In the following, the oxygen sticking coefficients on Pd nanoparticles deposited on Fe3O4 will be discussed. The Pd/Fe3O4catalysts have been prepared according to the procedure, described in Chapter 6. After preparation, the Pd/Fe3O4 catalysts were annealed to 580 K-600 K, Direct-ly following preparation, the adsorption measurements were carried out. Figure 8.2 shows the
Abbildung 8.2: Sticking coefficient S(NO) of O2 measured at 300 K plotted versus the number of adsorbed oxygen atoms on Pd nanoparticles with the Pd deposition coverage 0.6 Å (2.3 nm), 1.5 Å (2.9 nm), 4 Å (3.4 nm) and 7 Å (6.4 nm), supported on Fe3O4/Pt(111). A molecular flux of≈1.1·1014cm–2s–1and a pulse time of 266 ms has been used. The error bars correspond to the error of the mean
sticking coefficient at 300 K as a function of the number of adsorbed atoms for four different Pd deposition coverages. The number of adsorbed atoms is the total number of oxygen atoms which is initially adsorbed on the surface, regardless of subsequent processes.
These results were obtained as the average of three to five independent measurements, carried out in an identical way to the experiments on Pd(111). The sticking coefficient of O2on Fe3O4
has been determined to be 0 at 300 K, thus sticking on Pd/Fe3O4can be attributed to sticking on the nanoparticles. The initial sticking coefficient has been discussed in the previous section and has been explained with a model that includes the capture zone effect.
For the deposition coverage 7 Å Pd (6.4 nm), the initial sticking probability is 0.53±0.05 and gradually decreases until the number of adsorbed atoms is 0.10·1015cm–2and S∼0.51. At higher oxygen exposures, S decreases more sharply until the number of adsorbed atoms is≈0.44·1015 cm–2. An additional adsorption regime can be identified in Fig. 8.2 (a) in which the number of adsorbed O atoms decays more gradually is in the range 0.44·1015- 1.35·1015cm–2. The latter regime is termed regime II, whereas the regime in which S decays gradually and subsequently sharply is termed regime I. To determine the boundary between the two regimes, the region of the sharp decrease in S in regime I and the region of the gradual decay of S in regime II are fitted linearly. These linear fits are indicated with gray dotted lines in Fig. 8.2 (a). The intersection between the two lines is defined to be the boundary between regime I and regime II.
A gentle decay at the smallest oxygen exposures, a more prominent decay at higher oxygen ex-posures and a very gradual decay of the sticking coefficient at high oxygen exex-posures can be also identified in the sticking data for the deposition coverage 4 Å Pd (3.4 nm), shown in Fig.
8.2 (b). For the Pd deposition coverage 1.5 Å, a pronounced decay of the sticking probability is already observed at the lowest oxygen coverages. The more gradual decay of the sticking coeffi-cient in regime II occurs after the decay of the sticking probability to≈0.13, where the number of adsorbed oxygen atoms is≈0.23·1015cm–2. The boundary between regime I and II for the two smaller Pd nanoparticle sizes has been determined with the same procedure as the boundary for the largest particle size, described above. Due to the more gradual transition between the two regimes for the Pd deposition coverage 1.5 Å, this determination may be less accurate in this case. The two regimes are shown for all three cases in Fig. 8.2.
For the Pd deposition coverage 0.6 Å Pd (2.3 nm), the oxygen sticking probability is initially 0.25±0.08 and strongly decays with increasing oxygen exposure until saturation is reached at approximately 0.10·1015oxygen atoms cm–2.
A similar coverage dependence of the sticking coefficient as observed in regime I for the two largest nanoparticles has been measured in oxygen sticking measurements on Pd single crystals [63, 91, 97, 232–236, 236]. The gradual decay of the sticking coefficient at low exposures is attributed to precursor mediated adsorption and trapping in the capture zone. The latter effect is not expected to be dependent on the oxygen coverage. Regime II will be discussed in detail in the next chapter. Evidence will be given that this regime corresponds to the formation of a species other than adsorbed oxygen on the Pd nanoparticles.
Although it would be an exciting challenge to model the coverage dependent adsorption pro-bability as a function of the particle size, more information on the adsorption dynamics on the different adsorption sites and processes such as diffusion on and between the adsorption sites and diffusion from the support are necessary to perform such a task in a meaningful way.
In the following, the coverage at which oxygen saturates the surface Pd sites is discussed, which is assumed to be reached after the prominent decay of the sticking probability in regime I. This assumption is based on the following observations: in contrast to regime II, regime I has been observed in oxygen adsorption experiments on Pd single crystal facets at conditions in which only the formation of an adsorbate layer on Pd occurs. The high number of oxygen atoms which is adsorbed in regime II cannot be explained with the saturation of surface Pd atoms without the
Abbildung 8.3: Number of on Pd adsorbed oxygen atoms (a) and the number of on Pd adsorbed oxygen atoms divided by the number of surface Pd atoms. The particle size is given next to the data point for each respective deposition coverage
occupation of additional sites.
The number of on Pd adsorbed oxygen atoms is plotted for the four different Pd deposition co-verages and for Pd(111) in Figure 8.3 (a). As the number of surface Pd atoms decreases with decreasing Pd deposition coverage, the total number of on Pd adsorbed oxygen atoms follows the same trend. The oxygen coverage on the nanoparticles can be obtained by dividing the num-ber of adsorbed O atoms by the numnum-ber of Pd atoms at the particle surface, which is shown in Figure 8.3 (b). Details on how the number of surface Pd atoms has been estimated are given in Chapter 6.
Figure 8.3 (b) shows, that the estimated oxygen coverage at saturation is quite similar for the four different Pd particle sizes, the average oxygen coverage is 0.38±0.04.
STM experiments have been performed at 300 K on Pd nanoparticles supported onα-Al2O3and TiO2with a diameter of more than 8 nm: it could be shown, that the oxygen overlayer structures are the same on the facets of the Pd nanoparticles and on the corresponding single crystal facets [32, 33]. Therefore, the saturation coverage of the Pd/Fe3O4systems can be estimated based on the ratio of (111) facets (80%) to (100) facets (20%) and the saturation coverage on these facets.
In most studies, a p(2x2) structure with a saturation coverage of 0.25 has been found on Pd(111) [88, 166, 233], although large O2 exposures have been found to result in higher O coverages (Θ>0.36) [91, 233]. Hansen et al. observed the formation of a p(2x2)O structure on∼7.5 nm Pd particles, supported by Al2O3 after an oxygen exposure that was higher than in our studies.
Therefore, the formation of a p(2x2)O phase is expected to form on the Pd(111) facets with the conditions, used here. Based on the literature, the saturation coverage on the Pd(100) facet
is assumed to be 0.50 [93, 94, 249]. This gives an average saturation coverage of 0.30 on the Pd nanoparticles. In the present sticking probability measurements, described above, an oxygen coverage of 0.30-0.35 has been determined on Pd(111). When using this for the estimation, an oxygen coverage of 0.34-0.38 is obtained.
This range of oxygen coverages is in agreement with the experimentally obtained oxygen cover-age of 0.38±0.04 on Pd nanoparticles. Henry et al. obtained a saturation covercover-age of 0.40±0.08 [77] on Pd nanoparticles, supported on MgO(100), which also agrees quite well with the oxygen coverage, determined here.