Modelling of Desorption from Small Metal Clusters

TL;DR

This paper models desorption processes from small metal clusters, highlighting size-dependent effects on adsorption sites and spectra, and compares simulations with experimental data for platinum clusters on zeolite.

Contribution

It introduces a model accounting for size-dependent adsorption site transformation and desorption energy in small metal clusters, validated by TPD spectrum simulations.

Findings

Desorption energy depends explicitly on cluster size.

IR spectra show an extra band varying with cluster size.

Simulation results align with experimental TPD spectra.

Abstract

It is found out that there are physical effects, typical of small metal clusters deposited on a non-adsorbing support, that cause a difference between the adsorption properties of sites at the metal cluster surface and those of sites at the boundary of metal-support. This difference gives rise to an extra band in the IR spectra of the adsorbate whose major property is that its intensity changes with cluster size. The migration of chemisorbed molecules (atoms, ions) and molecules (atoms, ions) in any excited state over adsorption sites of different type results in changes in the state of the molecule (atom, ion) since the latter adjusts its state to the set of levels of another type of site. This can be described figuratively as a transformation of the type of adsorption site. Thus, a single type of adsorption site, called next surface effective site (SES), is formed. The energy of desorption from SES site retains an explicit dependence on the metal cluster size. A simulation of TPD spectra of oxygen desorption from small Pt clusters deposited on NaX zeolite was carried out. A comparison to experimental TPD spectra obtained by Jaeger and co-workers for the same system is made.