A Raman and ultraviolet diffuse reflectance spectroscopic investigation of alumina-supported molybdenum oxide
The Journal of Physical Chemistry
8791 alkali-metal and molybdenum oxide. 5. In situ heating of 6.4 wt 96 Mo/SiOz destroyed the polymolybdate clusters and led to the formation of a new Mo6+/Si02 structure in which the Mo6+ cations are isolated. The isolated Mo6+ species reformed a polymolybdate cluster upon rehydration of the surface. 6. Previously reported UVDRS bands attributed to tetrahedrally coordinated Mo were observed, but the assignment to Mo(TJ is inconsistept with the LRS results which showed only Mo(Oh) to be
... o(Oh) to be present. Acknowledgment. This work was supported by the Robert A. Welch Foundation and the U S . Department of Energy, Office of Basic Energy Sciences (C.C.W. and J.G.E), and by the Texaco Philanthropic Foundation (F.D.H.). We acknolwedge Prof. Y. Iwasawa for supplying one of the silica supports. Registry No. Molybdenum oxide, 13 13-27-5; calcium, 7440-70-2; sodium, 7440-23-5. Laser Raman spectroscopy and ultraviolet-visible diffuse reflectance spectroscopy were used to characterize alumina-supported molybdenum oxide prepared from Mo2(q3-C3H5)" H2(Mo03C204).2H20, and (NH4)6M07024.4H20 at loadings ranging from 0.67 to 13.3 wt 9% Mo. The structure of the calcined Mo6+/A1203, under ambient conditions, was found to be independent of the molybdenum precursor and the preparation pH. The hydrated surface molybdate structure was found to be governed by the inherent acid-base properties of the molybdena/alumina system and the molybdenum weight loading. At low Mo loadings (<I Mo atom/nm2), isolated MOO, tetrahedra dominate although a low degree of polymerization was observed. At 2-2.5 Mo atoms/nm2, the majority of the molybdenum was incorporated into octahedrally coordinated molybdenum polyanions. Crystalline Moo3 was detected above monolayer coverage ( 5-6 Mo atoms/nm2). Marcinkowska, K.; Adnot, A.; Roberge, P. C.; Kaliaguine, S.; Stencel, J. M.; Makovsky, L. E.; Diehl, J. R. J. Phys. Chem. 1986, 90, 2690. (6) Rodrigo, L.; Adnot, A.; Roberge, P. C.; Kaliaguine, S. J. Caral. 1987, IOS, 175. (7) Rodrigo, L.; Marcinkowska, K.; Lafrance, C. P.; Rokrge, P. C.; Kaliaguine, S. (IO) Kasztelan, S.; Grimblot, J.; Bonnelle, J. P.; Payen, E.; Toulhoat, H.; Jaquin, Y. Appl. Cataf. 1983, 7, 91.