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Silica Precursor Effect on the Physical and Chemical Properties of Cobalt Incorportated MCM-41 Catalysts and Their Performance Towards Single Wall Carbon Nanotubes

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Silica Precursor Effect on the Physical and Chemical Properties of Cobalt Incorportated MCM-41 Catalysts and Their Performance Towards Single Wall Carbon Nanotubes Journal of C Carbon Research Article Silica Precursor Effect on the Physical and Chemical Properties of Cobalt Incorportated MCM-41 Catalysts and Their Performance towards Single Wall Carbon Nanotubes Frank Ramírez Rodríguez 1,* ID , Luis Fernando Giraldo 2 and Betty Lucy Lopez 1 1 Grupo de Investigación Ciencia de los Materiales, Universidad de Antioquia, Calle 70 N◦ 52-21, Medellín 050010, Colombia; [email protected] 2 Laboratorio de Investigación en Polímeros, Universidad de Antioquia, Calle 70 N◦ 52-21, Medellín 050010, Colombia; [email protected] * Correspondence: [email protected]; Tel.: +574-219-6546 Received: 22 December 2017; Accepted: 2 February 2018; Published: 22 February 2018 Abstract: In this work, mesoporous silica (MCM-41) and a cobalt-incorporated catalyst (Co-MCM-41) were prepared using colloidal silica Cab-O-Sil, sodium silicate and tetraethylorthosilicate (TEOS) as the silica sources and cobalt nitrate as the cobalt source. Their physicochemical properties were analyzed, and their catalytic performance for the synthesis of single-wall carbon nanotubes (SWCNT) during chemical vapor deposition (CCVD) of methane was evaluated. When Cab-O-Sil was used, it was possible to incorporate 3% (nominal) cobalt with a good dispersion and without losing mesoporosity, resulting in minimal formation of superficial cobalt oxide. In contrast, the other catalysts product superficial cobalt oxide, according to the temperature programmed reduction (TPR) analysis. Co-MCM-41 prepared using Cab-O-Sil showed the best performance during the formation of SWCNT with a good regularity and selectivity without forming multi-wall carbon nanotubes or amorphous carbon structures. Keywords: heterogeneous catalysis; cobalt incorporated MCM-41; chemical vapor deposition; single wall carbon nanotubes 1. Introduction Mesoporous silica, containing pores less than 50 nm [1], has been used as catalytic support for a variety of processes, such as methane reforming [2], organic chemistry transformations [3], and single-wall carbon nanotube preparation [4]. These kinds of transformations are possible given the strong chemical interactions between transition metal cations and oxygen within the silica framework that enables dispersing and stabilizing the cations in the support. The incorporation of transition metal cations can be achieved during an in-situ process in which the cation salts and silica source are mixed together in the presence of a soft-template [5]. This process leads to the isomorphic replacement of silicon by the transition metal cations in the siloxane network while maintaining the regularity of the silica structure. Specifically, cobalt, nickel, and iron [6–8] have been incorporated in mesoporous silica (MCM-41) mesoporous silica and used as catalysts for the production of single- and multi-wall carbon nanotubes [9–13]. MCM-41 supports have the ability to disperse metallic species inside their pore walls [8,10,14,15] and drive the formation of small metallic clusters confined inside their cylindrical pores [16] when the incorporated cobalt species (Cox+) is partially reduced [17]. MCM-41 has a narrow pore-size distribution [18,19] that is within the mesoporous range, and its large surface area enables good dispersion of the incorporated metal cations. Cobalt-incorporated MCM-41 catalysts can be prepared using either colloidal silica Cab-O-Sil [20], sodium silicate [6] or C 2018, 4, 16; doi:10.3390/c4010016 www.mdpi.com/journal/carbon C 2018, 4, x FOR PEER REVIEW 2 of 15 C 2018, 4, 16 2 of 15 41 catalysts can be prepared using either colloidal silica Cab-O-Sil [20], sodium silicate [6] or tetraethylorthosilicate (TEOS) (TEOS) [21] [21 ]as as the the silica silica source. source. As As far far as we as weknow, know, a comparison a comparison between between Co- Co-MCM-41-incorporatedMCM-41-incorporated catalysts catalysts prepared prepared using using diff differenterent silica silica sources sources has has not not been been performed; therefore, in this study, MCM-41-based mesoporous catalysts we werere synthesized using sodium sodium silicate, silicate, colloidal silica Cab-O-Sil, and TEOS as silica sources, and they were doped with cobalt by means of in-situ incorporation.incorporation. Then,Then, eacheach catalyst catalyst was was tested tested during during the the chemical chemical vapor vapor deposition deposition (CCVD) (CCVD) of methaneof methane to produceto produce single-wall single-wall carbon carbon nanotubes nanotube (SWCNT),s (SWCNT), and and the the support support role role of theof the catalysts catalysts on theon the selectivity selectivity and and yield yield of theof the SWCNT SWCNT synthesis synthesis was was determined. determined. 2. Results and Discussion 2.1. Silica Characterization Figure1 1 shows shows thethe adsorptionadsorption isothermsisotherms ofof thethe silicasilica synthesizedsynthesized startingstarting atat pHpH 10.010.0 andand 11.511.5 with the different silicasilica sources;sources; all nitrogennitrogen adsorptionadsorption profilesprofiles correspondedcorresponded to type–IV isotherms according to the International Union of Pure and Ap Appliedplied Chemistry (IUPAC) (IUPAC) assignation [1], [1], that is, the materials had a narrow mesoporousmesoporous porepore size,size, exceptexcept forfor thethe S11.5S11.5 sample.sample. Figure 1. (A)N2 adsorption isotherms of the silica supports; (B) Barrett-Joyner-Halenda (BJH) pore size distributions. Figure 1. (A) N2 adsorption isotherms of the silica supports; (B) Barrett-Joyner-Halenda (BJH) pore size distributions. The S11.5 did not show a capillary condensation step (P/P0 0.25–0.35); this suggests that the structureThe S11.5 of this did silica not was show not a mesoporous, capillary condensation this behavior step may (P/P be0 related 0.25–0.35); to a lowthis polycondensationsuggests that the degreestructure conducting of this silica to anwas amorphous not mesoporous, material this after behavior organic may materials be related removal to a low [22]. polycondensation However, for the synthesisdegree conducting at pH 10.0, to the an mesostructureamorphous material was highly after ordered, organic asmaterials confirmed removal from the [22]. results However, summarized for the insynthesis Table1, inat whichpH 10.0, the slopethe meso of thestructure capillary was condensation highly ordered, step ofas S10.0 confirmed is the highest; from the the results slope issummarized directly related in Table to the1, in homogeneity which the slope of the of porousthe capi structure.llary condensation In contrast, step the of S10.0 mesoporous is the highest; silicas preparedthe slope withis directly TEOS related exhibited to the a narrow homogeneity pore size of distribution the porous structure. and large slope,In contrast, with athe larger mesoporous slope for thesilicas synthesis prepared at pHwith 11.5. TEOS However, exhibited the a surfacenarrow areaspore ofsize C11.5 distribution and S10.5 and had large the largestslope, with surface a larger area, whichslope for is an the important synthesis factor at pH in 11.5. designing However, mesoporous the surface materials areas withof C11.5 high and metal S10.5 dispersion had the and largest thus thesurface needed area, activity which towards is an important SWCNT [ 14factor]. in designing mesoporous materials with high metal dispersion and thus the needed activity towards SWCNT [14]. Table 1. Nitrogen physisorption results. FWHM: full width at maximum height. Table 1. Nitrogen physisorption2 −1 results. FWHM: full width at maximum height. Silica SA (m g ) Dp (nm) DP FWMH (nm) C11.5Silica SA 1239.6 (m2g−1)Dp (nm) D 2.3P FWMH (nm) 0.48 S10.0C11.5 1239.6 1080.2 2.3 2.50.48 0.25 T10.0S10.0 1080.2 902.3 2.5 2.60.25 0.16 T11.5 899.6 2.5 0.16 T10.0 902.3 2.6 0.16 T11.5 899.6 2.5 0.16 Figure2 shows the X-ray diffraction (XRD) patterns of mesoporous silica MCM-41. We can observe the characteristic reflections, (100), (110), and (200), and the higher in-plane reflection (210). C 2018, 4, x FOR PEER REVIEW 3 of 15 C 2018, 4, 16 3 of 15 Figure 2 shows the X-ray diffraction (XRD) patterns of mesoporous silica MCM-41. We can observe the characteristic reflections, (100), (110), and (200), and the higher in-plane reflection (210). ThisThis last last reflection reflection is only is only present present in the in highly the high orderedly ordered structures, structures, such as thesuch T10.0 as the and T10.0 T11.5 samples.and T11.5 Thesamples. intensity The and intensity the full widthand the at maximumfull width height at ma (FWMH)ximum height are parameters (FWMH) thatare areparameters directly relatedthat are withdirectly the structural related with regularity. the structur The higheral regularity. the intensity The ishigher and the the lower intensity the FWMH is and the is, the lower more the ordered FWMH andis, homogeneousthe more ordered are and the homogeneous silica pore. According are the silica to the pore. nitrogen According adsorption to the nitrogen and X-ray adsorption diffraction and patterns,X-ray diffraction the prepared patterns, silicas T10.0the prepared and T11.5 silicas showed T10.0 a higher and T11.5 structural showed order, a whilehigher S10 structural was a poorly order, organizedwhile S10 mesoporous was a poorly silica organized and S11.5 mesoporous showed a non-mesoporoussilica and S11.5 showed structure. a non-mesoporous structure. FigureFigure 2. 2.X-ray X-ray diffraction diffraction (XRD) (XRD) patterns patterns of of the the silica silica supports. supports. TableTable2 shows 2 shows the the cell cell parameters parameters and and wall wall thicknesses thicknesses calculated calculated for for each each MCM-41 MCM-41 sample. sample. AlthoughAlthough the the pore pore diameter diameter distributions distributions were were similar similar for for all all the the mesoporous mesoporous supports, supports, there there were were significantsignificant differences differences in in the the wall wall thicknesses thicknesses where where C11.5 C11.5 has has the the larger larger wall wall thickness.
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