CO adsorption and dissociation on Ru(0001) at elevated pressures

► We use Ambient Pressure X-ray Photoelectron to study CO adsorption on Ru(0001). ► We provide evidence for a previously unobserved adsorption state. ► This state is observed at 300 K and pressures > 10− 6 Torr; it is stable to ~ 400 K. ► Based on O 1s binding energies this state is attributed to bridge-bound CO. ► Above 520 K we observe the build-up of carbon on the surface due to CO dissociation.