Identifying Site-dependent Reactivity in Oxidation Reactions on Single Pt Particles

Catalytic nanoparticles are heterogeneous in their nature and even within the simplest particle
various surface sites clearly exist and influence the catalytic reactivity. A direct analysis of
site-dependent reactivity cannot be easily obtained by conducting averaging, ensemble-based
spectroscopy measurements. Thus, detailed chemical information at the nanoscale is essential
for understanding how surface properties and reaction conditions direct the reactivity of
various sites on the surface of catalytic nanoparticles. In this work, hydroxyl functionalized
N-heterocyclic carbene molecules (NHCs) were anchored to the surface of Pt particles and
utilized as chemical markers to detect reactivity variations between different surface sites
under liquid and gas phase oxidizing conditions. Differences in the chemical reactivity of
surface-anchored NHCs were identified using synchrotron-radiation-based infrared
nanospectroscopy with a spatial resolution of 20 nanometers. By conducting IR
nanospectroscopy measurements, along with complementary ensemble-based measurements,
we have identified that enhanced reactivity occurs on the particles’ periphery under both gas
and liquid phase oxidizing conditions. Under gas phase reaction conditions, the NHCs’
hydroxyl functional groups were preferably oxidized into acid along the perimeter of the
particle. Exposure of the sample to harsher, liquid phase oxidizing conditions induced
deformation of NHCs, which was located as well at the particle’s periphery. These results
demonstrate, based on single particle measurements, the high reactivity of low coordinated
surface sites, which are located on the nanoparticle’s periphery and the influence of various
reaction conditions on site-dependent reactivity.