Effect of Synthesis Parameters on the Characteristics of FeâB

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Ind. Eng. Chem. Res. 2007, 46, 2034-2041

Effect of Synthesis Parameters on the Characteristics of Fe-B Nanoalloys for Dehydrogenation of Ethanol Baskaran Rajesh, Natarajan Sasirekha, and Yu-Wen Chen* Department of Chemical and Materials Engineering, Nanocatalysis Research Center, National Central UniVersity, Chung-Li 320, Taiwan, Republic of China

Shao-Pai Lee Department of Chemical Engineering, Dai-Hwa Institute of Technology, Hsinchu 300, Taiwan, Republic of China

Fe-B amorphous nanoalloy materials have been synthesized by chemical reduction method using various iron precursors and preparation mediums. The samples have been characterized by X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, N2 sorption, transmission electron microscopy, X-ray photoelectron spectroscopy, differential scanning calorimetry, and electron diffraction. A series of studies have been performed to elucidate the influences of preparation parameters on the properties of Fe-B nanoalloys. The characterization results indicated that the amorphous nature of Fe-B materials remained up to 400 °C and the presence of boron retarded the crystallization of Fe sample. The iron precursor and preparation medium play a critical role in determining the structure, morphology, and composition of Fe-B nanoalloys. The iron precursors have a significant influence on the oxidation states of iron and boron species in Fe-B nanoalloys. The catalytic activities of Fe-B nanoalloys have been investigated by subjecting them for dehydrogenation of ethanol as a probe reaction. The results indicated that Fe72.8B27.2, prepared using FeCl3 in aqueous medium, showed high activity for dehydrogenation of ethanol owing to its high surface area and turnover frequency. 1. Introduction Amorphous alloy catalysts have attracted a great deal of attention due to their excellent activities and selectivities in many hydrogenation reactions.1,2 The classical work of Masumoto and co-workers3 on hydrogenation of carbon monoxide motivated rapid growth in amorphous alloy catalysis research. Amorphous alloys facilitate adsorption and surface reactions easier than the corresponding crystalline catalysts due to the presence of high concentrations of highly coordinated unsaturated sites on amorphous alloys. Moreover, the effects of intraparticle limitations on surface reactions are effectively eliminated owing to their nonporous nature. The physical, mechanical, and chemical properties of amorphous alloys are quite different from those of crystalline alloys with the same compositions. However, one of the important limitations is the low surface area of amorphous alloys, which limits their application as industrial catalysts. Several new methods have been developed to prepare nanosized amorphous alloy catalysts that combine the characteristics of amorphous alloys with a large disorder and nanometer materials with a small size to facilitate high surface area. Studies on ultrafine amorphous alloy particles have attracted much attention because of their interesting intrinsic properties, e.g., short-range order, long-range disorder, and high dispersion, as well as their potential applications, e.g., in powder metallurgy, magnetic materials, catalysts, and ferrofluids. The ultrafine amorphous alloy powders combine the features of amorphous and ultrafine powder and have properties that are of interest in catalysis: (i) the presence of a large number of surface coordinating unsaturated sites, (ii) the lack of crystal defects, and (iii) an isotropic, single-phase nature. Examples of ultrafine * To whom correspondence should be addressed. Tel.: (886) 3 422751, ext 34203. Fax: (886) 3 4252296. E-mail: ywchen@ cc.ncu.edu.tw.

amorphous alloy particles include M-B (M ) Fe, Co, or Ni);4-13 Ni-P;12-17 Co-P;18 Fe-P-B;19,20 Ni-P-B;19,21,22 Fe-P-B (M ) Cr, Co or Ni); Fe-Co-B;23 Fe-Ni-B;24,25 Ni-Co-B26 as well as ultrafine crystalline particles of Co, Co2B, or Co(BO2)2; and Fe, Fe2B, Mg, Al, Ni, Pd,27 and Cu. Various amorphous alloys have been prepared and employed in catalytic reactions including hydrogenation,27-31 hydrogenolysis,32 oxidation,33 and isomerization.34 The most widely used techniques for the preparation of amorphous alloy catalysts are the rapid quenching method and chemical reduction method. Each method has both advantages and disadvantages. In the chemical reduction method, the composition and hence the properties of ultrafine amorphous nanoalloys are more adjustable; in particular, the ratio of different elements is not limited to the eutectic composition of the alloy. One of the most influential methods is to prepare the amorphous alloys by chemical reduction of the metallic ions with hypophosphite (H2PO2-) or borohydride (BH4-) to form ultrafine metal boride or phosphide amorphous alloy particles, as first reported by van Wonterghem et al.35 and developed by Linderoth and Mørup et al.9,10,36 The powder samples obtained by chemical reduction are highly dispersed and can be compacted to serve different purposes. Chemical reduction ensures large surface areas of the resultant amorphous alloy catalysts; e.g., the ultrafine Ni-B amorphous alloy obtained by chemical reduction in ethanolic solution had a surface area of up to 200 m2/g. Okamoto et al.37 characterized the surface of Ni-B and Ni-P ultrafine catalysts prepared by a chemical reduction method with X-ray photoelectron spectroscopy (XPS), indicating that a variation in 3d electron density on the nickel metal induced by boron or phosphorus would modify the activity and selectivity of the nickel catalyst for hydrogenation. The objective of the present study is to achieve an understanding of the effects of preparative methods on the surface

10.1021/ie061552e CCC: $37.00 © 2007 American Chemical Society Published on Web 03/06/2007

Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 2035

Figure 3. XRD patterns of Fe65.3B34.7 upon various treatment temperatures.

Figure 1. XRD patterns of Fe-B catalysts.

Figure 4. DSC curves of Fe72.8B27.2. Figure 2. Electron image of Fe65.3B34.7.

chemistry of Fe-B amorphous alloy catalysts. Ethanol dehydrogenation was chosen as a test reaction to probe the catalytic behaviors. Based on various characterization techniques, the influence of iron precursors and preparation medium (H2O, H2O/ EtOH, isopropyl alcohol (IPA)) on the morphology and catalytic activity was discussed. 2. Experimental Section 2.1. Catalyst Preparation. Fe-B ultrafine amorphous alloy particles were systematically synthesized by chemical reduction method. A solution of NaBH4 with a concentration of 1 M was used as the reducing agent. Various iron salts such as FeCl2‚ 4H2O, FeCl3‚6H2O, FeCl2‚4H2O, and Fe(OAc)2 were used in

this study. NaBH4 solution was added dropwise to the solution of iron salt (0.1 M) under ultrasonic agitation. The solution was kept under an ice bath and a black precipitate formed immediately. The resulting precipitate was washed thoroughly, first with deionized water and then with a 95% ethanol (EtOH) solution. It was then stored in a 95% EtOH solution for further studies. 2.2. Catalyst Characterization. X-ray diffraction (XRD) experiments were performed using a Siemens D500 powder diffractometer. The XRD patterns were collected using Cu KR1 radiation (1.5405 Å) at a voltage and current of 40 kV and 30 mA, respectively. The sample for XRD was prepared as thin layers on a sample holder and was scanned over the range 2θ ) 20-60° at a rate of 0.05°/min to identify the crystalline structure. Electron diffraction was also used to identify the

Table 1. Composition and Surface Areas of Fe-B Samples under Various Preparation Conditions preparation conditions solvent salt H2O H2O H2O/EtOH (1:1) H2O/EtOH (1:1) H2O/IPA (1:1) a

FeCl2‚4H2O FeCl3‚6H2O FeCl2‚4H2O Fe(OAc)2 Fe(OAc)2

bulk compositiona (atomic ratio)

Fe/Bb (atomic ratio)

surf. compositionc (atomic ratio)

surf. area (m2/g)

Fe65.3B34.7 Fe72.8B27.2 Fe53.9B46.1 Fe63.7B36.3 Fe61.2B38.8

1.9 2.7 1.2 1.8 1.6


94 83 40 75 74

Determined by ICP-AES. b Fe/B ) 1/3 in the starting material. c Determined by XPS.

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Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007

Figure 6. TEM of Fe72.8B27.2 prepared using FeCl3 in H2O.

Figure 5. TEM of Fe65.3B34.7 prepared using FeCl2 in H2O.

structure of the samples. Differential scanning calorimetry (DSC) measurements were conducted under N2 (99.99%) atmosphere on a Perkin-Elmer DSC-7. The samples were scanned over the range 50°-550° at a rate of 20°/min to investigate the crystallization processes of the amorphous structure. Elemental analysis using inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Jobin-Yvon Co., France, JY-24) was carried out on the Fe-B samples to study the effect of preparation method on the compositions of the samples. In general, the weighed samples were dissolved in nitric acid and diluted with deionized water to the concentration within the calibration range of each element. Standard solutions purchased from Merck were diluted and used to establish the calibration curves. Wavelengths (in nm) used for elemental analysis were 259.94 and 249.77 for Fe and B, respectively. N2 sorption isotherms were measured at -197 °C using a Micromeritics ASAP 2010. Prior to the experiments, the samples were

Figure 7. TEM of Fe53.9B46.1 prepared using FeCl2 in H2O/EtOH.

dehydrated at 100 °C until the vacuum pressure was below 5 µmHg. The measurement of the surface areas of the samples was achieved by the Brunauer-Emmett-Teller (BET) method for relative pressures in the range P/P0 ) 0.05-0.2. The morphology and particle size of the samples were determined by transmission electron microscopy (TEM) on a JEM-1200


Figure 8. TEM of Fe63.7B36.3 prepared using Fe(OAc)2 in H2O/EtOH.

EX II operated at 160 kV. Initially, a small amount of sample was placed into the sample tube filled with a 95% EtOH solution. After agitating under ultrasonic environment for 10 min, one drop of the dispersed slurry was dipped onto a carboncoated copper mesh (300#) (Ted Pella Inc.) and dried in an oven at 100 °C for 1 h. XPS spectra were recorded with a Thermo VG Scientific Sigma Probe spectrometer. The XPS patterns were collected using Al KR radiation at a voltage and current of 20 kV and 30 mA, respectively. The base pressure in the analyzing chamber was maintained on the order of 10-9 Torr. The spectrometer was operated at 23.5 eV pass energy. The binding energy of XPS was corrected by contaminant carbon (C 1s ) 285.0 eV) to facilitate the comparisons of the values among the catalysts and the standard compounds. 2.3. Catalytic Activity. The dehydrogenation reaction was carried out in a continuous, U-shaped, quartz microreactor. About 40 mg of fresh catalyst was placed on a layer of quartz wool. The catalyst was first reduced with 5% H2 in Ar at 250 °C for 30 min. A saturator containing 99.8% EtOH was kept at a constant temperature of 22 °C. Nitrogen was used as a carrier gas at a constant flow rate (40 mL/min for catalysts). The experiments were carried out at a constant temperature of 250 °C under atmospheric pressure. To prevent possible condensation of reactant and products, all connection gas lines and valves were analyzed by a China Chromatography 8900F gas chromatograph equipped with a thermal conductivity detector using a Hyesep D column maintained at 150 °C. The reactant and products were identified by comparison with authentic samples. Product gas concentrations were determined with a SCSC2.01 integrator by comparing the peak areas to those for a standard mixture. The catalytic activity of Fe-B samples was calculated in terms of catalytic activity per gram of the catalyst and the specific activity per surface area of ethanol dehydrogenation. 3. Results and Discussion 3.1. Catalyst Characterization. Figure 1 illustrates the XRD patterns of Fe-B samples. A single broad peak around 2θ ) 45° indicates the amorphous nature of the samples. This is

Figure 9. TEM of Fe61.2B38.8 prepared using Fe(OAc)2 in H2O/IPA. Table 2. Particle Sizes of Fe-B Samples under Various Preparation Conditions particle size (nm) sample (atomic ratio) Fe65.3B34.7 Fe72.8B27.2 Fe53.9B46.1 Fe63.7B36.3 Fe61.2B38.8

TEM

estd av size from surf. area

10-20 50-60 20-30 20-30 60-70

8.1 9.2 19.1 10.2 10.3

further evidenced by the diffuse Debye rings of the electron diffraction image in the selected area of Fe65.3B34.7 sample as shown in Figure 2 that are assigned to the amorphous state of iron-metalloid alloy. Figure 3 depicts the XRD patterns of Fe65.3B34.7 catalyst at different treatment temperatures (as-

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Figure 10. XPS of Fe65.3B34.7 (a) before Ar+ sputtering and (b) after 10 min Ar+ sputtering.

synthesized, 400 °C, and 500 °C) for 120 min. There is no significant decrease in the amorphous nature of the sample calcined at 400 °C in comparison with the as-synthesized sample. However, the crystallinity of the sample increases after 400 °C as evidenced by the presence of Fe(110) peak in the XRD pattern of Fe65.3B34.7 catalyst calcined at 500 °C. Table 1 shows the effect of preparation variables on the surface areas of various catalysts. The Fe65.3B34.7 sample prepared in the aqueous solution with Fe/B ratio of 1/3 in the primary solution has the largest surface area of 94 m2/g. The Fe53.9B46.1 sample prepared in 50% EtOH solution with Fe/B ratio 1/3 in the primary solution possesses a surface area of 40 m2/g, which can be attributed to the solvent effect. It can also be observed that 50% EtOH solution and 50% IPA solution have no significant effect on the surface area of Fe-B nanoalloys prepared by Fe(OAc)2 precursor. DSC was carried out on Fe72.8B27.2 sample (Figure 4) to characterize the crystallization process. The single exothermal peak at 447 °C of the sample is different from that reported by Shen et al.19 for Fe87B13 (484 °C), which suggests that the presence of boron can retard the crystallization of Fe sample. TEM images of the prepared catalysts are shown in Figures 5-9, which show the distinct differences in the morphology and particle size of the samples influenced by the preparation variables. The Fe-B samples, prepared with FeCl2 and FeCl3 in aqueous medium and in H2O/EtOH, such as Fe65.3B34.7, Fe72.8B27.2, and Fe53.9B46.1 show a spherical or chainlike morphology with apparent boundary as shown in Figures 5-7. However, Fe-B sample prepared with Fe(OAc)2 using H2O/

EtOH, Fe63.7B36.3, shows a spherical or square morphology and the sample Fe61.2B38.8 prepared with IPA/H2O depicts a triangular morphology as shown in Figures 8 and 9, respectively. The results of Fe-B particle size from TEM micrographs and estimated average size are presented in Table 2. It can be observed that Fe65.3B34.7 has the largest surface area (94 m2/g) and the smallest particle size (8.1 nm). The XPS spectra of Fe-B catalysts are shown in Figures 10-14. The binding energies of Fe-B catalysts before and after Ar+ sputtering are compared with standard samples as given in Table 3. In Figure 10, two main peaks of iron with binding energies of 707.7 and 719 eV are attributed to the Fe 2p3/2 and Fe 2p1/2 levels, respectively. The binding energy of 707.7 eV for Fe 2p3/2 is consistent with the values for pure iron metal.37 In addition, a shoulder peak with a binding energy of around 711.4 eV can be assigned to the oxidized iron species on the surface. The relative areas of the main and shoulder peaks of the Fe 2p3/2 level suggest that iron is mainly in its metallic state on the surface of the sample. As shown in the pattern of B 1s for Fe65.3B34.7, the peak with higher binding energy can be attributed to the oxidized boron species on the surface of the sample.37 The relative areas of the two peaks indicate that boron exists mainly in the oxidized state at the surface of the sample. In comparison with pure boron (187.3 eV), the standard binding energy of the elemental boron in Fe65.3B34.7 (187.6 eV) positively shifted about 0.3 eV, indicating electron transfer from boron to iron. The XPS spectra of Fe72.8B27.2 are similar to that of Fe65.3B34.7. However, Fe53.9B46.1, Fe63.7B36.3, and Fe61.2B38.8 did not show the presence of elemental iron and boron even after


Figure 11. XPS of Fe72.8B27.2 (a) before min Ar+ sputtering.

Ar+

sputtering and (b) after 10




sputtering for 10 min, as shown in Figures 12-14. Moreover, a shift in Fe 2p1/2 can be observed after 10 min Ar+ sputtering in the case of Fe53.9B46.1, Fe63.7B36.3, and Fe61.2B38.8 samples. In conclusion, it can be stated that the starting material of iron salt has a significant influence on the properties of the prepared materials as it is evident that metallic iron species is obtained when FeCl3 is used as the starting material while FeCl2 and Fe(OAc)2 did not result in elemental iron species. 3.2. Catalytic Activity. Dehydrogenation of ethanol to acetaldehyde and hydrogen is an endothermic reaction. The reaction conversions on various Fe-B catalysts versus time on stream at 250 °C are shown in Figure 15. Among the catalysts, Fe63.7B36.3 showed no deactivation and Fe65.3B34.7 demonstrated

the highest initial conversion. Since the selectivity of acetaldehyde on all the catalysts are nearly 100% in this study, only the activity is considered for discussion as exhibited in Figure 16. Table 4 summarizes the effect of Fe/B mole ratio in the primary solution on the catalytic activities of the ultrafine amorphous alloy catalysts. The catalytic activity per gram of the catalyst is in the following order: Fe72.8B27.2 > Fe63.7B36.3 > Fe53.9B46.1 > Fe61.2B38.8 > Fe65.3B34.7. The manifested order of the specific activity per surface area of ethanol dehydrogenation is Fe72.8B27.2 > Fe53.9B46.1 > Fe63.7B36.3 > Fe61.2B38.8 > Fe65.3B34.7. Among the catalysts, Fe72.8B27.2, prepared using FeCl3 and H2O, shows the highest activity based on gram and surface area of the catalyst. This can be attributed to both high surface

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Table 3. XPS Binding Energy of Fe-B Catalysts Before and After Ar+ Sputtering and Standard Samples Fe (eV) samplea

2p3/2

2p1/2

B 1s (eV)

Standard Fe metal FeO R- Fe2O3 R- FeOOH γ- FeOOH Fe3O4 B B2O3

a

707 709.4 710.9 711.8 711.3 710.4

720

187.3 192


Catalysts before Ar+ Sputtering 711.4 724.8 711.0 724.8 710.3 724.0 711.3 724.8 711.1 724.8

192.1 191.3 191.8 191.7 191.8


Catalysts after 10 min Ar+ Sputtering 707.7 719.9 706.8 719.8 709.8 723.1 709.8 723 709.8 723.0

187.6 187.5 192.0 192.0 191.7

The binding energy was corrected by carbon (C 1s ) 285.0 eV).

Figure 15. Reaction conversion of Fe-B catalysts versus time on stream (reaction conditions: 25 °C, feed ) 0.01725 mol of ethanol/(g of catalyst‚ h)).

area and high activity per surface area (here we named it the quasi-turnover frequency (QTOF)). The high QTOF of this catalyst is possibly due to its high metal content on the surface and most of the iron atoms are in the metallic state. Fe72.8B27.2 sample had high metal content on the surface. In other words, it had higher active sites per surface area. According to our results, Fe/B molar ratio in the starting materials significantly affects the concentration of boron bonded to the iron metal, subsequently affecting the activity of the catalysts. Experimental results indicated that the different preparation procedures significantly affect the surface area and surface composition of these catalysts, accordingly varying their catalytic activities. 4. Conclusion Amorphous Fe-B nanoalloy catalysts were prepared by chemical reduction method using various iron salts such as FeCl2‚4H2O, FeCl3‚6H2O, FeCl2‚4H2O, and Fe(OAc) under different preparation mediums, viz., H2O, H2O/ethanol, and IPA. The catalysts were characterized by XRD, TEM, DSC, and

Figure 16. Dehydrogenation activity of Fe-B catalysts (reaction conditions: 250 °C, 1 atm). Table 4. Activity of Fe-B Catalysts preparation conditions catalyst

solvent

iron salt


H2O H2O H2O/EtO H (1:1) H2O/EtO H (1:1) H2O/IPA (1:1)

FeCl2‚4H2O FeCl3‚6H2O FeCl2‚4H2O Fe(OAc)2 Fe(OAc)2

catalytic activity surf. area mol × 109/ mol × 1010/ (m2/g) (g‚s) (m2‚s) 94 83 40 75 74

34.5 103 43.1 43.8 36.9

3.7 12.4 10.8 5.8 5.0

electron microscopy to confirm the amorphous alloy structure. The Fe-B sample shows amorphous structure up to 400 °C, after which it starts to crystallize. The influence of different preparation parameters on the composition and surface morphology of the alloy particles was investigated in some detail; especially systematic studies of the influence of iron precursor (FeCl2, FeCl3, Fe(OAc2)) and preparation medium (H2O, EtOH/ H2O, IPA) were performed and found to be of vital importance on the physicochemical properties of amorphous Fe-B nanoalloys. XPS results of the catalysts revealed the significant influence of the starting materials of iron salt on the properties of the prepared materials as evidenced by the metallic iron species obtained when FeCl3 is used, while FeCl2 and Fe(OAc)2 did not result in elemental iron species. Dehydrogenation of ethanol was tested over these catalysts as a probe reaction to test their catalytic activities. The selectivity of acetaldehyde on all these catalysts was nearly 100%. Among the catalysts, Fe72.8B27.2, prepared using FeCl3 and H2O, showed the highest activity due to both high surface area and high TOF. The activities of the catalysts were found to be affected by the Fe/B molar ratio in the starting materials and by the different preparation procedures. Acknowledgment This research was supported by the Ministry of Economical Affairs, Taiwan, Republic of China, under Contract No. 94EC-17-A09-S1-022. Literature Cited (1) Deng, J. F.; Li, H.; Wang, W. Progress in design of new amorphous alloy catalysts. Catal. Today 1999, 51, 113. (2) Liu, Z.; Dai, W.-L.; Liu, B.; Deng, J-F. The effect of boron on selective benzene hydrogenation to cyclohexene over ruthenium boride powders J. Catal. 1999, 187, 253. (3) Komiyama, H.; Yokoyama, A.; Inoue, H.; Masumoto, T.; Kimura, H. Sci. Rep. Res. Inst. Tohoku UniV. 1980, A28, 217. (4) Li, H.; Zhao, Q.; Wan, Y.; Dai, W.; Qiao, M. Self-assembly of mesoporous Ni-B amorphous alloy catalysts. J. Catal. 2006, 244, 251.

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ReceiVed for reView December 3, 2006 ReVised manuscript receiVed January 28, 2007 Accepted February 2, 2007 IE061552E

Effect of Synthesis Parameters on the Characteristics of FeâB

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