Elsevier

Carbon

Volume 48, Issue 7, June 2010, Pages 1932-1938
Carbon

The effect of powder sintering on the palladium-catalyzed formation of carbon nanofibers from ethylene–oxygen mixtures

https://doi.org/10.1016/j.carbon.2010.01.060Get rights and content

Abstract

Carbon nanofiber growth on palladium particles from ethylene–oxygen mixtures was investigated with respect to thermal history. Electron microscopy, combined with focused ion beam cross-sectioning show particles sinter quickly, but can be stabilized by the addition of a short carbon deposition step at a temperature below the general reaction temperature. This step generates a thin layer of carbon on the catalyst which reduces sintering once the temperature is raised to the optimal reaction temperature. For example, high temperature (e.g. 500 °C) catalyst pre-treatment leads to catalyst particle sintering, and subsequent fiber growth produces large diameter fibers. In contrast, small diameter fibers form on catalyst particles pretreated at low temperature (ca. 350 °C), even if the fibers are grown at a temperature at which deposition rates are faster (e.g. 550 °C). These results led to the development of unique multiple temperature fiber growth protocols that produce smaller diameter fibers while improving the deposition rate.

Introduction

Carbon nanofibers, because of their low weight and high strength [1], are desirable in applications such as composite materials [2], [3], [4], but they can also be beneficial in electronic components, catalyst support, and gas storage [1], [5], [6]. The properties are very size dependent, and control of the fiber morphology will determine the applicability to various functions. Consequently, the production process is vital in the commercialization of carbon nanofibers.

Many methods for producing carbon nanofibers have been demonstrated. By far, the most common method is catalytic chemical vapor deposition (CCVD) at, or near, atmospheric pressure. Many procedures use metal particles, usually nanoscale, for the catalyst. Formation is commonly achieved by either depositing a thin layer of metal (e.g. by sputtering) and then annealing it to form discrete, supported particles [7], or by precipitation of a metal precursor (e.g. nickel nitrate) or coprecipitation of multiple metals, again followed by an annealing process to encourage particle growth and separation [8], [9], [10]. Recently, ethylene–oxygen mixtures were found to generate carbon nanofibers over a wide range of temperatures when using commercially-purchased palladium powder as the catalyst [11], but fiber yields from bulk powders can be difficult to predict due to sintering effects.

The present work is intended to develop additional means to optimize carbon nanofiber growth. Protocols for improving deposition rate and sintering resistance are desirable since catalyst particle size correlates with carbon fiber size [5], [6], [12], [13], and powder sintering is a significant obstacle in maintaining consistent particle size. Although keeping temperatures low will reduce sintering, it is important to grow fibers at relatively high temperatures since growth rates are strongly temperature dependent. Therefore, a balance between deposition rate and sintering extent must be found for optimization. Ideally, an optimized protocol should maintain the growth rate of a higher temperature while minimizing sintering leading to increased fiber size disparity.

Even though soak times at high temperature are typically short for catalytic carbon deposition (<2 h), solid-state sintering can be significant since diffusion between particles leading to neck formation progresses more quickly for small neck radii, occurring early in the sintering process [14], making it an important consideration even for short reactions. Although this can be avoided by using a supported catalyst where crystallite or atomic migration (Ostwald ripening) are the dominant means of sintering [15], the simplicity of a powder catalyst is highly desirable, especially if more control of the carbon characteristics can be gained. This end can be achieved by maintaining a low temperature until a stabilizing carbon layer is established, and then raising the temperature to an optimal growth rate regime.

The catalysts investigated were palladium sub-micron powder (<1 μm) and palladium nanopowder (<25 nm). Carbon deposition was from fuel rich ethylene-oxygen mixtures in all cases. Experiments were designed to allow correlation of catalyst size with pre-treatment, carbon fiber diameter with initial catalyst particle size, and the influence of multi-temperature protocols on fiber size and growth rates. All observations, including scanning (SEM) and transmission (TEM) electron microscopy studies, and deposition rate studies are easily explained by a simple postulate: fiber characteristics are controlled by catalyst size at the onset of filament growth, and growth initiated at a low temperature acts to stabilize the particles, even if deposition temperature is increased.

Section snippets

Experimental

Two Pd powders were used in this study: sub-micron powder (<1 μm, 99.9%) and nanopowder (<25 nm, 99.9%). Both sizes of Pd powder were purchased from Sigma Aldrich and used without modification.

Powder sintering was investigated at three temperatures: 300 °C, 550 °C, and 900 °C. These three temperatures represent the lower, common, and upper temperatures used in catalyzing carbon deposition from palladium catalysts. The powders were allowed to soak for 2 h at the specified temperatures. Two gaseous

Results

In order to understand the impact of different protocols on the nature of fiber morphology and growth rate, both the free sintering of palladium particles, and the nature of fiber structure and growth rates as a function of treatment protocol were studied.

Discussion

Without any carbon deposition, sintering was found to occur even when the temperature was maintained well below the Tamman temperature for Pd (∼780 °C). Liu et al., report coalescence at temperatures as low as 500 °C as a consequence of high atom mobility within small, alumina supported palladium clusters if chance contact occurs [16]. Sintering by surface-induced melting [17] and surface diffusion processes, dominant for fine powders [18], are the likely mechanism for the powders since they are

Conclusions

The primary lesson of this work on fiber growth from palladium powder using ethylene–oxygen mixtures is that most catalyst particle sintering occurs prior to carbon deposition. Another major observation is that sintering is far more strongly affected by the maximum temperature than by soak time under these conditions. These findings led to a protocol for the rapid growth of small diameter fibers. First, carbon deposition is conducted at the lowest possible temperature (e.g. 350 °C) for a short

Acknowledgments

The authors gratefully acknowledge the support of the New Mexico Space Grant Consortium. This work was completed in part at the University of New Mexico Manufacturing Training and Technology Center.

Cited by (14)

  • Multiscale design of nanofibrous carbon aerogels: Synthesis, properties and comparisons with other low-density carbon materials

    2017, Carbon
    Citation Excerpt :

    For instance, in previous work on Pd powder, foil and sputtered films [45,46], sputtered films and sub-micron and nanoscale powders readily produced fibers, but foils resulted in microns thick carbon films being deposited. Additionally, concerns with sintering and strategies for controlling fiber diameter with bulk powder, as used here, have been described [47]. In this work powder was distributed very finely, which reduces the likelihood of sintering, and the fiber characteristics were found to be similar throughout the monoliths.

  • Direct synthesis and characterization of a nonwoven structure comprised of carbon nanofibers

    2013, Carbon
    Citation Excerpt :

    In the present work, the emphasis was on controlling the bulk density. This was done by modifying the catalyst charge, but it is clear from earlier work that changes in fiber diameter [24], basic fiber morphology [9] and other properties [25,26] can be controlled as well. The direct synthesis procedure also provides a vehicle for utilizing the constituent carbon nanofibers (CNFs).

  • Accelerated growth of carbon nanofibers using physical mixtures and alloys of Pd and Co in an ethylene-hydrogen environment

    2011, Carbon
    Citation Excerpt :

    True alloys of Pd and Co produced for this study did show growth rates much higher than that of either metal separately, but the synergism was less than that observed for physical mixtures and the product fibers were clearly different in structure. It has been previously found that particle sintering can reduce the fiber growth rate and increase average fiber diameter [11], and this may be a factor in the poorer performance of the alloy. However, a larger average fiber diameter was not found to be associated with the alloy than that of the physical mixture.

  • Carbon nanotubes and helical carbon nanofibers grown by chemical vapour deposition on C<inf>60</inf> fullerene supported Pd nanoparticles

    2011, Carbon
    Citation Excerpt :

    In this report we use fullerene supported nano Pd particles to synthesize carbon nanostructures. Nano sized Pd particles have been used for both the growth of YCNT [23], HCNTs [24], and HCNFs [22,25]. It has also been used as co catalyst together with Co to lower the growth temperature of MWCNT [26].

View all citing articles on Scopus
View full text