During my postgraduate studies my research interests were about surface catalysis. The research project has been carried out at SISSA-ISAS (International school for advanced studies), in the group of Stefano Baroni, in collaboration with the group of Renzo Rosei at ELETTRA, the synchrotron light source based in Trieste (Italy).

Transition metals are particular important in surface catalysis. In particular, rhodium is one of the best catalysts for the reaction 2CO + 2NO -> 2CO2  + N2 . This reaction (schematically shown in the figure on the right) is particularly important because eliminates the two poisonous CO and NO gases from the combustion engines exhaust gas.

In order to understand the mechanism of a surface reaction, a detailed knowledge of its elemental steps is required. For example, how does NO dissociate? What are the N and O adsorption sites after the NO dissociation? What happens to the surface upon adsorption of these atoms/molecules?

I focused in particular on the behaviour of two particular high symmetry rhodium surfaces, the Rh(110) and the Rh(100), upon adsorption of Carbon-monoxide (CO), Oxygen, Nitrogen and Carbon, which I have studied using first-principles techniques.

• On Rh(110) Carbon-monoxide adsorbs with carbon below, and the most stable, low temperature, structure is the (2x1)p2mg, with the Carbon-monoxide molecules adsorbed in the short bridge sites and alternatively tilted along the normal to the surface. We parametrized the interaction between the Carbon-monoxide molecules with an effective potential energy fitted to ab initio calculations, and we studied the finite temperature properties of the system using montecarlo simulations on the effective potential. We found that this structure undergoes an order-disorder transition at a temperature of about 300 K, in agreement with experimental findings.  Ref 0, 1.

• On Rh(100) oxygen adsorbs in the center of the fourfould sites formed by the rhodium first layer surface atoms. The adsorption of oxygen induces a `clock' reconstruction of the surface, where the sites filled with oxygen are deformed into rombii, and the empty sites undergo a clockwise and anticlockwise rotation (right figure on the right). This reconstruction is very similar to the one induced by carbon and nitrogen adsorption on Ni(100), where, on the contrary,  the rotating squares are those filled with the adsorbed atoms ( left figure on the right). Because of this similarity, the reconstruction induced by oxygen on Rh(100) was originally misinterpreted in the experiments, which assigned to the system O/Rh(100) the same reconstruction of the systems C/Ni(100) and N/Ni(100) (left figure on the right).  We actually found that in this reconstruction the oxygen atoms get off the center of the rombii, becoming essentially threefold coordinated (figure on the left). This geometry was not recognized untill very recently in the work by Baraldi et al. at ELETTRA. See our papers, Ref 0, 2, 3.

1. D. Alfè and S. Baroni,  "The structure and phase stability of CO adsorbates on Rh(110)", Surface
Science, 382, L666-L671 (1997).

2. D. Alfè, S. de Gironcoli and S. Baroni,  "The reconstruction of the Rh(001) surface upon Oxygen
adsorption", Surface Science,  407 , 151-157 (1998).

3. D. Alfè, S. de Gironcoli, and S. Baroni, "The reconstruction of Ni and Rh (001) upon Carbon, Nitrogen
or Oxygen adsorption", Surface Science,  437, 18-28 (1999).
 
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