Ultramicroscopy 29 (1989) 233-246 233 North-Holland, Amsterdam TEM AND IN-SITU EM STUDY OF THE DISPERSION OF MoO 3 ON SiO 2 A. DATTA and J.R. REGALBUTO Department of Chemical Engineerin~ University of Illinois at Chicago, Chicago, Illinois 60680, USA and C.W. ALLEN Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA Received at Editorial Office October-November 1988; presented at Conference May 1988 Molybdenum trioxide supported on nonporous silica spheres and planar silica substrates were used with TEM and in-situ EM (environmental cell and heating stage), respectively, to study the spreading behavior of the hexagonal and orthorhombic forms of MoO 3 on SiO2. At temperatures below 500 o C, supported orthorhombic crystallites are stable, while supported hexagonal crystaUites, formed by an initial calcination of the precursor at 300 o C, spread drastically and in doing so produce a novel, multilayered and amorphous silica-supported morphology for MoO 3. The mechanism of spreading appears to be the formation of mobile species on hexagonal crystallites, and subsequent migration across the SiO2 surface. 1. Introduction Supported molybdenum or its trioxide has a wide variety of uses in heterogeneous catalysis, for catalytic reactions such as hydrodesulfurization, hydrogenation and dehydrogenation, reforming, and hydrocracking, to name a few. The dispersion and morphology of the trioxide, often a precursor phase of the final catalyst, have much to do with final catalytic activity. A number of trioxide forms exist over various support materials. At low load- ings, the material is generally completely dispersed and of a monolayer type while at higher loadings, bulk crystallites form [1]. Some supports such as alumina and titania exhibit stronger interactions with MoO3 such that intermetallic oxide phases form [2]. Over SiO2, however, only the monolayer and bulk crystalline (orthorhombic) forms have been reported; these formed with an initial calcination of the impregnated precursor always greater than 450 ° C. At this high initial calcina- tion temperature, surface loadings less than I atom Mo/nm 2 [1] yield the well dispersed catalyst, whose morphology is only slightly altered by sub- sequent treatments [3], while loadings greater than this value result in bulk crystallites. There appears to be no way to redisperse bulk, orthorhombic crystallites of MoO 3 on SiO3 once they form [4], short of extremely high calcination temperatures, at which trioxide losses occur due to volatilization [5]. In recent characterization of MoO3/SiO 2 cata- lysts with X-ray diffraction (XRD) among other methods [6], it has been shown that the dispersion of high-loading MoO 3 (4 atoms Mo/nm 2) can be controlled through the initial formation of the little-studied hexagonal crystalline phase. A summary of XRD results from this study is shown in fig. 1. The supported hexagonal phase, formed by an initial 300 °C air calcination, is shown in pattern a and is compared to the orthorhombic phase formed by a 500 °C calcination (pattern b). A bulk crystalline (poorly dispersed) ortho- rhombic phase (pattern c) was formed by a 500 ° C calcination of the poorly dispersed hexagonal phase, while continued calcination at 300°C 0304-3991/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)