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

Abstract

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.

Surface science under reaction conditions: CO oxidation on Pt and Pd model catalysts

“…135 Two main reaction pathways were observed, at 330 and 520 K. Furthermore, IRRA spectra recorded after increasingly higher annealing temperature indicated that the low-temperature pathway involved a reaction between CO(ads) and the pristine PdO(101) surface, resulting in O v . The remaining CO shifted in adsorption site to atop sites on Pd 3-fold in the O v vicinity.…”

“…130 The kMC model showed, furthermore, that while the CO oxidation barrier is rate limiting for the reaction on the metallic surface, the CO adsorption strength is limiting the reactivity of the surface oxide. However, CO binding could be much stronger if O vacancies are present in the surface oxide, 135 which would increase the reactivity of the oxide. Alternatively, reactions at the boundary between (metastable) CO(ads) islands and the surface oxide 69 could account for the reactivity observed in the PLIF experiment.…”

“…137 Nørskov and coworkers confirmed the stability of the PdO(100) and PdO(101) surfaces and studied their interaction with CO. 132 The PdO(100) surface does not bind CO, and CO oxidation proceeds via a MvK-ER mechanism. The PdO (101) 132,135 This barrier decreases to 54 kJ mol À1 or 0.56 eV for a saturated CO coverage. 135 A MvK-ER reaction was also considered but, although a similar barrier was found (68 kJ mol À1 or 0.7 eV), 132 was not deemed viable, due to a low pre-exponential factor.…”

“…The PdO (101) 132,135 This barrier decreases to 54 kJ mol À1 or 0.56 eV for a saturated CO coverage. 135 A MvK-ER reaction was also considered but, although a similar barrier was found (68 kJ mol À1 or 0.7 eV), 132 was not deemed viable, due to a low pre-exponential factor. This, in turn, is related to the unfavorable loss of entropy during the reaction.…”

Promotion of CO oxidation on PdO(101) by adsorbed H2O

“…These prior investiga-67 tions signal a need for further study of the effects of water on CO oxida-68 tion over transition metal oxides, particularly experimental work using 69 oxide surfaces with well-defined structure. 70 We have previously investigated CO adsorption and oxidation as 71 well as water adsorption on a crystalline PdO(101) thin film [19][20][21][22][23]. 72 We find that the oxidation of CO is facile on PdO(101) as more than 73 70% of the CO molecules in a saturated layer are oxidized to CO 2 during 74 temperature programmed reaction spectroscopy (TPRS) via reaction 75 with the oxide surface [21].…”