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Notes Structure and Reactivity of Ultrafine Fe-Mo Oxide Particles Prepared by the Sol-Gel Method Wenxing Kuang, Yining Fan, and Yi Chen* Institute of Mesoscopic Solid State Chemistry, Department of Chemistry, Nanjing University, Nanjing 210093, China Received April 12, 1999. In Final Form: February 22, 2000
Introduction Fe-Mo oxides have been widely employed as catalysts for the partial oxidation of alcohols and hydrocarbons. The active component is generally considered to be the defective ferric molybdate, and the optimum activity of the catalyst is usually achieved at a low Fe/(Fe + Mo) atomic ratio ranging from ca. 0.20 to 0.38.1-7 It has been realized that the excess MoO3 in the Fe-Mo oxides can promote the formation of the stoichiometric ferric molybdate, i.e., the active species for the partial oxidation, and the excess Fe2O3 leads to the complete oxidation of hydrocarbons. In contrast to the large amount of research carried out in the presence of molecular oxygen, studies on the fundamental nature and reactivity of iron molybdates in the absence of molecular oxygen are rather limited. In the past decade, with the enforcement of environmental regulations, increasing attention has been paid to the application of lattice oxygen for the partial oxidation of hydrocarbons, as it might increase the selectivity to the main products and decrease pollution.8-11 Noteworthy is the fact that although the Fe-Mo oxides have been extensively studied,1-7,12-18 relatively little work has been done on the structure and reactivity of ultrafine Fe-Mo oxide particles. It is expected that due to the large surface (1) Kolovertnov, G. D.; Boreskov, G. K.; Dzis’Ko, V. A.; Popov, B. I.; Tarasova, D. V.; Belugina, G. G. Kinet. Catal. 1965, 6, 950. (2) Leroy, J. M.; Peirs, S.; Tridot, G. C. R. Acad. Sci., Ser. C 1971, 218. (3) Peirs, S.; Leroy, J. M. Bull. Soc. Chim. Fr. 1972, 1241. (4) Pernicone, N. J. Less-Common Met. 1974, 36, 289. (5) Abaulina, L. I.; Kustova, G. N.; Klevtsova, R. F.; Popov, B. I.; Bibin, V. N.; Melekhina, V. A.; Kolomiychuk, V. N.; Boreskov, G. K. Kinet. Catal. 1976, 17, 1307. (6) De La Torre, I.; Acosta, G.; Hernandez, M. Rev. Inst. Mex. Pet. 1979, 11, 68. (7) Okamoto, Y.; Morikawa, F.; Oh-Hiraki, K.; Imanaka, T.; Teranishi, S. J. Chem. Soc., Chem. Commun. 1981, 1018. (8) Patience, G. S.; Mills, P. L. Stud. Surf. Sci. Catal. 1994, 82, 1. (9) Emig, G.; Uihlein, K.; Hacker, C. J. Stud. Surf. Sci. Catal. 1994, 82, 243. (10) Haggin, J. Chem. Eng. News 1995, 73 (April 3), 20. (11) Abon, M.; Bere, K. E.; Delichere, P. Catal. Today 1997, 33, 15. (12) Trifiro, F.; Vecchi, V. D.; Pasquon, I. J. Catal. 1969, 15, 8. (13) Boreskov, G. K.; Muzykantov, V. S.; Panov, G. I.; Popovskii, V. V. Kinet. Katal. 1969, 10, 1043. (14) Trifiro, F.; Notarbartolo, S.; Pasquon, I. J. Catal. 1971, 22, 324. (15) Boreskov, G. K. Kinet. Katal. 1973, 14, 7. (16) Germain, J. E.; Laugier, R. C. R. Acad. Sci., Ser. C 1973, 276, 1349. (17) Carbucicchio, M.; Trifiro, F. J. Catal. 1976, 45, 77. (18) Klissurski, D. G.; Kancheva, M. M. J. Chem. Soc., Faraday Trans. 1981, 77, 1795.
Figure 1. XRD patterns of the ultrafine Fe-Mo oxides prepared by the sol-gel method: (a) Fe/(Fe + Mo) ) 1/3; (b) Fe/(Fe + Mo) ) 2/5; (c) Fe/(Fe + Mo) ) 1/2; (d) Fe/(Fe + Mo) ) 3/5; (e) Fe/(Fe + Mo) ) 4/5. Table 1. Mo1 ssbauer Parameters of the Fresh Ultrafine Fe-Mo Oxides Mo¨ssbauer parameters sample IS QS H (Fe/(Fe + Mo)) (mm‚S-1) (mm‚S-1) (kOe) 1/3 2/5 1/2 3/5 4/5
a
0.41 0.41 0.39 0.40 0.35 0.41 0.40 0.39 0.39
0 0 0.90 0 0.69 0 0.02 0.86 0
0 0 0 0 0 0 517 0 0
iron species
relative area (%)
Fe2(MoO4)3 Fe2(MoO4)3 Fe2O3a Fe2(MoO4)3 Fe2O3a Fe2(MoO4)3 Fe2O3b Fe2O3a Fe2(MoO4)3
100 100 34 66 43 57 14 70 16
Superparamagnetic Fe2O3. b Magnetic Fe2O3.
area and unique properties, the ultrafine oxide particles should have great potential applications in catalysis.29-31 (19) Gai, P. L.; Labun, P. A. J. Catal. 1985, 94, 79. (20) Wataru, U. Mater. Res. Soc. Symp. Proc. 1988, 111, 315. (21) Ma, Y. H.; Kmiotek, S. J.; J. Catal. 1988, 109, 132. (22) Zhang, H.; Li, Z.; Fu, X. Cuihua Xuebao 1988, 9, 331. (23) Weng, L. T.; Ma, S. Y.; Ruiz, P.; Delmon, B. J. Mol. Catal. 1990, 61, 99. (24) Wilson, J. H.; Hill, C. G., Jr.; Dumesic, J. A. J. Mol. Catal. 1990, 61, 333. (25) Hill, C. G., Jr.; Wilson, J. H. J. Mol. Catal. 1990, 63, 65. (26) Hill, C. G., Jr.; Wilson, J. H. J. Mol. Catal. 1991, 67, 57. (27) Zhang, H.; Shen, J.; Ge, X. J. Solid State Chem. 1995, 117, 127. (28) Ge, X.; Shen, J.; Zhang, H. Sci. China (Ser. B) 1995, 25, 785. (29) Barnard, K. R. J. Catal. 1990, 125, 265. (30) Zhong, Z.; Yan, Q.; Fu, X.; Gong, J. J. Chem. Soc., Chem Commun. 1996, 1745.
10.1021/la990418e CCC: $19.00 © 2000 American Chemical Society Published on Web 04/14/2000
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Notes
Table 2. Reactivity of the Ultrafine Fe-Mo Oxides in the Presence of Molecular Oxygen Fe/(Fe + Mo) atomic ratio conversion of toluene (mol %) benzaldehyde yield × 102 (µmol‚s-1‚m-2)
0.0 36.3 0.57
0.23 85.0 1.04
Among various methods for the preparation of mixed oxides, sol-gel method is unique in obtaining samples mixed at a molecular scale providing possibilities for tailoring the properties of oxide catalysts.32-35 In this Note, the structure and reactivity of ultrafine Fe-Mo oxide particles prepared by the sol-gel method are studied and compared with those of corresponding oxide particles prepared by coprecipitation. Experimental Section Sample Preparation. Ultrafine Fe-Mo oxide particles with various Fe/(Fe + Mo) atomic ratios as well as the Fe2O3 and MoO3 samples were prepared by the sol-gel method reported previously.36 For comparison, the oxide samples were also prepared by coprecipitation as described by Hill et al.25 Characterization. X-ray diffraction (XRD) patterns were collected on a Shimadzu-3A diffractometer with Fe KR radiation (0.19373 nm). The shape and size of the oxide particles were elucidated by a JEM-100S transmission electron microscope. The BET surface area measurements were performed on a micrometrics ASAP-2000 instrument (N2 adsorption at 77 K). Mo¨ssbauer spectroscopy was conducted with a constant acceleration spectrometer using a 10 mCi 57Co/Pd source. The temperatureprogrammed reduction (TPR) profiles were measured by heating the sample in a quartz U-type reactor at a rate of 10 K‚min-1 under a flowing H2-Ar mixture (5.0 vol % hydrogen) at a rate of 30 mL‚min-1. Partial Oxidation of Toluene. To evaluate the reactivity of the ultrafine Fe-Mo oxides37 in the presence of molecular oxygen, the reaction was carried out in a quartz fixed bed microreactor with 250 mg of sample at 0.1 MPa, 673 K, and at a helium flowing rate of 20 mL‚min-1 with a volume ratio of He/O2/toluene ) 50/ 5/1; while in the absence of molecular oxygen, a pulse microreactor was run at 623 K, 0.2 MPa, with a helium flowing rate of 40 mL‚min-1 and 1.45 µmol of toluene was injected for each pulse.
Results and Discussion Structure of the Ultrafine Fe-Mo Oxides. TEM results have shown that the Fe-Mo oxide particles prepared by the sol-gel method are ultrafine ranging from 20 to 80 nm. BET surface area of the samples with Fe/(Fe + Mo) atomic ratios of 0.23, 0.33, 0.40, 0.50, 0.60, and 0.80 is 10.1, 13.9, 20.2, 46.3, 46.4, and 44.7 m2‚g-1, respectively. Noticeably, with the increase of Fe/(Fe + Mo) atomic ratio from 0.23 to 0.50, the surface area increases dramatically from 10.1 to 46.3 m2‚g-1 and no obvious change afterward. XRD patterns (Figure 1) reveal the coexistence of MoO3 and Fe2(MoO4)3 in sample with an Fe/(Fe + Mo) ratio of 0.33 as well as 0.23, and only Fe2(MoO4)3 is observed when the ratio is increased to 0.40. Correspondingly only a singlet of Fe2(MoO4)3 is detected in their Mo¨ssbauer spectra (Table 1). The results are similar to those obtained from samples prepared by coprecipitation.22 For samples with Fe/(Fe + Mo) ratios of 0.50 and 0.60, XRD profiles reveal only the presence of Fe2(MoO4)3 phase, and Mo¨ssbauer spectra show the coexistence of Fe2(MoO4)3 and a super(31) Liang, Q.; Chen, K.; Hou, W.; Yan, Q. Appl. Catal., A: General 1998, 166, 191. (32) Pajonk, G. M. Appl. Catal., A: General 1991, 72, 217. (33) Lakeman, C. D. E.; Payne, D. A. Mater. Chem. Phys. 1994, 38, 305. (34) Ward, D. A.; Ko, E. I. Ind. Eng. Chem. Res. 1995, 34, 421. (35) Schneider, M.; Baiker, A. Catal. Rev.-Sci. Eng. 1995, 37, 515. (36) Kuang, W.; Fan, Y.; Chen, Y. J. Colloid Interface Sci. 1999, 215, 364. (37) Kuang, W.; Fan, Y.; Chen, K.; Chen, Y. J. Catal. 1999, 186, 310.
0.33 85.0 0.63
0.40 65.8 0.63
0.50 74.3 0.12
0.60 75.0 0.12
0.80 70.5 0.06
1.00 73.3 0.0
Table 3. Reactivity of the Fe-Mo Oxides with an Fe/(Fe + Mo) Atomic Ratio of 0.23 in the Presence of Molecular Oxygen preparation method particle size (nm) surface area (m2‚g-1) conversion of toluene (mol %) benzaldehyde selectivity (%) benzaldehyde yield (mol %)
Fe-Mo
Fe-Mo
coprecipitation 200-400 2.2 45.5 20.2 9.2
sol-gel 40-80 10.1 85.0 26.5 22.5
Table 4. Reactivity of Fe2(MoO4)3 in the Absence of Molecular Oxygen Fe2(MoO4)3 preparation method particle size (nm) surface area (m2‚g-1) total conversion of toluene (µmol‚g-1) benzaldehyde selectivity (%) total benzaldehyde yield (µmol‚g-1‚m-2)
Fe2(MoO4)3
coprecipitation 200-400 2.7 0.012
sol-gel 20-60 20.2 1.05
91.2 0.004
92.0 0.05
paramagnetic Fe3+ species, in contrast there is no highly dispersed Fe3+ species that can be detected in samples with similar composition but prepared by coprecipitation. On further increase of the Fe/(Fe + Mo) ratio to 0.80, the appearance of an additional Fe2O3 bulk phase is confirmed by both XRD and Mo¨ssbauer spectroscopy. Apparently, the preparation method has a significant influence on the morphology and structure of the Fe-Mo oxides. Partial Oxidation of Toluene in the Presence of Molecular Oxygen. The reaction usually reaches a steady state after ca. 120 min on stream, and no obvious bulk change has been found for the ultrafine Fe-Mo oxides after reaction. As shown in Table 2, the composition of the ultrafine Fe-Mo oxides has a strong impact on the reactivity for the conversion of toluene. With the increase of Fe/(Fe + Mo) atomic ratio, the yield of the benzaldehyde increases at first, then drops rapidly, and finally reaches zero for Fe2O3. The maximum yield reaches the vicinity of Fe/(Fe + Mo) ) 0.23, i.e., the catalyst consisting of both MoO3 and Fe2(MoO4)3. The result that the excess MoO3 is a good promoter for the partial oxidation is consistent with the fact that the excess MoO3 in Fe-Mo oxides promotes the formation of stoichiometric ferric molybdate, which is the active species for partial oxidation.1-7 On increase of the Fe/(Fe + Mo) atomic ratio to values higher than 0.40, the reactivity of samples composed of Fe2O3 and Fe2(MoO4)3 drops rapidly. Apparently the excess Fe2O3 has an opposite effect to that of excess MoO3 in promoting the complete oxidation and reducing the selectivity of the catalysts, which may correlate to the strong electrophilic oxidation ability of the oxygen species adsorbed on the surface of Fe2O3. For comparison, the reactivity of samples with a Fe/(Fe + Mo) ratio of 0.23 prepared by the sol-gel method and by coprecipitation is measured and listed in Table 3. It is interesting to note that ultrafine Fe-Mo oxide particles have an obviously higher conversion of toluene and a better selectivity to benzaldehyde than the corresponding larger particles prepared by coprecipitation. Partial Oxidation of Toluene in the Absence of Molecular Oxygen. Toluene oxidation in the absence of molecular oxygen was used to probe the reactivity of the
Notes
Langmuir, Vol. 16, No. 11, 2000 5207 Table 5. Reactivity of the Ultrafine Fe-Mo Oxides in the Absence of Molecular Oxygen
Fe/(Fe + Mo) atomic ratio total conversion of toluene (µmol‚g-1) total benzaldehyde yield (µmol‚g-1‚m-2)
0.0 0.0 0.0
0.23 0.07 0.01
0.33 0.08 0.01
0.40 1.05 0.05
0.50 24.3 0.50
0.60 10.7 0.23
0.80 8.12 0.18
1.00 0.0 0.0
Figure 2. XRD patterns of the ultrafine Fe-Mo oxides after reaction in the absence of molecular oxygen: (a) Fe/(Fe + Mo) ) 1/3; (b) Fe/(Fe + Mo) ) 2/5; (c) Fe/(Fe + Mo) ) 1/2; (d) Fe/(Fe + Mo) ) 3/5. Table 6. Mo1 ssbauer Parameters of the Ultrafine Fe-Mo Oxides after Reaction in the Absence of Molecular Oxygen Mossbauer parameters sample IS QS H (Fe/(Fe + Mo)) (mm‚S-1) (mm‚S-1) (kOe) 1/3a 2/5b 1/2c
3/5d
d
0.41 0.41 1.12 1.11 0.39 0.40 1.06 1.08 0.38 0.42
0 0 0.96 2.54 0.90 0 1.17 2.65 0.68 0
0 0 0 0 0 0 0 0 0 0
iron species
relative area (%)
Fe2(MoO4)3 Fe2(MoO4)3 β-FeMoO4 β-FeMoO4 Fe2O3e Fe2(MoO4)3 β-FeMoO4 β-FeMoO4 Fe2O3e Fe2(MoO4)3
100 100 36 34 11 19 15 11 40 34
a After the 3rd pulse. b After the 6th pulse. c After the 16th pulse. After the 12th pulse. e Superparamagnetic Fe2O3.
lattice oxygen in the ultrafine Fe-Mo oxides. With the increase of pulse numbers the conversion of toluene decreases remarkably, but little change has been found in the selectivity to benzaldehyde. The reactivity of the ultrafine Fe2(MoO4)3 prepared by the sol-gel method and corresponding larger particles obtained by coprecipitation is presented in Table 4. The about 10-fold higher yield of benzaldehyde for the ultrafine particles indicates that the reactivity of the lattice oxygen is greatly improved by reducing the particle sizes of the Fe-Mo oxides to nanometer scale. The composition of the ultrafine Fe-Mo oxide also has a strong influence on the reactivity for the conversion of toluene as shown in Table 5. For Fe2O3 or MoO3 alone, no product is detected, indicating that both of them are not active for the oxidation of toluene under the reaction
Figure 3. TPR profiles of the oxides: (a) Fe2O3; (b) Fe2(MoO4)3; (c) MoO3.
conditions applied. With the increase of Fe/(Fe + Mo) atomic ratio, the reactivity of the ultrafine Fe-Mo oxides increases slowly at first, and a big jump is observed around an atomic ratio of 0.5, i.e., for the oxide composed of Fe2(MoO4)3 and Fe2O3, and then decreases rapidly. The result is completely different from that obtained in the presence of molecular oxygen described above, indicating that the reactivity of the ultrafine Fe-Mo oxides is greatly influenced by the reaction conditions. XRD and Mo¨ssbauer results for the ultrafine Fe-Mo oxides after reaction are shown in Figure 2 and Table 6, respectively. For samples with the Fe/(Fe + Mo) ratio of 0.33 and 0.40, no new phase appears after reaction, suggesting that only lattice oxygen on the surface of these two samples has entered into the reaction. However, when the Fe/(Fe + Mo) ratio is increased to 0.50 and 0.60, both XRD and Mo¨ssbauer results point to the appearance of a new species, i.e., crystalline β-FeMoO4.38,39 In comparison with the Mo¨ss(38) Sleight, A. W.; Chamberland, B. L.; Weiher, J. F. Inorg. Chem. 1968, 7, 1093. (39) Carbucicchio, M.; Trifiro F.; Vaccari, A. J. Catal. 1982, 75, 207.
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bauer results for the fresh samples as listed in Table 1, one can conclude that the appearance of the β-FeMoO4 after reaction is based on the consumption of the Fe2(MoO4)3 and the highly dispersed Fe2O3 species in the samples, and the process is consistent with the following reaction stoichiometric reported elsewhere40
2Fe2(MoO4)3 + Fe2O3 f 6β-FeMoO4 + 3[O] where [O] denotes the lattice oxygen ion that is active for the partial oxidation of toluene. It is obvious that the best mole ratio of Fe2(MoO4)3 to Fe2O3 for producing β-FeMoO4 is 2:1, which is equivalent to an Fe/(Fe + Mo) ratio of 0.5 and is in good agreement with the distinguished reactivity of this sample discussed above. On further increase of the Fe/(Fe + Mo) ratio to 0.60 and 0.80, the decrease of Fe2(MoO4)3 species in the fresh samples as well as the decrease of β-FeMoO4 in samples after reaction might account for their low yield of benzaldehyde. The low yield of benzaldehyde for sample with the atomic ratio of 0.40 might be attributed to the absence of highly dispersed Fe2O3, and the lower yield for samples with the Fe/(Fe + Mo) ratios of 0.23 and 0.33 might be because part of the surface is covered by the excess MoO3. Reduction Behavior of the Ultrafine Fe-Mo Oxides. TPR profiles of the Fe2O3, MoO3, and Fe2(MoO4)3 are shown in Figure 3. For Fe2O3, two hydrogen consumption peaks around 660 and 857 K can be assigned to the reduction of Fe2O3 f Fe3O4 f R-Fe, respectively.41 For MoO3, three reduction peaks around 953, 1026, and 1143 K can be attributed to the reduction of MoO3 f MoO2.8 f Mo4O11 f MoO2, respectively.42 Fe2(MoO4)3 has three reduction peaks around 928, 975, and 1139 K, which (40) Kuang, W.; Fan, Y.; Chen K.; Chen, Y. J. Chem. Res. (s) 1997, 366. (41) Shen, J.; Zhang, S.; Liang, D. Cuihua Xuebao 1988, 9, 115. (42) Smith, M. R.; Ozakan, U. S. J. Catal. 1993, 141, 124.
Notes
can be mainly assigned, respectively, to the following steps:27,28
Fe2(MoO4)3 f β-FeMoO4 + Mo4O11 f Fe2Mo3O8 + MoO2 f Fe0 + Mo0 + Fe3Mo The reducibility sequence of the three oxides, i.e., Fe2O3 > Fe2(MoO4)3 > MoO3, seems to suggest that the lattice oxygen ions in Fe2O3 and in MoO3 have the highest and lowest mobility, respectively, in this series of compounds. It seems reasonable to consider that in the absence of molecular oxygen, the lattice oxygen ions should play a critical role for the oxidation reaction to occur, and the presence of the excess Fe2O3 or excess MoO3 in the ultrafine Fe-Mo oxides might result in a positive or a negative effect, respectively, on the reactivity of Fe2(MoO4)3 for the partial oxidation of toluene to benzaldehyde. Conclusions Composition, preparation method, and reaction conditions have strong influence on the structure and reactivity of the Fe-Mo oxides. In comparison with the corresponding oxide particles obtained by coprecipitation, the ultrafine Fe-Mo oxides prepared by the sol-gel method exhibit higher reactivity for the partial oxidation of toluene to benzaldehyde. In the presence of molecular oxygen, the ultrafine Fe-Mo oxide with an Fe/(Fe + Mo) atomic ratio of 0.23 has the highest yield of benzaldehyde, and the excess MoO3 in the oxides works as a promoter for the partial oxidation of toluene, while in the absence of molecular oxygen, the Fe/(Fe + Mo) ratio for the ultrafine oxides to have the highest yield goes to 0.50, and the excess Fe2O3 becomes the promoter instead of MoO3. Acknowledgment. The support of the National Natural Science Foundation of China is gratefully acknowledged. LA990418E
