Thermal Stability of Pt Nanoparticles Supported on WO x /Al 2 O 3 for n-Heptane Hydroconversion
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Thermal Stability of Pt Nanoparticles Supported on WOx/Al2O3 for n-Heptane Hydroconversion J. L. Contreras*,1, G.A. Fuentes2, J. Salmones3, B. Zeifert4 1 Universidad Autónoma Metropolitana-Azco.CBI-Energía Av.San Pablo 180.C.P.02200, México D. F., Mexico. 2 Universidad Autónoma Metropolitana-Iztapalapa CBI, IPH, Apartado Postal 55534, México, D.F. 3 Instituto Politécnico Nacional, Lab. de Catálisis y Materiales ESIQIE. C.P. 07738, México D. F. 4 Instituto Politécnico Nacional, UPALM-ESIQIE. C.P. 07738, México D. F. * Corresponding autor: [email protected] ABSTRACT The thermal stabilization of γ-Al2O3 using W+6 ions has been found useful to the synthesis of Pt/Al2O3 catalysts. The sequential impregnation method was used to study the effect of W6+ upon Pt/ γ-Al2O3 reducibility, Pt dispersion, Raman spectroscopy and n-heptane hydroconversion. The W/Pt atomic ratios varied from 3.28 to 75. We found that the W6+ ions delayed reduction of a fraction of Pt+4 atoms beyond 773 K. At the same time, W6+ inhibited sintering of the metallic crystallites once they were formed on the surface. For the sample with a W/Pt atomic ratio of 3.28, W6+ did not inhibit the H2 reduction of Pt oxides even below of 773 K, the Pt oxides were reduced completely, however, the Pt dispersion decreased for this sample with respect to the Pt/γ-Al2O3 catalyst. After reduction at 1073 K, sequential samples impregnating Pt on WOx/γAl2O3 were more active and stable during n-heptane hydroconversion than monometallic Pt/γAl2O3 catalyst. Selectivities for dehydrocyclization, isomerization and Hydrocracking changed significantly when the W/Pt atomic ratio and reduction temperature increased. Initial and final reaction rates were more sensitive to reduction temperature. W6+ ions promoted high thermal stability of Pt crystallites when sequential catalysts were reduced at 1073 K and deactivation of bimetallic catalysts reduced at 773 K and 1073 K was less than the deactivation of Pt/Al2O3 catalyst. INTRODUCTION The loss of active surface area produced by sintering is an important cause of catalyst deactivation during operation in catalytic reforming and has evident economic implications, particularly in the case of noble metal catalysis [1]. As a result, stabilization of the catalytically active surface groups during harsh operating conditions is a primary design consideration in the development of industrial catalysts. Different synthetic strategies are employed in order to stabilize the metallic sites. One of the most common methods is the use of textural or structural promoters which modify either the support or the metallic phase [1]. Promoters such as W [2,34,5-10], Re [11], Sn [12 ,13], Ge [14], and Ir [15] interact with platinum or with the Al2O3 support to get more stability [13,8] and to increase the octane number of the products [12]. The bimetallic catalysts can be performed under better operation conditions in the reforming process, such as lower pressure and high reaction temperature [13, 12] [16 -22]. Among the different options, the sub-mon
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