Determination of the maximum monolayer dispersion and dispersed state for WO 3 on γ‐Al 2 O 3

The maximum monolayer dispersion (the threshold) for WO 3 on γ‐Al 2 O 3 calcined at 500°, 550°, 600°, and 640°C has been determined quantitatively by XRD (amount of crystalline phase) and XPS (intensity ratios I w4f / I Al2 ). The results show that if the amount of WO 3 loaded is lower than the maximum monolayer dispersion, WO 3 will react with γ‐Al 2 O 3 to form surface compound due to mutual ionic interaction, and will be dispersed on γ‐Al 2 O 3 surface as monolayer then. In case the amount is higher than this value, the residual crystalline WO 3 will remain. The maximum monolayer dispersion (threshold) is 0.21 g and 0.20 g WO 3 /100 m 2 γ‐Al 3 O 3 by XRD and XPS respectively. It agrees with the value (0.189 g WO 3 /100 m 2 or 4.90 × 10 −18 W atoms/m 2 ) calculated from the model on assumption that the WO 3 is dispersed as a closed‐packed monolayer on γ‐Al 2 O 3 surface. Inasmuch as WO 3 /γ‐Al 2 O 3 system is stable up to higher temperature, e.g. 700°C, than MoO 3 /γ‐Al 2 O 3 system, WO 3 seems unfavorable to form new bulk compound with γ‐Al 2 O 3 at that temperature. However, Al 2 (MoO 4 ) 3 forms perceptibly in MoO 3 /γ‐Al 2 O 3 system at 500°C. Besides, the size of residual crystalline WO 3 in WO 3 /γ‐Al 2 O 3 is much smaller than that of MoO 3 in MoO 3 /γ‐Al 2 O 3 . It might be the reason that WO 3 /γ‐Al 2 O 3 catalyst is superior to MoO 3 /γ‐Al 2 O 3 in hydrodesulfurization (HDS) or hydrodenitrogenation (HDN) in some cases.