Adsorption energy

Abbildung 10.4: CO adsorption energy as a function of the number of adsorbed CO molecules at 110 K for the Pd deposition coverage 4 Å (3.4 nm) and 1.5 Å (2.9 nm), supported on Fe₃O₄. The black square scatters show the adsorption energies on the freshly prepared systems, the gray triangular scatters show the adsorp-tion energy after O₂ exposure at P_O2=5 ·10^-7mbar for t=5min directly after preparation.

The adsorption energy of CO on bare Pd/Fe₃O₄and on oxygen covered Pd/Fe₃O₄ is shown in Figure 10.4 for the Pd deposition coverages 4 Å (3.4 nm) and 1.5 Å (2.9 nm). Analogous re-sults for adsorption experiments on Pd(111) are shown in Fig. 10.2. The data corresponds to the same experimental data used to calculate the sticking coefficients, discussed above. The in-itial CO adsorption energies of 142±6 kJ/mol and 134±4 kJ/mol are in reasonable agreement with the adsorption energies of 134±4 kJ/mol and 125±5 kJ/mol, measured previously with SCAC at 300 K on Pd/Fe₃O₄ systems. The Pd/Fe₃O₄ catalysts, used for the studies at 300 K have been prepared with the identical preparation procedure as the systems which are used for the present studies at 110 K [205, 229]. Figure 10.4 shows that the CO adsorption energy on Pd/Fe₃O₄ decreases strictly monotonically with an increasing number of adsorbed molecules, whereas it is constant until the number of adsorbed CO molecules reaches a value of 0.55·10¹⁵ cm^–2 on Pd(111). The CO adsorption energy on O/Pd/Fe₃O₄ is reduced by 47±6 kJ/mol and 42±5 kJ/mol for the Pd deposition coverages 4 Å (3.4 nm) and 1.5 Å (2.9 nm) with respect to the bare catalysts. This decrease is the result of adsorbate-adsorbate interactions between CO and oxygen.

It is remarkable that the slope of the CO adsorption energy as a function of the number of adsorbed CO molecules on oxygen covered Pd/Fe₃O₄ is roughly the same as the one on bare Pd/Fe₃O₄. On bare Pd(111), the adsorbate structures transform from a p(√

3x√

3)R30^◦ phase into a c(2x4) phase, a c(2x2)3-CO phase is formed at the highest CO exposures [14, 82, 146,

146, 161–165, 275]. On oxygen covered Pd(111), where a p(2x2)O structure is present, separa-te√

3x√

3)R30^◦O and√ 3x√

3)R30^◦CO domains are formed upon CO exposure. At the highest CO exposures, a high density c(2x1) phase was observed which was found to be accompanied by a p(√

3x√

3)R30^◦CO phase [14, 69, 70, 106–109]. Thus, CO adsorption on an oxygen covered Pd(111) surface leads to significantly different CO adsorbate phases than on bare Pd(111). If it is assumed that similar adsorbate transformations also occur on the Pd nanoparticles, a different coverage dependence of the CO adsorption energy would be expected on bare and on the oxygen covered Pd nanoparticles.

But Fig. 10.4 shows a very similar dependence of the CO adsorption energy on the number of adsorbed molecules on bare and on oxygen covered Pd nanoparticles.

Figure 10.5 shows the difference in the CO adsorption energy between pristine Pd and oxygen

Abbildung 10.5: Difference between E_ads for CO on bare Pd and after O₂ exposure at P_O2=5

·10^-7mbar for t=5 min (Ediff) as a function of the number of adsorbed CO molecules. The black square scatters, the dark gray triangular scatters and the bright gray circles show Ediff for adsorption on Pd(111), 4 Å Pd/Fe₃O₄ and 1.5 Å Pd/Fe₃O₄

covered Pd (Ediff) as a function of the coverage. Ediff is a measure for the change of the CO binding strength due to the presence of oxygen on the catalyst. Ediff is initially 40-50 kJ/mol and only slightly differs for the three systems. As discussed above, CO adsorption on oxygen cover-ed Pd(111) leads to the formation of a dense coadsorbate structure atΘCO=0.17 on O/Pd(111), which causes a decay in the CO adsorption energy, whereas an analogous decrease in the adsorp-tion energy is observed at higher coverages on bare Pd(111). As a consequence, Ediff is roughly constant until the number of adsorbed CO molecules is 0.17, reaches a maximum shortly before E_adsdecays on Pd(111), and decreases at higher CO exposures.

On the supported systems, Ediff is roughly constant until CO saturation is reached for both Pd

deposition coverages. Accordingly, the difference in the CO adsorption energy due to saturation with oxygen is largely independent ofΘCOon Pd/Fe₃O₄.

10.5 Summary

CO-O coadsorption experiments on Pd(111) and Pd/Fe₃O₄ have been discussed in this chapter.

The experiments were performed at a temperature of 110 K at which no CO₂evolution occurs.

The sticking data shows that CO adsorbs via precursor states on all three substrates. On oxy-gen covered and bare Pd/Fe₃O₄, transient CO adsorption occurs in a weakly bound, physisorbed state on Fe₃O₄close to CO saturation, although it cannot be excluded that transient CO adsorp-tion occurs on the Pd nanoparticles as well. The CO desorpadsorp-tion rate was modeled based on the assumption that CO desorption is a first order process and that the CO desorption energy is con-stant during the adsorption of∼7·10¹³molecules cm^–2. The desorption rate is determined to be

∼2.1 s^–1.

The measured CO coverage on bare Pd(111) and on oxygen covered Pd(111) is in good agree-ment with the literature data. Based on the number of adsorbed CO molecules on Pd/Fe₃O₄ catalysts, it can be estimated that the CO coverage on the nanoparticle facets is comparable to the one on the respective single crystals in both cases.

The initial CO adsorption energy on pristine Pd(111) and on Pd/Fe₃O₄ is in agreement with earlier SCAC measurements. Due to CO-O interactions, the CO adsorption energy decreases by 35±4 kJ/mol on Pd(111). This energy difference increases at an intermediate CO coverage and decreases again close to CO saturation. In contrast, CO-O interactions cause a decrease in the CO adsorption energy by 40-50 kJ/mol on oxygen covered Pd/Fe₃O₄ catalysts. This value is largely independent of the CO coverage until saturation.

11 CO oxidation

Catalytic oxidation of CO on platinum group metals is one of the most widely studied surface reactions, partially due to its relatively simple mechanism but also due to its practical importan-ce. Nevertheless, information from experimental studies on the energetics of the various reaction steps on single crystal facets and especially on supported nanoparticles is still incomplete.

The energetics of oxygen adsorption and CO adsorption on the oxygen covered catalysts, inves-tigated with the current SCAC setup, has been discussed in the previous chapters. In this chapter, our results on transient CO₂-evolution and the energetics of the reaction steps of the CO oxida-tion reacoxida-tion on oxygen covered catalyst will be discussed.

In the first section of this chapter, the CO oxidation experiments on Pd(111) and Pd/Fe₃O₄ ca-talysts with the Pd deposition coverage 4 Å Pd (3.4 nm) and 1.5 Å Pd (2.9 nm) are discussed.

In the second section, an estimate for the surface reaction energies of the three systems is given and the relative reaction energies on Pd(111) and Pd/Fe₃O₄are compared.

11.1 Introduction

An introduction on the CO oxidation reaction on Pd facets and Pd nanoparticles has already been given in chapter 2. The overall reaction, which occurs via a Langmuir-Hinshelwood-mechanism, can be separated into several reaction steps: (i) the dissociative adsorption of O₂, (ii) the mole-cular adsorption of CO, (iii) the recombination of adsorbed CO and O and (iv) the desorption of CO₂. The literature on CO oxidation shows that the reaction constant is strongly dependent on the chemical composition of the surface and the surface concentration of CO and oxygen.

The activation energy for CO oxidation on Pd(111) was found to be ∼105 kJ/mol for small CO and oxygen coverages [65, 66],∼67 kJ/mol forΘCO<0.25 on an oxygen covered surface [65, 66, 68] and∼41 kJ/mol at the highest CO and oxygen exposures [70].

The various mixed CO-O adsorbate structures, introduced in Chapter 10, were observed to ex-hibit different activities in the CO oxidation reaction. At 300 K on Pd(111), no reaction could be detected before or during compression of the oxygen p(2x2) phase into the (√

3x√ 3)R30^◦ structure and reaction in the (√

3x√

3)R30^◦ phase has not been observed below 200 K, while reaction of the mixed c(2x1) adsorbate structure was found at temperatures as low as 136 K [69, 70, 108]. Also on Pd(100), an increased reactivity towards CO oxidation has been observed at higher adsorbate coverages: increasing the oxygen coverage from 0.04 to 0.24 at a fixed CO coverage ofΘCO=0.05 leads to a decrease of the TPR peak from 422 K to 360 K [94].

Structure, size and support effects for CO oxidation on Pd have been observed. Matolin et al.

found an activation barrier of 32 kJ/mol and 19 kJ/mol for CO oxidation on oxygen covered Pd nanoparticles with the particle sizes 27 nm and 2.5 nm, supported onα-Al₂O₃. An activation energy of 45 kJ/mol was found on oxygen covered Pd(111) [34, 68]. Due to the capture zone

effect, the turnover rate on Pd nanoparticles of different sizes supported on alumina have been observed to vary by one order of magnitude [26, 27]. Matolin et al. studied the reactivities of Pd nanoparticles of similar sizes supported on alumina supports which have been either oxidi-zed, reduced or annealed prior to the experiment. Qualitative changes in the reactivity have been observed on these different supports [73]. For Pd nanoparticles with a smaller diameter than 4 nm, supported by mica [20, 201, 202], MgO [203] orγ-Al₂O₃[29, 44, 72, 74], dissociative CO adsorption could be detected in several studies, whereas this has not been observed for larger particles. Dissociative CO adsorption may lead to carbon poisoning during the reaction.

Libuda et al. studied the particle size dependence of the the CO oxidation reaction with molecu-lar beam experiments as a function of the relative fluxes of CO and O₂on Pd/Al₂O₃[78–80, 82].

At a low relative CO flux, where the nanoparticles are mainly covered by oxygen, CO adsorpti-on is the rate limiting step: In this regime, the turnover rate was found higher adsorpti-on 6 nm-sized Pd nanoparticles compared to 1.8 nm-sized Pd nanoparticles by a factor of more than 2. At a high relative CO flux, the surface is almost completely CO covered and dissociative O₂adsorption is the rate limiting step: In this regime, the turnover rate was observed to be higher on the smaller Pd nanoparticles. On 6 nm-sized Pd nanoparticles, strong changes in the IRAS features were observed as a function of the relative CO flux whereas these changes were found to be signifi-cantly weaker on Pd nanoparticles with the average size 1.8 nm. Communication effects could be observed in transient measurements on Pd/Al₂O₃[76, 80, 81].