γ-Mo_2N催化剂上H_2及NO吸附性质的TPD-MS研究

TPD-MS Study of H_2 and NO Adsorption Properties of γ-Mo_2N

采用TPD-MS方法研究了H2及NO在γ-Mo2N上的吸附状况．单独的H2-TPD结果表明，当H2在673K吸附时，在443K、573K及723K得到了三个H2脱附峰，表明γ-Mo2N上有三种不同能量的H2吸附位．NO－TPD结果表明，NO吸附后亦有三个脱附峰（383K、493K、543K），对应着γ-Mo2N上三种不同能量的NO吸附位：低、中、高能吸附位．NO既可以以解离状态，又可以以一种NO三聚态（dimerordinitrosyl）的形式吸附在γ－Mo2N上，这些吸附物种在脱附过程中产生大量的N2及少量的N2O.对比NO吸附在不同处理条件的γ-Mo2N上的TPD结果可知，NO是吸附在γ-Mo2N上的MO的配位不饱和中心上，这些吸附中心既可通过还原催化剂，又可通过在773K抽空钝化态的γ－Mo2N而产生，H2和NO共吸附的结果表明，预吸附H2再吸附NO后，H2和NO的脱附量均大大减少，且只有两个脱附峰出现.NO只在363K及493K出现两个脱附峰，表明预吸附氢占据了NO的强吸附位，且NO很难取代它，从而使NO只能吸附在能量较低的吸附位上；而H2只在523K及723K出现两个脱附峰，且伴随着H2的脱出有N2和H2O的产生，表明在γ－Mo2N上NO可能与预吸附氢形成了一种复合相MoHx（NO）y,它在脱附时分解为H2、N2及H2O.

TPD-MS technique has been used to characterize the H2 and NO adsorptionproperties of γ-Mo2N. Three categories of adsorbed H2 can be well distinguished when H2was adsorbed at 673K. Mo coordinatively unsaturated sites (cus), on which NO can be adsorbed, can be produced by either reducing or evacuating the passivated Mo2N at elevatedtemperature. A comparison of NO adsorbed on evacuated, reduced and H2 preadsorbedMo2N revealed that there were three categories of artive sites on reduced Mo2N surfacethat edsorbed No: low, medium and high energy sites on which NO was desorbed withpeak maxima at 383, 493 and 543K respectively. NO can be adsorbed in both a dissociative and a dimer or dinitrosyl state. The results of NO adsorbed on H2 preadsorbed Mo2Nshowed that NO can only be adsorbed on low and medium energy sites when hydrogenpreadsorbed at 673K. NO cannot displace the most strongly adsorbed hydrogen. It is alsosuggested that NO can form some stable complexes with the adsorbed hydrogen. Thecomplexes can only be desorbed at temperatures at which hydrogen can be desorbed, andthe desorbed complexes decompose to N2, H2O and H2.