Confining the Synthesis of Palladium Nanoparticles in Mesoporous Silicas for CO Oxidation: The Role of the Support

The synthesis of palladium (Pd) nanoparticles (NPs) in the highly constrained pore network of mesoporous silicas is systematically investigated for the first time, comparing the synthesis outcomes with a reference sample of Pd NPs on the outer surface of nonporous silica. Two different families of mesoporous silica (SBA-15 and KIT-6), with different pore arrangements and connectivity, as well as two different pore sizes within both porous systems (i.e., 7 and 9 nm), are investigated as supports for Pd NPs synthesis, and the effect of mesopore confinement is explored. In particular, the mesoporous matrixes allow one to obtain smaller and homogeneously distributed Pd NPs with respect to the nonporous support, and their actual location within the mesopore-constrained environment is confirmed by electron tomography. Moreover, the application of Pd NPs on mesoporous and nonporous silicas as heterogeneous catalysts for CO oxidation is explored, and the noticeable stability of Pd NPs on the supports is assessed by probing the accessibility and dynamic response of Pd species by operando Pd K-edge X-ray absorption spectroscopy measurements. Oxidized Pd species are found to be the active phase of the catalytic reaction, and the complex interplay of several factors occurring in regulating the catalytic activity of the supported NPs is discussed.