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ZrO2 nanoparticles by flame spray pyrolysis and their catalytic properties in CO hydrogenation
Catalysis Communications 12 (2011) 917–922
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Effects of Co dopants and flame conditions on the formation of Co/ZrO2 nanoparticles by flame spray pyrolysis and their catalytic properties in CO hydrogenation Choowong Chaisuk a, Pornpoj Boonpitak a, Joongjai Panpranot b, Okorn Mekasuwandumrong a,⁎ a b
Department of Chemical Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhonpathom, Thailand Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e
i n f o
Article history: Received 23 November 2010 Received in revised form 17 January 2011 Accepted 21 January 2011 Available online 31 January 2011 Keywords: Flame spray pyrolysis Nanopowder Co/ZrO2 Catalyst
a b s t r a c t The Co/ZrO2 catalysts with various Co loadings (5–10 wt.%) were prepared by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by XRD and TEM, all the resulting Co/ZrO2 nanoparticles were composed of single-crystalline particles exhibiting the characteristic tetragonal structure of ZrO2. Varying the amount of Co dopants during FSP synthesis did not alter the primary particle size of ZrO2 which was determined to be ca. 14 nm. On the other hand, increasing precursor feed rate from 3 to 8 ml/min resulted in an increase of ZrO2 crystallite size from 10 to 19 nm. The higher precursor feed rate produced higher enthalpy of flame and longer residence times, which increased coalescence and sintering of the particles. Compared to the Co/ZrO2 prepared by conventional impregnation method, the catalytic activities of the FSP-made catalysts were much higher. Moreover, the hydrogenation rates of the FSP-made Co/ZrO2 catalysts were increased with increasing Co loading and precursor feed rate. According to H2 chemisorption and H2 temperature program reduction results, the improvement of catalytic activity and C2–C6 selectivities of the FSP-made catalysts in the CO hydrogenation was attributed to the higher number of Co metal active sites and lower interaction between Co/CoO and ZrO2 support obtained via the FSP synthesis. © 2011 Published by Elsevier B.V.
1. Introduction Flame synthesis is an established commercial process to make nanoparticles such as fume silica, titania, and carbon black [1]. Flame spray pyrolysis (FSP) is a technique that can produce a wide variety of product compositions including the high surface area and highly efficient noble metal laden catalysts [2–4]. The applications of supported metal catalysts synthesized via FSP method have been reported continuously. For examples, flame-made Pt–Ba/Al2O3 and Pt/Ba/CexZr1−xO2 have been investigated in lean-NOx storage-reduction [3]. High surface area FSP-derived Ag/ZnO has been reported to exhibit high photocatalytic performance in UV-photodegradation of methylene blue [4]. Baiker et al. [5–7] successfully applied the FSP method for synthesis of various Al2O3 supported noble metal catalysts. The flame-made Pt/Al2O3 showed an improved turnover frequency in the hydrogenation of ethyl pyruvate compared to conventional porous catalysts. Our recent studies also show that Pd/SiO2 and Pt–Sn/Al2O3 synthesized in one-step by the FSP method exhibited higher hydrogenation activity in the liquid-phase selective hydrogenation of 1-heptyne and dehydrogenation of propane comparing to the ones prepared by conventional impregnation method [8–10]. The structural differences of the flame-made and conventionally prepared catalysts have often been explained as the reasons for the differences in their catalytic behaviors. ⁎ Corresponding author. E-mail address: [email protected] (O. Mekasuwandumrong). 1566-7367/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.catcom.2011.01.016
Zirconia nanoparticles have been produced by FSP with good control of particle size, morphology, and crystallinity [11,12]. In the present study, the Co/ZrO2 nanocomposites were made by the FSP method under various flame conditions and employed as catalysts for CO hydrogenation. Due to their high activities, high selectivities for long chain paraffins, low water–gas shift activities, and their relatively low price compared to Ru, supported Co-based catalysts are widely used in the CO hydrogenation or the Fischer–Tropsch synthesis (FTS). Addition of ZrO2 to Co catalysts has shown to improve catalyst reducibility and capability of hydrogen adsorption via a spillover mechanism [13,14]. The FSP-derived Co/ZrO2 nanoparticles were synthesized with various Co loadings and under different flame conditions (varying precursor feed rates). The catalysts were characterized by N2 physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), H2 chemisorption, and H2-temperature program reduction (TPR). Their catalytic activities were evaluated in the CO hydrogenation reaction at 220 °C and atmospheric pressure. 2. Experimental 2.1. Powder preparation The Co/ZrO2 catalysts were synthesized using the FSP method using cobalt naphthenate (Sigma-Aldrich) and zirconium n-butoxide (SigmaAldrich) dissolved in xylene (99.8+%, PRA grade, Sigma-Aldrich) as starting materials. The total metal ion concentration was maintained at
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Fig. 2. The XRD crystallite size (dXRD) (□) and particle size (dTEM) (●), together with the BET equivalent particle size (dBET) (○) for flame-made Co/ZrO2 catalysts, as a function of Co loadings (a) and as a function of feed rate (b).
Fig. 1. The XRD patterns of FSP-made Co/ZrO2 catalysts prepared with Co loadings between 5 and 10 wt.% (a) and precursor feed rate between 3 and 8 ml/min (b).
0.5 mol/l for all experiments. The Co concentration with respect to the total metal ion concentration was varied from 5 to 10 wt.%. The FSP gasassist nozzle has a radially symmetric configuration with a stainless-steel capillary tube (ID 0.41 mm; OD 0.71 mm) at the central axis serving as the liquid feed nozzle. Immediately surrounding the capillary tube is a narrow annular gap, of adjustable cross-sectional area, that issues 5 l/min of oxygen for spray atomization of the liquid feed. The pressure drop across the nozzle was maintained at 1.5 bar during FSP operation. A narrow concentric orifice ring (0.15 mm spacing, 6 mm radius from the nozzle axis) was supplied with a mixture of CH4 (1.5 l/min) and O2 (3.2 l/min) to serve as a premixed flamelet for ignition and support of the spray flame. A sheath gas flow of 5 l/min of oxygen was issued through an annular sintered metal frit (8 mm width, inner radius 9 mm from the nozzle axis) to stabilize and contain the spray flame. The precursor liquid feed rate was varied from 3 to 8 ml/min using a rate-controlled syringe pump and all gas flows (Thai Industrial Gas, N99.95%) were metered using mass flow controllers (Alborg). A water-cooled, stainless-steel filter housing supported a glassfiber filter (Whatman GF/D; 15 cm diameter) for collection of the flame-p. The products are designed as *F** where the first asterisk represents the percent cobalt loading and the latter two asterisks represent the precursor liquid feed rate used during FSP.
2.2. Catalyst characterization XRD patterns were recorded with a SIEMENS D5000 spectrometer (30 kV, 30 mA) at 2θ (Cu-Kα) 20° to 70° and a step size of 0.04°. The
crystallite size (dXRD) was determined using the Scherrer equation. The value of the shape factor, K, was taken to be 0.9 and α-alumina was used as an external standard. The BET powder-specific surface area (SSA) was measured by nitrogen adsorption at 77 K (BEL SORP mini II) after degassing the sample for 3 h at 150 °C in nitrogen flow. The equivalent average primary particle diameter (dBET) was calculated using dBET =6 /(SSA × ρ) where ρ is the powder correcting density. Powders were analyzed by TEM, combined with energy dispersive X-ray spectroscopy (EDXS) for elemental mapping. The Sauter-mean diameter (dTEM) of the particle-size distribution as derived from TEM-images was calculated according to
dTEM =
∑ni d3i : ∑ni d2i
Pulse hydrogen chemisorption was carried out to determine the cobalt active sties using a Micromeritic Chemisorb 2750 automated system attached with ChemiSoft TPx software at room temperature. Prior to hydrogen chemisorption, the samples were reduced at 350 °C for 3 h. The TPR profiles of supported cobalt catalysts were obtained by temperature programmed reduction using the Micromeritic Chemisorb Table 1 Physiochemical properties of the flame-made Co/ZrO2 catalysts. Sample
XRD crystallite size (nm)
BET surface area (m2/g)
Active sites (⁎1019 H atom/g cat)
n.d. n.d. n.d.
58 60 59
1.7 1.9 2.2
Effect of precursor feed rate 5F3 10.4 n.d. 5F5 11.7 n.d. 5F8 22.5 5.3
92 58 38
1.8 1.7 1.6
ZrO2 Effect of Co loading 5F5 11.7 8F5 12.1 10F5 11.9
Co3O4
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Fig. 3. The transmission electron microscope (TEM) images and selective area electron diffraction (SAED) patterns for flame-made Co/ZrO2 powders.
2750 automated system attached with the ChemiSoft TPx software with a temperature ramp of 10 °C/min from 35 to 800 °C in a flow of 3% H2 in argon. The hydrogen consumption was calibrated using TPR of silver oxide (Ag2O) at the same conditions. 2.3. Catalytic testing CO hydrogenation was carried out at 220 °C and 1 atm total pressure according to Ref. [15]. Prior to CO hydrogenation, 0.2 g of catalyst was reduced in a fixed bed reactor with H2 (50 ml/min) at 350 °C for 3 h. The reactant gas had a ratio of H2/CO = 10/1. The total gas flow rate was 49.5 ml/min. After start up, the catalyst was applied for 3 h and analyzed by online gas chromatography. 3. Results and discussion The Co/ZrO2 catalysts with various Co loadings (5, 8, and 10 wt.%) were synthesized in one-step by the FSP method using the precursor liquid feed rate 5 ml/min. In addition, the precursor feed rates were varied at 3, 5, and 8 ml/min in order to obtain different Co/ZrO2 particle sizes (5 wt.% Co loading). The XRD patterns of the FSP-made Co/ZrO2 catalysts prepared with various Co loadings and various
precursor feed rates are shown in Fig. 1(a) and (b), respectively. All the samples except 5F8 exhibited the XRD characteristic peaks of the tetragonal ZrO2 structure at 2θ = 29.8°, 34.2°, 49.6°, and 59.5° (JCPDS no. 50-1089) without any contamination of other phases such as cobalt oxide phase or monoclinic or cubic zirconia, indicating high dispersion of cobalt particles on the ZrO2 supports. For the 5F8 powder which was prepared using the highest precursor feed rate of 8 ml/min, an additional peak corresponding to the diffraction maximum (311) of the cobalt oxide phase is observed at d = 2.44 Å (JCPDS no. 42-1467). This indicates the growth of cobalt oxide particle with increasing the precursor feed rate. Fig. 2 shows the XRD crystallite size (dXRD) (□) and particle size (dTEM) (●), together with the BET equivalent particle size (dBET) (○) for the flame-made Co/ZrO2 catalysts, as a function of Co loading (Fig. 2a) and as a function of feed rate (Fig. 2b). It is clearly seen that the amount of cobalt dopants had no effect on the crystallite size of the ZrO2. The dXRD was steady at around 12 nm. The specific surface area (SSA) for the flame-made powders was ranged between 57 and 60 m2/g from pure 5 wt.% Co/ZrO2 to 20 wt.% Co/ZrO2 (see Table 1). The equivalent average primary particle diameter dBET for the powders was also in the range of 11 to 13 nm. In Fig. 2b, for each of the flame configurations, the ZrO2 primary particles exhibited dXRD of
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Fig. 4. The elemental mappings performed by energy dispersive X-ray spectroscopy (EDXS) for the FSP-made 10 wt.% Co/ZrO2 catalyst.
approximately 10, 12 and 22 nm for the precursor feed rates of 3, 5 and 8 ml/min, respectively. The specific surface area (SSA) for the flame-made powders decreased from 92 to 38 m2/g with increasing precursor feed rate from 3 to 8 ml/min (shown in Table 1). The effect of precursor feed rates has been reported in previous studies of the flame-synthesized metal oxides such as TiO2 [16], ZnO [17], SiO2 [18], CeO2 [19], and Ag/ZnO [4]. Increasing the precursor feed flow rate while keeping the oxygen flow rate constant results in higher enthalpy of flame, longer residence times and hotter flames, which increased coalescence and sintering of the particles [16–18]. It was also found that the particle sizes of ZrO2 support calculated from three methods were in good agreement indicating that the particles were single crystalline. Fig. 3 shows the TEM images and SAED patterns of the flame-made Co/ ZrO2 powders. The flame-made powders (Fig. 3a, b, c, d, and e) consist of particles with primarily a spherical shape. The Sauter-mean diameter size for the particles was in the range of 10–19 nm. The particle size increased with increasing precursor feed rate. In contrast, the Sauter-mean diameter size was not changed with increasing amount of Co dopants. These values were consistent with the particle size calculated by both BET and XRD results. However, the cobalt metal and/or cobalt oxide phases were not distinguishable for all the samples due to the poor contrast between the Co and Zr atoms. The SAED patterns of ZrO2 and Co/ZrO2 catalysts are also shown in Fig. 3. All the samples exhibited the spot patterns, suggesting the formation of Co/ZrO2 single crystals. Elemental mappings performed by EDXS are shown in Fig. 4. The lighter or white patches in the micrographs represent cobalt oxide species on the ZrO2 surface. It can be seen that the cobalt oxide species were well dispersed and distributed (shown on mapping). The EDXS elemental composition signals were assigned to Co, O and Zr atoms on the catalyst surface. TPR was performed in order to identify the reduction behaviors of the Co/ZrO2 catalysts. The TPR profiles for the FSP-made catalysts with different Co loadings and those prepared with different feed rates are shown in Fig. 5a and b, respectively. No hydrogen consumption was found on the bare FSP-made ZrO2, indicating that the flame-made ZrO2 samples without cobalt dopants were not reducible under the TPR conditions. The 5 wt.% Co/ZrO2 exhibited three main reduction peaks and one shoulder at temperature around 190, 210, 550 and 400 °C, respectively. The first two peaks were assigned to a two-step
reduction of weak interaction cobalt oxide from Co3O4 → CoO → Co0. The other main peak could be assigned to the more difficult to reduce CoxOy–ZrO2 species similar to those found on alumina support as CoxOy–Al2O3 [20,21] or the formation of a solid solution or a zirconate phase between unreduced cobalt oxides and zirconia [22]. An additional small shoulder at around 450 °C was observed when the precursor feed rate was increased to 5 and 8 ml/min. This probably can be assigned to the highly dispersed Co3O4 clusters that were stabilized by the ZrO2 matrix which required a higher reduction temperature due to high surface energy of the resulting metallic cobalt [23,24]. With increasing of the Co loading, the first peak at 190 °C had disappeared and the second peak was slightly shifted to a higher temperature. This peak was assigned to the combination of the two-step reduction of Co3O4 to CoO and to Co0 metal, respectively. The small reduction shoulder and high temperature reduction peak became larger. When the precursor feed rate increased from 3 to 8 ml/min, the reduction peaks and shoulder in the TPR profiles were shifted towards a lower temperature. The result indicates a weaker interaction between cobalt oxide and ZrO2 support with increasing the precursor feed rate. Decreasing the interaction would be explained by the growth of cobalt oxide particle as increasing the precursor feed rate resulted in a lower surface contact between cobalt oxide with the support [25]. Table 1 summarizes the physical and chemical properties of flamemade Co/ZrO2 catalysts. The relative amounts of active surface Co metal after reduction on the catalyst samples were calculated from H2 chemisorption experiments at room temperature. The amounts of H2 chemisorption increased from 1.7 to 2.2 × 1019 sites/gcat as the Co loading increased from 5 to 10 wt.%. On the other hand, the amounts of H2 chemisorption decreased from 1.8 to 1.6 × 1019 sites/gcat as the precursor feed rate increased from 3 to 8 ml/min. Decreasing the metal active site would probably be due to the reduction of metal dispersion by growth of cobalt oxide particles. The catalytic performances of flame-made Co/ZrO2 catalysts were evaluated in the CO hydrogenation. The Co/ZrO2 catalysts prepared by conventional impregnation of the cobalt precursor on the FSP-made ZrO2 prepared by the same flame condition of 5F5 sample (BET surface area=63 m2/g) were used for comparison. The catalytic performances in terms of CO hydrogenation rate (a) and product distribution (b) of flameand impregnation-made Co/ZrO2 catalysts are shown in Fig. 6. It was
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feed rate during FSP synthesis. Compared to the two catalysts prepared with 5 and 10 wt.% Co by the conventional impregnation method on FSPmade ZrO2 (I5/F5-Zr and I10/F5-Zr) and reference ZrO2 (I5/Ref-Zr and I5/ Ref-Zr) with equivalent surface area, the FSP-made catalysts exhibited higher catalytic activity and selectivity to long chain hydrocarbons. 4. Conclusions Nanopowders Co/ZrO2 catalysts have been successfully synthesized using flame spray pyrolysis (FSP) — a rapid and readily scalable process. The flame-made Co/ZrO2 catalyst nanopowders were composed of single-crystalline particles exhibiting the characteristic tetragonal structure of ZrO2 with a primary particle size between 10 and 12 nm for Co doping concentrations between 5 and 10 wt.%. For similar Co loading (5 wt.%), the particle size increased from 10 to 20 nm with increasing precursor feed rate from 3 to 8 ml/min. An additional peak corresponding to Co3O4 was observed only for the flame-made Co/ZrO2 catalyst with precursor feed rate 8 ml/min. Increasing of Co contents and precursor feed rate improved the rate of reaction of the flame-made catalysts in CO hydrogenation reaction. Improvement of the catalytic activity and C2–C6 selectivities was attributed to the increasing of the Co metal active site and lower interaction between Co/CoO and ZrO2 support, respectively. The FSP-made catalyst exhibited a higher catalytic activity than those of the impregnation-made ones supported on both flame-made and commercial zirconia. Acknowledgements The authors are grateful to the Thailand Research Fund, the Office of Higher Education Commission and the Institute of Technology Silpakorn University for the financial support. References
Fig. 5. The TPR profiles for the FSP-made Co/ZrO2 catalyst prepared with various Co loadings (a) and various feed rates (b).
found that increasing of Co contents improved the rate of reaction of all catalysts. Improvement on the catalytic activity would probably be due to the increasing of the Co metal active site. However, the selectivities to heavy hydrocarbon decreased. With increasing the feed flow rate, both the CO hydrogenation rate and the selectivities to heavy hydrocarbon increased. It is likely that the selectivities to long chain hydrocarbon did not depend solely on the hydrogenation activities but the interaction between Co and ZrO2 support could play an important role. Generally, in the FSP process the particle was formed as follows: the sprayed droplets of the precursor solution were evaporated and combusted when they met the flame at a very high temperature and released the metal atoms, then nucleation and growth of particles by coagulation and condensation occurred along the axial direction of the flame. With this process, increasing the feed rate during the FSP synthesis resulted in an increasing of the supplied fuel energy to the flame so that larger particle size of Co/CoO and ZrO2 support and lower interaction could be obtained. The TPR results of FSP-made Co/ZrO2 in this study support such mechanism. The shift to a lower temperature of high temperature TPR peaks was observed with increasing of the precursor
[1] W.J. Stark, S.E. Pratsinis, Powder Technology 126 (2002) 103. [2] M. Piacentini, R. Strobel, M. Maciejewski, S.E. Pratsinis, A. Baiker, Journal of Catalysis 243 (2006) 43. [3] R. Strobel, F. Krumeich, S.E. Pratsinis, A. Baiker, Journal of Catalysis 243 (2006) 229. [4] M.J. Height, S.E. Pratsinis, O. Mekasuwandunrong, P. Praserthdam, Applied Catalysis B: Environmental 63 (2006) 305. [5] R. Strobel, W.J. Stark, L. Madler, S.E. Pratsinis, A. Baiker, Journal of Catalysis 213 (2003) 296. [6] R. Strobel, F. Krumeich, W.J. Stark, S.E. Pratsinis, A. Baiker, Journal of Catalysis 222 (2004) 307. [7] S. Hannemann, J.-D. Grunwaldt, P. Lienemann, D. Gunther, F. Krumeich, S.E. Pratsinis, A. Baiker, Applied Catalysis A: General 316 (2007) 226. [8] S. Somboonthanakij, O. Mekasuwandumrong, J. Panpranot, T. Nimmanwudtipong, R. Strobel, S.E. Pratsinis, P. Praserthdam, Catalysis Letters 119 (2007) 346. [9] O. Mekasuwandumrong, S. Somboonthanakij, P. Praserthdam, J. Panpranot, Industrial and Engineering Chemistry Research 48 (2009) 2819. [10] S. Pisduangdaw, J. Panpranot, C. Methastidsook, C. Chaisuk, K. Faungnawakij, P. Praserthdam, O. Mekasuwandumrong, Applied Catalysis A: General 370 (2009) 1. [11] M.C. Heine, L. Madler, R. Jossen, S.E. Pratsinis, Combustion and Flame 144 (2006) 809. [12] R. Jossen, M.C. Heine, S.E. Pratsinis, M.K. Akhtar, Chemical Vapor Deposition 12 (2006) 614. [13] B. Jongsomjit, J. Panpranot, J.G. Goodwin Jr., Journal of Catalysis 215 (2003) 66. [14] J. Panpranot, N. Taochaiyaphum, B. Jongsomjit, P. Praserthdam, Catalysis Communications 7 (2006) 192. [15] B. Jongsomjit, S. Kittiruangrayub, P. Praserthdam, Materials Chemistry And Physics 105 (2007) 14. [16] A. Teleki, S.E. Pratsinis, K. Kalyanasundaram, P.I. Goumab, Sensors and Actuators B: Chemical 119 (2006) 683–690. [17] O. Mekasuwandumrong, P. Pawinrat, P. Praserthdam, J. Panpranot, Chemical Engineering Journal 164 (2010) 77–84. [18] H. Briesen, A. Fuhrmann, S.E. Pratsinis, Chemical Engineering Science 53 (1998) 4105–4112. [19] L. Mädler, W.J. Stark, S.E. Pratsinis, Flame-made ceria nanoparticles, Journal of Materials Research 17 (2002) 1356–1362. [20] B. Jongsomjit, J. Panpranot, J.G. Goodwin Jr., Journal of Catalysis 204 (2001) 98. [21] B. Jongsomjit, J. Panpranot, J.G. Goodwin Jr., Journal of Catalysis 215 (2003) 68. [22] D.I. Enache, M. Roy-Auberger, R. Revel, Applied Catalysis A: General 268 (2004) 51. [23] Z. Tao, Catalysis Communications 7 (2006) 1061. [24] A.M. Saib, A. Borgna, J. van de Loosdrecht, P.J. van Berge, J.W. Geus, J.W. Niemantsverdriet, Journal of Catalysis 239 (2006) 326. [25] Y. Zhang, M. Koike, R.Q. Yang, S. Hinchiranan, T. Vitidsant, N. Tsubaki, Applied Catalysis A: General 292 (2005) 252.
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CO hydrogenation rate (106 CO mol/gcat.s)
a
10 9 8 7 6 5 4 3 2 1 0 5F5
b
8F5
10F5
5F3
5F8
I5/F5-Zr
I5/Ref-Zr I10/F5-Zr I10/Ref-Zr
100 90 80
% Selectivity
70 C7+
60
C2-C8
50
CH4 40 30 20 10 0 5F5
8F5
10F5
5F3
5F8
I5/F5-Zr I5/Ref-Zr I10/F5-Zr I10/Ref-Zr
Fig. 6. The catalytic performances in terms of CO hydrogenation rate (a) and product distribution (b) of flame- and impregnation-made Co/ZrO2 catalysts.
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t c o m
Short Communication
Effects of Co dopants and flame conditions on the formation of Co/ZrO2 nanoparticles by flame spray pyrolysis and their catalytic properties in CO hydrogenation Choowong Chaisuk a, Pornpoj Boonpitak a, Joongjai Panpranot b, Okorn Mekasuwandumrong a,⁎ a b
Department of Chemical Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhonpathom, Thailand Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e
i n f o
Article history: Received 23 November 2010 Received in revised form 17 January 2011 Accepted 21 January 2011 Available online 31 January 2011 Keywords: Flame spray pyrolysis Nanopowder Co/ZrO2 Catalyst
a b s t r a c t The Co/ZrO2 catalysts with various Co loadings (5–10 wt.%) were prepared by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by XRD and TEM, all the resulting Co/ZrO2 nanoparticles were composed of single-crystalline particles exhibiting the characteristic tetragonal structure of ZrO2. Varying the amount of Co dopants during FSP synthesis did not alter the primary particle size of ZrO2 which was determined to be ca. 14 nm. On the other hand, increasing precursor feed rate from 3 to 8 ml/min resulted in an increase of ZrO2 crystallite size from 10 to 19 nm. The higher precursor feed rate produced higher enthalpy of flame and longer residence times, which increased coalescence and sintering of the particles. Compared to the Co/ZrO2 prepared by conventional impregnation method, the catalytic activities of the FSP-made catalysts were much higher. Moreover, the hydrogenation rates of the FSP-made Co/ZrO2 catalysts were increased with increasing Co loading and precursor feed rate. According to H2 chemisorption and H2 temperature program reduction results, the improvement of catalytic activity and C2–C6 selectivities of the FSP-made catalysts in the CO hydrogenation was attributed to the higher number of Co metal active sites and lower interaction between Co/CoO and ZrO2 support obtained via the FSP synthesis. © 2011 Published by Elsevier B.V.
1. Introduction Flame synthesis is an established commercial process to make nanoparticles such as fume silica, titania, and carbon black [1]. Flame spray pyrolysis (FSP) is a technique that can produce a wide variety of product compositions including the high surface area and highly efficient noble metal laden catalysts [2–4]. The applications of supported metal catalysts synthesized via FSP method have been reported continuously. For examples, flame-made Pt–Ba/Al2O3 and Pt/Ba/CexZr1−xO2 have been investigated in lean-NOx storage-reduction [3]. High surface area FSP-derived Ag/ZnO has been reported to exhibit high photocatalytic performance in UV-photodegradation of methylene blue [4]. Baiker et al. [5–7] successfully applied the FSP method for synthesis of various Al2O3 supported noble metal catalysts. The flame-made Pt/Al2O3 showed an improved turnover frequency in the hydrogenation of ethyl pyruvate compared to conventional porous catalysts. Our recent studies also show that Pd/SiO2 and Pt–Sn/Al2O3 synthesized in one-step by the FSP method exhibited higher hydrogenation activity in the liquid-phase selective hydrogenation of 1-heptyne and dehydrogenation of propane comparing to the ones prepared by conventional impregnation method [8–10]. The structural differences of the flame-made and conventionally prepared catalysts have often been explained as the reasons for the differences in their catalytic behaviors. ⁎ Corresponding author. E-mail address: [email protected] (O. Mekasuwandumrong). 1566-7367/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.catcom.2011.01.016
Zirconia nanoparticles have been produced by FSP with good control of particle size, morphology, and crystallinity [11,12]. In the present study, the Co/ZrO2 nanocomposites were made by the FSP method under various flame conditions and employed as catalysts for CO hydrogenation. Due to their high activities, high selectivities for long chain paraffins, low water–gas shift activities, and their relatively low price compared to Ru, supported Co-based catalysts are widely used in the CO hydrogenation or the Fischer–Tropsch synthesis (FTS). Addition of ZrO2 to Co catalysts has shown to improve catalyst reducibility and capability of hydrogen adsorption via a spillover mechanism [13,14]. The FSP-derived Co/ZrO2 nanoparticles were synthesized with various Co loadings and under different flame conditions (varying precursor feed rates). The catalysts were characterized by N2 physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), H2 chemisorption, and H2-temperature program reduction (TPR). Their catalytic activities were evaluated in the CO hydrogenation reaction at 220 °C and atmospheric pressure. 2. Experimental 2.1. Powder preparation The Co/ZrO2 catalysts were synthesized using the FSP method using cobalt naphthenate (Sigma-Aldrich) and zirconium n-butoxide (SigmaAldrich) dissolved in xylene (99.8+%, PRA grade, Sigma-Aldrich) as starting materials. The total metal ion concentration was maintained at
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Fig. 2. The XRD crystallite size (dXRD) (□) and particle size (dTEM) (●), together with the BET equivalent particle size (dBET) (○) for flame-made Co/ZrO2 catalysts, as a function of Co loadings (a) and as a function of feed rate (b).
Fig. 1. The XRD patterns of FSP-made Co/ZrO2 catalysts prepared with Co loadings between 5 and 10 wt.% (a) and precursor feed rate between 3 and 8 ml/min (b).
0.5 mol/l for all experiments. The Co concentration with respect to the total metal ion concentration was varied from 5 to 10 wt.%. The FSP gasassist nozzle has a radially symmetric configuration with a stainless-steel capillary tube (ID 0.41 mm; OD 0.71 mm) at the central axis serving as the liquid feed nozzle. Immediately surrounding the capillary tube is a narrow annular gap, of adjustable cross-sectional area, that issues 5 l/min of oxygen for spray atomization of the liquid feed. The pressure drop across the nozzle was maintained at 1.5 bar during FSP operation. A narrow concentric orifice ring (0.15 mm spacing, 6 mm radius from the nozzle axis) was supplied with a mixture of CH4 (1.5 l/min) and O2 (3.2 l/min) to serve as a premixed flamelet for ignition and support of the spray flame. A sheath gas flow of 5 l/min of oxygen was issued through an annular sintered metal frit (8 mm width, inner radius 9 mm from the nozzle axis) to stabilize and contain the spray flame. The precursor liquid feed rate was varied from 3 to 8 ml/min using a rate-controlled syringe pump and all gas flows (Thai Industrial Gas, N99.95%) were metered using mass flow controllers (Alborg). A water-cooled, stainless-steel filter housing supported a glassfiber filter (Whatman GF/D; 15 cm diameter) for collection of the flame-p. The products are designed as *F** where the first asterisk represents the percent cobalt loading and the latter two asterisks represent the precursor liquid feed rate used during FSP.
2.2. Catalyst characterization XRD patterns were recorded with a SIEMENS D5000 spectrometer (30 kV, 30 mA) at 2θ (Cu-Kα) 20° to 70° and a step size of 0.04°. The
crystallite size (dXRD) was determined using the Scherrer equation. The value of the shape factor, K, was taken to be 0.9 and α-alumina was used as an external standard. The BET powder-specific surface area (SSA) was measured by nitrogen adsorption at 77 K (BEL SORP mini II) after degassing the sample for 3 h at 150 °C in nitrogen flow. The equivalent average primary particle diameter (dBET) was calculated using dBET =6 /(SSA × ρ) where ρ is the powder correcting density. Powders were analyzed by TEM, combined with energy dispersive X-ray spectroscopy (EDXS) for elemental mapping. The Sauter-mean diameter (dTEM) of the particle-size distribution as derived from TEM-images was calculated according to
dTEM =
∑ni d3i : ∑ni d2i
Pulse hydrogen chemisorption was carried out to determine the cobalt active sties using a Micromeritic Chemisorb 2750 automated system attached with ChemiSoft TPx software at room temperature. Prior to hydrogen chemisorption, the samples were reduced at 350 °C for 3 h. The TPR profiles of supported cobalt catalysts were obtained by temperature programmed reduction using the Micromeritic Chemisorb Table 1 Physiochemical properties of the flame-made Co/ZrO2 catalysts. Sample
XRD crystallite size (nm)
BET surface area (m2/g)
Active sites (⁎1019 H atom/g cat)
n.d. n.d. n.d.
58 60 59
1.7 1.9 2.2
Effect of precursor feed rate 5F3 10.4 n.d. 5F5 11.7 n.d. 5F8 22.5 5.3
92 58 38
1.8 1.7 1.6
ZrO2 Effect of Co loading 5F5 11.7 8F5 12.1 10F5 11.9
Co3O4
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Fig. 3. The transmission electron microscope (TEM) images and selective area electron diffraction (SAED) patterns for flame-made Co/ZrO2 powders.
2750 automated system attached with the ChemiSoft TPx software with a temperature ramp of 10 °C/min from 35 to 800 °C in a flow of 3% H2 in argon. The hydrogen consumption was calibrated using TPR of silver oxide (Ag2O) at the same conditions. 2.3. Catalytic testing CO hydrogenation was carried out at 220 °C and 1 atm total pressure according to Ref. [15]. Prior to CO hydrogenation, 0.2 g of catalyst was reduced in a fixed bed reactor with H2 (50 ml/min) at 350 °C for 3 h. The reactant gas had a ratio of H2/CO = 10/1. The total gas flow rate was 49.5 ml/min. After start up, the catalyst was applied for 3 h and analyzed by online gas chromatography. 3. Results and discussion The Co/ZrO2 catalysts with various Co loadings (5, 8, and 10 wt.%) were synthesized in one-step by the FSP method using the precursor liquid feed rate 5 ml/min. In addition, the precursor feed rates were varied at 3, 5, and 8 ml/min in order to obtain different Co/ZrO2 particle sizes (5 wt.% Co loading). The XRD patterns of the FSP-made Co/ZrO2 catalysts prepared with various Co loadings and various
precursor feed rates are shown in Fig. 1(a) and (b), respectively. All the samples except 5F8 exhibited the XRD characteristic peaks of the tetragonal ZrO2 structure at 2θ = 29.8°, 34.2°, 49.6°, and 59.5° (JCPDS no. 50-1089) without any contamination of other phases such as cobalt oxide phase or monoclinic or cubic zirconia, indicating high dispersion of cobalt particles on the ZrO2 supports. For the 5F8 powder which was prepared using the highest precursor feed rate of 8 ml/min, an additional peak corresponding to the diffraction maximum (311) of the cobalt oxide phase is observed at d = 2.44 Å (JCPDS no. 42-1467). This indicates the growth of cobalt oxide particle with increasing the precursor feed rate. Fig. 2 shows the XRD crystallite size (dXRD) (□) and particle size (dTEM) (●), together with the BET equivalent particle size (dBET) (○) for the flame-made Co/ZrO2 catalysts, as a function of Co loading (Fig. 2a) and as a function of feed rate (Fig. 2b). It is clearly seen that the amount of cobalt dopants had no effect on the crystallite size of the ZrO2. The dXRD was steady at around 12 nm. The specific surface area (SSA) for the flame-made powders was ranged between 57 and 60 m2/g from pure 5 wt.% Co/ZrO2 to 20 wt.% Co/ZrO2 (see Table 1). The equivalent average primary particle diameter dBET for the powders was also in the range of 11 to 13 nm. In Fig. 2b, for each of the flame configurations, the ZrO2 primary particles exhibited dXRD of
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Fig. 4. The elemental mappings performed by energy dispersive X-ray spectroscopy (EDXS) for the FSP-made 10 wt.% Co/ZrO2 catalyst.
approximately 10, 12 and 22 nm for the precursor feed rates of 3, 5 and 8 ml/min, respectively. The specific surface area (SSA) for the flame-made powders decreased from 92 to 38 m2/g with increasing precursor feed rate from 3 to 8 ml/min (shown in Table 1). The effect of precursor feed rates has been reported in previous studies of the flame-synthesized metal oxides such as TiO2 [16], ZnO [17], SiO2 [18], CeO2 [19], and Ag/ZnO [4]. Increasing the precursor feed flow rate while keeping the oxygen flow rate constant results in higher enthalpy of flame, longer residence times and hotter flames, which increased coalescence and sintering of the particles [16–18]. It was also found that the particle sizes of ZrO2 support calculated from three methods were in good agreement indicating that the particles were single crystalline. Fig. 3 shows the TEM images and SAED patterns of the flame-made Co/ ZrO2 powders. The flame-made powders (Fig. 3a, b, c, d, and e) consist of particles with primarily a spherical shape. The Sauter-mean diameter size for the particles was in the range of 10–19 nm. The particle size increased with increasing precursor feed rate. In contrast, the Sauter-mean diameter size was not changed with increasing amount of Co dopants. These values were consistent with the particle size calculated by both BET and XRD results. However, the cobalt metal and/or cobalt oxide phases were not distinguishable for all the samples due to the poor contrast between the Co and Zr atoms. The SAED patterns of ZrO2 and Co/ZrO2 catalysts are also shown in Fig. 3. All the samples exhibited the spot patterns, suggesting the formation of Co/ZrO2 single crystals. Elemental mappings performed by EDXS are shown in Fig. 4. The lighter or white patches in the micrographs represent cobalt oxide species on the ZrO2 surface. It can be seen that the cobalt oxide species were well dispersed and distributed (shown on mapping). The EDXS elemental composition signals were assigned to Co, O and Zr atoms on the catalyst surface. TPR was performed in order to identify the reduction behaviors of the Co/ZrO2 catalysts. The TPR profiles for the FSP-made catalysts with different Co loadings and those prepared with different feed rates are shown in Fig. 5a and b, respectively. No hydrogen consumption was found on the bare FSP-made ZrO2, indicating that the flame-made ZrO2 samples without cobalt dopants were not reducible under the TPR conditions. The 5 wt.% Co/ZrO2 exhibited three main reduction peaks and one shoulder at temperature around 190, 210, 550 and 400 °C, respectively. The first two peaks were assigned to a two-step
reduction of weak interaction cobalt oxide from Co3O4 → CoO → Co0. The other main peak could be assigned to the more difficult to reduce CoxOy–ZrO2 species similar to those found on alumina support as CoxOy–Al2O3 [20,21] or the formation of a solid solution or a zirconate phase between unreduced cobalt oxides and zirconia [22]. An additional small shoulder at around 450 °C was observed when the precursor feed rate was increased to 5 and 8 ml/min. This probably can be assigned to the highly dispersed Co3O4 clusters that were stabilized by the ZrO2 matrix which required a higher reduction temperature due to high surface energy of the resulting metallic cobalt [23,24]. With increasing of the Co loading, the first peak at 190 °C had disappeared and the second peak was slightly shifted to a higher temperature. This peak was assigned to the combination of the two-step reduction of Co3O4 to CoO and to Co0 metal, respectively. The small reduction shoulder and high temperature reduction peak became larger. When the precursor feed rate increased from 3 to 8 ml/min, the reduction peaks and shoulder in the TPR profiles were shifted towards a lower temperature. The result indicates a weaker interaction between cobalt oxide and ZrO2 support with increasing the precursor feed rate. Decreasing the interaction would be explained by the growth of cobalt oxide particle as increasing the precursor feed rate resulted in a lower surface contact between cobalt oxide with the support [25]. Table 1 summarizes the physical and chemical properties of flamemade Co/ZrO2 catalysts. The relative amounts of active surface Co metal after reduction on the catalyst samples were calculated from H2 chemisorption experiments at room temperature. The amounts of H2 chemisorption increased from 1.7 to 2.2 × 1019 sites/gcat as the Co loading increased from 5 to 10 wt.%. On the other hand, the amounts of H2 chemisorption decreased from 1.8 to 1.6 × 1019 sites/gcat as the precursor feed rate increased from 3 to 8 ml/min. Decreasing the metal active site would probably be due to the reduction of metal dispersion by growth of cobalt oxide particles. The catalytic performances of flame-made Co/ZrO2 catalysts were evaluated in the CO hydrogenation. The Co/ZrO2 catalysts prepared by conventional impregnation of the cobalt precursor on the FSP-made ZrO2 prepared by the same flame condition of 5F5 sample (BET surface area=63 m2/g) were used for comparison. The catalytic performances in terms of CO hydrogenation rate (a) and product distribution (b) of flameand impregnation-made Co/ZrO2 catalysts are shown in Fig. 6. It was
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feed rate during FSP synthesis. Compared to the two catalysts prepared with 5 and 10 wt.% Co by the conventional impregnation method on FSPmade ZrO2 (I5/F5-Zr and I10/F5-Zr) and reference ZrO2 (I5/Ref-Zr and I5/ Ref-Zr) with equivalent surface area, the FSP-made catalysts exhibited higher catalytic activity and selectivity to long chain hydrocarbons. 4. Conclusions Nanopowders Co/ZrO2 catalysts have been successfully synthesized using flame spray pyrolysis (FSP) — a rapid and readily scalable process. The flame-made Co/ZrO2 catalyst nanopowders were composed of single-crystalline particles exhibiting the characteristic tetragonal structure of ZrO2 with a primary particle size between 10 and 12 nm for Co doping concentrations between 5 and 10 wt.%. For similar Co loading (5 wt.%), the particle size increased from 10 to 20 nm with increasing precursor feed rate from 3 to 8 ml/min. An additional peak corresponding to Co3O4 was observed only for the flame-made Co/ZrO2 catalyst with precursor feed rate 8 ml/min. Increasing of Co contents and precursor feed rate improved the rate of reaction of the flame-made catalysts in CO hydrogenation reaction. Improvement of the catalytic activity and C2–C6 selectivities was attributed to the increasing of the Co metal active site and lower interaction between Co/CoO and ZrO2 support, respectively. The FSP-made catalyst exhibited a higher catalytic activity than those of the impregnation-made ones supported on both flame-made and commercial zirconia. Acknowledgements The authors are grateful to the Thailand Research Fund, the Office of Higher Education Commission and the Institute of Technology Silpakorn University for the financial support. References
Fig. 5. The TPR profiles for the FSP-made Co/ZrO2 catalyst prepared with various Co loadings (a) and various feed rates (b).
found that increasing of Co contents improved the rate of reaction of all catalysts. Improvement on the catalytic activity would probably be due to the increasing of the Co metal active site. However, the selectivities to heavy hydrocarbon decreased. With increasing the feed flow rate, both the CO hydrogenation rate and the selectivities to heavy hydrocarbon increased. It is likely that the selectivities to long chain hydrocarbon did not depend solely on the hydrogenation activities but the interaction between Co and ZrO2 support could play an important role. Generally, in the FSP process the particle was formed as follows: the sprayed droplets of the precursor solution were evaporated and combusted when they met the flame at a very high temperature and released the metal atoms, then nucleation and growth of particles by coagulation and condensation occurred along the axial direction of the flame. With this process, increasing the feed rate during the FSP synthesis resulted in an increasing of the supplied fuel energy to the flame so that larger particle size of Co/CoO and ZrO2 support and lower interaction could be obtained. The TPR results of FSP-made Co/ZrO2 in this study support such mechanism. The shift to a lower temperature of high temperature TPR peaks was observed with increasing of the precursor
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CO hydrogenation rate (106 CO mol/gcat.s)
a
10 9 8 7 6 5 4 3 2 1 0 5F5
b
8F5
10F5
5F3
5F8
I5/F5-Zr
I5/Ref-Zr I10/F5-Zr I10/Ref-Zr
100 90 80
% Selectivity
70 C7+
60
C2-C8
50
CH4 40 30 20 10 0 5F5
8F5
10F5
5F3
5F8
I5/F5-Zr I5/Ref-Zr I10/F5-Zr I10/Ref-Zr
Fig. 6. The catalytic performances in terms of CO hydrogenation rate (a) and product distribution (b) of flame- and impregnation-made Co/ZrO2 catalysts.