Metal dispersion, accessibility and catalytic activity in methane oxidation of mesoporous templated aluminosilica supported palladium Original Research Article

Palladium catalysts using templated mesostructured porous silicas and aluminosilicas of MCM-41 type as supports were synthesized with various metal loadings and particle sizes as well as different metal accessibilities to the reactants. The metal was deposited by reacting an aqueous solution of [Pd(NH3)4]2+ complexes with the support, the template of which was partially extracted. The evolution of the support characteristics was monitored at different steps of preparation using X-ray diffraction (XRD), N2 physisorption and transmission electron microscopy (TEM). Particle size and metal accessibility were evaluated combining information from XRD line broadening, TEM observations, UV–visible spectra and H2 uptakes. The PdO reducibility was investigated using temperature-programmed reduction (TPR). In the templated mesoporous aluminosilicate, 27Al-MAS-NMR revealed that Al3+ occupied mostly tetrahedral sites, a fraction of which adopted an octahedral environment in the presence of palladium. According to the light-off curves, the catalytic methane oxidation activity is enhanced in pure silica supports where partial pore wall collapse has occurred. In contrast, for aluminosilica supports the beneficial effect of Al3+ on metal dispersion and catalytic activity was counterbalanced by partial metal encapsulation. Optimizing palladium particle size and avoiding as much as possible metal encapsulation give rise to catalysts more active than the conventional alumina supported palladium catalysts.