Surface engineering and palladium dispersion in MCM-41 for methane oxidation

Highly dispersed palladium oxide nanoparticles (Ø ∼ 2 nm) were obtained in mesostructured silica of MCM-41 type from divalent palladium complexes tethered from grafted propylamine ligands themselves highly dispersed using a surface engineering approach based on a molecular stencil patterning technique (MSP). A partial surface coverage obtained by incomplete extraction of the surfactant produces a self-assembled and patterned molecular mask that is fixed by silanol capping. After removal of the masking surfactant, the unreacted silanol groups are reacted with 3-(aminopropyl) trimethoxysilane ligands, which are further metallated using an acetonitrile solution of palladium trifluoroacetate Pd(O2CCF3)2. Then, calcination yields PdO nanoparticles of ca. 2 or 6 nm depending of the masking percentage of the surface. In some case, calcination was accompagnied by a partial collapse that is undergone by the structure. The structural and textural characteristics of these materials were studied by means of different physico-chemical techniques such as XRD, N2 sorption isotherms, TGA, elemental analysis (EA) by ICP, 29Si-NMR, UV–visible, FT-IR, and Raman spectroscopies. Light-off reactivity curves of methane combustion reveals that these catalysts possess no external PdO phase and follow the gold rule of higher activity for higher active phase dispersion. Eventually, intermediate structural collapse that decreases the molecular diffusion limitation may invert this tendency.