J. Phys. Chem. 1987, 91, 2875-2883
2875
EPR Investigation of the Coordination Sphere of Mo5+ Ions on Thermally Reduced Silica-Supported Molybdenum Catalysts Prepared by the Grafting Method Catherine Louis and Michel Che* Laboratoire de RPactivitP de Surface et Structure, U A I 1 06 CNRS, Universite P. et M . Curie, 75252 Paris Cedex 05, France (Received: August 13, 1986)
Thermally reduced Mo/Si02 catalysts prepared by the grafting method, Le., by air- and water-free reaction between MoClS and hydroxyl groups of silica, present three EPR MoS+signals, clearly identified by means of their third-derivative spectrum (gl = 1.944, gl,= 1.892 for Mo;: g, = 1.957, gll = 1.866 for Mo::; and g , = 1.926, gll= 1.755 for Moz). The coordination sphere of the corresponding species was studied with water and carbon monoxide as probe molecules, which only act as ligands. The transformation of the EPR signals upon water adsorption, the analysis of the superhyperfine structure induced by "CO adsorption, as well as the analysis of the various g tensors have led to attribute these signals to molybdenyl ions in tetra(Mog), penta- (Mo::), and hexacoordination (Moz). This is the first positive identification by EPR of surface tetrahedral M o z ions produced by thermal reduction. It has been made possible owing to the high Mo dispersion on the grafted samples. The earlier data found in the literature on the g values of supported Mo5+ions are rationalized on the basis of the present results.
Introduction which allows one to obtain well-dispersed molybdenum even after thermal treatment, owing to the strong Mc-support interactions The structure and reactivity of supported molybdenum oxide induced by the preparation. The grafting involves a chemical catalysts are currently the subject of numerous investigations.' reaction, under air- and water-free conditions, between surface But the different steps of their preparation by the conventional silanol groups of silica and MoClS used as vapor'8-20or dissolved method of impregnation make both the formation of the metalin cyclohexane.'6~'8 Silica was selected as the support since it is support bonds and the dispersion of the active phase difficult to rather neutral in comparison with acid-base properties of other control. Recent studies24 have shown that the relative values of oxides. The use of paramagnetic MoCls vapors is particularly the pH of the ammonium heptamolybdate solutions used for interesting in that changes of the coordination sphere of Mo5+ impregnation, on the one hand, and of the zero charge or isoelectric can be monitored by EPR throughout the grafting process.18-20 point of the support,' (IEPS) on the other, determine the formation The next preparation step is performed in order to eliminate, by of either a homogeneous monolayer or clusters of Moo3. A washing with water'6J8 or ammonia solution,I8 the loosely bonded monolayer can also be obtained by solid/solid adsorption, owing molybdenum. This second decisive step leaves on the support only to the ability of a number of oxides to disperse spontaneously onto the really grafted molybdenum. The washing step was not persupports upon thermal t r e a t m e r ~ t . ~ ? ~ formed in earlier studies21,22dealing with molybdenum catalysts Well-controlled preparations can be achieved by reacting orby the grafting method. ganometallic complexes of molybdenum, e.g., M O ( T - C ~ H ~ ) ~ , ' * ~ prepared ~ This work, which is the continuation of earlier studies on grafted with supports. However, Roberge et al.ls have recently shown molybdenum catalyst^,'^-^^^^^-^^ deals with the characterization that part of the molybdenum was not strongly held on the support by EPR of the coordination sphere of thermally produced MoS+ since it could be eliminated by washing. We have recently deions. This has been achieved by means of probe molecules such veloped a different approach'&20 based on the grafting method as water and carbon monoxide. I3COhas proved to be particularly adapted to determine the number of vacancies present in the (1) Massoth, F. E. Ado. Catal. 1978, 27, 265 and references therein. coordination sphere, via the superhyperfine structure induced by (2) Brunelle, J. P. Pure Appl. Chem. 1978, 50, 121 1. (3) Li Wang; Hall, W. K. J . Catal. 1980, 66, 25. of I3CO with the interaction of the 13Cnuclear spin ( I = (4) Hall, W. K. In Proceedings of the 4th International Conference on unpaired electron of MoS+or Ni+ ions, as described previou~ly.l~*~~ Chemistry and Uses of Molybdenum; Climax Molybdenum: Ann Arbor, MI, Earlier EPR studies concerned Mo/Si02 samples prepared by 1982; p 224. or M o ( C O ) ~ , ~ ' impregnation from Mo7024(NH4)6.4H202',27-30 (5) Li Wang; Hall, W. K. J . Catal. 1982, 77, 232. (6) Leyrer, J.; Vielhaber, B.; Zaki, M. I.; Shuxian, Z.; Weitkamp, J.; but as shown in Table I, both the MoS+ g values and their inKnozinger, H. Mater. Chem. Phys. 1985, 13, 301. terpretations are at variance. This work represents an attempt (7) Parks, G.A. Chem. Rev. 1965,65, 117. (8) Xie Youchang; Gui Linlin; Lin Yingjun; Zhang Yufen; Zhao Biying; Yang Naifang; Guo Qinlin; Duan Lianyun; Huang Huizhong; Cai Xiaohai; Tang Youchi In Adsorption and Catalysis on Oxide Surfaces; Che, M., Bond, G.C., Eds.; Elsevier: Amsterdam, 1985; p 139. (9) Haber, J., personal communication. (10) Yermakov, Yu. I. Catal. Rev.-Sci. Eng. 1976, 13, 77. (11) Iwasawa, Y.; Nakano, Y.; Ogasawara, S. J. Chem. SOC.,Faraday Trans. 1 1978, 74, 2968. (12) Iwasawa, Y.; Ogasawara, S. J. Chem. SOC.,Faraday Trans. 1 1979, 75, 1465. (13) Rodrigo, L.; Marcinkowska, K.; Lafrance, C. P.; Roberge, P. C.; Kaliaguine, S. Proc. 9th Ibero-American Symp. Cafal.,Lisboa 1984, 675. (14) Marcinkowska, D.; Kaliaguine, S.; Roberge, P. C. J . Catal. 1984, 90,
49 ._.
(15) Marcinkowska, K.; Rcdrigo, L.; Kaliaguine, S.; Roberge, P. C. J . Cafal. 1985, 97, 15. (16) Louis, C.; Che, M.; Bozon Verduraz, F. J. Chim. Phys. Phys.-Chim. Biol. 1982, 79, 803. (1 7) Louis, C.; Che, M. In Reactiuity o j s o l i d s ; Series Materials Science Monograph: Barret, P., Dufour, L. C., Eds.: Elsevier: Amsterdam, 1985: Vol. B, p ioosi. (18) Louis, C. Doctoral Thesis, Paris, 1985
0022-3654/87/2091-2875$01.50/0
(19) Che, M.; Bonneviot, L.; Louis, C.; Kermarec, M. Mater. Chem. Phys. 1985. -13. 201. - ,~ - (20) Che, M.; Louis, C.; Tatibouet, J. M. Polyhedron 1986, 5 , 123. (21) Che, M.; Figueras, F.; Forissier, M.; McAteer, J. C.; Perrin, M.; Portefaix, J. L.; Praliaud, H. Proc: Int. Congr. Catal., bth, 1976 1977, 1, 261. (22) Fricke, R.; Hanke, W.; Ohlmann, G.J . Catal. 1983, 79, 1. (23) Che, M.; Dyrek, K.; Louis, C. J . Phys. Chem. 1985, 89, 4526. (24) Che, M.; Dyrek, K.; Louis, C. J . Phys. Chem. 1985,89, 4531. ( 2 5 ) Louis, C.; Tatibouet, J. M.; Che, M., submitted for publication in J . Catal. (26) Bonneviot, B.; Olivier, D.; Che, M. J . Mol. Catal. 1983, 21, 415. (27) Vorotyntsev, V. M.; Shvets, V. A,; Kazanskii, V. B. Kinef. Katal. 1971, 12, 1249. (28) Giamello, E.; Theobald, F.; Naccache, C.; Vedrine, J. J . Chim. Phys. Phys.-Chim. Biol. 1978, 75, 305. (29) Narayana, M.; Zhan, R. Y.; Kevan, L. J . Phys. Chem. 1985,89,639. (30) Zhan, R. Y.; Narayana, M.; Kevan, L. J . Chem. SOC.,Faraday Tram. I 1985, 81, 2083. (31) Howe, R. F.; Leith, I. R. J. Chem. SOC.,Faraday Tram. 1 1973, 69, ~~
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1961.
0 1987 American Chemical Society
2876
Louis and Che
The Journal of Physical Chemistry, Vol. 91, No. 11, I987
TABLE I: Mo5+ P Values Observed on Mo/SiO, and Mo-Bi/SiO, Catalvsts
method precursor reduction Mo, wt % g, gll
g, gll
ref
PG" MoCI, H2/500 O C
impregn
0.5 1.941 1.885 1.958 1.856 21
2 1.944 1.886
b, c
b
H2/500 O
21
Mo/Si02 impregn
impregn C
CO/SOO 2 1.942 1.891 1.961 1.861 21
impregn
b O C
H2/5OO O
b H2/500-600
C
0.01-0.1-1 1.93
Mo(CO), O C
1.865 29, 30
OPG = pseudografting. This method was so-called because we have previously
H2/500
O C
3.6 1.948 1.884 1.952 1.862 31
1.95 1 1.895
27
Mo-Bi/Si02 impregn
impregn
b H2/500-600
