Mechanism of Catalytic CNTs Growth in 400–650 °C Range: Explaining Volcano Shape Arrhenius Plot and Catalytic Synergism Using both Pt (or Pd) and Ni, Co or Fe

Abstract

The Arrhenius plot of catalytic carbon formation from olefins on Ni, Co, and Fe has a volcano shape in the range 400–550 °C with reaction orders 0 (at lower T: Below ~500 °C) and one (at higher T: Above ~500 °C) at each side of the maximum rate. The reaction follows a catalytic route with surface decomposition of the gas (olefin) on the catalyst nanoparticle, followed by the bulk diffusion of carbon atoms and carbon nanotube growth on the opposite side. At the higher temperature region (500–550 °C), the initial surface reaction step controls the rate and the reaction order is one, both in olefins and hydrogen (H). This confirms that H is essential for the surface reaction to occur. This is very valuable information to get faster CNT growth rate at relatively low temperatures. The apparent activation energy observed must correspond with the surface reaction Ea corrected for the temperature dependence of the two molecules involved (olefin and H). Adding a noble metal (Pt, Pd) to the carbon formation catalyst is frequently found to increase the reaction rate further. This effect has been described as an H spillover since 1964. However, there is evidence that the bulk diffusion of H atoms prevails and does not “spillover” the surface diffusion. Diffusion of H atoms through the solids involved is easy, and the H atoms remain single (“independent”) until emerging on a surface.

Kinetics of Carbon Nanotubes and Graphene Growth on Iron and Steel: Evidencing the Mechanisms of Carbon Formation

“…The Arrhenius plots shown in Figure 1 evidence the change of rate determining step at about 700 °C. Explanation of that change has been recently reminded [ 51 ]. Every point in the Arrhenius plot is a register of a steady-state deposition rate experiment (kinetic linearity observed) [ 5 ], that is, a full steady state C formation experiment, lasting 2 or 3 h. The volcano shape maximum with Fe and steel occurs at higher temperatures than with Ni and Co.…”

“…The two sides of the volcano correspond to the same mechanism (Route I, Catalytic), but with a change of the rate-determining step from C bulk diffusion to surface reaction decomposition of the gas reactant (C 2 H 2 and low olefins, only). The other C formation gases only operate at higher temperatures by impingement of pyrolytic formed carbon (C 2 ,C 3 …), C atoms entering the bulk of the catalyst and growing catalytically on the other side of the nano-particle [ 51 ]. In the studies of Puretzky et al, the reason a volcano shape was observed in the plot of the variation of the growth rates as a function of temperature was attributed to acetylene flow rates [ 57 ].…”

Carbon Formation at High Temperatures (550–1400 °C): Kinetics, Alternative Mechanisms and Growth Modes

“…In a recent review, three kinetic routes for chemical vapor deposition (CVD) were distinguished: catalytic, hybrid and pyrolytic [24]. The volcano shape Arrhenius plot, found at lower temperatures (400-650 • C), has been clarified, based on a survey of kinetic studies [25].…”

“…Those results were always inserted in an Arrhenius plot scale to evidence the activation energy. The Arrhenius plots are 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 also in a recent short review [24]. A simplified scheme of the alternative routes was proposed by Figueiredo and co-workers [27] and used by Ashik et al [21].…”

Explaining Bamboo-Like Carbon Fiber Growth Mechanism: Catalyst Shape Adjustments above Tammann Temperature

“…"Optimizing industrial production of carbon nanotubes is an important objective nowadays. Knowing the mechanisms operating helps to control properties, optimize production and reduce costs more easily" [76].…”