Determination of the maximum monolayer dispersion and dispersed state for WO3 on γ-Al2O3

The maximum monolayer dispersion(the threshold)for WO_3 on γ-Al_2O_3 calcined at 500°,550°,600°,and 640℃ has been determined quantitatively by XRD(amount of crystalline phase)and XPS(intensity ratios I_(W4f)/I_(Al2s)).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_2O_3 to form surface com- pound due to mutual ionic interaction,and will be dispersed on γ-Al_2O_3 surface as monolayer then. In case the amount is higher than this value,the residual crystalline WO_3 will remain.The max- imum monolayer dispersion(threshold)is 0.21 g and 0.20 g WO_3/100 m~2 γ-Al_2O_3 by NRD 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)cal- culated from the model on assumption that the WO_3 is dispersed as a closed-packed monolayer on γ-Al_2O_3 surface.Inasmuch as WO_3/γ-Al_2O_3 system is stable up to higher temperature,e.g.700℃, than MoO_3/γ-Al_2O_3 system,WO_3 seems unfavorable to form new buIk compound with γ-Al_2O_3 at that temperature.However,Al_2(MoO_4)_3 forms perceptibly in MoO_3/γ-Al_2O_3 system at 500℃. Besides,the size of residual crystalline WO_3 in WO_3/γ-Al_2O_3 is much smaller than that of MoO_3 in MoO_3/γ-Al_2O_3.It might be the reason that WO_3/γ-Al_2O_3 catalyst is superior to MoO_3/γ-Al_2O_3 in hydrodesulfurization(HDS)or hydrodenitrogenation(HDN)in some cases.