Jean-Mario Nhut

A few-layer graphene–graphene oxide composite containing nanodiamonds as metal-free catalysts

We report a high yield exfoliation of few-layer-graphene (FLG) with up to 17% yield from expanded graphite, under 5 h sonication time in water, using graphene oxide (GO) as a surfactant. The aqueous dispersion of GO attached FLG (FLG–GO), with less than 5 layers, is used as a template for further decoration of nanodiamonds (NDs). The hybrid materials were self-organized into 3D-laminated nanostructures, where spherical NDs with a diameter of 4–8 nm are homogeneously distributed on the surface of the FLG–GO complex (referred to as FLG–GO@NDs). It was found that GO plays a dual role, it (1) mediated exfoliation of expanded graphite in aqueous solution resulting in a FLG–GO colloid system, and (2) incorporated ND particles for the formation of composites. A high catalytic performance in the dehydrogenation of ethyl-benzene on FLG–GO@ND metal-free catalyst is achieved; 35.1% of ethylbenzene conversion and 98.6% styrene selectivity after a 50 h reaction test are observed which correspond to an activity of 896 mmolST gcatalyst−1 h−1, which is 1.7 and 5 times higher than those of the unsupported NDs and traditional catalysts, respectively. The results demonstrate the potential of the FLG–GO@ND composite as a promising catalyst for steam-free industrial dehydrogenation applications.

Silicon carbide foam as a porous support platform for catalytic applications

This review provides an overview of the use of foam-structured SiC as a porous support platform in some typical catalytic processes both for gas-phase and liquid-phase reactions, such as H2S selective oxidation, Friedel–Crafts benzoylation and Fischer–Tropsch synthesis, where traditional catalysts have shown their weaknesses. The macroscopic thermally conductive SiC material could be efficiently employed as a support for controlling the active phase, i.e. metal and zeolite, and anchoring the powder-foam nanocarbons in the field of catalysis. In light of the results, one can state that silicon carbide foam could be regarded as an ideal alternative support, which provides a great enhancement of both the catalytic performance and the catalytic stability compared to that of the traditional catalysts, in several gas- and liquid-phase catalytic processes.

Nitrogen-doped carbon nanotubes on silicon carbide as a metal-free catalyst

Abstract A hierarchical metal-free catalyst consisting of nitrogen-doped carbon nanotubes decorated onto a silicon carbide (N-CNTs/SiC) macroscopic host structure was prepared. The influence of N-CNTs incorporation on the physical properties of the support was evaluated using different characterization techniques. The catalyst was tested as a metal-free catalyst in the selective oxidation of H 2 S and steam-free dehydrogenation of ethylbenzene. The N-CNTs/SiC catalyst exhibited extremely good desulfurization performance compared to a Fe 2 O 3 /SiC catalyst under less conducive reaction conditions such as low temperature, high space velocity, and a low O 2 -to-H 2 S molar ratio. For the dehydrogenation of ethylbenzene, a higher dehydrogenation activity was obtained with the N-CNTs/SiC catalyst compared to a commercial K-Fe/Al 2 O 3 catalyst. The N-CNTs/SiC catalyst also displayed good stability as a function of time on stream for both reactions, which was attributed to the strong anchoring of the nitrogen dopant in the carbon matrix. The extrudate shape of the SiC support allowed the direct macroscopic shaping of the catalyst for use in a conventional fixed-bed reactor without the problems of catalyst handling, transportation, and pressure drop across the catalyst bed that are encountered with nanoscopic carbon-based catalysts.

One‐Pot Synthesis of a Nitrogen‐Doped Carbon Composite by Electrospinning as a Metal‐Free Catalyst for Oxidation of H2S to Sulfur

A macroscopic composite consisting of nitrogen‐doped carbon fibers (N@CFs) was synthesized by electrospinning. The as‐prepared N@CF material was further applied as a metal‐free catalyst in the catalytic oxidation of H2S to sulfur, which is one of the most important purification processes for raw chemical resources (that is, biogas, natural gas, and petrochemical compounds). The catalyst, after a carbonization step at T=800u2009°C, exhibits a high and stable desulfurization activity for more than 100u2005h of testing with 57u2009% H2S conversion and 95u2009% sulfur selectivity at T=230u2009°C, which is two times higher than that of the most active metal‐based catalyst (Fe2O3/SiC). The desulfurization performance could also be improved by changing the reactant velocity. Moreover, the macroscopic shaping with an inner hierarchical structure network allows the avoidance of problems linked with the transport and handling of nanoscopic carbon‐based materials and also enhances the mass diffusion during the oxidation reaction.

New catalysts based on silicon carbide support for improvements in the sulfur recovery: new silicon carbide nanotubes as catalyst support for the trickle-bed H2S oxidation

Silicon carbide nanotubes were prepared via a gas-solid reaction between SiO vapor and carbon nanotubes. The NiS2 active phase on this support displayed both a high catalytic activity and high solid sulfur storage capacity in the trickle-bed selective oxidation of H2S into elemental sulfur as compared to the grain-based SiC catalyst. The hypothesis of a confinement effect inside the SiC nanotubes has been put forward to explain the catalytic results. An artificial increase in the H2S partial pressure inside the tubes when compared to the H2S partial pressure outside the tubes would lead to an increase in the oxidation rate, due to the first order reaction rate toward H2S. The SiC nanotube supported catalyst displayed very high resistance to the sulfur loading, due to a peculiar mode of sulfur evacuation by condensed steam which allows the continuous cleaning of the active site. The high solid sulfur storage capacity was due to a much larger void volume between each SiC nanotube available for the sulfur storage, than the void volume of SiC support with a grain size morphology.

New catalysts based on silicon carbide support for improvements in the sulfur recovery. Silicon carbide as support for the selective H2S oxidation

As fases ativas NiS2 e Fe2O3 suportadas em b-carbeto de silicio (SiC) com media area especifica mostraram alta atividade, seletividade e estabilidade na oxidacao direta do H2S a enxofre elementar. Os catalisadores foram testados a temperaturas que variaram da temperatura ambiente, no caso do Ni em reator de leito gotejante, ate temperaturas superiores a do ponto de orvalho do enxofre, no caso do Fe em reator de leito fixo. Para ambos os casos, foi proposta a formacao de uma fase bastante ativa de oxisulfeto de Ni ou de Fe, formada pela oxidacao do NiS2 e pela sulfuracao do Fe2O3. A ausencia de microporosidade no suporte contribuiu a alta seletividade do catalisador. A grande estabilidade ao carregamento de enxofre solido, apresentada pelos catalisadores suportados em SiC em temperaturas inferiores a 100 oC, foi explicada pela maneira especial da deposicao do enxofre, a qual depende do papel da agua presente na reacao e do carater heterogeneo (hidrofilico e hidrofobico) da superficie do suporte.

Nitrogen-Doped Carbon Composites as Metal-Free Catalysts

This chapter provides an overview of recent developments of functionalized and doped carbon-based materials as potential metal-free catalysts in several fields of application. The panel of applications of those carbon nanomaterials is wide, such as medicine, advanced materials, and electronics, because of their unique physical and chemical properties. A particular attention is paid in this overview to their utilization in catalysis since they offer an alternative with much higher efficiency than conventional transition metal or oxide-based catalysts due to their architecture, which improves heat and mass transfer phenomena. Their high chemical inertness allows them to withstand operations in aggressive media. Surface functionalization or matrix doping with N and O species can finely tune the surface reactivity. These carbon-based metal-free catalysts also display an exceptionally high resistance toward deactivation due to the strong localization of the active sites within the catalyst matrix. Future challenges are likely related to the selection of appropriate synthesis route to get maximum concentration of N-containing species through the use of nontoxic and abundant raw materials. Today the development of more friendly environmental synthesis routes represents an outstanding outcome for further upscaling. The peculiar catalytic properties of those materials have been illustrated through relevant examples such as hydrogen oxidation, desulfurization, and liquid-phase transesterification reactions. The integration of those materials in the reactor design through controlled macroscopic shaping is also an important issue.