CNT‐Based Electrocatalysts Active in the Conversion of CO2 to Fuels

Fe oxide nanoparticles show enhanced electrocatalytic performance in the reduction of CO(2) to isopropanol when deposited on an N-functionalized carbon nanotube (CNT) support rather than on a pristine or oxidized CNT support. XRD and high-resolution TEM were used to investigate the nanostructure of the electrocatalysts, and CO(2) adsorptive microcalorimetry was used to study the chemical nature of the interaction of CO(2) with the surface sites. Although the particles always present the same Fe(3)O(4) phase, their structural anisotropy and size inhomogeneity are consequences of the preparation method of the carbon surface. Two types of chemisorption sites have been determined by using microcalorimetry: irreversible sites (280 kJ mol(-1)) at the uncoordinated sites of the facets and reversible sites (120 kJ mol(-1)) at the hydrated oxide surface of the small nanoparticles. N-Functionalization of the carbon support is advantageous, as it causes the formation of small nanoparticles, which are highly populated by reversible chemisorbing sites. These characteristic features correlate with a higher electrocatalytic performance.

Labeling and monitoring the distribution of anchoring sites on functionalized CNTs by atomic layer deposition

The chemical inertness of graphite and, in the case of tubes, of rolled up few layer graphene sheets, requires some degree of “defect engineering” for the fabrication of carbon based heterostructured materials. It is shown that atomic layer deposition provides a means to specifically label anchoring sites and can be used to characterize the surface functionality of differently treated carbon nanotubes. Direct observation of deposited titania by analytical transmission electron microscopy reveals the location and density of anchoring sites as well as structure related concentrations of functional groups on the surface of the tubes. Controlled functionalization of the tubes therefore allows us to tailor the distribution of deposited material and, hence, fabricate complex heterostructures.