Dynamics of Pd on nanocarbon in the direct synthesis of H2O2

This work aims to clarify the nano-structural transformation accompanying the loss of activity and selectivity for H2O2 synthesis of Pd and AuPd nanoparticles supported on N-functionalized carbon nanotubes (NCNTs). High resolution XPS allowed discriminating metallic Pd, electronically-modified metallic Pd hosting impurities and cationic Pd. This is paralleled by morphological heterogeneity observed by HRTEM where nanoparticles with 2 nm average size coexist with very small Pd clusters. The morphological distribution of Pd is modified after reaction through sintering and dissolution/redeposition pathways. The loss of selectivity is correlated to the extent of these processes occurring as a result of the particle instability at the carbon surface. We assign beneficial activity in the selective hydrogenation of oxygen to the Pd clusters with a modified electronic structure as compared to Pd metal or Pd oxides. These beneficial species are formed and stabilized on carbons modified with N atoms in substitutional positions. The formation of larger metallic Pd particles not only reduces the number of active sites for the synthesis but enhances the activity for the deep hydrogenation to water. The structural instability of the active species is thus detrimental in a dual way. Minimizing the chance of sintering of Pd clusters by all means is thus the key to better performing catalysts.