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

We have employed synchrotron based high-resolution XPS and thermal molecula r 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 temperatur e, with initial sticking probabilities of 0.5 and 0.4, respectively. Precur sor states play a strong role in the adsorption of CO; kinetic analysis ind icates 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 sh ift in binding energy of both the C 1s and O 1s peaks at a coverage which i s associated with the reconstruction of the surface to the (1 x 2) missing row. Similar experiments for O-2 adsorbed on Pd(110) show diffusion of oxyg en 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 highe r 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 pro duction curve shows two maxima in rate which we relate to the influence of surface reconstructions. In comparison, the reverse clean-off reaction wher e 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 dissociati ve 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. (C) 1999 Elsevie r Science B.V. All rights reserved.