Energetics of elementary reaction steps relevant for CO oxidation: CO and O2 adsorption on model Pd nanoparticles and Pd(111)

Abstract

The energetics of elementary surface processes relevant for CO oxidation, particularly CO and 02 adsorption, were investigated by a direct calorimetric method on model Pd nanoparticles and on the extended Pd(111) single crystal surface. The focus of this study lies on a detailed understanding of how a nanometer scale confinement of matter affects the binding strength of gaseous adsorbates. We report adsorption energies and sticking coefficients of CO and 02 measured as a function of the adsorbate surface coverage both on pristine and O-covered Pd surfaces. The reduced dimensions of the Pd substrate were found to affect the binding strength of the adsorbates in two principle ways: (i) via the change of the local adsorption environment that can result e.g. in stronger adsorbate bonding at the particle's low coordinated surface sites and (ii) via the contraction of the Pd lattice in small clusters and a concomitant weakening of chemisorptive interaction. Particularly for 02 adsorption, the change of the adsorption site from a three-fold hollow on Pd(111) to the edge site on Pd nanoparticles (approximately 4 nm sized on average) was found to result in a strong increase of the Pd-O bond strength. In contrast, CO adsorbs weaker on Pd nanoparticles as compared to the extended Pd(111) surface. In total, the binding energies of adsorbates on Pd and with this their surface coverages turn out to depend in a non-monotonic way on the particular structure of Pd surfaces, including the local structure of the adsorption site as well as the global properties of the small clusters arising e.g. from the lattice contraction.

Influence of support morphology on the bonding of molecules to nanoparticles

“…This is because the Pd particle size of 20-30 nm probed here already reaches the bulk limit of Pd(111) as reflected in temperature-programmed desorption and single-crystal adsorption calorimetry measurements of adsorbed CO. Naturally, our results therefore have no implications on this controversial issue (15). As for the chemical composition, our X-ray photoelectron spectroscopy (XPS) data suggest that the nanoparticles are composed of only Pd.…”

“…For example, Peter et al (15) point out that Pd nanoparticles less than 8 nm in diameter are compressively strained, and this leads to weaker CO adsorption. However, unlike the strain that arises from the particle size effect (15) or that from lattice mismatch, which is periodic and incremental in nature, the strain that is caused by particle growth across the substrate step edges is highly localized and abrupt. It may therefore have a more profound effect on gas adsorption in the vicinity of the underlying substrate steps or other lateral discontinuities.…”

“…All these sites have been shown to play a crucial role in some reactions (10, 11). Reducing the particle size can also lead to a decrease in the interatomic bond length in small metal clusters (12, 13), which in the case of Pd nanoparticles results in lower adsorption energies for both CO (14,15) and O 2 (16), although such weakening of CO binding on the nanoparticle can also arise from other factors such as encapsulation of the nanoparticles by the support (17).…”

“…All these sites have been shown to play a crucial role in some reactions (10, 11). Reducing the particle size can also lead to a decrease in the interatomic bond length in small metal clusters (12, 13), which in the case of Pd nanoparticles results in lower adsorption energies for both CO (14,15) and O 2 (16), although such weakening of CO binding on the nanoparticle can also arise from other factors such as encapsulation of the nanoparticles by the support (17).The role of the support in modifying nanoparticle properties has also been recognized. For instance, the strong metal support interaction has been known for some time (2) and charge transfer either to or from the nanoparticles can lead to enhanced reactivity (18).…”

Kinetics of CO oxidation on palladium using microkinetics coupled with reaction route analysis

“…UHV studies (Conrad et al, 1974;Peter et al, 2013a) suggest a sticking coefficient of 0.8 -1 and 0.7 − 0.8 for CO adsorption on Pd (111) surface at zero coverage (ș CO = 0), respectively. The pre-exponential of CO desorption can be coverage dependent and is ϭϯ proposed to be 10 15.3 s −1 for CO desorption on clean Pd (111) surface under UHV conditions .…”