# CO Adsorption on Pd Nanoparticles: Assignment of Experimental C–O Vibrational Frequencies by DFT Calculations

## TL;DR

This paper uses DFT calculations to better understand CO adsorption on Pd nanoparticles and improve the interpretation of experimental vibrational frequencies.

## Contribution

A frequency-dependent scaling method is introduced to align DFT calculations with experimental CO vibrational frequencies on Pd nanoparticles.

## Key findings

- CO adsorption on bridge-edge sites of Pd nanoparticles is energetically feasible.
- CO–CO interactions increase the C–O stretching frequency by up to 100 cm–1.
- The saturation frequency of 1990 cm–1 matches experimental observations for Pd particles ≥3 nm.

## Abstract

Adsorption of CO probe molecules on metal catalysts is
widely used
to characterize the surface reactivity and morphology of these nanomaterials
by assigning measured C–O vibrational frequencies to particular
surface sites. Density-functional calculations of the corresponding
CO adsorption complexes provide key complementary data for such characterization.
However, even for the adequate structural models, the calculated frequencies
do not quantitatively match the experimental values due to approximations
in conventional generalized-gradient exchange–correlation functionals.
We proposed a frequency-dependent scaling of the density-functional
C–O frequencies for adsorption on different sites of nanostructured
Pd catalysts, enabling quantitative agreement with the reference experimental
values. Then, we computationally studied coverage-dependent bridge
CO adsorption on edge sites of Pd nanoparticles, which revealed the
energetic feasibility of the full CO occupation of these sites. Due
to the static and dynamic CO–CO interactions, the calculated
C–O stretching frequency grows by as much as 100 cm–1 from the singleton CO adsorbed value with the number of coadsorbates
at the neighboring bridge-edge sites. The saturation frequency approaches
1990 cm–1, quantitatively matching the value experimentally
observed for moderately large Pd particles. Using our frequency scaling,
such particles are estimated to be at least 3 nm large.