We investigated the interaction of CO with graphene/Ni(1 1 1) and the Boudouard reaction at 3.7 mbar by Near Ambient Pressure X-Ray Photoemission Spectroscopy (NAPXPS), i.e. at one order of magnitude higher pressure than previously explored in-operando conditions. In this regime, CO intercalates under the graphene layer causing its partial detachment from the Ni substrate. The so-obtained high local CO coverage opens the way to CO₂ formation via the Boudouard reaction. Its onset is witnessed by observing physisorbed CO₂ accumulating below the graphene cover. The so-generated additional carbon atoms transform carbide into graphene, causing the expansion of the graphene islands. In addition, CO adsorption occurs on the strongly interacting areas of the graphene layer, confirming previous results obtained by some of us at low temperatures and in ultra-high vacuum conditions.