Reverse Spillover Dominating CO Adsorption on Single\nCobalt Atoms in Graphene Divacancies

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