# Reverse Spillover Dominating CO Adsorption on Single Cobalt Atoms in Graphene Divacancies

## TL;DR

This study shows that carbon monoxide (CO) adsorbs onto single cobalt atoms in graphene mainly through a reverse spillover mechanism, significantly increasing the sticking probability.

## Contribution

The paper introduces a new understanding of CO adsorption on single cobalt atoms in graphene via reverse spillover, not direct impingement.

## Key findings

- CO adsorption on single Co atoms in graphene occurs mainly (up to 97%) through reverse spillover.
- Reverse spillover increases the sticking probability by up to two orders of magnitude compared to direct impingement.
- The study determines key energy barriers for CO diffusion and adsorption on graphene and cobalt.

## Abstract

The kinetics of molecular
adsorption and desorption can unveil
the details of the adsorption potential that impact, for instance,
the overall sticking probability. This information is of particular
importance for catalysis and gas sensing. We investigate the room-temperature
CO adsorption on a model single-atom catalyst consisting of single
Co atoms trapped in graphene (Gr) double carbon vacancies during Gr
growth by chemical vapor deposition (CVD) on Ni(111). The study is
conducted by combining a thermal desorption spectroscopy (TDS) instrument
that allows the study of systems with a very low surface density of
active sites, of the order of 10–3 monolayers (MLs)
with variable-temperature scanning tunneling microscopy (VT-STM).
Our findings show that CO adsorption onto the single Co atoms occurs
mainly (up to 97%) through a reverse spillover mechanism, rather than
through direct impingement from the gas phase. This mechanism involves
CO physisorption and diffusion on pristine Gr, followed by lateral
adsorption onto Co atoms. The reverse spillover channel effectively
increases the sticking probability, by up to 2 orders of magnitude,
compared with direct impingement. We use kinetic models to determine
the relevant energies, such as the diffusion barrier for CO on Gr
(68 ± 15 meV), the energy barrier for lateral CO adsorption on
Co (174 ± 2 meV), and the chemisorption energy of CO on Co (0.97
± 0.02 eV).