Infrared Study of the Thermal Decomposition of Methanol Over a Pd/Ga2O3 Catalyst

S.E. COLLINS, M.A. BALTANÁS, A.L. BONIVARDI

Foz do Iguazú, Brasil

Congreso; 13° Congresso Brasileiro de Catálise y 3° Congresso de Catálise do MERCOSUR; 2005

Sociedade Brasileira do Catálise

The thermal decomposition of adsorbed methanol on 2 wt% Pd/silica, 2 wt% Pd/gallia and pure gallia, was studied by temperature-programmed surface reaction (TPSR) between 323 and 723 K under He flow using FTIR spectroscopy. After methanol adsorption on Pd/silica at 323 K, the concentration of methoxy species on silica decreased during the TPRS experiment but some methoxy groups still remained on this support even at 723 K. Simultaneously, methanol decomposed over metallic Pd to yield, stepwise, HCO and COS. On clean gallium oxide, methanol is Lewis-bound adsorbed to the surface as well as dissociatively adsorbed as methoxy (CH3O), but the position of the infrared bands indicates a stronger interaction of these species on gallium oxide than on silica. Methoxy species on gallia are decomposed to (mono- and bi-dentate) formate groups (m- and b-HCOO) at T > 473 K. We suggest that CO and CO2 are further produced by the non-stoichiometric transformation of those formates leading to the release of atomic hydrogen on the surface of the oxide, as detected by the Ga-H stretching infrared band. In the presence of Pd on the gallia surface, the dehydrogenation of CH3O species goes faster than over the pure oxide, and we propose the following mechanism for methanol decomposition: i) methanol reacts with OH groups on the gallia surface to produce methoxy species and water, ii) the dehydrogenation of the latter carbonaceous group leads to methylenbisoxy, first, and then to m-HCOO and b-HCOO, and iii) the hydrogen atoms released in the previous steps are transferred from gallia to the Pd surface where they recombine and desorb as H2(g).