Preparation, Characterization and Catalytic Performance of Carbon Nanotubes Promoted Ni-B Amorphous Alloy

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1 Journal of the Chinese Chemical Society, 2007, 54, Preparation, Characterization and Catalytic Performance of Carbon Nanotubes Promoted Ni-B Amorphous Alloy Chang-Yuan Hu a,b, * ( ), Xiao-Ning Liao c ( ), Feng-Yi Li b ( ), Rong-Bin Zhang b ( ), Rong-Fa Zhang a ( ) and Ting-Zhi Liu a ( ) a Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science and Techology Normal University, Nanchang, , P. R. China b Department of Chemistry, Nanchang University, Nanchang , P. R. China c College of Science, Jiangxi Agriculture University, Nanchang , P. R. China Ni-B and Ni-B/CNTs amorphous alloy catalysts were prepared by chemical reduction and impregnation-chemical reduction methods, respectively, and characterized by TEM, ICP, XPS, XRD, BET and CO chemisorption techniques. Their catalytic activities were evaluated in acetylene selective hydrogenation reaction. Based on characterizations, the effects of carbon nanotubes on Ni-B amorphous alloy were attributed to both its structure effect, dispersing Ni-B particles, leading to bigger surface area of active nickel and enhancing the thermal stability, and the electronic effect, resulting in electron-rich nickel centers. Therefore, the superior thermal stability and acetylene selective hydrogenation activities of Ni- B/CNTs to Ni-B amorphous catalyst were obtained in the present study. Keywords: CNTs; Ni-B amorphous alloy catalyst; Acetylene selective hydrogenation; Thermal stability. INTRODUCTION As promising novel catalytic materials, Ni-B amorphous alloys have attracted much attention. 1-2 Their unique isotropic structure and high concentration of coordinatively unsaturated sites lead to superior catalytic activity and selectivity to their crystalline counterparts. 2-3 However, amorphous alloys show low thermal stability, which results from the high surface energy of ultrafine amorphous alloy particles, and thus limit their practical applications. 4 It has been reported that depositing amorphous alloys on supports such as silica, 5 alumina, 6 and bentonite 7 is a useful way to improve their thermal stability and catalytic activity. Since the discovery and large-scale synthesis of carbon nanotubes, considerable attempts have been concentrated on their potential applications in many aspects, such as composites, field emission, fuel cells, hydrogen storage, catalyst support, etc. 8 As a new class of advanced materials for catalytic applications, carbon nanotubes used as catalyst supports induce peculiar activity and selectivity in catalytic reactions. 9 Up to now, however, little attention has been paid to carbon nanotube supported amorphous alloy catalysts How to prepare well-dispersed Ni-B amorphous nanoparticles on nanotubes still remains to be investigated. The reason may be as follows. On one hand, CNTs are a persistent van der Waals aggregation of individual nanotubes into nanotubes bundles, which are poorly dispersed in solvent. On the other hand, the surface of nanotubes has hydrophobic properties. Hence, dispersing nanotubes in solvent and increasing hydrophilic properties are necessary for preparing well-dispersed Ni-B amorphous nanoparticles on nanotubes. In the present work, Triton x was applied to functionalize the surface of ammonia-treated CNTs, 13 for preparing high dispersion and homogeneous Ni-B particles on the nanotubes. And then the effects of nanotubes on dispersion, thermal stability, and acetylene selective hydrogenation of Ni-B amorphous alloy catalyst have been systematically studied. EXPERIMENTAL Treatment of CNTs and Preparation of Ni-B Catalysts CNTs were synthesized by catalytic decomposition of * Corresponding author. Tel: ; hcychem@hotmail.com

2 1472 J. Chin. Chem. Soc., Vol. 54, No. 6, 2007 Hu et al. CH 4 on the Ni-Cu-Al catalyst following our previous work. 14 After the heat treatment of CNTs with ammonia, 13 Triton x-100 adsorption on CNTs was conducted through several processes according to the literature. 11 Ni-B/CNTs amorphous alloy catalyst was prepared by impregnationchemical reduction method (the theoretical nickel loading is 25 percent). 10 Ni-B amorphous alloy catalyst was prepared by chemical reduction elsewhere. 2 Characterization of Catalyst The morphology of the catalysts was determined by transmission electron microscopy (H-600, Hitachi, Japan). The Ni loading and bulk compositions of samples were analyzed by inductively coupled plasma (ICP). The total surface area (S BET ) was measured by N 2 adsorptionat77k. The surface area of active nickel (S Ni ) was determined by carbon monoxide chemisorption. 15 The XPS measurements were made on a KRATOS Analytical AXISHSi spectrometer with a monochromatized Al K X-ray source ( ev photons). The binding energy scale was calibrated by the Au 4f 7/2 peak at 83.9 ev as well as Cu 2p 3/2 peak at 76.5 and ev. The XRD patterns of the samples were obtained via a Rigaku Automatic diffractometer (D/MAX- RA, Japan). Selective Hydrogenation 100 mg catalyst was loaded into the micro-reactor and reduced in situ at 493 K by H 2 for 30 minutes. After that, the gaseous reactant (C 2 H 2 :H 2 :N 2 = 2:4:94, V/V) flowed into the micro-reactor at a rate of 50 ml/min. The reactant and products were determined with an online gas chromatograph equipped with TCD, using a Porapak N column. As a consequence, the acetylene conversion and ethylene selectivity were calculated as follows: 0 0 X =(C C2H2 C C2H2 )/C C H = C /(C 0 C2 H 4 C 2 H 2 C C H )% %, S C H HereC C2H2 is the initial concentration of acetylene in the reactant mixture, C C2H2 and C C2H4 the concentration of acetylene and ethylene in the product, respectively. RESULTS AND DISCUSSION Characterization of Ni-B and Ni-B/CNTs catalysts TEM was performed to observe micrographs and distributions of Ni-B amorphous particles. Due to the extremely high specific surface energy of the prepared ultrafine Ni-B amorphous particles, Ni-B alloys are inclined to self-aggregation for decreasing their surface energy. 16 Therefore, as shown in Fig. 1a, most Ni-B alloys were in the form of high agglomeration except for a few separated particles. For comparison, the mean size of 10 nm Ni-B amorphous particles were well dispersed on the outside of individual nanotubes treated by ammonia and Triton x-100, as can be seen from Fig. 1b. This implies that nanotubes promote the dispersion of Ni-B alloy effectively, which can be verified further by the S BET and S Ni in Table 1. This action can be called a structure effect. Fig. 1. TEM pictures of catalysts: a Ni-B, b Ni-B/CNTs.

3 Preparation, Characterization and Catalytic Performance J. Chin. Chem. Soc., Vol. 54, No. 6, Table 1. Some characteristics of the samples determined by ICP, BET and CO chemisorption Samples Ni-Content (wt%) Composition (molar ration) S BET (m 2 /g) SNi (m 2 /g) CNTs 2.2* * Ni-B Ni B Ni-B/CNTs 18.05** Ni B ** * Value of Ni residue in CNTs, ** Value subtracted from Ni residue in CNTs. Bulk composition, Ni-content, total surface area and active surface area of the fresh catalysts are listed in Table 1. The S BET and S Ni for Ni-B amorphous alloy catalyst were 18.3 m 2 /g and 3.5 m 2 /g, respectively. The values of S BET and S Ni, however, increased to 93.3 m 2 /g and 29.9 m 2 /g after Ni-B catalyst was supported on nanotubes. Clearly, the introduction of a nanotube carrier enhances the dispersion of Ni-B particles, which is in good agreement with the results of TEM. According to the result of composition, the Ni/B molar ratio of Ni-B was smaller than that of Ni-B/CNTs, indicating that the support also influences the composition of the amorphous alloy. As shown in Fig. 2, the XRD patterns indicate the graphite structure of prepared CNTs. From Fig. 2, cubic nickel was present in the CNTs sample after it was treated with ammonia. The nickel could be assigned to the Ni residue from the Ni-Cu-Al catalyst used for synthesizing CNTs. It seems that nickel residue in CNTs is difficult to remove by heat treatment with ammonia. Compared with the XRD spectra of CNTs, spectra of Ni-B/CNTs did not exhibit other crystalline peaks. This implies that the Ni-B/CNTs are of an amorphous structure catalyst. 10 Additionally, a visible decrease of graphite and cubic nickel peaks was found in the XRD spectra of Ni-B/CNTs, which mainly resulted from the interactions between the nanotubes and Ni-B amorphous alloys. 17 Table 2 shows XPS results of Ni-B and Ni-B/CNTs catalysts. Peaks at and ev were observed for the CNTs carrier. The peak at ev can be attributed to the characteristic peak of a C-C sp 2 hybrid bond. 18 The other peak at ev corresponds to the peak of C-N- H 18 or C-C sp 3 hybrid bond. 19 In comparison with the standard binding energy (BE) of a C-C sp 2 hybrid, however, the BE of ev shifted positively by 0.3~0.45 ev, 18 showing partial electron transfer from carbon nanotubes to Ni in the Ni-B/CNTs amorphous alloy Concerning the XPS spectra in Ni 2p3/2 level, it was found that nickel species were present both in metallic state, whose BE at ev (Ni- B), ev (Ni-B/CNTs) and in oxidized state, whose BE at ev (Ni-B), ev (Ni-B/CNTs), respectively. 22 Compared with Ni-B, the BE of metallic nickel in Ni-B/CNTs amorphous alloy catalyst shifted by ev, implying higher electron density of nickel centers. These results indicate that metallic Ni accepts electrons not only from B 2 but also from nanotubes in the preparation of Ni- B/CNTs catalyst, making Ni electron-enriched. The thermal stability of Ni-B amorphous alloy is essential for practical applications. Fig. 3 shows the XRD spectra of Ni-B and Ni-B/CNTs catalysts with and without Fig. 2. Powder X-ray diffraction patterns of samples. a: Ni-B, b: Ni-B/CNTs, c: CNTs. Table 2. Results of XPS for Ni-B and Ni-B/CNTs catalyst Sample BE (ev) C-C(sp 2 ) [18] Ni-B Ni-B/CNTs E(eV) C 1s ~ ~0.45 Ni 2p3/ E: Chemical shift

4 1474 J. Chin. Chem. Soc., Vol. 54, No. 6, 2007 Hu et al. heat treatment under a N 2 flow for 1 h. For a Ni-B catalyst, after heat treatment at 573 K, Ni(111) and Ni(200) peaks appeared in the XRD spectra, indicative of the occurrence of crystallization of the amorphous alloy. At higher temperatures, Ni 3 BandNi 2 B diffraction peaks appeared and the intensity of Ni(111) and Ni(200) increased significantly, as shown in Figs.3c-d. Comparing Fig. 3c and Fig. 3d, it is found that there is nearly no difference in the two spectra. Therefore, it is concluded that Ni-B amorphous alloy is completely crystallized at 773 K. For Ni-B/CNTs catalyst, no significant effect on the XRD patterns of the heat treatment was observed as long as the treating temperature was below 573 K. However, when the Ni-B/CNTs sample was treated at elevated temperatures above 573 K, the original broad peak disappeared and diffraction peaks corresponding to Ni(111) and Ni(200) appeared gradually, showing that the crystallization of the Ni-B/CNTs amorphous alloy occurred at high temperature. The crystallization degree increased with the increase of the treating temperature and reached completion at 873 K. Therefore, the crystallization temperature of Ni-B/CNTs is at least 100 degrees higher than that of Ni-B. The above results exhibit the stabilizing effect of nanotubes on the Ni-B amorphous structure. It is understood that nanotubes disperse Ni-B particles well (as seen in Fig. 1) and thus inhibit the aggregation of Ni-B particles by their interaction with Ni-B alloys, while aggregation is the prerequisite for crystallization. Catalytic performance 100 mg Ni-B and Ni-B/CNTs were used to investigate the acetylene selective hydrogenation reaction. The amount of nickel from 100 mg Ni-B is much larger than that from 100 mg Ni-B/CNTs, which can be seen from the ICP results. However, Ni-B/CNTs show higher catalytic activity and C 2 H 4 selectivity in acetylene selective hydrogenation. It seems that the amount of nickel in the catalysts isn t the predominant element affecting the hydrogenation performance of Ni-B amorphous alloys. As a control experiment, acetylene hydrogenation didn t take place on the CNTs carrier without loading Ni-B amorphous alloy in the temperature range 343 K to 383 K. The nickel residue in CNTs shows deactivation at a relatively low temperature. Concerning the activity enhancement of Ni-B/CNTs, it can be understood as follows. It is well known that active nickel species are the active sites of hydrogenation. The bigger the surface area of active nickel, Fig. 3. XRD patterns of Ni-B and Ni-B/CNTs amorphous alloy catalysts treated at different temperatures. (1) Ni-B, (2) Ni-B/CNTs (a: fresh sample, b: 573 K, c: 773 K, d: 873 K, N 2,1h). Fig. 4. C 2 H 2 conversion and C 2 H 4 selectivity of Ni-B and Ni-B/CNTs.

5 Preparation, Characterization and Catalytic Performance J. Chin. Chem. Soc., Vol. 54, No. 6, the higher the catalytic activity. It was observed that the active nickel surface area of Ni-B increased by nearly 8-fold after the introduction of nanotubes to Ni-B catalyst (seen in Table 1). The result indicates that carbon nanotubes are helpful to disperse nickel particles and then promote hydrogenation activity. From thermal stability analysis, welldispersed Ni-B particles along nanotubes do not like to aggregate each other because of the interactions between Ni-B particles and nanotubes and then the Ni-B/CNTs amorphous alloy is difficult to be crystallized by heat treatment. Thus, more Ni-B/CNTs alloys will maintain an amorphous alloy state during the reaction process. Undoubtedly, this favors the higher activity of Ni-B/CNTs since amorphous alloy catalysts show better activity than their crystalline counterparts. 7 All these changes are beneficial for improving the catalytic activity of acetylene hydrogenation. To understand the selectivity improvement of Ni-B/ CNTs catalysts over a wide range of comparative conversions, it is necessary to consider the reaction mechanism for acetylene hydrogenation. In acetylene hydrogenation, the reaction proceeds via three major routes according to previous studies Route one is the hydrogenation of acetylene to ethylene followed by desorption from the surface or further hydrogenation. Route two is the hydrogenation of acetylene to intermediates, such as ethylidyne, which do not yield ethylene but are directly hydrogenated to ethane. Route three is the polymerization of ethylene to produce polymer that eventually covers the surface leading to the catalyst deactivation. Considering the reaction routes of acetylene hydrogenation, we may explain the boost of selectivity as follows. As discussed in the TEM section, well-dispersed Ni-B amorphous nanoparticles are formed on CNTs treated by ammonia and Triton x-100. This indicates that Ni-B spreads highly on the surface of nanotubes and the existence of multiple coordinated sites of nickel is greatly reduced. The well-dispersed Ni-B on nanotubes suppresses routes two and three so that the production of ethane and polymer is decreased and the ethylene selectivity is improved. 23,24,26 The electronic interactions between boron, nanotubes and nickel, that boron and nanotubes transfer electrons to nickel in a Ni-B/CNTs amorphous catalyst, on the other hand, results in electron-rich metallic nickel. This facilitates desorption of ethylene produced on the catalyst surface by route one Consequently, the higher ethylene selectivity is obtained on the Ni-B/CNTs catalysts. Therefore, the combination of structure and electronic effect of nanotubes resulted in a better catalytic performance of Ni-B/CNTs amorphous alloy in the selective acetylene hydrogenation. CONCLUSION Compared to the Ni-B amorphous alloy catalyst, carbon nanotube supported Ni-B amorphous alloy leads to bigger total surface area and surface area of active nickel, and the improvement of thermal stability as well as better activity and higher ethylene selectivity in selective acetylene hydrogenation. The effects of nanotubes were attributed to both its structural effect, dispersing Ni-B particles, and its electronic effect, resulting in electron-rich nickel. ACKNOWLEDGMENTS This project was financially supported by the National Natural Science Foundation of China (No ) and the Natural Science Fund of Jiangxi Province in China (No ). Received April 20, REFERENCES 1. Guo, H.-B.; Li, H.-X.; Zhu, J.; Ye, W.-H.; Qiao, M.-H.; Dai, W. L. J. Mol. Catal. A: Chem. 2003, 200, Li,H.-X.;Li,H.;Dai,W.-L.;Qiao,M.-H. Appl. Catal. A: Gen. 2003, 238, Wang, M.-H.; Li, H.-X.; Wu, Y.-D.; Zhang, J. Mater. Lett. 2003, 57, Li, H.-X.; Wu, Y.-D.; Luo, H.-S.; Wang, M.-H.; Xu, Y. P. J. Catal. 2003, 214, Chen, X.-F.; Li, H.-X.; Dai, W.-L.; Wang, J.; Ran, Y.; Qiao, M.-H. Appl. Catal. A: Gen. 2003, 253, Hou, Y.-J.; Wang, Y.-Q.; He, F.; Mi, W.-L.; Li, Z.-H.; Mi, Z.-T.; Wu, W.; Min, E.-Z. Appl. Catal. A: Gen. 2004, 259, Zhang, R.-B.; Li, F.-Y.; Zhang, N.; Shi, Q. J. Appl. Catal. A: Gen. 2003, 239, Ayajan, P.-M. Chem. Rev. 1999, 99, Nhut, J.-M.; Pesant, L.; Tessonnier, J.-P.; Wine, G..; Guille, J.; Pham-Huu, C.; Ledoux, M.-J. Appl. Catal. A: Gen. 2003, 54, Xu, S.-X.; Li, F.-Y.; Wei, R.-Z. Carbon. 2005, 43, 861.

6 1476 J. Chin. Chem. Soc., Vol. 54, No. 6, 2007 Hu et al. 11. Hu, C.-Y.; Li, F.-Y.; Zhang, R.-B.; Liao, X.-N.; Hua, L. J. Chin. Chem. Soc. 2007, 54(2), Shim, M.; Kam, N.-W.-S.; Chen, R.-J.; Li, Y.-M.; Dai, H.-J. Nano. Lett. 2002, 2, Jiang, L.-Q.; Gao, L. Carbon. 2003, 41, Ju,Y.;Li,F.-Y.;Wei,R.-Z. J. Serb. Chem. Soc. 2005, 70, Xie, S.-H.; Li, H.-X.; Li, H.; Deng, J. F. Appl. Catal. A: Gen. 1999, 189, Dai, W.-L.; Qiao, M.-H.; Deng, J. F. Appl. Sur. Sci. 1997, 120, Huang, Y.; Lin, S.; Tong, Y. J.; Chen, S. Z. J. Fujian Teachers Univ. 1998, 14, Pkpalugo, T.-I.-T; Papakonstantinou, P; Murphy, H. Carbon, 2005, 43, Paloniemi, H.; Aaritao, T.; Laiho, T. J. Phys. Chem. B 2005, 109, Wang, M.-W.; Li, F.-Y.; Zhang, R.-B. Catal. Today. 2004, 93-95, Menon, M.; Andriotis, A.-N; Froudakis, G.-E. Chem. Phys. Lett. 2000, 320, Li,J.;Qiao,M.H.;Deng,J.F. J. Mol. Catal. A: Chem. 2001, 169, Kang, J.-H.; Shin, E.-W.; Kim, W.-J.; Park, J.-D.; Moon, S.-H. Catal. Today. 2000, 63, Kang, J.-H.; Shin, E.-W.; Kim, W.-J.; Park, J.-D.; Moon, S.-H. J. Catal. 2002, 208, Ngamsom, B.; Bogdanchikova, N.; Borja, M. A.; Praserthdam, P. Catal. Commun. 2004, 243, Kim, W. J.; Kang, J. H.; Ahn, I.-Y.; Moon, S. H. Appl. Catal. A: Gen. 2004, 268, 77.