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

The maximum monolayer dispersion（the threshold）for WOon γ-AlOcalcined at500°,550°,600°,and 640℃ has been determined quantitatively by XRD（amount of crystallinephase）and XPS(intensity ratios I/I).The results show that if the amount of WOloadedis lower than the maximum monolayer dispersion,WOwill react with γ-AlOto form surface com-pound due to mutual ionic interaction,and will be dispersed on γ-AlOsurface as monolayer then.In case the amount is higher than this value,the residual crystalline WOwill remain.The max-imum monolayer dispersion（threshold）is 0.21 g and 0.20 g WO/100 mγ-AlOby NRD andXPS respectively.It agrees with the value(0.189 g WO/100 mor 4.90×10W atoms/m)cal-culated from the model on assumption that the WOis dispersed as a closed-packed monolayer onγ-AlOsurface.Inasmuch as WO/γ-AlOsystem is stable up to higher temperature,e.g.700℃,than MoO/γ-AlOsystem,WOseems unfavorable to form new buIk compound with γ-AlOatthat temperature.However,Al（MoO）forms perceptibly in MoO/γ-AlOsystem at 500℃.Besides,the size of residual crystalline WOin WO/γ-AlOis much smaller than that of MoOinMoO/γ-AlO.It might be the reason that WO/γ-AlOcatalyst is superior to MoO/γ-AlOinhydrodesulfurization（HDS）or hydrodenitrogenation（HDN）in some cases.