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

摘要

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>1wt% 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-0 bond strength with the decrease of the Pt crystallite size could also inhibit the reaction.