To obtain information on the CO-O interaction on Pd(111), CO adsorption experiments on oxy-gen covered Pd(111) have been performed at T∼110 K. At such conditions, no CO2evolution has been observed previously [14, 70, 108]. CO adsorption on bare Pd(111) is compared with CO adsorption on oxygen covered Pd(111), to obtain information on the CO-O interaction. Oxygen saturation has been achieved by exposure at PO2=5·10-7mbar at 300 K for≈5 minutes. Directly afterwards, the sample was cooled down to perform the CO adsorption experiments at 110 K.
A pulse time of 266 ms and a CO molecular beam flux of 1.2·1014has been chosen for these experiments.
The sticking coefficient and the adsorption energy as a function of the CO coverage is given in Figure 10.2. All shown results are the average of at least three independent measurements. Due to transient CO adsorption during the pulse and CO desorption in between the pulses at high CO exposures, which will be discussed in more detail below, the sticking coefficient remains rela-tively high close to saturation. On pristine Pd(111) at 110 K, the initial CO sticking coefficient
Abbildung 10.2: Sticking probability and adsorption energy for CO, measured at 110 K, plotted versus the CO coverage on bare Pd(111) (black square scatters) and after 5 min oxygen exposure at PO2=5·10-7mbar at 300 K (red circular scatters). The green triangular scatters show the sticking coefficient and adsorption energy for CO, measured at 300 K on bare Pd(111).
of 0.84 is slightly lower than the literature value of 0.92-0.95, measured under similar conditi-ons [242, 269, 283]. The initial adsorption energy of 149±2 kJ/mol is in reasonable agreement with the binding energy, measured by Ertl et al. (142 kJ/mol) with TPD [269] and in excellent agreement with earlier SCAC-results (149±3 kJ/mol) [205].
The CO sticking coefficient on Pd(111) is approximately constant until a CO coverage of 0.35, which indicates precursor mediated adsorption. Examples for such a behavior can be widely found in the literature [156, 284]. Whereas the sticking coefficient is approximately constant until a coverage of 0.35, the adsorption energy drops to a lower value at a coverage of 0.23. A sudden decrease in the desorption energy atΘCO=0.33 has been observed in TPD studies by Ertl et al. and associated with a transformation of the adsorbate structure. The prominent decay of Eadsand S at the coverage 0.35 can be rationalized with the transformation of the p(√
3x√ 3)R30◦ phase into the c(2x4) phase atΘCO≥0.33, observed in previous studies [163–165]. A lower ad-sorbate binding energy in the more dense c(2x4) phase is expected due to stronger adad-sorbate- adsorbate-adsorbate interactions.
The adsorption energy reaches a constant value at a coverage of 0.64±0.05 and the sticking probability remains constant at a CO coverage of 0.7-0.8. It can be additionally considered at which coverage the heat, which is evolved per pulse on the sample, reaches a constant value. At ΘO>0.75, the heat release changes by less than 4 % with respect to the initial heat release. In
good agreement between the sticking measurements and the consideration of the heat release per pulse, saturation of the surface is reached atΘCO≈0.75. This value agrees with the literature [82, 146, 146, 162, 165, 275, 275]. As mentioned before, the relatively high sticking coefficient of≈0.4 at saturation can be explained with transient CO adsorption of∼1013molecules cm–2 during the pulse and the same amount of molecules desorbing in between the pulses.
S(0) of CO on O/Pd(111) is slightly lower (0.81±0.01) and the adsorption energy is significantly lowered by 35±4kJ/mol compared to CO adsorption on bare Pd(111). The sticking coefficient is constant until the CO coverage reaches a value of about 0.2 and decays at higher coverages, whereas the adsorption energy already starts to decay at higher CO coverages than 0.08. At a coverage of∼0.4, Eads and the sticking coefficient remain constant. T0.17 Thus saturation of the Pd surface sites is reached at this coverage. The measured heat release per pulse remains constant at a coverage of∼0.46.
In the following, changes in the sticking coefficient and the adsorption energy as a function of the CO coverage on oxygen covered Pd(111) are compared with changes in the adsorbate phases observed in previous structural and spectroscopic studies.
The saturation coverage of CO is in agreement with the structural model of a mixed c(2x1) phase together with a pure p(√
3x√
3)R30◦CO phase, as one expects a total CO coverage of 0.42 from the adsorbate phases, determined with microscopic studies [14, 69, 70]. If the (2x1) phase was a pure oxygen phase as has been suggested by other authors [108, 109], the total CO coverage was∼0.17, which is definitely not the case.
During the transformation of the oxygen p(2x2) overlayer into the p(√ 3x√
3)R30◦phase, which is completed atΘCO=0.08, the sticking coefficient and the adsorption energy are approximately constant. Upon further CO exposure, the c(2x1) structure is formed, which coincides with a de-crease of the sticking coefficient at a CO coverage which is higher than 0.08. From the literature data, it is not clear if the mixed c(2x1) is formed in a stepwise process, i.e. initial formation of the c(2x1)O phase and subsequent CO adsorption in this phase or if these two processes happen simultaneously.
In the first case, saturation of the c(2x1)O phase would occur atΘCO=0.17. Figure 10.2 shows a pronounced decay of the CO sticking probability and the adsorption energy as the CO coverage reaches∼0.2. This indicates, that the mixed c(2x1) phase is formed in a stepwise process. Fur-ther investigations with IRAS or PES are necessary to verify this hypothesis, however.
As a comparison to the above results, the adsorption energy and sticking coefficient as a functi-on of the coverage, obtained at 300 K functi-on bare Pd, is shown in Figure 10.2. The initial sticking coefficient of 0.73 is lower than the sticking probability of 0.85±0.01 measured at 110 K. In comparison to the measurements at 110 K, the sticking coefficient and adsorption energy decay prominently atΘCO∼0.2. The heat release per pulse reaches a constant value at a coverage of 0.47-0.53, the sticking probability remains constant atΘCO>0.53. Accordingly, the saturation coverage, determined in the present studies, is ∼0.5. This is in good agreement with the CO saturation coverage of 0.5 found in the literature for CO adsorption at 300 K. The adsorption energy at this coverage is 86±5 kJ/mol.
Compared to the measurements at 110 K, the sticking coefficient, measured at 300 K, starts to decay at lower coverages. This could be explained by a shorter lifetime of the precursor state at higher temperatures. Accordingly, less ordered adsorption structures would be formed, resulting in a decay of Eads at lower CO coverages [165]. Due to the less compact adsorbate structures
close to saturation, the adsorption energy in this regime is higher at 300 K. Significantly less transient CO adsorption/desorption at 300 K results in a decay of the CO sticking coefficient to lower values close to saturation at this temperature.