![]() This suggests that there is a lack ofġ8 O in the p(2x2) layer for the associative desorption at the beginning.Īpparently, 18O2 only desorbs when enough subsurface 18O has migrated up At the same time, desorption for massģ6 starts at slightly higher temperature. There is significant exchange between subsurface 18O (adsorbed at 700K)Īnd overlayer 16O during desorption. It is obvious from the 16O18O (mass 34) TPD trace that Experiments were performed as described for the data presented inįigure 3.3, with the difference that 18O2 was dosed at 700 K and 16O2 was Labeled O2, to investigate the exchange between subsurface and surface Graphs are offset vertically for clarity.įinally, we have carried out initial experiments with isotopically Figure 6a contains TPD traces of O2 for comparison. Pt(111), and the reactivity of subsurface O towards CO oxidation similar toįigure 3.6 TPR spectra for a) O2 b) CO2 and c) CO recorded after CO adsorption on Pt(111), pre-exposed to O2 at different temperatures. Oxygen at 760 K, which is lower but comparable to what we observe for Furthermore, they observed desorption of the subsurface In work function for Pt(100) when the oxygen-covered surface was heatedįrom 360 to 600 K. Suggested thermodynamically stable subsurface oxygen to explain a change Suggests that sub-surface oxygen is thermodynamically quite stable even in The observation that some oxygen remains unreacted even when CO isĪvailable on the surface and the temperature favors the oxidation reaction, Considering that unreacted COĭesorbs at lower temperatures than unreacted O, these findings support ourĬlaim that the additional oxygen is absorbed in subsurface sites. This clearly indicates that part of the atomic O created by high temperatureĪdsorption is not available for CO oxidation. Unreactive isolated oxygen on the Pt(111) surface exposed to O2 at 600 K. Platinum surface, yet they refuse to react even at elevated temperatures! This is most unusual: we seem to have both O and CO on At 700 K, even more CO desorbs at the expense of the CO2įormation. However, when the surface is initially oxidized at 500 K, some amount ofĬO as well as O2 are left on the surface after the same amount of CO2 hasīeen formed. O at 300 K, all CO as well as most of the oxygen react to form CO2. Previous studies, we observe that for the surface saturated with Partial pressures of m/e = 32 (O2), 28 (CO) and 44 (CO2). Subsequently, the sample temperature was ramped up while recording the Onto the Pt(111) surface pre-exposed to O2 at different temperatures. That covering the Pt(111) surface with atomic oxygen above 0.25 ML blocksįor the TPR traces shown in Figure 3.6, CO was adsorbed at ≤90 K Temperature activates the reaction that produces CO2. CO and O co-adsorb in a 1:1 ratio and an increase in surface Oxygen on Pt(111) does not react with CO at temperatures below 150 K. From previous studies it is known that the 0.25 ML of atomic Oxygen adsorbed on Pt(111) at various temperatures reacts with carbon Temperature programmed reaction (TPR) measurements to check how Support the subsurface oxygen hypothesis. An unusual reactivity of the additional adsorbed atomic oxygen would
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