CO偶联临氢反应Pd-Fe/Al_2O_3催化剂的XPS研究

XPS Study of Pd-Fe/Al_2O_3 Catalyst for CO Coupling Reaction with Hydrogen

利用XPS及氩离子溅射等技术对CO偶联和临氢反应中所用催化剂表面活性组分和助剂的含量及其化学状态进行了分析 ,并通过测定氢在催化剂表面的化学吸附 ,以及氢浓度对催化剂活性的影响 ,探讨了CO偶联反应中催化剂临氢失活的主要原因 .XPS表征结果表明 ,CO偶联反应中催化剂活性组分以Pd0 和Pd2 + 形式共存 ;而临氢反应后仅以Pd0 形式存在 ,助剂FeO从催化剂的内部向表面迁移且有少量Fe2 + 转变为Fe3 + .催化剂临氢失活的主要原因是H2 在活性组分Pd及助剂Fe(主要是FeO)表面均可形成解离吸附 ,形成的金属氢化物可在低活化能条件下发生迁移 .这种迁移有利于副产物乙醇的生成 ,从而削弱了CO偶联主反应 ,催化剂表面活性组分Pd的相对含量减少 ,并几乎处于钝化状态 ,导致临氢反应中CO转化率、草酸二乙酯选择性及空时收率均下降 .停止通入H2 后 。

The catalytic activity of Pd-Fe/Al2O3 catalyst for CO Coupling to oxalate is influenced by H-2 as an impurity, which brings about the decrease in CO conversion, selectivity for diethyl oxalate (DEO) and space-time yield of DEO with the increase of H-2 concentration and the production of by-product C2H5OH. The deactivated catalyst and the normal catalyst were characterized by XPS. The results indicated that the active component Pd exists as both Pd2+ and Pd-0 in the reaction, while the deactivated catalyst was in passivation state. In addition, the XPS result proved the migration of iron oxide and iron hydride from inside surface to outside surface of the catalyst, which causes the increase in Fe concentration on the outside surface of the catalyst. Iron oxide catalyzed the sub-reaction of H-2 with C2H5ONO to C2H5OH, which inhibited the objective reaction. When removing H-2 from the system, the deactivated catalyst can regenerated quickly. According to XPS characterization and experimental results, the deactivation mechanism of the catalyst by H-2 was inferred.