Thermodynamic Analysis of Carbon Nucleation on a Metal Surface
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Thermodynamic Analysis of Carbon Nucleation on a Metal Surface Vladimir L. Kuznetsov, Anna N. Usoltseva, Andrew L. Chuvilin, Elena D. Obraztsova1, JeanMarc Bonard2 Boreskov Institute of Catalysis, Lavrentieva 5, 630090 Novosibirsk, Russia. 1 Institute of General Physics, 38 Vavilov Street, 117942 Moscow, Russia. 2 Institut de Physique Expérimentale, Ecole Polytechnique Fédérale de Lausanne, CH1015 Lausanne, Switzerland.
ABSTRACT
A thermodynamic analysis of the carbon nucleation on the metal surface was performed. From the consideration of the catalytic mechanisms of carbon deposit formation on the metal surface we concluded that majority of these mechanisms include some common steps. The most important of them is the step of nucleation of carbon deposit on the metal surface. The master equation for the dependence of critical radius of the nucleus on reaction parameters was obtained. This equation demonstrates that a variation of the reaction parameters, such as the temperature, the nature of metal catalyst and the degree of supersaturation of the metal-carbon solution, can lead to the formation of different carbon deposits, such as filamentous carbon, multi-wall nanotubes (MWNT) or single-wall nanotubes (SWNT). The analysis performed allows us to conclude that for selective production of SWNT the nucleation must proceed at high temperature on the surface of liquid metal particles (Fe, Co, Ni). For solid metal particles (Mo) a high degree of supersaturation with carbon is also required.
INTRODUCTION
The processes based on carbon deposition on a metal surface attract great attention because they provide the production of a variety of carbon materials, namely carbon nanotubes, filaments, metal particles encapsulated in graphitic shells, etc. [1,2]. The type of carbon deposit formed depends on the reaction conditions (temperature, pressure of reaction gases), the catalyst composition, the nature of the carbon source (hydrocarbons, CO, carbon plasma etc.). An understanding of the formation mechanisms of the carbon deposit is very important for the development of controlled production of pure nanocarbon materials. Thermodynamics control of the carbon deposit growth can dominate in the conditions where diffusion limitations are negligible. Here we perform a thermodynamic analysis of the carbon nucleation on metal surfaces to estimate the influence of reaction parameters on the nucleation step, which is a crucial stage for the formation of different carbon deposits.
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THEORY
The majority of the formation mechanisms of carbon deposits include the following principal stages: (1) formation of a metal particle saturated with carbon, (2) diffusion of carbon atoms to the metal surface, (3) nucleation of carbon species on the metal surface and (4) growth of carbon deposits (see references in [1, 2]). Carbon nucleation on a metal surface is the most important step for any type of formation of catalytic carbon deposits, namely, carbon fibers, filaments, multi- and single-wall nanotubes. It determines the critical size
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