J. Phys. Chem. B 2002, 106, 2415-2422
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In Situ Time-Resolved Energy-Dispersive X-ray Absorption Fine Structure Study on the Decarbonylation Processes of Mo(CO)6 Entrapped in NaY and HY Zeolites Aritomo Yamaguchi,†,‡ Akane Suzuki,† Takafumi Shido,† Yasuhiro Inada,§ Kiyotaka Asakura,| Masaharu Nomura,⊥ and Yasuhiro Iwasawa*,† Department of Chemistry, Graduate School of Science, the UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, Research Center for Materials Science, Nagoya UniVersity, Nagoya 464-8602, Japan, Catalysis Research Center, Hokkaido UniVersity, Kita-ku, Sapporo 060-0811, Japan, and Photon Factory, Institute of Materials Structure Science, KEK, Ibaraki 305-0801, Japan ReceiVed: September 17, 2001; In Final Form: December 11, 2001
Temperature-programmed decarbonylation processes of Mo(CO)6 entrapped in NaY and HY zeolites were investigated in situ by the time-resolved energy-dispersive X-ray absorption fine structure (DXAFS) technique. The DXAFS study revealed that the decarbonylation of Mo(CO)6 in both NaY and HY proceeded through molybdenum subcarbonyl species and that the subsequent formation of decarbonylated species is affected by the stability of the subcarbonyl intermediate species. In the case of Mo(CO)6/NaY, stable Mo(CO)3(OL)3 (OL, oxygen atoms of the zeolite framework) species were formed at 400-500 K and Mo(II) oxocarbide dimeric species Mo2(C)Ox were formed above 500 K. In the case of Mo(CO)6/HY, on the other hand, unstable Mo(CO)3(OL)x (x ) 1-2) species were formed around 450 K and they reacted easily with protons in zeolite supercages to be converted to Mo(II) monomer species. An unstable intermediate species was observed, and its structure was determined by DXAFS for the first time.
1. Introduction Preparation of highly dispersed and uniform active structures is crucial to create efficient catalysis with high performances. Chemical design of a variety of active structures has been attempted to develop a new class of catalysts by metal complex.1,2 Although the efforts on the design of excellent catalysts have been acutely difficult challenges, molecular-level catalyst preparation has become realistic on the basis of modern physical techniques as well as definite precursors and crystalline supports.3 Zeolites are crystalline porous materials with high surface areas, which are often used as supports to provide uniform environments for catalytically active metal sites. Numerous studies have been done to prepare highly dispersed metal species with uniform structures in zeolites.4,5 Decarbonylation processes of Mo(CO)6 in Y zeolites have been characterized by IR,6-9 extended X-ray absorption fine structure (EXAFS),3,10 and temperature-programmed desorption (TPD).11 Okamoto et al. claimed that Mo(CO)3(OL)3 (OL, oxygen atoms of the zeolite framework) species in NaY zeolite were formed by the decarbonylation at 373 K.10 Asakura et al. revealed that Mo(II) oxocarbidic dimer species, Mo2(C)Ox, were formed in NaY zeolite by the decarbonylation at 573 K of Mo(CO)6 adsorbed at saturation at 298 K.3 The produced Mo2(C)Ox species in NaY supercages showed a unique catalytic property.12 On the other hand, the decarbonylation process of Mo(CO)6/HY was studied * To whom correspondence should be addressed. Fax: 81-3-5800-6892. E-mail:
[email protected]. † University of Tokyo. ‡ Present address: Faculty of Science and Technology, Science University of Tokyo, Noda, Chiba 278-8510, Japan. § Nagoya University. | Hokkaido University. ⊥ Institute of Materials Structure Science.
by Abdo and Howe,7,13 who observed H2 evolution during the decarbonylation and indicated that Mo(0) species were oxidized by protons to form Mo(II) species. An EXAFS study revealed that Mo monomer species coordinated by three lattice oxygen atoms were formed by the decarbonylation of Mo(CO)6/HY.14 Despite the past extensive study, the chemistry of the decarbonylation processes is not clear yet. This is mainly due to lack of information on the time-dependent structures of Mo species during the dynamic decarbonylation processes. Local structures of noncrystalline dispersed metal species have been extensively studied by EXAFS because it does not require longrange order of the metal species.15,16 However, it takes several tens of minutes to measure an EXAFS spectrum for dispersed species by the conventional EXAFS technique. So, the conventional technique cannot monitor time-dependent structures during dynamic chemical processes. Recently, a time-resolved EXAFS technique, called energy-dispersive EXAFS (DXAFS), has been developed,17,18 which is applicable to the study of structural changes of metal sites during the chemical processes.19-22 DXAFS is a technique to measure the whole range of X-ray absorption spectra using a bent crystal and a position sensitive detector (1024 channel photodiode arrays). Because no mechanical motion is required in this technique, EXAFS spectra can be measured every second.19,20 In the previous paper, we demonstrated that in the temperature-programmed decarbonylation of Mo(CO)6 in NaY zeolite Mo2(C)Ox species were formed via Mo(CO)3(OL)3 species by the DXAFS technique.23 The relation of these two species, Mo2(C)Ox and Mo(CO)3(OL)3, was not clear previously. Only DXAFS can provide the information on the structural change during the temperature-programmed decarbonylation of dispersed Mo(CO)6. In this paper, we report the detail of the timeresolved information on the local structure around Mo atoms in the decarbonylation of Mo(CO)6 in NaY. We also report the
10.1021/jp0135499 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/07/2002
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DXAFS study on the decarbonylation process of Mo(CO)6 in HY zeolite. The DXAFS results on the Mo(CO)6/NaY and Mo(CO)6/HY zeolites are compared to examine the effect of cations in Y zeolites on the decarbonylation mechanism of Mo(CO)6. 2. Experimental Section Mo(CO)6/Y samples were prepared as described in the previous papers.3,12 Briefly, Mo(CO)6 vapor was admitted to a wafer of HY or NaY zeolite at room temperature for 24 h. The wafer was mounted in a stainless steel EXAFS cell and was calcined at 723 K before the exposure to Mo(CO)6. The Mo loadings and the edge jumps were 3 wt % (determined by inductively coupled plasma spectrometry (ICP)) and ca. 0.5, respectively. The ramping rate and the final temperature of the temperature-programmed (TP) decarbonylation were 5 K min-1 and 623 K, respectively. The DXAFS spectra were measured in situ during the TP decarbonylation as described in the previous paper.19,20 Briefly, DXAFS measurements at the Mo K edge were carried out using synchrotron radiation at BL-9C of KEK-PF. A triangle-shaped Si(311) bent crystal was used to focus the polychromatic X-ray beam. The sample in the in situ EXAFS cell was placed at the focus position. The diverging X-rays after the sample were detected by a photodiode array detector (Hamamatsu Photonics S3904-1024FX SPL3402). Energy calibration was carried out using a spectrum of Mo foil. The energy resolution was estimated to be 5 eV from the comparison of X-ray absorption spectra for Mo foil at the DXAFS set up and at the conventional XAFS set up. The energy range of the spectra was 19 80020 450 eV. Each spectrum was recorded in a 1.5 s acquisition time (300 ms × 5 scans) for Mo(CO)6/NaY or in 3.0 s (300 ms × 10 scans) for Mo(CO)6/HY. The ratios of initial, intermediate, and final species produced during the TP decarbonylation were estimated by X-ray absorption near edge structure (XANES) analysis.24,25 The XANES spectra during the TP decarbonylation were reproduced by linear combination of reference spectra for initial, intermediate, and final species.
Xobs )
∑i CiXi
(1)
where Xobs, Ci, and Xi are an observed spectrum, a coefficient of i-th reference, and the i-th reference spectrum, respectively. The XANES spectra of the same sample before and after the TP decarbonylation were taken as the reference spectra for initial and final species, respectively. In addition, the XANES spectrum of Mo(CO)6/NaY at 473 K was used as the reference spectrum for an intermediate species during the decarbonylation of both Mo(CO)6/NaY and Mo(CO)6/HY. The coefficients, Ci, were obtained by a linear least-squares fitting. The k ranges of the Fourier transformation of DXAFS data for Mo(CO)6/NaY and Mo(CO)6/HY were 30-90 and 30-105 nm-1, respectively, and the fitting R range was 0.11-0.32 nm in both samples. EXAFS data (both DXAFS and conventional) were analyzed by the UWXAFS package.26 After background subtraction using AUTOBK,27 k3-weighted EXAFS functions were Fourier transformed into R space and fitted in the R space. The backscattering amplitudes and phase shifts were calculated by the FEFF8 code.28 The energy resolution of the spectrometer (5 eV) was taken into account to calculate the parameters by using the “EXCHANGE” flag of an input file in the FEFF8 code. The option is originally used to specify the energydependent exchange correlation, but it can also be used to
Figure 1. Energy-dispersive X-ray absorption spectra at the Mo K edge for Mo(CO)6/NaY (a) and Mo(CO)6/HY (b) during the temperature-programmed decarbonylation processes under vacuum from 293 to 623 K at a heating rate of 5 K min-1. The data acquisition time is 1.5 s.
estimate instrumental broadening. To fit Mo-CO contributions, multiple scattering was taken into account and four pathways such as Mo f C f Mo, Mo f O f Mo, Mo f C f O f Mo (and Mo f O f C f Mo), and Mo f C f O f C f Mo were the EXAFS fitting. The interatomic distances and DebyeWaller factors of the second, third, and fourth pathways and the coordination numbers of the first, second, third, and fourth pathways were set to be equal values. To analyze DXAFS data, Debye-Waller factors calculated from the conventional EXAFS data measured at several temperatures were used. Threshold energy correction (∆E0) was fixed at a certain value, and the same value was used in the analysis of whole data. ∆E0 was determined so that the average residual factor became minimum. Conventional EXAFS spectra at the Mo K edge were measured in a transmission mode using a Si(311) channel-cut monochromator at BL-10B of KEK-PF. X-ray intensities before and after the sample were monitored by ion chambers. The spectra were measured at several temperatures to estimate Debye-Waller factors as a function of temperature. The Einstein model was applied to estimate the Debye-Waller factors at a certain temperature.29 The k range of the Fourier transformation and the R range used for fitting were 30-145 nm-1 and 0.120.32 nm, respectively. 3. Results 3.1. XANES Analysis. Figure 1 shows energy-dispersive X-ray absorption spectra at the Mo K edge for Mo(CO)6/NaY
Decarbonylation Processes of Mo(CO)6
J. Phys. Chem. B, Vol. 106, No. 9, 2002 2417 TABLE 1: Structural Parameters for the Molybdenum Species in Y-Zeolites during the Course of the Temperature-Programmed Decarbonylation Process Determined by Conventional EXAFSa abs-scat
CN
R, nm
σ2, σ02, 10-5 nm2 10-5 nm2 ΘE, K
Mo(CO)6/NaY at 293 K (∆E0 ) 0.9 eV, Rf ) 3.0%) Mo-C 5.1 ( 0.7 0.206 ( 0.001 1.3 -7.5 Mo-(C)-O 5.1 ( 0.7 0.324 ( 0.002 2.6 0
460 830
Mo(CO)6/NaY at 473 K (∆E0 ) 1.9 eV, Rf ) 1.6%) Mo-C 3.3 ( 0.4 0.192 ( 0.006 2.8 2.4 Mo-(C)-O 3.3 ( 0.4 0.309 ( 0.006 3.7 2.1 Mo-O 2.9 ( 1.0 0.231 ( 0.008 9.5 9.3
2200 2500 1000
Mo(CO)6/NaY at 623 K (∆E0 ) 2.2 eV, Rf ) 3.1%) 1.0 ( 0.3 0.195 ( 0.001 7.0 -3.2 2.1 ( 1.0 0.205 ( 0.001 4.7 3.4 1.0 ( 0.3 0.278 ( 0.002 10.8 6.4
690 610 580
Mo(CO)6/HY at 293 K (∆E0 ) 1.1 eV, Rf ) 2.7%) Mo-C 5.2 ( 0.7 0.207 ( 0.001 2.9 -7.5 Mo-(C)-O 5.2 ( 0.7 0.321 ( 0.002 4.1 0
460 830
Mo-C Mo-O Mo-Mo
Mo-O
Mo(CO)6/HYY at 623 K (∆E0 ) 3.2 eV, Rf ) 7.1%)b 1.8 ( 0.6 0.206 ( 0.003 6.5 3.0
670
a
The conventional EXAFS spectra were measured at 293 K after the decarbonylation at given temperatures. b The R range used for fitting was 0.11-0.22 nm.
Figure 2. Existence ratios of molybdenum species (coefficient) and the residual factor (Rf) as a function of decarbonylation temperature obtained by linear least-squares fitting of the XANES spectra: (O) initial species; (0) intermediate species; (4) final species; (×) residual factor. Panel a shows the fitting results for Mo(CO)6/NaY with three references; the observed XANES spectra were fitted by the XANES spectra of Mo(CO)6/NaY measured at 293 K (initial species), 473 K (intermediate species), and 623 K (final species). Panel b shows the fitting results for Mo(CO)6/HY with two references; the observed XANES spectra were fitted by the XANES spectra of Mo(CO)6/HY measured at 293 K (initial species) and 623 K (final species). Panel c shows the fitting results for Mo(CO)6/HY with three references; the observed XANES spectra were fitted by the XANES spectra of Mo(CO)6/HY measured at 293 K (initial species) and 623 K (final species) together with that of Mo(CO)6/NaY measured at 473 K (intermediate species).
(a) and Mo(CO)6/HY (b) during the TP decarbonylation. In Mo(CO)6/NaY, the XANES shape changed at ca. 400 and 550 K. The results indicate that the decarbonylation of Mo(CO)6/NaY proceeds through two steps and a stable intermediate exists in the temperature range 400-550 K. On the other hand, in the TP decarbonylation process of Mo(CO)6/HY, the XANES shape changed once around 450 K. There is no stable intermediate species in the case of Mo(CO)6/HY. Figure 2 shows the coefficients for Mo reference species (initial, intermediate, and final species) delivered by the XANES analysis as presented in eq 1. For Mo(CO)6/NaY (Figure 2a), the XANES spectra measured at 293, 473, and 623 K were used
as the reference spectra for initial, intermediate, and final species, respectively, and other spectra were represented by a linear combination of the reference spectra. As shown in Figure 2a, all of the XANES spectra for Mo(CO)6/NaY during the TP decarbonylation were basically reproduced well by the linear combination of the reference spectra as indicated by small residual factors (Rf) below 1%. The Rf’s in 500-580 K were relatively large, in which temperature range a fractional amount of unstable intermediate might exist, but the XANES analysis cannot say anything about such fractional species. In Figure 2b, a series of the XANES spectra for Mo(CO)6/HY during the TP decarbonylation were fitted by a linear combination of the XANES spectra for the initial and final species (measured at 300 and 623 K). The Rf’s were large at 400-550 K, indicating that an intermediate species exists in this temperature range. Thus, the XANES spectrum of Mo(CO)6/NaY measured at 473 K was taken as a third reference spectrum for an intermediate species. The fitting results by the three references are shown in Figure 2c. The fitting quality was improved by including the third reference as indicated by the reduction of Rf’s. The Rf’s by the three-reference analysis were half of those by the tworeference analysis. Thus, the decarbonylation of Mo(CO)6/HY may proceed via an intermediate similar to the case of the Mo(CO)6/NaY decarbonylation. However, the Rf’s were a little larger than those in the fitting of Mo(CO)6/NaY. 3.2. Conventional EXAFS Measurement. We measured conventional EXAFS of initial and final species in the decarbonylation of Mo(CO)6/HY and Mo(CO)6/NaY together with the intermediate species for Mo(CO)6/NaY at different temperatures to obtain a statistical Debye-Waller factor (σ0) and Einstein temperature (ΘE), which were used to analyze the DXAFS data during the TP decarbonylation processes. The interatomic distances (R) hardly depended on the measuring temperature; that is to say, thermal expansion for R could not be observed. As suggested from the XANES analysis in Figure 2a, a stable intermediate exists in the TP decarbonylation of Mo(CO)6/NaY, which means that the structure of the stable intermediate can be studied by conventional EXAFS. Table 1 shows structural parameters determined by a curve-fitting analysis of the
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Figure 3. Typical k3-weighted EXAFS functions measured by the DXAFS setup for Mo(CO)6/HY before (a) and after (b) the decarbonylation. The acquisition time is 1.5 s.
conventional EXAFS data for Mo(CO)6/NaY evacuated at 293 (a), 473 (b), and 623 K (c) and for Mo(CO)6/HY evacuated at 293 (d) and 623 K (e). Coordination numbers (CNs) and R’s in Table 1 are obtained from the EXAFS data measured at 293 K. Before the decarbonylation, Mo-C and Mo-(C)-O bond distances were observed at 0.206 ( 0.001 and 0.324 ( 0.002 nm, respectively, for Mo(CO)6/NaY and at 0.207 ( 0.001 and 0.321 ( 0.002 nm, respectively, for Mo(CO)6/HY. The CNs were 5.1 ( 0.7 for Mo(CO)6/NaY and 5.2 ( 0.7 for Mo(CO)6/ HY. The Mo-C and Mo-(C)-O bond distances were similar to those of Mo(CO)6 in the crystal (0.208 and 0.323 nm, respectively). The results indicate that Mo(CO)6 molecules were entrapped in NaY and HY supercages, retaining the structure, which agrees with the previous work.3 After the decarbonylation at 623 K, the structures of the molybdenum species in NaY and HY were different from each other. In NaY, Mo-C or Mo-O bonds were observed at 0.195 and 0.205 nm. In addition, a Mo-Mo bond was observed at 0.278 nm. The results are consistent with the previous work,3 which demonstrated that molybdenum(II) oxocarbidic dimers were formed after the decarbonylation of Mo(CO)6/NaY at 623 K. The existence of Mo-C contributions has been demonstrated by O2 titration, which shows that the estimated Mo/C ratios in the samples with different Mo loadings are always 2, independent of Mo loadings.3 In HY, on the other hand, Mo-O bonds were observed at 0.206 nm with a CN of 1.8, and no Mo-Mo bonding was observed. The results indicate that Mo monomer species were formed in HY after the decarbonylation at 623 K. In the case of Mo(CO)6/NaY evacuated at 473 K, Mo-C and Mo-(C)-O contributions of molybdenum carbonyls were observed at 0.192 and 0.309 nm with a CN of 3.3. In addition, a Mo-O contribution was observed at 0.231 nm with a CN of 2.9. The curve-fitting results were identical to those reported by Okamoto et al.,10 who indicate that Mo(CO)3(OL)3 subcarbonyl species are formed in NaY supercages. 3.3. DXAFS Analysis. Figure 3 shows typical k3-weighted EXAFS oscillations for Mo(CO)6/HY measured by the DXAFS setup. The quality of the spectra was reasonably good up to 105 nm-1. The resolution of the DXAFS setup was estimated
Figure 4. Fourier-transformed k3-weighted EXAFS functions during the TP decarbonylation processes of Mo(CO)6/NaY (a) and Mo(CO)6/ HY (b).
to be 3 eV by comparing the EXAFS data of the Mo foil obtained at the DXAFS setup and a conventional EXAFS setup. Figure 4 shows a series of Fourier-transformed k3-weighted EXAFS functions during the TP decarbonylation processes of Mo(CO)6/NaY (a) and Mo(CO)6/HY (b). Phase shifts were not corrected in this figure. In Mo(CO)6/NaY, two peaks were observed at 0.14 and 0.26 nm due to Mo-C and Mo-(C)-O bonds of Mo carbonyl species. The intensity of these peaks decreased with temperature up to 400 K. At ca. 400 K, these two peaks shifted to shorter distances and got broader. The intensity of the new peaks increased with temperature up to 450 K and decreased above 450 K. These peaks disappeared at ca. 500 K and new peaks appeared at 0.16 and 0.26 nm. The feature of the change in the DXAFS indicates that the decarbonylation process occurred via two steps and that a stable intermediate species exists in the temperature range 400-500 K. These results agree well with the XANES analysis. In Mo(CO)6/HY, two peaks were observed at 0.14 and 0.26 nm due to Mo-C and Mo-(C)-O bonds of Mo carbonyl species. The intensity of these peaks decreased with temperature up to 420 K. At ca. 450 K, the two peaks shifted to 0.12 and 0.24 nm, respectively, and they disappeared at ca. 500 K. A new peak appeared at 500 K at 0.12 nm, and the intensity of the new peak increased with temperature. The DXAFS results indicate that a short-lived intermediate may exist around 450 K, which agrees with the results of XANES analysis in Figure 2b,c. Figure 5 shows CNs, R’s and Rf’s determined by a curvefitting analysis of the DXAFS spectra during the decarbonylation of Mo(CO)6/NaY. Free parameters for the curve-fitting analysis were CN and R of each shell, and the Debye-Waller factor (σ) was fixed at the value calculated from σ0 and ΘE obtained by the analysis of conventional EXAFS spectra measured at several temperatures.
Decarbonylation Processes of Mo(CO)6
Figure 5. CN (a), R (b), and Rf (c) during the decarbonylation of Mo(CO)6/NaY as a function of temperature: (2) Mo-C of carbonyl ligands in the initial species; (O) Mo-(C)-O of carbonyl ligands in the initial species; (1) Mo-C of carbonyl ligands in the intermediate species; (0) Mo-(C)-O of carbonyl ligands in the intermediate species; ()) Mo-O in the intermediate species; (4) Mo-Mo in the final species; (() Mo-C in the final species; (3) Mo-O in the final species.
At 293-410 K, Mo-C and Mo-(C)-O bonds were observed at 0.210 and 0.328 nm, respectively. The CNs of these bonds decreased with the decarbonylation temperature. At 368 K, additional Mo-C and Mo-(C)-O bonds of different molybdenum carbonyls and a Mo-O bond were observed at 0.192, 0.313, and 0.236 nm, respectively. The CN of the new carbonyl and Mo-O bonds increased with temperature until 460 K and decreased with temperature after that. These bondings disappeared at 510 K. Above 530 K Mo-Mo, Mo-C, and Mo-O bonds were observed at 0.278, 0.195, and 0.206 nm, respectively. We could not fit the DXAFS spectra obtained at 510-530 K because there existed six shells in the spectra and the fitting was tricky. The structural parameters of the Mo species before and after the decarbonylation and the stable intermediate subcarbonyl species, calculated from the DXAFS data, were consistent with the parameters calculated from a conventional EXAFS data. In addition, the behaviors of the CNs of incipient Mo carbonyl species, subcarbonyl species, and decarbonylated species against the decarbonylation temperature were compatible with the ratios of these species calculated from the XANES analysis. Figure 6 shows CNs, R’s, and Rf’s delivered by a curve-fitting analysis of the DXAFS spectra during the decarbonylation of
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Figure 6. CN (a), R (b), and Rf (c) during the decarbonylation of Mo(CO)6/HY as a function of temperature. Panels a and b use the following symbols: (2) Mo-C of carbonyl ligands in the initial species; (O) Mo-(C)-O of carbonyl ligands in the initial species; (1) Mo-C of carbonyl ligands in the intermediate species; (0) Mo-(C)-O of carbonyl ligands in the intermediate species; ()) Mo-O in the intermediate species; (4) Mo-O in the final species. In panel c, (O) and (b) indicate Rf’s of the curve-fitting analysis with and without the intermediate subcarbonyl species.
Mo(CO)6/HY. As an intermediate species, we assume Mo subcarbonyl species similar to Mo(CO)3(OL)3/NaY and include Mo-C, Mo-(C)-O, and Mo-OL to fit the data. Free parameters for the curve-fitting analysis were CN and R of each shell. The σ values of each shell for Mo(CO)6/HY and Mo(II)/HY were fixed at the values calculated from conventional XAFS data. In addition, the σ’s of Mo-C, Mo(C)-O, and Mo-OL bonds of the intermediate subcarbonyl species were fixed at the values calculated from the conventional EXAFS for Mo(CO)3(OL)3/NaY. As shown in Figure 6c, the Rf at 460 K was reduced by including the intermediate species in the fitting. The Rf values with and without the intermediate were 4% and 14%, respectively. The substantial reduction of the Rf values also indicates that an intermediate species exists at ca. 460 K in the TP decarbonylation of Mo(CO)6/HY. At 293-460 K, Mo-C and Mo-(C)-O bonds at 0.207 and 0.325 nm were observed. The CNs decreased with temperature. At 406 K, additional Mo-C and Mo-(C)-O bonds of different molybdenum carbonyl and a Mo-O bond were observed at 0.192, 0.309, and 0.229 nm, respectively. The CNs of the new
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carbonyl and Mo-OL bonds increased with temperature until 460 K and decreased with temperature after that. Above 480 K, a Mo-O bond at 0.208 nm was observed. The coordination number of the Mo-O bond increased with temperature up to 510 K and became constant after that. The structural parameters of the molybdenum species before and after the decarbonylation calculated from the DXAFS data are consistent with the parameters calculated from conventional EXAFS data. The behavior of the CNs of incipient Mo carbonyl species, subcarbonyl species, and decarbonylated species against the decarbonylation temperature were compatible with the ratios of these species calculated from XANES analysis in Figure 2c. 4. Discussion 4.1. Analysis of DXAFS Data. Because there are several species during the TP decarbonylation process and because the number of independent parameters in the DXAFS spectra is limited to 10 and 12 for Mo(CO)6/NaY and Mo(CO)6/HY, respectively, the number of fitting parameters should be reduced. The number of independent parameters, Nidp, in EXAFS spectra is restricted by the following formula (Niquest law).30
Nidp ) 2∆k∆R/π + 2
(2)
Because the k and R ranges in the present DXAFS study, for example, for Mo(CO)6/NaY are 30-90 nm-1 and 0.11-0.32 nm, the Nidp is calculated to be 10. The strategy to reduce the number of the fitting parameters in this study is to use Debye-Waller factors of the stable species such as Mo(CO)6/NaY, Mo(CO)6/HY, Mo(CO)3(OL)3/NaY, Mo2(C)Ox/NaY, and Mo(II)/HY calculated from conventional EXAFS data measured at several temperatures. The EXAFS spectra measured at different temperatures were fitted simultaneously to estimate the statistical Debye-Waller factors (σ0) and the Einstein temperature (ΘE) of each shell. Because the bonding mode of the incipient adsorbed Mo(CO)6 is of molecule and the bonding of Mo-CO in [Mo(CO)3(OL)3] species can be regarded as a molecular type than a continuous bulk, the Einstein model was used to calculate the Debye-Waller factor of the Mo-CO contributions.29,31 Debye-Waller factors at temperature T were calculated by the following formula.
σ2 ) σT(T,ΘE)2 + σ02
(3)
where ΘE is Einstein temperature. Thus, Debye-Waller (DW) factors of Mo(CO)6/NaY, Mo(CO)6/HY, Mo(CO)3(OL)3/NaY, Mo2(C)Ox/NaY, and Mo(II)/HY can be fixed at the value calculated by eq 3, and free parameters for these species are R and CN of each shell. In the curve fitting of DXAFS data during the TP decarbonylation of Mo(CO)6/NaY, the DW factors of all shells were calculated from conventional EXAFS data. At 350-420 K, the observed spectra were fitted by five shells, two Mo-C, two Mo-(C)-O of carbonyl ligands, and one Mo-OL bonds. Because the CNs of Mo-C and Mo-(C)-O of carbonyl ligands should be equal, the number of free parameters required to fit a carbonyl (Mo-C and Mo-(C)-O) is three (R of Mo-C, R of Mo-(C)-O, and CN of Mo-C (Mo-(C)-O)). Thus, the number of free parameters required to fit the DXAFS data at 350-420 K during the TP decarbonylation of Mo(CO)6/NaY is eight, which is smaller than Nidp (10 in this case). The number of shells required to fit the DXAFS data in other temperature ranges is smaller, and the whole spectra can be fitted with the number of free parameters less than Nidp.
The intermediate species during the decarbonylation of Mo(CO)6/HY seems to be a Mo subcarbonyl species (Mo(CO)x(OL)y) because the residual factor of the XANES fit was reduced by including the XANES of Mo(CO)3(OL)3/NaY as a reference as shown in Figure 2c. If a subcarbonyl species is an intermediate, 13 parameters (three for Mo(CO)6/HY, two for Mo(II)/HY, and eight for the intermediate) are required to fit the DXAFS data, but this is larger than Nidp. However, the DW factor of Mo-CO ligands in the subcarbonyl intermediate would be similar to those of Mo(CO)3(OL)3/NaY, because the DW factors of Mo-C and Mo-(C)-O bonds of Mo(CO)6/NaY, Mo(CO)6/ HY, and Mo(CO)3(OL)3/NaY were similar to each other. In addition, the distances of Mo-C and Mo-(C)-O of Mo(CO)6/ HY and Mo-O of Mo(II)/HY can be fixed at the value determined by EXAFS before and after the decarbonylation. Hence, the number of fitting parameters is eight, which is smaller than the value, 12, of Nidp in this case. In conclusion, the number of fitting parameters can be reduced by using the curve-fitting results of conventional EXAFS data of the initial and final species, and only structural parameters for the intermediate species are required to fit the DXAFS data. 4.2. Decarbonylation Process of Mo(CO)6/NaY. As shown in Figure 2a, all XANES spectra during the TP decarbonylation of Mo(CO)6/NaY can be reproduced by a linear combination of the reference XANES spectra measured at 293, 473, and 623 K. The XANES spectra measured at 293 and 623 K are regarded as the XANES spectra for the initial and final species. On the other hand, the XANES spectrum measured at 473 K could be a mixture of initial, final, and intermediate species. However, it is reasonable to claim that the intermediate species is a predominant species at 473 K by the following two reasons. The coefficient for the intermediate species is almost unity in a wide temperature range (420-480 K) as shown in Figure 2a, which indicates that the intermediate species is predominant. As shown in Figure 4a, a clear change was observed in the EXAFS function at 400 and 500 K, which also indicates the preferential formation of the intermediate species. If not, such a clear change in the EXAFS function cannot be observed. Thus, it is concluded that a stable intermediate exists predominantly in the temperature range 400-500 K. As shown in Table 1, the structural parameters for the intermediate species indicate that Mo(CO)3(OL)3 species are formed. The curve-fitting analysis of the DXAFS data also support this. The structural parameters around 473 K agree with those obtained by conventional EXAFS. Thus, it is concluded that the decarbonylation of Mo(CO)6/NaY proceeds through Mo(CO)3(OL)3. This is also supported by TPD experiments.11 In the TPD experiments, half of the coordinated CO desorbed in the temperature range 350-420 K and the other half desorbed in the range 450-480 K. The TPD profile agrees with the XANES analysis (Figure 2a) and the EXAFS analysis (Figure 5). There are two possibilities for the first step of the decarbonylation at 300-440 K. One is that the decarbonylation proceeds step-by-step, Mo(CO)6 f Mo(CO)5 f Mo(CO)4 f Mo(CO)3. The other is that three CO ligands in the Mo(CO)6 desorb simultaneously and only the initial and stable intermediate species exist in the first step, where the ratio of the intermediate to the initial species increases with the extent of decarbonylation. The results of the XANES analysis suggest the latter possibility. This does not exclude the first possibility, but the steps Mo(CO)5 f Mo(CO)4 f Mo(CO)3 may be much faster than the step Mo(CO)6 f Mo(CO)5. As shown in Figure 2a, the XANES spectra observed at 300-440 K are reproduced well by the linear
Decarbonylation Processes of Mo(CO)6
J. Phys. Chem. B, Vol. 106, No. 9, 2002 2421
SCHEME 1: The Structural Transformations during the Temperature-Programmed Decarbonylation of Mo(CO)6/NaY (a) and Mo(CO)6/HY (b)
combination of those for the initial and intermediate species with small Rf (
