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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.

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19The CO oxidation behavior under excess oxygen and near stoichiometric conditions over the surface 20 of Pd3Au(100) has been studied by combining near-ambient pressure X-ray photoelectron 21 spectroscopy and quadrupole mass spectrometry and compared to Pd(100). During heating and 22 cooling cycles, normal hysteresis in the CO2 production, i.e. with the light-off temperature being 23 higher than the extinction temperature, is observed for both surfaces. On both Pd3Au(100) and 24Pd(100) the (√5x√5)R27° surface oxide structure is present during CO2 production under excess 25 oxygen conditions (O2:CO = 10:1), while at near stoichiometric conditions (O2:CO = 1:1) the surfaces 26 are covered with atomic oxygen. Au as alloying element hence induces only minor differences in the 27 observed hysteresis and the active phase compared to pure Pd. Alloying with Au thus yields a 28 different behavior compared to Ag, where reversed hysteresis is observed for CO2 production over 29 Pd75Ag25(100) at similar conditions [Fernandes et al., ACS Catal. (2016)

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Highlights-H2 dissociation on Co is promoted by undercoordinated sites -But hydrogen atoms bind less strongly there compared to flat terrace sites -In FTS steps and kinks may be the dominant source of surface hydrogen atoms