CO oxidation on Pd(110): a high-resolution XPS and molecular beam study

Abstract

We have employed synchrotron based high-resolution XPS and thermal molecular beams to investigate the interaction and reaction of CO and O 2 on Pd(110). CO and O 2 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 O 2 adsorbed on Pd(110) show diffusion of oxygen into the subsurface region during the temperature ramp, at a temperature significantly below the onset of O 2 desorption. The CO oxidation reaction has been studied in a transient mode where CO/(O 2) is dosed onto an O 2/(CO) pre-dosed surface. At low temperatures, CO is immediately adsorbed with a long lifetime state on the O 2 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 CO 2. At 375 K, the CO 2 production curve shows two maxima in rate which we relate to the influence of surface reconstructions. In comparison, the reverse clean-off reaction where O 2 is dosed onto different CO pre-coverages shows that O 2 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, O 2 can adsorb on lower CO pre-coverages and there is fast reaction for CO coverages below 0.3 ML.