Heterogeneous catalysis: Methanol oxidation
J.R.B. Gomes and J.A.N.F. Gomes
CEQUP/Faculdade de Ciências da Universidade do Porto
Rua do Campo Alegre, 687, 4069-007 Porto, Portugal
email: [email protected]
Methanol chemistry is of significance to industry, exemplified by its partial oxidation to formaldehyde over silver or copper based catalysts1. These considerations have motivated numerous surface science investigations of methanol2, some emphasising intermediates generated upon adsorption and during subsequent thermal processing. Wachs and Madix3 showed that formaldehyde formation on an oxygen precovered Cu (110) surface proceeds via a methoxy, CH3O(a), surface intermediate. This pioneering work has been followed by extensive investigations that employed surface science tools (e. g., X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), infrared spectroscopy (IR), etc.) to characterise oxygenates on a variety of single-crystal, polycrystalline and metal oxide surfaces.
Despite the enormous experimental effort, much more still needed to be done since these kind of heterogeneous catalytic reactions are too complicated. There are several examples in the literature where the analysis of the experimental results was very difficult. One example is the adsorption of formate on copper surfaces4. In the middle eighties it were published various papers about formate adsorption with conflicting results derived from a difficult analysis of the experimental results5-10. Differences concerning the adsorption site that stabilise more efficiently the formate species, distance of the adsorbate to the surface and internal geometry of the adsorbate were registered.
In cases like the one above, theoretical studies assume major importance as an helping tool. The clarification of experimental findings such as vibrational frequencies, adsorbate geometry on the different adsorption sites could be retrieved.
The importance of methanol in the chemical industry and the difficulty to understand the methanol reaction mechanism has motivated our theoretical study of methanol and methanol oxidation intermediates on the noble metal surfaces of copper, silver and gold.
For that purpose, we use quantum chemical calculations with the density functional theory (DFT) methodology. The three-parameter hybrid method proposed by Becke is employed. It includes a mixture of Hartree-Fock (HF) and Density Functional Theory (DFT) exchange terms, associated with the gradient corrected correlation functional of Lee, Young and Parr. We have used DFT in order to include some correlation effects with minor computational effort.
We have studied how molecules such as the methoxy radical (CH3O)11,12, formaldehyde (H2CO), dioxymethylene (H2CO2)13, formate (HCO2)14 and, of course, methanol, adsorb on these surfaces. The interest on the copper group is due to the use in chemical industry of a copper/zinc oxide catalyst to obtain methanol from syn-gas and of a silver catalyst to the production of formaldehyde from methanol.
In this poster, theoretical results about the adsorption of the species referred above will be presented and compared, where possible, with experimental data. These results include vibrational frequencies, adsorption geometry and charges and the reaction mechanism for the oxidation of the methoxy radical to formaldehyde on a clean copper surface and on a pre-adsorbed hydroxyl copper surface.
Acknowledgements
We thank JNICT and PRAXIS XXI for financial support.
References