NMR relaxometry investigation about the role of a protective silica layer on Pd/C catalyst

This work explored the activity and selectivity of Pd nanoparticles (NPs) supported on carbon black (Pd/CB) in the presence of a protective mesoporous silica layer (Pd/CB@SiO2). The performances of these catalysts were compared for the hydrogenation of benzaldehyde, observing, in addition to the strong stabilizing impact of the presence of silica layer which maintained its activity up to 4 reaction cycles, differences in the reaction kinetic, particularly concerning product formation and selectivity. Indeed, besides its proper function of protecting Pd NPs, the porous layer of SiO2 could influence the catalyst-reactant interaction, which in turn modifies the selectivity of the reaction. Therefore, in parallel to the conventional analysis of the catalytic experimental data, the effect of the presence of the silica layer has been studied by NMR relaxometry, as a non-conventional approach of interface characterization operating in the presence of solvent. Considering the hydrogenation of benzaldehyde to benzyl alcohol and toluene, the presence of the SiO2 layer did not influence the activity of the catalyst in terms of reaction rate, but it changed the formation of the products. NMR analyses disclosed the reason on the basis of such difference, showing that the silica layer tends to increase the interaction of the aromatic ring with the catalyst surface, thereby controlling how the molecule approaches the surface of the catalyst active sites.