Positron annihilation spectroscopic investigation of pillared clay-based catalytic hybrid nanocomposites

Positron annihilation lifetime spectra for pillared montmorillonite clay (PILC), as encapsulated with chitosan (CS) chains, supported with Pd nanoparticles, fresh and recovered PILC-CS@Pd heterogeneous catalysts, along with the starting montmorillonite clay (MC) were measured. After pillaring modification, as the interlayer nano-space expanded, the sub nanometer level free-volume nanohole size increases obviously from 0.182 nm (MC) to 0.246 nm (PILC). Then, as the CS chains and active Pd species encapsulated within the interlayer nano-space, the o-Ps lifetime (τ3) and intensity (I) undergo a continuous decrease, indicating the decrease of the free-volume nanohole size and concentration. Meanwhile, in practical catalytic application scenarios, it is found that the variations of the free-volume parameters show close correlation with the recycling aging behavior and catalytic performance decline of the catalytic nanocomposites. The decrease in density of sub nanometer level molecular stacking due to the leaching of Pd species and degradation of CS chains in the interlayer galleries nano-space of the recovered PILC-CS@Pd hybrid nanocomposite has been sensitively detected by the o-Ps probe.