CO Chemisorption on Ultrathin MgO-Supported Palladium Nanoparticles

Abstract

The adsorption of CO on Pd nanoparticles (NP) supported on MgO ultrathin films is studied by sum frequency generation (SFG) vibrational spectroscopy at room temperature as a function of CO coverage for NP lateral sizes ranging from 3 to 6 nm. While the spectroscopic signature of CO on (100) top facets dominates the spectra, other new bands are attributed to (111) facets, edges, and defects. CO remains strongly bonded for all sizes, but spectroscopic and kinetic parameters evolve as NP size decreases. It is only for the smallest NPs ∼3 nm in diameter that the deviation from Pd(100) single crystal is important. Modeling of adsorption kinetics, dipolar coupling and DFT calculations allow rationalizing the observations: the size dependent Pd–Pd bond extension by the substrate is the main parameter that controls CO frequency in the low coverage limit. SFG sensitivity is found to increase up to 10–4 ML as NP size decreases. This makes SFG particularly well suited to probe molecular adsorption in the submonolayer range on small NPs of 3 nm in diameter and below. These results motivate one to probe CO under catalytic conditions and to follow electron transfer in pump–probe experiments.