|Puguh Setyopratomo||Universitas Indonesia|
|Praswasti P.D.K. Wulan||Universitas Indonesia|
|Mahmud Sudibandriyo||Universitas Indonesia|
Carbon nanotubes (CNT) were synthesized from liquefied petroleum gas by a chemical vapor deposition method using a Fe-Co-Mo/MgO supported catalyst. Metal loading was varied from 2.5 to 20 wt%. The catalyst with metal loading of 10 wt% produced the highest CNT yield, at 4.55 g CNT/g catalyst. This high CNT yield was attributed to the high pore volume of the catalyst. The diameter of the CNT was quite variable: the outer diameter ranged from about 4 to 12 nm, while the inner diameter ranged from about 2 to 5 nm. The catalyst with 10 wt% metal loading produced CNT with the highest surface area and the largest total pore volume. XRD analysis detected the existence of highly oriented pyrolytic graphite, C(002), at 2 theta ? 26o, which was attributed to the CNT.
Carbon nanotubes; Chemical vapor deposition; Liquefied petroleum gas; Metal loading; Supported catalyst
Various methods are available for carbon nanotube (CNT) synthesis, but chemical vapor deposition (CVD) is viewed as having the most potential for use in large scale production. This is because CVD is easily controlled and less expensive than other CNT synthesis methods, and it can be operated at atmospheric pressure and a lower temperature (i.e., 500–1000°C) (Tapasztó et al., 2005).
In CNT production by CVD, the performance of the catalysts is often more efficient if mixtures of transition metals are used, rather than a single metal alone. The reaction temperature also can be lowered for mixtures of two or more metals (Dupuis, 2005). Ago et al. (2006) reported that catalyst activity increased in accordance with the metal used, in the order of Fe > Co > Ni. Although Fe is more active than Co, Co is superior to Fe for producing CNT with respect to the degree of graphitization and the CNT structure (Fonseca et al., 1996; Hernadi et al., 2000).
Molybdenum (Mo) is usually added to the Fe or Co catalyst to increase its activity. Mo does not play a role as an active catalyst; instead, it serves as a promoter or activator to enhance the catalyst performance. Mo also acts as an inhibitor to prevent rapid deactivation of the catalyst (Dupuis, 2005; Ago et al., 2006). Mo also improves dispersion and prevents the sintering of Fe nanoparticles (Zhang et al., 2011). An advantage of using MgO as a catalyst support is that it increases the CNT yield and it can be separated easily from the CNT product (Tsoufis et al., 2007) by a simple acid treatment, thereby facilitating the purification of CNT (Ago et al., 2006).
A significant factor for controlling catalyst performance is the level of metal loading (Tsoufis et al., 2007). The size of metal nanoparticles dispersed on a catalyst support is affected by the level of metal loading, as a lower metal loading results in a smaller size of the metal nanoparticles. By contrast, increased metal loading may result in sintering of the metal particles (Zhang et al., 2011). The metal loading also substantially affects the extent of dispersion of the metal nanoparticles on the support (Wei et al., 2008).
The main disadvantage of using a single metal as an active catalyst component is the low performance, as described above. The aim of the present study was to examine the use of a tri-metallic supported catalyst (Fe-Co-Mo/MgO) for CNT synthesis by the CVD method, focusing on the effect of metal loading on the catalyst performance. The main observed parameter of the catalyst performance was CNT yield, which was determined by the mass of CNT produced per unit mass of catalyst. The quality of the CNT produced was also analyzed.
We observed a significant influence of the catalyst composition and characteristics on the yield and properties of the produced CNT. The experimental results showed that the high CNT yield was attributed to the high pore volume of the catalyst. This confirmed that the pore volume of the catalyst plays an important role in the growth of CNT. Mesopores dominated the pore distribution of the CNT product. A high yield of CNT with high surface area and pore volume was produced with a 10 wt% metal loading. The Fe-Co-Mo/MgO catalyst successfully facilitated the formation and growth of multi-walled CNT with ordered structures and a high degree of graphitization.
The authors wish to thank The Directorate Research and Community Services, Universitas Indonesia, for providing the financial support through The Research Cluster Grant 2015- contract number: 1875/UN2.R12/HKP.05.00/2015.
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