Multi-Scale Simulation of Catalysed Surface Reactions

Significant Achievements, Anticipated Outcomes and Future Work

We have selected finite cluster models to represent the infinite nature of real silver catalyst. Cluster size dependence of the atomic oxygen adsorption energy is studied using models varying in size to address cluster edge effects and to select an adequate cluster model for methanol adsorption. The stability of oxygen atoms on the silver surface is rationalized by analyzing the binding energy on different local symmetry points of the silver models. We found that oxygen atoms are strongly chemisorbed to the Ag(111) surface. The highest co-ordination adsorption sites (three-hollow sites) have the largest binding energy and the stability is not affected considerably by increasing/decreasing the size of the cluster models within the number of silver atoms employed (10-22 silver atoms).

The stability of several of the surface species that are present during the oxy-dehydrogenation of methanol was calculated on a silver cluster model having 19 silver atoms. The stability of these species on a clean silver surface increases in the order of formaldehyde (≈ 2 kcal/mol) < methanol (≈ 5 kcal/mol) < methoxy (33 kcal/mol) < hydroxyl (50.5 kcal/mol) < oxygen (65.0 kcal/mol). The presence of oxygen atoms on the silver surface increases the stability of the surface species in the order of methanol (≈ 13.6 kcal/mol ) < formaldehyde ( 30.0 kcal/mol) < methoxy ( 36.2 kcal/mol).

By analyzing the binding energy, we found that on oxygen-free silver the methoxy group is strongly chemisorbed but methanol and formaldehyde are weakly adsorbed in the form of physisorption. On a pre-oxidized silver surface, methoxy is weakly perturbed by oxygen atoms but methanol and formaldehyde are stabilized. The hydroxyl hydrogen of methanol bonds to the co-adsorbed atomic oxygen stabilizing the methanol molecule. Atomic oxygen acts as a Brönsted base abstracting protons from methanol and forming methoxide groups. Formaldehyde is strongly stabilized by oxygen atoms by forming formate groups (-OCH₂O-). These theoretical results agree with the experimental fact that methanol only reacts with pre-oxidized silver catalyst and that methoxide is one of the most stable surface species on silver surface. The predicted local geometry of the carbon-oxygen species is within the experimental uncertainty.

We plan to study the effect of sub-surface oxygen on the stability and local geometry of the surface species. Sub-surface oxygen has been suggested from experimental studies to play an important role in the reaction mechanism, but very little is known about the chemistry of this species. We expect to understand at a molecular level the effect of different oxygen species on the reaction mechanism and be able to define a reaction coordinate for the dehydrogenation of methanol. We plan to use periodic boundary calculations to analyze the effect of surface relaxation.

Computational Techniques Used

Computational Quantum Chemistry studies are performed by means of first-principles density functional cluster model. The GGA-mPW91 functional is used to calculate the geometry and binding energy of surface species. The semi-relativistic LANL2DZ effective core potential is used for the Ag atoms to replace the Ar core. Electrons of the oxygen and hydrogen atoms are described with a 6-31G(d) basis set.

Publications, Awards and External Funding

External Funding and Awards

University of Sydney, Sesqui Postdoctoral Fellowship

Publications

None.