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 ∼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 Pdcus/Ov sites above about 400 K.
DFT calculations show that a strong enhancement in binding (∼70
kJ/mol) is responsible for the rapid migration of CO molecules from
Pdcus/Ocus sites to Pdcus/Ov 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 Ocus atoms, wherein
the apparent reaction barriers are nearly identical in each pathway.
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