Search

The coupling mechanism of two propyne molecules on the Cu{111} surface has been studied by means of a DFT cluster model approach. The gas-phase dimerization is highly unfavored because of the energy cost to activate propyne by promoting molecules to the triplet state. However, on the surface, propyne is adsorbed with a geometry very close to that of gas-phase propyne in the triplet state and, therefore, activation of the reacting molecules does not incur any additional energy cost. Moreover, isomerization to vinylcarbene is necessary to allow head-to-tail or head-to-head coupling resulting in 1,4- and 1,3-cyclohexadiene intermediates. Vinylcarbene biradicals are present at the surface because the isomerization process proceeds at practically no (thermodynamic) cost. Both head-to-tail and head-to-head interactions suggested by experiment are possible. Both cyclohexadiene intermediates can dehydrogenate to yield benzene and H-2 with a moderate energy cost. An alternative head-to-head interaction, without interacting tails, yields two C-6 noncyclic intermediates which, upon H-2 addition, can be regarded as being responsible for the 82 amu product observed in the reaction.

Coadsorption of K and CO at the Cu(100) surface and adsorption and reaction of CO and CO2 at surface and defect sites of MgO are reviewed. The cluster technique is used to determine optimal adsorption geometries, the physical properties of the adsorbed species, in particular the vibrational frequencies, and the nature of the surface chemical bond. It is shown that the large red-shift of the CO ωe when K is coadsorbed on a metal surface can be explained only with very short K-CO distances. Chemisorption at oxide surfaces is also discussed and the important role of the surface defects in determining the reactivity of the MgO surface is shown.

Ab initio cluster model calculations on the interaction of CO and CO2 with NiO and MgO surface and defect sites are reviewed. NiO behaves very similarly as the non-transition metal oxide MgO towards CO chemisorption because the bonding of CO to these surfaces is dominated by electrostatic terms and involves a very modest σ donation or, in the NiO case, π back donation. The role of the surface defects in determining the reactivity of oxides toward CO and CO2 chemisorption is shown.

The surface chemistry of metal oxides is of considerable importance in catalysis. A large fraction of industrial catalytic processes involves reactions occuring at the surface of transition metal oxides.1,2 For this reason, there is a need for the development of analytical surface techniques for the determination of the physico-chemical properties of the oxide surfaces. An important problem is the determination of the formal oxidation state of the surface cations. This can be indirectly deduced from measures of the Lewis’s acidity of the cationic surface sites. A variety of physico-chemical methods have been used to characterize the surface of an oxide: thermal desorption, microcalorimetry, Potentiometric titrations, spectroscopy. Usually, these techniques make use of a probe molecule adsorbed on the surface. The response of this adsorbed molecule to the experimental technique applied provides a measure of the chemical nature of the adsorption sites.