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SiO2 Fischer–Tropsch catalyst
Catalysis Communications 11 (2010) 834–838
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Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c a t c o m
Highly active and stable catalytic performance on phosphorous-promoted Ru/Co/Zr/SiO2 Fischer–Tropsch catalyst Jong Wook Bae ⁎, Seung-Moon Kim, Seon-Ju Park, Yun-Jo Lee, Kyoung-Su Ha, Ki-Won Jun ⁎ Petroleum Displacement Technology Research Center, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yuseong, Daejeon, 305-600, Republic of Korea
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Article history: Received 31 January 2010 Received in revised form 1 March 2010 Accepted 2 March 2010 Available online 15 March 2010 Keywords: Fischer–Tropsch synthesis Cobalt Zirconium–phosphorous species Aggregation Spatial confinement
a b s t r a c t A novel doubly promoted Ru/Co/ZrP/SiO2 catalyst with zirconium–phosphorous species (ZrP) reveals high activity and stability on Fischer–Tropsch synthesis (FTS). The surface modification of SiO2 with ZrP species containing appropriate amount of phosphorous component, has an effect on the suppression of cobalt particle aggregation during FTS reaction. The spatial confinement of cobalt particles in ZrP matrix could be significantly enhanced when P/(Zr + P) molar ratio is around 0.13 (CoZrP(0.5)). © 2010 Elsevier B.V. All rights reserved.
1. Introduction The Fischer–Tropsch Synthesis (FTS) is a commercially proven technology to produce environmentally-benign fuels and chemicals from alternative energy sources [1]. Cobalt-based catalysts deposited on inorganic oxide supports such as Al2O3, TiO2 and SiO2 are frequently applied for low temperature FTS which exhibits high catalytic performance to linear paraffin formation. These supports help to disperse the cobalt particles better, and they alter the nature of supported cobalt species due to the variation of metal–support interaction. Since the moderate interaction of cobalt particle with support is important to achieve high catalytic performance, the commercial available FTS catalysts are further modified with some other promoters such as Zr, Ti and/or La components on Co/SiO2 catalyst [1,2]. These promoters are differently contributed to catalytic activity by increasing reducibility of cobalt particles or by preventing catalyst deactivation which is mainly induced from the sintering of metallic cobalt or oxidation by water generated. These promoters are expected not only to increase the FTS activity but also to help to retain high activity for longer duration by decreasing deactivation rate. Zirconium component is frequently used as one of the promoters for Co/SiO2 catalyst, however, the method for preventing abrupt deactivation at the beginning of FTS reaction is still one of the challengeable tasks for commercial-scale application. With this view in mind, we have further investigated the effects of phosphorous incorporation on Ru/Co/Zr/SiO2 system. In general, phosphorous-modified Co/Al2O3 catalyst revealed a low activity due ⁎ Corresponding authors. Bae is to be contacted at Tel.: +82 42 860 7383; fax: +82 42 860 7388. Jun, Tel.: +82 42 860 7671; fax: +82 42 860 7388. E-mail addresses: fi[email protected] (J.W. Bae), [email protected] (K.-W. Jun). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.03.003
to the hard reducibility of cobalt particles by forming strong interaction of cobalt with phosphorous component [3]. Our previous work about phosphorous-modified γ-Al2O3 support on Co/Al2O3 FTS catalyst revealed the high resistance to sintering and wax formation by the local surface transformation to aluminum phosphate with appropriate phosphorous content [4]. Even though the effect of phosphorous modification on Co/Al2O3 was already reported by our previous work, this is the first time to use phosphorous component as a promoter on Co/Zr/SiO2 catalytic system to the best of our knowledge. The properties of Co/SiO2 catalyst with the previous modification of zirconium–phosphorous species (ZrP) on SiO2 have not well established to date. In the present communication, we report the use of ZrP promotion not only to increase activity but also to enhance catalytic stability by preventing aggregation of cobalt particle during FTS reaction. 2. Catalyst preparation, activity test and characterization The ZrP-modified Ru/Co/SiO2 catalyst was prepared by stepwise impregnation with metal nitrate precursors such as zirconium oxynitrate, cobalt(II) nitrate, ruthenium(III) nitrosyl nitrate and phosphoric acid on SiO2 support (Davisil; pore size of 15.0 nm and surface area of 300 m2/g). with 3 steps as followings; (1) The 10 wt.% metals using aqueous solution of zirconium oxynitrate and phosphoric acid with a different P/(P + Zr) molar ratio were co-impregnated on SiO2 and the sample was dried at 110 °C for 12 h. (2) The cobalt precursor was successively impregnated in the slurry of the uncalcined ZrP-modified SiO2 support with 20 wt.% cobalt metal without further calcination. (3) The final catalyst was prepared by impregnation of Ru precursor with 0.5 wt.%Ru metal using previously
J.W. Bae et al. / Catalysis Communications 11 (2010) 834–838
prepared Co/ZrP/SiO2 catalyst. The final catalyst with 3.0 g was dried and calcined at 400 °C h for 12 h at a final step with an air flow rate of 50 ml/min with a heating rate of 10 °C/min. The final catalyst was denoted as CoZrP(×), which is representing the catalyst composition of 0.5 wt.%Ru/16.6 wt.%Co/7.5 wt.%ZrP/75.4 wt.%SiO2, with a different weight ratio of P/(P + Zr) from 0 to 0.1 (equivalent P/(P + Zr) molar ratio of 0, 0.03, 0.13 and 0.25). The × digit in CoZrP(×) catalyst stands for the values of (P/(P + Zr) × 10) in 10 wt.%ZrP/SiO2 support. Prior to activity tests, the catalysts were activated at 400 °C with a heating rate of 10 °C/min in a fixed-bed reactor (I.D. = 12.7 mm) for 12 h with a flow rate of 5%H2/He gas mixture around 20 ml/min. The activity test was carried out at differential reactor by adopting a catalyst loading of around 0.3 g with a particle size of 80–120 μm and its bed-height of around 5 mm. The activity test was conducted for around 70 h under the following reaction conditions; T = 220 °C; P = 2.0 MPa; SV (L/kgcat/h) = 2000; feed composition of H2/CO/CO2/ Ar (mol%) = 57.3/28.4/9.3/5.0 which was directly prepared by combined steam and carbon dioxide reforming of methane. The effluent gas was analyzed by an online gas chromatograph (YoungLin Acme 6000 GC) employing GS-GASPRO capillary column connected with a flame ionization detector and Porapak Q/molecular sieve (5A) packed column connected with thermal conductivity detector for the analysis of carbon oxides and hydrogen with an internal standard gas of Ar. The BET surface area (Sg; m2/g) was estimated from nitrogen desorption isotherm obtained at −196 °C using a constant–volume adsorption apparatus (Micromeritics, ASAP-2400). The surface morphology and the cobalt particle size distribution for the fresh (i.e. calcined) and used CoZrP catalysts was characterized by using transmission electron microscope (TEM) and energy electron loss spectroscopy (EELS) (TECNAI G2 instrument) analysis operating at 200 kV. The dispersion and particle size of metallic cobalt was calculated from H2 chemisorption method at 100 °C under static condition using micromeritics ASAP 2000 instrument equipped with a high vacuum pump system providing 10−6 torr. The particle size of cobalt was calculated with the assumption of H/Co metal's stoichiometry of 1. The reduction degree was determined by O2 titration on the reduced catalyst at 400 °C for 12 h and then the particle size of metallic cobalt was corrected by considering the degree of reduction. The FT-IR spectra of fresh CoZrP catalysts were recorded on a Nicolet Protege 460 FT-IR spectrometer equipped with a MCT detector which was cooled with liquid nitrogen and has a spectral resolution of 2 cm−1. The surface cobalt species after calcination and FTS reaction were characterized by using XPS (ESCALAB MK-II) analysis. The used catalyst was previously passivated with 1 vol.%O2 balanced with He gas at the temperature of FTS reaction before being exposed in air environment. The catalyst was pressed to a thin pellet before XPS analysis and the binding energy (BE) was corrected with the reference BE of C1s (284.4 eV).
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3. Results and discussion The catalytic performance of ZrP modified Ru/Co/SiO2 catalyst is summarized in Table 1. The detailed catalytic performance such as a full-range hydrocarbon distribution at two different reaction times is shown in Supplementary materials (Table S1). The surface area of CoZrP increased with the increase of phosphorous content from 217 to 232 m2/g. The particle size of cobalt was observed above 8 nm in average, calculated from H2-chemisorption method, and the degree of reduction on all CoZrP catalysts showed similar values above 73%. The CoZrP catalyst with proper amount of phosphoric acid such as CoZrP (0.1) and CoZrP(0.5) showed a stable activity after around 60 h on stream with a low decline of activity. Although all catalysts showed a high CO conversion above 90% at the very beginning of the reaction, fast deactivation was observed on CoZrP(0) and CoZrP(1) catalysts. In general, CO conversion and product distribution during FTS reaction are mainly affected by the cobalt particle size, degree of reduction, reconstruction of metallic cobalt and wax/coke deposition [5,6]. However, the cobalt-based catalyst possessing a particle size above 6– 8 nm is known to show a little effect on enhancing intrinsic FTS activity [7]. Because of high reducibility and cobalt particle size above 8 nm on all CoZrP catalysts, high initial activity above 82% at the very beginning of reaction was observed on those catalyst with high C5+ selectivity. The CO conversion and product distribution at steady-state around 70 h on stream are compared to elucidate the differences in catalytic performance. Higher CO conversion around 85.3% and C5+ selectivity around 87.9% at steady-state are attributed to the redispersion of cobalt species, low mobility of cobalt particles [8] and high reducibility on CoZrP(0.5) catalyst. Interestingly, the variation of CH4 selectivity is largely altered before and after FTS reaction. On CoZrP(0) catalyst, CH4 selectivity steadily increased with time on stream from 6.8 to 14.6% (Table S1) due to the possible reconstruction of cobalt species to form small cobalt particles below 8 nm. As reported by Bezemer et al. [7], the smaller cobalt particle size below 8 nm is responsible for showing low intrinsic CO conversion and high CH4 selectivity due to the possible character of small particle with strong interaction with support. However, all the phosphoroustreated CoZrP catalyst showed a decreased CH4 selectivity (increase of C5+ selectivity) with time on stream and this beneficial effect is further explained by the suppressed reconstruction and aggregation of cobalt particles with the help of ZrP species on SiO2 surface. The different catalytic performance on CoZrP catalyst at steadystate could be related to the degree of aggregation and mobility of cobalt particle due to the different characteristics of ZrP species on SiO2 surface by varying phosphorous content. The site-time yield (converted CO moles/surface-atom/s, s−1) on supported cobalt catalyst such as Al2O3, SiO2 and TiO2 supports [9] is generally reported in the range of 1.6 × 10−2 − 3.0 × 10−2 s−1, the value on CoZrP(0.5)
Table 1 CO conversion and product distribution on CoZrP catalysts and their physicochemical properties.a Notationb
P/(P + Zr) ratio
Sg (m2/g)
D (%)/dp (nm)
CoZrP(0) CoZrP(0.1) CoZrP(0.5) CoZrP(1)
0 0.03 0.13 0.25
217 219 231 232
11.2/8.9 11.8/8.5 8.8/11.3 9.0/11.1
a
c
Dred (%)d
CO conve (10 h → 70 h)
RDe
C5+e
Ratio of ICo/IZr
72.9 73.4 75.9 77.5
82.8 → 50.5 97.7 → 80.5 98.9 → 85.3 96.5 → 53.8
0.54 0.29 0.23 0.71
69.4 84.9 87.9 79.8
5.42 10.42 13.53 8.98
f
Reaction conditions: T = 220 °C, Pg = 2.0 MPa, SV (L/kgcat/h) = 2000, feed compositions (H2/CO/CO2/Ar; mol%) = 57.3/28.4/9.3/5.0. The notation of CoZrP(×) FT catalysts stands for the Ru/Co/ZrP/SiO2 catalysts repaired by stepwise impregnation with different wt.% of Zr/P form 10/0 to 9/1 at fixed Co concentration of 20 wt.% and 0.5 wt.% of Ru with 100 wt.%SiO2. The composition of final catalyst is 0.5 wt.%Ru/16.6 wt.%Co/7.5 wt.%ZrP/75.4 wt.%SiO2. The × digit in CoZrP(×) catalyst stands for the values of (P/(P + Zr) × 10) in 10 wt.%ZrP/SiO2 support from 0 to 0.1 of P/(P + Zr) weight ratio. c The dispersion (D) and cobalt particle size (dp) were calculated from H2 chemisorption and it was further corrected by considering the degree of reduction. d The degree of reduction (Dred; %) was calculated on the reduced catalyst at 400 °C for 12 h by O2 titration method from the amount of O2 consumption divided by the theoretical O2 consumption by using the equation of 3Co + 2O2 → Co3O4. e The decline of activity by deactivation (RD) is calculated from the difference between CO conversion at 10 h on stream and steady-state conversion at 70 h on stream; RD = (CO conversion at 10 h on stream − CO conversion at 70 h on stream)/CO conversion at 10 h on stream. The selectivity to C5+ is an averaged value at steady-state after 60 h on stream for 10 h. f The intensity ratio of I(used)/II(fresh) of ICo/IZr was recalculated from XPS analysis to elucidate the relative interaction between Co and Zr particle before and after FTS reaction. b
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catalyst is around 2 times higher as 5.53 × 10−2 s-1 on CoZrP(0.5) catalyst. Even though CoZrP(0) and CoZrP(1) catalysts showed a high reducibility and proper cobalt particle size after activation, the lower CO conversion and C5+ selectivity at steady-state are mainly attributed to the significant aggregation of cobalt particles with bimodal size distribution. The smaller cobalt particle size below 8 nm is also known to contribute the low catalytic activity and C5+ selectivity because of high formation rate of CH4. The FT-IR analysis on the calcined CoZrP catalysts is carried out to elucidate the chemical states of ZrP species on SiO2 surface and the results are shown in Fig. 1. The well-dispersed cobalt oxides on CoZrP catalysts are suggested by the intense vibration band at 665 and 565 cm−1 which is assigned to the Co–O adsorption band [10]. The band at around 1240 and 470 cm−1 could be mainly attributed to the Si–O stretching vibration mode on CoZrP catalyst and partially attributed to the triply degenerate P–O stretching vibration mode or to the triply degenerate O–P–O bending vibration mode of tetrahedral (PO4)3− [11]. The increase of intensity at 1240 cm−1 and the shift to lower frequency at the band of 1105 cm−1 are possibly attributed to the deteriorated silica framework. It is also attributed to the Zr4+ interaction on SiO2 surface due to the abundant presence of phosphorous component [12]. Furthermore, the presence of the band at 980 cm−-1 and the shift to lower frequency at 1105 cm−1 is also indicative of the increase of Zr–O–Si population on the SiO2 surface due to the phase segregation of Si–O–Si linkages [13]. Although the heterogeneity (phase segregation) of SiO2 surface induced from Si–O–Si linkages is generally suppressed with the increase of zirconium content on ZrO2–SiO2 by substitution of Si–O–Zr linkages, the heterogeneity on CoZrP catalyst increases with the increase of P/(P+Zr) molar ratio due to the possible modification of SiO2 surface. From the results of FT-IR, the observed well-defined crystalline cobalt oxides on CoZrP catalysts are responsible for the good dispersion and high reducibility which is beneficial for obtaining high FTS activity. The differences in catalytic performance and effect of ZrP species on the aggregation of cobalt particles are further understood by characterizing the used CoZrP catalysts with X-ray photoelectron microscopy (XPS) analysis. Ru promoter is well known to enhance the reducibility of Co3O4 with a proper amount (around 0.5 wt.%) due to the well-mixed cobalt and ruthenium particles without significant segregation as previously reported from our investigation [14]. Therefore, the effect of Ru promoter on CoZrP catalysts with respect to the variation in chemical state of cobalt particle and catalytic performance is not considered in the present investigation. The summarized catalytic performance at steady-state and their correlation with XPS results are shown in Table 1 and Fig. 2. The characteristic Co2p3/2 and Zr3d5/2 peaks were observed in all catalysts at the binding energy (BE) of around 776 and 178 eV respectively as shown in Figs. S1 and S2 in Supplementary materials (Fig. S1 for
Fig. 1. FT-IR spectra of the fresh calcined CoZrP FTS catalysts.
Fig. 2. Correlation between ICo/ISi and IZr/ISi ratios analyzed from XPS and CO conversion with respect to P/(P + Zr) molar ratio.
calcined catalyst and Fig. S2 for used one). The ZrP-modified used CoZrP catalysts show the BE of Co2p3/2 around 776.5–777.0 eV. The BE of Co2p3/2 slightly shifted to lower value of 775.7 eV on CoZrP(0.5) compared to that on CoZrP(0) around 775.9 eV without the presence of satellite peak on the higher BE side which is assigned to the localized amorphous Co2+ phase. The BE of Zr3d5/2 peaks on CoZrP catalysts reveals the presence of Zr4+ species. Unlike other CoZrP catalysts, the BE of Zr3d5/2 at 177.8 eV in used CoZrP(0.5) catalyst is not much altered at the end of reaction. The observed ZrP species is generally known as the hydrogen phosphate of Zr(H2PO4)2 and H2PO− 4 species which is known to be stable up to high temperature by the transformation of H2PO− 4 to pyrophosphate species with a low thermal mobility up to 1350 °C [15]. Therefore, the strong interaction of Si–P on CoZrP(1) due to the presence of unstable SiO2 surface is responsible for the shift to higher binding energy of Zr3d5/2 peak and the formation of larger ZrP particles. The small amount of phosphorous content on Zr/SiO2 such as CoZrP(0.1) and CoZrP(0.5) catalysts significantly enhances the homogeneous particle size distribution of Zr(H2PO4)2 species with a strong interaction between ZrP species and SiO2. This is confirmed by shift to lower BE side around 177.6– 177.8 eV of Zr3d5/2. These characteristics help to increase dispersion of cobalt particles with a low aggregation of cobalt particles during FTS reaction. The intensity ratio of ICo/ISi on the used CoZrP catalysts could be also related with the order of cobalt particle size after FTS reaction. With the increase of ICo/ISi ratio, the particle size of cobalt is generally getting smaller with the assumption of monolayer coverage of cobalt species on SiO2 support [1,16,17]. The higher value of ICo/ISi around 1.22 is observed on CoZrP(0.5) catalyst. The calculated intensity ratio of IZr/ISi (ratio between fresh and used ones) reveals that stable ZrP species are well dispersed on CoZrP(0.5) catalyst. It is suggested by the high value of 3.83 on CoZrP(0.5) catalyst with the formation of small ZrP particles on SiO2 surface. The low value of IZr/ISi ratio around 2.88 on CoZrP(1) suggests that the large ZrP particle formation and its formation were found to be less efficient to suppress cobalt aggregation during FTS reaction. Furthermore, the intensity ratio of ICo/IZr (ratio of used to fresh catalyst as shown in Table 1) which is calculated from the values of ICo/ISi divided by that of IZr/ISi by using the values of fresh and used CoZrP catalysts separately is also suggesting the degree of adjacent contact of Co and Zr species [16,17]. The higher value of ICo/IZr on CoZrP(0.5) around 13.53 reveals that the cobalt particles are well contacted with ZrP species with a high dispersion of cobalt particles, and vice versa on CoZrP(0) catalyst. From the XPS results on the used CoZrP catalysts, it could be concluded that the high catalytic stability and performance with
J.W. Bae et al. / Catalysis Communications 11 (2010) 834–838
respect to CO conversion and C5+ selectivity on CoZrP(0.5) catalyst is mainly attributed to the formation of proper cobalt particles as well as ZrP species on SiO2 surface which is helping to reduce the aggregation of cobalt species by the effective spatial confinement because of presence of well-dispersed ZrP species. The catalytic deactivation during FTS reaction is mainly attributed to the aggregation [1,9,18,19] and reoxidation [20,21] of metallic cobalt particle, however, the deactivation by reoxidation by water is known to be insignificant on cobalt particle size above 4 nm. Therefore the catalyst deactivation on CoZrP catalyst, which showed the average particle size above 8 nm, is mainly explained by aggregation of cobalt particles resulting in suppressing the amount of active sites. The cobalt particles are homogeneously dispersed on ZrP modified SiO2 surface confirmed by mapping of elemental metal dispersion (Fig. S3 in Supplementary materials) by using TEM and EELS. The representative dark field images with a mapping of elemental metal species revealed that the ZrP species are well dispersed in close proximity on SiO2 surface and the cobalt particles are formed between ZrP species. The particle size of fresh CoZrP catalysts such as CoZrP(0) in Fig. 3(A-1) and CoZrP(0.5) in Fig. 3(B-1) is not significantly different, however, the behavior of cobalt particle size variation on these catalysts is considerably different after FTS reaction with the average particle size of 12 and 18 nm separately. In the case of CoZrP(0.5) as shown in Fig. 3(B-2), the cobalt particle size is not much altered after FTS reaction due to the spatial confinement effect by ZrP species. However, bimodal particle size distribution with the presence of aggregated large cobalt particles as well as redispersion of cobalt particles with small size [9] are observed on CoZrP(0) catalyst in Fig. 3(A-2). The lower C5+ selectivity on CoZrP(0) catalyst is mainly attributed to the co-presence of small cobalt particles as reported previously [4,6,7]. The ZrP modification of SiO2 surface with an appropriate molar ratio of P/(P + Zr) around 0.03–0.13 is found to be good for suppressing the aggregation of cobalt particles. This modification
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also helps to distribute cobalt particles homogeneously and eventually results in showing a high catalytic performance. The decline of activity by deactivation on CoZrP catalysts is found to be in the order of CoZrP(1) N CoZrP(0) N CoZrP(0.1) N CoZrP(0.5), showing a lowest value on CoZrP(0.5) around 0.23 and highest value on CoZrP(1) around 0.71. The thermally stable zirconium phosphate species of Zr (H2PO4)2 could be partially formed on SiO2 surface because of lower P/ (P + Zr) molar ratio than stoichiometry and the decline of activity was minimized at optimum P/(P + Zr) molar ratio with a low aggregation of cobalt particles during FTS reaction.
4. Conclusions The effect of ZrP on Ru/Co/SiO2 is mainly responsible for enhancing the catalytic performance with a concomitant suppression of catalyst deactivation. It is mainly induced from the prevention of cobalt aggregation with low mobility during FTS reaction. This superior activity is achieved by spatial confinement of cobalt particles in the ZrP matrix and the optimum molar ratio of P/(Zr + P) is around 0.03– 0.13 (CoZrP(0.1) and CoZrP(0.5)). Detailed characterization to elucidate the roles of ZrP on the suppressed aggregation of cobalt particles and the chemical states of ZrP species on SiO2 surface is still under study.
Acknowledgements The authors would like to acknowledge the financial support of KEMCO and GTL Technology Development Consortium (Korea National Oil Corp., Daelim Industrial Co., LTD, Doosan Mecatec Co., LTD, Hyundai Engineering Co. LTD and SK Energy Co. LTD) under “Energy & Resources Technology Development Programs” of the Ministry of Knowledge Economy, Republic of Korea.
Fig. 3. The variation of cobalt particle size on CoZrP catalysts before and after FTS reaction; (A-1) for the fresh CoZrP(0) and (A-2) for the used CoZrP(0); (B-1) for the fresh CoZrP(0.5) and (B-2) for the used CoZrP(0.5) which are characterized by TEM analyses.
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.catcom.2010.03.003. References [1] A.Y. Khodakov, W. Chu, P. Fongarland, Chem. Rev. 107 (5) (2007) 1692. [2] G. Jacobs, T.K. Das, Y. Zhang, J. Li, G. Racoillet, B.H. Davis, Appl. Catal. A 233 (2002) 263. [3] J. Quartararo, M. Guelton, M. Rigole, J.P. Amoureux, C. Fernandez, J. Grimblot, J. Mater. Chem. 9 (1999) 2637. [4] J.W. Bae, S.M. Kim, Y.J. Lee, M.J. Lee, K.W. Jun, Catal. Commun. 10 (2009) 1358. [5] S.J. Park, J.W. Bae, J.H. Oh, K.V.R. Chary, P.S. Sai Prasad, K.W. Jun, Y.W. Rhee, J. Mol. Catal. A 298 (2009) 81. [6] J.W. Bae, S.M. Kim, S.H. Kang, K.V.R. Chary, Y.J. Lee, H.J. Kim, K.W. Jun, J. Mol. Catal. A 311 (2009) 7. [7] G.L. Bezemer, J.H. Bitter, H.P.C.E. Kuipers, H. Oosterbeek, J.E. Holewijn, X. Xu, F. Kapteijn, A. Jos van Dillen, K.P. de Jong, J. Am. Chem. Soc. 128 (2006) 3956. [8] Y.J. Lee, J.Y. Park, K.W. Jun, J.W. Bae, P.S. Sai Prasad, Catal. Lett. 130 (2009) 198.
[9] E. Iglesia, Appl. Catal. A 161 (1997) 59. [10] H. Xiong, Y. Zhang, W. Wang, J. Li, Catal. Commun. 6 (2005) 512. [11] F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero, G. Colon, J.A. Navio, M. Macias, J. Catal. 179 (1998) 483. [12] P. Salas, J.A. Wang, H. Armendariz, C. Angeles-Chavez, L.F. Chen, Mater. Chem. Phys. 114 (2009) 139. [13] Z. Zhan, H.C. Zeng, J. Non-Cryst. Solids 243 (1999) 26. [14] S.H. Song, S.B. Lee, J.W. Bae, P.S. Sai Prasad, K.W. Jun, Catal. Commun. 9 (13) (2008) 2282. [15] A.A.S. Alfaya, Y. Gushikem, S.C. de Castro, Micropor. Mesopor. Mater. 39 (2000) 57. [16] A.Y. Khodakov, J. Lynch, D. Bazin, B. Rebours, N. Zanier, B. Moisson, P. Chaumette, J. Catal. 168 (1997) 16. [17] A.Y. Khodakov, A. Griboval-Constant, R. Bechara, F. Villain, J. Phys. Chem. B 105 (2001) 9805. [18] A. Tavasoli, R.M. Malek Abbaslou, A.K. Dalai, Appl. Catal. A 346 (2008) 58. [19] D.J. Moodley, J. van de Loosdrecht, A.M. Saib, M.J. Overett, A.K. Datye, J.W. Niemantsverdriet, Appl. Catal. A 354 (2009) 102. [20] E. van Steen, M. Claeys, M.E. Dry, J. van de Loosdrecht, E.L. Viljoen, J.L. Visagie, J. Phys. Chem. B 109 (2005) 3575. [21] A.M. Saib, A. Borgna, J. van de Loosdrecht, P.J. van Berge, J.W. Niemantsverdriet, Appl. Catal. A 312 (8) (2006) 12.
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Highly active and stable catalytic performance on phosphorous-promoted Ru/Co/Zr/SiO2 Fischer–Tropsch catalyst Jong Wook Bae ⁎, Seung-Moon Kim, Seon-Ju Park, Yun-Jo Lee, Kyoung-Su Ha, Ki-Won Jun ⁎ Petroleum Displacement Technology Research Center, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yuseong, Daejeon, 305-600, Republic of Korea
a r t i c l e
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Article history: Received 31 January 2010 Received in revised form 1 March 2010 Accepted 2 March 2010 Available online 15 March 2010 Keywords: Fischer–Tropsch synthesis Cobalt Zirconium–phosphorous species Aggregation Spatial confinement
a b s t r a c t A novel doubly promoted Ru/Co/ZrP/SiO2 catalyst with zirconium–phosphorous species (ZrP) reveals high activity and stability on Fischer–Tropsch synthesis (FTS). The surface modification of SiO2 with ZrP species containing appropriate amount of phosphorous component, has an effect on the suppression of cobalt particle aggregation during FTS reaction. The spatial confinement of cobalt particles in ZrP matrix could be significantly enhanced when P/(Zr + P) molar ratio is around 0.13 (CoZrP(0.5)). © 2010 Elsevier B.V. All rights reserved.
1. Introduction The Fischer–Tropsch Synthesis (FTS) is a commercially proven technology to produce environmentally-benign fuels and chemicals from alternative energy sources [1]. Cobalt-based catalysts deposited on inorganic oxide supports such as Al2O3, TiO2 and SiO2 are frequently applied for low temperature FTS which exhibits high catalytic performance to linear paraffin formation. These supports help to disperse the cobalt particles better, and they alter the nature of supported cobalt species due to the variation of metal–support interaction. Since the moderate interaction of cobalt particle with support is important to achieve high catalytic performance, the commercial available FTS catalysts are further modified with some other promoters such as Zr, Ti and/or La components on Co/SiO2 catalyst [1,2]. These promoters are differently contributed to catalytic activity by increasing reducibility of cobalt particles or by preventing catalyst deactivation which is mainly induced from the sintering of metallic cobalt or oxidation by water generated. These promoters are expected not only to increase the FTS activity but also to help to retain high activity for longer duration by decreasing deactivation rate. Zirconium component is frequently used as one of the promoters for Co/SiO2 catalyst, however, the method for preventing abrupt deactivation at the beginning of FTS reaction is still one of the challengeable tasks for commercial-scale application. With this view in mind, we have further investigated the effects of phosphorous incorporation on Ru/Co/Zr/SiO2 system. In general, phosphorous-modified Co/Al2O3 catalyst revealed a low activity due ⁎ Corresponding authors. Bae is to be contacted at Tel.: +82 42 860 7383; fax: +82 42 860 7388. Jun, Tel.: +82 42 860 7671; fax: +82 42 860 7388. E-mail addresses: fi[email protected] (J.W. Bae), [email protected] (K.-W. Jun). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.03.003
to the hard reducibility of cobalt particles by forming strong interaction of cobalt with phosphorous component [3]. Our previous work about phosphorous-modified γ-Al2O3 support on Co/Al2O3 FTS catalyst revealed the high resistance to sintering and wax formation by the local surface transformation to aluminum phosphate with appropriate phosphorous content [4]. Even though the effect of phosphorous modification on Co/Al2O3 was already reported by our previous work, this is the first time to use phosphorous component as a promoter on Co/Zr/SiO2 catalytic system to the best of our knowledge. The properties of Co/SiO2 catalyst with the previous modification of zirconium–phosphorous species (ZrP) on SiO2 have not well established to date. In the present communication, we report the use of ZrP promotion not only to increase activity but also to enhance catalytic stability by preventing aggregation of cobalt particle during FTS reaction. 2. Catalyst preparation, activity test and characterization The ZrP-modified Ru/Co/SiO2 catalyst was prepared by stepwise impregnation with metal nitrate precursors such as zirconium oxynitrate, cobalt(II) nitrate, ruthenium(III) nitrosyl nitrate and phosphoric acid on SiO2 support (Davisil; pore size of 15.0 nm and surface area of 300 m2/g). with 3 steps as followings; (1) The 10 wt.% metals using aqueous solution of zirconium oxynitrate and phosphoric acid with a different P/(P + Zr) molar ratio were co-impregnated on SiO2 and the sample was dried at 110 °C for 12 h. (2) The cobalt precursor was successively impregnated in the slurry of the uncalcined ZrP-modified SiO2 support with 20 wt.% cobalt metal without further calcination. (3) The final catalyst was prepared by impregnation of Ru precursor with 0.5 wt.%Ru metal using previously
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prepared Co/ZrP/SiO2 catalyst. The final catalyst with 3.0 g was dried and calcined at 400 °C h for 12 h at a final step with an air flow rate of 50 ml/min with a heating rate of 10 °C/min. The final catalyst was denoted as CoZrP(×), which is representing the catalyst composition of 0.5 wt.%Ru/16.6 wt.%Co/7.5 wt.%ZrP/75.4 wt.%SiO2, with a different weight ratio of P/(P + Zr) from 0 to 0.1 (equivalent P/(P + Zr) molar ratio of 0, 0.03, 0.13 and 0.25). The × digit in CoZrP(×) catalyst stands for the values of (P/(P + Zr) × 10) in 10 wt.%ZrP/SiO2 support. Prior to activity tests, the catalysts were activated at 400 °C with a heating rate of 10 °C/min in a fixed-bed reactor (I.D. = 12.7 mm) for 12 h with a flow rate of 5%H2/He gas mixture around 20 ml/min. The activity test was carried out at differential reactor by adopting a catalyst loading of around 0.3 g with a particle size of 80–120 μm and its bed-height of around 5 mm. The activity test was conducted for around 70 h under the following reaction conditions; T = 220 °C; P = 2.0 MPa; SV (L/kgcat/h) = 2000; feed composition of H2/CO/CO2/ Ar (mol%) = 57.3/28.4/9.3/5.0 which was directly prepared by combined steam and carbon dioxide reforming of methane. The effluent gas was analyzed by an online gas chromatograph (YoungLin Acme 6000 GC) employing GS-GASPRO capillary column connected with a flame ionization detector and Porapak Q/molecular sieve (5A) packed column connected with thermal conductivity detector for the analysis of carbon oxides and hydrogen with an internal standard gas of Ar. The BET surface area (Sg; m2/g) was estimated from nitrogen desorption isotherm obtained at −196 °C using a constant–volume adsorption apparatus (Micromeritics, ASAP-2400). The surface morphology and the cobalt particle size distribution for the fresh (i.e. calcined) and used CoZrP catalysts was characterized by using transmission electron microscope (TEM) and energy electron loss spectroscopy (EELS) (TECNAI G2 instrument) analysis operating at 200 kV. The dispersion and particle size of metallic cobalt was calculated from H2 chemisorption method at 100 °C under static condition using micromeritics ASAP 2000 instrument equipped with a high vacuum pump system providing 10−6 torr. The particle size of cobalt was calculated with the assumption of H/Co metal's stoichiometry of 1. The reduction degree was determined by O2 titration on the reduced catalyst at 400 °C for 12 h and then the particle size of metallic cobalt was corrected by considering the degree of reduction. The FT-IR spectra of fresh CoZrP catalysts were recorded on a Nicolet Protege 460 FT-IR spectrometer equipped with a MCT detector which was cooled with liquid nitrogen and has a spectral resolution of 2 cm−1. The surface cobalt species after calcination and FTS reaction were characterized by using XPS (ESCALAB MK-II) analysis. The used catalyst was previously passivated with 1 vol.%O2 balanced with He gas at the temperature of FTS reaction before being exposed in air environment. The catalyst was pressed to a thin pellet before XPS analysis and the binding energy (BE) was corrected with the reference BE of C1s (284.4 eV).
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3. Results and discussion The catalytic performance of ZrP modified Ru/Co/SiO2 catalyst is summarized in Table 1. The detailed catalytic performance such as a full-range hydrocarbon distribution at two different reaction times is shown in Supplementary materials (Table S1). The surface area of CoZrP increased with the increase of phosphorous content from 217 to 232 m2/g. The particle size of cobalt was observed above 8 nm in average, calculated from H2-chemisorption method, and the degree of reduction on all CoZrP catalysts showed similar values above 73%. The CoZrP catalyst with proper amount of phosphoric acid such as CoZrP (0.1) and CoZrP(0.5) showed a stable activity after around 60 h on stream with a low decline of activity. Although all catalysts showed a high CO conversion above 90% at the very beginning of the reaction, fast deactivation was observed on CoZrP(0) and CoZrP(1) catalysts. In general, CO conversion and product distribution during FTS reaction are mainly affected by the cobalt particle size, degree of reduction, reconstruction of metallic cobalt and wax/coke deposition [5,6]. However, the cobalt-based catalyst possessing a particle size above 6– 8 nm is known to show a little effect on enhancing intrinsic FTS activity [7]. Because of high reducibility and cobalt particle size above 8 nm on all CoZrP catalysts, high initial activity above 82% at the very beginning of reaction was observed on those catalyst with high C5+ selectivity. The CO conversion and product distribution at steady-state around 70 h on stream are compared to elucidate the differences in catalytic performance. Higher CO conversion around 85.3% and C5+ selectivity around 87.9% at steady-state are attributed to the redispersion of cobalt species, low mobility of cobalt particles [8] and high reducibility on CoZrP(0.5) catalyst. Interestingly, the variation of CH4 selectivity is largely altered before and after FTS reaction. On CoZrP(0) catalyst, CH4 selectivity steadily increased with time on stream from 6.8 to 14.6% (Table S1) due to the possible reconstruction of cobalt species to form small cobalt particles below 8 nm. As reported by Bezemer et al. [7], the smaller cobalt particle size below 8 nm is responsible for showing low intrinsic CO conversion and high CH4 selectivity due to the possible character of small particle with strong interaction with support. However, all the phosphoroustreated CoZrP catalyst showed a decreased CH4 selectivity (increase of C5+ selectivity) with time on stream and this beneficial effect is further explained by the suppressed reconstruction and aggregation of cobalt particles with the help of ZrP species on SiO2 surface. The different catalytic performance on CoZrP catalyst at steadystate could be related to the degree of aggregation and mobility of cobalt particle due to the different characteristics of ZrP species on SiO2 surface by varying phosphorous content. The site-time yield (converted CO moles/surface-atom/s, s−1) on supported cobalt catalyst such as Al2O3, SiO2 and TiO2 supports [9] is generally reported in the range of 1.6 × 10−2 − 3.0 × 10−2 s−1, the value on CoZrP(0.5)
Table 1 CO conversion and product distribution on CoZrP catalysts and their physicochemical properties.a Notationb
P/(P + Zr) ratio
Sg (m2/g)
D (%)/dp (nm)
CoZrP(0) CoZrP(0.1) CoZrP(0.5) CoZrP(1)
0 0.03 0.13 0.25
217 219 231 232
11.2/8.9 11.8/8.5 8.8/11.3 9.0/11.1
a
c
Dred (%)d
CO conve (10 h → 70 h)
RDe
C5+e
Ratio of ICo/IZr
72.9 73.4 75.9 77.5
82.8 → 50.5 97.7 → 80.5 98.9 → 85.3 96.5 → 53.8
0.54 0.29 0.23 0.71
69.4 84.9 87.9 79.8
5.42 10.42 13.53 8.98
f
Reaction conditions: T = 220 °C, Pg = 2.0 MPa, SV (L/kgcat/h) = 2000, feed compositions (H2/CO/CO2/Ar; mol%) = 57.3/28.4/9.3/5.0. The notation of CoZrP(×) FT catalysts stands for the Ru/Co/ZrP/SiO2 catalysts repaired by stepwise impregnation with different wt.% of Zr/P form 10/0 to 9/1 at fixed Co concentration of 20 wt.% and 0.5 wt.% of Ru with 100 wt.%SiO2. The composition of final catalyst is 0.5 wt.%Ru/16.6 wt.%Co/7.5 wt.%ZrP/75.4 wt.%SiO2. The × digit in CoZrP(×) catalyst stands for the values of (P/(P + Zr) × 10) in 10 wt.%ZrP/SiO2 support from 0 to 0.1 of P/(P + Zr) weight ratio. c The dispersion (D) and cobalt particle size (dp) were calculated from H2 chemisorption and it was further corrected by considering the degree of reduction. d The degree of reduction (Dred; %) was calculated on the reduced catalyst at 400 °C for 12 h by O2 titration method from the amount of O2 consumption divided by the theoretical O2 consumption by using the equation of 3Co + 2O2 → Co3O4. e The decline of activity by deactivation (RD) is calculated from the difference between CO conversion at 10 h on stream and steady-state conversion at 70 h on stream; RD = (CO conversion at 10 h on stream − CO conversion at 70 h on stream)/CO conversion at 10 h on stream. The selectivity to C5+ is an averaged value at steady-state after 60 h on stream for 10 h. f The intensity ratio of I(used)/II(fresh) of ICo/IZr was recalculated from XPS analysis to elucidate the relative interaction between Co and Zr particle before and after FTS reaction. b
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catalyst is around 2 times higher as 5.53 × 10−2 s-1 on CoZrP(0.5) catalyst. Even though CoZrP(0) and CoZrP(1) catalysts showed a high reducibility and proper cobalt particle size after activation, the lower CO conversion and C5+ selectivity at steady-state are mainly attributed to the significant aggregation of cobalt particles with bimodal size distribution. The smaller cobalt particle size below 8 nm is also known to contribute the low catalytic activity and C5+ selectivity because of high formation rate of CH4. The FT-IR analysis on the calcined CoZrP catalysts is carried out to elucidate the chemical states of ZrP species on SiO2 surface and the results are shown in Fig. 1. The well-dispersed cobalt oxides on CoZrP catalysts are suggested by the intense vibration band at 665 and 565 cm−1 which is assigned to the Co–O adsorption band [10]. The band at around 1240 and 470 cm−1 could be mainly attributed to the Si–O stretching vibration mode on CoZrP catalyst and partially attributed to the triply degenerate P–O stretching vibration mode or to the triply degenerate O–P–O bending vibration mode of tetrahedral (PO4)3− [11]. The increase of intensity at 1240 cm−1 and the shift to lower frequency at the band of 1105 cm−1 are possibly attributed to the deteriorated silica framework. It is also attributed to the Zr4+ interaction on SiO2 surface due to the abundant presence of phosphorous component [12]. Furthermore, the presence of the band at 980 cm−-1 and the shift to lower frequency at 1105 cm−1 is also indicative of the increase of Zr–O–Si population on the SiO2 surface due to the phase segregation of Si–O–Si linkages [13]. Although the heterogeneity (phase segregation) of SiO2 surface induced from Si–O–Si linkages is generally suppressed with the increase of zirconium content on ZrO2–SiO2 by substitution of Si–O–Zr linkages, the heterogeneity on CoZrP catalyst increases with the increase of P/(P+Zr) molar ratio due to the possible modification of SiO2 surface. From the results of FT-IR, the observed well-defined crystalline cobalt oxides on CoZrP catalysts are responsible for the good dispersion and high reducibility which is beneficial for obtaining high FTS activity. The differences in catalytic performance and effect of ZrP species on the aggregation of cobalt particles are further understood by characterizing the used CoZrP catalysts with X-ray photoelectron microscopy (XPS) analysis. Ru promoter is well known to enhance the reducibility of Co3O4 with a proper amount (around 0.5 wt.%) due to the well-mixed cobalt and ruthenium particles without significant segregation as previously reported from our investigation [14]. Therefore, the effect of Ru promoter on CoZrP catalysts with respect to the variation in chemical state of cobalt particle and catalytic performance is not considered in the present investigation. The summarized catalytic performance at steady-state and their correlation with XPS results are shown in Table 1 and Fig. 2. The characteristic Co2p3/2 and Zr3d5/2 peaks were observed in all catalysts at the binding energy (BE) of around 776 and 178 eV respectively as shown in Figs. S1 and S2 in Supplementary materials (Fig. S1 for
Fig. 1. FT-IR spectra of the fresh calcined CoZrP FTS catalysts.
Fig. 2. Correlation between ICo/ISi and IZr/ISi ratios analyzed from XPS and CO conversion with respect to P/(P + Zr) molar ratio.
calcined catalyst and Fig. S2 for used one). The ZrP-modified used CoZrP catalysts show the BE of Co2p3/2 around 776.5–777.0 eV. The BE of Co2p3/2 slightly shifted to lower value of 775.7 eV on CoZrP(0.5) compared to that on CoZrP(0) around 775.9 eV without the presence of satellite peak on the higher BE side which is assigned to the localized amorphous Co2+ phase. The BE of Zr3d5/2 peaks on CoZrP catalysts reveals the presence of Zr4+ species. Unlike other CoZrP catalysts, the BE of Zr3d5/2 at 177.8 eV in used CoZrP(0.5) catalyst is not much altered at the end of reaction. The observed ZrP species is generally known as the hydrogen phosphate of Zr(H2PO4)2 and H2PO− 4 species which is known to be stable up to high temperature by the transformation of H2PO− 4 to pyrophosphate species with a low thermal mobility up to 1350 °C [15]. Therefore, the strong interaction of Si–P on CoZrP(1) due to the presence of unstable SiO2 surface is responsible for the shift to higher binding energy of Zr3d5/2 peak and the formation of larger ZrP particles. The small amount of phosphorous content on Zr/SiO2 such as CoZrP(0.1) and CoZrP(0.5) catalysts significantly enhances the homogeneous particle size distribution of Zr(H2PO4)2 species with a strong interaction between ZrP species and SiO2. This is confirmed by shift to lower BE side around 177.6– 177.8 eV of Zr3d5/2. These characteristics help to increase dispersion of cobalt particles with a low aggregation of cobalt particles during FTS reaction. The intensity ratio of ICo/ISi on the used CoZrP catalysts could be also related with the order of cobalt particle size after FTS reaction. With the increase of ICo/ISi ratio, the particle size of cobalt is generally getting smaller with the assumption of monolayer coverage of cobalt species on SiO2 support [1,16,17]. The higher value of ICo/ISi around 1.22 is observed on CoZrP(0.5) catalyst. The calculated intensity ratio of IZr/ISi (ratio between fresh and used ones) reveals that stable ZrP species are well dispersed on CoZrP(0.5) catalyst. It is suggested by the high value of 3.83 on CoZrP(0.5) catalyst with the formation of small ZrP particles on SiO2 surface. The low value of IZr/ISi ratio around 2.88 on CoZrP(1) suggests that the large ZrP particle formation and its formation were found to be less efficient to suppress cobalt aggregation during FTS reaction. Furthermore, the intensity ratio of ICo/IZr (ratio of used to fresh catalyst as shown in Table 1) which is calculated from the values of ICo/ISi divided by that of IZr/ISi by using the values of fresh and used CoZrP catalysts separately is also suggesting the degree of adjacent contact of Co and Zr species [16,17]. The higher value of ICo/IZr on CoZrP(0.5) around 13.53 reveals that the cobalt particles are well contacted with ZrP species with a high dispersion of cobalt particles, and vice versa on CoZrP(0) catalyst. From the XPS results on the used CoZrP catalysts, it could be concluded that the high catalytic stability and performance with
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respect to CO conversion and C5+ selectivity on CoZrP(0.5) catalyst is mainly attributed to the formation of proper cobalt particles as well as ZrP species on SiO2 surface which is helping to reduce the aggregation of cobalt species by the effective spatial confinement because of presence of well-dispersed ZrP species. The catalytic deactivation during FTS reaction is mainly attributed to the aggregation [1,9,18,19] and reoxidation [20,21] of metallic cobalt particle, however, the deactivation by reoxidation by water is known to be insignificant on cobalt particle size above 4 nm. Therefore the catalyst deactivation on CoZrP catalyst, which showed the average particle size above 8 nm, is mainly explained by aggregation of cobalt particles resulting in suppressing the amount of active sites. The cobalt particles are homogeneously dispersed on ZrP modified SiO2 surface confirmed by mapping of elemental metal dispersion (Fig. S3 in Supplementary materials) by using TEM and EELS. The representative dark field images with a mapping of elemental metal species revealed that the ZrP species are well dispersed in close proximity on SiO2 surface and the cobalt particles are formed between ZrP species. The particle size of fresh CoZrP catalysts such as CoZrP(0) in Fig. 3(A-1) and CoZrP(0.5) in Fig. 3(B-1) is not significantly different, however, the behavior of cobalt particle size variation on these catalysts is considerably different after FTS reaction with the average particle size of 12 and 18 nm separately. In the case of CoZrP(0.5) as shown in Fig. 3(B-2), the cobalt particle size is not much altered after FTS reaction due to the spatial confinement effect by ZrP species. However, bimodal particle size distribution with the presence of aggregated large cobalt particles as well as redispersion of cobalt particles with small size [9] are observed on CoZrP(0) catalyst in Fig. 3(A-2). The lower C5+ selectivity on CoZrP(0) catalyst is mainly attributed to the co-presence of small cobalt particles as reported previously [4,6,7]. The ZrP modification of SiO2 surface with an appropriate molar ratio of P/(P + Zr) around 0.03–0.13 is found to be good for suppressing the aggregation of cobalt particles. This modification
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also helps to distribute cobalt particles homogeneously and eventually results in showing a high catalytic performance. The decline of activity by deactivation on CoZrP catalysts is found to be in the order of CoZrP(1) N CoZrP(0) N CoZrP(0.1) N CoZrP(0.5), showing a lowest value on CoZrP(0.5) around 0.23 and highest value on CoZrP(1) around 0.71. The thermally stable zirconium phosphate species of Zr (H2PO4)2 could be partially formed on SiO2 surface because of lower P/ (P + Zr) molar ratio than stoichiometry and the decline of activity was minimized at optimum P/(P + Zr) molar ratio with a low aggregation of cobalt particles during FTS reaction.
4. Conclusions The effect of ZrP on Ru/Co/SiO2 is mainly responsible for enhancing the catalytic performance with a concomitant suppression of catalyst deactivation. It is mainly induced from the prevention of cobalt aggregation with low mobility during FTS reaction. This superior activity is achieved by spatial confinement of cobalt particles in the ZrP matrix and the optimum molar ratio of P/(Zr + P) is around 0.03– 0.13 (CoZrP(0.1) and CoZrP(0.5)). Detailed characterization to elucidate the roles of ZrP on the suppressed aggregation of cobalt particles and the chemical states of ZrP species on SiO2 surface is still under study.
Acknowledgements The authors would like to acknowledge the financial support of KEMCO and GTL Technology Development Consortium (Korea National Oil Corp., Daelim Industrial Co., LTD, Doosan Mecatec Co., LTD, Hyundai Engineering Co. LTD and SK Energy Co. LTD) under “Energy & Resources Technology Development Programs” of the Ministry of Knowledge Economy, Republic of Korea.
Fig. 3. The variation of cobalt particle size on CoZrP catalysts before and after FTS reaction; (A-1) for the fresh CoZrP(0) and (A-2) for the used CoZrP(0); (B-1) for the fresh CoZrP(0.5) and (B-2) for the used CoZrP(0.5) which are characterized by TEM analyses.
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.catcom.2010.03.003. References [1] A.Y. Khodakov, W. Chu, P. Fongarland, Chem. Rev. 107 (5) (2007) 1692. [2] G. Jacobs, T.K. Das, Y. Zhang, J. Li, G. Racoillet, B.H. Davis, Appl. Catal. A 233 (2002) 263. [3] J. Quartararo, M. Guelton, M. Rigole, J.P. Amoureux, C. Fernandez, J. Grimblot, J. Mater. Chem. 9 (1999) 2637. [4] J.W. Bae, S.M. Kim, Y.J. Lee, M.J. Lee, K.W. Jun, Catal. Commun. 10 (2009) 1358. [5] S.J. Park, J.W. Bae, J.H. Oh, K.V.R. Chary, P.S. Sai Prasad, K.W. Jun, Y.W. Rhee, J. Mol. Catal. A 298 (2009) 81. [6] J.W. Bae, S.M. Kim, S.H. Kang, K.V.R. Chary, Y.J. Lee, H.J. Kim, K.W. Jun, J. Mol. Catal. A 311 (2009) 7. [7] G.L. Bezemer, J.H. Bitter, H.P.C.E. Kuipers, H. Oosterbeek, J.E. Holewijn, X. Xu, F. Kapteijn, A. Jos van Dillen, K.P. de Jong, J. Am. Chem. Soc. 128 (2006) 3956. [8] Y.J. Lee, J.Y. Park, K.W. Jun, J.W. Bae, P.S. Sai Prasad, Catal. Lett. 130 (2009) 198.
[9] E. Iglesia, Appl. Catal. A 161 (1997) 59. [10] H. Xiong, Y. Zhang, W. Wang, J. Li, Catal. Commun. 6 (2005) 512. [11] F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero, G. Colon, J.A. Navio, M. Macias, J. Catal. 179 (1998) 483. [12] P. Salas, J.A. Wang, H. Armendariz, C. Angeles-Chavez, L.F. Chen, Mater. Chem. Phys. 114 (2009) 139. [13] Z. Zhan, H.C. Zeng, J. Non-Cryst. Solids 243 (1999) 26. [14] S.H. Song, S.B. Lee, J.W. Bae, P.S. Sai Prasad, K.W. Jun, Catal. Commun. 9 (13) (2008) 2282. [15] A.A.S. Alfaya, Y. Gushikem, S.C. de Castro, Micropor. Mesopor. Mater. 39 (2000) 57. [16] A.Y. Khodakov, J. Lynch, D. Bazin, B. Rebours, N. Zanier, B. Moisson, P. Chaumette, J. Catal. 168 (1997) 16. [17] A.Y. Khodakov, A. Griboval-Constant, R. Bechara, F. Villain, J. Phys. Chem. B 105 (2001) 9805. [18] A. Tavasoli, R.M. Malek Abbaslou, A.K. Dalai, Appl. Catal. A 346 (2008) 58. [19] D.J. Moodley, J. van de Loosdrecht, A.M. Saib, M.J. Overett, A.K. Datye, J.W. Niemantsverdriet, Appl. Catal. A 354 (2009) 102. [20] E. van Steen, M. Claeys, M.E. Dry, J. van de Loosdrecht, E.L. Viljoen, J.L. Visagie, J. Phys. Chem. B 109 (2005) 3575. [21] A.M. Saib, A. Borgna, J. van de Loosdrecht, P.J. van Berge, J.W. Niemantsverdriet, Appl. Catal. A 312 (8) (2006) 12.