Abstract
This Note aims at clarifying the alternative mechanisms of carbon formation from gases at temperatures above 550 °C. Both the growth of carbon nanotubes (CNTs) by a hybrid route, and of graphene layers deposition by a pyrolytic route are analyzed: the transition had no influence in apparent kinetics, but the carbon structure was totally different. The transition temperature from hybrid to pyrolytic growth varies with the gas pressure: higher temperature transition was possible using lower active gas pressures. The rate-determining step concept is essential to understanding the behavior. In catalytic and hybrid carbon formation, the slower step controls and determines kinetics. In the pyrolytic region, the faster step dominates, and carbon bulk diffusion is blocked: layers of graphene cover the external catalyst surface. It is easier to optimize CNTs growth (rate, shape, properties) knowing the details of the alternative mechanisms operating.
Highlights
Carbon formation from C-containing gases has been studied in some detail by many researchers from 1950–1970
Arrhenius plots are aalways vacuum microbalance, as shownplot in Figure and in recent short reviewThe based in successive insertion of points corresponding to many steady-state rates of carbon formation measurements using a vacuum microbalance, as shown in Figure 3 and in a recent short review
Faster lower gas pressures carbon nanotubes (CNTs) growth can operate at higher temperatures rates
Summary
Carbon formation from C-containing gases has been studied in some detail by many researchers from 1950–1970. Studies of carbon formation from gases provide useful kinetic information [2,3]. Diffusion of gases or atoms in solids was extensively studied in the period 1920–1940 Barrer summarizes this new branch of science in a book that includes theoretical and experimental results discussed in detail [4]. Details of the kinetic behavior and alternative mechanisms operating require clarification. The aim of this Note is to offer a simple perspective of the alternative routes and the reasons for the mechanistic transition. The volcano shape Arrhenius plot, found at lower temperatures (400–650 ◦ C), has been clarified, based on a survey of kinetic studies [25].
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