CEQUP/Dept. Química, Faculdade de Ciências da Universidade do Porto

Rua do Campo Alegre, 687

4069-007 Porto

Portugal

The adsorption of a large variety of molecules on metal surfaces has been studied in the last two decades by means of experimental as well as theoretical techniques [1-4].

The reaction of methanol on transition metal surfaces is one of the most studied reactions in surface science [1]. The major reasons are the importance of methanol in the chemical industry (synthesis from syn-gas and oxidation to formaldehyde) and also because methanol adsorption could be regarded as a model for the adsorption of larger chain alcohols.

On metal surfaces, methanol could be adsorbed molecularly or suffer decomposition to a large variety of products depending on the nature of the metal to which it is bound, as well as the presence of co-adsorbates. On clean surfaces, the decomposition products are a function of the inherent reactivity of the metal toward C-H and C-O activation. On the relatively inert copper, silver and gold surfaces only partial dehydrogenation to formaldehyde and hydrogen is found. In the latter two surfaces, co-adsorbed atomic oxygen on the surface is needed for the occurrence of methanol O-H bond scission. Methyl formate is also found on these surfaces. On more reactive metals (Ni, Pt, Pd, Ru, Rh) it is observed as products CO and H₂ while on Fe and Mo some methane is also found formed indicating C-O bond scission.

Several reaction intermediates were found also. The adsorbed methoxy and formate species were extensively studied. A few other species such as dioxymethylene have been postulated as reaction intermediates.

In order to understand at the atomic level the methanol oxidation, we have employed the density functional theory (DFT) to study the interaction of the isolated species show above with copper, silver and gold surfaces. The effect of co-adsorbed species is also analysed. Vibrational frequencies, adsorption energies, adsorption geometries and oxidation mechanism obtained with the DFT based B3LYP method will be reported [5-7].

Financial support from Fundação para a Ciência e Tecnologia is acknowledged. J. R. B. G. thanks PRAXIS XXI for a Ph. D. grant.

[1]- C. M. Friend and X. Xu, Annu. Rev. Phys. Chem. 42 (1991) 251.

[2]- N. D. S. Canning and R. J. Madix, J. Phys. Chem. 88 (1984) 2437.

[3]- C. M. Friend, Scientific American, Apr (1993) 42.

[4]- J. Sauer, Chem. Rev. 89 (1989) 199.

[5]- J. R. B. Gomes and J. A. N. F. Gomes, Theochem, 463 (1999) 163.

[6]- J. R. B. Gomes and J. A. N. F. Gomes, Surf. Sci., accepted.

[7]- J. R. B. Gomes and J. A. N. F. Gomes, Electrochim. Acta., accepted.