Active Surface Oxygen for Catalytic CO Oxidation on Pd(100) Proceeding under Near Ambient Pressure Conditions

Abstract

Catalytic CO oxidation reaction on a Pd(100) single-crystal surface under several hundred mTorr pressure conditions has been studied by ambient pressure X-ray photoelectron spectroscopy and mass spectroscopy. In-situ observation of the reaction reveals that two reaction pathways switch over alternatively depending on the surface temperature. At lower temperatures, the Pd(100) surface is covered by CO molecules and the CO2 formation rate is low, indicating CO poisoning. At higher temperatures above 190 °C, an O-Pd-O trilayer surface oxide phase is formed on the surface and the CO2 formation rate drastically increases. It is likely that the enhanced rate of CO2 formation is associated with an active oxygen species that is located at the surface of the trilayer oxide.

Reversed Hysteresis during CO Oxidation over Pd75Ag25(100)

“…Although some studies indicate a chemisorbed oxygen phase as the active surface, 32,44 the emergent trend points to the surface oxide 33,35,38,39 as the most active phase under oxygen rich conditions. The CO oxidation reaction under ultrahigh vacuum (UHV) conditions can be described by a Langmuir-Hinshelwood mechanism.…”

“…[45][46][47][48][49] Under more realistic conditions, several studies indicate that the mechanism is of Mars van Krevelen type, where the catalyst surface is oxidized and gas phase CO interacts with the oxide to form CO 2 . [30][31][32][33]35,[37][38][39][40]50 In the present work, we have investigated the influence of Ag as alloying element in Pd catalysts through comparison of the CO oxidation reaction over Pd(100) and Pd 75 Ag 25 (100).…”

“…[30][31][32][33][34][35][36][37][38][39][40] In the case of Pd(100), it has been shown experimentally that the higher activity of this surface towards CO oxidation at these conditions coincides with the presence of the √ 5 surface oxide. 30,32,35,38,39,41 Kinetic Monte-Carlo simulations support the view that a surface oxide on Pd(100) could be responsible for increased reactivity.…”

Near Ambient Pressure XPS Investigation of CO Oxidation Over Pd3Au(100)

“…Surface oxides rather than surfaces covered by chemisorbed oxygen have been 35 observed as the most active towards CO oxidation under near ambient as well as more realistic 36 conditions (above ambient pressure) [5][6][7][8][9][10][11][12][13][14][15][16]. However, some studies also indicate Pd surfaces covered 37 by atomic oxygen as highly active [8,17], and generally both will exhibit activity.…”

“…However, some studies also indicate Pd surfaces covered 37 by atomic oxygen as highly active [8,17], and generally both will exhibit activity. For Pd(100), the 38 presence of a (√5x√5)R27° surface oxide (henceforth denoted √5) is found to exist when the surface 39 is highly active towards CO oxidation [5,6,8,10,14,17,18], and this is consistent with the reaction 40 following a Mars-van Krevelen mechanism with gas-phase CO reacting with the surface oxide to 41 form CO2 [5][6][7]9,11,[13][14][15]19]. The presence of the surface oxide during high CO2 production is also 42 supported by kinetic Monte-Carlo simulations [20,21].…”