- Email: [email protected]
Preparation of MoO3 by spray reaction method and photometathesis of C3H6
Studies m Surface Science and Catalysis 132 Y. Iwasawa, N. Oyama and H. Kunieda (Editors) (c) 2001 Elsevier Science B.V. All rights reserved.
793
Preparation of M0O3 by spray reaction method and photometathesis of C3H, H. Murayama,N. Ichikuni, S. Shimazu and T. Uematsu Faculty of Engineering, Chiba University, Inage-ku, Chiba 263-8522, Japan Molybdenum oxide catalysts prepared by the spray reaction method had anisotropy in their crystal structures. In the present paper, the surface structure and the photoactivity for propen metathesis of the spray catalyst compared with the commercial M0O3 were investigated. In the XPS, the main peak of the spray catalysts was similar to that of the commercial M0O3 with the other characteristic peaks of Mo 3d^,-,^ and 3d^^f^y The activity of spray catalysts showed higher than that of the commercial M0O3, and increased with increasing the T^^y suggesting the formation of the characteristic active species due to the preparation method, Le. rapid heating and quenching steps, for the spray reaction method. 1. INTRODUCTION Most of photocatalystshave been successfully prepared by tailoring the active species on the supports with high surface area These surface species dispersed as atomic or molecular unit showed high photoactivity. For example, the supported molybdenum oxide species on SiOj show photocatalyticactivity in the oxidativedehydrogenation of alcohols [1], the oxidation of hydrocarbons [2] and the metathesis of alkenes [3]. On the other hand, the bulk molybdenum oxide catalysthas no photocatalysisfor the QH^ metathesis reaction [4]. Sprayed particles, which were prepared by rapid heating and quenching process, can provide the characteristic properties in various catalyses [5-7] due to nano structures in the fine composites with metastable surface states [8]. The unsupported bulk oxides prepared from the spray reaction method were expected to show high photocatalysis as well as the supported catalysts. In this study, molybdenum oxide photocatalysts were prepared by the spray reaction method, and their characterization of the active site structure and the catalytic behavior in the photometathesis of C^H^ were investigated. 2. EXPERIMENTAL 2.1. Catalysts preparation The spray molybdenum oxide catalysts were prepared as follows: The 0.15 mol/1 molybdenum aqueous solution was prepared by dissolving M0O3 (Junsei Chemical Co., Ltd.) in dilute NH3 solution, and used as a starting solution. Atomized droplets from the starting solution were dried and calcined in a quartz tube reactor heated at the spray reaction temperature (T^pr) between 673-973 K. The catalyst was designated with T^^ as spr 673. 2.2. Characterization The catalyst was pretreated under O2 at 673 K for 120 min, followed by evacuation for 15
794 min prior to use for each of measurements and reactions. Characterization was carried out by methods of BET surface area measurement, XRD, XPS and Raman spectroscopy. The Raman spectra were taken with the 514.5 nm line of an Ar* laser (JASCO NRS 2100). The XPS was measured with using Mg-K^^. The pretreated catalyst was sealed in a pyrex glass tube, it was opened in a Ar glove box before setting it on a holder. The number of active sites was quantified by the CO photooxidation reaction performed under the same condition of the C3Hg photometathesisreaction. 23.
Reaction The photocatalytic metathesis reaction was performed at 275 K with 3.3 kPa of C^H^ in a quartz cell connected with a closed circulating system. UV irradiation was carried out by using a 75 W high pressure Hg lamp with a water filter. The reaction products (CjH^ and C4H8) were analyzed by a GC. The TON was calculated from the amount of formed C2H4 for 180 min divided by the number of photoactive sites. 3. RESULTS AND DISCUSSION 3.1. XRD As shown in Fig. 1, the interesting differences in the relative intensity of XRD peaks of a-MoOg {20 = 23.4, 25.8, 27.4, 33.8 and 39.0 degree) between the spray catalyst and the commercial M0O3 were observed. The peaks at 25.8 and 39.0 degree were diffracted from (040) and (060) plane, respectively. The ratio of these two peaks {26 = 25.8 and 39.0 degree) to the peaks at 26 = 23.4 and 27.4 degree was much smaller for the spray catalysts than that for the conunercial M0O3 w^^ich has the ratio over unity. These observations indicate that the anisotropic growth of crystals [9] was promoted by the spray reaction method. With increasing T,^, the anisotropic profile of the spray catalysts was more distinguished. The XRD peaks of spr 673, spr 773 and the commercial M0O3 were assigned to only aM0O3. As T^p^ raised to 873 K, two phases, a-MoOj and P-M0O3, were coexistent The impregnation catalyst (Mo/SiOj) showed no P-M0O3. Since P-M0O3 is stable below 623 K [10], it is unusual that the spray catalysts prepared at 7,^^ above 873 K contained P-M0O3. The rapid heating and quenching steps for the spray reaction may form metastable sites on the surface. Thus, the surface of the spray molybdenum oxides could be partly converted to the different stable phase as |J-MoQ. 3.2. Raman spectra A band due to the Mo-O-Mo bridged bond of a-Mo03 was observed at 820 cm' in the Raman spectra of spr 973 as shown in Fig. 2. Moreover, a band observed at 1000 cm' was assigned to the terminal Mo=0 stretching vibration of a-MoOj. It suggests that the pseudoisolated molybdenum oxide species existed on the spray catalyst The p- M0O3 phase was also observed at 775 and 850 cm *, which was in good agreement with the XRD results. The intensity of peak at 1000 cm^ reduced after the reaction as shown in Fig. 2 (b). However, in the case of spr 773, there is little difference in intensity between before and after the reaction. The different photocatalytic activity between spr973 and spr773 was expected.
795 S.
^MoO^
I
\ i \
20
i J L L »^^ 1 25
30 35 26 I degree
(a)
40
Figure 1. XRD patterns for Mo oxide catalysts; (a) the commercial MoQ, (b) spr 673 ,(c) spr 773, (d) spr 873 and (e)spr973.
45
'i
M W\
(a) ••
A\
(b) 11
J
1000
^'•---'''^^
.'--"^
Vwx/N.^^^^-^ 800
600
400
Raman shift / cm'
Figure 2. Raman spectra for spr 973; (a) pretreatedand (b) after the photometathesis.
3.3. XPS The surface species of the spray catalysts were characterized by the XPS. As for spr 973, binding energy (BE) of Mo 3d(3/2) and 3d(5/2) were higher by 0.15 eV and that of 01s was lower by 0.2 eV, respectively, than those of the commercial M0O3. Since the FWHM was broadened over the spr 973 compared to the commercial M0O3, it was deconvoluted into two peaks by gauss functions, as shown in Fig. 3. The main peaks at 235.6 and 232.5 eV 230 236 234 232 238 corresponded to those of the commercial binding energy / eV M0O3. The other peaks (BE = 234.4 and Figure 3. XPS spectra of Mo (3d3;2 ^^^ 231.3 eV) were shifted to lower BE by 1.2 3d5,2) level for (a)the commercial M0O3 and eV, and observed solely for the spray (b)spr973. catalysts. The energy gap between Mo 3d(5/2) and 3d(3/2) and the peak ratio of Mo 3d(5/2) and'^^^,2)were almost constant for the deconvoluted peaks. These new lower BE peaks could be contributed metastable surface structures of the spray molybdenum oxide. It can be said that these new BE peaks were formed by the rearrangement of the surface structure during the spray reaction.
796 Table 1 Activity for QH^ photometathesis reaction catalyst TON (C,H^) number of active sites /10 ^ mol g,^' spr673 27 1.7 spr 773 29 3.0 spr 873 36 2.2 spr 973 48 2.7 MoQ* 13 OT * Purchased from Junsei Chemical Co., Ltd. 3.4. Photometathesis reaction of CJl^ Although all samples had almost equal BET surface areas (ca 20 m-g'), the spray catalysts had much more active sites than the commercial M0O3 as shown in Table 1. The number of active sites were in order of spr 773 > spr 973 > spr 873 > spr 673 » the commercial M0O3. ^^ decrease in active sites for spr 973 and spr 873 was caused by the content of P-M0O3 which had lower amount of active sites. From the results of the XRD and the Raman spectra, it is suggested that the anisotropic growth of a-Mo03 on the spray catalysts resulted in the production of molybdenyl double bonds associated with the active site. The TON became higher with increasing T^^^. Considering die fact that the higher T^^^ could produce much more unstable surface, these unstable states may have high efficiency of the charge transfer process. It can be speculated that molybdenyl double bonds on the individual surface of the unstable a - MoC^ would show the higher activity. REFERENCES 1. T. Ono, M. Anpo and Y. Kubokawa, J. Phys. Chem., 90 (1986) 4780. 2. K. Marcinkowska, S. Kaliaguineand P. C. Roberge, J. Catal., 90 (1984) 49. 3. M. Anpo, M. Kondo and Y. Kubokawa, J. Chem. Soc., Faraday Trans. 1, 84 (1988) 2771. 4. Y. Kubokawa and M. Anpo, in "Adsorption and Catalysis on Oxide Surface", Eds. M. Che and G. C. Bond, Elsevier, Amsterdam, (1985) 127. 5. D. Li, N. Ichikuni, S. Shimazu and T. Uematsu, Appl. Catal. ArGeneral, 172 (1998) 352. 6. D. Li, N. Ichikuni, S. Shimazu and T. Uematsu, Appl. Catal. A:General, 180 (1999) 227. 7. N. Ichikuni, D. Murata, S. Shimazu and T. Uematsu, Catal. Lett., 69 (2000) 33. 8. T. Tsuchiya, N. Ichikuni, S. Shimazu and T. Uematsu, Chem. Lett., (2000) 652. 9. S. Li and J. S. U e , J. Catal., 162 (19%) 76. 10. E. McCarron, J. Chem. Soc, Chem. Commun., (1986) 336.
793
Preparation of M0O3 by spray reaction method and photometathesis of C3H, H. Murayama,N. Ichikuni, S. Shimazu and T. Uematsu Faculty of Engineering, Chiba University, Inage-ku, Chiba 263-8522, Japan Molybdenum oxide catalysts prepared by the spray reaction method had anisotropy in their crystal structures. In the present paper, the surface structure and the photoactivity for propen metathesis of the spray catalyst compared with the commercial M0O3 were investigated. In the XPS, the main peak of the spray catalysts was similar to that of the commercial M0O3 with the other characteristic peaks of Mo 3d^,-,^ and 3d^^f^y The activity of spray catalysts showed higher than that of the commercial M0O3, and increased with increasing the T^^y suggesting the formation of the characteristic active species due to the preparation method, Le. rapid heating and quenching steps, for the spray reaction method. 1. INTRODUCTION Most of photocatalystshave been successfully prepared by tailoring the active species on the supports with high surface area These surface species dispersed as atomic or molecular unit showed high photoactivity. For example, the supported molybdenum oxide species on SiOj show photocatalyticactivity in the oxidativedehydrogenation of alcohols [1], the oxidation of hydrocarbons [2] and the metathesis of alkenes [3]. On the other hand, the bulk molybdenum oxide catalysthas no photocatalysisfor the QH^ metathesis reaction [4]. Sprayed particles, which were prepared by rapid heating and quenching process, can provide the characteristic properties in various catalyses [5-7] due to nano structures in the fine composites with metastable surface states [8]. The unsupported bulk oxides prepared from the spray reaction method were expected to show high photocatalysis as well as the supported catalysts. In this study, molybdenum oxide photocatalysts were prepared by the spray reaction method, and their characterization of the active site structure and the catalytic behavior in the photometathesis of C^H^ were investigated. 2. EXPERIMENTAL 2.1. Catalysts preparation The spray molybdenum oxide catalysts were prepared as follows: The 0.15 mol/1 molybdenum aqueous solution was prepared by dissolving M0O3 (Junsei Chemical Co., Ltd.) in dilute NH3 solution, and used as a starting solution. Atomized droplets from the starting solution were dried and calcined in a quartz tube reactor heated at the spray reaction temperature (T^pr) between 673-973 K. The catalyst was designated with T^^ as spr 673. 2.2. Characterization The catalyst was pretreated under O2 at 673 K for 120 min, followed by evacuation for 15
794 min prior to use for each of measurements and reactions. Characterization was carried out by methods of BET surface area measurement, XRD, XPS and Raman spectroscopy. The Raman spectra were taken with the 514.5 nm line of an Ar* laser (JASCO NRS 2100). The XPS was measured with using Mg-K^^. The pretreated catalyst was sealed in a pyrex glass tube, it was opened in a Ar glove box before setting it on a holder. The number of active sites was quantified by the CO photooxidation reaction performed under the same condition of the C3Hg photometathesisreaction. 23.
Reaction The photocatalytic metathesis reaction was performed at 275 K with 3.3 kPa of C^H^ in a quartz cell connected with a closed circulating system. UV irradiation was carried out by using a 75 W high pressure Hg lamp with a water filter. The reaction products (CjH^ and C4H8) were analyzed by a GC. The TON was calculated from the amount of formed C2H4 for 180 min divided by the number of photoactive sites. 3. RESULTS AND DISCUSSION 3.1. XRD As shown in Fig. 1, the interesting differences in the relative intensity of XRD peaks of a-MoOg {20 = 23.4, 25.8, 27.4, 33.8 and 39.0 degree) between the spray catalyst and the commercial M0O3 were observed. The peaks at 25.8 and 39.0 degree were diffracted from (040) and (060) plane, respectively. The ratio of these two peaks {26 = 25.8 and 39.0 degree) to the peaks at 26 = 23.4 and 27.4 degree was much smaller for the spray catalysts than that for the conunercial M0O3 w^^ich has the ratio over unity. These observations indicate that the anisotropic growth of crystals [9] was promoted by the spray reaction method. With increasing T,^, the anisotropic profile of the spray catalysts was more distinguished. The XRD peaks of spr 673, spr 773 and the commercial M0O3 were assigned to only aM0O3. As T^p^ raised to 873 K, two phases, a-MoOj and P-M0O3, were coexistent The impregnation catalyst (Mo/SiOj) showed no P-M0O3. Since P-M0O3 is stable below 623 K [10], it is unusual that the spray catalysts prepared at 7,^^ above 873 K contained P-M0O3. The rapid heating and quenching steps for the spray reaction may form metastable sites on the surface. Thus, the surface of the spray molybdenum oxides could be partly converted to the different stable phase as |J-MoQ. 3.2. Raman spectra A band due to the Mo-O-Mo bridged bond of a-Mo03 was observed at 820 cm' in the Raman spectra of spr 973 as shown in Fig. 2. Moreover, a band observed at 1000 cm' was assigned to the terminal Mo=0 stretching vibration of a-MoOj. It suggests that the pseudoisolated molybdenum oxide species existed on the spray catalyst The p- M0O3 phase was also observed at 775 and 850 cm *, which was in good agreement with the XRD results. The intensity of peak at 1000 cm^ reduced after the reaction as shown in Fig. 2 (b). However, in the case of spr 773, there is little difference in intensity between before and after the reaction. The different photocatalytic activity between spr973 and spr773 was expected.
795 S.
^MoO^
I
\ i \
20
i J L L »^^ 1 25
30 35 26 I degree
(a)
40
Figure 1. XRD patterns for Mo oxide catalysts; (a) the commercial MoQ, (b) spr 673 ,(c) spr 773, (d) spr 873 and (e)spr973.
45
'i
M W\
(a) ••
A\
(b) 11
J
1000
^'•---'''^^
.'--"^
Vwx/N.^^^^-^ 800
600
400
Raman shift / cm'
Figure 2. Raman spectra for spr 973; (a) pretreatedand (b) after the photometathesis.
3.3. XPS The surface species of the spray catalysts were characterized by the XPS. As for spr 973, binding energy (BE) of Mo 3d(3/2) and 3d(5/2) were higher by 0.15 eV and that of 01s was lower by 0.2 eV, respectively, than those of the commercial M0O3. Since the FWHM was broadened over the spr 973 compared to the commercial M0O3, it was deconvoluted into two peaks by gauss functions, as shown in Fig. 3. The main peaks at 235.6 and 232.5 eV 230 236 234 232 238 corresponded to those of the commercial binding energy / eV M0O3. The other peaks (BE = 234.4 and Figure 3. XPS spectra of Mo (3d3;2 ^^^ 231.3 eV) were shifted to lower BE by 1.2 3d5,2) level for (a)the commercial M0O3 and eV, and observed solely for the spray (b)spr973. catalysts. The energy gap between Mo 3d(5/2) and 3d(3/2) and the peak ratio of Mo 3d(5/2) and'^^^,2)were almost constant for the deconvoluted peaks. These new lower BE peaks could be contributed metastable surface structures of the spray molybdenum oxide. It can be said that these new BE peaks were formed by the rearrangement of the surface structure during the spray reaction.
796 Table 1 Activity for QH^ photometathesis reaction catalyst TON (C,H^) number of active sites /10 ^ mol g,^' spr673 27 1.7 spr 773 29 3.0 spr 873 36 2.2 spr 973 48 2.7 MoQ* 13 OT * Purchased from Junsei Chemical Co., Ltd. 3.4. Photometathesis reaction of CJl^ Although all samples had almost equal BET surface areas (ca 20 m-g'), the spray catalysts had much more active sites than the commercial M0O3 as shown in Table 1. The number of active sites were in order of spr 773 > spr 973 > spr 873 > spr 673 » the commercial M0O3. ^^ decrease in active sites for spr 973 and spr 873 was caused by the content of P-M0O3 which had lower amount of active sites. From the results of the XRD and the Raman spectra, it is suggested that the anisotropic growth of a-Mo03 on the spray catalysts resulted in the production of molybdenyl double bonds associated with the active site. The TON became higher with increasing T^^^. Considering die fact that the higher T^^^ could produce much more unstable surface, these unstable states may have high efficiency of the charge transfer process. It can be speculated that molybdenyl double bonds on the individual surface of the unstable a - MoC^ would show the higher activity. REFERENCES 1. T. Ono, M. Anpo and Y. Kubokawa, J. Phys. Chem., 90 (1986) 4780. 2. K. Marcinkowska, S. Kaliaguineand P. C. Roberge, J. Catal., 90 (1984) 49. 3. M. Anpo, M. Kondo and Y. Kubokawa, J. Chem. Soc., Faraday Trans. 1, 84 (1988) 2771. 4. Y. Kubokawa and M. Anpo, in "Adsorption and Catalysis on Oxide Surface", Eds. M. Che and G. C. Bond, Elsevier, Amsterdam, (1985) 127. 5. D. Li, N. Ichikuni, S. Shimazu and T. Uematsu, Appl. Catal. ArGeneral, 172 (1998) 352. 6. D. Li, N. Ichikuni, S. Shimazu and T. Uematsu, Appl. Catal. A:General, 180 (1999) 227. 7. N. Ichikuni, D. Murata, S. Shimazu and T. Uematsu, Catal. Lett., 69 (2000) 33. 8. T. Tsuchiya, N. Ichikuni, S. Shimazu and T. Uematsu, Chem. Lett., (2000) 652. 9. S. Li and J. S. U e , J. Catal., 162 (19%) 76. 10. E. McCarron, J. Chem. Soc, Chem. Commun., (1986) 336.