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用CNDO/2方法定量计算氨在HY(Si/Al=2.4)沸石上的吸附热,其值24.4kcal/mol与实验值符合较好。质子在表面与NH_3间迁移,有两个能量极小值;吸附时,质子从接近表面的极小迁移到接近NH_3极小时,要越过势垒9.40kcal/mol;反之要越过势垒11.9kcal/mol。400℃时,质子在表面与NH_3间跳动频率为1.92×10~9sec~(-1)。 NH_3吸附后,NH键长增加,加以质子迁移,导致NH_8上负电荷密度向沸石表面迁移,使H上的电子集居数减少,H更质子化。
The CNDO / 2 method was used to quantitatively calculate the heat of adsorption of ammonia on HY (Si / Al = 2.4) zeolite. The value of 24.4 kcal / mol was in good agreement with the experimental data. When proton migrates from the surface to NH_3, there are two minimum energies. At the time of adsorption, the proton will cross the barrier 9.40kcal / mol when the proton migrates from the near surface to the extremely near NH_3. On the contrary, the proton will cross the barrier 11.9kcal / mol. At 400 ℃, the bouncing frequency of proton between surface and NH_3 was 1.92 × 10 ~ 9sec ~ (-1). NH_3 adsorption, the NH bond length increases, to proton migration, resulting in negative charge density on the NH_8 migration to the zeolite surface, reducing the number of electrons on the H, H more protonated.