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 2.5 0.48 0.25 T10.0S10.0 1080.2 902.3 2.5 2.6 0.25 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. thickness. Larger Larger wall wall thicknessthickness may may confer confer a highera higher thermal thermal stability stability of of the the siloxane siloxane network network which which may may support support high high temperaturetemperature processes; processes; the the colloidal colloidal silicasilica Cab-O-SilCab-O-Sil is an aggregate aggregate of of nanometric nanometric particles particles that that are arehydrolyzed hydrolyzed in in the the reaction reaction medium, medium, but but the hydrolysishydrolysis ofof thethe colloidalcolloidal particlesparticles is is not not entirely entirely completecomplete resulting resulting in in minor-sized minor-sized particles particles that that exhibit exhibit slower slower polycondensation, polycondensation, and and may may contribute contribute toto a largera larger wall wall thickness thickness and, and, hence, hence, less less organization. organization. TEOS TEOS is is an an organic organic compound compound with with low low solubilitysolubility in in water, water, to to hydrolyze hydrolyze the the EtO-Si EtO-Si bond bond is is necessary necessary to to have have the the species species in in solution solution thus thus the the polycondensationpolycondensation when when using using TEOS TEOS may may be be the the slowest slowest among among all all silica silica sources sources leading leading to to a porousa porous structurestructure construction construction more more organized organized as the as hydrolyzed the hydrolyzed species species condense condense in a more in organized a more organized fashion duefashion to the due lack to of the high lack amounts of high of amounts silicate species of silicate in solution. species Asin solution. a matter ofAs fact, a matter the XRD of fact, patterns the XRD of patterns of the TEOS silicas show higher peak intensities and (200) and (210) reflections than the other the TEOS silicas show higher peak intensities and (200) and (210) reflections than the other supports. supports. These findings are in agreement with the well-ordered structure of these supports. These findings are in agreement with the well-ordered structure of these supports.

2 = λCu = 2d ;= − = Kα = √100 = − d1002 a0 3 ; Wt a0 Dp 2senθ100 √ 3

Table 2. Pore diameters, Bragg’s parameters, and wall thicknesses of the silica supports. Table 2. Pore diameters, Bragg’s parameters, and wall thicknesses of the silica supports. Silica Dp (nm) FWMH (nm) 2Θ (°) d100 (nm) a0 (nm) Wt (nm) ◦ SilicaC11.5 Dp (nm)2.3 FWMH (nm) 0.48 2Θ 2.22( ) d100 3.97(nm) 4.58 a0 (nm) 2.3 Wt (nm) C11.5S10.0 2.32.51 0.48 0.25 2.22 2.45 3.6 3.97 4.16 4.58 1.7 2.3 S10.0 2.51 0.25 2.45 3.6 4.16 1.7 T10.0 2.58 0.16 2.17 4.06 4.69 2.1 T10.0 2.58 0.16 2.17 4.06 4.69 2.1 T11.5T11.5 2.49 2.49 0.16 0.16 2.15 2.15 4.10 4.10 4.74 4.74 2.2 2.2

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2.2. Catalysts Characterization 2.2. CatalystsFigure Characterization 3A shows the nitrogen adsorption isotherms and Figure 3B shows the pore size distributionFigure3A showsof each the prepared nitrogen catalyst, adsorption All isotherms the catalysts and Figureshow type3B shows IV isotherms, the pore size except distribution for CS11.5, ofwhich each prepared did not show catalyst, an Allappreciable the catalysts capillary show condensation type IV isotherms, step, such except as forsilica CS11.5, S11.5. which did not show an appreciable capillary condensation step, such as silica S11.5.

FigureFigure 3. (3.A )(A Nitrogen) Nitrogen adsorption adsorption isotherms. isotherms. (B )(B BJH) BJH pore pore size size distribution distribution of theof the catalysts. catalysts.

ForFor the the catalyst catalyst prepared prepared with with Cab-O-Sil Cab-O-Sil (CC11.5) (CC11.5) as as the the silica silica source, source, the the nitrogen nitrogen adsorption adsorption profileprofile was was nearly nearly the the same same as as that that of of silica, silica, with with a defineda defined capillary capillary step step and and a narrowa narrow pore pore size size distribution.distribution. Nevertheless, Nevertheless, the the average average pore pore size size was was different; different; the the pore pore size size of of the the catalyst catalyst was was larger larger byby approximately approximately 0.2 0.2 nm, nm, and and the the behavior behavior can can be be explained explained considering considering the the bond bond lengths lengths of of Si-O Si-O andand Co-O: Co-O: the the latter latter is approximatelyis approximately 200 200 pm, pm, while while the the first first is 174is 174 pm, pm, therefore therefore when when one one cobalt cobalt cationcation replaces replaces one one silicon silicon atom atom the the porous porous structure structure and and size size changes changes expanding expanding the the pore pore dimension. dimension. Ultimately,Ultimately, the the catalyst catalyst has has 215 215 m2 gm−21gless−1 less surface surface area area than than the the support, support, and and this this result result is inis in agreement agreement withwith the the inverse inverse relationship relationship between between the the pore pore size size and and the the surface surface area area [23 ].[23]. TheThe catalyst catalyst prepared prepared using using sodium sodium silicate silicate as the silicaas the source silica and source a pH and of 10.0 a pH (CS10.0) of 10.0 exhibited (CS10.0) a lowerexhibited superficial a lower areasuperficial and the area same and pore the sizesame distribution pore size distribution but with less but intensity;with less theseintensity; results these indicateresults that indicate the quantity that the ofquantity the pores of the was pores lower, was and lower, therefore, and therefore, the surface the areasurface was area lower was as lower well. as TEOSwell. catalysts TEOS catalysts (CT10.0) and(CT10.0) (CT11.5) and exhibited (CT11.5) aexhibited well-defined a well-defined capillary condensation capillary condensation step, unlike thestep, Cab-O-Silunlike the catalyst, Cab-O-Sil and thecatalyst, pore sizeand distributionthe pore size was distribution nearly the was same nearly in both the cases. same Tablein both3 shows cases. theTable real3 shows concentration the real ofconcentration cobalt in the of mesoporous cobalt in the supports, mesoporous where supports, catalysts where CT10.0, catalysts CT11.5 CT10.0, and CC11.5 CT11.5 exhibitand CC11.5 a larger exhibit actual cobalt a larger concentration, actual cobalt possibly concentrat dueion, to the possibly slower due polycondensation to the slower polycondensation kinetics. When usingkinetics. TEOS When as the using silica sourceTEOS as at the pH silica 11.5 more source cobalt at pH was 11.5 incorporated more cobalt compared was incorporated to the sample compared at pH to 10.0the since sample at higher at pHpH 10.0 values, since at more higher TEOS pHhydrolyzed values, more species TEOS arehydrolyzed present. Hence,species theare cobaltpresent. cations Hence, werethe already cobalt cations interacting were with already thehydrolyzed interacting TEOSwith th priore hydrolyzed to the polycondensation; TEOS prior to the therefore, polycondensation; once the polycondensationtherefore, once stepthe polycondensation began, the cobaltwas step incorporated began, the morecobalt efficiently. was incorporated Tetramethylammonium more efficiently. silicateTetramethylammonium and sodium silicate providedsilicate and the hydrolyzedsodium silicate silicic provided acid, and the the condensationhydrolyzed silicic of the acid, in-solution and the silicatecondensation anions that of the interacted in-solution with silicate the cobalt anions cations that interacted permitted with larger the incorporation. cobalt cations permitted Nevertheless, larger sodiumincorporation. silicate at Nevertheless, pH 10.0 did not sodium show silicate the same at behaviorpH 10.0 did as Cab-O-Sil.not show the Since same the behavior difference as betweenCab-O-Sil. bothSince processes the difference is the soluble between sililica both sourceprocesses it is is possible the soluble that sililica sodium source cations it is compete possible with that cobalt sodium cationscations to compete form SiO-Na with cobalt bonds cations instead to form of SiO-Co, SiO-Na leading bonds instead to a lower of SiO-Co, cobalt leading incorporation to a lower in cobalt the siloxaneincorporation network. in the siloxane network. FigureFigure4 shows 4 shows the XRDthe XRD patterns patterns of all of catalysts. all catalysts. For each For catalysteach catalyst only theonly diffraction the diffraction peaks ofpeaks the of planesthe planes (100), (110), (100), and (110), (200) and are (200) visible are except visible the exce CS11.5pt the catalyst CS11.5 which catalyst does which not show does the not diffraction show the peaksdiffraction characteristics peaks characteristics of MCM-41 materials of MCM-41 due to materials its amorphous due to nature. its amorphous The peak nature. that corresponds The peak tothat thecorresponds plane (210) reflectionto the plane was not(210) observed, reflection which was corroboratesnot observed, the which fact that corroborates the incorporation the fact of cobaltthat the generatedincorporation distortions of cobalt in the generated mesoporous distorti silicaons framework. in the mesoporous silica framework. C 2018, 4, 16 5 of 15 C 2018, 4, x FOR PEER REVIEW 5 of 15

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FigureFigure 4. XRD 4. XRD patterns patterns of of the the catalysts. catalysts.

Table 3 shows the surface area and pore size distributions of both the silica and catalysts. Table3 shows the surface area and pore size distributions of both the silica and catalysts. Table 3. Surface areaFigure and 4. pore XRD size patterns distributi of theons catalysts. of both the silica and catalysts. Table 3. Surface area and pore size distributions of both the silica and catalysts. TableCatalyst 3 shows Catalyst the surface SA (m area2g− 1and) ∆ poreSA (m size2g− distributions1) Silica Dp of(nm) both Catalystthe silica Dandp (nm) catalysts. Co (%) # CC11.5 1024.42 −1 215.22 −1 2.29 2.51 2.02 # Catalyst Catalyst SA (m g ) ∆SA (m g ) Silica Dp (nm) Catalyst Dp (nm) Co (%) CS10.0 Table 3. Surface957.1 area and pore size 123.1 distributi ons of 2.49 both the silica and 2.46catalysts. 1.66 CC11.5CS11.5 1024.4NA 215.2 NA 2.29 NA NA 2.51 1.90 2.02 Catalyst Catalyst SA (m2g−1) ∆SA (m2g−1) Silica Dp (nm) Catalyst Dp (nm) Co (%) # CS10.0CT10.0 957.1810.9 123.1 91.4 2.49 2.55 2.41 2.46 1.99 1.66 CS11.5CC11.5 NA1024.4 215.2 NA 2.29 NA 2.51 NA 2.02 1.90 CT11.5 990.2 -90.6 2.45 2.39 2.18 CT10.0CS10.0 810.9957.1 123.191.4 2.49 2.55 2.46 2.41 1.66 1.99 # Final concentration of cobalt determined from atomic absorption data. CT11.5CS11.5 990.2NA -90.6 NA NA 2.45 NA 2.39 1.90 2.18 CT10.0 # 810.9 91.4 2.55 2.41 1.99 Figure 5 showsFinal the concentration diffuse reflectance of cobalt determined UV-Vis (D fromRS–UV–Vis) atomic absorption spectra data.of the catalysts prepared CT11.5 990.2 -90.6 2.45 2.39 2.18 using different silica sources. The spectra of all catalysts exhibited profiles that were consistent with # Final concentration of cobalt determined from atomic absorption data. Figureabsorption5 shows due the to diffusethe 4A2→ reflectance4T1(P) electronic UV-Vis transition (DRS–UV–Vis) of the d7 cobalt spectra valence of the electrons catalysts when prepared they using differentoccupied silicathe tetrahedral sources. Theholes spectra in the silica of all wall, catalysts and the exhibited tripartition profiles of the thatsignal were (525 consistent nm, 585 nm, with Figure 5 shows the diffuse reflectance UV-Vis (DRS–UV–Vis) spectra of the catalysts prepared and 624 nm) was4 due→ 4to spin-orbit coupling. However, Torbjørn7 Vrålstad stated that ultraviolet absorptionusing different due to thesilicaA sources.2 T1 (P)The electronicspectra of all transition catalysts ofexhibited the d cobaltprofiles valence that were electrons consistent when with they spectroscopy is not sufficient for determining the state of cobalt in siloxane networks [24]. Hence, our occupiedabsorption the tetrahedral due to the holes4A2→4T in1(P) the electronic silica wall, transition and the of the tripartition d7 cobalt valence of the signalelectrons (525 when nm, they 585 nm, results provide an estimation to corroborate the presence of tetrahedral cobalt but not the absence of and 624occupied nm) wasthe tetrahedral due to spin-orbit holes in the coupling. silica wall, However, and the tripartition Torbjørn of Vrålstad the signal stated (525 nm, that 585 ultraviolet nm, octahedral cobalt. Cobalt with an octahedral symmetry occurs in CoO and a combination of distorted spectroscopyand 624 nm) is not was sufficient due to forspin-orbit determining coupling. the However, state of cobalt Torbjørn in siloxaneVrålstad networksstated that [24 ultraviolet]. Hence, our tetrahedral and octahedral environment in Co3O4; therefore, under this condition and given that all resultsspectroscopy provide an is estimation not sufficient to for corroborate determining the the presence state of cobalt of tetrahedral in siloxane cobaltnetworks but [24]. not Hence, the absence our of the ultraviolet profiles are nearly the same, we cannot discard the possibility of cobalt (II) and cobalt results provide an estimation to corroborate the presence of tetrahedral cobalt but not the absence of octahedral(II) oxides cobalt. formation Cobalt with on the an surface octahedral during symmetry catalyst synthesis. occurs in However, CoO and by a combinationmeans of a temperature of distorted octahedral cobalt. Cobalt with an octahedral symmetry occurs in CoO and a combination of distorted tetrahedralprogrammed and octahedral reduction environment (TPR) analysis in it Cowas3O possible4; therefore, to obtain under indirect this information condition andabout given the cobalt that all tetrahedral and octahedral environment in Co3O4; therefore, under this condition and given that all the ultravioletspecies formed. profiles Cobalt are nearly (II) oxide the same,is reduced we cannotbelow 400 discard °C, while the possibilitythe cobalt silicate of cobalt showed (II) anda higher cobalt the ultraviolet profiles are nearly the same, we cannot discard the possibility of cobalt (II) and cobalt (II) oxidesreduction formation resistance, on the over surface 500 °C during [25] catalyst synthesis. However, by means of a temperature (II) oxides formation on the surface during catalyst synthesis. However, by means of a temperature programmedprogrammed reduction reduction (TPR) (TPR) analysis analysis it it was was possiblepossible to obtain obtain indirect indirect information information about about the cobalt the cobalt ◦ speciesspecies formed. formed. Cobalt Cobalt (II) (II) oxide oxide is is reduced reduced belowbelow 400 °C,C, while while the the cobalt cobalt silicate silicate showed showed a higher a higher ◦ reductionreduction resistance, resistance, over over 500 500C[ °C25 [25]]

Figure 5. Diffuse reflectance UV-Vis spectra of the catalysts.

FigureFigure 5. 5.Diffuse Diffuse reﬂectance reflectance UV-Vis spectra spectra of of the the catalysts. catalysts. C 20182018,, 44,, x 16 FOR PEER REVIEW 6 of 15

Figure 6 shows the temperature-programmed reduction profiles for each group of catalysts, i.e., sodiumFigure silicate,6 shows Cab-O-Sil the temperature-programmed and TEOS as the prec reductionursor. The profiles TEOS-based for each catalysts group ofshow catalysts, different i.e., sodiumprofiles silicate,with the Cab-O-SilpH change. and Figure TEOS 7A as shows the precursor. the cobalt The cation TEOS-based interaction catalysts behavior show prior different to the profilespolycondensation with the pH step change. and Figure Figure 7B the7A cobalt shows cation the cobalt incorporation cation interaction degree. The behavior maximum prior reduction to the polycondensationtemperature of CT11.5 step andwas Figure approximately7B the cobalt 110 cation°C high incorporationer than that degree.of CT10.0. The This maximum change reductionis related ◦ temperatureto the Co incorporation of CT11.5 was level approximately inside the silica 110 wall.C higher During than the thatpolycondensation of CT10.0. This process, change the is related cobalt tocations the Co may incorporation be captured level by hydrolyzed inside the silicaSi-OH wall. species. During Once the the polycondensation cation is captured, process, it might the remain cobalt cationsoutside mayof the be silica captured wall byas hydrolyzedsurface cobalt Si-OH oxide, species. or as Oncesub-surface the cation cobalt is captured, silicate species it might (shallow remain outsideincorporation) of the silicaor in the wall middle as surface of the cobalt silica oxide,wall [26]. or asInner sub-surface cobalt cations cobalt are silicate harder species to reduce (shallow than incorporation)sub-surface or outer or in cobalt the middle cations of because the silica more wall ener [26].gy Inner is required cobalt for cations hydrogen are harder molecules to reduce to diffuse than sub-surfacethrough the orsilica outer wall cobalt to reach cations the because cobalt cations. more energy Therefore, is required at pH for11.5, hydrogen the incorporated molecules cobalt to diffuse may throughbe located the deeper silica wallthan to the reach cobalt the at cobalt pH 10.0 cations.. Upon Therefore, the partial at hydrolysis pH 11.5, the of incorporated TEOS, (EtO) cobalt3Si-O− mayspecies be − locatedare generated, deeper once than thethese cobalt species at pH are 10.0. produced Upon thethey partial are able hydrolysis to catch ofcobalt TEOS, cations (EtO) 3fromSi-O thespecies solution. are generated,Since the equilibrium once these speciesdepends are on produced the pH, higher they are pH able va tolues catch lead cobalt to higher cations content from of the charged solution. silicate Since thein the equilibrium solution dependsthat forms on SiO-Co the pH, resulting higher pH in values a higher lead cobalt to higher degree content of incorporation of charged silicate and inmore the solutionuniformity that in forms the cobalt SiO-Co cations resulting distribution in a higher inside cobalt of degree the walls. of incorporation Another feature and more found uniformity in the TPR in theprofile cobalt of CT11.5 cations is distribution a small signal inside located of the at walls.497 °C. Another This peak feature was attributed found in theto the TPR reduction profile of of CT11.5 cobalt ◦ isoxide a small species signal anchored located to at the 497 MCM-41C. This wall peak through was attributed electrostatic to the interactions reduction of with cobalt the oxideincorporated species anchoredcobalt [27]. to theThe MCM-41 sodium wallsilicate-based through electrostatic catalysts (CS10.0 interactions and withCS11.5) the incorporatedexhibited nearly cobalt the [27 ].same The sodiummaximum silicate-based reduction temperatures catalysts (CS10.0 (771 and CS11.5)776 °C), exhibitedand the Co nearly incorporation the same degree maximum was reductionhigher at ◦ temperatureshigher synthesis (771 pH and values. 776 TheC), andCS11.5 the presents Co incorporation a small broad degree peak was below higher 400 at °C higher which synthesis is most ◦ pHlikely values. related The to cobalt CS11.5 oxide, presents as the a small surface broad particles peak belowwere physically 400 C which attached is most to the likely support. related The to cobalt oxide,oxide was as the entirely surface reduced, particles leading were physically to large cobalt attached clusters to the that support. did not The have cobalt any oxideselectivity was entirelytoward reduced,SWCNT deposition leading to large[17]; cobalttherefore, clusters poor that or didnoncatalytic not have anyperformance selectivity was toward expected. SWCNT In depositioncontrast, the [17 catalyst]; therefore, prepared poor wi orth noncatalytic colloidal silica performance Cab-O-Sil was as the expected. precursor In contrast,showed a the maximum catalyst preparedreduction withtemperature colloidal near silica 725 Cab-O-Sil °C. This as theprocess precursor was associated showed a maximumwith the reduction reduction of temperature the cobalt ◦ nearsilicate 725 speciesC. This produced process wasduring associated the polycondensation with the reduction step.of The the reduction cobalt silicate process species occurred produced in a duringnarrow therange polycondensation of temperatures, step. whic Theh reductionwas indicative process of occurreduniform inCo a incorporation narrow range ofthroughout temperatures, the whichentire support. was indicative After comparing of uniform the Co cobalt incorporation environments, throughout Figure the 7B, entire for the support. Cab-O-Sil After (CC11.5) comparing and thesodium cobalt silicate environments, (CS11.5)-based Figure catalysts,7B, for the we Cab-O-Sil expected (CC11.5)roughly the and same sodium incorporation silicate (CS11.5)-based degree. Both catalysts,silicas sources we expected were soluble roughly in the the reaction same incorporation medium, and degree. the cobalt Both cations silicas sourcespreferably were interacted soluble in with the reactionsilicate species medium, instead and the of cobaltbromide. cations When preferably the poly interactedcondensation with step silicate occurred, species insteadcobalt cations of bromide. that Whenwere bonded the polycondensation to the silicate became step occurred, trapped cobaltin the cationssiloxane that network, were bonded leading toto thea greater silicate extent became of trappedincorporation. in the siloxane network, leading to a greater extent of incorporation.

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FigureFigureFigure 6.6. TemperatureTemperature programmedprogrammed programmed reductionreduction reduction ((TPR)TPR) (TPR) profilesprofiles proﬁles ofof of thethe the Co-MCM-41Co-MCM-41 Co-MCM-41 catalystscatalysts catalysts usingusing using ((AA)) (Cab-O-SilACab-O-Sil) Cab-O-Sil ( (BB)) (sodium Bsodium) sodium silicate, silicate, silicate, and and and ( (CC)) (tetraethylorthosilicate Ctetraethylorthosilicate) tetraethylorthosilicate (TEOS) (TEOS) (TEOS) as as precursors. asprecursors. precursors.

FigureFigureFigure 7.7. 7. (( A(A))) CobaltCobalt Cobalt cationcation cation interactioninteraction interaction behaviorbehavior behavior priorprior prior toto to thethe the polycondensationpolycondensation polycondensation step.step. step. (( B(BB))) CobaltCobalt Cobalt cationcation cation incorporationincorporationincorporation degree.degree. degree.

AmongAmongAmong allallall catalystscatalystscatalysts analyzedanalyzed here,here, CC11.5CC11.5 hadhad thethe lowestlowest amountamount ofof surfacesurface cobaltcobalt oxide,oxide, demonstratingdemonstratingdemonstrating thatthat Cab-O-SilCab-O-SilCab-O-Sil andandand tetramethylammoniumtetramethylammoniumtetramethylammonium silicatesilicate cancan incorporateincorporateincorporate cobaltcobaltcobalt moremoremore efﬁcientlyefficientlyefficiently thanthan than thethe the otherother other catalystscatalysts catalysts analyzedanalyzed analyzed inin in thisthis this study.study. study.

2.3.2.3.2.3. CatalystCatalyst PerformancePerformance TheTheThe preparedprepared catalysts catalystscatalysts were were testedtested tested duringduring during thethe the CCVD CCVD CCVD synthesissynthesis synthesis ofof of SWCNTSWCNT SWCNT withwith with methane methane methane as as thethe as thecarboncarbon carbon source source source under under under the the same thesame same conditions: conditions: conditions: temperatur temperatur temperature,e,e, flow flow rate, ﬂowrate, and rate,and time. time. and The time.The bar bar The diagram diagram bar diagram in in Figure Figure in Figure88 depictsdepicts8 depicts thethe total total the carboncarbon total carbon yield.yield. The yield.The highesthighest The highest carboncarbon carbon massmass depositeddeposited mass deposited waswas 3.5%3.5% was forfor 3.5% thethe CC11.5 forCC11.5 the CC11.5 catalystcatalyst catalystafterafter thethe after reaction,reaction, the reaction, whilewhile thethe while carboncarbon the carbon massmass depositeddeposited mass deposited byby thethe byotherother the catalysts othercatalysts catalysts waswas notnot was higherhigher not higher thanthan 1.5%.1.5%. than 1.5%.AccordingAccording According to to the the TPR toTPR the results, results, TPR results, each each catalyst catalyst each catalyst exhi exhibitedbited exhibited a a different different a different hydrogen-reduction hydrogen-reduction hydrogen-reduction profile. profile. Among proﬁle.Among Amongallall thethe catalysts,catalysts, all the catalysts, CC11.5CC11.5 CC11.5 showedshowed showed thethe highesthighest the highest homogeneityhomogeneity homogeneity inin termsterms in terms ofof thethe of cobaltcobalt the cobalt incorporationincorporation incorporation andand oneone of of the the highest highest levels levels of of cobalt cobalt content. content. The The co cobaltbalt oxide oxide and and surface surface cobalt cobalt species species exhibited exhibited low- low- temperaturetemperature reductionreduction behavior,behavior, andand theythey werewere rereducedduced earlierearlier thanthan thethe incorporatedincorporated cobalt.cobalt. AsAs C 2018, 4, 16 8 of 15

C 2018, 4, x FOR PEER REVIEW 8 of 15 and one of the highest levels of cobalt content. The cobalt oxide and surface cobalt species exhibited low-temperaturementioned before, reduction when this behavior, cobalt species and they was werereduced, reduced the large earlier cobalt than clusters the incorporated lost their selectivity cobalt. Astowards mentioned SWCNT. before, However, when this in cobalt the case species of CC11.5, was reduced, the catalyst the large did cobaltnot show clusters evidence lost their of cobalt selectivity oxide. towardsTherefore, SWCNT. CC11.5 However, presented in the the case highest of CC11.5, cataly thetic catalystactivity. did Cobalt-incorporated not show evidence ofMCM-41 cobalt oxide. using Therefore,sodium silicate CC11.5 presentedas the silica the highestsource catalyticwas only activity. active Cobalt-incorporatedwhen the synthesis MCM-41 pH was using 10.0, sodium and a silicatemesostructured as the silica catalyst source was was confirmed. only active CS11.5 when show the synthesised a significant pH was amount 10.0, andof cobalt a mesostructured oxide and did catalyst was confirmed. CS11.5 showed a significant amount of cobalt oxide and did not exhibit a not exhibit a mesostructure or a high cobalt content according to TPR and N2-adsorption. This catalyst mesostructuredid not show orany a highcatalytic cobalt activity content toward according the SWCNT to TPR andbecause N2-adsorption. the sintering This process catalyst could did not not be showcontrolled any catalytic to avoid activity large towardparticle thesizes SWCNT in the absenc becausee of the mesoporosity sintering process to confine could notthe becobalt controlled clusters toformed avoid largeduring particle the reduction sizes in step the [7]. absence The CT10.0 of mesoporosity and TEOS tocatalysts confine showed the cobalt a low clusters activity formed during duringSWCNT the deposition. reduction step This [ 7behavior]. The CT10.0 was attributed and TEOS to catalysts the fact showed that the aprereduction low activity duringtemperature SWCNT was deposition.higher than This the behavior maximum was reduction attributed temperature. to the fact that According the prereduction to the TPR temperature results, a was large higher amount than of thereduced maximum cobalt reduction silicates temperature. formed during According the prereduction to the TPR results, process, a largeleading amount to large of reduced particle cobalt sizes. silicatesTherefore, formed the activity during thewas prereduction lost. CT11.5 process,had a higher leading reduction to large temperature particle sizes. than Therefore, CT10.0. Hence, the activity large wasparticle lost. CT11.5sizes could had abe higher avoided reduction during temperature the prereduc thantion CT10.0. process, Hence, leaving large some particle cobalt sizes inside could of bethe avoidedsilica framework during the to prereduction anchor the process,formed leavingcobalt clus someters cobalt and insideleading of to the asilica higher framework activity toward to anchor the theSWCNT. formed cobalt clusters and leading to a higher activity toward the SWCNT.

FigureFigure 8. 8.Total Total carbon carbon deposited deposited over over the the catalysts. catalysts.

FigureFigure9 shows9 shows the the Raman Raman spectra spectra of of all all carbon carbon deposition deposition over over the the catalysts catalysts without without further further purification.purification. The The Raman Raman spectrum spectrum of of the the carbon carbon nanotubes nanotubes consists consists ofthree of three main main bands. bands. The The G band G band at −1 approximatelyat approximately 1580 1580 cm− 1cmwas was attributed attributed to the to tangential the tangential vibration vibration mode mode of benzene, of benzene, such as such the fused as the ringsfused in rings the graphite in the graphite sheet, and sheet, was directlyand was related directly to therelated structural to the order structural in the order carbon in nanotubes. the carbon Thenanotubes. D band appearedThe D band when appeared there was when partial there or totalwas pa destructionrtial or total of thedestruction symmetry of of the benzene, symmetry such of asbenzene, fused rings, such i.e.,as fused structural rings, defects i.e., structural or the presence defects or of the amorphous presence carbon.of amorphous This band carbon. is located This band at −1 approximatelyis located at approximately 1350 cm−1. Finally, 1350 the cm most. Finally, important the bandmost wasimportant the radial band breathing was the mode radial (RBM) breathing that ismode characteristic (RBM) that of SWCNT is characteristic and is related of SWCNT to the and SWCNT is related diameter. to the In SWCNT addition, diameter. the wavelength In addition, of the the apparitionwavelength is inverselyof the apparition proportional is inversely to the diameter proportional of the to SWCNT. the diam Thus,eter itof is the possible SWCNT. to determine Thus, it is thepossible diameter to determine distribution the using diameter a simple distri equationbution using [28]. a simple equation [28]. 234cm nm = −1 +10cm Bundle of SWCNT 234 cm nm −1 ωRBM = + 10 cm Bundle of SWCNT dt The ratio of intensity G/D is an important parameter that indicates the structural quality of graphiticThe ratio materials, of intensity such as G/D SWCNT. is an The important higher parameterthe ratio of thatG/D, indicates the higher the is structuralthe structural quality order of in graphiticthe graphitic materials, structure. such as SWCNT. The higher the ratio of G/D, the higher is the structural order in the graphiticOnly the structure. carbon deposition over the CC11.5, CS11.5, and CT10.0 catalysts, which exhibited the highestOnly content the carbon of carbon, deposition were an overalyzed the since CC11.5, the CS11.5,other catalysts and CT10.0 did not catalysts, show good which Raman exhibited spectra theFigure highest 9A shows content the of G carbon, and D bands. were analyzed Notably, sincethe ca therbon other deposition catalysts over did the not CC11.5 show catalyst good Raman showed a lower D band intensity and higher G band intensity; therefore, it presented the highest structural order among the other catalysts. In contrast, we found that catalyst CS10.0 also showed an intense D band, even though it showed a prominent G band. Then, the SWCNT produced with this catalyst C 2018, 4, 16 9 of 15 spectra Figure9A shows the G and D bands. Notably, the carbon deposition over the CC11.5 catalyst showed a lower D band intensity and higher G band intensity; therefore, it presented the highest Cstructural 2018, 4, x FOR order PEER among REVIEW the other catalysts. In contrast, we found that catalyst CS10.0 also showed9 of 15 an intense D band, even though it showed a prominent G band. Then, the SWCNT produced with exhibitedthis catalyst larger exhibited structural larger disorder. structural It has disorder. been proven It has beenthat the proven presence that theof sodium presence in ofCo-MCM-41 sodium in destroysCo-MCM-41 its selectivity destroys its toward selectivity SWCNT toward deposition SWCNT [25]. deposition Finally, [25 the]. Finally,CT11.5 thecatalyst CT11.5 also catalyst showed also a prominentshowed a prominentG band; however, G band; however,there was there a significant was a significant loss in lossthe instructural the structural order orderas well. as well. An Anexamination examination of the of radial the radial breathing breathing mode mode(RBM) (RBM) zone, in zone, Figure in 9B, Figure allows9B, us allows to confirm us to confirmthe presence the presenceof SWCNT. of SWCNT.The signals The of signals the CC11.5 of the CC11.5catalyst catalyst range from range 175 from cm 175−1 to cm 275−1 tocm 275−1, cmwhile−1, whilethe other the catalystsother catalysts show showdistributions distributions below below 150 cm 150−1. The cm− lower1. The energy lower energyvibrations vibrations in the RBM in the zone RBM are zone related are torelated large-diameter to large-diameter SWCNT, SWCNT, even double-wall even double-wall carbon nanotubes. carbon nanotubes. Thus, CC11.5 Thus, has CC11.5 the highest has the activity highest andactivity the andnarrowest the narrowest diameter diameter distribution. distribution.

Figure 9. Raman spectra of the carbon depositions over the Co-MCM-41 catalysts: (A) D and G bands, Figure 9. Raman spectra of the carbon depositions over the Co-MCM-41 catalysts: (A) D and G bands, (B) radial breathing mode zone. (B) radial breathing mode zone.

A more detailed analysis of the RBM zone was made by converting converting the maximum maximum intensity peak decomposition into Gaussian-like functions to estimateestimate the SWCNT diameter distribution. Figure 10 shows thethe deconvolution deconvolution of allof spectra,all spectra, and Tableand 4Table summarizes 4 summarizes the approximate the approximate populations populations calculated calculatedusing the total using area the of total the peaksarea of within the peaks the radial within breathing the radial mode breathing zone and mode each peakzone associatedand each peak with associatedeach diameter with of each the SWCNT.diameter From of the the SWCNT. obtained Fr results,om the the obtained SWCNT results, growth the over SWCNT the CC11.5 growth catalyst over theshowed CC11.5 a high catalyst selectivity showed towards a high SWCNT selectivity with atowards diameter SWCNT of 1.1 nm, with as calculated a diameter with of the 1.1 previous nm, as calculatedequation. Approximatelywith the previous 60% equation. of the diameters Approximately of the total 60% SWCNT of the diameters produced wereof the close total to SWCNT 1.1 nm. producedNotably, approximately were close to 1.1 89% nm. of theNotably, total ofapproximately SWCNT was 89% within of the 0.9 nmtotal and of SWCNT 1.1 nm, whilewas within 11% was 0.9 withinnm and the 1.1 diameter nm, while range 11% between was within 1.7 nmthe anddiameter 1.8 nm. range The between CS10.0 catalyst 1.7 nm showedand 1.8 anm. more The disperse CS10.0 catalystSWCNT showed diameter a more distribution disperse than SWCNT the SWNCTsdiameter distribution deposited over than the the CC11.5 SWNCTs catalyst. deposited In addition, over the CC11.5the Raman catalyst. spectrum In addition, did not the show Raman signals spectrum for SWCNT did not with show diameters signals for less SWCNT than 1 with nm, whilediameters that lessfor thethan SWCNT 1 nm, while grown that over for the the CC11.5 SWCNT catalyst grown did. over The the CT11.5CC11.5 catalyst,catalyst unlikedid. The the CT11.5 other catalyst, catalyst, unlikeshowed the even other less catalyst, uniformity showed in the even diameter less distribution;uniformity in approximately the diameter 22%distribution; of the contribution approximately of the 22%SWCNT of the had contribution diameters largerof the than SWCNT 1.9 nm. had The diamet RBMers deconvolution larger than 1.9 for nm. CT11.5 The depositionRBM deconvolution presented fornonselectivity CT11.5 deposition toward apresented narrow SWCNT nonselectivity diameter toward distribution. a narrow In SWCNT summary, diameter the CT11.5 distribution. catalyst is In a propersummary, material the CT11.5 for growing catalyst SWCNT is a proper with amaterial narrow diameterfor growing distribution SWCNT withwith aa selectivitynarrow diameter toward distributiondiameters near with 1.1 a nm, selectivity which is toward a really diameters close diameter near for1.1 CCVDnm, which produced is a really SWCNT. close diameter for CCVD produced SWCNT. C 2018, 4, 16 10 of 15 C 2018, 4, x FOR PEER REVIEW 10 of 15

Figure 10. Radial breathing mode (RBM) deconvoluted spectra of the total deposited single-wall carbon nanotubes (SWCNT); (A): CC11.5, (B): CS10.0, and (C): CT11.5 catalysts. Figure 10. Radial breathing mode (RBM) deconvoluted spectra of the total deposited single-wall Table 4. Single-wall carbon nanotubes (SWCNT) diameters and populations. carbon nanotubes (SWCNT); (A): CC11.5, (B): CS10.0, and (C): CT11.5 catalysts.

Radial Breathing Mode (RBM) Peaks of the SWNCT Deposited on CC11.5 Table 4. Single-wall carbon nanotubes (SWCNT) diameters and populations. Raman Shift Tube Diameter Peak Area Peak Number Population (%) Radial Breathing(cm Mode−1) (RBM) Peaks(nm) of the SWNCT(cm−1. Deposited Intensity) on CC11.5 1 138.76 1.8 1.82Peak Area 7.6 Peak Number Raman Shift (cm−1) Tube Diameter (nm) Population (%) 2 150.67 1.7(cm 0.82−1. Intensity) 3.4 1 3138.76 201.87 1.2 1.8 3.06 1.82 12.8 7.6 4 212.87 1.2 2.53 10.6 2 5150.67 223.10 1.1 1.7 3.22 0.82 13.4 3.4 3 6201.87 231.00 1.1 1.2 11.45 3.06 47.712.8 4 7212.87 265.01 0.9 1.2 1.08 2.53 4.5 10.6 5 223.10RBM peaks of the SWNCT 1.1 deposited on CS10.0 3.22 13.4 6 Raman231.00 shift Tube diameter 1.1 Peak area 11.45 47.7 Peak number Population (%) 7 265.01(cm− 1) (nm) 0.9 (cm−1. Intensity) 1.08 4.5 1RBM 136.86 peaks of the SWNCT 1.8 deposited on 2.10 CS10.0 9.9 2 147.39 1.7 0.69Peak area 3.2 Peak number Raman shift (cm−1) Tube diameter (nm) Population (%) 3 199.39 1.2(cm 1.48−1. Intensity) 7.0 1 4136.86 212.05 1.2 1.8 2.14 2.10 10.0 9.9 5 222.00 1.1 2.96 13.9 2 6147.39 230.53 1.1 1.7 11.90 0.69 56.0 3.2 3 199.39 1.2 1.48 7.0 4 212.05 1.2 2.14 10.0 5 222.00 1.1 2.96 13.9 6 230.53 1.1 11.90 56.0 RBM peaks of the SWNCT deposited on CT11.5 C 2018, 4, 16 11 of 15

C 2018, 4, x FOR PEER REVIEW Table 4. Cont. 11 of 15

RBM peaks of the SWNCT deposited on CT11.5Peak area Peak number Raman shift (cm−1) Tube diameter (nm) Population (%) Raman shift Tube diameter Peak(cm− area1. Intensity) Peak number Population (%) 1 136.34(cm− 1) (nm) 1.9 (cm−1. Intensity) 6.59 22.2 2 1148.66 136.34 1.9 1.7 6.59 0.97 22.2 3.3 3 2182.19 148.66 1.7 1.4 0.97 0.76 3.3 2.6 4 3193.68 182.19 1.4 1.3 0.76 0.99 2.6 3.3 4 193.68 1.3 0.99 3.3 5 5199.24 199.24 1.2 1.2 3.53 3.53 11.911.9 6 6213.13 213.13 1.2 1.2 8.10 8.10 27.327.3 7 7223.75 223.75 1.1 1.1 2.96 2.96 10.010.0 8 8232.00 232.00 1.1 1.1 5.79 5.79 19.519.5

Since a higherhigher selectivityselectivity and yield of thethe CC11.5CC11.5 catalystcatalyst waswas observedobserved over thethe CT11.5CT11.5 andand CS10.0 toward SWCNT synthesis duringduring the chemical vapor deposition of methane, it was chosenchosen forfor transmissiontransmission electronelectron microscopymicroscopy (TEM)(TEM) analysisanalysis withoutwithout puriﬁcation.purification. Figure 11 shows the TEM imageimage ofof the the obtained obtained SWCNT. SWCNT. Clearly, Clearly, multiwall multiwall carbon carbon nanotubes, nanotubes, as structures as structures with large with diameters, large candiameters, not be can seen. not The be sampleseen. The consisted sample consisted of a few isolatedof a few SWCNT, isolated SWCNT, but most but of the most sample of the showed sample bundlesshowed ofbundles SWCNT. of SWCNT.

Figure 11. 11. TransmissionTransmission electron electron microscopy microscopy (TEM) (TEM) images images of the of SWCNT the SWCNT growth growth on the onCC11.5 the CC11.5catalyst. catalyst.

3. Materials and Methods

3.1. Co-MCM-41 Synthesis Using Colloidal SilicaSilica Cab-O-Sil asas thethe PrecursorPrecursor (CC11.5)(CC11.5) The synthesis of the Co-MCM-41 catalyst was carriedcarried out by following the procedure described by Lim Sangyun et al. [[29].29]. First, 2.50 g of colloidal silica Cab-O-Sil (99.5%) Sigma-Aldrich (St.(St. Louis, MO, USA)USA) andand 10 10 mL mL of of tetramethylammonium tetramethylammonium silicate silicate Sigma-Aldrich Sigma-Aldrich (St. ( Louis,St. Louis, MO, MO, USA) USA (15–20%)) (15– 2 (TMASiO20%) (TMASiO2) were) stirredwere stirred for 30 for min 30 in min 50 mLin 50 of mL deionized of deio water;nized water; then, thethen, proper the proper amount amount of the cobaltof the . 3 2. 2 precursorcobalt precursor [Co.(NO [Co.(NO3)2 6H2O]) 6H MerckO] Merck (Kenilworth, (Kenilworth, NJ, USA) NJ, (99.3%) USA) ((99.3%) was added was to added reach ato 3% reach nominal a 3% massnominal of cobalt.mass of Thecobalt. mixture The mixture was kept was under kept under constant constant stirring stirring for 30 for min, 30 min, and and then, then, two two drops drops of antifoamof antifoam A Sigma-Aldrich A Sigma-Aldrich (St. Louis, (St. Louis, MO, USA) MO, and USA 28.79) and gof 28.79 cetyltrimethylammonium g of cetyltrimethylammonium hydroxide (CTMAOH)hydroxide (CTMAOH) were added. were Next, added. the pH Next, was adjustedthe pH was to 11.5adjusted using to glacial 11.5 using acetic glacial acid Merck acetic (Kenilworth, acid Merck NJ,(Kenilworth, USA) (99%). NJ, The CTMAOHUSA) (99%). was previouslyThe CTMA preparedOH usingwas 20%previously cetyltrimethylammonium prepared using bromide 20% Sigma-Aldrichcetyltrimethylammonium (St. Louis, bromide MO, USA) Sigma-Aldrich (CTMABr, 99%) (St. Louis, and an MO, ionic USA exchange) (CTMABr, resin, 99%) Ambersep and an ionic 900® exchange resin, Ambersep 900® Sigma-Aldrich (St. Louis, MO, USA), in the proportion of 1 mmol CTMABr: 1 mL Ambersep 900®. Previous works have shown that both MCM-41 [20] and the Co- MCM-41 catalyst [30] have good thermal stability and pore uniformity when the synthesis is conducted under these conditions. C 2018, 4, 16 12 of 15

Sigma-Aldrich (St. Louis, MO, USA), in the proportion of 1 mmol CTMABr: 1 mL Ambersep 900®. Previous works have shown that both MCM-41 [20] and the Co-MCM-41 catalyst [30] have good thermal stability and pore uniformity when the synthesis is conducted under these conditions.

3.2. Co-MCM-41 Synthesis Using Sodium Silicate as the Precursor Catalysts prepared using sodium silicate as silica source are named as CSX where C is for catalyst, S for silicate, and X is the synthesis pH. Co-MCM-41 catalyst synthesized at pH 10.0 (CS10.0) MCM-41 synthesized at pH 11.5 (CS11.5). The synthesis of the catalysts using sodium silicate as the silica precursor was carried out by following the procedure described by Giraldo et al. [18]. First, 2.00 g of CTMABr was dissolved in 10 mL of 0.1 M HCl (Merck), then the proper amount of the cobalt precursor was added to reach a 3% nominal mass of cobalt in the silica support. Next, 6.00 g of a sodium silicate solution Sigma-Aldrich (St. Louis, MO, USA), (25.5–28.5% SiO2), previously dissolved in 30 mL of deionized water, was added and, the reaction was left under stirring for 30 min. Finally, the pH in the sol–gel process was adjusted to 10 and 11.5. This synthesis has the advantage of requiring a less expensive silica source.

3.3. Co-MCM-41 Synthesis Using TEOS as the Precursor Catalysts prepared using TEOS as silica source are named as CTX where C is for catalyst, T for TEOS and X is the synthesis pH. Co-MCM-41 catalyst synthesized at pH 10.0 (CT10.0) MCM-41 synthesized at pH 11.5 (CT11.5) The synthesis of catalysts using TEOS as the silica precursor was carried out following the procedure described by M. Grün, K. et al. [23]. First, 2.00 g of CTMABr were added in 50 mL of deionized water and maintained under magnetic stirring until reaching complete dissolution. The . proper amount of the cobalt precursor [Co.(NO3)2 6H2O] to reach a 3% nominal mass of cobalt in the silica support was added to this solution, and then 8 mL ammonia (27% w/v) was incorporated. The mixture was left under magnetic stirring for 30 min. Then, 5.00 g of tetraethylortho silciate (TEOS) Sigma-Aldrich (St. Louis, MO, USA) (99%) was added dropwise to this solution, and the pH was adjusted to 10.0 (CT10.0) and 11.5 (CT11.5) All syntheses were subjected to hydrothermal treatment in an autoclave for three days at 100 ◦C to promote silica polycondensation into siloxane networks around the surfactant aggregates. Once the hydrothermal treatment was completed, the obtained materials were calcined at 540 ◦C under air atmosphere.

3.4. MCM-41 Silica Synthesis The mesoporous MCM-41 silica was synthesized following the procedures described above, except for the cobalt incorporation step, using the following silica precursors: sodium silicate, Cab-O-Sil and TEOS, MCM-41 sample names use similar acronyms as catalyst, XY, where X is C for Cab-O-Sil, T for TEOS and S for silicate and Y is for synthesis pH, thus, C11.5 is for MCM-41 Cab-O-Sil synthesized at pH 11.5, S10.0, and S11.5 for sodium silicate MCM-41 synthesized at pH 10.0 and 11.5 respectively, T10.0 and T11.5 for TEOS MCM-41 synthesized at pH 10.0 and 11.5 respectively.

3.5. Catalytic Performance Evaluation Each catalyst was tested during the CCVD of methane to produce SWCNT. First, 200 mg of the catalyst was set into a 20 mm diameter circle on a fritted quartz disk inside of a 50 cm diameter quartz tube and placed into a vertical oven that was heated to 700 ◦C under a nitrogen flow rate of 50 cm3min−1 at 20 ◦C min−1. Then the nitrogen flux was replaced with a mixture of 150 cm3min−1: 3 −1 50 cm min N2:H2 as a reducing agent for 30 min. Once the reduction was completed, the oven was ◦ 3 −1 3 −1 rapidly heated to 800 C under nitrogen flux; then, a mixture of 150 cm min : 50 cm min CH4:N2 was used as a carbon source. The carbon deposition was carried out for a 30 min period, and finally, C 2018, 4, 16 13 of 15 the methane and nitrogen mixture was switched to pure nitrogen. The oven was left to cool, and the blackish product was collected and characterized without any further purification.

3.6. Characterization of the Co-MCM-41 Catalysts Nitrogen physisorption on MCM-41 and Co-MCM-41 was carried using a Micromeritics Accelerated Surface Area and Porosimetry System (ASAP) 2010 (Norcross, GA, USA) at the nitrogen normal boiling point (77K) to evaluate their adsorption–desorption isotherms, total surface area, and pore size distributions. Diffuse reflectance ultraviolet–visible spectroscopy (DRS–UVVis) measurements were conducted using a Perkin–Elmer Lambda 35 (Waltham, MA, USA) to determine the presence of cobalt in the catalysts [31]. Temperature programmed reduction (TPR) data were collected to examine the cobalt cation incorporation degree within the silica structures [26] using a Micromeritics AutoChem II 2920 (Norcross, GA, USA) chemisorption analyzer equipped with a ◦ thermal conductivity detector (TCD) using 50 mg of a degassed catalyst at 250 C under 5% H2/Ar at a heat rate of 5 ◦C/min from atmosphere temperature to 1000 ◦C. The temperature was maintained for one hour to ensure total cobalt (II) and cobalt (III) reduction. The, catalysts and silica mesostructures were evaluated with X–ray diffraction measurements using a Siemens (Berlin, Germany) D5000 Cu–Kα ◦ ◦ ◦ (λα = 1.54 Å scanning 2θ from 0.7 to 10 step size 0.048 ), and finally, the total cobalt amount was quantified by atomic absorption spectroscopy (AAS) Thermo Fisher Scientific (Waltham, MA, USA) Unicam 929 using a cobalt lamp and a cobalt (II) calibration curve from 1–5 mg/mL.

3.7. CCVD Deposited Products Characterization The amount of carbon deposition was measured using the TGA technique with a TA Instruments (New Castle, DE, USA) thermogravimetric analyzer (Q500) that was heated at 10 ◦Cmin−1 from room temperature to 800 ◦C under an oxidant atmosphere. The structural quality of the SWCNT was investigated with Raman microscopy measurements using a Horiba (Kyoto, Kyoto Prefecture, Japan) Labram HR with a 732 nm excitation wavelength, spectral deconvolution was performed using Originlab with Lorentzian functions to determine the intensity maximum of each peak and area for population analysis. Transmission electron micrographs were collected with a Thermo Fisher Scientiﬁc (Waltham, MA, USA) Tecnai F20 200 KV, and the carbon deposition was characterized without any puriﬁcation procedure.

4. Conclusions In this work, we determined that Co. catalysts prepared with colloidal silica Cab-O-Si MCM-41 exhibit better cobalt incorporation capabilities within the siloxane network without signiﬁcant loss in mesoporosity or the physical integrity of the support compared with the catalysts prepared using sodium silicate and TEOS as precursors under the same conditions. In addition, cobalt was successfully incorporated without forming cobalt oxide on the silica walls. When the catalytical properties were evaluated during SWCNT preparation, the CC11.5 catalyst exhibited the best performance and produced graphite-like materials with high structural regularity and the highest yield among all catalysts tested in this study. Moreover, the SWCNT growth on the CC11.5 catalyst showed a narrower diameter distribution among all the deposited materials on the several catalysts. From the TEM analysis, neither amorphous carbon nor multiwall carbon nanotubes were formed. In general, from this work, we can conclude that the Co-MCM-41 catalyst prepared using colloidal silica Cab-O-Sil as the precursor and methane as the carbon source is an economic alternative for producing SWCNT with good structural regularity. Nevertheless, the CO disproportion SWCNT synthesis offers higher yield (higher than 3.0% here attained) and therefore optimization of the methane CCVD synthesis over Cab-O-Sil based catalyst could be improved to reach higher yields.

Acknowledgments: Frank Ramírez Rodríguez thanks Colciencias Colombia for the program “Francisco José de Caldas Convocatoria Nacional para Estudios de Doctorado en Colombia” N◦ 511. The authors declare no conﬂict of interest. C 2018, 4, 16 14 of 15

Author Contributions: Luis Fernando Giraldo and Frank Ramírez Rodríguez designed the experiments; Frank Ramírez Rodríguez performed the experiments and data analysis; Luis Fernando Giraldo, Betty Lucy López and Frank Ramírez Rodríguez wrote the manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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