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

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).