2330
J. Phys. Chem. 1996, 100, 2330-2333
Surface Coordinate Geometry of Iron Catalysts: Distinctive Behaviors of Fe/Al2O3 in CO Hydrogenation Yuan Kou,* Hong-li Wang, Jian-zhong Niu, and Wei-jie Ji State Key Laboratory for Oxo Synthesis and SelectiVe Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ReceiVed: September 1, 1995X
Two well-defined iron catalysts, Fe(II)/Al2O3 (4.5 wt %) and Fe2O3/Al2O3 (1.5 wt %), have been studied using Fe K-edge X-ray absorption fine structure (XAFS). The total coordination number of the nearest oxygen neighbors is found to be 4.9 for the former and 5.7 for the latter, but the average Fe-O bond length remains the same, 1.98 Å. The nearest iron neighbors at 2.95-2.97 Å are observed for both the catalysts, but for the Fe2O3/Al2O3, further two iron neighbors at 3.42 and 3.86 Å are also present. The fact that the shell radius of 3.42 Å is 2/3x3 times longer than that of the first nearest irons is consistent with what is expected from the para- and the meta-positions surrounding the same oxygen on the γ-Al2O3 (111) plane but is at variance with the estimation made for the (110) plane. Two geometrical models proposed for the catalysts stoichiometrically explain why the higher metal loading only leads to one type of the iron neighbors but the lower metal loading unexpectedly leads to three types of iron neighbors.
Introduction Iron catalysts offer intriguing prospects in catalysis, especially in the hydrogenation of carbon oxides. Cluster-derived Fe(0)/ Al2O3 exhibited a good selectivity (>80%) for lower olefins in CO hydrogenation but the activity was poor.1 To distinguish the zerovalent iron ensembles from the oxide species, Fe(II)/ Al2O3 was prepared by calcinating the Fe(0)/Al2O3 in air at 923 K.2 Soon after the calcination, some catalytic activity in CO hydrogenation was observed. Further studies revealed that, by increasing the iron loading to 4.5 wt %, the Fe(II)/Al2O3 showed much higher CO conversion (>30%), although the selectivity to lower olefins was unsatisfactory. In fact, it is a catalyst for the Fischer-Tropsch synthesis. Well-defined ferric oxide catalyst Fe2O3/Al2O3 was previously investigated in this laboratory; however, no catalytic activity in CO hydrogenation was observed.3 In this paper, the relationship between catalytic activity and surface structure of supported iron catalysts is continuously studied by using X-ray absorption fine structure (XAFS). Understanding of a molecular species on the surface is particularly relevant to the description and design of working catalysts. Following the approach established by Kno¨zinger and Zhao4 and as a result of the development of XAFS technique, more work concerning the cluster-derived species has been recently reported.1-3,5 Among them, the support surface itself, which favors or disfavors some specific structures, was emphasized in our work.1-3,6,7 From this paper, we will extend the approach to the supported iron oxide catalysts. Experimental Details The γ-Al2O3 (industrial grade, 0.707 g/mL, Wenzhou Chemical Industry) was unpretreated but purged by purified nitrogen (99.99% purity) before use. The BET surface was 160-170 m2/g. The average pore diameters were in the range of 30-50 Å. The preparation of Fe(II)/Al2O3 (4.5 wt % Fe) started with the green sample Fe3(CO)12/Al2O3.1 After decarbonylation in vacuum at 373 K for 1 h, the catalyst was obtained by further * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, January 1, 1996.
0022-3654/96/20100-2330$12.00/0
treatment in air at 973 K. Fe2O3/Al2O3 (1.5 wt % Fe) was prepared by conventional incipient wetness impregnation of the γ-Al2O3 with an aqueous solution of ferric nitrate. The resulting catalyst was dried in air for 10 h at 273 K and calcined in air for 1 h at 723 K.3 Samples were checked by X-ray diffraction, UV-vis diffuse reflectance, and laser Raman spectroscopies to ensure that no iron-containing microcrystalline materials remained on the surface. X-ray absorption spectra were obtained using the BL-7C facilities at the Photon Factory (Tsukuba, Japan) with a positron beam energy of 2.5 GeV and an average stored current of 250 mA. Data were recorded with a Si(111) double-crystal (Sagittal focusing) monochromator in transmission mode for pure compounds and in fluorescence mode for supported samples. Spectra were recorded in four energy regions about the Fe K-edge: -100 to -30 eV in 15-eV steps, -30 to 70 eV in 0.5-eV steps, 70 to 700 eV in 4-eV steps, and above 700 eV in 8-eV steps. The energy calibration was set as 7111.0 eV at the Fe foil 3d pre-edge feature. Specimens maintained under nitrogen were made by directly applying the fine powders on Scotch tape. The data were processed on an AST 486 microcomputer with the Program Library for EXAFS Data Processing written by the Institute of Physics, Chinese Academy of Sciences. The XANES (X-ray absorption near edge structure) data were normalized within 70 eV (-20 to 50 eV) about the edge. The absorption threshold ET used as the zero of energy was taken with respect to the first inflection point while the routine E0 was taken at the maximum in the derivative spectrum. A general procedure for the EXAFS (extended XAFS) analysis has been given previously.1,2 The raw data collected for the Fe(II)/Al2O3 and Fe2O3/Al2O3 are shown in Figure 1. Results and Discussion Fe K-edge XANES (X-ray absorption near edge structure) of both catalysts has been briefly discussed in a preliminary paper.2 The derivative spectra revealed an E0 of 11.0 eV for the Fe(II)/Al2O3 and 9.5 eV for the Fe2O3/Al2O3. The values were thought to be comparable with those in bulk oxides such as 12.9 eV in Fe3O4 and 9.5 eV in Fe2O3. The intensity of the pre-edge feature, and the energy positions of the E0 and the absorption maximum for both catalysts © 1996 American Chemical Society
Surface Coordinate Geometry of Iron Catalysts
J. Phys. Chem., Vol. 100, No. 6, 1996 2331
Figure 3. Fourier transforms of the K-edge EXAFS of the catalysts.
Figure 1. Raw data of Fe K-edge XAFS collected for the catalysts.
Figure 4. Best fits to Fourier-filtered Fe K-edge EXAFS of Fe(II)/ Al2O3 (in the range of 0.75-3.31 Å) and Fe2O3/Al2O3 (in the range of 0.77-3.73 Å) with the weight of k3.
TABLE 2: EXAFS-Derived Coordination Number (CN), Shell Radius (R), and Debye-Waller Factor (DW) for Both Catalysts sample
shell
CN
R (Å)
DW (Å-1)
R factor
Fe(II)/Al2O3
Fe-O Fe-O Fe-Fe Fe-O Fe-O Fe-Fe Fe-Fe Fe-Fe
2.4(3) 2.5(3) 3.5a 3.4(6) 2.3(8) 1.8a 1.3a 3.3a
1.91(1) 2.05(1) 2.95(9) 1.93(2) 2.05(4) 2.97(9) 3.42(4) 3.86(9)
0.003 0.003 0.036 0.007 0.015 0.021 0.009 0.030
0.07
Fe2O3/Al2O3 Figure 2. Fe K-edge XANES spectra of the catalysts compared with those of bulk oxides (for the derivative spectra of the catalysts, see ref 2).
TABLE 1: Intensity and Energy Position of Fe K-Edge XANES Features feature
pre-edge (intensity)
edge (position, eV)
maximum (position, eV)
assignment FeO Fe2O3 Fe3O4 Fe(III)/Al2O3 Fe2O3/Al2O3
1s f 3d 0.07 0.07 0.10 0.08 0.07
E0 12.0 9.5 12.9 11.0 9.5
1s f 4p 18.0 21.0 19.8 20.4 21.0
compared with those of bulk oxides, are summarized in Figure 2 and Table 1. The iron ions in both FeO and Fe2O3 are in octahedral interstices and thus show very weak pre-edge peaks because the 1s to 3d transition is dipole forbidden.8-10 Meanwhile, half-filled 3d orbitals of Fe3+ in Fe2O3 cause a smaller E0. The smallest E0 in energy is also observed when Fe2O3 is compared with M2O3 (M ) Mn, Co, and Ni).11 The more stable the d-shell is, the greater energy difference between 3d and 4p levels will be. A larger energy difference between the absorption threshold and the maximum is therefore certain for bulk Fe2O3. Among the three bulk oxides, Fe3O4, a mixed Fe2+ and Fe3+ system, is quite special and offers a good example to understand the XANES of a mixture. The Fe3+ ions in Fe3O4 are half in tetrahedral interstices. The addition of these tetrahedral units into mainly octahedral coordinates results in a more intense pre-edge peak for Fe3O4, which is 0.10 in relative height but is 0.07 for FeO and Fe2O3. In a site-geometry sense, Table 1 shows that the data observed for Fe2O3/Al2O3 are almost not different from those of Fe2O3, indicating that the surface species on Fe2O3/Al2O3 consists of octahedral FeO6 units. On
a
0.10
Errors are greater than (1.0.
the other hand, the data observed for Fe(II)/Al2O3 appears to be an average between Fe2O3 and Fe3O4, indicating that the species on Fe(II)/Al2O3 may probably be a mixture of octahedral FeO6 and tetrahedral FeO4 units involving both Fe2+ and Fe3+ ions. The Fourier transforms of the K-edge EXAFS (extended XAFS) of Fe(II)/Al2O3 and Fe2O3/Al2O3 are shown in Figure 3. The most intense peaks centered at 1.58 Å (not corrected for phase shift) are contributed by the nearest oxygen neighbors while the remaining contributions ranged from 2 to 3 Å are mainly caused by the nearest iron neighbors. Interestingly, the higher iron loading in Fe(II)/Al2O3 (4.5 wt %) only gives several weak peaks in the range of 2-3 Å but the lower iron loading in Fe2O3/Al2O3 (1.5 wt %) causes more intense and complicated contributions in this region. The best fits to the Fourier-filtered Fe K-edge EXAFS are summarized in Table 2 and Figure 4. The best fits indicate the following: (1) Although the oxidation states of the irons on both catalysts are found to be somewhat different from each other, the average Fe-O bond lengths remain the same, 1.98 ( 0.02 Å. It is noteworthy that this value is almost identical to that observed for zerovalent Fe(0)/Al2O3, 1.97 ( 0.02 Å. It is thought to reflect the covalent feature of the iron atoms and/ or ions on the surface. (2) Distinct coordination numbers (CNs) are obtained for the nearest oxygens. The total CN of the oxygens is found to be 4.9 for Fe(II)/Al2O3 and 5.7 for Fe2O3/ Al2O3. According to the XANES results, it is evident that the irons on the former are alternated between 4- and 6-coordina-
2332 J. Phys. Chem., Vol. 100, No. 6, 1996
Figure 5. An estimation for the possible Fe sites on the γ-Al2O3: (A) (111) plane, (B) (110) plane.
tions while the irons on the latter are most probably appeared at “oxygen saturated” 6-coordinate sites. (3) Distinct iron neighbors are observed. For Fe(II)/Al2O3, only the iron neighbors at 2.95 ( 0.09 Å with a CN of 3.5 are present. But for Fe2O3/Al2O3, besides the iron neighbors at 2.97 ( 0.09 Å, further two neighbors at 3.42 ( 0.04 and 3.86 ( 0.09 Å, respectively, are observed. It is of particular interest that the EXAFS-derived parameters are highly consistent with the γ-Al2O3 crystal face. The surface of small particles of crystalline γ-Al2O3 has been determined to be predominantly (111) in orientation based on electron diffraction12,13 on the basis of energy consideration,14 but there are still arguments that the (110) is the favored orientation.15,16 The difference between the faces and the influence to the EXAFS observation have been discussed.1 Transition metal atoms or ions in small ensembles generally tend to occupy the multiple bridging sites on the low-index faces of Al2O3.4 On the (111) surface, only the threefold position, a potentially octahedral site,1 is available. It can be seen from Figure 5A that there are six threefold positions surrounding one surface oxygen. The six threefold sites are somewhat similar to the six carbons in benzene; three types of positional relationships between any two freely chosen sites can thus be defined in analogy with those used in describing benzene, i.e., ortho-, meta-, and para-positions as illustrated in the figure. The distance between the meta-positions is identical to the O-O distance, which is approximately 2.80-2.85 Å on the γ-Al2O3 crystal surface.4,17 The distance between the para-positions is 2 /3x3 times the distance between the meta-positions and calculated to be 3.23 Å when O-O distance is 2.80 Å. Two orthopositions calculated to be about 1.6 Å apart will be too close to each other and are not expected to simultaneously accommodate two irons because of strong repulsion. It can be seen from Figure 5A that the sites with a distance of 2.80, 3.23, 4.28, and 4.85 Å on the (111) surface are geometrically allowed positions for adjacent iron ions. Figure 5B illustrates the possible distances between the irons occupying the different sites on the (110) surface. At first sight, the twofold symmetry hollow site surrounded by four oxygens may be reconstructed into a fourfold site and accommodate an iron ion, analogous to the situation observed in the S/Fe (110) system.18 These reconstructed fourfold sites (not shown in the figure) are about 3.8 Å apart in the (110) direction. Meanwhile, two irons simultaneously sharing any two threefold positions surrounding one next layer oxygen is also not available, analogous to the situation for the ortho-positions on the (111) plane. Therefore, only the positions surrounding the same oxygen on the top layer, i.e., the tetrahedral positions on the (110) plane, are suitable for such an arrangement. Detailed analysis (side view, as partly showed in Figure 5B) based on the O-O distance of 2.80 Å and an
Kou et al. average Fe-O distance of 1.98 Å reveals that the sites which give the Fe-Fe distance of 2.62, 2.80, 3.04, 3.80, 3.83, and 4.72 Å, respectively, are potential positions for iron ions. Furthermore, because the iron cations will be covered to a certain extent by adventitious oxygens from stoichiometric considerations, it is clear from the discussion herein that the irons on the (111) surface are likely to have a six-coordination while those on the (110) surface are probably present in predominant four-coordination. Undoubtedly, the former is highly consistent with the XAFS results. It is noteworthy that the shell radius of 3.42 Å observed for the second nearest iron neighbors on Fe2O3/Al2O3 is 2/3x3 times longer than that of the first nearest irons (2.97 Å). Taking the iron neighbors at 2.95 Å on either Fe(II)/Al2O3 or Fe(0)/ Al2O31 into consideration, the resulting data definitely suggest that the γ-Al2O3 surface for the samples used in this investigation is (111) in orientation and the O-O distance on the surface is approximately 2.95 Å. The longer O-O distance than that on a well-crystallized and unsupported alumina may arise from the presence of iron species on the Al2O3. There are 22/3 site vacancies over the 24 cation positions in the unit cell of γ-Al2O3,17 but may be more on the surface. They may significantly increase the O-O distance since in the EXAFS measurements, only the average values are observed. And, just because the EXAFS-derived parameters are average, slight deviation of an iron location, for example, slight deviation from the center of the threefold position as well as local reconstruction of the Al2O3 surface, may significantly increase the error of the data observed. It is thus not surprising that the distance of 2.95 Å as the shell radius of the first iron neighbors has an unexpectedly large error of (0.09 Å. In accordance with the EXAFS-suggested values, it can be concluded that the possible Fe-Fe distance on the (111) plane in our samples must be 2.95, 3.41, 4.51, and 5.12 Å, respectively, with the O-O distance on Al2O3 taken to be 2.95 Å. Each iron in this case should approximately have an octahedral site symmetry and on average share 5-6 oxygens. In the case of (110) plane, the possible Fe-Fe distance will be expected to be 2.71, 2.95, 3.20, 4.00, 4.04, and 4.97 Å, respectively, and each iron may on average share only 4 oxygens. Experimentally, however, the latter is not the case observed. Two geometrical models proposed have been shown in Figure 6. The irons in the stoichiometric model for Fe(II)/Al2O3 are alternated in 4-, 5-, and 6-coordinations, forming a monolayer with a two-dimensional size about 20 Å. It is the reason why a higher metal loading such as 4.5 wt % in this sample leads to only one type of the nearest iron neighbors at 2.95 Å. The model-derived average CN for the iron neighbors is 4.2, close to the value of 3.5 observed. The top layer oxygens on the iron ensemble are also monolayered but with some defects. The model-derived CN for the oxygen neighbors is 5.1 on average, also close to the total of 4.9 determined by EXAFS. It can be interestingly seen from the model that, without the oxygen defects, the CN of the oxygens must be 6.0. It will lead to a perfect model with an apparent formula of Fe2O3 (Fe20O30). In that case, by increasing the ensemble size, the ratio between the irons and the top layer oxygens may decrease gradually, and finally in an extreme case, reach the value of 1:1. It is likely that the two-dimensional growth of iron oxide not only leads to the monolayer film with an apparent formula of FeO but may also change the coordination tendency (from octahedral to tetrahedral) of the iron sites. It is hard to obtain such a monolayer on alumina surface, but a model proposed for the iron oxide on Pt(111) surface19 seems very close to this estimation. In that case, the iron does not obey the multiple-
Surface Coordinate Geometry of Iron Catalysts
J. Phys. Chem., Vol. 100, No. 6, 1996 2333
Figure 6. Geometrical models proposed for the catalysts. (A) The model for Fe(II)/Al2O3. Model-derived parameters: Fe-O shell at 1.98 Å, CN ) 5.1; Fe-Fe shell at 2.95 Å, CN ) 4.2. (B) The model for Fe2O3/Al2O3. Model-derived parameters: Fe-O shell at 1.98 Å, CN ) 5.7; Fe-Fe shell at 2.95 Å, CN ) 1.4; Fe-Fe shell at 3.41 Å, CN ) 1.0. The γ-Al2O3 (111) surface is shown by the close-packed circles (fine line), which represent the positions of the surface oxygens on the substrate without concerning the real radius. To avoid exact assignments of Fe2+ and Fe3+, the iron and adventitious oxygen are drawn to scale with the metal radius of iron (1.26 Å) and the covalent radius of oxygen (0.74 Å).
fold positional arrangement at all. It may imply that the coordinate behavior of the iron atom/ion defined in small ensembles deviates significantly from that in monolayers. The iron in small ensembles may retain much more molecule-like, not exhibit crystal-like, character on the surface. The model for Fe2O3/Al2O3 explains why a low metal loading unexpectedly leads to three types of iron neighbors. It can be seen from the model that the higher dispersion of the irons on Fe2O3/Al2O3 is achieved by simultaneously sharing the oxygens on Al2O3 surface in both meta- and para-positions, so the iron neighbors at 3.41 Å are becoming EXAFS-observable (3.42 Å). Accordingly, both the iron neighbors at 2.97 and 3.42 Å are accompanied by lower CN, 1.5 and 1.0, respectively. It matches the EXAFS results (1.8 and 1.3) very well. Since the iron ions on Fe2O3/Al2O3 are almost saturated by adventitious oxygens (top layer), the total CN for Fe-O shell is 5.5, very close to the EXAFS-derived data (5.7). An attempt to arrange the EXAFS-observed iron neighbors at 3.86 ( 0.09 Å onto the (111) plane was unsuccessful. We therefore tentatively propose that these irons may caused by some randomly deposited, isolated iron ions. In the high dispersion case as shown by Fe2O3/Al2O3, it is very common to occur on the surface. Surface reconstruction may be also closely related. As stated earlier in this paper, the γ-Al2O3 surface may be significantly different from one to another because of distinct method of preparation. Factors to be considered here include the starting material, the route of synthesis, as well as the subsequent treatment, and possibly others. It therefore should not be surprising that in actuality the γ-Al2O3 used in industrial catalysts may expose more than on crystal face only in different proportions. This may rationalize the different findings reported in the literature. But, for the samples used in this study, the (111) plane appears to be predominant, and the method established herein may be used advantageously to determine the preferred exposed face of the sample containing γ-alumina in question. Conclusion Two γ-alumina-supported iron catalysts have been investigated by XAFS analyses. It has been found that the average Fe-O bond length keeps the same value when the sample changes from Fe(0)/Al2O3, Fe(II)/Al2O3 to Fe2O3/Al2O3 with an increase in coordination number of the nearest oxygens (about 3, 5, and 6, respectively). In order to better understand the distinctive catalytic behaviors in CO hydrogenation, an approach to the catalyst surface based on the XAFS analyses and the
surface coordinate geometry consideration is developed. The resulting models reveal that the iron ions on Fe(II)/Al2O3 are nearly monolayered and are a mixture of FeO4, FeO5, and FeO6 units alternated by ferric and ferrous sites. The catalytic activity of Fe(II)/Al2O3 is thought to be caused by the oxygenunsaturated surface, namely, the outer oxygen defects. The irons on Fe2O3/Al2O3 are almost fully saturated by adventitious oxygens. The higher dispersion of the irons is achieved by locating a part of the irons onto the para-positions of the surface. The higher dispersion does not seem to be beneficial to the increase in catalytic activity. In fact, the Fe2O3/Al2O3 shows no observable activity in CO hydrogenation. It can therefore be concluded that the arrangement of iron oxide units, i.e., the irregularly close-packed iron monolayer with considerable outer oxygen defects, seems to be more important than the higher dispersion in improving the catalytic activity of iron catalysts. Acknowledgment. The project was supported by the National Natural Science Foundation, China. We are very grateful to the Photon Factory in Tsukuba, Japan, for use of the BL-7C facilities. We thank Dr. T. Tanaka and Professor M. Nomura for experimental assistance. We thank Professors H. Kno¨zinger and D. W. Goodman for helpful discussion. References and Notes (1) Kou, Y.; Wang, H.-L.; Te, M.; Tanaka, T.; Nomura, M. J. Catal. 1993, 141, 660. (2) Kou, Y.; Suo, Z.-H.; Wang, H.-L. J. Catal. 1994, 149, 247. (3) Ji, W.; Shen, S.; Li, S.; Wang, H.-L. Stud. Surf. Sci. Catal. 1991, 63, 517. (4) Kno¨zinger, H.; Zhao, Y., J. Catal. 1981, 71, 337. (5) Gates, B. C.; Koningsberger, D. C. CHEMTECH 1992, May, 300. (6) Ji, W.; Kou, Y.; Shen, S.; Li, S.; Wang, H.-L. Stud. Surf. Sci. Catal. 1993, 75B, 2059. (7) Kou, Y.; Suo, Z.-H.; Niu, J.-Z.; Wang, H.-L. Catal. Lett. 1995, 35, 271, 279. (8) Shulman, R. G.; Yafet, Y.; Eisenberger, P.; Blumberg, W. E. Proc. Natl. Acad. Sci. 1976, 73, 1384. (9) Bart, J. C. J. AdV. Catal. 1986, 34, 203. (10) Bianconi, A. Top. Curr. Chem. 1988, 145, 29. (11) Grunes, L. A. Phys. ReV. B 1983, 27, 2111. (12) Iijima, S. Jpn. J. Appl. Phys. 1984, 23, L347. (13) Iijima, S. Surf. Sci. 1985, 156, 1003. (14) Kno¨zinger, H.; Ratnasamy, P. Catal. ReV.-Sci. Eng. 1978, 17, 31. (15) Reller, A.; Cocke, D. L. Catal. Lett. 1989, 2, 91. (16) Chen, Y.; Zhang, L. Catal. Lett. 1992, 12, 51. (17) Foger, K. In Catalysis: Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1984; Vol. 6, p 231. (18) Shin, H. E.; Jona, F.; Jepsen, D. W.; Marcus, P. M. Phys. ReV. Lett. 1981, 46, 731. (19) Galloway, H. G.; Benı´tez, J. J.; Salmeron, M. Surf. Sci. 1993, 298, 127.
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