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XAFS analysis of europium species supported on aluminium oxide surface
Applied Surface Science 156 Ž2000. 65–75 www.elsevier.nlrlocaterapsusc
XAFS analysis of europium species supported on aluminium oxide surface Tomoko Yoshida a,) , Tsunehiro Tanaka a , Satohiro Yoshida a , Satoru Hikita b, Toshihide Baba b, Takafumi Hinode b, Yoshio Ono b a
b
Department of Molecular Engineering, Kyoto UniÕersity, Kyoto 606-01, Japan Department of Chemical Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo, 152, Japan Received 19 March 1999; accepted 26 August 1999
Abstract Europium amide species prepared by dissolving Eu in a liquid ammonia were supported on alumina, and the samples ŽEurAl 2 O 3 . were evacuated at various temperatures. The samples exhibited the activity for the isomerization of 2,3-dimethylbut-1-ene to 2,3-dimethylbut-2-ene, and the catalytic activity changed remarkably with the evacuation temperature and the loading amount of Eu. X-ray absorption near edge structure ŽXANES. spectra of the low loading Ž4 wt.%. and high loading Ž8 wt.%. EurAl 2 O 3 showed that Eu species in these samples are the mixture of Eu2q and Eu3q ions and the composition of di- and trivalent ions changes with the evacuation temperature. Structural analyses by extended X-ray absorption fine structure ŽEXAFS. spectra indicated that Eu ions preferentially react with OH groups on an Al 2 O 3 surface to form the isolated Eu species in 4 wt.% EurAl 2 O 3 , while the additional Eu species consisting of the Eu atoms bridged by the nitrogen atom are formed in 8 wt.% EurAl 2 O 3. It has been revealed that the former Eu species is inactive for the isomerization of 2,3-dimethylbut-1-ene, but the latter is active. These results were in good agreement with those obtained from IR measurements. EXAFS analysis also clarified close relationship between the catalytic activity and the degree of the aggregation of the active Eu species. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Among the lanthanide metals, only europium ŽEu. and ytterbium ŽYb. metals dissolve into liquid ammonia to yield a blue solution, characteristic of solvated electron w1–3x, and di- and trivalent amide
)
Corresponding author. Center for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464-8603, Japan. Tel.: q81-52-789-5124; fax: q81-52-789-5158. E-mail address: [email protected] ŽT. Yoshida..
species of these metals are formed w4–7x. We have studied the catalytic property of the Eu or Yb species supported on potassium-exchanged Y-zeolite ŽEurK-Y or YbrK-Y. which was prepared by impregnation with an ammonia solution of each metal, followed by heating in vacuo. The Eu or Yb species exhibited the activity for isomerization of olefin, functioning as a base catalyst. The catalytic activity changed remarkably according to the loading amount of the metal and the evacuation temperature, i.e., the highest activity was obtained when 7–8 wt.% EurKY or YbrK-Y evacuated at around 450 K was used
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 3 5 6 - 6
T. Yoshida et al.r Applied Surface Science 156 (2000) 65–75
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for the reaction. The characterization using X-ray absorption near edge structure ŽXANES., infrared spectroscopy ŽIR. and temperature programmed desorption ŽTPD. has been carried out for these samples and the following have been drawn at the present stage. 1. Both Eu and Yb species in these samples are the mixtures of their divalent and trivalent ions and the compositions of di- and trivalent ions change with the evacuation temperature. 2. The chemical states of Eu and Yb species also vary with elevating the evacuation temperature; amide imide nitride.
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In the previous works, we proposed that the change in the catalytic activity is attributed to the change in the valence and the chemical state of Yb or Eu species and the amide or imide species of divalent Yb or Eu are active species w8–12x. However, the structures or the dispersion state of Eu or Yb species are still unclear. Recently, we found that Eu species supported on alumina by the same preparation method ŽEurAl 2 O 3 . also functioned as a base catalyst and exhibited the activity for isomerization of olefin. The activity of EurAl 2 O 3 changed remarkably with the evacuation temperature and the loading amount of Eu in a similar matter as EurK-Y. However, the catalytic activity of EurAl 2 O 3 was much higher than that of EurK-Y and the dependence of the catalytic activity on the loading amount of Eu was appreciable for EurAl 2 O 3 samples. On the basis of these results, we considered that the study on EurAl 2 O 3 system should make clearer the correlation among the valence, chemical state of Eu species and the catalytic property of Eu catalysts. Furthermore, we expected that the study would give some information about the effects of the structure or the dispersion state of Eu species on the catalytic property which have not been examined yet. Therefore, in the present work, structural analysis by extended X-ray absorption fine structure ŽEXAFS. was added to the characterization by XANES and IR to clarify the local structure of Eu species supported on alumina. On the basis of the results obtained from these analyses, we discuss the factors generating and controlling the catalytic activity.
2. Experimental 2.1. Preparation of samples EurAl 2 O 3 samples were prepared by an impregnation method as described elsewhere w8x. Alumina was heated under vacuum to 10y3 Pa at 773 K for 3 h. The Al 2 O 3 was impregnated with an Eu metal solution of liquid ammonia under an inert condition, followed by evacuation to remove NH 3 at room temperature. The sample was evacuated at a given temperature for 1 h. The EurAl 2 O 3 samples evacuated at T ŽT : 353–973 K. are referred to as EurAl 2 O 3 ŽT . hereinafter. 2.2. Reaction procedures The isomerization of 2,3-dimethylbut-1-ene was performed in a conventional gas-circulating system of volume 422 cm3 w8x. 2,3-Dimethylbut-1-ene was purified by distillation before reaction. The reaction temperature was 314 K and the initial pressure of 2,3-dimethylbut-1-ene was 10.5 kPa. The reaction mixtures were analyzed by a gas chromatograph with 50 m column of OV-101. 2.3. IR measurements IR spectra were recorded on a JASCO FTIR 7000 spectrometer at room temperature by using a homemade IR cell w9x. The most important characteristic of the IR cell is that a self-supported disk placed on the holder can be impregnated in a metal–ammonia solution and then heat-treated in situ. The sample for IR measurements was prepared as follows: Al 2 O 3 was pressed into thin self-supported disk. After a disk of Al 2 O 3 was heated in vacuo at 773 K for 3 h, a piece of the ingot of Eu metal was placed in the bottom of the cell under nitrogen. The IR cell was evacuated to 10y3 Pa at room temperature, and ammonia was then liquefied into the bottom of the cell cooled with a mixture of dry-ice and ethanol to dissolve Eu metal. The disk of Al 2 O 3 was immersed into the ammonia solution of Eu for 1 h. The disk placed on the holder was then withdrawn from the ammonia solution and ammonia remaining in the bottom was removed by evacuation at room
T. Yoshida et al.r Applied Surface Science 156 (2000) 65–75
temperature. Finally, the disk was again moved down to be heat-treated under vacuum at a prescribed temperature. 2.4. X-ray absorption experiments X-ray absorption ŽXA. experiments were carried out at the beam line 7C ŽBL7C. station at Photon Factory in Institute of Materials Structure Science ŽKEK-PF. with a ring energy, 2.5 GeV and stored current, 250–350 mA. XA data of samples were collected by the EXAFS facilities installed at BL7C in a fluorescence mode at room temperature with an SiŽ111. two-crystal monochromator. Each XA spectrum was recorded using an in situ cell made of Pyrex glass with a Kapton window without exposing the sample to air. XA data of Eu 2 O 3 reference sample was collected in a transmission mode at room temperature. Normalization of X-ray absorption spectra and the extraction of their EXAFS oscillations were performed as described elsewhere w13,14x. The extracted EXAFS spectrum was Fourier-transformed in the ˚ y1 region to obtain the radial structure 2.5–11.0 A function ŽRSF.. The nonlinear curve-fitting analysis was performed for the Fourier-filtered EXAFS. For Fourierfiltering, the peaks in an RSF were isolated with a ˚ and Hanning window in the region of ca. 1.0–4.0 A, was inversely Fourier-transformed. The curve-fitting analysis was carried out using the following Eq. Ž1..
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The phase shifts of Ni–O and Ni–Mg shells were extracted from the EXAFS of a NiO reference sample and a Ni 0.02 Mg 0.98 O solid solution w16x, respectively. For the backscattering amplitude of Al, we also adopted the parameter of Ni–Mg shell. The Ni 0.02 Mg 0.98 O solid solution has the NaCl rock structure and the magnesium atoms exist in second neighboring shells around a Ni atom. Since Ni–O– Mg atoms are not collinear, it is improbable that Ni–Mg backscattering amplitude and phase shift are influenced by forward scattering. In the curve-fitting method, three parameters, Nj , r j and D sj 2 per shell were analyzed. The accuracy of the distance r j and uncertaincy of the coordination number Nj which are evaluated by the present ˚ and "10%, recurve-fitting method are "0.02 A spectively.
3. Results and discussion 3.1. Isomerization of 2,3-dimethylbut-1-ene oÕer Eu r Al 2 O3 samples Fig. 1 shows the influence of the evacuation temperature on the activity of 8 wt.% EurAl 2 O 3 for the isomerization of 2,3-dimethylbut-1-ene. For com-
k x Ž k . s Ý A j Ž k . = Njrr j2 = exp Ž y2 Dsj 2 k 2 . =sin Ž 2 kr j q f j Ž k . . ,
Ž 1.
where, Nj is the coordination number of scattering atoms at distance r j , and D sj 2 is the difference between the Debye–Waller factors of an interested sample and a reference sample. The amplitude AŽ k . and phase shift f Ž k . for Eu–O or Eu–N ŽEu–O,N. were extracted from the EXAFS of cubic C-type Eu 2 O 3 . For the AŽ k . and f Ž k . for the Eu–Eu shell, we used those compiled in FEFF package w15x. As there were no suitable compounds for Eu–Al shell, we adopted the phase shift Ž f . of an Eu–Mg shell which was obtained by calculating from Eu–O, Ni–O and Ni–Mg shells, i.e., f Ž Eu–Al . ( f Ž Eu–Mg . s f Ž Eu–O . y f Ž Ni–O . q f Ž Ni–Mg . .
Fig. 1. Influence of the evacuation temperature on the catalytic activity for isomerization of 2,3-dimethylbut-1-ene over Ža. 8 wt.% EurK-Y and Žb. 8 wt.% EurAl 2 O 3 and on Žc. the fraction of Eu2q species in 8 wt.% EurAl 2 O 3 .
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carried out several analyses concentrated on the high loading Ž8 wt.%. and low loading Ž4 wt.%. EurAl 2 O 3 samples as mentioned below. 3.2. The Õalence analysis for Eu species by XANES
Fig. 2. Influence of the loading amount of Eu on the catalytic activity for isomerization of 2,3-dimethylbut-1-ene over EurAl 2 O 3 samples.
parison, the variation of the activity of 8 wt.% EurK-Y ŽK-Y; Kq ion-exchanged Y-type zeolite. for the isomerization with the evacuation temperature w10x is also shown. The activity of 8 wt.% EurAl 2 O 3 increased with elevating the evacuation temperature and reached a maximum around 520 K. At a higher evacuation temperature, the catalytic activity was suppressed as the evacuation temperature increased. The similar relationship between the catalytic activity and the evacuation temperature was observed for 8 wt.% EurK-Y, although the catalytic activity reached a maximum at a little lower evacuation temperature than 520 K. However, the maximum catalytic activity for 8 wt.% EurAl 2 O 3 was about 15 times that for 8 wt.% EurK-Y, and the variation of the activity with the evacuation temperature was remarkable for 8 wt.% EurAl 2 O 3 . Fig. 2 shows the dependence of the catalytic activity of EurAl 2 O 3 on the loading amount of Eu. All EurAl 2 O 3 samples were evacuated at 523 K before the isomerization of 2,3-dimethylbut-1-ene. The EurAl 2 O 3 loading Eu less than 5 wt.% was inactive, while above 5 wt.%, the catalytic activity increased with the Eu loading. This clearly indicates the absence of the active Eu species for this reaction in EurAl 2 O 3 loading Eu less than 5 wt.%. Thus, the catalytic activity depended strongly on the loading amount of Eu. On the basis of these results, we
Fig. 3Ža. – Že. shows Eu L 3-edge XANES spectra of 8 wt.% EurAl 2 O 3 evacuated at various temperatures. The spectra exhibit two absorption maxima at 6972 and 6980 eV, so-called white lines, indicating the presence of two valence states of the Eu species. These energy values are very close to those of Eu2q and Eu3q in EuCo 2 Si 2yxGe x w17x and Eu 2 Ni 3 Si 5 w18x which are attributed to 2p 3r2 –5d electron transition in Eu2q and Eu3q, respectively w19x. The ratio of the peak heights differs from each EurAl 2 O 3 , indicating that the valence of Eu ions in EurAl 2 O 3 varies with the evacuation temperature. To estimate the fraction of Eu2q and Eu3q, we carried out deconvolution w11,12,20x of each XANES spectrum with two sets of a Lorentzian for a white line and an arctangent function for a continuum absorption w21,22x. Fig. 4 shows the deconvoluted spectrum of 8 wt.% EurAl 2 O 3 Ž573 K. as an example. The sum of the areas of two white lines was found to be constant for all samples Žca. 16.5 eV unit. so that the ratio of areas can be regarded to be the ratio of Eu2q and
Fig. 3. Eu L 3 -edge normalized XANES spectra of 8 wt.% EurAl 2 O 3 evacuated at Ža. 353, Žb. 423, Žc. 473, Žd. 573 and Že. 973 K and those of 4 wt.% EurAl 2 O 3 evacuated at Žf. 423 and Žg. 523 K. Energy offset is taken to be 6950.0 eV.
T. Yoshida et al.r Applied Surface Science 156 (2000) 65–75
Eu3q. The fractions of Eu2q and Eu3q estimated by this method are summarized in Table 1. By elevating the evacuation temperature, the fraction of Eu2q increases up to ca. 500 K and it decreases at the temperature above 573 K. It is noteworthy that the fraction of Eu2q is ca. 90%–100% in EurAl 2 O 3 evacuated at around 500 K. Fig. 3Žf. and Žg. shows Eu L 3-edge XANES spectra of 4 wt.% EurAl 2 O 3 evacuated at 423 and 523 K. These spectra clearly indicate that Eu species in the low loading EurAl 2 O 3 are a mixture of Eu2q and Eu3q ions and the fraction of Eu2q changes with the evacuation temperature. It is also noteworthy that the Eu2q fraction is about 90% for 4 wt.% EurAl 2 O 3 Ž423 K.. A definitive difference in the electronic state of Eu species between the high and low loading EurAl 2 O 3 is not detected, i.e., the valence state of Eu species is di- and trivalent regardless of Eu loading. 3.3. IR measurements To get information on surface Eu species on alumina, IR spectra of the catalyst were recorded. When alumina was heated under a vacuum at 773 K for 3 h, three IR bands were observed at ca. 3700 cmy1 ŽFig. 5Ž1a... These bands are due to OH groups on alumina w23x. The alumina pellet was immersed in Eu ammoniacal solution after dissolving 0.008 g of Eu in 3.8 cm3 of ammonia. The IR
Fig. 4. Eu L 3 -edge normalized XANES and its deconvoluted spectra of 8 wt.% EurAl 2 O 3 Ž573 K.. Energy offset is taken to be 6950.0 eV.
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Table 1 Calculated fractions of Eu2q and Eu3q species in 8 wt.% EurAl 2 O 3 samples Evacuation temperaturerK
Eu2q speciesr%
Eu3q speciesr%
353 423 473 573 973
71.8 96.0 92.5 85.4 65.5
28.2 4.0 7.5 14.6 34.5
spectrum was measured after the sample was evacuated at 298 K for 20 min ŽFig. 5Ž1b... The IR bands due to OH groups at ca. 3700 cmy1 disappeared upon supporting Eu, and three bands at 3302, 3242 and 1520 cmy1 appeared. This result indicates that some Eu atoms react with surface OH grops of an Al 2 O 3 . The bands at 3302 and 3242 cmy1 are attributed to asymmetrical and symmetrical stretching vibrations of N–H, respectively and the band at 1520 cmy1 is attributed to the bending vibration of H–N–H. Linde and Juza reported that IR bands due to the EuŽNH 2 . 2 were observe at 3263, 3200 and 1503 cmy1 w24x. Three IR bands as shown in Fig. 5Ž1b. agree reasonably well with those of europium amide, indicating that Eu supported on alumina exists as the amide upon evacuating the sample at 298 K. The Eu amide observed in this sample will be denoted as Eu amide-I. The second sample was prepared by immersing the alumina disk in a more concentrated solution of 0.20 g of Eu in 3.8 cm3 of liquid ammonia. The IR spectra of the sample evacuated at various temperatures are shown in Fig. 5Ž2.. The new three bands were observed at 3268, 3218 and 1560 cmy1 together with the bands observed in the spectrum of the first sample at 3302, 3242, and 1520 cmy1 . The new bands are also attributed to Eu amide w25x, which is denoted as Eu amide-II. The intensity of the peaks due to Eu amide-I decreased by increasing the evacuation temperature and the band disappeared at 673 K ŽFig. 5Ž2... On the other hand, the intensity of the peak due to Eu amide-II started to decrease around 623 K, and the bands disappeared at 773 K. Thus, Eu amide-II is thermally more stable than Eu amide-I. As mentioned above, it is noteworthy that only one type of Eu amide species ŽEu amide-I. would be formed in
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the low loading EurAl 2 O 3 , while two types of Eu amide species ŽEu amide-I and II. for the high loading EurAl 2 O 3 . 3.4. Structural analysis of Eu species by EXAFS We applied EXAFS analysis to clarify the difference in the structure of Eu species between the high
and low loading EurAl 2 O 3 . For the sample including the metal ions with more than two valence states, the structural analysis by EXAFS is very difficult, if not impossible, is not expected to be a reliable one. Therefore, we carried out the structural analyses for 8 wt.% EurAl 2 O 3 evacuated at around 500 K and 4 wt.% EurAl 2 O 3 Ž423 K. where the dominant valence state of Eu is divalent. Fig. 6 shows k-weighted Eu L 3-edge EXAFS spectra of 4 wt.% EurAl 2 O 3 Ž423 K., 8 wt.% EurAl 2 O 3 Ž473 and 573 K. and Eu 2 O 3 . There is a difference between EXAFS spectra of EurAl 2 O 3 samples and that of Eu 2 O 3 . In the spectra of the EurAl 2 O 3 samples, the distinct oscillation in the ˚ y1 and the weak oscillation in the range of 4–7 A higher k region are observed. On the other hand, the ˚ y1 is distinct and oscillation in the region of 8–11 A intense in the spectrum of Eu 2 O 3 . These results indicate that the degree of the aggregation of Eu species is low in EurAl 2 O 3 samples. The EXAFS spectra of 4 wt.% EurAl 2 O 3 Ž423 K., 8 wt.% EurAl 2 O 3 Ž473 K. and 8 wt.% EurAl 2 O 3 Ž573 K. may resemble each other. However, the prominent ˚ y1 is observed in EXAFS specshoulder around 5 A trum of 8 wt.% EurAl 2 O 3 Ž473 K., while no shoulder is seen in that of 4 wt.% EurAl 2 O 3 Ž423 K.. Although the shoulder can be seen in the EXAFS spectrum of 8 wt.% EurAl 2 O 3 Ž573 K., the shoulder is weak compared with that in the EXAFS spectrum of 8 wt.% EurAl 2 O 3 Ž473 K.. The local structures of Eu2q species became visually clearer when a Fourier transform was performed ˚ y1 region, on the EXAFS spectra in the 2.5–11.0 A to obtain the RSF. The RSFs are shown in Fig. 7. ˚ is attributed to The peak appearing at around 1.8 A Eu–O or Eu–N bonds ŽEu–O,N., and the peaks at ˚ show the presence of the neighboring around 2.8 A
Fig. 5. Ž top . IR spectra of the low loading EurAl 2 O 3 . Ža. Al 2 O 3 evacuated at 773 K for 3 h. Žb. Al 2 O 3 wafer was immersed in Eu ammoniacal solution dissolving 0.008 g of Eu in 3.8 cm3 of liquid ammonia, and then was evacuated at 298 K for 20 min. Ž bottom. IR spectra of the high loading EurAl 2 O 3 . The samples were prepared by immersing the Al 2 O 3 wafer in a solution of 0.20 g of Eu in 3.8 cm3 of liquid ammonia. Ža. Al 2 O 3 evacuated at 773 K for 3 h. Žb. The sample evacuated at 298 K for 20 min. Žc–g. The samples evacuated at Žc. 423, Žd. 473, Že. 523, Žf. 573 and Žg. 673 K, respectively for 1 h.
T. Yoshida et al.r Applied Surface Science 156 (2000) 65–75
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Fig. 6. k-Weighted EXAFS spectrum of Ža. 4 wt.% EurAl 2 O 3 sample evacuated at 423 K and those of 8 wt.% EurAl 2 O 3 samples evacuated at Žb. 473 and Žc. 573 K and that of Žd. Eu 2 O 3 .
metal atoms. Note that the peaks appearing at around ˚ in the RSF of 8 wt.% EurAl 2 O 3 Ž473 K. 2.8 A shown in Fig. 7Žb. is higher than those of 8 wt.% EurAl 2 O 3 Ž573 K. ŽFig. 7Žc.. and 4 wt.% EurAl 2 O 3 Ž423 K. ŽFig. 7Ža... We also found a difference in the imaginary part of RSFs. In particular, the imagi˚ shows an nary part of the peak appearing at 1.8 A ˚ unsymmetric pattern containing the shoulder at 2.4 A Ž . Ž Ž .. for 8 wt.% EurAl 2 O 3 473 K Fig. 7 b , while in the RSF of 8 wt.% EurAl 2 O 3 Ž573 K. ŽFig. 7Žc.., the shoulder is weak and the pattern of imaginary part becomes close to the symmetric one for 4 wt.% EurAl 2 O 3 Ž423 K.. The variation observed in RSFs would correspond to the variation of the shoulder at ˚ y1 seen in their EXAFS spectra. These results 5 A clearly indicate that the local structure of Eu2q species in 8 wt.% EurAl 2 O 3 changes during the elevation of the evacuation temperature from 473 K
to 573 K. It is also expected that there is a difference in the local structure of Eu2q species between the low loading Ž4 wt.%. EurAl 2 O 3 Ž423 K. and especially the high loading Ž8 wt.%. EurAl 2 O 3 Ž473 K.. The structure of Eu2q species in the high loading Ž8 wt.%. EurAl 2 O 3 Ž573 K. would be the intermediate one between them. 3.5. CurÕe-fitting analysis We carried out further analysis of the EXAFS oscillations by employing nonlinear curve-fitting against the Fourier-filtered EXAFS spectrum of 4 wt.% EurAl 2 O 3 Ž423 K.. The results are summarized in Table 2. The curve-fitting analysis revealed that there are short and long Eu–O or Eu–N bonds ˚ and the interatomic distances of these are ca. 2.13 A
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Fig. 7. Fourier transforms of k-weighted EXAFS spectra of Ža. 4 wt.% EurAl 2 O 3 sample evacuated at 423 K and those of 8 wt.% EurAl 2 O 3 samples evacuated at Žb. 473 and Žc. 573 K and that of Žd. Eu 2 O 3 .
˚ As for the atoms neighboring in the and 2.32 A. second shell around the Eu center, the curve-fitting analysis indicates the presence of Al atoms at the ˚ On the basis of the results of distance of ca. 3.28 A. IR measurements in Section 3.3, one of the two kinds of Eu–O,N bonds would be assigned to the nitrogen atoms of amide groups bonded to Eu atoms, and the other Eu–O,N bond would indicate Eu–O–Al bond. The curve-fitting results clearly show that Eu amide species fixed on Al 2 O 3 are formed preferentially in the low loading Ž4 wt.%. EurAl 2 O 3 . The Eu amide species would be isolated on Al 2 O 3 , since the presence of the neighboring Eu atoms is not detected by curve-fitting analysis. As for the high loading Ž8 wt.%. EurAl 2 O 3 Ž473 and 573 K., the curve-fitting analyses indicate the presence of short and long Eu–O or Eu–N bonds and the Al atoms neighboring in the second shell
around the Eu center as summarized in Table 2. These would be assignable to the nitrogen atoms of amide groups bonded to Eu atoms and the Eu–O–Al bond, in the similar manner to the low loading EurAl 2 O 3 Ž423 K.. In addition, as for the high loading EurAl 2 O 3 , the curve-fitting analyses show the presence of the neighboring Eu atoms in the second shell. For the Fourier-filtered EXAFS spectra of 8 wt.% EurAl 2 O 3 Ž473, 573 K., the curve-fitting without Eu–Eu shells were also performed, but the resultant parameters were unacceptable, i. e., the Debye–Waller factors became too large. In addition, the presence of the neighboring Eu atom was confirmed by carrying out inversely Fourier transforms for the secondary peaks in RSFs of 8 wt.% EurAl 2 O 3 Ž473, 573 K. and the low loading Ž4 wt.%. EurAl 2 O 3 Ž423 K.. The envelope of the inversely Fourier-transformed spectra for 8 wt.% EurAl 2 O 3 Ž473, 573 K.
T. Yoshida et al.r Applied Surface Science 156 (2000) 65–75 Table 2 The curve-fitting results of EurAl 2 O 3 samples evacuated at various temperatures. N: Coordination number, R: interatomic distances and s 2 : Debye–Waller factors Samples
Shell
N
˚ R rA
˚2 s 2 rA
8 wt.% EurAl 2 O 3 Ž473 K.
Eu–O,N Eu–O,N Eu–Al Eu–Eu Eu–O,N Eu–O,N Eu–Al Eu–Eu Eu–O,N Eu–O,N Eu–Al
2.2 2.1 1.3 1.5 1.6 1.6 0.9 0.7 2.2 2.6 1.4
2.17 2.35 3.29 3.48 2.14 2.33 3.29 3.49 2.13 2.32 3.28
y0.0010 a 0.0018 a 0.0050 a 0.0082 b y0.0010 a y0.0033 a 0.0067 a 0.0075 b y0.0012 a y0.0023 a 0.0072 a
8 wt.% EurAl 2 O 3 Ž573 K.
4 wt.% EurAl 2 O 3 Ž423 K. a
Relative Debye–Waller factors against those of reference samples. b Debye–Waller factors.
maximized at higher k value than that for 4 wt.% EurAl 2 O 3 Ž423 K., indicates the presence of the heavy Eu atoms around a Eu atom in former samples. The presence of the neighboring Eu atom demonstrates that Eu amide species fixed on Al 2 O 3 are not isolated in the high loading Ž8 wt.%. EurAl 2 O 3 . As shown in Table 2, the coordination number of the neighboring Eu atoms are evaluated to 1.5 and 0.7 for 8 wt.% EurAl 2 O 3 Ž473 K. and EurAl 2 O 3 Ž573 K., respectively. The values 1.5 and 0.7 may represent the formation of Eu dimer or trimer in EurAl 2 O 3 Ž473 K. and that of Eu monomer or dimer in EurAl 2 O 3 Ž573 K.. In addition, the curvefitting analysis also detected the decrease in the number of Eu–O,N bond as the evacuation temperature rises from 473 K to 573 K. The decrease in the number of Eu–O,N bond would correspond to the decrease in the number of the neighboring Eu atom from 1.5 to 0.7. In the temperature region of 473–573 K, we observed the desorption of NH 3 gas from the EurAl 2 O 3 sample by the TPD measurement. On the basis of these results, it can be expected that the neighboring Eu atom in Table 2 demonstrates the presence of Eu atoms bridged by nitrogen atom ŽEu–N–Eu.. Therefore, the decrease of the number of the neighboring Eu atom in EurAl 2 O 3 Ž573 K. probably results from the desorption of the bridged nitrogen atoms as NH 3 .
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Here, we conclude for the high loading Ž8 wt.%. EurAl 2 O 3 as follows. Eu amide species fixed on Al 2 O 3 are not isolated due to the formation of Eu atoms bridged by nitrogen atoms. Eu dimer or trimer probably form in EurAl 2 O 3 Ž473 K., while the Eu dimer or trimer changed to Eu monomer or dimer during the rise of evacuation temperature to 573 K by the desorption of the bridging nitrogen atoms. 3.6. The factors causing and controlling the catalytic actiÕity On the basis of the results revealed by XANES, IR and EXAFS analyses, in this section, we discuss the dependence of the catalytic activity on the electronic state, chemical state and local structure of Eu species in EurAl 2 O 3 . 3.6.1. The relationship between the catalytic actiÕity and the electronic state of Eu species In Fig. 1Žc., the fraction of Eu2q in 8 wt.% EurAl 2 O 3 is plotted against the evacuation temperature. The fraction was obtained from the XANES analysis as mentioned in Section 3.2. The fraction of Eu2q reached a maximum value Žca. 90%–100%. in 8 wt.% EurAl 2 O 3 evacuated at around 500 K, and the samples exhibited high activity for the isomerization of 2,3-dimethylbut-1-ene, indicating that Eu2q species is the active species for the isomerization of 2,3-dimethylbut-1-ene. This result is consistent with that for EurK-Y w10,11x. However, the change of the catalytic activity is not correlated to the valence variation of Eu species in EurAl 2 O 3 . For example, the fractions of Eu2q in 8 wt.% EurAl 2 O 3 evacuated at 473 and 573 K are almost the same Ž93% and 85%, respectively., but the catalytic activity of 8 wt.% EurAl 2 O 3 Ž473 K. is much higher by a factor of 10 than that of 8 wt.% EurAl 2 O 3 Ž573 K.. In addition, as shown in Fig. 3Žf., XANES spectrum showed that the fractions of Eu2q species is close to 90% for the low loading Ž4 wt.%. EurAl 2 O 3 Ž423 K. which sample was inactive for the isomerization of 2,3-dimethylbut-1-ene. These results indicate that the difference in the catalytic activity does not arise mainly from the difference in the number of Eu2q species. Although the active species for the isomerization of 2,3-dimethylbut-1-ene is Eu2q species,
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both active and inactive Eu2q species exist in EurAl 2 O 3 samples. 3.6.2. The relationship between the catalytic actiÕity and the chemical state of Eu species It has been found from the IR measurement that at least two types of Eu amide species are present in the high loading EurAl 2 O 3 . The IR bands at 3302, 3242 and 1520 cmy1 and those at 3268, 3218 and 1560 cmy1 were due to the two types of Eu amide species. On the other hand, only one type of Eu amide species was detected for the low loading EurAl 2 O 3 as represented by bands at 3302, 3242 and 1520 cmy1 . Taking into account the difference in the catalytic activity for the isomerization of 2,3-dimethylbut-1-ene between the high and low loading EurAl 2 O 3 , it is likely that the Eu amide represented by the bands at 3302, 3242 and 1520 cmy1 and those at 3268, 3218 and 1560 cmy1 are catalytic inactive and active species, respectively. By noticing the band at 1560 cmy1 in Fig. 5, it is found that the intensity of the band drops by evacuation above 673 K where the activity for the isomerization of 2,3-dimethylbut-1-ene reduces. This result supports the above consideration. Thus, the IR spectra suggest that both active and inactive Eu amide species coexist in the high loading EurAl 2 O 3 while only inactive Eu amide species exist in the low loading EurAl 2 O 3 . The active and inactive Eu amide species should correlate with the active and inactive Eu2q species discussed in the Section 3.6.1. However, the fundamental difference in the state of active and inactive Eu2q amide species is still unclear. Moreover, we cannot explain the reason why the catalytic activity of 8 wt.% EurAl 2 O 3 drops abruptly by elevating the evacuation temperature from 473 K to 573 K, since the variation of IR spectrum is very slight. To elucidate these, Section 3.6.3, we discuss the influence of the structure of Eu species on the catalytic activity. 3.6.3. The relationship between the catalytic actiÕity and the local structure of Eu species On the basis of EXAFS analysis, we concluded that Eu species fixed on Al 2 O 3 ŽEu–O–Al. are formed preferentially in 4 wt.% EurAl 2 O 3 Ž423 K., while both Eu–O–Al and Eu atoms bridged by the nitrogen atoms ŽEu–N–Eu. are formed in 8 wt.%
EurAl 2 O 3 . This would closely relate to the fact that IR spectra reflected the presence of one type Eu amide species for the low loading EurAl 2 O 3 and two types for the high loading EurAl 2 O 3 samples. That is, the preferential formation of Eu–O–Al leads to the generation of isolated Eu amide species in the low loading EurAl 2 O 3 samples and this is responsible for the absence of the catalytic property for the isomerization of 2,3-dimethylbut-1-ene. Imanura et al. w26,27x prepared EurSiO 2 samples with the similar preparation method as ours, and reported that the lanthanide selectively reacts with the hydroxyl groups on SiO 2 to form highly dispersed SiO–EuNH 2 species in low loading sample. Their results are in good agreement with ours, supporting our conclusion for the low loading EurAl 2 O 3 sample. On the other hand, as for high loading EurAl 2 O 3 , the formation of Eu–N–Eu bond in addition to that of Eu–O–Al bond has been detected. The Eu–N–Eu bond would reflect the local structure of active Eu amide species detected by the IR measurement. Actually, the Eu– N–Eu bond would be associated with the activity for the isomerization of 2,3-dimethylbut-1-ene, since the number of Eu–N–Eu bond formed in the low active 8 wt.% EurAl 2 O 3 Ž573 K. is small compared with that formed in the high active 8 wt.% EurAl 2 O 3 Ž473 K.. As mentioned above, the decrease in the number of Eu–N–Eu sites would be caused by the desorption of the bridging nitrogen atoms. This is the reason why the catalytic activity of 8 wt.% EurAl 2 O 3 drops abruptly by elevating the evacuation temperature from 473 K to 573 K. The degree of aggregation of Eu amide species is also the important factor controlling the catalytic activity. Finally, on the basis of all the results, we can here conclude that the Eu2q amide species in a form of appropriate size of the cluster is an active species for isomerization of 2,3-dimethylbut-1-ene, i.e., the site generating the property as solid base.
4. Conclusion Ž1. Eu species in EurAl 2 O 3 samples are the mixture of divalent and trivalent Eu ions, and Eu2q amide species is the active site for the isomerization of 2,3-dimethylbut-1-ene, i.e., the site exhibits basic-
T. Yoshida et al.r Applied Surface Science 156 (2000) 65–75
ity. These results are consistent with those for EurK-Y samples. Ž2. Among the divalent Eu amide species, there are the active and inactive Eu species for the isomerization of 2,3-dimethylbut-1-ene. In the low loading EurAl 2 O 3 samples, divalent Eu ions preferentially react with OH group on Al 2 O 3 surface to form the isolated Eu species which exhibits no activity for the isomerization of 2,3-dimethylbut-1-ene. In the high loading EurAl 2 O 3 samples, not only the inactive Eu species, but also the active Eu species which consists of the Eu atoms bridged by the nitrogen atom are formed. Ž3. The catalytic activity as a solid base is controlled by the degree of the aggregation of the active Eu species mentioned in Ž2.. The difference in the aggregation of the active Eu species is one of the important reasons to explain why the catalytic activity as solid base changes with the evacuation temperature and the loading amount of Eu.
Acknowledgements X-ray absorption experiments were performed under the approval of the Photon Factory Program Advisory Committee ŽProposal no. 91-175 and 93G168..
References w1x w2x w3x w4x
J.C. Warf, W.J. Korst, J. Phys. Chem. 60 Ž1956. 1590. J.K.A. Gshneidner, J. Less-Common Met. 17 Ž1969. 13. M.C. Symons, Q. Rev. ŽLondon. 13 Ž1959. 99. A. Iandelli, A. Palenzona, J. Less-Common Met. 29 Ž1972. 293.
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XAFS analysis of europium species supported on aluminium oxide surface Tomoko Yoshida a,) , Tsunehiro Tanaka a , Satohiro Yoshida a , Satoru Hikita b, Toshihide Baba b, Takafumi Hinode b, Yoshio Ono b a
b
Department of Molecular Engineering, Kyoto UniÕersity, Kyoto 606-01, Japan Department of Chemical Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo, 152, Japan Received 19 March 1999; accepted 26 August 1999
Abstract Europium amide species prepared by dissolving Eu in a liquid ammonia were supported on alumina, and the samples ŽEurAl 2 O 3 . were evacuated at various temperatures. The samples exhibited the activity for the isomerization of 2,3-dimethylbut-1-ene to 2,3-dimethylbut-2-ene, and the catalytic activity changed remarkably with the evacuation temperature and the loading amount of Eu. X-ray absorption near edge structure ŽXANES. spectra of the low loading Ž4 wt.%. and high loading Ž8 wt.%. EurAl 2 O 3 showed that Eu species in these samples are the mixture of Eu2q and Eu3q ions and the composition of di- and trivalent ions changes with the evacuation temperature. Structural analyses by extended X-ray absorption fine structure ŽEXAFS. spectra indicated that Eu ions preferentially react with OH groups on an Al 2 O 3 surface to form the isolated Eu species in 4 wt.% EurAl 2 O 3 , while the additional Eu species consisting of the Eu atoms bridged by the nitrogen atom are formed in 8 wt.% EurAl 2 O 3. It has been revealed that the former Eu species is inactive for the isomerization of 2,3-dimethylbut-1-ene, but the latter is active. These results were in good agreement with those obtained from IR measurements. EXAFS analysis also clarified close relationship between the catalytic activity and the degree of the aggregation of the active Eu species. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Among the lanthanide metals, only europium ŽEu. and ytterbium ŽYb. metals dissolve into liquid ammonia to yield a blue solution, characteristic of solvated electron w1–3x, and di- and trivalent amide
)
Corresponding author. Center for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464-8603, Japan. Tel.: q81-52-789-5124; fax: q81-52-789-5158. E-mail address: [email protected] ŽT. Yoshida..
species of these metals are formed w4–7x. We have studied the catalytic property of the Eu or Yb species supported on potassium-exchanged Y-zeolite ŽEurK-Y or YbrK-Y. which was prepared by impregnation with an ammonia solution of each metal, followed by heating in vacuo. The Eu or Yb species exhibited the activity for isomerization of olefin, functioning as a base catalyst. The catalytic activity changed remarkably according to the loading amount of the metal and the evacuation temperature, i.e., the highest activity was obtained when 7–8 wt.% EurKY or YbrK-Y evacuated at around 450 K was used
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 3 5 6 - 6
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for the reaction. The characterization using X-ray absorption near edge structure ŽXANES., infrared spectroscopy ŽIR. and temperature programmed desorption ŽTPD. has been carried out for these samples and the following have been drawn at the present stage. 1. Both Eu and Yb species in these samples are the mixtures of their divalent and trivalent ions and the compositions of di- and trivalent ions change with the evacuation temperature. 2. The chemical states of Eu and Yb species also vary with elevating the evacuation temperature; amide imide nitride.
™
™
In the previous works, we proposed that the change in the catalytic activity is attributed to the change in the valence and the chemical state of Yb or Eu species and the amide or imide species of divalent Yb or Eu are active species w8–12x. However, the structures or the dispersion state of Eu or Yb species are still unclear. Recently, we found that Eu species supported on alumina by the same preparation method ŽEurAl 2 O 3 . also functioned as a base catalyst and exhibited the activity for isomerization of olefin. The activity of EurAl 2 O 3 changed remarkably with the evacuation temperature and the loading amount of Eu in a similar matter as EurK-Y. However, the catalytic activity of EurAl 2 O 3 was much higher than that of EurK-Y and the dependence of the catalytic activity on the loading amount of Eu was appreciable for EurAl 2 O 3 samples. On the basis of these results, we considered that the study on EurAl 2 O 3 system should make clearer the correlation among the valence, chemical state of Eu species and the catalytic property of Eu catalysts. Furthermore, we expected that the study would give some information about the effects of the structure or the dispersion state of Eu species on the catalytic property which have not been examined yet. Therefore, in the present work, structural analysis by extended X-ray absorption fine structure ŽEXAFS. was added to the characterization by XANES and IR to clarify the local structure of Eu species supported on alumina. On the basis of the results obtained from these analyses, we discuss the factors generating and controlling the catalytic activity.
2. Experimental 2.1. Preparation of samples EurAl 2 O 3 samples were prepared by an impregnation method as described elsewhere w8x. Alumina was heated under vacuum to 10y3 Pa at 773 K for 3 h. The Al 2 O 3 was impregnated with an Eu metal solution of liquid ammonia under an inert condition, followed by evacuation to remove NH 3 at room temperature. The sample was evacuated at a given temperature for 1 h. The EurAl 2 O 3 samples evacuated at T ŽT : 353–973 K. are referred to as EurAl 2 O 3 ŽT . hereinafter. 2.2. Reaction procedures The isomerization of 2,3-dimethylbut-1-ene was performed in a conventional gas-circulating system of volume 422 cm3 w8x. 2,3-Dimethylbut-1-ene was purified by distillation before reaction. The reaction temperature was 314 K and the initial pressure of 2,3-dimethylbut-1-ene was 10.5 kPa. The reaction mixtures were analyzed by a gas chromatograph with 50 m column of OV-101. 2.3. IR measurements IR spectra were recorded on a JASCO FTIR 7000 spectrometer at room temperature by using a homemade IR cell w9x. The most important characteristic of the IR cell is that a self-supported disk placed on the holder can be impregnated in a metal–ammonia solution and then heat-treated in situ. The sample for IR measurements was prepared as follows: Al 2 O 3 was pressed into thin self-supported disk. After a disk of Al 2 O 3 was heated in vacuo at 773 K for 3 h, a piece of the ingot of Eu metal was placed in the bottom of the cell under nitrogen. The IR cell was evacuated to 10y3 Pa at room temperature, and ammonia was then liquefied into the bottom of the cell cooled with a mixture of dry-ice and ethanol to dissolve Eu metal. The disk of Al 2 O 3 was immersed into the ammonia solution of Eu for 1 h. The disk placed on the holder was then withdrawn from the ammonia solution and ammonia remaining in the bottom was removed by evacuation at room
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temperature. Finally, the disk was again moved down to be heat-treated under vacuum at a prescribed temperature. 2.4. X-ray absorption experiments X-ray absorption ŽXA. experiments were carried out at the beam line 7C ŽBL7C. station at Photon Factory in Institute of Materials Structure Science ŽKEK-PF. with a ring energy, 2.5 GeV and stored current, 250–350 mA. XA data of samples were collected by the EXAFS facilities installed at BL7C in a fluorescence mode at room temperature with an SiŽ111. two-crystal monochromator. Each XA spectrum was recorded using an in situ cell made of Pyrex glass with a Kapton window without exposing the sample to air. XA data of Eu 2 O 3 reference sample was collected in a transmission mode at room temperature. Normalization of X-ray absorption spectra and the extraction of their EXAFS oscillations were performed as described elsewhere w13,14x. The extracted EXAFS spectrum was Fourier-transformed in the ˚ y1 region to obtain the radial structure 2.5–11.0 A function ŽRSF.. The nonlinear curve-fitting analysis was performed for the Fourier-filtered EXAFS. For Fourierfiltering, the peaks in an RSF were isolated with a ˚ and Hanning window in the region of ca. 1.0–4.0 A, was inversely Fourier-transformed. The curve-fitting analysis was carried out using the following Eq. Ž1..
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The phase shifts of Ni–O and Ni–Mg shells were extracted from the EXAFS of a NiO reference sample and a Ni 0.02 Mg 0.98 O solid solution w16x, respectively. For the backscattering amplitude of Al, we also adopted the parameter of Ni–Mg shell. The Ni 0.02 Mg 0.98 O solid solution has the NaCl rock structure and the magnesium atoms exist in second neighboring shells around a Ni atom. Since Ni–O– Mg atoms are not collinear, it is improbable that Ni–Mg backscattering amplitude and phase shift are influenced by forward scattering. In the curve-fitting method, three parameters, Nj , r j and D sj 2 per shell were analyzed. The accuracy of the distance r j and uncertaincy of the coordination number Nj which are evaluated by the present ˚ and "10%, recurve-fitting method are "0.02 A spectively.
3. Results and discussion 3.1. Isomerization of 2,3-dimethylbut-1-ene oÕer Eu r Al 2 O3 samples Fig. 1 shows the influence of the evacuation temperature on the activity of 8 wt.% EurAl 2 O 3 for the isomerization of 2,3-dimethylbut-1-ene. For com-
k x Ž k . s Ý A j Ž k . = Njrr j2 = exp Ž y2 Dsj 2 k 2 . =sin Ž 2 kr j q f j Ž k . . ,
Ž 1.
where, Nj is the coordination number of scattering atoms at distance r j , and D sj 2 is the difference between the Debye–Waller factors of an interested sample and a reference sample. The amplitude AŽ k . and phase shift f Ž k . for Eu–O or Eu–N ŽEu–O,N. were extracted from the EXAFS of cubic C-type Eu 2 O 3 . For the AŽ k . and f Ž k . for the Eu–Eu shell, we used those compiled in FEFF package w15x. As there were no suitable compounds for Eu–Al shell, we adopted the phase shift Ž f . of an Eu–Mg shell which was obtained by calculating from Eu–O, Ni–O and Ni–Mg shells, i.e., f Ž Eu–Al . ( f Ž Eu–Mg . s f Ž Eu–O . y f Ž Ni–O . q f Ž Ni–Mg . .
Fig. 1. Influence of the evacuation temperature on the catalytic activity for isomerization of 2,3-dimethylbut-1-ene over Ža. 8 wt.% EurK-Y and Žb. 8 wt.% EurAl 2 O 3 and on Žc. the fraction of Eu2q species in 8 wt.% EurAl 2 O 3 .
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carried out several analyses concentrated on the high loading Ž8 wt.%. and low loading Ž4 wt.%. EurAl 2 O 3 samples as mentioned below. 3.2. The Õalence analysis for Eu species by XANES
Fig. 2. Influence of the loading amount of Eu on the catalytic activity for isomerization of 2,3-dimethylbut-1-ene over EurAl 2 O 3 samples.
parison, the variation of the activity of 8 wt.% EurK-Y ŽK-Y; Kq ion-exchanged Y-type zeolite. for the isomerization with the evacuation temperature w10x is also shown. The activity of 8 wt.% EurAl 2 O 3 increased with elevating the evacuation temperature and reached a maximum around 520 K. At a higher evacuation temperature, the catalytic activity was suppressed as the evacuation temperature increased. The similar relationship between the catalytic activity and the evacuation temperature was observed for 8 wt.% EurK-Y, although the catalytic activity reached a maximum at a little lower evacuation temperature than 520 K. However, the maximum catalytic activity for 8 wt.% EurAl 2 O 3 was about 15 times that for 8 wt.% EurK-Y, and the variation of the activity with the evacuation temperature was remarkable for 8 wt.% EurAl 2 O 3 . Fig. 2 shows the dependence of the catalytic activity of EurAl 2 O 3 on the loading amount of Eu. All EurAl 2 O 3 samples were evacuated at 523 K before the isomerization of 2,3-dimethylbut-1-ene. The EurAl 2 O 3 loading Eu less than 5 wt.% was inactive, while above 5 wt.%, the catalytic activity increased with the Eu loading. This clearly indicates the absence of the active Eu species for this reaction in EurAl 2 O 3 loading Eu less than 5 wt.%. Thus, the catalytic activity depended strongly on the loading amount of Eu. On the basis of these results, we
Fig. 3Ža. – Že. shows Eu L 3-edge XANES spectra of 8 wt.% EurAl 2 O 3 evacuated at various temperatures. The spectra exhibit two absorption maxima at 6972 and 6980 eV, so-called white lines, indicating the presence of two valence states of the Eu species. These energy values are very close to those of Eu2q and Eu3q in EuCo 2 Si 2yxGe x w17x and Eu 2 Ni 3 Si 5 w18x which are attributed to 2p 3r2 –5d electron transition in Eu2q and Eu3q, respectively w19x. The ratio of the peak heights differs from each EurAl 2 O 3 , indicating that the valence of Eu ions in EurAl 2 O 3 varies with the evacuation temperature. To estimate the fraction of Eu2q and Eu3q, we carried out deconvolution w11,12,20x of each XANES spectrum with two sets of a Lorentzian for a white line and an arctangent function for a continuum absorption w21,22x. Fig. 4 shows the deconvoluted spectrum of 8 wt.% EurAl 2 O 3 Ž573 K. as an example. The sum of the areas of two white lines was found to be constant for all samples Žca. 16.5 eV unit. so that the ratio of areas can be regarded to be the ratio of Eu2q and
Fig. 3. Eu L 3 -edge normalized XANES spectra of 8 wt.% EurAl 2 O 3 evacuated at Ža. 353, Žb. 423, Žc. 473, Žd. 573 and Že. 973 K and those of 4 wt.% EurAl 2 O 3 evacuated at Žf. 423 and Žg. 523 K. Energy offset is taken to be 6950.0 eV.
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Eu3q. The fractions of Eu2q and Eu3q estimated by this method are summarized in Table 1. By elevating the evacuation temperature, the fraction of Eu2q increases up to ca. 500 K and it decreases at the temperature above 573 K. It is noteworthy that the fraction of Eu2q is ca. 90%–100% in EurAl 2 O 3 evacuated at around 500 K. Fig. 3Žf. and Žg. shows Eu L 3-edge XANES spectra of 4 wt.% EurAl 2 O 3 evacuated at 423 and 523 K. These spectra clearly indicate that Eu species in the low loading EurAl 2 O 3 are a mixture of Eu2q and Eu3q ions and the fraction of Eu2q changes with the evacuation temperature. It is also noteworthy that the Eu2q fraction is about 90% for 4 wt.% EurAl 2 O 3 Ž423 K.. A definitive difference in the electronic state of Eu species between the high and low loading EurAl 2 O 3 is not detected, i.e., the valence state of Eu species is di- and trivalent regardless of Eu loading. 3.3. IR measurements To get information on surface Eu species on alumina, IR spectra of the catalyst were recorded. When alumina was heated under a vacuum at 773 K for 3 h, three IR bands were observed at ca. 3700 cmy1 ŽFig. 5Ž1a... These bands are due to OH groups on alumina w23x. The alumina pellet was immersed in Eu ammoniacal solution after dissolving 0.008 g of Eu in 3.8 cm3 of ammonia. The IR
Fig. 4. Eu L 3 -edge normalized XANES and its deconvoluted spectra of 8 wt.% EurAl 2 O 3 Ž573 K.. Energy offset is taken to be 6950.0 eV.
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Table 1 Calculated fractions of Eu2q and Eu3q species in 8 wt.% EurAl 2 O 3 samples Evacuation temperaturerK
Eu2q speciesr%
Eu3q speciesr%
353 423 473 573 973
71.8 96.0 92.5 85.4 65.5
28.2 4.0 7.5 14.6 34.5
spectrum was measured after the sample was evacuated at 298 K for 20 min ŽFig. 5Ž1b... The IR bands due to OH groups at ca. 3700 cmy1 disappeared upon supporting Eu, and three bands at 3302, 3242 and 1520 cmy1 appeared. This result indicates that some Eu atoms react with surface OH grops of an Al 2 O 3 . The bands at 3302 and 3242 cmy1 are attributed to asymmetrical and symmetrical stretching vibrations of N–H, respectively and the band at 1520 cmy1 is attributed to the bending vibration of H–N–H. Linde and Juza reported that IR bands due to the EuŽNH 2 . 2 were observe at 3263, 3200 and 1503 cmy1 w24x. Three IR bands as shown in Fig. 5Ž1b. agree reasonably well with those of europium amide, indicating that Eu supported on alumina exists as the amide upon evacuating the sample at 298 K. The Eu amide observed in this sample will be denoted as Eu amide-I. The second sample was prepared by immersing the alumina disk in a more concentrated solution of 0.20 g of Eu in 3.8 cm3 of liquid ammonia. The IR spectra of the sample evacuated at various temperatures are shown in Fig. 5Ž2.. The new three bands were observed at 3268, 3218 and 1560 cmy1 together with the bands observed in the spectrum of the first sample at 3302, 3242, and 1520 cmy1 . The new bands are also attributed to Eu amide w25x, which is denoted as Eu amide-II. The intensity of the peaks due to Eu amide-I decreased by increasing the evacuation temperature and the band disappeared at 673 K ŽFig. 5Ž2... On the other hand, the intensity of the peak due to Eu amide-II started to decrease around 623 K, and the bands disappeared at 773 K. Thus, Eu amide-II is thermally more stable than Eu amide-I. As mentioned above, it is noteworthy that only one type of Eu amide species ŽEu amide-I. would be formed in
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the low loading EurAl 2 O 3 , while two types of Eu amide species ŽEu amide-I and II. for the high loading EurAl 2 O 3 . 3.4. Structural analysis of Eu species by EXAFS We applied EXAFS analysis to clarify the difference in the structure of Eu species between the high
and low loading EurAl 2 O 3 . For the sample including the metal ions with more than two valence states, the structural analysis by EXAFS is very difficult, if not impossible, is not expected to be a reliable one. Therefore, we carried out the structural analyses for 8 wt.% EurAl 2 O 3 evacuated at around 500 K and 4 wt.% EurAl 2 O 3 Ž423 K. where the dominant valence state of Eu is divalent. Fig. 6 shows k-weighted Eu L 3-edge EXAFS spectra of 4 wt.% EurAl 2 O 3 Ž423 K., 8 wt.% EurAl 2 O 3 Ž473 and 573 K. and Eu 2 O 3 . There is a difference between EXAFS spectra of EurAl 2 O 3 samples and that of Eu 2 O 3 . In the spectra of the EurAl 2 O 3 samples, the distinct oscillation in the ˚ y1 and the weak oscillation in the range of 4–7 A higher k region are observed. On the other hand, the ˚ y1 is distinct and oscillation in the region of 8–11 A intense in the spectrum of Eu 2 O 3 . These results indicate that the degree of the aggregation of Eu species is low in EurAl 2 O 3 samples. The EXAFS spectra of 4 wt.% EurAl 2 O 3 Ž423 K., 8 wt.% EurAl 2 O 3 Ž473 K. and 8 wt.% EurAl 2 O 3 Ž573 K. may resemble each other. However, the prominent ˚ y1 is observed in EXAFS specshoulder around 5 A trum of 8 wt.% EurAl 2 O 3 Ž473 K., while no shoulder is seen in that of 4 wt.% EurAl 2 O 3 Ž423 K.. Although the shoulder can be seen in the EXAFS spectrum of 8 wt.% EurAl 2 O 3 Ž573 K., the shoulder is weak compared with that in the EXAFS spectrum of 8 wt.% EurAl 2 O 3 Ž473 K.. The local structures of Eu2q species became visually clearer when a Fourier transform was performed ˚ y1 region, on the EXAFS spectra in the 2.5–11.0 A to obtain the RSF. The RSFs are shown in Fig. 7. ˚ is attributed to The peak appearing at around 1.8 A Eu–O or Eu–N bonds ŽEu–O,N., and the peaks at ˚ show the presence of the neighboring around 2.8 A
Fig. 5. Ž top . IR spectra of the low loading EurAl 2 O 3 . Ža. Al 2 O 3 evacuated at 773 K for 3 h. Žb. Al 2 O 3 wafer was immersed in Eu ammoniacal solution dissolving 0.008 g of Eu in 3.8 cm3 of liquid ammonia, and then was evacuated at 298 K for 20 min. Ž bottom. IR spectra of the high loading EurAl 2 O 3 . The samples were prepared by immersing the Al 2 O 3 wafer in a solution of 0.20 g of Eu in 3.8 cm3 of liquid ammonia. Ža. Al 2 O 3 evacuated at 773 K for 3 h. Žb. The sample evacuated at 298 K for 20 min. Žc–g. The samples evacuated at Žc. 423, Žd. 473, Že. 523, Žf. 573 and Žg. 673 K, respectively for 1 h.
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Fig. 6. k-Weighted EXAFS spectrum of Ža. 4 wt.% EurAl 2 O 3 sample evacuated at 423 K and those of 8 wt.% EurAl 2 O 3 samples evacuated at Žb. 473 and Žc. 573 K and that of Žd. Eu 2 O 3 .
metal atoms. Note that the peaks appearing at around ˚ in the RSF of 8 wt.% EurAl 2 O 3 Ž473 K. 2.8 A shown in Fig. 7Žb. is higher than those of 8 wt.% EurAl 2 O 3 Ž573 K. ŽFig. 7Žc.. and 4 wt.% EurAl 2 O 3 Ž423 K. ŽFig. 7Ža... We also found a difference in the imaginary part of RSFs. In particular, the imagi˚ shows an nary part of the peak appearing at 1.8 A ˚ unsymmetric pattern containing the shoulder at 2.4 A Ž . Ž Ž .. for 8 wt.% EurAl 2 O 3 473 K Fig. 7 b , while in the RSF of 8 wt.% EurAl 2 O 3 Ž573 K. ŽFig. 7Žc.., the shoulder is weak and the pattern of imaginary part becomes close to the symmetric one for 4 wt.% EurAl 2 O 3 Ž423 K.. The variation observed in RSFs would correspond to the variation of the shoulder at ˚ y1 seen in their EXAFS spectra. These results 5 A clearly indicate that the local structure of Eu2q species in 8 wt.% EurAl 2 O 3 changes during the elevation of the evacuation temperature from 473 K
to 573 K. It is also expected that there is a difference in the local structure of Eu2q species between the low loading Ž4 wt.%. EurAl 2 O 3 Ž423 K. and especially the high loading Ž8 wt.%. EurAl 2 O 3 Ž473 K.. The structure of Eu2q species in the high loading Ž8 wt.%. EurAl 2 O 3 Ž573 K. would be the intermediate one between them. 3.5. CurÕe-fitting analysis We carried out further analysis of the EXAFS oscillations by employing nonlinear curve-fitting against the Fourier-filtered EXAFS spectrum of 4 wt.% EurAl 2 O 3 Ž423 K.. The results are summarized in Table 2. The curve-fitting analysis revealed that there are short and long Eu–O or Eu–N bonds ˚ and the interatomic distances of these are ca. 2.13 A
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T. Yoshida et al.r Applied Surface Science 156 (2000) 65–75
Fig. 7. Fourier transforms of k-weighted EXAFS spectra of Ža. 4 wt.% EurAl 2 O 3 sample evacuated at 423 K and those of 8 wt.% EurAl 2 O 3 samples evacuated at Žb. 473 and Žc. 573 K and that of Žd. Eu 2 O 3 .
˚ As for the atoms neighboring in the and 2.32 A. second shell around the Eu center, the curve-fitting analysis indicates the presence of Al atoms at the ˚ On the basis of the results of distance of ca. 3.28 A. IR measurements in Section 3.3, one of the two kinds of Eu–O,N bonds would be assigned to the nitrogen atoms of amide groups bonded to Eu atoms, and the other Eu–O,N bond would indicate Eu–O–Al bond. The curve-fitting results clearly show that Eu amide species fixed on Al 2 O 3 are formed preferentially in the low loading Ž4 wt.%. EurAl 2 O 3 . The Eu amide species would be isolated on Al 2 O 3 , since the presence of the neighboring Eu atoms is not detected by curve-fitting analysis. As for the high loading Ž8 wt.%. EurAl 2 O 3 Ž473 and 573 K., the curve-fitting analyses indicate the presence of short and long Eu–O or Eu–N bonds and the Al atoms neighboring in the second shell
around the Eu center as summarized in Table 2. These would be assignable to the nitrogen atoms of amide groups bonded to Eu atoms and the Eu–O–Al bond, in the similar manner to the low loading EurAl 2 O 3 Ž423 K.. In addition, as for the high loading EurAl 2 O 3 , the curve-fitting analyses show the presence of the neighboring Eu atoms in the second shell. For the Fourier-filtered EXAFS spectra of 8 wt.% EurAl 2 O 3 Ž473, 573 K., the curve-fitting without Eu–Eu shells were also performed, but the resultant parameters were unacceptable, i. e., the Debye–Waller factors became too large. In addition, the presence of the neighboring Eu atom was confirmed by carrying out inversely Fourier transforms for the secondary peaks in RSFs of 8 wt.% EurAl 2 O 3 Ž473, 573 K. and the low loading Ž4 wt.%. EurAl 2 O 3 Ž423 K.. The envelope of the inversely Fourier-transformed spectra for 8 wt.% EurAl 2 O 3 Ž473, 573 K.
T. Yoshida et al.r Applied Surface Science 156 (2000) 65–75 Table 2 The curve-fitting results of EurAl 2 O 3 samples evacuated at various temperatures. N: Coordination number, R: interatomic distances and s 2 : Debye–Waller factors Samples
Shell
N
˚ R rA
˚2 s 2 rA
8 wt.% EurAl 2 O 3 Ž473 K.
Eu–O,N Eu–O,N Eu–Al Eu–Eu Eu–O,N Eu–O,N Eu–Al Eu–Eu Eu–O,N Eu–O,N Eu–Al
2.2 2.1 1.3 1.5 1.6 1.6 0.9 0.7 2.2 2.6 1.4
2.17 2.35 3.29 3.48 2.14 2.33 3.29 3.49 2.13 2.32 3.28
y0.0010 a 0.0018 a 0.0050 a 0.0082 b y0.0010 a y0.0033 a 0.0067 a 0.0075 b y0.0012 a y0.0023 a 0.0072 a
8 wt.% EurAl 2 O 3 Ž573 K.
4 wt.% EurAl 2 O 3 Ž423 K. a
Relative Debye–Waller factors against those of reference samples. b Debye–Waller factors.
maximized at higher k value than that for 4 wt.% EurAl 2 O 3 Ž423 K., indicates the presence of the heavy Eu atoms around a Eu atom in former samples. The presence of the neighboring Eu atom demonstrates that Eu amide species fixed on Al 2 O 3 are not isolated in the high loading Ž8 wt.%. EurAl 2 O 3 . As shown in Table 2, the coordination number of the neighboring Eu atoms are evaluated to 1.5 and 0.7 for 8 wt.% EurAl 2 O 3 Ž473 K. and EurAl 2 O 3 Ž573 K., respectively. The values 1.5 and 0.7 may represent the formation of Eu dimer or trimer in EurAl 2 O 3 Ž473 K. and that of Eu monomer or dimer in EurAl 2 O 3 Ž573 K.. In addition, the curvefitting analysis also detected the decrease in the number of Eu–O,N bond as the evacuation temperature rises from 473 K to 573 K. The decrease in the number of Eu–O,N bond would correspond to the decrease in the number of the neighboring Eu atom from 1.5 to 0.7. In the temperature region of 473–573 K, we observed the desorption of NH 3 gas from the EurAl 2 O 3 sample by the TPD measurement. On the basis of these results, it can be expected that the neighboring Eu atom in Table 2 demonstrates the presence of Eu atoms bridged by nitrogen atom ŽEu–N–Eu.. Therefore, the decrease of the number of the neighboring Eu atom in EurAl 2 O 3 Ž573 K. probably results from the desorption of the bridged nitrogen atoms as NH 3 .
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Here, we conclude for the high loading Ž8 wt.%. EurAl 2 O 3 as follows. Eu amide species fixed on Al 2 O 3 are not isolated due to the formation of Eu atoms bridged by nitrogen atoms. Eu dimer or trimer probably form in EurAl 2 O 3 Ž473 K., while the Eu dimer or trimer changed to Eu monomer or dimer during the rise of evacuation temperature to 573 K by the desorption of the bridging nitrogen atoms. 3.6. The factors causing and controlling the catalytic actiÕity On the basis of the results revealed by XANES, IR and EXAFS analyses, in this section, we discuss the dependence of the catalytic activity on the electronic state, chemical state and local structure of Eu species in EurAl 2 O 3 . 3.6.1. The relationship between the catalytic actiÕity and the electronic state of Eu species In Fig. 1Žc., the fraction of Eu2q in 8 wt.% EurAl 2 O 3 is plotted against the evacuation temperature. The fraction was obtained from the XANES analysis as mentioned in Section 3.2. The fraction of Eu2q reached a maximum value Žca. 90%–100%. in 8 wt.% EurAl 2 O 3 evacuated at around 500 K, and the samples exhibited high activity for the isomerization of 2,3-dimethylbut-1-ene, indicating that Eu2q species is the active species for the isomerization of 2,3-dimethylbut-1-ene. This result is consistent with that for EurK-Y w10,11x. However, the change of the catalytic activity is not correlated to the valence variation of Eu species in EurAl 2 O 3 . For example, the fractions of Eu2q in 8 wt.% EurAl 2 O 3 evacuated at 473 and 573 K are almost the same Ž93% and 85%, respectively., but the catalytic activity of 8 wt.% EurAl 2 O 3 Ž473 K. is much higher by a factor of 10 than that of 8 wt.% EurAl 2 O 3 Ž573 K.. In addition, as shown in Fig. 3Žf., XANES spectrum showed that the fractions of Eu2q species is close to 90% for the low loading Ž4 wt.%. EurAl 2 O 3 Ž423 K. which sample was inactive for the isomerization of 2,3-dimethylbut-1-ene. These results indicate that the difference in the catalytic activity does not arise mainly from the difference in the number of Eu2q species. Although the active species for the isomerization of 2,3-dimethylbut-1-ene is Eu2q species,
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both active and inactive Eu2q species exist in EurAl 2 O 3 samples. 3.6.2. The relationship between the catalytic actiÕity and the chemical state of Eu species It has been found from the IR measurement that at least two types of Eu amide species are present in the high loading EurAl 2 O 3 . The IR bands at 3302, 3242 and 1520 cmy1 and those at 3268, 3218 and 1560 cmy1 were due to the two types of Eu amide species. On the other hand, only one type of Eu amide species was detected for the low loading EurAl 2 O 3 as represented by bands at 3302, 3242 and 1520 cmy1 . Taking into account the difference in the catalytic activity for the isomerization of 2,3-dimethylbut-1-ene between the high and low loading EurAl 2 O 3 , it is likely that the Eu amide represented by the bands at 3302, 3242 and 1520 cmy1 and those at 3268, 3218 and 1560 cmy1 are catalytic inactive and active species, respectively. By noticing the band at 1560 cmy1 in Fig. 5, it is found that the intensity of the band drops by evacuation above 673 K where the activity for the isomerization of 2,3-dimethylbut-1-ene reduces. This result supports the above consideration. Thus, the IR spectra suggest that both active and inactive Eu amide species coexist in the high loading EurAl 2 O 3 while only inactive Eu amide species exist in the low loading EurAl 2 O 3 . The active and inactive Eu amide species should correlate with the active and inactive Eu2q species discussed in the Section 3.6.1. However, the fundamental difference in the state of active and inactive Eu2q amide species is still unclear. Moreover, we cannot explain the reason why the catalytic activity of 8 wt.% EurAl 2 O 3 drops abruptly by elevating the evacuation temperature from 473 K to 573 K, since the variation of IR spectrum is very slight. To elucidate these, Section 3.6.3, we discuss the influence of the structure of Eu species on the catalytic activity. 3.6.3. The relationship between the catalytic actiÕity and the local structure of Eu species On the basis of EXAFS analysis, we concluded that Eu species fixed on Al 2 O 3 ŽEu–O–Al. are formed preferentially in 4 wt.% EurAl 2 O 3 Ž423 K., while both Eu–O–Al and Eu atoms bridged by the nitrogen atoms ŽEu–N–Eu. are formed in 8 wt.%
EurAl 2 O 3 . This would closely relate to the fact that IR spectra reflected the presence of one type Eu amide species for the low loading EurAl 2 O 3 and two types for the high loading EurAl 2 O 3 samples. That is, the preferential formation of Eu–O–Al leads to the generation of isolated Eu amide species in the low loading EurAl 2 O 3 samples and this is responsible for the absence of the catalytic property for the isomerization of 2,3-dimethylbut-1-ene. Imanura et al. w26,27x prepared EurSiO 2 samples with the similar preparation method as ours, and reported that the lanthanide selectively reacts with the hydroxyl groups on SiO 2 to form highly dispersed SiO–EuNH 2 species in low loading sample. Their results are in good agreement with ours, supporting our conclusion for the low loading EurAl 2 O 3 sample. On the other hand, as for high loading EurAl 2 O 3 , the formation of Eu–N–Eu bond in addition to that of Eu–O–Al bond has been detected. The Eu–N–Eu bond would reflect the local structure of active Eu amide species detected by the IR measurement. Actually, the Eu– N–Eu bond would be associated with the activity for the isomerization of 2,3-dimethylbut-1-ene, since the number of Eu–N–Eu bond formed in the low active 8 wt.% EurAl 2 O 3 Ž573 K. is small compared with that formed in the high active 8 wt.% EurAl 2 O 3 Ž473 K.. As mentioned above, the decrease in the number of Eu–N–Eu sites would be caused by the desorption of the bridging nitrogen atoms. This is the reason why the catalytic activity of 8 wt.% EurAl 2 O 3 drops abruptly by elevating the evacuation temperature from 473 K to 573 K. The degree of aggregation of Eu amide species is also the important factor controlling the catalytic activity. Finally, on the basis of all the results, we can here conclude that the Eu2q amide species in a form of appropriate size of the cluster is an active species for isomerization of 2,3-dimethylbut-1-ene, i.e., the site generating the property as solid base.
4. Conclusion Ž1. Eu species in EurAl 2 O 3 samples are the mixture of divalent and trivalent Eu ions, and Eu2q amide species is the active site for the isomerization of 2,3-dimethylbut-1-ene, i.e., the site exhibits basic-
T. Yoshida et al.r Applied Surface Science 156 (2000) 65–75
ity. These results are consistent with those for EurK-Y samples. Ž2. Among the divalent Eu amide species, there are the active and inactive Eu species for the isomerization of 2,3-dimethylbut-1-ene. In the low loading EurAl 2 O 3 samples, divalent Eu ions preferentially react with OH group on Al 2 O 3 surface to form the isolated Eu species which exhibits no activity for the isomerization of 2,3-dimethylbut-1-ene. In the high loading EurAl 2 O 3 samples, not only the inactive Eu species, but also the active Eu species which consists of the Eu atoms bridged by the nitrogen atom are formed. Ž3. The catalytic activity as a solid base is controlled by the degree of the aggregation of the active Eu species mentioned in Ž2.. The difference in the aggregation of the active Eu species is one of the important reasons to explain why the catalytic activity as solid base changes with the evacuation temperature and the loading amount of Eu.
Acknowledgements X-ray absorption experiments were performed under the approval of the Photon Factory Program Advisory Committee ŽProposal no. 91-175 and 93G168..
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