CO oxidation on PdO surfaces

Abstract

Density functional calculations were performed in order to investigate CO oxidation on two of the most stable bulk PdO surfaces. The most stable PdO(100) surface, with oxygen excess, is inert against CO adsorption, whereas strong adsorption on the stoichiometric PdO(101) surface leads to favorable oxidation via the Langmuir-Hinshelwood mechanism. The reaction with a surface oxygen atom has an activation energy of 0.66 eV, which is comparable to the lowest activation energies observed on metallic surfaces. However, the reaction rate may be limited by the coverage of molecular oxygen. Actually, the reaction with the site blocking molecular oxygen is slightly more favorable, enabling also possible formation of carbonate surface species at low temperatures. The extreme activity of strongly bonded surface oxygen atoms is more greatly emphasized on the PdO(100)-O surface. The direct reaction without adsorption, following the Eley-Rideal mechanism and taking advantage of the reaction tunnel provided by the adjacent palladium atom, has an activation energy of only 0.24 eV. The reaction mechanism and activation energy for the palladium activated CO oxidation on the most stable PdO(100)-O surface are in good agreement with experimental observations.

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

“…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.…”

“…This is supported by DFT calculations predicting that the barrier for the reaction between CO(ads) and an oxidic O is lower than the energy needed to desorb. [130][131][132] Alternatively, at higher temperature, the CO oxidation rate could become too fast compared to the replenishment rate of O vacancies, leading to a partial decomposition of the surface oxide. This would result in a lower concentration of available O and a lower CO 2 production.…”

“…Restoring oxygen on the PdO(100) surface is more problematic and it was proposed to occur on metallic Pd after partial reduction. 132 …”

“…However, this Wulff construction is in variance with experimentally grown PdO single crystals, which show large PdO(001) and PdO(110) facets. 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.…”

“…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.…”