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 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, 29 Si 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 LaO X 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.