Principles and applications of CVD powder technology

Chemical vapor deposition (CVD) is an important technique for surface modification of powders through either grafting or deposition of films and coatings. The efficiency of this complex process primarily depends on appropriate contact between the reactive gas phase and the solid particles to be treated. Based on this requirement, the first part of this review focuses on the ways to ensure such contact and particularly on the formation of fluidized beds. Combination of constraints due to both fluidization and chemical vapor deposition leads to the definition of different types of reactors as an alternative to classical fluidized beds, such as spouted beds, circulating beds operating in turbulent and fast-transport regimes or vibro-fluidized beds. They operate under thermal but also plasma activation of the reactive gas and their design mainly depends on the type of powders to be treated. Modeling of both reactors and operating conditions is a valuable tool for understanding and optimizing these complex processes and materials. In the second part of the review, the state of the art on materials produced by fluidized bed chemical vapor deposition is presented. Beyond pioneering applications in the nuclear power industry, application domains, such as heterogeneous catalysis, microelectronics, photovoltaics and protection against wear, oxidation and heat are potentially concerned by processes involving chemical vapor deposition on powders. Moreover, simple and reduced cost FBCVD processes where the material to coat is immersed in the FB, allow the production of coatings for metals with different wear, oxidation and corrosion resistance. Finally, large-scale production of advanced nanomaterials is a promising area for the future extension and development of this technique

Large scale production of multi-walled carbon nanotubes by fluidized bed catalytic chemical vapor deposition : a parametric study

A parametric study investigating the impact of temperature, run duration, total pressure, and composition of the gaseous phase on the catalytic growth of multi-walled carbon nanotubes (MWNT) has been performed. MWNT have been produced very selectively on the multi gram scale by catalytic chemical vapor deposition from ethylene in a fluidized bed reactor. The kinetics of MWNT growth is fast and, with the catalyst used, no induction period has been observed. The kinetic law is positive order in ethylene concentration and the process is limited by internal diffusion in the porosity of the catalyst. The formation of MWNT in the macroporosity of the catalyst induces an explosion of the catalyst grains. Such a process, thanks to the absence of temperature gradient and to the efficient mixing of the grains allows a uniform and selective treatment of the catalyst powder leading to very high selectivity towards MWNT formation. High purity MWNT have been obtained after catalyst dissolution. Depending on the temperature of production, the specific surface area of this material ranged between 95 and 455 m2/g

An original growth mode of MWCNTs on alumina supported iron catalysts

Multi-walled carbon nanotubes (MWCNTs) have been produced from ethylene by Fluidized Bed Catalytic Chemical Vapor Deposition (FB-CCVD) on alumina supported iron catalyst powders. Both catalysts and MWCNTs-catalyst composites have been characterized by XRD, SEM-EDX, TEM, Mössbauer Spectroscopy, TGA and nitrogen adsorption measurements at different stages of the process. The fresh catalyst is composed of amorphous iron (III) oxide nanoparticles located inside the porosity of the support and of a micrometric crystalline &-iron (III) oxide surface film. The beginning of the CVD process provokes a brutal reconstruction and simultaneous carburization of the surface film that allows MWCNT nucleation and growth. These MWCNTs grow aligned between the support and the surface catalytic film, leading to a uniform consumption and uprising of the film. When the catalytic film has been consumed, the catalytic particles located inside the alumina porosity are slowly reduced and activated leading to a secondary MWCNT growth regime, which produces a generalized grain explosion and entangled MWCNT growth. Based on experimental observations and characterizations, this original two-stage growth mode is discussed and a general growth mechanism is proposed

Carbon supported platinum catalysts for catalytic wet air oxidation of refractory carboxylic acids

Carbon supported platinum (1% wt) catalysts were prepared by the incipient wetness impregnation method and by organometallic chemical vapor deposition. Catalyst characterization was carried out by means of adsorption and thermogravimetric techniques, and by electron microscopy. The catalyst with higher metal dispersion was produced by incipient wetness impregnation. The catalysts were tested in the catalytic wet air oxidation (200 C and 6.9 bar of oxygen partial pressure) of aqueous solutions containing low molecular weight (C2 to C4) carboxylic acids. Significant conversions (greater than 60% over 2 h) and 100% selectivity towards water and non-carboxylic acid products were observed for both systems. The initial reaction rate was used to compare the performance of the two catalytic materials and direct correspondence to the metal dispersion was found. No metal leaching was observed during reaction and no significant deactivation occurred in three successive catalytic oxidation runs. A kinetic model based on the Langmuir–Hinshelwood formulation was applied and the results were analyzed in terms of a heterogeneously catalyzed free radical mechanism

Liquid-phase hydrogenation of unsaturated aldehydes: enhancing selectivity of multiwalled carbon nanotube- supported catalysts by thermal activation

Platinum and iridium organometallic precursors are used to prepare nanosized, thermally stable multiwalled carbon nanotube- supported catalysts. The materials are characterized by N2 adsorption at 77 K, temperature-programmed desorption coupled with mass spectrometry, H2 chemisorption, transmission electron microscopy and thermogravimetric analysis; they are tested in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol under mild conditions (363 K and 1 MPa). A thermal activation at 973 K is found to have a very positive effect over both activity and selectivity, leading to selectivities of approximately 70%, at 50% conversion, regardless of the active metal phase (Pt or Ir). Since no noticeable differences in the metal particle sizes are detected, the results are interpreted in light of an enhanced metal/support interaction. This effect, induced by the removal of oxygenated surface groups, is thought to change the adsorption mechanism of the cinnamaldehyde molecule

Deposição fotoquímica de platina sobre nanotubos de carbono: preparação de catalisadores para a hidrogenação selectiva de aldeídos insaturados

Um catalisador de Pt suportado sobre nantotubos de carbono (1% p/p de metal), preparado pelo método da deposição fotoquímica revelou-se extremamente eficiente para na reacção de hidrogenação selectiva do cinamaldeído ao correspondente álcool insaturado. A importância da temperatura de redução da fase metálica do catalisador foi estudada em detalhe. A importância da natureza do suporte foi investigada utilizando catalizadores preparados pelo mesmo método, mas suportados em TiO2. A eficiência dos catalisadores foi avaliada por comparação dos desempenho com um catalisador de Pt suportado em nanotubos de carbono, mas preparado pelo método tradicional da impregnação incipiente. Com base nos resultados obtidos procurou-se explicar a quimio-selectividade observada (40% a 50% de conversão) na hidrogenação da ligação carbonilo

Liquid-phase hydrogenation of unsaturated aldehydes: enhancing selectivity of MWCNT catalysts by thermal activation

The integration of nanotechnology into bioassays is having a great impact with the development of new nanostructures, nanodevices, nanomaterials or, in general, nanoparticles (NPs), such as nanoshells, nanowires, nanotubes and nanobarcodes, of a variety of shapes, sizes and composition [1-4]. These NPs, which exhibit new properties that are not shown by the bulk matter, are being considered as an alternative to conventional reagents, such as enzymes or organic molecules, often used in bioassays. The main reasons of this success can be ascribed to their ability to improve the features of these assays, allowing their miniaturization and expeditiousness, reducing reagent and sample consumption, and facilitating the performance of heterogeneous formats. NPs present a larger surface area for the display of receptors than flat surfaces and the reactions are faster and more sensitive

Photochemical deposition of Platinum over MWNT: making catalysts for selective hydrogenation of cinnamaldehyde

In the present study, the photochemical deposition method was compared against the usual incipient wetness procedure in the preparation of supported Pt catalysts. Particular attention was given to parameters such as nature of the support (multiwalled carbon nanotubes, MWNT vs. titanium dioxide, TiO2) and reduction temperature. Supported 1% wt Pt catalysts treated in hydrogen at 773K showed improved activity and selectivity towards cinnamyl alcohol (50.8 and 57.4% were observed in MWNT and TiO2 - SMSI effect, respectively) when comparing to those untreated (45.8 and 28.9, MWNT and TiO2, respectively). The best catalytic performance was achieved by a 5% wt Pt supported in TiO2 catalyst treated in hydrogen at 773K. Selectivity to unsaturated alcohol as high as 63.2% at 80% conversion was observed

Carbon supported iridium catalysts in the catalytic wet air oxidation of carboxylic acids: kinetics and mechanistic interpretation

Carbon-supported iridium catalysts were prepared by different incipient wetness impregnation methods and by organometallic chemical vapor deposition. The catalysts were characterized by N2 adsorption, TPD, SEM and H2 chemisorption measurements. The results obtained indicate a clear dependency of the metal-phase dispersion on the pre-treatment of the carbon support and the impregnation method. Their activity for catalytic wet air oxidation of butyric and iso-butyric acid aqueous solutions was investigated in a stirred reactor at 473K and 0.69MPa of oxygen partial pressure. The conversions obtained after 2 h were 43 and 52%,with respect to each carboxylic acid, when the most active catalysts were used. The measured conversions and initial reaction rates correlate well with the exposed metal area. A rate equation was determined from measurements of the initial reaction rates at different oxygen partial pressures, temperatures and catalyst mass loads. The results were modeled considering a heterogeneously catalyzed free-radical mechanism