Cortés-Jácome M A, Toledo J A, Angeles-Chavez C, Aguilar M, Wang J A
Instituto Mexicano del Petróleo, Prog. Ingeniería Molecular, Eje Central L. Cardenas # 152, 07730 México DF, México.
J Phys Chem B. 2005 Dec 8;109(48):22730-9. doi: 10.1021/jp053255d.
WO3-ZrO2 catalysts were synthesized by precipitating the aqueous solutions of zirconium oxynitrate and ammonium metatungstate with ammonium hydroxide. The white slurry precipitate was treated under three different conditions. In the as-made materials, the amorphous phase was formed in the aged and refluxed samples, while well-crystallized tetragonal and monoclinic phases were obtained in the hydrothermally treated sample. The real amount of tungsten loaded in the samples was similar for the three samples, independently of the treatments; however, the tungsten surface atomic density in the annealed WO3-ZrO2 samples varied between 6 and 9 W atoms/nm2. Two different contrast types of aggregates were determined by scanning electron microscopy, the white particles which are rich in W, and the gray ones which are rich in zirconium; both of them were formed in the calcined solids prepared under aging or reflux condition. A very high dispersion of tungsten species on the zirconia surface was achieved in the hydrothermally treated sample. The degree of the interaction between WO(x) and ZrO2 surface strongly modified the Zr-O bond lengths and bond angles in the structure of tetragonal zirconia as proved by X-ray diffraction analysis and the Rietveld refinement. The catalyst obtained under hydrothermal condition exhibited the highest dispersion of tungsten species in the zirconia, which in turn causes strong structural deformation of the tetragonal ZrO2 phase responsible of the strongest surface acidity and, consequently, the optimum catalytic activity for n-hexane isomerization.
通过用氢氧化铵沉淀硝酸氧锆和偏钨酸铵的水溶液来合成WO₃-ZrO₂催化剂。对白色淤浆沉淀物进行了三种不同条件的处理。在刚制备的材料中，老化和回流样品中形成了非晶相，而水热处理样品中获得了结晶良好的四方相和单斜相。这三个样品中负载的钨的实际量相似，与处理方式无关；然而，退火后的WO₃-ZrO₂样品中钨的表面原子密度在6至9个W原子/纳米²之间变化。通过扫描电子显微镜确定了两种不同对比度类型的聚集体，富含W的白色颗粒和富含锆的灰色颗粒；它们都在老化或回流条件下制备的煅烧固体中形成。在水热处理的样品中，钨物种在氧化锆表面实现了非常高的分散。X射线衍射分析和Rietveld精修证明，WO(x)与ZrO₂表面之间的相互作用程度强烈改变了四方氧化锆结构中的Zr-O键长和键角。在水热条件下获得的催化剂在氧化锆中表现出最高的钨物种分散度，这反过来又导致四方ZrO₂相发生强烈的结构变形，从而产生最强的表面酸性，因此对正己烷异构化具有最佳的催化活性。
