Ordered Mesoporous Silica-Based Precious Metal Catalysts for NO Reduction

Ordered mesoporous silicas (OMS), e.g., MCM-41, MCM-48, SBA-15, SBA-3, IITM-56, were used as supports for the incorporation of noble metals such as Ru, Rh, Pd and Pt. The supported catalysts were prepared by homogeneous deposition precipitation (Hdp), deposition precipitation (Dp) and impregnation (Imp) methods. All the samples were systematically characterized by various analytical, spectroscopic and imaging techniques. These studies suggest that the deposition of the noble metal does not affect the pore structure of the various silica supports. In particular, TEM investigations indicate the presence of highly ordered pore structures and that the deposited noble metals are well-dispersed with nearly uniform particle size distribution. However, it was noticed that the particle size of the noble metals depends not only on the method of catalyst preparation but also on the type of metals used. For example, Rh and Pd deposited catalysts, viz., Rh/OMS and Pd/OMS, demonstrate the formation of very small particle sizes (3-4 nm) as compared to that of silica-Ru/OMS and Pt/OMS catalysts. The catalytic activity of all the supported systems was tested for the reduction of NO by CO. It was observed that the performance of the studied catalysts depends on the method of preparation, the noble metal employed as well as the OMS used as support. Among the various supports, the three-dimensional pore systems, i.e., MCM-48, preferred choice for the metal loading. Further, among the different preparation processes employed, the Hdp method was found to be most active and that the Rh/MCM-48 is effective even at lower temperature. This study also demonstrates that the catalytic activity of the various supported systems follow the following order: Rh/OMS > Ru/OMS > Pd/OMS > Pt/OMS. These finding are in good agreement with our previous theoretical analyses based on the activation energy for the dissociation of the adsorbed NO.