BINDING OF RADICAL SPECIES TO SURFACES: CLUSTER MODEL STUDY OF CH3O AND OH CO-ADSORPTION ON COPPER (111)
J. R. B. Gomes1, F. Illas2 and J. A. N. F. Gomes1
1CEQUP / Faculdade de Ci�ncias da Universidade do Porto
Rua do Campo Alegre, 687 - 4150 Porto
2Departament de Qu�mica F�sica da Universitat de Barcelona
C/ Mart� i Franqu�s,1 - 08028 Barcelona
The adsorption of small molecular radicals at metal surfaces has considerable experimental and theoretical interest since they are present and can occur as reaction intermediates in heterogeneous catalytic processes. Two examples of these species are the hydroxy, OH, and methoxy, CH3O, radicals. The OH radical is found, e. g., as a component in the catalytically activated hydrogen-oxygen reaction1 or in the water-gas shift reaction2. The CH3O is the most stable intermediate found in the methanol oxidation or methanol synthesis reactions which are catalysed by metal surfaces3. Both species can exist simultaneously adsorbed on a metal surface and it is thought that such reaction is the responsible for the CH3O to formaldehyde, H2CO, reaction step. Since H2CO is one of the most manufactured products in chemical industry, there is a large interest in the mechanism of this oxidation reaction.
The structure of the two isolated species adsorbed on Cu (111) is well characterised by means of a large variety of experimental techniques4 and also by theoretical studies5. Both species are adsorbed on metal surfaces through their oxygen atoms on fcc hollow sites and the CO axis of CH3O is normal to the metal surface. Also, there is a trend for both species to accumulate electron charge which is transferred from the metal surface in order to form a closed-shell system.
Adsorbed methanol oxidates in a great extent and yields as major products H2O, H2CO, H2 and also, in the presence of pre-adsorbed oxygen, CO2. It is known that oxygen increases the formation of the CH3O radical3. However, its role in the hydrogen abstraction from the methoxy radical (reaction scheme below) is not well understood.
CH3O (a) + O (a) => H2CO (a) + OH (a)
CH3O (a) + OH (a) => H2CO (a) + H2O (a)
Then, H2CO and H2O are desorbed from the surface:
H2CO (a) => H2CO (g)
H2O (a) => H2O (g)
In order to understand the role of atomic oxygen and hydroxy in the CH3O oxidation, we have performed DFT calculations on the co-adsorption of CH3O and OH on a Cu (111) surface. The results are compared with the ones obtained for the methoxy adsorption on the copper clean surface. The DFT method used is the Becke3LYP which has proven to be very good to treat systems involving transition metal atoms.
In this communication, will be presented very recent results of the interaction of the isolated species with the copper surface and also of the interaction of the OH and CH3O species co-adsorbed on the metal surface. Four different metal clusters were used to simulate the interaction of both radicals with the different adsorption sites on the 111 surface (top, bridge and hollow sites). The size of the metal clusters was varied from 20 to 27 metal atoms.
Financial support from FCT and project PRAXIS XXI is acknowledged. We thank also C4, CESCA-CEPBA for enabling access to the SP2 through TMR programme. JRBG thanks PRAXIS for a Ph. D. grant.
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5- K. Hermann, M. Witko, L. G. M. Pettersson and P. Siegbahn, J. Chem. Phys., 99 (1993) 610; J. R. B. Gomes and J. A. N. F. Gomes, Theochem, accepted; J. R. B. Gomes and J. A. N. F. Gomes, submitted to Surf. Sci.