ABSTRACT
Another metal on the surface of which coadsorption of oxygen and carbon monoxide was studied was platinuln. Behm et ale [24] followed the changes of LEED patterns when Pt (100) surface covered with preadsorbed CO or 0 was exposed to the pressures of 02 or CO of 10-5 to 10-8 Torr. The temperature of the experiments (355 K) was low enough to prevent the reconstruction of the surface and to keep Dads immobile while the surface diffusion of CO was rapid. Under these conditions the separate islands of Dads and COads species can coexist and react with the rate, enabling continu0us registration of the intensity of diffraction spots with a fast video system. The only surface phase of CO which appears on (100) face is c (2 x 2). The changes of its coverage were characterized by the intensity of (1/2, 1/2) diffraction spot. Oxygen was forming a (3 xl) structure monitored by the (0, 1/3) beam. Figure 6.10 shows the behavior of the surface saturated with preadsorbed carbon monoxide after exposure to oxygen for different periods of time. At saturation the COads coverage was about 0.65, exceeding the value 0.5 corresponding to the c (2 x 2) structure. It is seen
p
300 400 500 T(K) FIGURE 6.9 Thermal desorption spectra of C02 from the Pd (111) surface after successive exposures to 02 and CO: (a) 0.4 L 02 + 0.05 L CO; (b) 0.1 L 02 + 0.1 L CO; (c) 0.1 L 02 + 0.15 LeO; (d) 0.1 L 02 + 0.2 L CO; (e) 0.1 L 02 + 0.5 L CO; (f) 0.1 L 02 + 1 L CO [8].
