J. MATER. CHEM., 1993,3(12), 1313-1318 1313 Surface Dispersion of Molybdena Supported on Silica, Alumina and Titania Margarita del Arco,' Silvia R. G. Carrazan," Cristina Martin,' lnes Martin," Vicente Rives*" and Pilar Maletb a Departamento de Quimica Inorganica, Universidad de Salamanca, Facultad de Farmacia, 3700 7-Sa la mama , Spa in Spain Departamento de Quimica Inorganica, Universidad de Sevilla, Facultad de Quimica, 41 01 2-Sevilla, Surface species formed upon impregnation of silica, alumina and titania with aqueous solutions of ammonium molybdate and calcination at 770 K have been studied by X-ray diffraction, specific surface area assessment, scanning electron microscopy, laser Raman spectroscopy and temperature-programmed reduction. Dispersion decreases in the order TiO,> AI,O,>>SiO,. Polymolybdate species are formed for low loadings, that lead to bulk MOO, for loadings of 0.4 monolayer on SO,, but persist for higher loadings on titania and alumina. On this last support, isolated molybdate species have been also found. Keywords: Molybdena ; Supported catalyst; Laser Raman spectroscopy Supported molybdena is an active catalyst in the oxidation of propene ' and methanol,2 in hydrotreating processe~,~-~ alcohol synthesis and photochemical reactions.6-8 'Monolayer-type' catalysts are formed when a high degree of dispersion of the active component is achieved on the surface of the support, and it is assumed that the supported phase exists as a monolayer on the support surface. However, their catalytic properties are not only determined by their high degree of dispersion, but also by the structure of the active phase. In this way, the activity and selectivity have been related to the actual nature of the active phase, which itself depends on several factors, such as the type of support and the method of preparation of the catalyst.' In spite of the great deal of work on the alumina and titania-supported Mo catalysts, silica-supported Mo catalysts have attracted much less attention. One reason may be that the catalytic activity was considered to be much lower than that for alumina- and titania-supported catalysts. However, silica-supported Mo catalysts have high activities for alcohol synthesis and photochemical reactiom6 In the reactions mentioned above for all these catalysts, the reactivity is very sensitive to the structure and oxidation state of the supported Mo species. Usually, most of the supported Mo atoms are in tetrahedral or octahedral coordination (isolated MOO: - species or isopolyanions such as poly- molybdate, Mo70Z;, depending on the preparation pro- cedures, drying and calcination conditions) at low loadings, and these Mo ions play a significant role in the reactions mentioned above, but the dispersed Mo species and crystalline MOO, formed at high loadings are not active in these reactions. Taking all this into account, in this paper an investigation and comparison of the interaction of molybdenum oxide with various metal oxide supports, commonly used as catalysts and catalyst supports, has been attempted. In particular, the work is focused on the phenomenon of spreading and on the identification of the Mo species formed on the supports. The systems prepared by impregnation techniques were M003,A1203, MoO,/TiOz and Mo03/SiOz. X-Ray diffraction (XRD) and temperature-programmed reduction (TPR) have been used to obtain additional infor- mation on the spreading of MOO, on these supports and laser Rarnan spectroscopy (LRS) has been applied in order to identify Mo species formed on the supports with different concentrations of the supported compound. Scanning electron microscopy (SEM) and specific surface area measurements of the samples were also carried out. Experimental Materials and Sample Preparation The supports were A1203 (y-alumina, ref. RVOOS), Ti02 (P- 25) and SiOz (Aerosil) from Degussa (Germany), and were calcined in air overnight at 770 K to eliminate adsorbed organic impurities. Samples were prepared by impregnating the supports with an aqueous solution of (NH4)6Mo,02,-4H20 (HMA, Carlo Erba) and then drying them at 373 K overnight. The samples were finally calcined at 770 K in a static, uncontrolled atmos- phere in an open crucible in an oven for 2 h. After this treatment, chemical analysis indicated no loss of molybdena through sublimation. In all cases, the relative amounts of HMA and the supports ranged from 0.4 to 2 monolayers upon calcination, as calcu- lated from the specific surface areas of the supports calcined at 770 K [SBET (A1203)=105 m2 g-', SBET (Ti02)= 50 m2 g-' and SBET (SiO2)=2o7 m2 g-'1 and the area covered" by a 'molecule' of MOO,, 15 x lo4 pm'. With this, one monolayer equals 16.81 g MOO, per 100 g A1203, 7.96 g MOO, per 100 g Ti02 and 33.14g MOO, per 1OOg Si02. In the case of Mo03/SiOz systems, samples with 0.1 and 0.2 monolayer were also prepared in order to identify 'interaction species' which apparently do not exist at MOO, contents >0.4 monolayer. The naming of the samples, according to the molybdena content and the nature of the support, is summarized in Table 1. Samples are designated as MX-n, M = MOO,, X =A, T, S for A1203, TiOz or SO2, respectively, and n=M003 content, given as the number of monolayers. Experimental Techniques XRD patterns were obtained by reflection from powder packed in a sample holder with a Siemens-500 diffractometer using Cu-Ka, radiation (A = 154.05 pm) with a graphite mono- Downloaded on 11 February 2011 Published on 01 January 1993 on http://pubs.rsc.org | doi:10.1039/JM9930301313 View Online