Role of Support in Lean DeNOxCatalysis

Mesoporous Pt–SiO2 and Pt–SiO2–Ta2O5 Catalysts Prepared Using Pt Colloids as Templates

Sol-gel synthesis of silica and silica-tantalum oxide embedded platinum nanoparticles is carried out using Pt colloids as templates. These colloids are prepared by reduction with Na[AlEt(3)H] and stabilized with different ligands (ammonium halide derivatives, non-ionic surfactants with polyether chains, and 2-hydroxy-propionic acid). The aim of the present study is to prepare mesoporous silica embedded Pt colloids combining the "precursor concept" with the model of catalyst preparation using preformed spheres. Nanoparticles of Pt incorporated in high surface area mesoporous materials are formed after calcination. Further, it is observed that calcination of these catalysts causes partial aggregation and oxidation of the parent colloids, a process that is largely dependent on the nature of the stabilizing ligands. Several methods have been used for characterization of these materials: adsorption-desorption isotherms at 77 K, H(2) chemisorption, X-ray diffraction(XRD), (29)Si and (13)C magic angle spinning (MAS) NMR, ammonia diffuse reflectance Fourier transform infrared spectroscopy (NH(3)-DRIFT), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). It is found that both metal oxide systems exhibit Brønsted acidity (weaker for silica and quite strong for silica-tantalum oxide). In addition, NH(3)-DRIFT experiments demonstrate the oxidative properties of the surface. Part of the adsorbed NH(4) (+) species is oxidized to N(2)O. Testing these catalysts in the reduction of NO and NO(2) with isopentane under lean conditions indicate that the activity of these catalysts is indeed dependent on the size of the platinum particles, with those of size 8-10 nm demonstrating the best results. The support likely contributes to this effect, particularly after Ta incorporation into silica.