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The maximum monolayer dispersion(the threshold)for WO_3 on γ-Al_2O_3 calcined at500°,550°,600°,and 640℃ has been determined quantitatively by XRD(amount of crystallinephase)and XPS(intensity ratios I_(W4f)/I_(Al2s)).The results show that if the amount of WO_3 loadedis 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 andXPS 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 onγ-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 atthat 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 inMoO_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 inhydrodesulfurization(HDS)or hydrodenitrogenation(HDN)in some cases.
The maximum monolayer dispersion (the threshold) for WO_3 on γ-Al_2O_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_ Al2s)). The results show that if the amount of WO_3 loadedis 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 andXPS 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, eg 700 ° C, than MoO_3 / γ-Al_2O_3 system, WO_3 seems unfavorable to form new buIk compound with γ-Al 2 O 3 atthat 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 inMoO_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 inhydrodesulfurization (HDS) or hydrodenitrogenation (HDN) in some cases.