  Adsorption of CO at the Sr$_3$Ru$_2$O$_7$(001) surface was studied with
low-temperature scanning tunneling microscopy (STM) and density functional
theory. In situ cleaved single crystals terminate in an almost perfect SrO
surface. At 78 K, CO first populates impurities and then adsorbs above the
apical surface O with a binding energy E$_\mathrm{ads}$=-0.7 eV. Above 100 K,
this physisorbed CO replaces the surface O, forming a bent CO2 with the C end
bound to the Ru underneath. The resulting metal carboxylate (Ru-COO) can be
desorbed by STM manipulation. A low activation (0.2 eV) and high binding (-2.2
eV) energy confirm a strong reaction between CO and regular surface sites of
Sr$_3$Ru$_2$O$_7$; likely, this reaction causes the "UHV aging effect" reported
for this and other perovskite oxides.
