Sticking coefficient and coverage

Abbildung 10.3: CO sticking probability on Pd/Fe₃O₄ at 110 K for the Pd deposition coverage 1.5 Å (2.9 nm) and 4 Å (3.4 nm), plotted versus the number of adsorbed CO molecules. The black and green scatters show the sticking probability on the Pd/Fe₃O₄systems as prepared and after an oxygen exposure of 4·10¹⁶ mole-cules cm^–2at 300 K.

Figure 10.3 shows the CO sticking coefficient on supported Pd nanoparticles with the Pd depo-sition coverages 4 Å (3.4 nm) and 1.5 Å (2.9 nm) after preparation (black squares) and after oxygen exposure at P_O2=5 ·10^-7mbar for t=5min. The modulation of the molecular beam is similar to the equivalent measurements on Pd(111).

Based on the results of chapter 7, the diffusion length of CO on iron oxide was estimated to be in the micrometer range at 110 K. Accordingly, the capture zones would completely cover the Fe₃O₄surface.

The initial sticking coefficient of 0.82±0.02 and 0.79±0.02 for the Pd deposition coverages 4 Å (3.4 nm) and 1.5 Å (2.9 nm) are similar to the sticking coefficient of 0.84±0.01 on Pd(111).

The sticking coefficient is constant over a wide coverage range, both on oxygen covered and on bare Pd/Fe₃O₄.

The sticking probability strongly decays as the number of adsorbed CO molecules reaches 0.56·10¹⁵and 0.39·10¹⁵on bare Pd nanoparticles for the Pd deposition coverages 4 Å (3.4 nm) and 1.5 Å (2.9 nm). On the oxygen covered Pd nanoparticles, the sticking coefficient decays as the number of adsorbed CO molecules reaches 0.33·10¹⁵and 0.22·10¹⁵for the Pd deposition

System k_des/ s^–1 System k_des/ s^–1

Pd(111) 1.1±0.1 O/Pd(111) 1.1±0.2

4 Å Pd/Fe₃O₄ 2.1±0.1 O/4 Å Pd/Fe₃O₄ 2.1±0.2 1.5 Å Pd/Fe₃O₄ 2.3±0.2 O/1.5 A Pd/Fe₃O₄ 2.2±0.3

Fe₃O₄ 2.1±0.2 O/Fe₃O₄ 2.0±0.3

Tabelle 10.1: Desorption rate of CO from Pd(111), Pd/Fe₃O₄and from bare Fe₃O₄per time and number of adsorbed molecules at T=110 K on the bare surfaces and with oxygen exposure at T=300 K prior to the measurements (O/”catalyst”)

coverages 4 Å and 1.5 Å . The sticking probability decays to a constant, non-zero value which is the same for the two particle sizes and is not changed by the oxygen treatment. Due to transient CO adsorption close to saturation, a quasi-steady state regime is reached where the number of adsorbed molecules during the pulse equals the number of molecules which desorb in between the pulses.

In this regime, the QMS intensity, which is measured after closing the beam shutter until the next pulse impinges on the surface can be used to model the CO desorption rate. Details on this evaluation are given in Chapter 5. Shortly, the QMS intensities on the sample and on the gold reference are compared to separate the decay of the QMS intensity due to desorption and due to the chamber behavior. To determine the desorption rate, it is assumed that CO desorbs in a first order process. Furthermore, it is assumed that the coverage dependence of the desorption rate can be neglected for the increase in the CO coverage within one pulse.

The desorption rate for CO on Pd(111) and Pd/Fe₃O₄with and without O₂exposure is given in Table 10.1. O/”catalyst” indicates, that the model systems have been exposed to 4·10¹⁶O₂ mo-lecules cm^–2 at 300 K prior to the measurement. Each value is the average of typically three to six independent measurements. To model the desorption rate, it is assumed that CO desorption is a first order process and that the coverage dependence of the desorption rate can be neglected.

The latter assumption is justified by the fact that only∼7·10¹³CO molecules adsorb during one pulse on the model catalysts in the CO saturation regime. It has been often observed, that the desorption rate in the first∼1000 ms after the chopper closes is slightly higher than the subse-quent desorption rate. Table 10.1 shows the initial desorption rate after the chopper closes.

The oxygen desorption rates from Pd/Fe₃O₄are identical within the error to the desorption rates from Fe₃O₄. Accordingly, transient CO adsorption on Pd/Fe₃O₄during the on time of the pulse can be attributed to CO adsorption on Fe₃O₄. The observation that the desorption rate is the same from oxygen covered Pd/Fe₃O₄ and from pristine Pd/Fe₃O₄would be in line with this result. It is also possible that CO additionally desorbs from the Pd nanoparticles with a similar desorption rate. The determined desorption rate of 1.1±0.1 s^–1from Pd(111) and from O/Pd(111) is signi-ficantly lower than the desorption rate from the Pd/Fe₃O₄catalysts of∼2.1 s^–1. As discussed in chapter 7, the desorption prefactor for CO from Fe₃O₄is estimated to beνdes=2·10^-11-2·10^-12 s^–1by assuming that the measured adsorption energy of 25 kJ/mol equals the desorption barrier.

Saturation of the Pd/Fe₃O₄model catalysts can be assumed to be reached after the pronounced decay of the CO sticking probability in Fig. 10.3. The total number of adsorbed CO molecules may be rationalized by considering the abundance of the different Pd facets on the Pd

nanopar-Adsorbed CO molecules / 10¹⁵cm²

Tabelle 10.2: Estimated number of adsorbed CO molecules based on the CO coverage on the single crystal facets and the particle morphology (see text) compared with the mea-sured number of adsorbed CO molecules on Pd catalysts.

ticles and their respective CO saturation coverages.

Recent STM studies on 20-30 nm large Pd nanoparticles, supported on TiO₂, showed that the CO adsorbate structures on the (111) facet of supported particles are identical to the ones on the Pd(111) single crystal [33]. No structural studies on the Pd(100) facet of Pd nanoparticles have been performed however. It is unclear if ordered adsorbate structures are also formed on the significantly smaller Pd nanoparticles which are used in the present studies. It is interesting though to consider the case that the CO phases which have been observed on the Pd(111) and Pd(100) facets are also formed on the respective facets of the Pd nanoparticles and compare the corresponding number of adsorbed molecules with the experimentally observed ones.

Based on this consideration, the CO saturation coverage on the Pd nanoparticles can be esti-mated by taking into account the ratio of (111) facets (80 %) to (100) facets (20 %) and the saturation coverage of CO at 110 K on these facets. On Pd(100), a saturation coverage of 0.75 has been determined by various groups [146, 276–281] and a saturation coverage of 0.75 has been determined on Pd(111) in recent studies [82, 146, 162, 165, 275]. Based on the abundance of the (111) and (100) facets, the CO saturation coverage on these facets and the number of sur-face Pd atoms, the number of CO molecules that are expected to adsorb on the freshly prepared Pd/Fe₃O₄catalysts was calculated and is given in Table 10.2. Note, that the number of adsorbed CO molecules is given in 10¹⁵cm^–2in contrast to Figure 10.2.

Table 10.2 shows, that the estimated CO coverage on Pd(111) of 1.14·10¹⁵cm^–2 is in a good agreement with the measured value of∼1.19·10¹⁵ cm^–2. For the Pd deposition coverage 4 Å (3.4 nm), the estimated number of adsorbed CO molecules of 0.90·10¹⁵ cm^–2 is in reasona-ble agreement with the experimentally determined value of ∼0.84·10¹⁵ CO molecules cm^–2.

∼0.54·10¹⁵CO molecules cm^–2 are adsorbed for the Pd deposition coverage 1.5 Å (2.9 nm).

This value is significantly higher than the estimated number of adsorbed CO molecules of 0.40·10¹⁵ cm^–2. The larger discrepancy between the measured and the estimated number of adsorbed CO molecules for the smallest particle size can be explained with a larger error in the estimation of the CO coverage or of N_Pd,surf. But as mentioned above, it is also possible that no ordered adsorbate structures are formed on the smaller Pd nanoparticles.

Due to the saturation of the Pd catalysts with O₂, the number of adsorbed CO molecules de-creases. The difference in the number of adsorbed molecules between the bare catalysts and the oxygen covered catalysts is determined to be ∼0.6·10¹⁵ CO molecules cm^–2 , 0.29·10¹⁵ CO molecules cm^–2and 0.18·10¹⁵cm^–2on Pd(111), 4 Å Pd/Fe₃O₄(3.4 nm) and 1.5 Å Pd/Fe₃O₄

(2.9 nm).