Ni-W/Al_2O_3表面NiWO_4化合物的生成及其对加氢反应活性的影响

FORMATION OF NiWO_4 AND ITS EFFECT ON HYDROGENATION ACTIVITY OF Ni-W/Al_2O_3 CATALYSTS

采用PASCA,ESCA,TA和微型反应色谱等技术研究了Ni-W/Al_2O_3催化体系。本工作是文献[4]工作的继续。由于固体表面在原子水平上的不均匀性,钨分散在氧化铝的“活性表面”上,并形成“微晶岛”。当钨以一个单分子层分散时,W/Al_2O_3表面仍然存在未被钨占据的“惰性表面”。在这种情况下,当镍分散到W/Al_2O_3中时,镍将优先占据这些“惰性表面”,并同其相邻的钨原子结合生成NiWO_4化合物。NiWO_4是硫化催化剂中“Ni-W-S”活性相的前身物。由于“位阻”作用,某些钨原子处于镍“不易接近”的位置。因此,造成最佳Ni/W原子比小于1.O。适当降低钨的担载量可消除这种位阻作用,使钨在氧化铝表面上处于“最佳”的分散状态,Ni-W/Al_2O_3催化剂的最佳(即具有最高相对加氢活性)Ni/W原子比正好为1.O。

PASCA (positron annihilation spectroscopy for chemical analysis), ESCA, TA and micro-reactor have been used to study the Ni-W/Al2O3 catalysts with various Ni/W atomic ratios (R) .An interaction model for interpreting the states of Ni and W on alumina surface is presented, which is similar to the 'island model' proposed by Clausen and Hall.The alumina surface can be divided into two parts: active area, where exists a number of surface sites, and inactive area.When loaded as a PASCA monolayer, the W reacts with surface sites on the active area to form metal-support interaction complexes.When Ni is introduced onto the W/A12O3 system, it will load on the inactive area, interacting with its neighboring W and forming NiWO4 which has been considered as the precursor of the active phase, Ni-W-S, on the surface.Some of the W atoms may locate on the center of the islands, which are unable to be reached by the Ni atoms.This is why the optimal R value may be less than 1.0.The optimal R value may be increased up to 1.0 by slightly reducing the W loading.