The effect of the use of lanthanum-doped mesoporous SBA-15 on the performance of Pt/SBA-15 and Pd/SBA-15 catalysts for total oxidation of toluene

Abstract

Abstract Platinum and palladium were supported on a mesoporous SBA-15 for total oxidation of toluene. The effects of the use of lanthanum-doped SBA-15 on the performance of Pt/SBA-15 and Pd/SBA-15 catalysts and on the total oxidation of toluene were evaluated. Lanthanum-substituted mesoporous SBA-15 molecular sieve has been prepared by direct synthesis and post-synthesis procedures using lanthanum nitrate as source. By post-synthesis procedure, La was ion-exchanged on calcined silica. Characterizations by atomic absorption, X-ray diffraction, nitrogen adsorption, 29Si MAS NMR and FT-IR spectroscopy have been carried out to evaluate the efficiency of the incorporation of lanthanum by these procedures. Lanthanum was well incorporated into the SBA-15 structure only by post-synthesis procedure. Structural and textural evolutions of LaSBA-PS (post-synthesis) material were studied according to the Si/La ratio. These LaSBA-15 mesoporous molecular sieves retain the hexagonal order and physical properties of siliceous SBA-15 material. After removing the surfactant molecules by calcinations, 1 wt% of platinum or 0.5 wt% of palladium was impregnated on SBA-15 or LaSBA-PS (Si/La = 20) via incipient wetness technique using respectively hexachloroplatinic acid and palladium nitrate as the Pt and Pd sources. The activity for toluene total oxidation of the pre-reduced samples follows this order: 0.5 wt% Pd/SBA-15 > 1 wt% Pt/SBA-15 > 0.5 wt% Pd/LaSBA-PS > 1 wt% Pt/LaSBA-PS. 0.5 wt% Pd/SBA-15 was found to be a very interesting catalyst for VOC oxidation. However, the lower activity of the lanthanum catalysts could be explained by patches of LaOX residing on the surface of the metal particles (particularly on Pd) and/or to the interaction between the metal and LaSBA-PS which modifies the electron density of the metal. For the Pt-based catalyst the observed higher dispersion of platinum species for Pt/LaSBA-PS and thus to the increase of the Pt–O bond strength with the decrease of the Pt crystallite size could also inhibit the reaction.