Abstract
Thermodynamics must be favorable for the growth of carbon nanotubes (CNTs) and graphene to take place, but a kinetic study is required to find the operating mechanism. In fact, thermodynamics indicates whether a reaction is possible; however, the route prevailing is not necessarily the most thermodynamically favorable, but the fastest one. Detailed kinetic studies state that there are three alternative routes operating under different temperature and pressure rates. The modes and rates of diffusion of carbon (C) atoms and noble metals have been known since the 1930s, but proof of C bulk diffusion operating in CNT growth came from detailed kinetic studies performed in the early 1970s, when reversible versus irreversible C formation was discussed with examples. The reason for interstitial C bulk diffusion in transition metals is evidenced based on the values of covalent radius. The reason for operating under steady-state conditions (linearity of the weight versus time) when searching for the operating mechanism is discussed herein. The steady-state C formation process operates sometimes with two different solid phases at each side of the catalyst particle (e.g., Ni and Ni3C), with thicknesses proportional to 1/D of the respective C bulk diffusivities when the carbon bulk diffusion step is the rate-determining one.
Highlights
It is vital to understand the role of thermodynamics versus kinetics in a given reaction.Thermodynamics is important for understanding the ranges in which a given reaction can occur, and it must be favorable
The mechanisms of carbon nanotubes (CNTs) and graphene layer formation are better understood when solid-state reaction kinetics is studied in detail (Table 1), and C and H bulk diffusion in transition and noble metals are considered [2,3]
We intended to correct the approach frequently used: that the operating mechanism in carbon formation can be found-based in thermodynamic information
Summary
It is vital to understand the role of thermodynamics versus kinetics in a given reaction. Detailed kinetics is an essential approach for understanding the mechanism operating in given conditions. The mechanisms of CNT and graphene layer formation are better understood when solid-state reaction kinetics is studied in detail (Table 1), and C and H bulk diffusion in transition and noble metals are considered [2,3]. Thermodynamics may may be indicate which reaction conditions are favorable for carbonfor formation. Kinetics kinetics is the appropriate approach for mapping active reaction conditions carbon formation. Is the appropriate approach forthe mapping the active reaction prevailing, in order to understand the operating mechanism and to maximize production. When a mechanism well2–H understood, detailed kinetic studies of carbon formation versus gasification [7]. Only kinetics can reveal the active reaction conditions prevailing and to understand operating mechanism.
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