Abstract
An x-ray photoelectron spectroscopic study of MoO/sub 3//SiO/sub 2/ hydrodesulfurization catalysts was carried out to understand their physiochemical and catalytic behavior and to compare the results with a previous study of MoO/sub 3//Al/sub 2/O/sub 3/ catalysts. With the oxidic MoO/sub 3//SiO/sub 2/ catalysts, it was found that Mo was well dispersed to form monolayers below 2 wt % MoO/sub 3/ content (ca. 0.4 x 10/sup 18/ Mo atoms/m/sup 2/), compared to ca. 13 wt % MoO/sub 3/ (4 x 10/sup 18/ Mo atoms/m/sup 2/) for alumina-supported catalysts. Multilayered Mo species and microcrystalline MoO/sub 3/ developed as the MoO/sub 3/ content increased to ca. 9 wt %. Above 13 wt % MoO/sub 3/, crystalline MoO/sub 3/ was detected by XRD. At high MoO/sub 3/ content, no Mo species interacting strongly with SiO/sub 2/ was observed by XPS. When sulfided, MoO/sub 3//SiO/sub 2/ catalysts were characterized by more vigorous surface segregations and subsequent sinterings of Mo and by lower degrees of Mo sulfidation, compared to those for MoO/sub 3//Al/sub 2/O/sub 3/ catalysts, except for the extremely low MoO/sub 3/ content catalysts. It was found that the extent of the sulfidation of Mo was restricted within the formation of double layers of amore » two-dimensional MoS/sub 2/ layer over MoO/sub 2/ at atmospheric pressure and within the formation of double layers of a two-dimensional MoS/sub 2/ layer over MoO/sub 2/ at atmospheric pressure and 400/sup 0/C. The discrepancies in the physiochemical and catalytic features between SiO/sub 2/..mu.. and Al/sub 2/O/sub 3/-supported catalysts are well understood in terms of the strength of Mo-support interactions and resultant Mo species on sulfidation. Differential charging effects in the XPS measurements are pointed out to be very important for the interpretations of the XPS results on MoO/sub 3//SiO/sub 2/ catalysts.« less
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