| Popis: |
We have employed synchrotron based high-resolution XPS and thermal molecular beams to investigate the interaction and reaction of CO and O2 on Pd(110). CO and O2 stick on this surface with high efficiency at room temperature, with initial sticking probabilities of 0.5 and 0.4, respectively. Precursor states play a strong role in the adsorption of CO; kinetic analysis indicates a greater precursor effect in comparison with that for CO adsorption on Rh(110). Temperature-programmed XPS of CO from Pd(110) shows a sharp shift in binding energy of both the C 1s and O 1s peaks at a coverage which is associated with the reconstruction of the surface to the (1×2) missing row. Similar experiments for O2 adsorbed on Pd(110) show diffusion of oxygen into the subsurface region during the temperature ramp, at a temperature significantly below the onset of O2 desorption. The CO oxidation reaction has been studied in a transient mode where CO/(O2) is dosed onto an O2/(CO) pre-dosed surface. At low temperatures, CO is immediately adsorbed with a long lifetime state on the O2 pre-covered surface, in contrast to higher temperatures (>420 K) where there is no net uptake of CO at the beginning of the reaction owing to reactive loss of CO as CO2. At 375 K, the CO2 production curve shows two maxima in rate which we relate to the influence of surface reconstructions. In comparison, the reverse clean-off reaction where O2 is dosed onto different CO pre-coverages shows that O2 cannot adsorb on a CO-saturated surface. We have determined that there is a critical CO coverage of 0.60 (±0.06) ML of CO adsorbed above which oxygen dissociative adsorption is blocked. However, O2 can adsorb on lower CO pre-coverages and there is fast reaction for CO coverages below 0.3 ML. |
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