Support effect on carbon nanotube growth by methane chemical vapor deposition on cobalt catalysts

Abstract

A influência do suporte na produção de nanotubos de carbono por deposição química a vapor (CVD) de metano sobre catalisadores de cobalto foi investigada. As técnicas de fisissorção de N 2 , difratometria de raios X (XRD), redução à temperatura programada (TPR) e quimissorções de H 2 e CO foram usadas na caracterização dos catalisadores de cobalto suportados em diferentes óxidos (Al 2 O 3 , SiO 2 , Nb 2 O 5 e TiO 2 ). As técnicas de espectroscopia Raman, oxidação a temperatura programada (TPO) e microscopia eletrônica de varredura (SEM) foram utilizadas na caracterização e quantificação de espécies de carbono produzidas. No crescimento dos nanotubos, o catalisador produziu três espécies principais de carbono: carbono amorfo, nanotubos de carbono de parede única (SWNT) e nanotubos de carbono de parede múltiplas (MWNT). Técnicas de caracterização mostraram que a seletividade catalítica a cada tipo de nanotubo depende da distribuição do tamanho de partículas de cobalto, a qual depende das propriedades texturais do suporte. O catalisador Co/TiO 2 apresentou alta seletividade para a produção de nanotubos de parede única. Esta alta seletividade se deve a uma distribuição estreita do tamanho de partículas de cobalto sobre TiO 2 .The influence of the support on carbon nanotube production by methane chemical vapor deposition (CVD) on cobalt catalysts was investigated. N 2 physisorption, X-ray diffractometry (XRD), temperature programmed reduction (TPR) and H 2 and CO chemisorption techniques were used to characterize the structure of cobalt catalysts supported on different metal oxides (Al 2 O 3 , SiO 2 , Nb 2 O 5 and TiO 2 ). Raman spectroscopy, temperature programmed oxidation (TPO) and scanning electron microscopy (SEM) were used for the characterization and quantification of produced carbon species. On carbon nanotube growth, the catalyst produced three main carbon species: amorphous carbon, single walled carbon nanotubes (SWNT) and multi walled carbon nanotubes (MWNT). The characterization techniques showed that the catalyst selectivity to each kind of nanotube depended on the cobalt particle size distribution, which was influenced by the textural properties of the support. Co/TiO 2 showed the highest selectivity towards single wall nanotube formation. This high selectivity results from the narrow size distribution of cobalt particles on TiO 2 .

Direct syngas hydrogenation over a Co–Ni bimetallic catalyst: Process parameter optimization

“…According to the characteristic two step reduction process Co 3 O 4 -CoO-Co1, the monometallic Co oxide supported on SiO 2 presented with two reduction peaks at 250 1C and 300 1C with an intensity ratio close to 1:4. The third broad shoulder peak between 450 1C and 650 1C is associated with the reduction of the Co oxides interacted with the support SiO 2 material (Oliveira et al, 2012;vanSteen et al, 1996). The monometallic Ni oxide supported on SiO 2 profile showed the characteristic NiO reduction peak at 300 1C with the one step reduction process NiO-Ni1.…”

XPS and TPR study of sol-gel derived M/TiO2 powders (M=Co, Cu, Mn, Ni)

“…The profile of the Co/ТіО 2 shows weak hydrogen consumption up to 400 °C, and a predominant consumption is observed at region 400-800 °C, revealing two distinct reduction regions. Reduction behavior of Со 3 О 4 is controversial in the literature, some authors observed a single step for the Со 3 О 4 reduction [28,36], while many others reported about a twostep process, involving with CoO in the reduction [24,30,37,38]. The two-step reduction process consideration supposes that the ratio of the lowtemperature peak corresponding to Со 3 О 4 + Н 2 → 3СоО + Н 2 О reaction to the high-temperature peak corresponding to СоО + Н 2 → Со + Н 2 О reaction, should be 1:3 theoretically.…”

“…Since the shift of the cobalt reduction peaks to higher temperatures demonstrates an intensity of the cobalt/support interaction, this shift can be explained due to a strong interaction between cobalt and TiO 2 . It is known that the interaction increases with a higher dispersion of cobalt on the support surface [36], and at low-loading cobalt the titania has a tendency to stabilize CoO and to resist reduction [38]. Therefore, reduction behavior of cobalt oxide species in TiO 2 doped with small cobalt amounts is strongly affected by supports [24].…”

Selective growths of single‐walled carbon nanotubes from mesoporous supports via CO disproportionation

“…A single peak at 282 cm −1 suggests formation of semiconducting (7,5) SWCNTs, confirmed by E 22 transition in photoluminescence excitation (PLE). [15] A prereduction process in hydrogen flows also enhances carbon yields and G/D (disorder/tangential stretching) bands of SWCNTs (Table S3). In Figure 2b, RBM peaks suggested that hydrogen prereduction temperature alters diameter distributions for SWCNTs.…”

“…Quantitative evaluation of the nanotube population is another issue since there is a lack of knowledge of the optical efficiency as a function of (n,m). Since CVD is the most common method for commercial production of carbon nanotubes, the catalyst nanoparticles are embedded on various support, [13] such as MgO [14] and Al 2 O 3 , [15] to increase surface area for enhanced yields of the carbon feedstock with the metal particles. [16,17] Pfefferle and coworkers have selectively grown SWCNTs on Co-Mn catalysts in mesopores with high surface area (480 m 2 /g) at 600-800 C. [18,19] However, qualitative support effects toward chirality selectivity of SWCNTs have not been thoroughly studied.…”