Effects of surface orientation and crystallinity of alumina supports on the microstructures of molybdenum oxides and sulfides

The effects of surface orientation and crystallinity of the supports for catalysts on the microstructures of molybdenum oxides and sulfide clusters were investigated. X-ray photoelectron spectroscopic (XPS) analysis and transmission electron microscopy (TEM) were used to observe Mo model catalysts supported on three γ-Al2O3 single crystal thin films of different surface orientation ((1 0 0), (1 1 0), and (1 1 1)) and on one amorphous alumina film for different sulfidation temperatures. In the oxide states, the dispersion of Mo atoms, the amount of oxygen interacting with Mo atoms, and the Mo(VI) contribution to the valence of Mo atoms increased in the following order: (1 1 1)<(1 0 0)<(1 1 0). This order indicates that the differences in the surface oxygen arrangements among these three crystal planes significantly affect the number and quality of Mo–O–Al linkages and the network of Mo atoms, leading to differences in the Mo dispersion and the electrical states of Mo atoms. Our XPS results show that, for all of the catalysts, both the S/Mo ratio and the Mo(IV) contribution increased with increasing sulfidation temperature, whereas the O/Mo ratio decreased. At high sulfidation temperature (above 573 K), both the S/Mo ratio and the Mo(IV) contribution decreased in the following order: (1 1 0)>(1 0 0) ≧ (1 1 1). This order indicates that the differences in the number and quality of Mo–O–Al linkages and the network of Mo atoms in the oxide states among these three single crystal catalysts resulted in differences in the sulfidation mechanism of MoOx. Our TEM observation shows that the orientation of MoS2 clusters after sulfidation at high temperature was affected by the surface orientation of the supports. This orientation dependence can be explained by the presence of interfacial Mo atoms that helped maintain the arrangement in oxide states.