Reversed Hysteresis during CO Oxidation over Pd₇₅Ag₂₅(100)

Abstract

CO oxidation over Pd(100) and Pd 75 Ag 25 (100) has been investigated by a combination of near-ambient pressure X-ray photoelectron spectroscopy, quadrupole mass spectrometry, density functional theory calculations and micro-kinetic modeling. For both surfaces, hysteresis is observed in the CO 2 formation during heating and cooling cycles. Whereas normal hysteresis with higher light-off temperature than extinction temperature is present for Pd(100), reversed hysteresis is observed for Pd 75 Ag 25 (100).The reversed hysteresis can be explained from dynamic changes in the surface composition. At the beginning of the heating ramp, the surface is rich in palladium which gives a CO coverage that poisons the surface until the desorption rate becomes sufficiently high. The thermodynamic preference for an Ag rich surface in the absence of adsorbates promotes diffusion of Ag from the bulk to the surface as CO desorbs.During the cooling ramp, an appreciable surface coverage is reached at temperatures too low for efficient diffusion of Ag back into the bulk. The high concentration of Ag in the surface leads to a high extinction temperature and, consequently, the reversed hysteresis.

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].…”

“…The √5 surface oxide structure on Pd(100) is well documented [43,53] and is, as stated above, known 105 to be the active phase during CO oxidation in excess oxygen [19] at near ambient conditions. An 106 image of the LEED pattern for this oxide structure obtained by exposing the single crystal surface to 107 O2 at about 10 -5 mbar and ~ 320 °C is shown in Figure 1 (bottom).…”

“…for Pd(100) [19,35]. As mentioned, a Pd 3d5/2 interface component was assigned for the √5 surface 143 oxide on Pd(100) [19,43].…”

“…Previously reported CO oxidation experiments over Pd(100) at ratio 10:1 performed 87 at beamline I511 at MAX IV Laboratory [50] have also been included here, with the experimental 88 details described in ref. [19]. 89…”

“…The 121 previously analyzed spectrum for Pd(100) [19] is first discussed as a basis for the interpretation for 122…”