  The oxidation of CO on single Pd atoms anchored to MgO(100) surface oxygen
vacancies is studied with temperature-programmed-reaction mass-spectrometry and
infrared spectroscopy. In one-heating-cycle experiments CO$_2$, formed from
O$_2$ and CO preadsorbed at 90 K, is detected at 260 K and 500 K. Ab-initio
simulations suggest two reaction routes, with Pd(CO)$_2$O$_2$ and Pd(CO$_3$)CO
found as precursors for the low and high temperature channels, respectively.
Both reactions result in annealing of the vacancy and induce migration and
coalescence of the remaining Pd-CO to form larger clusters.
