Mecanismo de Síntesis de Metanol por Hidrogenación de CO2 sobre Paladio/Galia: Un Estudio FT-IR in situ

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

Merida, México

Simposio; XIX Simposio Iberoamericano de Catálisis; 2004

The temperature-programmed adsorption of CO2 (TPA-CO2) and the temperatureprogrammed reaction of a mixture of H2 and CO2 (TPR-H2/CO2) over palladium catalysts supported on two types of gallium(III) oxides [a mixture of Ga2O3 (á and â phases) and GaOOH (Ga2-xO3-xHx); and a monoclinic â-Ga2O3] were studied by in-situ FT-IR spectroscopy. In the TPA-CO2 experiments (0.1 MPa) showed that bicarbonate (HCO3-) and bidentate (b-CO3=) and polydentate carbonates (p-CO3=) were the main species over the surface of both clean gallium oxides. TPR-H2/CO2 experiments on clean Ga2O3 exhibited that these carbonates were sequentially hydrogenated to formate (m-HCOO, b-HCOO) and methoxy (CH3O) species after the dissociative chemisorption of H2 over GaxO (T > 500 K) had occurred. Over the Pd/gallia catalysts a similar mechanism of the hydrogenation of carbon dioxide to methanol is postulated; however, the hydrogenated species started to be form from 323 K onwards. It is suggested that the hydrogen spills-over from metallic palladium to gallium sites and reacts there with the carbonaceous intermediates from said low temperature. The production of COg via the reverse water gas shift reaction was observed over both Pd/gallia catalysts. Upon considering our previous results on Ga-Pd/SiO2 catalysts, a more comprehensive mechanism to methanol synthesis from CO2 and H2 over the Pd-Ga system is presented.