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Xanes, Exafs and Reaction Studies of Some Well-Dispersed Ferric Oxide Catalysts
Guai, L et al. (Editors), New Frontiers in Catalysis
Proccedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
XANES, E W S AND REACTION STUDIES OF SOME WELL-DISPERSED FERRIC OXIDE CATALYSTS W. Ji,
Y.kko, S. Shen, S. Li and H.Wang
State Key Laboratory for 0x0 Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 73oo00,China
INTRODUCTION When transition metal oxides are supported on and chemically interact with the surface of the typical oxide carriers such as Al203, Ti02 etc. , they form welldispersed monolayers in the form of two dimensional compounds, unlike those found at the surface of the unsupported oxide[l]. In this paper, the dispersion state of the well-dispersed monolayer ferric oxides on various supports ( y- A1203, Ti02, Zr02and MgO) is characterized by XANES and EXAFS. The modifications of the interaction on the reaction performance of these supported systems have also been studied by three model reactions. EXPERIMENTAL Details of the preparation chemistry have been reported elsewhereC2). The fluorescence Fe K-edge x-ray absorption measurements were performed at BL- 7C station of the photon factory in Japan. The data analysis was done first on a SUN386 MC with the program library written by the Institute of Physics, Chinese Academy of Sciences and then fitted on SUN workstation with FEFFS. 25 developed by J. J. Rehr et al. [3] ,using spherical-wave approximation to fit the EXAFS data in K-space. Pulse reaction of 2-C4H8and oxidation of CO with or without 0 2 supply and the succesive CO -I- HZreaction are chosen to study the influences of dispersion state on the reaction performances. RESULTS AND DISCUSSION Fig. 1 shows the XANE spectra of the Fe K- edge of the well- dispersed samples. It can be seen that a small peak due to 1s-3d transition appears at the preedge of all XANE spectra. This suggests that the coordination structure of Fe3+ site is either somewhat distorted or lacking an inversion center, otherwise these small
2060 peaks are forbidden by the selection rule and thus can rarely be observed. The differences in the intensity and pmition of the pre-edge as well as the shape of the post-edge of X A N E spectra reflect the differences in the degree of distortion and the electronic structure of the well- dispersed Fe3+. The covalent property of the surface Fe3+-@- bond is also responsible for the intensity of the 1s-3d peak. The more covalent the bond, the larger the peak. We have found in our XAS measurements that the Eo values of the Fe K-edge are all generally several eV hlgher than that of Fe3+ in the unsupported bulk aFezO3. Although the exact reason for this phemenological increase in binding energy of the inner electrons is not yet fully understood, we tentatively propose here that this increase may be brought about by the strong interactions existing in these well-dispersed systems. Fig. 2 shows the experimental and the theoretically fit Fourier transforms of the FeK-edge oscillations of the well-dispersed samples. The curve fitting results are listed in Table 1. The results indicate that the coordination structure of Fe3+in the monolayer FeOx species changes from one support to another and strongly depends on the chemical nature of the support used. This is ostensively different from the dispersion behavior of the typical monolayer oxide systems known (supported MoO3, WO3 and V2O6 ect. ). On some supports, for instance Ti02, Fe3+ is not dispersed in a simple manner. Instead, the coordination structure of F e 3 + i n relation to the 0 coordination shell-is very close to that of Fe3+ in Fe~TiO6[4]. The CN of 0 around the center Fe3+in FeOx on TiOz, ZrOzand MgO is higher than that of 0 in FeOx on y- A l 2 0 3 . The Fe3+ seems to disperse preferentially only on certain parts of the support surface in the submonolayer range, especially on y-Al~O3and Ti02 because of their relative small ratios of Fe loading/surface area of the support. If the corresponding surface area of support and the Fe loading are taken into account then the degree of spreading may be reasonably arranged in the following order : Fe3+-ZrOz> Fe3+- y-Al203 -Fe3+TiOz) Fe3+-Mg0. Theresults of pulse reaction of CO on FeOx/y-Al203 monolayer catalyst without O~supplyat 693K or with 0 2 supply at 623K showed that the lattice oxygen coordinated with surface Fes+ is less active as compared with that in the bulk ferric oxide for combustion of CO to produce C02. This inactivity is also shownby the behavior of adsorbed C2H4 on FeOx/ y- ,41203 and FeOx/ Ti02 monolayer catalysts. The adsorbed species do not change into water and carbonates at room temperature or formates at medium temperature. Both monolayer FeOx species and supported bulk ferric oxide are not active for CO+Hp reaction to yield CHI and other hydrocarbons during the initial period of reaction. It seems clear that the activity of methanation mainly depends on the valence state of iron rather than the local structure of Fe3+. This has been further comfirmed by the results of temperature programed reaction in syngas on FeOx/yA 2 0 3 and 15wt % Fe~03/y-A1~03 catalysts. In contrast to methanation, the monolayer FeOx species on various supports are still active for oxidative dehydrogenation of 2 - C4He. Thus an interesting
2061
conclusion can be reached that one atomic layer of Fe3+is enough for this reaction. From the EXAFS results we know the coordination structure of Fe3+in FeOx changes from one to another, particularly in the CV and the distance of the next neighbor Fe3+. However, the selectivity based on the normalized calculation is quite similar when the dispersion state is changed from two-dimensional to bulk on y-Al*Oa- and TiOa- supported samples. The uncovered support surface seems to have a major contribution to the formation of surface residues which decrease the selectivity based on the internal standard calculation. We tend to speculate that the structure requirement for the neighboring Fe3+ is not very demanding for the reaction process even though it is claimed to be "structure sensitive" [5].
Table 1 Curve fitting results of the well-dispersed samples Sample F A ( I )1.6 F A ( II ) 1 . 5 FT( II )O. 5 FM( I )O. 1st CN 2. 9 3. 0 4. 1 3. 5 Atomic R ( A ) 1.93 1. 94 1. 94 1. 93 0.0068 0.0063 Shell $(A2) 0. 0082 0. 0084 CN 2. 2 1. 6 2nd Atomic R ( A ) 2. 10 2. 06 Shell $(Az) 0.0037 0.0028 CN 2. 2 1. 4 2. 1 2. 5 3rd Atomic R ( A ) 3.38 3. 35 3. 09 3. 28 Shell $(Az) 0.0129 0.0108 0.0103 0.0105 Notes 1. The 1st and 2nd atomic shell are 0, the 3rd is Fe. Listed in the first Line represent the FeOx/y-AlZOs, FeOx/TiOZ, FeOx/Zr02respectively. The Roman numeral in ( ) refers preparation metheds[2]. Fe loading (wt%) is indicated after (
5
FZr( I )O. 5
3. 9 1. 93 0.0050 1.9 2. 06 0.0026 1.1 3.11 0.0104 2. The samples FeOx/MgO and to the different
>.
CONCLUSIONS
1. The curve fitting results of EXAES data as well as the features of the XANES indicate that the dispersion state of Fe'+changeS from one support to another. The very low CN of next neighboring Fes+with a relative long distance verifies that the Fe3+is basically dispersed in monolayers. 2. The Eo values of the Fe K-edge of the well-dispersed samples are probably an indicator showing the strong interaction existing in these systems. 3. The modification of the interaction on the reaction performance is not the same for the different reactions.
2062
REFERENCES 1 Y. Iwasawa, Adv. Catal. , 35(1987)187. 2 Weijie Ji, Shikong Shen, Shuben Li and Hongli Wang, in ”Studies in Surface Science and Catalysis 63’’ , Preparation of Catalysts V , G. Poncelt et al. (editors), Elsevier, Amsterdam, 1991, P. 517. 3 J. J. Rehr , FEFF Code(Version 3. 251, 1991, University of Washington. 4 C. L. Christ, Descriptive Crystal Chemistry, Weily &. Sons, 1989, P. 283. 5 B.L. Yang, F. Hong and H.H. Kung, J. Phys. Chem. , 88(1984)2531.
-20
50
0 E-Eo/eV
Fig.1. Fe K-XANES spectra of the well-dispersed samples. (a) FA( I N.6, (b) FA(1 11.6, (4FM( I 10.6, (d)FT( a )0.6, (e) FZr( I )0.6.
0
1
2
3
4
5
R 1s
Fig.2. Experimental(-) and theoretical fit(----) Fourier transforms of the well-dispersed samples. (4 FM( I )0.5, (b) Wll P.6, ( 4 FA(nl)1.6, ( 4 FA( I N.6, ( 4 FZr( I ) O h
Proccedings of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Hungary Q 1993 Elsevier Science Publishers B.V. All rights reserved
XANES, E W S AND REACTION STUDIES OF SOME WELL-DISPERSED FERRIC OXIDE CATALYSTS W. Ji,
Y.kko, S. Shen, S. Li and H.Wang
State Key Laboratory for 0x0 Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 73oo00,China
INTRODUCTION When transition metal oxides are supported on and chemically interact with the surface of the typical oxide carriers such as Al203, Ti02 etc. , they form welldispersed monolayers in the form of two dimensional compounds, unlike those found at the surface of the unsupported oxide[l]. In this paper, the dispersion state of the well-dispersed monolayer ferric oxides on various supports ( y- A1203, Ti02, Zr02and MgO) is characterized by XANES and EXAFS. The modifications of the interaction on the reaction performance of these supported systems have also been studied by three model reactions. EXPERIMENTAL Details of the preparation chemistry have been reported elsewhereC2). The fluorescence Fe K-edge x-ray absorption measurements were performed at BL- 7C station of the photon factory in Japan. The data analysis was done first on a SUN386 MC with the program library written by the Institute of Physics, Chinese Academy of Sciences and then fitted on SUN workstation with FEFFS. 25 developed by J. J. Rehr et al. [3] ,using spherical-wave approximation to fit the EXAFS data in K-space. Pulse reaction of 2-C4H8and oxidation of CO with or without 0 2 supply and the succesive CO -I- HZreaction are chosen to study the influences of dispersion state on the reaction performances. RESULTS AND DISCUSSION Fig. 1 shows the XANE spectra of the Fe K- edge of the well- dispersed samples. It can be seen that a small peak due to 1s-3d transition appears at the preedge of all XANE spectra. This suggests that the coordination structure of Fe3+ site is either somewhat distorted or lacking an inversion center, otherwise these small
2060 peaks are forbidden by the selection rule and thus can rarely be observed. The differences in the intensity and pmition of the pre-edge as well as the shape of the post-edge of X A N E spectra reflect the differences in the degree of distortion and the electronic structure of the well- dispersed Fe3+. The covalent property of the surface Fe3+-@- bond is also responsible for the intensity of the 1s-3d peak. The more covalent the bond, the larger the peak. We have found in our XAS measurements that the Eo values of the Fe K-edge are all generally several eV hlgher than that of Fe3+ in the unsupported bulk aFezO3. Although the exact reason for this phemenological increase in binding energy of the inner electrons is not yet fully understood, we tentatively propose here that this increase may be brought about by the strong interactions existing in these well-dispersed systems. Fig. 2 shows the experimental and the theoretically fit Fourier transforms of the FeK-edge oscillations of the well-dispersed samples. The curve fitting results are listed in Table 1. The results indicate that the coordination structure of Fe3+in the monolayer FeOx species changes from one support to another and strongly depends on the chemical nature of the support used. This is ostensively different from the dispersion behavior of the typical monolayer oxide systems known (supported MoO3, WO3 and V2O6 ect. ). On some supports, for instance Ti02, Fe3+ is not dispersed in a simple manner. Instead, the coordination structure of F e 3 + i n relation to the 0 coordination shell-is very close to that of Fe3+ in Fe~TiO6[4]. The CN of 0 around the center Fe3+in FeOx on TiOz, ZrOzand MgO is higher than that of 0 in FeOx on y- A l 2 0 3 . The Fe3+ seems to disperse preferentially only on certain parts of the support surface in the submonolayer range, especially on y-Al~O3and Ti02 because of their relative small ratios of Fe loading/surface area of the support. If the corresponding surface area of support and the Fe loading are taken into account then the degree of spreading may be reasonably arranged in the following order : Fe3+-ZrOz> Fe3+- y-Al203 -Fe3+TiOz) Fe3+-Mg0. Theresults of pulse reaction of CO on FeOx/y-Al203 monolayer catalyst without O~supplyat 693K or with 0 2 supply at 623K showed that the lattice oxygen coordinated with surface Fes+ is less active as compared with that in the bulk ferric oxide for combustion of CO to produce C02. This inactivity is also shownby the behavior of adsorbed C2H4 on FeOx/ y- ,41203 and FeOx/ Ti02 monolayer catalysts. The adsorbed species do not change into water and carbonates at room temperature or formates at medium temperature. Both monolayer FeOx species and supported bulk ferric oxide are not active for CO+Hp reaction to yield CHI and other hydrocarbons during the initial period of reaction. It seems clear that the activity of methanation mainly depends on the valence state of iron rather than the local structure of Fe3+. This has been further comfirmed by the results of temperature programed reaction in syngas on FeOx/yA 2 0 3 and 15wt % Fe~03/y-A1~03 catalysts. In contrast to methanation, the monolayer FeOx species on various supports are still active for oxidative dehydrogenation of 2 - C4He. Thus an interesting
2061
conclusion can be reached that one atomic layer of Fe3+is enough for this reaction. From the EXAFS results we know the coordination structure of Fe3+in FeOx changes from one to another, particularly in the CV and the distance of the next neighbor Fe3+. However, the selectivity based on the normalized calculation is quite similar when the dispersion state is changed from two-dimensional to bulk on y-Al*Oa- and TiOa- supported samples. The uncovered support surface seems to have a major contribution to the formation of surface residues which decrease the selectivity based on the internal standard calculation. We tend to speculate that the structure requirement for the neighboring Fe3+ is not very demanding for the reaction process even though it is claimed to be "structure sensitive" [5].
Table 1 Curve fitting results of the well-dispersed samples Sample F A ( I )1.6 F A ( II ) 1 . 5 FT( II )O. 5 FM( I )O. 1st CN 2. 9 3. 0 4. 1 3. 5 Atomic R ( A ) 1.93 1. 94 1. 94 1. 93 0.0068 0.0063 Shell $(A2) 0. 0082 0. 0084 CN 2. 2 1. 6 2nd Atomic R ( A ) 2. 10 2. 06 Shell $(Az) 0.0037 0.0028 CN 2. 2 1. 4 2. 1 2. 5 3rd Atomic R ( A ) 3.38 3. 35 3. 09 3. 28 Shell $(Az) 0.0129 0.0108 0.0103 0.0105 Notes 1. The 1st and 2nd atomic shell are 0, the 3rd is Fe. Listed in the first Line represent the FeOx/y-AlZOs, FeOx/TiOZ, FeOx/Zr02respectively. The Roman numeral in ( ) refers preparation metheds[2]. Fe loading (wt%) is indicated after (
5
FZr( I )O. 5
3. 9 1. 93 0.0050 1.9 2. 06 0.0026 1.1 3.11 0.0104 2. The samples FeOx/MgO and to the different
>.
CONCLUSIONS
1. The curve fitting results of EXAES data as well as the features of the XANES indicate that the dispersion state of Fe'+changeS from one support to another. The very low CN of next neighboring Fes+with a relative long distance verifies that the Fe3+is basically dispersed in monolayers. 2. The Eo values of the Fe K-edge of the well-dispersed samples are probably an indicator showing the strong interaction existing in these systems. 3. The modification of the interaction on the reaction performance is not the same for the different reactions.
2062
REFERENCES 1 Y. Iwasawa, Adv. Catal. , 35(1987)187. 2 Weijie Ji, Shikong Shen, Shuben Li and Hongli Wang, in ”Studies in Surface Science and Catalysis 63’’ , Preparation of Catalysts V , G. Poncelt et al. (editors), Elsevier, Amsterdam, 1991, P. 517. 3 J. J. Rehr , FEFF Code(Version 3. 251, 1991, University of Washington. 4 C. L. Christ, Descriptive Crystal Chemistry, Weily &. Sons, 1989, P. 283. 5 B.L. Yang, F. Hong and H.H. Kung, J. Phys. Chem. , 88(1984)2531.
-20
50
0 E-Eo/eV
Fig.1. Fe K-XANES spectra of the well-dispersed samples. (a) FA( I N.6, (b) FA(1 11.6, (4FM( I 10.6, (d)FT( a )0.6, (e) FZr( I )0.6.
0
1
2
3
4
5
R 1s
Fig.2. Experimental(-) and theoretical fit(----) Fourier transforms of the well-dispersed samples. (4 FM( I )0.5, (b) Wll P.6, ( 4 FA(nl)1.6, ( 4 FA( I N.6, ( 4 FZr( I ) O h