In Situ Photoemission Observation of Catalytic CO Oxidation Reaction on Pd(110) under Near-Ambient Pressure Conditions: Evidence for the Langmuir–Hinshelwood Mechanism

Abstract

CO oxidation reaction on a Pd(110) single crystal surface at various temperatures under near-ambient-pressure conditions has been investigated using in situ X-ray photoemission spectroscopy and mass spectroscopy. At lower temperature conditions, the CO2 formation rate is low, where the surface is covered by CO molecules (i.e., CO poisoning). Above a critical temperature 165 °C the Pd(110) surface converts to a catalytically active surface and is dominated by chemisorbed oxygen species. Further at the higher temperatures up to 320 °C, the CO2 formation rate is gradually decreased to about 80% of the maximum rate. At this moment, the amount of chemisorbed O was also decreased, which suggests that the CO oxidation reaction proceeds via the conventional Langmuir–Hinshelwood mechanism even under near-ambient pressure conditions.

Reversed Hysteresis during CO Oxidation over Pd75Ag25(100)

“…[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)

“…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].…”

“…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.…”

Comparison of AP-XPS and PLIF Measurements During CO Oxidation Over Pd Single Crystals

“…The reaction is therefore completely controlled by diffusion of the reactants, which makes the gas-phase composition highly important for the catalytic process. Gas-phase studies using AP-XPS have been performed and reported in the literature [10][11][12], but very few have been performed for a highly active catalyst during CO oxidation [13][14][15][16]. As mentioned previously, the formation of a CO 2 boundary layer changes the gas composition close to the sample surface, but how this is reflected in the AP-XPS gas-phase signal is not obvious.…”