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Yuan Chen,
Li Wei,
Bo Wang,
Sangyun Lim, Dragos Ciuparu,
Ming Zheng,
Jia Chen,
Codruta Zoican,
Yanhui Yang,
Gary L Haller,
Lisa D Pfefferle
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ABSTRACT: A mild, four-step purification procedure using NaOH reflux, HCl wash, and oxidation by 4 mol % molecular oxygen at 500 degrees C was developed to purify single-walled carbon nanotubes (SWCNTs) with narrow semiconducting (n,m) distribution produced from cobalt-incorporated MCM-41 (Co-MCM-41) in order to obtain bulk low-defect-density nanotubes. Three key features of Co-MCM-41 allow this mild purification technique: (1) ultrathin silica walls versus dense silica or other crystalline oxide supports are soluble in dilute NaOH aqueous solution, which avoids the damage to SWCNTs usually caused by using HF treatment to remove catalytic supports; (2) the small metallic particles are easily dissolved in HCl, a significantly milder chemical treatment compared to HF or HNO(3); (3) the high selectivity to SWCNTs with negligible multiwalled carbon nanotubes or graphite, which facilitates the removal of undesired carbon species by selective oxidation. The effectiveness of this purification procedure was evaluated by high-resolution transmission electron microscopy, scanning electron microscopy, Raman, UV-vis-NIR, and fluorescence spectroscopy, solution redox chemistry on fractionated (6,5) tubes, and SWCNT-based field effect transistor device performance. The results demonstrate that Co-MCM-41 catalyst not only provides tubes with narrow semiconducting (n,m) distribution but also allows a mild purification procedure and, therefore, produces SWCNTs with fewer defects.
ACS Nano 12/2007; 1(4):327-36. · 10.77 Impact Factor
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ABSTRACT: The effect of initial synthesis solution pH and tetramethylammonium silicate concentration in the synthesis solution on the physical and chemical properties of MCM-41 was systematically investigated using N(2) physisorption, X-ray diffraction, temperature-programmed reduction, in situ Fourier transform IR, UV-vis, and X-ray absorption spectroscopies. pH and tetramethylammonium (TMA) fraction affect the porosity of MCM-41 and the reducibility of incorporated Ni cations; higher pH and TMA concentration produced more porosity with higher stability against reduction, which is attributed to more metal ions locating in the interior of the silica walls. The control of the pore diameter of mesoporous MCM-41 at the sub-nanometer scale may be accomplished by adjusting the pH and TMA fraction. pH may be used to control the surface free silanol group density and nickel reduction degree as well, and this is useful in the design of a specific catalyst for particular reactions, such as CO methanation, which requires highly dispersed, stable metallic clusters with controllable size.
The Journal of Physical Chemistry B 04/2006; 110(12):5927-35. · 3.70 Impact Factor
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ABSTRACT: Chemometric tools were employed to analyze the in-situ dynamic X-ray absorption spectroscopy data to probe the state of Co-MCM-41 catalysts during reduction in pure hydrogen and under single-wall carbon nanotube synthesis reaction conditions. The use of the progressive correlation analysis established the sequence in which changes in the spectral features near the Co K edge occurred, and the evolving factor analysis provided evidence for the formation of an intermediate Co(1+) ionic species during reduction of the Co-MCM-41 catalyst in pure hydrogen up to 720 degrees C. This intermediate species preserves the tetrahedral environment in the silica framework and is resistant to complete reduction to the metal in H(2). While the Co(2+) species is resistant to reduction in pure CO, the intermediate Co(1+) species is more reactive in CO most likely forming cobalt carbonyl-like compounds with high mobility in the MCM-41. These mobile species are the precursors of the metallic clusters growing carbon nanotubes. Controlling the rates of each step of this two-stage reduction process is key to controlling the size of the metallic Co clusters formed in Co-MCM-41 catalysts.
The Journal of Physical Chemistry B 10/2005; 109(34):16332-9. · 3.70 Impact Factor
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ABSTRACT: Surface characterization and catalysis can significantly benefit from the application of generalized two-dimensional (2D) correlation analysis. This two-dimensional approach allows a better resolution of overlapping peaks, can reveal new features not readily observable in the raw spectra, gives clear evidence for spectral intensities that change as an effect of a perturbation applied to the system, and allows the establishment of time sequences for the changes occurring in different spectral features of interest for determining reaction intermediates and/or mechanisms. The interpretation of the synchronous and asynchronous plots was observed to lead to erroneous time sequences when spectral features change in a non-monotonic way, such as a biphasic or oscillatory behavior, under the influence of a perturbation. We propose a new approach to the 2D correlation analysis to avoid misinterpretation of the results calculated in the asynchronous plot. Progressive correlation analysis (ProCorA) calculates the synchronous plot from the first two spectra of the data matrix and one spectrum is added at every step of the analysis. The sequence of changes can be set up from the progressive evolution of peaks in both the synchronous and asynchronous plots.
Applied Spectroscopy 09/2005; 59(8):1060-7. · 1.66 Impact Factor
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ABSTRACT: Highly ordered Ni-MCM-41 samples with nearly atomically dispersed nickel ions were prepared reproducibly and characterized. Similar to the Co-MCM-41 samples, the pore diameter and porosity can be precisely controlled by changing the synthesis surfactant chain length. Nickel was incorporated by isomorphous substitution of silicon in the MCM-41 silica framework, which makes the Ni-MCM-41 a physically stable catalyst in harsh reaction conditions such as CO disproportionation to single wall carbon nanotubes or CO2 methanation. X-ray absorption spectroscopy results indicate that the overall local environment of nickel in Ni-MCM-41 was a tetrahedral or distorted tetrahedral coordination with surrounding oxygen anions. Hydrogen TPR revealed that our Ni-MCM-41 samples have high stability against reduction; however, compared to Co-MCM-41, the Ni-MCM-41 has a lower reduction temperature, and both the H2-TPR and in situ XANES TPR reveal that the reducibility of nickel is not clearly correlated with the pore radius of curvature, as in the case of Co-MCM-41. This is probably a result of nickel being thermodynamically more easily reduced than cobalt. The stability of the structural order of Ni-MCM-41 has been investigated under SWNT synthesis and CO2 methanation reaction conditions as both require catalyst exposure to reducing environments leading to formation of metallic Ni clusters. Nitrogen physisorption and XRD results show that structural order was maintained under both SWNT synthesis and CO2 methanation reaction conditions. EXAFS results demonstrate that the nickel particle size can be controlled by different prereduction temperatures but not by the pore radius of curvature as in the case of Co-MCM-41.
The Journal of Physical Chemistry B 08/2005; 109(27):13237-46. · 3.70 Impact Factor
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ABSTRACT: Both Ni- and Co-MCM-41 may be used for the synthesis of single-wall carbon nanotubes (SWNT). We present a comparative investigation that demonstrates that smaller diameter SWNT with a narrower distribution of diameters are produced using Co-MCM-41. Temperature-programmed reduction and x-ray absorption spectroscopy were used to measure the reducibility and metal cluster growth of Ni- and Co-MCM-41 in He, H(2) and under CO disproportionation reaction. The differences between these two catalysts can be attributed to a greater reducibility of and a greater CO affinity for Ni relative to Co.
Nanotechnology 07/2005; 16(7):S476-83. · 3.98 Impact Factor
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ABSTRACT: The adjustment of the initial synthesis solution pH affects the structure and stability of C16 Co-MCM-41. This pH effect was systematically investigated using N2 physisorption, TPR, in-situ FTIR, and X-ray absorption spectroscopy. Co-MCM-41 catalysts with the same pore size but greater porosity were synthesized with increasing pH from 10.5 to 12. The distribution of Co ions with respect to being on the pore wall or in the silica wall framework was changed by pH; higher pH produced Co ions mainly distributed just subsurface or in the interior of the silica wall. These pH effects significantly affected the reduction stability of the Co-MCM-41 sample similar to that of the pore radius of curvature effect (Lim et al, J. Phys. Chem. B, 109 (2005) 2285), resulting in stable and size controllable sub-nanometer Co clusters that are useful for catalyst design for specific reactions.
Topics in Catalysis 04/2005; 34(1):31-40. · 2.62 Impact Factor
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ABSTRACT: Single-wall carbon nanotubes (SWNT) were incorporated into an organic polymer monolith containing vinylbenzyl chloride (VBC) and ethylene dimethacrylate (EDMA) to form a novel monolithic stationary phase for high-performance liquid chromatography (HPLC) and capillary electrochromatography (CEC). The retention behavior of neutral compounds on this poly(VBC-EDMA-SWNT) monolith was examined by separating a mixture of small organic molecules using micro-HPLC. The result indicated that incorporation of SWNT enhanced chromatographic retention of small neutral molecules in reversed-phase HPLC presumably because of their strongly hydrophobic characteristics. The stationary phase was formed inside a fused-silica capillary whose lumen was coated with covalently bound polyethyleneimine (PEI). The annular electroosmotic flow (EOF) generated by the PEI coating allowed peptide separation by CEC in the counterdirectional mode. Comparison of peptide separations on poly(VBC-EDMA-SWNT) and on poly(VBC-EDMA) with annular EOF generation revealed that the incorporation of SWNT into the monolithic stationary phase improved peak efficiency and influenced chromatographic retention. The structures of pretreated SWNT and poly(VBC-EDMA-SWNT) monolith were examined by high-resolution transmission electron microscopy, Raman spectroscopy, scanning electron microscopy, and multipoint BET nitrogen adsorption/desorption.
Analytical Chemistry 04/2005; 77(5):1398-406. · 5.86 Impact Factor
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ABSTRACT: Samples of Co-MCM-41 with different pore diameters have been synthesized using organic templates with different alkyl chain lengths. The reducibility of cobalt in these highly stable samples was investigated by TPR and X-ray absorption spectroscopy. We have found that the reducibility correlates strongly with the pore diameter of the MCM-41, with the cobalt incorporated in the smaller pore MCM-41 being more resistant to complete reduction. It is proposed that the distribution of cobalt ions in the pore wall is affected by both the preparation procedure and the pore diameter. The size of the metallic Co clusters formed after different reducing treatments correlates linearly with the pore size, giving direct evidence for the effect of the radius of curvature on reducibility. Complete cobalt reduction after TPR causes an inverse variation of the cluster size with the pore size, resulting from differences in the density of Co clusters and from differences in the rate of Co migration and aggregation outside the pores of MCM-41 with different pore sizes.
The Journal of Physical Chemistry B 03/2005; 109(6):2285-94. · 3.70 Impact Factor
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ABSTRACT: Fe-substituted MCM-41 molecular sieves with ca. 1, 2, and 3 wt % Fe were synthesized hydrothermally using different sources of colloidal silica (HiSil and Cab-O-Sil) and characterized by ICP, XRD, N2 physisorption, UV-vis, EPR, TPR, and X-ray absorption. Catalysts synthesized from Cab-O-Sil showed higher structural order and stability than those from HiSil. The local environment of Fe in the mesoporous material as studied by UV-vis reveals the dominance of framework Fe in all the as-synthesized Fe-MCM-41 samples. Dislodgement of some Fe species to extraframework location occurs upon calcination, and this effect is more severe for Fe-MCM-41 (2 wt %) and Fe-MCM-41 (3 wt %), as confirmed by EPR and X-ray absorption. These materials have been used as catalytic templates for the production of carbon nanotubes (CNTs) by acetylene pyrolysis at atmospheric pressure. A relationship between the Fe loading in MCM-41 and the carbon species produced during this reaction has been established. Using our optimized conditions for this system, Fe-MCM-41 with ca. 2 wt % Fe showed the best results with particularly high selectivity for single-wall carbon nanotube (SWNT) production. This catalyst was selective for carbon nanotubes with a low amount of amorphous carbon for a narrow range of temperatures from 1073 to 1123 K. To account for the different selectivity of these catalysts for CNTs production, the local environment and chemical state of Fe in the used catalyst was further probed by X-band EPR.
The Journal of Physical Chemistry B 03/2005; 109(7):2645-56. · 3.70 Impact Factor
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ABSTRACT: Application of the generalized 2D correlation analysis to a series of in situ XANES spectra enabled the determination of additional useful information not readily available from the conventional spectra. In addition to the changes in the intensity of the white line and in the pre-edge feature, readily observable in the regular spectra, the generalized 2D correlation analysis clearly evidenced an otherwise imperceptible shift in the main edge energy caused by the gradual reduction of Co(2+) to metallic cobalt. The 2D correlation spectra also allowed the establishment of a time sequence for the changes occurring in the spectral features during hydrogen reduction, which provides valuable information on the reduction mechanism. While the generalized 2D correlation analysis was found to be very useful in obtaining supplementary information from the series of XANES spectra analyzed, interpretation of the correlation intensities should be checked for consistency with the general trend of each spectral feature, as spectral intensities that do not change monotonically may induce changes in the signs of the correlation intensities leading to inaccurately establishing sequences of changes among the spectral features in the series.
Journal of the American Chemical Society 03/2005; 127(6):1906-12. · 9.91 Impact Factor
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ABSTRACT: Cobalt silicate was studied as a model catalyst to seek an understanding of why Co-MCM-41 catalyst is not active for single wall carbon nanotubes (SWNT) upon treatment at high temperature. Cobalt orthosilicate was formed during high-temperature pretreatment of Co-MCM-41 (at 900 °C) in an oxidizing environment. Cobalt silicate did not produce any carbon species during the first reaction cycle because the high thermal stability of cobalt silicate did not result in sufficient metallic Co clusters responsible for carbon dissociation in CO disproportionation. With repeated reaction cycles, however, undesirable carbon species, i.e., amorphous carbon and graphite, were produced resulting from larger metallic cobalt clusters formed at the high reduction temperature and without the confinement and/or anchoring of the MCM-41 matrix. These relatively large clusters do not catalyze the growth of SWNT.
12/2004;
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ABSTRACT: The mechanisms controlling the reduction of cobalt and the growth of metallic cobalt clusters in Co-MCM-41 catalysts at different stages in the process of single-wall carbon nanotubes (SWNT) synthesis were investigated both by in-situ and ex-situ X-ray absorption spectroscopy. We have found that prereduction of the catalyst in hydrogen at temperatures below 700 °C does not reduce the cobalt ions to metallic cobalt, but removes hydroxyl groups and oxygen ions creating oxygen vacancies and/or a partially reduced cobalt species. The prereduction treatment, however, does increase the density of electrons at the Fermi level weakening the interaction of Co2+ with the silica framework. Subsequent exposure of the catalyst to CO at 750 °C causes CO to strongly interact with the cobalt clusters most likely by the transfer of electrons into the d orbitals of Co. This strong interaction makes the cobalt more mobile at the surface and allows it to nucleate into clusters capable of dissociating CO and initiating the growth of SWNT. Prior to exposure to CO, the reduced cobalt species strongly interacting with the silica framework do not nucleate into larger clusters in the presence of He or H2, preserving near atomic cobalt dispersion, as determined by EXAFS.
09/2004;
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ABSTRACT: We report here the first synthesis of pure boron single-wall nanotubes by reaction of BCl3 with H2 over an Mg−MCM-41 catalyst with parallel, uniform diameter (36 ± 1 Å) cylindrical pores. The composition of the tubular structures observed in TEM was confirmed by electron energy loss spectroscopy, and the tubular geometry was confirmed by the presence of the characteristic spectral features in the Raman breathing mode region.
03/2004;
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ABSTRACT: Single-walled carbon nanotubes of uniform diameter were grown in cobalt-substituted MCM-41 molecular sieves templated with C12 and C16 alkyl chains to result in pore diameters of 2.6 and 3.3 nm, respectively. The narrow diameter distribution of the tubes grown was probed by Raman, UV−visible, and NIR spectroscopy, as well as by high-resolution transmission electron microscopy. Tube diameters have been observed to vary with the size of the Co clusters formed during carbon deposition, as measured by extended X-ray absorption fine structure (EXAFS). It is proposed that the diameter of the carbon nanotubes grown in MCM-41 catalysts is controlled by the size of the metallic clusters formed in the template. Because MCM-41 catalysts of different pore diameter form Co clusters of different sizes, this mechanism can be exploited to grow carbon nanotubes of uniform, preselected diameters.
12/2003;
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ABSTRACT: Highly ordered cobalt substituted MCM-41 samples were synthesized and characterized for application as catalytic templates for producing aligned single walled carbon nanotubes (SWNT). Highly reproducible Co−MCM-41 samples were successfully synthesized using alkyl templates with 10, 12, 14, 16, and 18 carbon chain lengths by direct incorporation of cobalt into the siliceous MCM-41 framework using a hydrothermal method; the pore size and the pore volume can be controlled precisely. The local environment of cobalt as determined by UV−vis spectroscopy is a mixture of tetrahedral and distorted tetrahedral structures similar to those observed in Co3O4. Cobalt atoms are uniformly distributed in the pores (about 30−40/pore) at nearly atomic dispersion probed by XAFS. Incorporation of cobalt into siliceous MCM-41 improves the structure, most likely by dehydroxylation and/or knitting the defective structure of the amorphous silica polymer. The optimum crystallization temperature and time were 100 °C and 4 days for siliceous MCM-41 and 6 days for Co−MCM-41, respectively. Co−MCM-41 is very stable against reducing and oxidation conditions at temperatures under 750 °C. The catalytic templates showed over 90% selectivity to SWNT with up to 4 wt % carbon yield. The growth of SWNT in the pores of Co−MCM-41 was confirmed by Raman spectroscopy and TEM. The catalytic template maintained its structure after successive reaction cycles, which suggests that Co−MCM-41 is a very stable template for producing SWNT under harsh reaction conditions.
09/2003;
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ABSTRACT: Temperature programmed isotopic exchange (TPIE) of oxygen and pulsed methane combustion experiments were used to investigate the contribution of oxygen from the support in the methane combustion reaction over alumina- and zirconia-supported palladium. TPIE experiments show that PdO is more active than reduced palladium for oxygen exchange between the catalyst and gas phase. Experiments performed on initially reduced catalysts produced a partially oxidized palladium phase containing both oxygen isotopes, suggesting that oxygen exchange between solid phases proceeds simultaneously with Pd oxidation. Reactivity tests performed with labeled reaction mixture pulses showed an important contribution of the support in the overall oxygen balance. Adsorbed oxygen is much more rapidly exchanged on zirconia-supported Pd than on alumina-supported Pd, and the isotopic exchange between the gas phase and the catalyst is strongly accelerated in the presence of methane. This is assigned to an increase in the concentration of oxygen vacancies in the support via oxidation of reaction intermediates on the support.
03/2002;
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ABSTRACT: Transition metal incorporated MCM-41 materials were synthesized by careful control of multiple factors affecting the final product quality. Among them, the pH adjustment of the initial synthesis solution of Co-MCM-41 was crucial for the distribution of metallic ions in the silica framework. Higher pH significantly improved the physical structure of Co-MCM-41 and is accompanied by higher porosity. Each pore of Co-MCM-41, at the optimum pH, exhibited a hexagonal pore shaped entrance, not just the usual hexagonal array of pores of approximate circular shape. A mild acid treatment of Co-MCM-41 was a useful technique to remove surface metal oxide, which causes lowering of reduction stability by hydrogen spillover on the surface.
Microporous and Mesoporous Materials 101:200-206. · 3.29 Impact Factor
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ABSTRACT: The activity of ceria-zirconia supported palladium catalysts for methane combustion has been found to be strongly dependent on the oxygen content of the PdO catalyst particles. Reaction mixture pulses of 1% methane, 4% oxygen and helium balance were injected over partially reduced catalysts obtained by chemical reduction with methane, and over completely reduced catalysts produced by thermal decomposition of the PdO phase. Catalyst activity was observed to initially increase with the degree of reduction, reaching a maximum and then decreasing continuously as the oxygen is depleted. The degree of chemical reduction is temperature dependent and, at high temperatures, CO production is associated with extraction of subsurface oxygen.Reoxidation of the completely reduced catalyst is slow and strongly inhibited, while the partially reduced catalysts reoxidize at higher rate. However, if a completely reduced layer forms on top of the oxide core, the reoxidation inhibition phenomenon is present and the reoxidation rate considerably decreases. At low temperature reaction conditions oxygen exchange from the bulk can be faster than reoxidation from the gas phase.
Applied Catalysis A: General. 218:197-209.
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ABSTRACT: The effect of the support (γ-alumina and zirconia) and of the presence of water on the catalytic activity of palladium particles was investigated both on oxidized and reduced states using temperature programmed reaction (TPR). The support was found to influence the onset and the rate of the reoxidation process. The particle size of the palladium phase, however, strongly influenced the reduction temperature and the activity decay and recovery during TPR experiments. In the metallic form, the catalyst is not affected by the water presence; in oxide state we found that, independent of the support, the inhibitory effect of the water molecules is stronger at lower temperatures. Consequently, the reaction may take place in two different regimes: one in which the rate limiting step is water desorption from the catalyst surface, which is favored by low temperatures (<500°C) and high conversions and the reaction order with respect to water is approximately −1, and the second, at higher temperatures, in which the rate limiting step is the methane activation. For the last regime, when working with dry feed, the reaction order with respect to water becomes 0. An important observation is that the water poisoning effect is only slowly reversible, so TPR experiments predict water poisoning to be important to much higher temperature than pulse experiments.
Applied Catalysis A: General.
