Study of the Formation of WO x Nanostructures Supported on Zirconia Prepared by Different Synthesis Routes
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Study of the Formation of WOx Nanostructures Supported on Zirconia Prepared by Different Synthesis Routes Martha L. Hernández1, Ascención Montoya2, Paz Del angel2, Silvia P. Paredes1 and Sergio O. Flores1 1 Instituto Politécnico Nacional, ESIQIE, Av. IPN s/n Zacatenco, 07738 D. F., México 2 Instituto Mexicano del Petróleo, DIyP, Eje Central L. Cárdenas Norte 152, 07730 D.F., México
ABSTRACT Different synthesis procedures of Pt supported on tungstated zirconia catalysts (Pt/WOx-ZrO2) were investigated with the aim to elucidate the different WOx nanostructures developed on the zirconia surface depending on the preparation route. Pt/WOx-ZrO2 catalysts were synthesized by the coprecipitation and impregnation methods and pretreated by various procedures such as different calcinations temperatures or the use of reflux. The catalysts characterization was carried out by X-ray diffraction (XRD), Raman spectroscopy, transmission and scanning electron microscopy (TEM, SEM) and nitrogen physisorption, and the catalytic activity was evaluated in the n-hexane isomerization reaction. The results indicate that the development of active sites for isomerization of n-hexane is enhanced by the stabilization of the WOx nanostructures on the surface of zirconia, before the formation of the WO3 crystallites, and it largely depends on the synthesis method. INTRODUCTION In the last years, the isomerization processes have been extensively documented, mainly as a result of the growing demand for cleaner and high performance fuels, since the alkanes isomerization reactions provide gasoline of high octane number products [1-3]. Isomerization reaction is catalyzed by strong acids; it can be carried out using liquid acid catalysts such as HCl or H2SO4 as well as solid catalysts such as chlorinated alumina or Pt/mordenite, which present different problems of pollution and/or operation. The most promising alternatives to replace these systems are highly acidic solids such as sulfated zirconia (SZ) or tungstated zirconia (WZ). Although WZ is less active than SZ, tungstated zirconia catalysts offer higher stability under high-temperature treatments and reductive atmospheres, lower deactivation rates during catalysis, and easier regeneration [4]. In the case of the WZ catalysts, it has been found that the tungsten surface density (atoms W/nm2) determines the structure and catalytic activity [5]. It has been also proposed that the WOx species of intermediate size are reduced in the presence of H2 resulting in a delocalization of charge by interaction with the H-atom producing Brönsted acid sites. The surface density depends on the dispersion of the WOx species on the zirconia surface, and it has been reported that with a WOx coverage lower that the monolayer, the WOx species are difficult to reduce due to the strong interaction with the support, meanwhile when the tungsten loading increases, the WOx nanostructures become into large crystals of WO3 and therefore the active sites are inaccessible to the reactant and the catalytic activity decreases [4,5
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