Micro−Nano Hierarchal Web of Activated Carbon Fibers for Catalytic

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Micro-Nano Hierarchal Web of Activated Carbon Fibers for Catalytic Gas Adsorption and Reaction Riju Mohan Singhal, Ashutosh Sharma, and Nishith Verma* Department of Chemical Engineering and DST Unit on Nanoscience, Indian Institute of Technology Kanpur, Kanpur-208016, India

A hierarchal web of activated carbon fibers is synthesized in which carbon nanofibers (CNF) are grown on micro activated carbon fibers (ACF) used as a substrate. A bed of ACF was first impregnated with nickel nitrate hexahydrate which was subsequently reduced to metallic nickel. Catalytic chemical vapor deposition (CVD) was then carried out at 1023 K using benzene as carbon source, which resulted in the growth of CNF. Conditions of hierarchal web formation were optimized by employing two different types of reactorssa perforated tube reactor with radially outward flow of gases and a perforated disc reactor providing a parallel flow. Both of these configurations were used for carrying out reduction and CVD. Our results show that relatively larger amount and uniformity of CNF could be obtained using the perforated disc reactor. Various analytical techniques including atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed to characterize the carbon web. The resulting ACF web is a hierarchical structure, which was tested for its NO removal capability by reduction. We found these carbon structures to be more efficient compared to the original ACF or ACF impregnated with metal. 1. Introduction The utility of carbon nanofibers (CNF) as reinforcing agents in several matrixes including polymers, metals, and also brittle materials such as carbon and ceramics is highly acclaimed. Composites reinforced with CNF are shown to exhibit improvement in mechanical strength, electrical conductivity, or thermal stability over the original material.1-4 Carbon nanotubes (CNT) and CNF have also been reported to be promising candidates for hydrogen uptake. In this context, a number of investigations related to hydrogen adsorption capacities by CNT and CNF have been reported.5-10 The metal catalyst residing inside CNT or CNF pores was found to have a significant influence on the hydrogen adsorption, and it was shown that metal containing CNT and CNF displayed a higher hydrogen storage.8-10 Our previous experimental investigations considered potential applications of activated carbon fibers (ACF) and metalimpregnated ACF in the removal of a number of atmospheric air pollutants, such as volatile organic compounds, sulfur dioxide, and NO, by adsorption and/or catalytic effects.11-14 In this study we present synthesis of a hierarchical micro-nano carbon web in which CNF are grown directly on ACF (used here as a substrate). Further, we have explored two different reactor configurations with a view toward the production of superior webs. Finally, we characterize the web morphology and study its catalytic removal characteristics for nitric oxide (NO). The catalytic removal performance of the hierarchal web is compared to that of its parent materialssvirgin ACF and ACF impregnated with metal catalyst. A literature survey reveals that the commonly practiced route to producing CNF through catalytic chemical vapor deposition (CVD) generally involves catalyst supports like metal oxides or zeolites, which subsequently need to be removed for use in certain end applications.15,16 The removal is often a tedious and expensive task which is desired to be omitted without reducing the applicability of CNF. It is therefore useful to grow carbon * To whom correspondence should be addressed. Tel.: +91-5122597704. Fax: +91-512-2590104. E-mail: [email protected].

Figure 1. Tubular reactor used for catalyst impregnation and CVD.

nanostructures on ACF directly, as it would allow them to be used without any further postsynthesis processing in applications such as electrodes for fuel cells or supercapacitors. Moreover, an increase in the adsorption efficiency of the resulting ACF web may also be expected and is demonstrated in this work. In essence, the developed structure is a nanostructure (CNF)microstructure (ACF) integrated material. As an example, we show this hierarchical structure to be highly efficient in reactions involving adsorption-reduction of NO. The structure comprising ACF impregnated with metallic catalyst and decorated with CNF is thus found to be more efficient than untreated ACF and ACF impregnated with metallic catalyst. Further, two different types of reactors were designed and used for the reduction of metal oxide and CVD in this study. The first one

10.1021/ie071114n CCC: $40.75 © 2008 American Chemical Society Published on Web 01/18/2008

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Figure 2. Experimental setup for CVD.

is a perforated tube reactor which allows radial flow of gases through it. The second is a perforated disc reactor through which gas flow is parallel. These two reactors were compared for uniformity and quantity (of product) in the CNF growth. The latter reactor was observed to yield relatively more uniform CNF growth with higher quantities. 2. Experimental Section 2.1. Carbonization and Activation of Raw Carbon Fibers. The raw nonactivated carbon fibers samples based on phenolic resin precursor were procured from Nippon Kynol Inc., Japan. The ACF were prepared first by carbonizing the nonactivated fibers in an inert atmosphere and then by activation in the presence of an oxidizing agent (CO2 or steam). For carbonization, the raw samples were pyrolyzed at different temperatures between 773 and 1273 K. The samples were allowed to be heated up to the desired carbonization temperature at a constant rate of ∼3 K/min under a small N2 gas flow rate (0.3 slpm). Once the required temperature was achieved, the samples were further heated for 30-120 min. Following carbonization, activation was carried out for 30-120 min using either CO2 or steam as an oxidizing agent. During activation, the temperature and the flow rate of the oxidizing medium were maintained the same as during carbonization. After activation, the samples were allowed to cool slowly down to the room temperature at a rate of 3 K/min under a small N2 flow rate (0.3 slpm). Further details of the experimental setup used in carbonization and activation of the microfibers may be obtained in another study.12 In the same study, we also reported the typical characteristics of the ACF due to carbonization and activation including the BET area of the various samples, pore size distribution (PSD), and the surface oxygen group. Briefly, the BET areas of the activated samples were in the range of 1200-1600 m2/g and most of the pores were found to be in the micropore and mesopore range. The large amount of micropores and mesopores available in the ACF is primarily responsible for the uniform dispersion of the metal catalyst nanoparticles to provide active sites for the growth of the nanofibers. 2.2. Catalyst Preparation. The catalyst supported on ACF was prepared by impregnating the activated fabrics with nickel nitrate hexahydrate solution of concentrations ranging from 0.2 to 1 M with acetone as solvent under flow conditions. It has been shown that much better results of CNF growth are obtained when acetone rather than water is used as a solvent for metal nitrates owing to the hydrophobic nature of ACF.17 The use of acetone as solvent leads to a better dispersion of nickel nitrate compared to that obtained with water. The ACF were wrapped

Figure 3. Newly designed perforated disc reactor with parallel gas flow.

around a perforated tube (Figure 1) which was in turn fitted into another cylindrical shell. Nickel nitrate solution was passed through the tube and continuously recycled using a peristaltic pump. This process was carried out for 6 h. We observed that this method of impregnation of catalyst is more efficient in uniformly dispersing nickel nitrate in the micropores of ACF as compared to other traditional methods like impregnation by continuously stirring the ACF in salt solution or leaving ACF in salt solution under batch conditions. The impregnated ACF fabrics were dried in an oven for 6 h (at 40 °C), and the metal salt was subsequently decomposed in a calcination step at 573 K. The resulting metal oxide on ACF was then reduced to the corresponding metallic state by treating the loaded ACF with 0.16 slpm of hydrogen at 673 K for 1 h. For carrying out reduction, a perforated inconel-made tubular reactor (similar to the one used for impregnation) was fitted inside a vertical quartz tube (Figure 2). The inconel tube had holes that are uniformly distributed in rows around the tube surface. The ACF fabrics were wrapped around the perforated reactor (L ) 25 mm, i.d. ) 5 mm), and hydrogen flowed out radially through the perforations (1 mm holes). In a modification to the aforementioned reactor shape, another reactor was fabricated (Figure 3) in which the flow of gases was parallel (instead of radial). Here, a perforated disc of 20 mm diameter having uniformly distributed perforations (1 mm hole) was attached to one end of a threaded conical inconel shell. Gas inlet was provided at the opposite end of the shell. The advantage of using the latter reactor is discussed in the subsequent sections. The entire assembly (the reactor and the quartz tube) was heated using an electric furnace. The constant heating zone of the furnace ensured uniform temperature around the reactor holding the ACF. The spent gas exited from the bottom of the quartz reactor to the vent. 2.3. CVD of Benzene. After carrying out reduction, nitrogen was bubbled through benzene at 280 K and passed through the reactor packed with metal-impregnated ACF. A Freon refrigeration unit was used for controlling the temperature of the bubbler, thereby setting the desired saturation vapor pressure of the liquid

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Figure 4. SEM images of ACF for (a) 0.2, (b) 0.7, (c) 0.4, and (d) 1 M [Ni2+] after calcination and (e) grown CNF after CVD for 0.4 M catalyst impregnation.

benzene and concentration of the benzene vapor in the carrier gas. Benzene-saturated nitrogen was passed through the reactor maintained at a temperature of 1023 K. This resulted in the decomposition of benzene at the active catalyst sites to carbon and subsequent formation and growth of CNF. The CVD of benzene was continued for 1 h after which the system was allowed to cool down to the room temperature while being purged with nitrogen.

2.4. Reduction of NO. The NO reduction experiments were carried out on the grown hierarchical CNF web, and the results were compared to those obtained on the metal-impregnated ACF and virgin ACF alone. The reaction was carried out in a vertical stainless steel (SS)-made tubular reactor (L ) 150 mm, i.d. ) 14 mm). Briefly, two individual streams of gases, NO and N2, were mixed together in an SS-made tubular gas mixer. The concentrations of incoming NO and N2 to the gas mixer (L )

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Figure 5. General growth mechanism for CNF.

135 mm, i.d. ) 25 mm) were controlled by adjusting the flow rates of the individual streams with the aid of mass flow controllers (model PSFIC-I, Bronkhorst, The Netherlands) installed in each stream. The gas flow rates were adjusted to obtain the inlet NO concentration of 450 ppm in N2, and the mixture was delivered at a constant flow rate to the reactor packed with carbon fibers. The reactor was heated using a heating tape (1000 W) to 500 °C, and the temperature of the reactor was controlled with the aid of a PID temperature controller (model PXZ-4, Fuji Electric Co., Japan). A dedicated chemiluminescence NOx analyzer (42 C model Thermo Electron Co., U.S.A.) was used to measure the concentration of NO. 3. Results and Discussion 3.1. CNF Growth and Structure. The samples generated after CVD were examined by scanning electron microscopy (SEM). Figure 4a-d shows SEM images of ACF postcalcination (and before CVD), and Figure 4e shows an SEM image of CNF grown on ACF following CVD. As observed in Figure 4, parts a and c, the impregnation of Ni catalyst is uniform on the fibers under the metal concentrations used in the study, that is, 0.2 and 0.4 M, respectively. We have consistently observed that if the concentration of the nickel salt solution was in the vicinity of 1 M or higher, there was precipitation of excess metal salts on the surfaces of ACF as also observed in Figure 4, parts b and d. As observed in Figure 4e, the growth of CNF occurs everywhere on the ACF corresponding to 0.4 M of Ni salts. The internal structure of the grown CNF was observed with the aid of a transmission electron microscope (TEM). The specimens for the TEM analysis were prepared by dispersing the samples in acetone under ultrasonic wave input at room temperature. For this, a drop of the suspension was dripped onto a copper microgrid covered with a holey carbon thin film. Earlier studies have shown that CNF are produced as a result of decomposition of hydrocarbons on the surface of transition metal nanoparticles. During this reaction, the hydrocarbon first adsorbs and then decomposes on the surface of the metal particle. The resulting carbon atoms then dissolve in and diffuse through the metal particle. The precipitation of carbon from the saturated metal particle leads to the formation of CNF.18,19 The rationale for choosing these metals as catalyst for CVD growth of CNF lies in the phase diagrams for the metals and carbon. At high temperatures, carbon has finite solubility in these metals, which leads to the formation of metal-carbon solutions and, therefore, the aforementioned growth mechanism. In the majority of the cases, the catalyst particle is carried away from the surface of the support, and there is sufficient evidence to suggest that the CNF adhere very strongly to the support. The general CNF growth mechanism is shown in Figure 5. Typical representative examples of TEM images for carbon fibers deposited from benzene for 0.7 M catalyst concentration are shown in Figure 6. Images a and b in Figure 6 indicate a

cluster of newly formed CNF and nickel particles on ACF fabric support. The nickel particles catalyzing the benzene decomposition are present on the tips of CNF. It is clear that the diameter of the carbon fibers is roughly coincident with the size of nickel particles catalyzing the growth of fibers. In the CNF observed in Figure 6, the diameter ranges from 20 to 50 nm. Image c in Figure 6 shows a typical core-wall structure of the CNF. 3.2. Comparison of CNF Production in the Two Different Reactors. As pointed out in the preceding section on the experimental setup, two types of reactors were used in the study, one with radially outward flow of gases and the other with parallel flow. The CNF obtained using the two different reactors as mentioned in the previous section were also compared for uniformity in diameter. The diameters of about 40 nanofibers were determined using atomic force microscopy (AFM), and an average value was calculated for the samples produced using the same catalyst concentration. Figure 7, parts a and b, shows the AFM images obtained for the samples that were produced using the perforated tube and perforated disc reactors, respectively. From the AFM analysis, it was found that the average diameter of the nanofibers using the tubular reactor was 41 nm and ranged from 25 to 70 nm, whereas the average diameter was 46 nm and ranged from 32 to 58 nm in case of the disc reactor. The measurements suggest that the nanofibers produced using the disc reactor had a higher degree of uniformity compared to those produced using the tubular reactor. This was also corroborated by the SEM images taken for the various samples. Figure 8, parts a and b, shows the comparison between nanofiber yield using the perforated tube and perforated disc reactors at a [Ni2+] of 0.6 M. It can clearly be seen in these images that the growth of CNF was higher when the disc reactor was used compared to that obtained using the tubular reactor. The corresponding energy-dispersive X-ray (EDX) spectra are shown in Figure 8, parts c and d, respectively. In the case of CNF grown using the perforated tube reactor, 64% carbon by weight (wrt 36% nickel) is obtained, whereas in the case of perforated disc reactor, carbon content increases to 79.5% (wrt 20.5% nickel). Thus, a large part of this difference in carbon content must come from a higher CNF yield corresponding to the former reactor since the preparation technique was identical for both specimens. There are two main reasons why two reactors perform differently, as observed from Figures 7 and 8. There is a distinct difference between the flow patterns in the two types of reactors used for CVD in the study. As a consequence, the yield and uniformity of carbon deposition on the catalyst-impregnated ACF differed in the two reactor configurations. In the tubular reactor, the gas enters the tube from one end and flows radially outward into the shell through perforations on the tube’s walls, with the other end of the tube closed. ACF is wrapped over this tube. Since the gas flow rate is typically small, unsymmetric flow conditions exist due to channeling (or short-circuiting). Most of the flows are apparently through the first few perforations on the tube. As a result, the carbon deposition over the ACF wrapped in the vicinity of the tube entrance is more significant in comparison to that toward the end of the tube. In the second configuration (perforated disc reactor), the gas flow in the tube is axially downward into a concentric perforated disc on which the ACF is held on. As the gas enters into the space over the disc, flow is uniformly distributed over the ACF. Most of the ACF held on the disc receive the gas at the same rate since the perforations are essentially at the same horizontal level. Thus, the uniformity of deposition in the latter is superior to that in the former.

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Figure 6. TEM results showing (a and b) CNF at 0.7 M [Ni2+] and (c) the internal structure of the CNF produced with a hollow core.

Figure 7. AFM images obtained for the CNF produced at 0.4 M [Ni2+] using (a) the perforated tube reactor and (b) the perforated disc reactor.

The small diameter of the tube (5 mm in the present case) limited the number of perforations at a given cross section of the tube. Moreover, for a given quantity of ACF used the number of layers wrapped over the perforated tube was large

which resulted in variation in the gas compositions along the radial direction. However, in the case of the perforated disc reactor, a large number of perforations could easily be achieved on the disc. This way, there were only a few layers of ACF (in

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Figure 8. SEM images showing comparison between the CNF produced at 0.6 M [Ni2+] using (a) the tubular reactor and (b) the perforated disc reactor and the corresponding EDX spectra in (c) and (d), respectively.

some cases, just one). This resulted in a minimal change in gas composition across the disc and across ACF layers. In other words the ACF experienced nearly a uniform gas composition. To sum up, the difference was attributed to uniformity in both flow and concentration of the gas mixture (benzenenitrogen) in case of the perforated disc reactor compared to that in the perforated tube reactor. 3.3. Reduction of NO Using the Metal-Impregnated ACF and Micro-Nano Hierarchical Structures. Prior to testing the synthesized micro-nano hierarchal web of ACF for their NO removal capability, experiments were carried out on metalimpregnated ACF with a view to ascertaining the catalytic effects of the macrostructure alone in removal of NO. This establishes the base case to assess the performance of the micro-nano webs grown on the ACF as the substrate. Toward this end, ACF samples were impregnated with different transition metals (Cu, Ni, and Cr). The method of impregnation followed by calcination was identical to that adopted for the preparation of CNF. Figure 9 compares the catalytic activities of different transition metals (Cu, Ni, and Cr) impregnated ACF samples. As observed from the figure, the performance of Cu-dispersed ACF was superior to those of Ni- and Cr-impregnated ACF. The overall order of the catalytic activity of metal-ACF for the reduction of NO was observed to be in the following sequence: Cu- > Ni- > Cr- > ACF-without metal. The steady-state conversions were observed at ∼58%, 24%, 20%, and less than 4% for Cu-, Ni-, Cr-impregnated and untreated ACF samples, respectively. To further examine varying catalytic effects observed due to different transition metals, BET area and PSD were measured

Figure 9. Effects of types of metal on the catalytic reduction of NO by ACF (Q ) 0.1 slpm, T ) 300 °C, W ) 0.5 g).

for the ACF samples. The analysis showed that the BET area and total pore volume of ACF-Cu (1300 m2/g and 0.8 cm3/g) are much higher in comparison to those of ACF-Ni (1150 m2/g and 0.38 cm3/g) and ACF-Cr (1100 m2/g and 0.36 cm3/g). The

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Figure 11. Comparative performance of grown CNF relative to ACF for NO reduction (NO concentration ) 450 ppm, T ) 500 °C, W ) 0.5 g).

Figure 10. TDP profiles of ACF impregnated with different metals.

relatively superior performance of the Cu-ACF can therefore be attributed to large BET area and pore volume of the material. We also carried out the thermal programming desorption (TPD) analysis to characterize the oxygen surface groups of different metal-impregnated ACF samples with a view to ascertaining any correlation between the surface groups and their catalytic effects. The surface complexes desorb primarily CO and CO2 during the TPD process. The gases released consist predominantly of CO2 at low temperatures (500 °C), CO is released due to the decomposition of weak acid groups (lactone, phenol, and carbonyl groups). Figure 10 compares the TPD profiles of the ACF dispersed with Cu, Ni, and Cr oxides. It may be seen from the figure that the amounts of decomposing surface oxygen groups are different for the ACF samples with and without impregnation, suggesting that the surface functional groups have been altered or rearranged appreciably during the catalyst preparation, in particular during the calcination step. The extent of CO2-evolving groups in all the ACF samples (except in the Cr sample) is nearly the same, whereas that of CO-evolving groups is different. Sample ACF-Ni5 has a clear peak at 500 °C and another broad peak at ∼670 °C. For ACF-Cr5, there is a broad peak in the range of 600-750 °C with another shoulder peak at 850 °C. There is a similar broad peak in the range of 600-800 °C for ACFCu5. These results indicate that the sample ACF-Ni5 possesses the largest number of CO-evolving groups. The extent of COyielding groups thus has the following sequence: Ni > Cr > Cu. From the CO evolution curves, it appears that the metal impregnation enhances the formation of the weak acidic groups, which in turn adversely affects the catalytic removal of NO by reduction. From the foregoing arguments, we conclude that metalimpregnated ACF are effective in removing NO in the order Cu > Ni > Cr. However, for the growth of CNF on ACF the deposition of carbon from benzene by Cu was found to be considerably less in comparison to Ni due to weaker catalytic

activity of Cu for this particular reaction. Thus, Ni salts were used as precursors to impregnate ACF in this work for the synthesis of the CNF webs. Yet, despite this choice of the catalyst, the NO reduction by the Ni grown webs was found to be superior to the Cu-based ACF alone. Figure 11 describes the representative NO reduction data obtained for three samples, CNF on Ni-impregnated ACF, Niimpregnated ACF, and virgin ACF alone. As observed from the figure, nearly 100% conversion, defined as (Cinlet - Coutlet)/ Cinlet, of NO to N2 was obtained for more than 800 min when using CNF web, whereas in the case of Ni-impregnated ACF the concentration breakthrough occurred after approximately 300 min. As also seen, the ACF without catalyst impregnation showed very little catalytic effects with breakthrough occurring instantaneously and conversion decreasing to less than 20%. 4. Conclusions In this study, CNF were grown using an ACF substrate by taking advantage of the large surface area available for catalyst dispersion. CNF were synthesized uniformly and densely on nickel-impregnated ACF fabrics using the CVD technique in specially designed perforated tube and disc reactors. Owing to a more uniform gas composition, the perforated disc reactor was found to be superior in the production of CNF. The hierarchal micro-nano carbon composite with ACF support thus produced obviates the need for a host of postsynthesis processes because the web can be used directly as a catalytic adsorbent/ filter material. This is unlike most currently practiced techniques that require a metal oxide or zeolite as support which subsequently needs to be removed to obtain pure CNF. Pure CNF thus produced then have to be integrated with or transferred to another support for their effective use. Various analytical techniques such as AFM, TEM, and SEM were used to examine the internal structures of the grown CNF and the uniformity of growth. The average diameter was observed to be between 20 and 50 nm for most of the fibers having a crooked morphology and a three-dimensional network. The CNF were tested for their removal efficiency for NO and found to be superior to their parent materials ACF, and metal-impregnated ACF in catalytically reducing NO.

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Acknowledgment The authors thankfully acknowledge Nippon Kynol Inc., Japan for supplying the ACF samples for testing. Literature Cited (1) Hu, J. W.; Li, M. W.; Zhang, M. Q.; Xiao, D. S.; Cheng, G. S.; Rong, M. Z. Preparation of binary conductive polymer composites with very low percolation threshold by latex blending. Macromol. Rapid Commun. 2003, 24, 889. (2) Po¨tschke, P.; Fornes, T. D.; Paul, D. R. Rheological behavior of multiwalled carbon nanotube/polycarbonate composites. Polymer 2002, 43, 3247. (3) Zeng, J.; Saltysiak, B.; Johnson, W.; Schiraldi, D.; Kumar, S. Processing, structure, and properties of carbon nano fiber filled PBZT composite fiber. Composites, Part B 2004, 35, 173. (4) Higgins, B. A.; Brittain, W. J. Polycarbonate carbon nanofiber composites. Eur. Polym. J. 2005, 41, 889. (5) Ye, Y.; Ahn, C. C.; Witham, C.; Fultz, B.; Liu, J.; Rinzler, A. G.; Colbert, D.; Smith, K. A.; Smalley, R. E. Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes. Appl. Phys. Lett. 1999, 74, 2307. (6) Chambers, A.; Park, C.; Baker, R. T. K.; Rodriguez, N. M. Hydrogen storage in graphite nanofibers. J. Phys. Chem. B 1998, 122, 4253. (7) Chen, J.; Wu, F. Review of hydrogen storage in inorganic fullerenelike nanotubes. Appl. Phys. A 2004, 78, 989. (8) Hou, P. X.; Xu, S. T.; Ying, Z.; Yang, Q. H.; Liu, C.; Cheng, H. M. Hydrogen adsorption/desorption behavior of multi-walled carbon nanotubes with different diameters. Carbon 2003, 41, 2471. (9) Lueking, A.; Yang, R. T. Hydrogen spillover from a metal oxide catalyst onto carbon nanotubessimplications for hydrogen storage. J. Catal. 2002, 206, 165. (10) Browning, D. J.; Gerrard, M. L.; Lakeman, J. B.; Mellor, I. M.; Mortimer, R. J.; Turpin, M. C. Studies into the storage of hydrogen in carbon

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ReceiVed for reView August 14, 2007 ReVised manuscript receiVed November 7, 2007 Accepted November 12, 2007 IE071114N