CO titration experiments

To check, if the sites, which are covered in regime II on [Pd/Fe₃O₄]1 are irreversibly occupied, CO titration experiments have been performed.

Initially, the Pd surfaces have been exposed to 3·10¹⁶ O₂ molecules cm^–2. Subsequently, CO titration experiments are performed with a CO beam intensity of 1.8·10¹⁴molecules cm^–2s^–1, a chopper opening time of 500 ms and a pulse period of 8 s.

The number of CO molecules, which occupy the Pd surface sites and react with adsorbed oxygen to CO₂ can be determined from the CO adsorption experiments. The number of evolved CO₂ molecules can be evaluated from the QMS intensity data of CO₂.

In the following, the CO adsorption experiments are discussed. Upon CO adsorption at 300 K, two processes may occur: CO molecules either react with adsorbed oxygen atoms on the Pd nanoparticles to form CO₂ or adsorb without reaction. The total CO sticking probability is the fraction of the initially adsorbed CO molecules that undergo either process. Similarly, the num-ber of adsorbed CO molecules corresponds to the numnum-ber of CO molecules which react with oxygen and the number of CO molecules which are adsorbed without reaction.

As the CO titration experiments have been performed on catalysts which have been saturated with oxygen, these systems are referred to as O/[Pd/Fe₃O₄]1, O/[Pd/Fe₃O₄]2 and O/Pd(111) in the following.

Figure 9.2 shows the total CO sticking coefficient versus the number of adsorbed CO molecules on oxygen covered Pd(111) and on O/[Pd/Fe₃O₄]1 and O/[Pd/Fe₃O₄]2 for the Pd deposition co-verages 7 Å and 4 Å. On Pd(111), S(0) is 0.72 and decays with increasing number of adsorbed CO molecules. Saturation is reached as the number of adsorbed CO molecules reaches∼10¹⁵

Abbildung 9.2: CO sticking coefficient vs the number of adsorbed CO molecules following oxy-gen adsorption on Pd/Fe₃O₄]1 and on [Pd/Fe₃O₄]2 on Pd(111) and Pd nanopar-ticles of two different sizes. A chopper opening time of 500 ms and a pulse period of 8 s has been used

cm^–2.

Significant transient adsorption/desorption from Pd nanoparticles at high CO exposures, which cannot be quantified from the present measurements at the conditions used here [205, 273], hampers the determination of the exact number of adsorbed molecules at high CO coverages.

Nevertheless, the qualitative change in the total CO sticking coefficient as a function of the number of adsorbed CO molecules gives valuable information on the change in the adsorption behavior due to the cleaning procedure.

The initial total sticking probability of CO on O/[Pd/Fe₃O₄]1 with the Pd deposition coverages 7 Å Pd and 4 Å Pd is 0.6 and 0.67 and decays strictly monotonically with increasing CO expos-ure. On O/[Pd/Fe₃O₄]2, the initial total sticking coefficient of CO is reduced to 0.49 and 0.6 for the Pd deposition coverages 7 Å Pd and 4 Å Pd. It is also evident that the number of adsorbed CO molecules is significantly lower on the Pd nanoparticles after O₂and CO exposure at 300 K and cleaning at elevated temperatures (O/[Pd/Fe₃O₄]2).

These results can be compared with the oxygen sticking measurements: the higher number of adsorbed oxygen atoms on [Pd/Fe₃O₄]1 compared to [Pd/Fe₃O₄]2 agrees with a higher number of adsorbed CO molecules on O/[Pd/Fe₃O₄]1 in comparison to O/[Pd/Fe₃O₄]2. The large num-ber of adsorbed CO molecules on O/[Pd/Fe₃O₄]1 may occur due to CO adsorption and reaction

Abbildung 9.3: Intensity of the CO₂ signal as a function of the time, measured on Pd(111) (a) and Pd nanoparticles corresponding to the Pd deposition coverages 7 Å and 4 Å Pd after oxygen exposure. The black and red line correspond to the CO₂ intensities on O[PdFe₃O₄]1 and O/[PdFe₃O₄]2. A chopper opening time of 500 ms and a pulse period of 8 s has been used

with oxygen that adsorbs in regime II of the oxygen sticking data.

To find further evidence for this hypothesis, the CO₂ QMS signal, measured during the CO ti-tration measurements, can be considered. Figure 9.3 shows the CO₂signal, which is measured during CO exposure of oxygen covered Pd(111) and Pd nanoparticles of two different sizes. The corresponding total sticking probabilities for CO are given in Figure 9.2.

The background of the CO₂signal from O/Pd(111), shown in Fig. 9.2 (a), increases prominently, then is approximately constant for a few seconds, and then gradually decreases. This behavior has been measured and modeled by several authors on supported nanoparticles in molecular beam measurements with continuous beams. A discussion on this subject can be found in the references [20, 77–79]. The strong variations of the signal, which are most prominent during the rise of the CO₂ intensity occur due to an increasing CO₂ evolution during the on time of the pulse (500 ms) and a decay during the off time of the pulse. These variations can be clearly distinguished from the rise in the CO₂background in the inset of Fig. 9.2 (a).

Fig. 9.2 (b) and (c) show the CO₂signals on oxygen covered Pd nanoparticles for the Pd

depo-sition coverages 7 Å and 4 Å for the freshly prepared system (black line) and for the catalysts after one cycle of oxygen and CO exposure and cleaning at elevated temperatures (red line). It can be clearly seen that the CO₂release is significantly higher on O/[Pd/Fe₃O₄]1 compared to O/[Pd/Fe₃O₄]2. To obtain a quantitative measure for the amount of oxygen that reacts during CO exposure, the background substracted CO₂peak area was integrated. By calibrating the CO₂ peak area from O/[PdFe₃O₄]1 and O/[PdFe₃O₄]2 to the CO₂area from O/Pd(111), the number of evolved CO₂molecules has been determined. Note, that adsorbed oxygen atoms on Pd nano-particles can be completely removed upon CO exposure at 300 K on O/Pd(111) [65, 69, 70, 233].

The CO₂release, obtained in this manner can be compared to the number of adsorbed oxygen

Abbildung 9.4: Comparison of the number of adsorbed molecules on [PdFe₃O₄]1 (squares) and [PdFe₃O₄]2 (circles): the filled symbols indicate the number of adsorbed oxy-gen atoms which are obtained by sticking coefficient measurements, the hollow symbols show the measured CO₂ release, the error bars indicate the standard deviation

atoms on Pd/Fe₃O₄ systems, determined from the oxygen sticking measurements (Fig. 9.1).

Both quantities are shown for the investigated Pd deposition coverages on [Pd/Fe₃O₄]1 (black scatters) and on [PdFe₃O₄]2 (red scatters) in Figure 9.4. The filled black squares indicate the number of adsorbed oxygen atoms, the hollow black squares show the measured CO₂ release on [Pd/Fe₃O₄]1. The number of adsorbed oxygen atoms and the CO₂ release on [Pd/Fe₃O₄]1 is very similar for the three particle sizes, thus the major oxygen fraction can be removed upon CO exposure at 300 K. The number of adsorbed oxygen atoms is 1.26-1.47·10¹⁵cm^–2, 0.9-0.96

·10¹⁵cm^–2and 0.37-0.4·10¹⁵cm^–2for the deposition coverage 7 Å Pd , 4 Å Pd , 1.5 Å Pd on the freshly prepared system. The filled and the hollow red circles show the number of adsorbed oxygen atoms and the number of evolved CO₂molecules on [Pd/Fe₃O₄]2. Also in that case, the number of adsorbed oxygen atoms and the CO₂release is in good agreement but the number of

adsorbed O atoms is significantly lower than on [Pd/Fe₃O₄]1.