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CO hydrogenation over zirconia supported iron catalysts promoted with rare earth oxides
~
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 158 (1997) 215-223
CO hydrogenation over zirconia supported iron catalysts promoted with rare earth oxides Kaidong Chen, Qijie Yan* Department of Chemist~, Nanjing University, Nanjing 210093, China Received 11 July 1996; received in revised form 4 November 1996; accepted 22 November 1996
Abstract
Zirconia supported iron catalysts promoted with La203 and Get2, respectively, are prepared by sequential impregnation method. It is shown that the structure, reduction behavior, and the catalytic property for CO hydrogenation of these catalysts are quite different from that of the unpromoted Fe/ZrO2 sample, and are influenced significantly by the kinds of promoter elements. For the Fe/La/ZrO2 sample, the catalytic activity is only a little bit higher than that of the Fe/ZrO2 sample, but light olefins selectivity increases and methane formation is suppressed. For the Fe/Ce/ZrO2 sample, the catalytic activity is much higher than that of the unpromoted Fe/ZrO2 sample, while methane formation is suppressed and light olefins selectivity slightly increases. The roles of different rare earth oxide promoters in these catalysts are discussed. Keywords: CO hydrogenation; Iron-zirconia catalyst; Lanthanum promoter; Cerium promoter
1. Introduction
Recent research efforts in FT synthesis have been aimed towards more selective catalysts that demonstrate high selectivity for C2-C4 hydrocarbons [ 1]. It has been shown [2-7] that iron catalysts promoted with some transition metal oxides like MnO, T i t 2 and V205 show unusually high selectivity for lower alkenes and suppress methane formation. The promotion effects of these additives have been discussed in detail [2-7]. Recently, it has been discovered that some rare earth oxides like La203 and CeO2 exert an influence on CO hydrogenation over supported metal catalysts [8-11 ]. For instance, Barrault et al. [9] reported that lanthanum or cerium oxide promoters * Corresponding author. 0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 6 ) 0 0 4 16-4
216
K. Chen, Q. Yan/Applied Catalysis A: General 158 (1997) 215-223
improved the total activity and increased the selectivity to alkenes and higher hydrocarbons of carbon-supported Co and Ru catalysts. However, studies on lanthana or ceria used as promoters to enhance the selectivity to light olefins in hydrogenation of CO are rather limited. On the other hand, although iron-based catalysts are a very important system for selective synthesis of light olefins, few works have discussed rare earth oxides promoted iron catalysts in CO hydrogenation. We have shown that zirconia supported iron catalyst is effective in CO hydrogenation to light olefins [ 12]. In the present paper, we first report Fe/Ce/ZrO2 and Fe/La/ZrO2 catalysts for hydrogenation of CO. It is observed that the Fe/Ce/ ZrO2 catalyst exhibits much higher catalytic activity than the Fe/ZrO2 and Fe/La/ ZrO2 catalysts. For both Fe/Ce/ZrO2 and Fe/La/ZrO2 catalysts, methane formation is suppressed and light olefins selectivity increases. The roles of the rare earth oxides in these samples are discussed.
2. Experimental Various catalysts were prepared by sequential incipient wetness impregnation method. First, zirconium oxide (AR. Shanghai) with surface area of 39 mZ/g was impregnated with La(III) or Ce(III) nitrate solution, and then dried at 393 K and calcined at 773 K for 5 h. Second, the obtained samples were impregnated with iron(III) nitrate solution and then dried and calcined at the same above conditions. For all samples, Fe203 loading is ca. 7 wt% and the Fe/La or Fe/Ce atomic ratio is 2:1. X-ray diffraction (XRD), M6ssbauer spectroscopy, temperature-programmed reduction, X-ray photoelectron spectroscopy and CO hydrogenation reaction test were carried out as described in [12]. The reaction condition was 643 K, 1 atm, H2/CO= 1.7 and GHSV ca. 700 h -I . Temperature-programmed desorption experiments of CO were carried out in a U-type quartz reactor with a heating rate of 16 K/min, and a flow rate of He ca. 30 ml/min. Before CO-TPD experiment, the asprepared samples were reduced in pure H 2 at 673 K for 2 h, and then blown in pure He flow at 723 K for 1 h for removal of the adsorbed hydrogen, and then cooled to room temperature. CO adsorption was carried out by passing a flow of CO for 0.5 h, and then a He flow for 0.5 h for removal of the physisorbed gas. CO desorption was monitored by a thermal conductivity detector.
3. Results and discussion 3.1. Structure
XRD and Mrssbauer spectroscopy are used to study the structure of various catalysts. Mrssbauer spectra of various samples are shown in Fig. 1, the corre-
K. Chen, Q. Yan/Applied Catalysis A: General 158 (1997) 215-223
217
CO
Q_
0 O3 ..(3 <~
'8
-,z
-'4
'
i
'
Telocitylnnnnls 2
Fig. 1. M6ssbauer spectra of various catalysts: (a) Fe/ZrO2, (b) Fe/Ce/ZrO2, (c) Fe/La/ZrO2.
Table 1 M6ssbauer parameters of various catalysts Sample
Fe/ZrO2 Fe/La/ZrO2 Fe/Ce/ZrO z
M6ssbauer parameters IS (mnds)
QS (turn/s)
H(KOe)
0.35 0,35 0.31 0,38 0,31
0.25 0.94 1.01 0.22 0.94
514 0 0 516 0
Iron species
Relative area (%)
Fe 3+ Fe 3+ Fe 3+ Fe 3+ Fe 3+
28 72 100 71 29
sponding M6ssbauer parameters are listed in Table 1. For the unpromoted Fe/ZrO2 sample, weak XRD lines of a-Fe203 are observed in the XRD pattern. The appearance of the sextet in the M6ssbauer spectrum also indicates that 28% Fe203 large particles exist in this sample. For the sample Fe/La/ZrO2, XRD lines of ironoxide are not observed and only a doublet assigned to superparamagnetic Fe 3+ cations is shown in the M6ssbauer spectrum. This suggests that the La203 layer on the surface of zirconia significantly enhances the dispersion of Fe203 particles. This result is consistent with that of Zhou et al [10]. They studied structure and reduction properties of Fe/La/'y-AlzO3 samples, and confirmed that the La203 layer dispersed on "~-AI203 support could obviously enhance the dispersion of iron oxide. For Fe/Ce/ZrO2 sample, XRD lines of a-Fe203 are clearly observed, and M6ssbauer result indicates that there are 71% large particles Fe203, suggesting that
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K. Chen, Q. Yan/Applied Catalysis A: General 158 (1997) 215-223
difference from La203, the CeO2 layer on the surface of the zirconia decreases the dispersion of Fe203 particles. From the above results, we can conclude that the structure of the rare earth oxides promoted Fe/ZrO2 catalysts is significantly affected by the promoters. 3.2. Reduction behavior
TPR profiles of various samples are shown in Fig. 2. A very weak reduction peak at ca. 897 K is observed for pure La203, indicating that the reduction of La203 particles is difficult. But for pure CeO2, two reduction peaks at ca. 687 and 839 K, respectively, are observed, indicating that partial reduction of CeO2 particles is easier than that of La203. This phenomenon has also been observed by many workers [11,13,14]. As reported by Guerrero-Ruiz et al. [11], the hydrogen consumption at lower temperature can be due to the reduction of surface ions, while the reduction peak at higher temperature can be due to the elimination of bulk oxygen anions. For unpromoted Fe/ZrO2 sample, it has been shown [ 12] that the three reduction peaks correspond to Fe203 + H2 --~ Fe304 + H20
(1)
Fe304 ÷ H2 ~ FeO + H20
(2)
FeO+H2---+Fe+H20
and
Fe203+H2~Fe÷H20
(3)
respectively. From Fig. 2 it may be seen that the three reduction peak maxima of sample Fe/La/ZrO2 are much higher than their counterparts in unpromoted Fe/ZrO2, suggesting that the reduction of Fe 3+ to Fe ° becomes difficult owing
(a)
(6) (e) -(e) i
273
473
- -
i
i
673
873
Temperature (K)
07
I 73
Fig. 2. TPR profiles of various samples: (a) Fe]ZrO 2, (b) Fe/Ce/ZrO2, (c) Fe/La/ZrO2, (d) CeO2, (e) La203.
K. Chen, Q. Yan/Applied Catalysis A: General 158 (1997) 215-223
219
Table 2 M6ssbauer parameters of various reduced catalysts Species
Fe/ZrO2 Fe/La/ZrO2 Fe/Ce/ZrO2
M6ssbauer parameters IS (ram/s)
QS (ram/s)
H(KOe)
0.00 0.33 0.02 0.26 0.01 0.26
0.00 0.89 0.07 0.96 0.05 1.01
331 0 330 0 333 0
Iron species
Relative area (%)
Fe ° Fe 3Fe ° Fe 3+ Fe ° Fe 3+
77 23 45 55 78 22
to the addition of La203. Similar result was also obtained by Zhou et al. [10]. They studied the reduction properties of Fe/La/7-AI203 samples by using TPR and M6ssbauer spectroscopy, and concluded that the reduction of iron oxide for Fe/La/ "y-AI203 sample was much more difficult than that of the Fe/')'-AI203, only a little amount of iron oxides could be reduced to Fe°. The reduction process and peak maxima of sample Fe/Ce/ZrO2 are similar to that of the Fe/ZrO2 catalyst, three reduction peaks are observed too (Fig. 2). However, for the sample Fe/Ce/ZrO2, the relative area of the second peak is larger than that of the sample Fe/ZrO2, indicating the enhancement of the reduction of F e 3 0 4 to FeO in this sample due to the addition of ceria. M6ssbauer parameters of the Fe/ZrO2, Fe/Ce/ZrO2 and Fe/La/ZrO2 samples reduced in pure H2 at 673 K for 2 h are listed in Table 2. It is shown that ca. 77% Fe 3+ is reduced to Fe° for sample Fe/ZrO2 and Fe/Ce/ZrO2, while 45% Fe 3+ for the sample Fe/La/ZrO2 is reduced. This result also confirms that the reduction of Fe 3+ to Fe ° becomes difficult for sample Fe/La/ZrO2, and is consistent with that of the TPR results. The XRD and M6ssbauer results show that iron oxide particles highly disperse on the La203 layer for sample Fe/La/ZrO2. The strong interactions between Fe203 and La203 might lead to the difficult reduction of Fe 3+ to Fe °.
3.3. Co desorption properties CO-TPD profiles of various samples are shown in Fig. 3. It can be seen that the TPD profiles of Fe/La/ZrO2 and Fe/Ce/ZrO2 samples are quite different from that of the unpromoted Fe/ZrO2 sample. For unpromoted Fe/ZrO2, only a desorption peak at maximum ca. 673 K is observed. But for Fe/La/ZrO2 and Fe/Ce/ZrO2 samples, two or more consecutive desorption peaks are shown, and the main desorption peak shifts to higher temperature compared to that of the unpromoted Fe/ZrO2 sample, indicating an enhancement of Fe-C bond and a weakening of C-O bond. Since the surface characteristic of the catalysts might have been obviously changed by the addition of lanthana or ceria, several kinds of CO adsorption sites might appear for the Fe/La/ZrO2 and Fe/Ce/ZrO2 samples. On the
K. Chen, Q. Yan/Applied Catalysis A: General 158 (1997) 215-223
220
(c)
(b) (a) t
]
i
---7~
Gr3
I
873
Temperature (K)
075
Fig. 3. CO-TPD profiles of various catalysts: (a) Fe/ZrO2, (b) Fe/Ce/ZrO2, (c) Fe/La/ZrO2.
other hand, a s L a 3+ and C e 4+ ions show basic properties, formation of carbonate like structures by interaction of CO with adsorbed oxygen or lattice 0 2 - ions is expected to occur, and hence a little amount of CO2 is most likely formed upon thermal decomposition of surface carbonates. This phenomenon has already been found in previous studies on LaCoO3 [15], Ni/La/7-A1203 [16], Co/Ce/C and Ru/Ce/C [13] systems. Therefore the CO-TPD profiles of Fe/La/ZrO2 and Fe/Ce/ZrO2 samples are rather complex. Similar results have been reported by Rodrfguez-Ramos et al. [13]. However, It is clear, from the CO-TPD results, that the C-O bond is weakened by the addition of rare earth oxide promoters, and the dissociation and reaction of CO might become easier. This may result in the increase of the catalytic activity and light olefins selectivity for hydrogenation of CO.
3.4. Catalytic properties Table 3 lists the catalytic activity and selectivity of various catalysts for CO hydrogenation. The CO conversion of the sample Fe/La/ZrO2 is only a little bit Table 3 CO hydrogenation on various catalysts Catalyst
Fe/ZrO2 Fe/La/ZrO2 Fe/Ce/ZrO2
CO Conv. (%)
5.5 5.6 12.0
Hydrocarbon distribution (wt%) CI
C2
C3
C4
C5
C~5/C2-5 (%)
43.1 36.1 35.2
28.9 29.8 29.8
22.0 23.6 24.1
6,0 10.0 9.2
-1.5 1.7
73.8 78.4 74.4
K. Chen, Q. Yan/Applied Catalysis A: General 158 (1997) 215-223
221
higher than that of the unpromoted Fe/ZrO2 sample, but methane formation is suppressed and the light olefins selectivity increases. This result is similar to that of the Co/La/C [9] catalyst. As described in [9,16-20], the promotion effect of the lanthanum oxide can be explained by the basic character and the electronic effect of lanthana, which increases the electron density of active phase iron and weakens the C-O bond and enhances the dissociation of CO as shown in CO-TPD result, hence CO reactivity is enhanced, meanwhile, methane formation is suppressed and light olefins selectivity increases. However, for the Fe/La/ZrO2 catalyst, iron-oxide particles are highly dispersed on the surface of La203 layer, reduction of Fe 3+ to Fe ° becomes difficult owing to the strong interactions between iron oxide and La203, this may result in the decrease of the catalytic activity due to the decrease of the amount of active phase metallic iron [12]. As a total result, the CO conversion of this sample is only a little bit higher than that of the Fe/Zr sample, but methane formation is suppressed and light olefins selectivity increases. It is pertinent to note that for the Fe/Ce/ZrO2 sample, CO conversion is much higher than that of the unpromoted Fe/ZrO2 catalyst, methane formation is suppressed and light olefins selectivity slightly increases. XPS is used to study the surface species of Fe/Ce/ZrO2 sample. It is shown that for the as-prepared sample, the surface Ce/Fe atomic ratio is 1.11, but for the catalyst after reacted in syngas for 10 h, the surface Ce/Fe atomic ratio becomes 1.58. This result indicates that some Ce 4+ cations in the vicinity of iron particles might migrate onto the surface of metallic iron particles forming very small CeO2 islands during the reaction process. Our TPR result indicates that the surface C e O 2 can be reduced in the reaction condition, XPS results also show that Ce 3+ and Ce 4+ ions coexist on the surface of the catalyst after reacting in syngas, hence we suggest that new catalytic active sites (Fe-CeO2 ensembles) might be formed in this sample. CO can bond with its carbon atom to iron atom and with its oxygen atom to the adjacent partially reduced Ce203. This arrangement will result in a weakening of the C-O bond. CO can easily dissociate and the Ce203 particles are reoxidized to G e t 2 by the oxygen atom dissociated from CO. Being redox centers, ceria can rapidly transfer the oxygen atom dissociated from CO, thus the dissociation of C-O bond and reactivity of CO are markedly enhanced. Similar conclusions have also been obtained in Co/Ce/C [9,11] and Ru/Ce/C [11] systems. For the Fe/Ce/ZrO2sample, CO dissociation and reactivity are obviously enhanced owing to the formation of the new catalytic active sites, the concentration of the surface CHx species might increase too. This may result in the increase of the rate constant for polymerization of CHx species, hence the catalytic activity and light olefins selectivity increase, while methane formation is suppressed. For the Fe/La/ZrO2 sample, reduction of La203 is quite difficult, there are not new catalytic active sites formed, and the metallic iron content is rather low, hence the catalytic activity is lower than that of the Fe/Ce/ ZrO2 sample.
K. Chen, Q. Yan/Applied Catalysis A: General 158 (1997) 215-223
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4. Conclusion The results presented in this study show that the structure, reduction behavior, and the catalytic properties for CO hydrogenation of rare earth oxides promoted Fe/ZrO2 catalysts are quite different from that of the unpromoted Fe/ZrO2 sample, and are influenced significantly by the kinds of promoter elements. For the Fe/La/ ZrO2 sample, Fe203 particles highly disperse on the La203 layer, reduction of iron oxide becomes difficult owing to the strong interactions between iron oxide and La203. The catalytic activity is only a little bit higher than that of the Fe/ZrO2 sample, but light olefins selectivity increases and methane formation is suppressed. The promotion effect of lanthanum can be attributed to the basic character and the electronic effect of lanthana layer, which increases the electron density of the iron particles and enhances the dissociation of CO. For the Fe/Ce/ZrO2 sample, dispersion of iron oxide decreases by the addition of CeO2 layer. The catalytic activity is much higher than that of the unpromoted Fe/ZrO2 sample, while methane formation is suppressed and light olefins selectivity slightly increases. The promotion effect of ceria can be explained in terms of the formation of new catalytic active sites (Fe-CeO2 ensembles) at which the CO may bond with its carbon atom to iron atom and its oxygen atom to the adjacent partially reduced cerium oxide. This arrangement results in a weakening of the C-O bond, and the dissociation and reaction of CO is markedly enhanced.
Acknowledgements The support of the National Natural Science Foundation of China is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [ 11 ] [12] [13] [14]
M. Janardanro, Ind. Eng. Chem. Res., 29 (1990) 1735. J. Barrault and C. Renard, Appl. Catal., 14 (1985) 133. J. Barrault, C. Forquy and V. Perrichon, Appl. Catal., 5 (1983) 119. M.A. Vannice, J. Catal., 74 (1982) 199. H. Arai, K. Mitsuishi and T. Seiyama, Chem. Lett., (1984) 1291. U. Lochner, H. Papp and M. Baerns, Appl. Catal., 23 (1986) 339. B. Cornils, C.D. Frohning, K. Moraw, Proceedings of Eighth International Congress on Catalysis, Berlin, vol. 2, Verlag Chemie, Weinheim, 1984, p. 23. R. Pierantozzi, Chem. Ind., 22 (1985) 115. J. Barrault and A. Guilleminot, Appl. Catal., 21 (1986) 307. Y.M. Zhou and Y.R. Ding, CuiHua XueBao, 12 (1991) 167. A. Guerrero-Ruiz, A. Sep61veda-Escribano and I. Rodrfguez-Ramos, Appl. Catal., 120 (1994) 71. K.D. Chen, Y.N. Fan, Z. Hu and Q.J. Yan, Catal. Lett., 36 (1996) 139. I. Roddguez-Rarnos, A. Guerrero-Ruiz and J.L.G. Fierro, Appl. Surf. Sci., 40 (1989) 239. T. Arai, K. Maruya, K. Domen and T. Onishi, J. Catal., 141 (1993) 533.
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L. Gonz~ilez Tejuca, A.T. Bell, J.L.G. Fierro and M.A. Pena, Appl. Surf. Sci., 31 (1988) 301. X.H. Liu, Y. Miao and X.L. Li, Acta Physico-Chimica Sinica, 5 (1991) 217. J.S. Rieck and A.T. Bell, J. Catal., 96 (1985) 88. R.F. Hicks and A.T. Bell, J. Catal., 90 (1984) 205. R.E Hicks and A.T. Bell, J. Catal., 91 (1985) 105. M.P. Rosynek, J. Catal., 48 (1977) 417.
223
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 158 (1997) 215-223
CO hydrogenation over zirconia supported iron catalysts promoted with rare earth oxides Kaidong Chen, Qijie Yan* Department of Chemist~, Nanjing University, Nanjing 210093, China Received 11 July 1996; received in revised form 4 November 1996; accepted 22 November 1996
Abstract
Zirconia supported iron catalysts promoted with La203 and Get2, respectively, are prepared by sequential impregnation method. It is shown that the structure, reduction behavior, and the catalytic property for CO hydrogenation of these catalysts are quite different from that of the unpromoted Fe/ZrO2 sample, and are influenced significantly by the kinds of promoter elements. For the Fe/La/ZrO2 sample, the catalytic activity is only a little bit higher than that of the Fe/ZrO2 sample, but light olefins selectivity increases and methane formation is suppressed. For the Fe/Ce/ZrO2 sample, the catalytic activity is much higher than that of the unpromoted Fe/ZrO2 sample, while methane formation is suppressed and light olefins selectivity slightly increases. The roles of different rare earth oxide promoters in these catalysts are discussed. Keywords: CO hydrogenation; Iron-zirconia catalyst; Lanthanum promoter; Cerium promoter
1. Introduction
Recent research efforts in FT synthesis have been aimed towards more selective catalysts that demonstrate high selectivity for C2-C4 hydrocarbons [ 1]. It has been shown [2-7] that iron catalysts promoted with some transition metal oxides like MnO, T i t 2 and V205 show unusually high selectivity for lower alkenes and suppress methane formation. The promotion effects of these additives have been discussed in detail [2-7]. Recently, it has been discovered that some rare earth oxides like La203 and CeO2 exert an influence on CO hydrogenation over supported metal catalysts [8-11 ]. For instance, Barrault et al. [9] reported that lanthanum or cerium oxide promoters * Corresponding author. 0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 6 ) 0 0 4 16-4
216
K. Chen, Q. Yan/Applied Catalysis A: General 158 (1997) 215-223
improved the total activity and increased the selectivity to alkenes and higher hydrocarbons of carbon-supported Co and Ru catalysts. However, studies on lanthana or ceria used as promoters to enhance the selectivity to light olefins in hydrogenation of CO are rather limited. On the other hand, although iron-based catalysts are a very important system for selective synthesis of light olefins, few works have discussed rare earth oxides promoted iron catalysts in CO hydrogenation. We have shown that zirconia supported iron catalyst is effective in CO hydrogenation to light olefins [ 12]. In the present paper, we first report Fe/Ce/ZrO2 and Fe/La/ZrO2 catalysts for hydrogenation of CO. It is observed that the Fe/Ce/ ZrO2 catalyst exhibits much higher catalytic activity than the Fe/ZrO2 and Fe/La/ ZrO2 catalysts. For both Fe/Ce/ZrO2 and Fe/La/ZrO2 catalysts, methane formation is suppressed and light olefins selectivity increases. The roles of the rare earth oxides in these samples are discussed.
2. Experimental Various catalysts were prepared by sequential incipient wetness impregnation method. First, zirconium oxide (AR. Shanghai) with surface area of 39 mZ/g was impregnated with La(III) or Ce(III) nitrate solution, and then dried at 393 K and calcined at 773 K for 5 h. Second, the obtained samples were impregnated with iron(III) nitrate solution and then dried and calcined at the same above conditions. For all samples, Fe203 loading is ca. 7 wt% and the Fe/La or Fe/Ce atomic ratio is 2:1. X-ray diffraction (XRD), M6ssbauer spectroscopy, temperature-programmed reduction, X-ray photoelectron spectroscopy and CO hydrogenation reaction test were carried out as described in [12]. The reaction condition was 643 K, 1 atm, H2/CO= 1.7 and GHSV ca. 700 h -I . Temperature-programmed desorption experiments of CO were carried out in a U-type quartz reactor with a heating rate of 16 K/min, and a flow rate of He ca. 30 ml/min. Before CO-TPD experiment, the asprepared samples were reduced in pure H 2 at 673 K for 2 h, and then blown in pure He flow at 723 K for 1 h for removal of the adsorbed hydrogen, and then cooled to room temperature. CO adsorption was carried out by passing a flow of CO for 0.5 h, and then a He flow for 0.5 h for removal of the physisorbed gas. CO desorption was monitored by a thermal conductivity detector.
3. Results and discussion 3.1. Structure
XRD and Mrssbauer spectroscopy are used to study the structure of various catalysts. Mrssbauer spectra of various samples are shown in Fig. 1, the corre-
K. Chen, Q. Yan/Applied Catalysis A: General 158 (1997) 215-223
217
CO
Q_
0 O3 ..(3 <~
'8
-,z
-'4
'
i
'
Telocitylnnnnls 2
Fig. 1. M6ssbauer spectra of various catalysts: (a) Fe/ZrO2, (b) Fe/Ce/ZrO2, (c) Fe/La/ZrO2.
Table 1 M6ssbauer parameters of various catalysts Sample
Fe/ZrO2 Fe/La/ZrO2 Fe/Ce/ZrO z
M6ssbauer parameters IS (mnds)
QS (turn/s)
H(KOe)
0.35 0,35 0.31 0,38 0,31
0.25 0.94 1.01 0.22 0.94
514 0 0 516 0
Iron species
Relative area (%)
Fe 3+ Fe 3+ Fe 3+ Fe 3+ Fe 3+
28 72 100 71 29
sponding M6ssbauer parameters are listed in Table 1. For the unpromoted Fe/ZrO2 sample, weak XRD lines of a-Fe203 are observed in the XRD pattern. The appearance of the sextet in the M6ssbauer spectrum also indicates that 28% Fe203 large particles exist in this sample. For the sample Fe/La/ZrO2, XRD lines of ironoxide are not observed and only a doublet assigned to superparamagnetic Fe 3+ cations is shown in the M6ssbauer spectrum. This suggests that the La203 layer on the surface of zirconia significantly enhances the dispersion of Fe203 particles. This result is consistent with that of Zhou et al [10]. They studied structure and reduction properties of Fe/La/'y-AlzO3 samples, and confirmed that the La203 layer dispersed on "~-AI203 support could obviously enhance the dispersion of iron oxide. For Fe/Ce/ZrO2 sample, XRD lines of a-Fe203 are clearly observed, and M6ssbauer result indicates that there are 71% large particles Fe203, suggesting that
218
K. Chen, Q. Yan/Applied Catalysis A: General 158 (1997) 215-223
difference from La203, the CeO2 layer on the surface of the zirconia decreases the dispersion of Fe203 particles. From the above results, we can conclude that the structure of the rare earth oxides promoted Fe/ZrO2 catalysts is significantly affected by the promoters. 3.2. Reduction behavior
TPR profiles of various samples are shown in Fig. 2. A very weak reduction peak at ca. 897 K is observed for pure La203, indicating that the reduction of La203 particles is difficult. But for pure CeO2, two reduction peaks at ca. 687 and 839 K, respectively, are observed, indicating that partial reduction of CeO2 particles is easier than that of La203. This phenomenon has also been observed by many workers [11,13,14]. As reported by Guerrero-Ruiz et al. [11], the hydrogen consumption at lower temperature can be due to the reduction of surface ions, while the reduction peak at higher temperature can be due to the elimination of bulk oxygen anions. For unpromoted Fe/ZrO2 sample, it has been shown [ 12] that the three reduction peaks correspond to Fe203 + H2 --~ Fe304 + H20
(1)
Fe304 ÷ H2 ~ FeO + H20
(2)
FeO+H2---+Fe+H20
and
Fe203+H2~Fe÷H20
(3)
respectively. From Fig. 2 it may be seen that the three reduction peak maxima of sample Fe/La/ZrO2 are much higher than their counterparts in unpromoted Fe/ZrO2, suggesting that the reduction of Fe 3+ to Fe ° becomes difficult owing
(a)
(6) (e) -(e) i
273
473
- -
i
i
673
873
Temperature (K)
07
I 73
Fig. 2. TPR profiles of various samples: (a) Fe]ZrO 2, (b) Fe/Ce/ZrO2, (c) Fe/La/ZrO2, (d) CeO2, (e) La203.
K. Chen, Q. Yan/Applied Catalysis A: General 158 (1997) 215-223
219
Table 2 M6ssbauer parameters of various reduced catalysts Species
Fe/ZrO2 Fe/La/ZrO2 Fe/Ce/ZrO2
M6ssbauer parameters IS (ram/s)
QS (ram/s)
H(KOe)
0.00 0.33 0.02 0.26 0.01 0.26
0.00 0.89 0.07 0.96 0.05 1.01
331 0 330 0 333 0
Iron species
Relative area (%)
Fe ° Fe 3Fe ° Fe 3+ Fe ° Fe 3+
77 23 45 55 78 22
to the addition of La203. Similar result was also obtained by Zhou et al. [10]. They studied the reduction properties of Fe/La/7-AI203 samples by using TPR and M6ssbauer spectroscopy, and concluded that the reduction of iron oxide for Fe/La/ "y-AI203 sample was much more difficult than that of the Fe/')'-AI203, only a little amount of iron oxides could be reduced to Fe°. The reduction process and peak maxima of sample Fe/Ce/ZrO2 are similar to that of the Fe/ZrO2 catalyst, three reduction peaks are observed too (Fig. 2). However, for the sample Fe/Ce/ZrO2, the relative area of the second peak is larger than that of the sample Fe/ZrO2, indicating the enhancement of the reduction of F e 3 0 4 to FeO in this sample due to the addition of ceria. M6ssbauer parameters of the Fe/ZrO2, Fe/Ce/ZrO2 and Fe/La/ZrO2 samples reduced in pure H2 at 673 K for 2 h are listed in Table 2. It is shown that ca. 77% Fe 3+ is reduced to Fe° for sample Fe/ZrO2 and Fe/Ce/ZrO2, while 45% Fe 3+ for the sample Fe/La/ZrO2 is reduced. This result also confirms that the reduction of Fe 3+ to Fe ° becomes difficult for sample Fe/La/ZrO2, and is consistent with that of the TPR results. The XRD and M6ssbauer results show that iron oxide particles highly disperse on the La203 layer for sample Fe/La/ZrO2. The strong interactions between Fe203 and La203 might lead to the difficult reduction of Fe 3+ to Fe °.
3.3. Co desorption properties CO-TPD profiles of various samples are shown in Fig. 3. It can be seen that the TPD profiles of Fe/La/ZrO2 and Fe/Ce/ZrO2 samples are quite different from that of the unpromoted Fe/ZrO2 sample. For unpromoted Fe/ZrO2, only a desorption peak at maximum ca. 673 K is observed. But for Fe/La/ZrO2 and Fe/Ce/ZrO2 samples, two or more consecutive desorption peaks are shown, and the main desorption peak shifts to higher temperature compared to that of the unpromoted Fe/ZrO2 sample, indicating an enhancement of Fe-C bond and a weakening of C-O bond. Since the surface characteristic of the catalysts might have been obviously changed by the addition of lanthana or ceria, several kinds of CO adsorption sites might appear for the Fe/La/ZrO2 and Fe/Ce/ZrO2 samples. On the
K. Chen, Q. Yan/Applied Catalysis A: General 158 (1997) 215-223
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(c)
(b) (a) t
]
i
---7~
Gr3
I
873
Temperature (K)
075
Fig. 3. CO-TPD profiles of various catalysts: (a) Fe/ZrO2, (b) Fe/Ce/ZrO2, (c) Fe/La/ZrO2.
other hand, a s L a 3+ and C e 4+ ions show basic properties, formation of carbonate like structures by interaction of CO with adsorbed oxygen or lattice 0 2 - ions is expected to occur, and hence a little amount of CO2 is most likely formed upon thermal decomposition of surface carbonates. This phenomenon has already been found in previous studies on LaCoO3 [15], Ni/La/7-A1203 [16], Co/Ce/C and Ru/Ce/C [13] systems. Therefore the CO-TPD profiles of Fe/La/ZrO2 and Fe/Ce/ZrO2 samples are rather complex. Similar results have been reported by Rodrfguez-Ramos et al. [13]. However, It is clear, from the CO-TPD results, that the C-O bond is weakened by the addition of rare earth oxide promoters, and the dissociation and reaction of CO might become easier. This may result in the increase of the catalytic activity and light olefins selectivity for hydrogenation of CO.
3.4. Catalytic properties Table 3 lists the catalytic activity and selectivity of various catalysts for CO hydrogenation. The CO conversion of the sample Fe/La/ZrO2 is only a little bit Table 3 CO hydrogenation on various catalysts Catalyst
Fe/ZrO2 Fe/La/ZrO2 Fe/Ce/ZrO2
CO Conv. (%)
5.5 5.6 12.0
Hydrocarbon distribution (wt%) CI
C2
C3
C4
C5
C~5/C2-5 (%)
43.1 36.1 35.2
28.9 29.8 29.8
22.0 23.6 24.1
6,0 10.0 9.2
-1.5 1.7
73.8 78.4 74.4
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higher than that of the unpromoted Fe/ZrO2 sample, but methane formation is suppressed and the light olefins selectivity increases. This result is similar to that of the Co/La/C [9] catalyst. As described in [9,16-20], the promotion effect of the lanthanum oxide can be explained by the basic character and the electronic effect of lanthana, which increases the electron density of active phase iron and weakens the C-O bond and enhances the dissociation of CO as shown in CO-TPD result, hence CO reactivity is enhanced, meanwhile, methane formation is suppressed and light olefins selectivity increases. However, for the Fe/La/ZrO2 catalyst, iron-oxide particles are highly dispersed on the surface of La203 layer, reduction of Fe 3+ to Fe ° becomes difficult owing to the strong interactions between iron oxide and La203, this may result in the decrease of the catalytic activity due to the decrease of the amount of active phase metallic iron [12]. As a total result, the CO conversion of this sample is only a little bit higher than that of the Fe/Zr sample, but methane formation is suppressed and light olefins selectivity increases. It is pertinent to note that for the Fe/Ce/ZrO2 sample, CO conversion is much higher than that of the unpromoted Fe/ZrO2 catalyst, methane formation is suppressed and light olefins selectivity slightly increases. XPS is used to study the surface species of Fe/Ce/ZrO2 sample. It is shown that for the as-prepared sample, the surface Ce/Fe atomic ratio is 1.11, but for the catalyst after reacted in syngas for 10 h, the surface Ce/Fe atomic ratio becomes 1.58. This result indicates that some Ce 4+ cations in the vicinity of iron particles might migrate onto the surface of metallic iron particles forming very small CeO2 islands during the reaction process. Our TPR result indicates that the surface C e O 2 can be reduced in the reaction condition, XPS results also show that Ce 3+ and Ce 4+ ions coexist on the surface of the catalyst after reacting in syngas, hence we suggest that new catalytic active sites (Fe-CeO2 ensembles) might be formed in this sample. CO can bond with its carbon atom to iron atom and with its oxygen atom to the adjacent partially reduced Ce203. This arrangement will result in a weakening of the C-O bond. CO can easily dissociate and the Ce203 particles are reoxidized to G e t 2 by the oxygen atom dissociated from CO. Being redox centers, ceria can rapidly transfer the oxygen atom dissociated from CO, thus the dissociation of C-O bond and reactivity of CO are markedly enhanced. Similar conclusions have also been obtained in Co/Ce/C [9,11] and Ru/Ce/C [11] systems. For the Fe/Ce/ZrO2sample, CO dissociation and reactivity are obviously enhanced owing to the formation of the new catalytic active sites, the concentration of the surface CHx species might increase too. This may result in the increase of the rate constant for polymerization of CHx species, hence the catalytic activity and light olefins selectivity increase, while methane formation is suppressed. For the Fe/La/ZrO2 sample, reduction of La203 is quite difficult, there are not new catalytic active sites formed, and the metallic iron content is rather low, hence the catalytic activity is lower than that of the Fe/Ce/ ZrO2 sample.
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4. Conclusion The results presented in this study show that the structure, reduction behavior, and the catalytic properties for CO hydrogenation of rare earth oxides promoted Fe/ZrO2 catalysts are quite different from that of the unpromoted Fe/ZrO2 sample, and are influenced significantly by the kinds of promoter elements. For the Fe/La/ ZrO2 sample, Fe203 particles highly disperse on the La203 layer, reduction of iron oxide becomes difficult owing to the strong interactions between iron oxide and La203. The catalytic activity is only a little bit higher than that of the Fe/ZrO2 sample, but light olefins selectivity increases and methane formation is suppressed. The promotion effect of lanthanum can be attributed to the basic character and the electronic effect of lanthana layer, which increases the electron density of the iron particles and enhances the dissociation of CO. For the Fe/Ce/ZrO2 sample, dispersion of iron oxide decreases by the addition of CeO2 layer. The catalytic activity is much higher than that of the unpromoted Fe/ZrO2 sample, while methane formation is suppressed and light olefins selectivity slightly increases. The promotion effect of ceria can be explained in terms of the formation of new catalytic active sites (Fe-CeO2 ensembles) at which the CO may bond with its carbon atom to iron atom and its oxygen atom to the adjacent partially reduced cerium oxide. This arrangement results in a weakening of the C-O bond, and the dissociation and reaction of CO is markedly enhanced.
Acknowledgements The support of the National Natural Science Foundation of China is gratefully acknowledged.
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