Heterogeneous Catalysis and Surface Thermodynamics

Oxidation of Carbon Monoxide: Anomalous behavior over Ru

Introduction

Recent high-pressure catalytic reactor studies reported an anomalous behavior over the Ru(0001) surface 1: For oxidizing conditions, ruthenium yields the highest rate of CO₂ formation, but under ultra-high vacuum (UHV) conditions, the rate is by far the lowest; also the kinetic data is different to the other transition metal catalysts. Thus, the oxidation of CO over Ru(0001) appears to be a prime example of a process that exhibits so-called ``pressure gap'' behavior.

This figure shows an example of the CO2 turn-over frequency (TOF) over Ru(0001). In the same figure the TOF is given for the reaction over ``real working catalysts'' which contain a small percent of Ru metal. It can be seen the rates are very similar to over Ru(0001) which indicates that the latter represents a good model catalyst. Such single-crystal metal surfaces have the advantage that they can be very well characterized by surface science techniques. (Click for a larger image)

In recent years, due to improvements in calculation methods and increased computer power, it has become possible to undertake first-principles investigations for ``simple'' chemical reactions at surfaces. We have performed such studies for the catalytic oxidation of CO at ruthenium.

O at Ru(0001)

We first studied the adsorption of O on Ru(0001). It is known from experiments that under UHV conditions, at room temperature, dissociative adsorption of O₂ results in an (apparent) saturation coverage of half a monolayer. On the basis of our DFT calculations we predicted that even higher coverages should be attainable and that their formation under UHV conditions is only kinetically hindered 3; namely, we predicted formation of a full monolayer structure. This phase was subsequently verified experimentally 4. In order to achieve this, use of an oxygen carrying molecule that readily dissociates in the presence of O on the surface is necessary e.g. NO₂, or high gas pressures of O₂ need to be employed.

Adsorption structure where the interlayer spacing as obtained by low-energy electron diffraction is given. (Click for a larger image)

The adsorption energy of O on Ru(0001) is shown below, where it is given with respect to 1/2O₂. It can be seen that it decreases with increasing coverage, reflecting a strong adsorbate-adsorbate repulsion. It can also be seen that the hcp site is preferred over the fcc site for all coverages.

The yellow region indicates that in which dissociative chemisorption of O2 is kinetically hindered under UHV. (Click for a larger image)

As for above figure, but where the coverage regime is extended to 1.5 monolayers in which half a monolayer is located under the surface Ru layer in octahedral sites. (Click for a larger image)

The coadsorption of O and CO on Ru(0001) forms an array of ordered structures depending on the partial gas pressures. We have studied these systems as shown below 5. From left to right, the first three have been identified experimentally; on the basis of our calculations we predict the stability of the fourth one. The calculated atomic geometries are given along with the total valence electron density.

Scattering Reaction (Eley-Rideal)

The conditions under which highest rates of CO₂ production were experimentally reported were for oxidizing conditions. Under these conditions a high concentration (one monolayer) of O was proposed to be at the surface, and no CO could be detected on the surface; furthermore, an Eley-Rideal reaction mechanism was proposed to be active. In an Eley-Rideal mechanism a particle incident from the gas phase reacts with one that is adsorbed on the surface without itself adsorbing on the surface prior to reaction.

The interaction of CO with the surface with one monolayer of O coverage was therefore investigated theoretically 6. It was found that CO could not adsorb at any site on the surface. Thus we considered a scattering reaction of CO with adsorbed O, i.e. an Eley-Rideal mechanism as had been speculated.

In order to identify the transition state for the reaction and the corresponding activation barrier, an appropriate cut through the high-dimensional potential energy surface (PES) was constructed, as shown below.

PES for CO at O-covered Ru(0001).  (Click for a larger image)

The image on the left shows the identified transition state for CO2 formation. The corresponding energy barrier was about 1.1 eV. (Click for a larger image)

On the basis of these results an energy diagram can be constructed as below. (Click for a larger image)

Using an Arrhenius-type equation with the determined energy barrier, and a prefactor corresponding to the number of CO molecules hitting the surface per site per second, an estimate of the rate can be obtained. It was found that the rate is notably less than the measured one which indicates that the scattering reaction mechanism is not the dominant one taking place.

We thus investigated an alternative reaction mechanism, namely, the Langmuir-Hinshelwood (L-H) mechanism (see below).

Langmuir-Hinshelwood Reaction

It is assumed that there exists O vacancies in the high-O coverage layer. We find that CO can adsorb in such a vacancy with a small energy barrier. To determine the reaction path for CO₂ formation we adopt a standard grid approach, investigating the PES for many positions of the O and CO particles 7. The lowest energy pathway is found for CO moving towards a neighboring O atom as illustrated below.

Reaction pathway for CO2 formation for CO adsorbed in an O vacancy of the high-coverage O adlayer. The associated energy barrier is about 1.5 eV. (Click for a larger image)

The corresponding energy diagram is given below. In an estimate of the rate, in this case the prefactor will be much higher than in the scattering mechanism since both adsorbates are adsorbed on the surface and the prefactor reflects the vibrational frequency.
(Click for a larger image)  

To summarize, ruthenium exhibits unusual behavior with respect to other transition metals in that it binds oxygen (and CO) particularly strongly and can support high O concentrations at the surface without undergoing a phase transition to surface oxide formation. At low oxygen coverages (as present under UHV conditions) a Ru catalyst dissociates O₂ efficiently, but it holds the oxygen (and CO) so strongly that reaction to CO₂ is inhibited. For oxygen at monolayer (or higher) coverage, the adsorption energy is significantly weaker and thus CO₂ formation enhanced; also for such high O coverages CO does not have to diffuse very far before finding reaction partner. Our theoretical results thus explain the anomalous dependence of the reaction on oxygen pressure, as only under sufficiently high oxygen pressure the monolayer structure attained.

Surface Thermodynamics and Kinetics

Introduction

Figure 1, click on the figure for an enlargement

Lattice gas Hamiltonian

We apply the present approach to O at Ru(0001) for which detailed experimental data exists. In order to determine the interaction parameters, we perform DFT calculations for many various atomic arrangements of O atoms on Ru(0001) as shown below.

Interaction Parameters

Temperature Programmed Desorption and Heat of Adsorption

The shift of the peak maximum to lower temperature for higher initial coverages is appropriate for repulsive interactions. For coverage 0.1, the peak in the theoretical curve occurs at a somewhat higher temperature, indicating that the theory predicts a stronger binding of O on Ru(0001) than experiment. In terms of energies, however, this amounts to a small difference. We point out that with the present state of DFT-GGA a better agreement is not to be expected.

Theoretical (left) and experimental (right) temperature programmed desorption spectra. (Click for a larger image)

We also calculate equilibrium properties, namely, as shown below the isosteric heat of adsorption for a few temperatures. At the highest temperature it exhibits a smooth decrease. At the lowest temperature, sharp peaks and dips are seen at coverages 0.25, 0.5, 0.75, and 1.0. These coverages, in fact, correspond to each of the ordered phases that form in nature. We point out that it is only recently that the structure at coverage 0.75 was discovered 10 11 yet our results clearly indicate its stability.
Thus, our calculations predict successfully all of the ordered phases of O on Ru(0001).

Isosteric heat of adsorption for several temperatures. (Click for a larger image)

To summarize, we have developed a first-principles based approach for calculation of the thermodynamics and kinetics of an adsorbate on a surface. We used density-functional theory to create a lattice gas hamiltonian from which we evaluated the partition function. Our theoretical results show very good overall agreement with available experimental results, providing confidence in our approach.

References

1. C. H. F. Peden and D. W. Goodman, J. Phys. Chem. 90 1360 (1986).

2. A. Böttcher, H. Niehus, S. Schwegmann, H. Over, and G. Ertl, J. Phys. Chem. 101, 711 (1997).

3. C. Stampfl and M. Scheffler, Phys. Rev. B 54, 2868 (1996).

4. C. Stampfl, S. Schwegmann, H. Over, M. Scheffler, and G. Ertl, Phys. Rev. Lett. 77, 3371 (1996).

5. C. Stampfl and M. Scheffler, Israel J. Chem., in press.

6. C. Stampfl and M. Scheffler, Phys. Rev. Lett. 78, 1500 (1997).

7. C. Stampfl and M. Scheffler, Surf. Sci. in press.

8. H. J. Kreuzer and S. H. Payne in Equilibria and dynamics of gas adsorption on heterogeneous solid surfaces, eds. W. Rudzinski, W. A. Steele, and G. Zgrablich, Vol. 104, Elsevier (1997).

9. P. Piercy, K. De'Bell, and H Pfnür, Phys. Rev. B 45, 1869 (1992).

10. Y. D. Kim, S. Wendt, S. Schwegmann, H. Over, and G. Ertl, in press.

11. M. Gsell, M. Stichler, P. Jakob, and D. Menzel, Israel J. Chem., in press.

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