CO Oxidation on PdO(101) during Temperature-Programmed Reaction Spectroscopy: Role of Oxygen Vacancies

We investigated the oxidation of CO on PdO(101) using temperature-programmed reaction spectroscopy (TPRS), reflection absorption infrared spectroscopy (RAIRS), and density functional theory (DFT). We find that about 71% of the CO molecules adsorbed in a saturated layer on PdO(101) transform to CO2 during TPRS, with the CO2 desorbing in two main features centered at 330 and 520 K. RAIRS shows that CO molecules initially adsorb in an atop configuration on coordinatively unsaturated (cus) Pd sites of PdO(101) located next to Ocus atoms, yielding a RAIRS peak at 2135 cm(-1), and that the oxidation of these species produces the CO2 TPRS peak at 330 K. Concurrent with reaction, a large fraction of CO molecules migrates to atop-Pdcus sites located next to Ocus atom vacancies (Ov) that are created during reaction, as evidenced by the appearance of a RAIRS peak centered at similar to 2085 cm(-1). Our RAIRS measurements demonstrate that oxidation of the CO-Pdcus/Ov species is responsible for the CO2 TPRS peak at 520 K, and further show that oxygen atoms from the subsurface readily fill Ov sites as CO molecules vacate the Pd-cus/O-v sites above about 400 K. DFT calculations show that a strong enhancement in binding (similar to 70 kJ/mol) is responsible for the rapid migration of CO molecules from Pd-cus/O-cus sites to Pd-cus/O-v sites as the PdO(101) surface is reduced at low temperature. DFT also predicts that both CO species can access facile pathways for oxidation on PdO(101) via reaction with O-cus atoms, wherein the apparent reaction barriers are nearly identical in each pathway.