添加钠对低镍甲烷化催化剂结构性能的影响

用活性测试、XRD和EXAFS方法研究了一系列含Ni 6wt％、添加不同量Na助剂的γ-Al_2O_3担载Ni甲烷化催化剂的结构性能。结果表明:Ni与Al_2O_3有较强的相互作用。添加不同量的Na助剂能增强或削弱这种作用,从而影响催化剂宏观性能,Na在Al_2O_3上是高度分散的。它的影响是通过改变载体表面性能从而改变Ni的状态及其分布,Ni的分散度和Ni-Al_2O_3相互作用而完成的。适量Na可提高Ni的分散度,优化Ni-Al_2O_3相互作用,从而提高催化剂活性。过量Na则会加强Ni-Al_2O_3相互作用,或者造成NiO自身聚集形成NiO晶相,从而降低Ni利用率,使催化剂活性降低。

The Effect of Na Additive on the Structure and Property of Low Weight Loading Nickel Based Methanation Catalyst

A family of Ni/ Al2O3 system promoted by different amount of Na additive was studied by methanation activity measurement, XRD and EXAFS methods. It was found that the methanation activity increased with increasing Na content and passed through a maximum at 0.141 - 0. 235 wt% for Ni/Al2O3 containig 6 wt% of Ni. The Na additive was highly dispersed on Al2O3 surface. A suitable amount of Na would give a good distribution as well as a desirable dispersion for the Ni and optimize the Ni-Al2O3 interaction , thus enhance the activity of the catalyst. An excess of Na would result in too strong Ni -Al2O3interaction or the accumulation of crystalline NiO,thus decrease the methanation activity. The favorable amount of Na additive would depend on the Ni/Na/Al2O3 ratio rather than the Ni/Na ratio. The local structure of the Ni derived from EXAFS data were used to explain the differences in the methanation activity. The catalyst on which the Ni atom had a low O coordination number N(O) and a low Ni coordination number N(Ni) exhibited a good methanation activity and methane selectivity, while for catalysts with similar O coordination, the one with a lower N(Ni) value would exhibit a better activity. The catalyst with higher N(O) value exhibited a lower activity with noticeable formation of CO2. The oxygen coordinated with Ni probably takes part in the catalytic cycle, thus promotes the formation of CO2 when the N(O) value is high.