氟硅酸铵处理对γ-Al_2O_3负载NiW催化剂性质的影响

EFFECT OF FLUORINATION WITH (NH 4) 2SiF 6 ON SURFACE PROPERTIES AND ACTIVITY OF NiW/Al 2O 3 CATALYST

应用ＩＲ，ＸＰＳ，ＴＰＲ，ＮＯ和ＣＯ２吸附及脉冲微反等技术研究了氟硅酸铵处理前后γ－Ａｌ２Ｏ３及负载ＮｉＷ催化剂的表面性质，探讨了其改性机理和它对催化剂活性相的影响．结果表明，经氟硅酸铵处理后，γ－Ａｌ２Ｏ３表面的不饱和四面体空位及其四配位Ａｌ３＋上的强碱性羟基均明显减少，改变了γ－Ａｌ２Ｏ３表面的酸碱性和Ｎｉ２＋在γ－Ａｌ２Ｏ３表面的分布，使ＮｉＯ倾向分布于γ－Ａｌ２Ｏ３的八面体空位．过量的氟硅酸铵处理将明显破坏γ－Ａｌ２Ｏ３表面的碱性羟基，导致ＷＯ３和ＮｉＯ的分散度下降，使改性后催化剂的噻吩加氢脱硫性能降低．

The surface properties of γ Al 2O 3 and NiW/Al 2O 3 samples before and after fluorination with (NH 4) 2SiF 6 have been studied by using IR, XRD, XPS and adsorption of NO and CO 2. Their activity for HDS of thiophene has been measured with pulse micro reaction system. The mechanism of fluorination and its influence on the active phase of the catalyst have also been discussed. The results show that the tetrahedral vacancies of γ Al 2O 3, which contain more net positive charge as well as its strongest basic OH groups cordinated to tetrahedral Al 3+ , decrease when γ Al 2O 3 is fluorinated with (NH 4) 2SiF 6. It causes the Ni species on NiW F/Al 2O 3 to prefer to occupy the octahedral vacancies of γ Al 2O 3. However, when γ Al 2O 3 is fluorinated excessively with (NH 4) 2SiF 6, its basic OH groups are replaced largely by F -, which reduces the dispersity of WO 3 and NiO. This disadvantage would cover up the advantage of fluorination for the distribution of Ni species and cut down HDS activity of fluorinated catalyst.