Colloidal Nanoparticles Embedded in Ceramers: Toward Structurally Designed Catalysts

Ceramers constitute a new type of porous hybrid ceramic which is obtained by pyrolysis of polysiloxane precursors with organic side groups at temperatures low enough that part of the polymer has not yet been decomposed (approximate to 500 degrees C). By using different precursors, interesting possibilities arise to change the structural and mechanical properties of this monolithic material over a wide range. Moreover, the surface chemical properties (such as basicity or hydrophilicity) can be modified by varying the organic side groups. In order to add a catalytic function to such a ceramer, different preparation techniques were employed in this study with the objective of incorporating nanoparticles into the ceramers (addition of ionic precursors vs preformed colloidally synthesized nanoparticles during the synthesis). The resulting materials were structurally characterized and catalytically studied using the oxidation of CO as a test reaction. Our results reveal that especially colloidally prepared nanoparticles provide attractive options to manufacture tailored catalysts since the particle sizes, as controlled by the colloidal synthesis, can be well-preserved. It could be shown that aminopropyltriethoxysilane (APTE) leads to a homogeneous distribution of nanoparticles in the ceramer matrix and an enhanced CO oxidation activity after sample activation, an effect that can be attributed to a more effective binding of the Pt nanoparticles to the precursors of the material hindering sintering and agglomeration. The mass transport limitation due to pore diffusion was characterized for samples in the form of larger grains and finely ground powders by determining the macro kinetics (reaction orders and activation energies) and calculating the Thiele modulus. The comparison reveals mass transport limitation due to microporosity. We discuss strategies to avoid such restrictions and optimize mass transport of gaseous reactants paving the way to ceramer-based monolithic catalyst with high structural stability and an optimized distribution of monodisperse nanoparticles.